GB2604011A

GB2604011A - Security device and method of manufacture thereof

Info

Publication number: GB2604011A
Application number: GB2117708.4A
Authority: GB
Inventors: Commander Lawrence; Godfrey John; Whiteman Robert
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2020-12-09
Filing date: 2021-12-08
Publication date: 2022-08-24
Anticipated expiration: 2041-12-08
Also published as: AU2021397868A1; GB202019383D0; GB2604011B; GB202117708D0; EP4259450A1; WO2022123241A1

Abstract

A security device 10 comprises a substrate 12; a relief structure 20 on a first side 12a of the substrate; and a colour filter 30. The relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source L to thereby project a caustic image CI. The colour filter is configured to overlap in use at least part of the relief structure and comprises one or more at least semi-transparent materials. At least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour. The caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light. The security device may be configured such that the caustic image projected by the security device when illuminated with white light exhibits a multi-tonal and/or multi-coloured appearance. The light transmitted by the colour filter varies in colour and/or tone between laterally offset regions of the colour filter.

Description

SECURITY DEVICE AND METHOD OF MANUFACTURE THEREOF

This invention relates to security devices such as may be used as a mark of authenticity associated with an object of value, such as a security document including banknotes, passports, certificates, licences and the like. Methods for manufacturing security devices are also disclosed.

Objects of value, and particularly documents of value such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such objects are provided with a number of security devices for checking the authenticity of the object. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, venetian blind devices, lenticular devices, moire interference devices and moire magnification devices, each of which generates a secure visual effect. Other known security devices include holograms, watermarks, embossings, perforations and the use of colour-shifting or luminescent / fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult, or impossible, to copy using available reproduction techniques such as photocopying. Security devices exhibiting non-visible effects such as magnetic materials may also be employed.

Recently, relief structures which project caustic images have been employed in security devices. A "caustic" is the envelope of light rays reflected or refracted by a curved surface or object, and the visualisation of that envelope of light rays is referred to herein as a "caustic image", projected by the relief structure (which defines the curved surface(s)) when illuminated by a light source. The image is formed as a result of the relief structure redirecting the incident light to form bright spots (where redirected light rays converge) and dark spots (where the redirected light rays are largely absent), the relative arrangement of the bright and dark spots combining to convey an image. Various methods for designing a relief structure which will project a certain caustic image have been disclosed in WO-A- 2019/063778, WO-A-2019/063779 and WO-A-2020/070304, in each of which the caustic image is a "real" image which is visualised by projection onto a suitable surface, such as a wall or screen. Similarly, WO-A-2020/070299 discloses techniques for forming a relief structure which generates a "virtual" caustic image, which does not require projection onto a surface but can be viewed directly by the naked eye.

While the ability to project a caustic image is a useful security feature, since a photocopy of the device will not contain the necessary relief structure and will therefore not project an image, as it stands the device is still prone to counterfeiting. For example, it is possible to simulate the projected image effect by simply printing a light-scattering varnish on a transparent substrate in regions corresponding to the dark spots of the desired image. When light is shone through the substrate, an image approximately corresponding to the areas of the substrate where the varnish is absent will be visualised. The image will not be as bright as a caustic image, and will be more obvious on the substrate when inspected under reflected light than would be the case for a caustic image -but it may still be sufficient to convince a layperson that the device is genuine. Further, if the caustic relief structure surface is left exposed on a genuine device in circulation, then it can readily be used as a master for contact copying, in which case a counterfeiter may be able to reproduce the relief and hence form near-identical devices.

It would therefore be desirable to increase the security level of such security devices.

In accordance with a first aspect of the invention, a security device is provided, comprising: a substrate; a relief structure on a first side of the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and a colour filter, the colour filter being configured to overlap in use at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby the caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light.

As noted above, a caustic image is the visualisation of the envelope of light rays reflected or refracted by a curved surface or object, here provided by the relief structure. That is, the relief structure includes one or more curved surfaces, configured as necessary to give rise to the desired caustic image. The caustic image may be a real image or a virtual image. The caustic image can take any desired form, such as defining alpha-numeric text, one or more symbols, a logo, a portrait or another graphic. A "colour filter" is something which selectively transmits only certain wavelengths of light, so that when the incident light is white, the light transmitted by the colour filter is non-white. This property of the one or more materials which form the colour filter could be intrinsic to the material itself (i.e. it is a bulk property of the or each material) -for example, the material could include a substance which absorbs certain wavelengths and transmits others. Alternatively, the wavelength-selective nature of the material(s) may be due to the or each material's structure and/or interactions between components of the or each material -for example, the material could be structured to generate plasmonic colour (e.g. providing an array of nanoholes or nanopillars) or could comprise multiple internal layers giving rise to colour by way of interference effects.

By providing a colour filter overlapping at least part of the relief structure (at least in use) as set out above, the security level of the device is thereby increased. This is because, as a minimum, when viewed in reflected light, the device will exhibit at least one non-white colour due to the colour filter, which is hard to imitate since a would-be counterfeiter will need to have access to one or more suitable at least semi-transparent materials of the right colour(s), which do not impede the generation of the caustic image, and to provide the at-least semi-transparent material(s) in the right position(s). The colour filter can also help to camouflage the presence of the caustic device. Depending on the configuration of the security device, the colour(s) exhibited by the caustic image may also include one or more non-white colours (as is preferred), which presents a further challenge to the counterfeiter seeking to imitate the device.

It should be noted that while the colour(s) exhibited by the caustic image might be the same as the respective colour(s) transmitted by each of the one or more at least semi-transparent materials forming the colour filter, this is not necessarily the case. As will be explained in more detail below, in some embodiments where the colour filter comprises a plurality of materials which transmit different colours, the caustic relief structure may result in mixing of two or more of those colours such that the caustic image exhibits a different, combined colour. For instance, the colour filter could comprise red, green and blue regions, which colours could be mixed by the caustic relief resulting in a white (or near-white) caustic image. Similarly even where the colour filter comprises only a single (non-white) coloured material (optionally alongside a colourless material), the caustic relief structure may result in display of multiple tones of that non-white colour in the caustic image. Hence, there is not necessarily a direct correspondence between the number (or selection) of colours in the colour filter and those exhibited in the caustic image -although such a direct correspondence may be preferred in some embodiments.

The term "colour" used herein includes any visible colour, including white, hence the use of the term "non-white colour" where white is to be excluded.

By the relief structure being "on" the substrate, it is meant that the relief structure is carried by the substrate (and may or may not be integral therewith). "On" does not require direct contact between the two items in question (although the contact may indeed be direct) -for example an intermediate layer such as a primer layer may exist between the relief structure and the substrate. Nor does the term "on" impose any limitation in terms of orientation, since for example an item may be "on" the underside, or "on" the lateral side, of another item to the same extent that it may be "on" top of that item. The term "on" should be interpreted in this manner wherever it appears herein unless otherwise stated.

In most embodiments, the colour filter will be fixed in overlapping relation with the relief structure and can be embodied in various different ways as will be explained below. In such cases, the first aspect of the invention provides a security device, comprising: a substrate; a relief structure on a first side of the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and a colour filter overlapping at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby the caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light.

However, it is also possible to provide the colour filter separate from the relief structure in such a way that the overlapping can be achieved by a user during examination of the security device in an authentication test. Examples will be provided below.

In some embodiments, the colour filter may be arranged to overlap only part of the relief structure, e.g. if only part of the caustic image is to appear non-white in colour or if a masking component is also provided elsewhere (see below).

However, preferably, in use the colour filter overlaps substantially all of the relief structure. The colour filter may also extend laterally beyond the relief structure in one or more directions.

The colour and/or tone of the light transmitted by the filter could be uniform over its full lateral extent. For instance, this will be the case if the whole filter is made of the same material (or overlapping set of materials) with constant thickness. However, in preferred embodiments, the light transmitted by the colour filter varies in colour and/or tone between laterally offset regions of the colour filter This increases the complexity and hence security level of the device.

The colour filter could consist of a single material, which material transmits a desired non-white colour, in which case at least part of the caustic image will typically exhibit the same non-white colour (although different tones of that nonwhite colour may be exhibited across the at least part of the caustic image).

However, to further increase the complexity and hence security level of the security device, it is preferred that the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of (only) one of the at least semitransparent materials, or an overlapping combination of two or more of the at least semi-transparent materials, whereby the colour of light transmitted by the colour filter varies across the colour filter in accordance with the regions. In other words, the colour transmitted by the colour filter is uniform across any one region, but differs from region to region of the colour filter. Each region could be formed of a single material or, in some embodiments, multiple overlapping materials depending on how the colour filter is constructed.

Alternatively or in addition, the colour filter may comprise a semi-transparent material which varies in thickness between regions of the colour filter, the tone of the transmitted light depending on the thickness of the semi-transparent material.

This is of particular relevance where the wavelength-selectivity is caused by a bulk property of the material, such as its absorption characteristics (rather than a surface structure for instance). While the colour (hue) of the transmitted light will be determined by the intrinsic characteristics of the material itself, the tone of the transmitted light will depend on the absolute volume of material through which the light passes. Hence, the greater the thickness of a semi-transparent material in a certain region, the darker the tone of the light transmitted by that region of the colour filter. The thickness of the at least one semi-transparent material in each respective region could be varied between a minimum (but non-zero) thickness corresponding to a minimum transmitted tone (regions of zero thickness, which are possible if the colour filter is implemented as a layer separate from that carrying the caustic relief, correspond to regions in which the colour filter is omitted) and a maximum thickness corresponding to a maximum transmitted tone (at which some light will still be transmitted by the filter). Elsewhere, the same at least one semi-transparent material could be provided at a still greater thickness, which does not transmit light, to form a masking component (discussed below).

It should be noted that the provision of regions with different transmitted colours and/or tones does not necessarily mean that the caustic image will appear multi- coloured or multi-tonal (although as noted below this is preferred) since the various transmitted colours and/or tones could become mixed by the caustic relief as mentioned above. This not only allows the appearance of the projected image to be made more complex, but also enables the colour filter itself to exhibit a non-projected image, arising from the arrangement of regions, which may or may not be the same as the caustic image as will be discussed further below. The non-projected image is that seen when the colour filter itself is viewed, either in reflected light or in transmitted light (as opposed to the projected caustic image, which will be generated elsewhere, e.g. on a projection screen, by light transmitted through the security device). In some preferred embodiments, discussed below, the non-projected image may appear in different colours depending on whether it is viewed in reflected or transmitted light, although its information content will be the same in both modes of viewing. The non-projected image could be multi-tonal and/or multi-coloured. If the colour filter is formed of a single material varying only in thickness from region to region, then the non-projected image will be multi-tonal but not multi-coloured (i.e. it will appear in multiple shades of one and the same colour).

In some preferred embodiments, the colour filter comprises a colourless transparent material forming one or more of the regions and a semi-transparent material transmitting only a subset of visible light wavelengths corresponding to a non-white colour forming another one or more of the regions. The use of a colourless transparent material alongside a semi-transparent material with a nonwhite colour enables the display of either a single colour caustic image (in which all the bright spots of the image have substantially the same hue and intensity), or a two-colour caustic image (containing parts with a non-white colour and other parts which appear white), or a multi-tonal caustic image On which different parts of the image have the same hue but different intensities).

In further preferred embodiments, the colour filter comprises at least two different semi-transparent materials each transmitting only a subset of visible light wavelengths corresponding to a different respective non-white colour, each forming one or more different respective (laterally offset) regions of the colour filter.

The use of at least two different materials alongside one another, each transmitting a different subset of visible light wavelengths and hence a different non-white colour, enables the display of images exhibiting more than one non-white colour although as noted above there is not necessarily a direct correspondence between the colours of the materials and the colours of the caustic image. Preferably, a colourless material may also be included in the colour filter (in one or more dedicated regions thereof), which allows for straightforward creation of white portions of the caustic image and/or multiple tones of individual hues. This could alternatively be achieved by omitting the colour filter in those dedicated region(s), if the colour filter is formed as a separate colour filter layer (see below).

Alternatively still, white portions of the caustic image could be created by mixing light transmitted through red, green and blue regions of the colour filter.

Advantageously, the security device is configured such that the caustic image projected by the security device when illuminated with white light exhibits a multi-tonal and/or multi-coloured appearance. "Multi-tonal" means that at least two different intensity levels of the same colour are exhibited simultaneously (e.g. light blue and dark blue), whereas multi-coloured means that at least two different nonwhite hues are exhibited simultaneously (e.g. red and yellow). In some cases this is achieved through configuration of the regions of the colour filter, e.g. differently coloured regions as described above. In such cases a multi-tonal and/or multicoloured appearance of the projected caustic image will be achieved where the caustic relief does not mix all the various colours transmitted by the different regions of the colour filter into a single colour of the caustic image. However, it is possible to achieve a multi-tonal appearance of the projected caustic image even where the colour filter is uniform and no regions as such are defined. This is because the caustic relief structure can be configured to redirect light in such a way that different parts of the image receive different amounts of same-coloured light resulting in different intensities and hence different apparent tones in the projected image.

The various regions and colours (or tones) of the colour filter could be arranged without any relation to the caustic relief structure, in which case the colouration/tonality of the caustic image may bear no relation to the image content. However, it is preferable to provide at least a coarse correlation between the arrangement of the material(s) in the colour filter and the caustic image that will ultimately be displayed, so that the colouration/tonality complements the image.

For example, where the caustic relief structure is configured to project an image of the number "20, it may be determined that the majority of the light contributing to the element "2" of the image comes from the left half of the relief structure, while the majority of the light contributing to the element "0" of the image comes from the right half. Hence by configuring the colour filter to have a region of red material overlapping the left half of the relief structure and a region of blue material overlapping the right half, the caustic image will exhibit a red "2" alongside a blue "0". It will be appreciated that the correlation between the arrangement of regions and the relief structure need not be exact, since even if a small proportion of light contributing to the "2" is coloured blue, it will be overwhelmed by the largely red light forming that element.

Hence, advantageously, a first group of one or more of the regions of the colour filter, which overlap one or more first areas of the relief structure configured to generate a first portion of the caustic image, are formed of a first set of one or more at least semi-transparent materials, such that the first portion of the caustic image exhibits a first colour which is determined by the colour(s) transmitted by the first set of materials, the first portion of the caustic image preferably being a first distinct element of the caustic image. (A "distinct element" of an image may be, for example, the whole of a contiguous bright part of the image, e.g. one letter or number in an image comprising alphanumeric text. Alternatively the elements may be distinguishable by context, e.g. the petals of a flower or face of a portrait).

In the simplest case, the first set of materials will comprise a single material which transmits the first colour (preferably non-white). Thus, each of the regions in the first group will be formed of one and the same material, which by itself determines the colour of the first portion of the caustic image. However, in other cases the first colour could be a mixed colour resulting from the use of a first set of more than one materials, each with a different colour. Each region will still be formed of a single material, but since the group of regions which contribute to the first portion of the caustic image now includes regions of different colour, the result is that the first portion exhibits a colour which is a mixture of the colours transmitted by those materials. For instance, if half of the regions in the first group are blue and the other half yellow, the first portion of the caustic image will appear green. It should further be noted that the regions forming the first group could be adjacent one another or could be spaced from one another (e.g. by other regions of the colour filter), as will be explained below.

Hence in some preferred implementations, the first set of materials comprises a single at least semi-transparent material, the first colour being the colour transmitted by the respective single at least semi-transparent material. In other preferred implementations, the first set of material comprises a plurality of different at least semi-transparent materials, each forming different region(s) of the colour filter, whereby the first colour is a colour arising from the mixture of colours transmitted by the materials in the respective set.

Comparable considerations apply to any other portions of the caustic image.

Thus, preferably, a second group of one or more of the regions of the colour filter, which overlap one or more second areas of the relief structure configured to generate a second portion of the caustic image, are formed of a second set of one or more at-least semi-transparent materials, such that the second portion of the caustic image exhibits a second colour which is determined by the colour(s) transmitted by the second set of materials, the second colour being different from the first colour, and the second portion of the caustic image preferably being a second distinct element of the caustic image. All the same considerations discussed in relation to forming the colour of the first portion of the caustic image immediately above apply similarly to forming the colour of the second portion here.

Preferably the second colour is a non-white colour. Third and subsequent portions of the caustic image can be coloured in an equivalent way.

It is also possible to vary the thickness of any of the material(s) in the first or second sets (and indeed in any additional sets) between regions and/or between groups to vary the tone of the transmitted light, as mentioned above. This could be instead of varying the colour from one portion of the caustic image to another -the first portion of the caustic image could have a first tone, and the second portion of the caustic image could have a second (different) tone (but the same colour).

The more exact the correlation between (i) the areas of the relief structure which contribute to the various portions of the caustic image, and (ii) the regions of the colour filter which impart colour(s) to the transmitted light, the more difficult it will be for a would-be counterfeiter to reproduce the effect and hence the greater the security level.

In some embodiments, the areas of the relief structure and the portions of the caustic image which each area generates have substantially the same lateral arrangement relative to one another in the relief structure and in the caustic image, respectively. That is, the area(s) of the relief structure which generate a certain element of the caustic image will be positioned in approximately the same location in the relief structure as that element is positioned within the caustic image. For instance, if the caustic image is of the number "20", the relief structure will typically contain areas with similar curved surfaces which are also arranged laterally in the approximate form of the number "20". This is the case, for example, for the caustic surface relief structures disclosed in each of WO-A-2019/063778, WO-A2019/063779, WO-A-2020/070304 and WO-A-2020/070299. For brevity, such relief structures may be referred to hereinafter as "non-redistributed" relief structures, including both continuous relief structures and segmented relief structures which have not been redistributed (explained below). In such cases, if (as is preferred) the colour filter comprises a plurality of regions of different colours (or tones), configured to overlap certain areas of the relief structure so as to give colour to desired portions of the caustic image, this has the result that the colour filter exhibits a non-projection image arising due to the arrangement of regions, the non-projection image being a version of the caustic image. As already mentioned above, the colour filter itself will have a visible appearance, which in this implementation will have the form of an image (the "non-projection image") due to the arrangement of regions corresponding to the caustic image. In general, the direct appearance of the colour filter will typically be faint since the materials used will generally have a low optical density (explained below) and will generate low optical scattering -so the non-projection image may be seen as a coloured sheen or gloss and the device may need to be held against a uniform, light background (such as white paper) for the non-projection image to be seen clearly. The non-protection image may be viewed in reflected or transmitted light. The non-projection image could be single-colour, multi-tonal or multi-coloured.

However, in other preferred embodiments, the security level of the device can be significantly enhanced by enabling the lateral arrangement of the regions of the colour filter (and hence any non-projection image exhibited) to be independent of the caustic image. In other words, the two-dimensional appearance exhibited directly by the device in reflected or transmitted light is decoupled from the caustic image it projects under appropriate (usually transmitted) illumination. This can be achieved if the relief surface is segmented, the segments being distributed such that areas of the relief structure and the portions of the caustic image which each area generates have different lateral arrangements relative to one another in the relief structure and in the caustic image, respectively. For example, the caustic relief structure necessary to generate a certain caustic image may be divided into segments (the light rays redirected by any one segment preferably contributing to a single corresponding portion of the caustic image), and those segments redistributed in some other order so that their relative arrangement is different from that in the original (undivided) relief structure. The segments may be the same size and/or shape as one another, but neither is essential. For instance, the array of segments may take the form of a regular 1D or 2D grid of segments (orthogonal, hexagonal or otherwise), each having substantially the same shape and size as one another, e.g. square, rectangular, parallelogram. The segments preferably tessellate with one another The segments' shapes can be adapted at boundaries of the non-projected image to give smoother curves in the non-projected image.

Redistribution of the segments does not significantly affect the caustic image since this depends primarily on the angles of the redirected light rays, rather than the particular position from which each light ray emanates from the security device. The caustic image projected by a redistributed caustic structure may contain some artefacts caused by discontinuities in the resulting relief structure but through careful design their visual impact can be minimised. Typically the redistribution will be done by computer modelling, prior to formation of any physical relief structure. It will be appreciated that the same result can alternatively be achieved starting from a desired two-dimensional arrangement of segments and associated cells, and then allocating a certain relief to each segment, configured to redirect light to a desired angle, whereby a caustic image can be built up accordingly.

Hence it is not essential to create (either in software or in reality) a continuous surface relief structure corresponding to an image before then rearranging it in segments. The redistribution is relative to the two-dimensional layout of the eventual projected caustic image, however it is formed. For brevity, such relief structures may be referred to hereinafter as "redistributed" relief structures. WO-A-2019/063778 discloses an example of a segmented (non-continuous) caustic relief structure and one way of performing segmentation, although not redistribution of the segments as presently preferred -hence this is an example of a non-redistributed relief structure.

The use of a redistributed relief structure gives rise to numerous possibilities for achieving different visual effects through different arrangements of the colour filter. Preferably, the relief structure is configured such that each segment thereof redirects light towards a single portion of the caustic image, each portion of the caustic image receiving light from one or multiple segments, and the regions of the colour filter are arranged in cells, each cell of the colour filter overlapping a single segment of the relief structure and each cell comprising one or more regions, more preferably a single region, of the colour filter, such that the colour of light contributed to the caustic image by a segment is determined by the colour(s) transmitted by the material(s) forming the region(s) in the corresponding cell of the colour filter. In other words, each segment of the relief structure has a corresponding cell of the colour filter overlapping it, such that the light redirected by that segment has the colour of that cell. Like the segments, the cells may have the same shape and/or size as one another, but this is not essential. Each cell may comprise multiple regions (if the regions are smaller than the cells) or more preferably may comprise a single region. If multiple adjacent cells are formed of the same material, they collectively can form a region. In preferred implementations there may be a one-to-one correspondence between the cells and the regions. If there are multiple regions in one cell, multiple colours may be transmitted by the cell which will be combined by the segment into a mixed colour.

In one particularly preferred embodiment, the segments of the relief structure are arranged laterally relative to one another such that the colour filter exhibits a non-projection image arising due to the corresponding arrangement of cells, the non-projection image being different from the caustic image. That is, the segments (and hence the cells) are redistributed according to an image which is different from the caustic image, and which will then be visible when the colour filter is viewed directly in reflected or transmitted light. Preferably, the non-projection image is a human-intelligible image, i.e. having information content. Advantageously, the non-projection image comprises any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure (e.g. a building or bridge), a person, animal or plant. The information content of the non-projection image is preferably different from the caustic image -that is, the non-projection image is a different image from the caustic image (and not merely in terms of colour).

For instance, the segments which correspond to a portion of the caustic image which is to have a first colour (e.g. a red "2") may be arranged to form a star-shaped area of the relief structure, while the segments which correspond to a portion of the caustic image which is to have a second colour (e.g. a blue "0") may be arranged to form a rectangular background surrounding the star. The cells of the colour filter will be arranged correspondingly. Hence, assuming that each of the colour filter materials reflects and transmits the same colour, the non-projection image will be of a red star against a blue background, while the caustic image will be the number "20" with the "2" appearing red and the "0" appearing blue (against a dark background). More generally, the non-projection image is preferably a multi-coloured image (although it could alternatively be a multi-tonal image as mentioned above). Typically, the non-projection image will have approximately the same selection of transmission colours and the same proportion of each colour as appear in the caustic image (although, nonetheless, some of the colours may appear mixed in the caustic image as mentioned previously).

In other preferred embodiments, it may be desirable for the direct appearance of the security device under reflected or transmitted light to exhibit no discernible image content. For example, this can help to disguise the presence of a security effect. Hence in some preferred implementations, the lateral arrangement of the segments of the relief structure relative to one another is random or pseudorandom such that the colour filter exhibits a substantially uniform colour across the colour filter, arising due to the corresponding random or pseudo-random arrangement of cells. (In other words, the non-projection "image" has no information content and just appears as a substantially uniform colour area). This is particularly effective if the cells are sufficiently small so as not to be individually resolvable by the naked eye, so that a mixed colour of the various cells is seen in reflected or transmitted light. For instance, the cells of the colour filter could form a uniform array of coloured lines or pixels, e.g. a RGB or CMYK grid.

In further preferred embodiments, the segments of the relief structure and the cells of the colour filter are configured such that at least some of the colours transmitted by the materials in the colour filter are mixed in the caustic image, whereby the number of colours exhibited by the non-projection image is greater than the number of colours exhibited by the caustic image. This unexpected effect acts as a strong security feature. Advantageously, there will be at least two different non-white colours exhibited in reflected light by the colour filter (i.e. the non-projection image appears multi-coloured). For example, the colour filter could display yellow, blue and red regions (e.g. in the form of an image or a pattern) while the relief structure could combine the yellow and blue such that the resulting caustic image appears green and red. Alternatively, the relief structure could mix the colours to result in green and purple parts of the image. In particularly preferred examples, all of the colours visible on the colour filter in reflected light may be combined such that the caustic image appears monochromatic, typically in a non-white colour -although if the colour filter contains red, green and blue regions it is also possible to arrange for the caustic image to appear white, or close to white, which is a surprising visual effect and one which is hard for a counterfeiter to emulate.

In some particularly preferred embodiments, segments of the relief structure which contribute to a first portion of the caustic image are arranged to alternate periodically with segments of the relief structure which contribute to a second portion of the caustic image, and the cells of the colour filter exhibit a corresponding periodic pattern. This can be extended to include any number of image portions: segments contributing to each image portion will be interleaved with those contributing to other image portions in a periodic manner. The periodicity can be one-dimensional or two-dimensional. This results in the direct appearance of the device (i.e. the non-projection image) being patterned or uniform (if the cells are too small to be individually resolved), which is markedly different from the caustic image.

Advantageously, the relief structure and the colour filter are spaced from one another in the direction normal to the plane of the security device, the colour filter preferably being provided on the second side of the substrate and the substrate being transparent, such that upon tilting of the security device relative to the light source, the colour(s) exhibited by the caustic image change and/or a different portion of the caustic image displays a non-white colour This works particularly well where the segments and cells are arranged periodically as discussed immediately above. In this scenario, preferably the ratio of the spacing between the relief structure and the colour filter to the lateral width of the cells in the colour filter is such that, on tilting, the first and second portions of the caustic image appear to switch colours. Particularly good colour changing effects can be achieved where the spacing between the relief structure and the colour filter is greater than or equal to the lateral width of the cells. Most preferably, the cells (and hence the segments) have a lateral width of 200 microns or less, preferably microns or less, more preferably 50 microns or less. This enables the optical spacing and hence the thickness of the security device to be kept suitably small as is advantageous for use of the security device in documents and the like.

In many implementations, the segments of the relief structure (whether redistributed or not) will abut one another directly. However, in other embodiments some or all of them may be spaced apart. In a particularly preferred case, at least some of the segments are laterally spaced from one another in the relief structure by scattering areas of the relief structure which substantially do not contribute to the caustic image. This can be useful to help ensure that each cell of the colour filter only overlaps its allocated segment and does not extend over any other segment, e.g. due to poor registration.

In many cases, the material(s) forming the colour filter will each reflect the same colour as they transmit. This will be the case, for instance, for typical dyes and pigments which operate on a wavelength absorption mechanism. In such cases, any non-projection image exhibited by the colour filter will have the same appearance whether it is viewed in reflected light or in transmitted light. However, to further increase the complexity of the device, in some preferred embodiments, at least one (preferably all) of the materials forming the colour filter transmits only a first subset of visible light wavelengths corresponding to a first non-white colour and reflects only a second subset of visible wavelengths corresponding to a second non-white colour, such that the colour exhibited by the material is different when viewed under reflected light and in transmitted light. For instance, this can be achieved through the use of materials comprising plasmonic pigments, plasmonic structures, interference pigments, interference structures or liquid crystals. This gives the surprising visual effect that at least one of the colours appearing in the projected caustic image will be different from the reflected colour of the corresponding material in the colour filter This can be put to particularly effective use where the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of one of the at least semi-transparent materials, and the colour filter exhibits a non-projected image arising due to the arrangement of regions. If at least one of the materials has different reflected and transmitted colours, then one or more colours in the non-projected image will be different when viewed under reflected light and in transmitted light. In particularly preferred embodiments, all of the colours in the non-projected image may be different under the two viewing conditions.

In still further embodiments, additional effects can be achieved by forming the colour filter out of a first material which has the same first colour in reflection and transmission, and a second material which has different colours in reflection and transmission. As previously described, the materials will be arranged in respective laterally offset regions. In this case, one of the regions will appear to change colour when the colour filter is viewed directly in reflected versus transmitted light, whereas the other region will not. If the second material is selected such that either its reflected colour or its transmitted colour is the same as the first colour exhibited by the first material, the two regions will match under one viewing condition and not match under the other viewing condition. By designing the regions appropriately, this can be used to make an image appear/disappear or change between reflection and transmission modes of direct viewing.

The colour filter can be implemented in various different ways. However, in all implementations it is advantageous if the colour filter is located on the same side of the relief structure as that on which the light source is positioned to project the caustic image. For instance, where the caustic relief structure is transparent and operates on refraction, such that the projected caustic image will be viewed on the opposite side of the device from that on which the light source is located, the colour filter is preferably located between the light source and the relief structure in use.

This achieves better results than locating the colour filter on the opposite side of the device from the light source, since having passed through the relief structure, the light rays will not be parallel and hence there may be some unintended mixing of colours.

As noted above, in most preferred embodiments the colour filter is fixed in overlapping relation to the surface relief. This is optimal since then the alignment of the colour filter and the relief structure can be accurately controlled. This can be achieved in various ways.

For example, in some implementations the colour filter is integral with the substrate, the substrate being formed of the one or more at least semi-transparent materials. This may be appropriate for instance where the colour filter consists of a single material overlapping the whole device, in which case the colour filter could be implemented as a tint throughout the substrate material. More complex multi- coloured configurations could be implemented by forming the substrate of a multi-layered laminate, with different coloured inserts at the appropriate locations.

In other implementations, the relief structure may be formed in the surface of an embossing layer applied to the substrate, the colour filter being integral with the embossing layer, the embossing layer being formed of the one or more at least semi-transparent materials. The embossing layer could comprise one or more polymeric materials which are deformable under heat and/or pressure to receive the relief structure in a surface thereof Alternatively, the embossing layer could be formed by cast-curing, in which case the embossing layer typically comprises one or more curable resins which have been cast into the desired relief structure shape and cured (e.g. by exposure to UV light). This will be described further below.

In other implementations the colour filter is a colour filter layer formed of the one or more at least semi-transparent materials, the colour filter layer being disposed on the first or second surface of the substrate, or on the surface relief. That is, the colour filter is provided by a separate layer rather than being integral with one of the components already described. Such a colour filter layer could be disposed on either side of the substrate, for instance on the first side of the substrate between the relief structure and the substrate, or on the relief structure surface itself an which case the colour filter layer either conforms to the relief structure and/or has a different refractive index from the material in which the relief structure is formed in order to maintain the necessary optical interfaces). Alternatively the colour filter layer could be located on the second side of the substrate.

It is also possible to provide any combination of the above and/or distribute the colour filter such that different lateral parts of it are provided by different components. For instance, more than one colour filter layer could be provided at different positions within the device structure (e.g. one on either side of the substrate) which may or may not partially or fully overlap one another. Similarly, a tinted substrate could be used in combination with one or more colour filter layers and/or an embossing layer formed of colour filter materials.

Whether the relief structure is a redistributed relief structure or not, it can take the form of one contiguous area of relief, or multiple discontinuous areas. In the latter case, the caustic relief areas will be spaced from one another by one or more areas of the relief structure which have a non-caustic surface and hence do not contribute to the caustic image. For instance, they may be flat areas or could have a light-scattering or matte surface structure. If the relief structure is formed in an embossing layer on the substrate, the embossing layer could be omitted in the one or more areas between the caustic relief areas. The multiple caustic relief areas could ultimately be located within a single window region of a security document, or they could be distributed between multiple window regions. For instance, the security device could comprise a first caustic relief area of its relief structure in a first window of a security document and a second caustic relief area of its relief structure in a second window of a security document. The first and second windows will be spaced from one another by a non-transparent region of the document, e.g. carrying one or more opacifying layers. However the first and second windows should of course be located sufficiently close to one another that they can be illuminated simultaneously by the same light source at substantially the same illumination angle.

Embodiments such as those mentioned above in which the colour filter and relief structure are fixed overlapping one another have the advantage that the relative positions of the colour filter and the relief structure can be carefully controlled during manufacture. However, in other embodiments it may be preferred to provide the two components separately, so that they are only overlapped during an authentication test. For example, the colour filter may be provided on the substrate at a first location and the surface relief is provided on the substrate at a second location (laterally offset from the first), the substrate being transparent and foldable such that the colour filter can be positioned so as to overlap the surface relief in use. To ensure the desired relative positioning of the colour filter and relief structure can be achieved by the user, appropriate alignment markers may be provided -for instance, each component may be arranged such that when the substrate is folded with its corners matching up, the colour filter and relief structure come into the desired alignment with one another Hence, the present invention also provides a security device assembly, comprising a substrate; a relief structure on a first side of the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and a colour filter laterally offset from the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby when the colour filter is arranged to overlap the relief structure, a security device of the sod disclosed above is formed and the caustic image projected by the security device when the security device is illuminated with white light exhibits one or more colour(s).

Preferably, whether the colour filter and relief structure are fixed in overlapping relation or not, the colour filter and the relief structure are registered to one another In other words, the colour filter and the relief structure will have the same relative position on every one of a plurality of the disclosed security devices. In devices that are designed to show a coloured non-projection image from the colour filter itself as well as a projected colour image, this registration between the caustic and the filter enables the projected colour elements to have defined colours -the caustic relief defines to where the light is re-directed whilst the region of the filter defines what colour that light will be. The level of registration required depends on the complexity of the non-projected image -a more complex non-projected image contains more boundaries between coloured regions and therefore a greater impact from mis-registration (as the boundaries are a greater proportion of the filter). As such, there is a trade-off between larger, easy to visualise imagery and the extra security achieved when using fine print. Easy to read (coarse) imagery may only require a registration tolerance of ±250pm, preferably ±150pm, whereas finer security print may require ±150pm, preferably ±75pm. Hence, in preferred embodiments, the colour filter and the relief structure are registered to one another to within ±250pm, more preferably ±150pm, still preferably ±75pm.

In preferred embodiments, the colour filter is a printed colour filter, preferably formed by offset printing, gravure printing, lithographic printing, flexographic printing or screen printing. This is relevant not only to implementations in which the colour filter is a colour filter layer, but also those in which the colour filter is integral with an embossing layer carrying the relief structure in its surface, since the material(s) forming the embossing layer may also be applied by printing.

Preferably, the or each material transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour comprises any of: an absorptive dye, an absorptive pigment, a plasmonic material (such as a plasmonic structure or a plasmonic pigment), an interference layer structure, an interference layer pigment or a liquid crystal material. The material(s) forming the colour filter must give rise to minimal (preferably zero) optical scattering, in order to avoid disrupting the light rays and hence inhibiting the caustic image. Hence if the material(s) derive their colour(s) from additives, dyes or very finely ground pigments or other particles are preferred. Preferably, the particle size is below 500 nm, more preferably equal to or less than 100nm. The additives may be carried in a suitable binder. Hence, in many examples the materials may comprise inks. However in other cases, the material(s) could comprise vapour-deposited layers, such as dielectric stacks, which derive their colour from interference between the layers, or structures such as nanopillars or nanoholes which provide plasmonic colour.

In some preferred embodiments, the security device may further comprise a masking component configured to overlap in use a sub-part of the relief structure, the masking component comprising one or more materials which are substantially opaque to visible light. Since the masking component is substantially opaque, parts of the relief structure which it overlaps will not contribute to the caustic image. However, the masking component will be visible when the security device is viewed directly On reflected or transmitted light) and so may supplement any non-projected image exhibited by the colour filter. Indeed, the masking component could be formed integrally with the colour filter or as a separate layer. If the masking component is integral with the colour filter, it could be formed by the same process and take the form of one or more regions of substantially opaque material(s) which are either laterally offset from the colour filter or partially overlap it. For instance, if the colour filter comprises a material of varying thickness, the masking component could be formed by the thickest area(s) of that material. The presence of the masking component will need to be taken into account in the design of the caustic relief.

As mentioned above, the relief structure may be configured to project a real caustic image which can be viewed on a projection screen, or to project a virtual caustic image which can be viewed directly by the naked eye. Examples of relief structures (and methods for designing them) which project real caustic images can be found in WO-A-2019/063778, WO-A-2019/063779 and WO-A- 2020/070304, while WO-A-2020/070299 discloses techniques for forming a relief structure which generates a "virtual" caustic image.

In particularly preferred implementations, the relief structure is a refractive light-redirecting relief structure, the relief structure and the substrate being transparent.

The security device is of course suitable for inspection by the naked eye, with a user being able to determine the authenticity of the device by illuminating it with a suitable light source (such as a torch on a smart phone, or a desk lamp) and observing the projected caustic image. This can if necessary be compared against a reference image, potentially provided on the same security document or separately. However, the security device can equally be inspected by machine.

Security documents are often inspected by machines as part of an automated process (e.g. banknote and cheque acceptors, passport e-gate inspection). This inspection often includes imaging of the document and comparison to a template which allows for the caustic element to be validated. The caustic relief will be illuminated by a controlled light source and the transmitted or reflected light will fall onto an electronic sensor. The illumination will be distorted by the caustic element either to form an image that would be the same as seen by the public or to form a distorted image (if there is not sufficient distance between the caustic and the sensor) which in either case will be characteristic of the caustic device and therefore useable for document validation. In some cases the sensor will be a linescan camera and the image will be built up from sequential scans as the document is moved in front of the linescan camera. The sensor can work in the visible spectrum but could equally work with infra-red light so long as the caustic element refracts or reflects light in infrared as well as the visible spectrum.

The overall thickness of the security device (comprising the substrate, caustic relief structure and any separate colour filter layer) is preferably 1 mm or less, more preferably 500 microns or less, still preferably 150 microns or less, most preferably 100 microns or less. Thus the security device is sufficiently thin and flexible to be incorporated into security documents and the like.

The invention also provides a security article comprising a security device as defined above, the security article preferably being a security thread, strip, patch, label, foil or insert.

Also provided is a security document comprising a security device or a security article, each as defined above, the security document preferably being a banknote, a passport, an identity document, a driver's licence, a certificate, a visa or a stamp. Such a security document may alternatively comprise a security device assembly as mentioned above.

Preferably, the security device is located in one or more first transparent window region(s) of the security document. Such a window region can be formed, for example, in a security document based on a transparent document substrate with one or more opacifying layers thereon which are omitted in the window region, or in a security document based on any type of substrate with an aperture provided therethrough corresponding to the window region. As mentioned above, it is also possible for parts of the security device to be distributed between multiple transparent window regions.

In a preferred embodiment, the security document further comprises a supplemental security feature which exhibits an image when viewed in transmitted light, the image of the supplemental security feature being the same as or complementary to the caustic image and/or exhibiting the same non-white colour(s) as the caustic image. For example, the supplemental security feature may be a watermark, pseudo-watermark, or transmissive optically variable device such as a lenticular device or moire magnifier. The supplemental security feature can be located in a non-window region, or in one of the one or more first transparent window region(s) or in a second transparent window region of the security document. This embodiment provides the advantage that both the visual effects generated by the security device on one hand and by the supplementary security feature on the other can be observed in the same viewing mode, preferably simultaneously, by the observer. By arranging the caustic image and the image exhibited by the supplementary security feature to be the same as one another or at least complementary, authenticity of the security document can be checked by comparing the two images against one another. For example, the images will be complementary if one provides a missing element of the other (e.g. "E" and "10"), or if both provide related information content (e.g. "0E11" and a portrait of the Queen). The use of corresponding colours provides a particularly strong visual effect.

The first aspect of the invention further provides a method of manufacturing a security device, comprising: providing a substrate; forming a relief structure on a first side of the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and providing a colour filter, the colour filter being configured to overlap in use at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby at least part of the caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light.

The result is a security device as disclosed above, with all of the attendant benefits. Any of the preferred features of the security device disclosed above can be provided by appropriate modification of the method.

The relief structure could be formed using any available technique, including direct embossing into the substrate, or embossing into a suitable layer applied thereto, using heat and/or pressure to deform the surface into the desired relief structure.

However, more preferably the relief structure is formed by cast-curing. Hence preferably, the relief structure is formed by applying an embossing layer comprising one or more curable materials to the first side of the substrate or to a casting relief on the surface of a casting tool, the casting relief defining the relief structure therein, bringing the casting tool into contact with the substrate with the embossing layer therebetween to thereby form the relief structure into the surface of the embossing layer, and curing the embossing layer to retain the relief structure. This could be performed, for instance, in a sheet-fed or a web-fed process. The one or more curable materials may preferably be applied by printing, e.g. gravure printing. If multiple curable materials are used, these will preferably be applied in laterally offset positions which collectively form the embossing layer, rather than mixed together or overlapping one another.

As mentioned above, in preferred embodiments, the colour filter may be integral with the embossing layer, in which case the curable material(s) of the embossing layer comprises the one or more at least semi-transparent materials. Hence the application of the various curable materials will be in accordance with the desired arrangement of regions of the colour layer (if any). This method of manufacture is particularly preferred since the colour filter and the relief structure will both be formed during one and the same cast-cure process and hence the register between them can be particularly accurately controlled.

In other preferred embodiments, the colour filter may be provided by applying a colour filter layer formed of the one or more at least semi-transparent materials, the colour filter layer being applied to the first or second surface of the substrate, or onto the surface relief. For instance, the colour filter layer may be applied by printing, preferably offset printing, gravure printing, lithographic printing, flexographic printing or screen printing. In order to achieve good register, desirably, the colour filter layer and the relief structure are applied to the substrate on the same processing line, while the substrate is being conveyed along the processing line in the machine direction. This could be a sheet-fed or a web-fed process. It is particularly advantageous if the colour filter layer is applied to the second surface of the substrate, the colour filter layer and the relief structure being applied simultaneously to opposite surfaces of the substrate at the same position along the machine direction. This assists in ensuring exact register between the colour filter and relief structure, since they are applied to the same part of the substrate at the same instant, so no deformation or slippage of the substrate can have taken place between the application of one component and the application of the other.

The colour filter can alternatively by provided integrally with the substrate, as mentioned previously.

Suitable apparatus, materials and methods for forming the relief structures disclosed herein, and (if desired) applying a separate colour filter layer (simultaneously or otherwise) are described in WO-A-2018/153840 and WO-A5 2017/009616 In preferred embodiments, the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of (only) one of the at least semi-transparent materials, whereby the colour of light transmitted by the colour filter varies across the colour filter in accordance with the regions. Additionally or alternatively the thickness of the at least one semi-transparent material may be different from one region to another to achieve different tones of the transmitted light. Desirably, the security device is configured such that the caustic image projected by the security device when illuminated with white light exhibits a multi-tonal and/or multi-coloured appearance.

Preferably, a first group of one or more of the regions of the colour filter, which overlap one or more first areas of the relief structure configured to generate a first portion of the caustic image, are formed of a first set of one or more at least semi-transparent materials, such that the first portion of the caustic image exhibits a first colour which is determined by the colour(s) transmitted by the first set of materials, the first portion of the caustic image preferably being a first distinct element of the caustic image.

This can be achieved in various ways. In a preferred example, the colour filter is created by: selecting a first portion of the caustic image and a first colour for the selected first portion; identifying the one or more first areas of the relief structure which generate the selected first portion, preferably by ray tracing; designating the one or more regions of the colour filter which overlap the identified first area(s) as the first group, and assigning the first set of one or more at least semi-transparent materials to the designated region(s), wherein the first set of one or more at least semi-transparent materials transmit in combination the first colour.

This of course can be extended to any number of portions of the caustic image and corresponding colours. The process will work for both non-redistributed and redistributed caustic relief structures.

Once the design of the colour filter has been created, the colour filter itself can then be realised in accordance with the design, e.g. by printing of appropriate material(s) in the appropriate arrangement.

In another implementation, the colour filter can be created first, in accordance with a desired non-projected appearance of the device, and then areas of the relief structure which contribute to certain portions of the caustic image can be allocated to the regions of the colour filter accordingly.

For example, the method may further comprise, prior to forming the relief structure: designing the relief structure by (in this order): providing a non-projection image to be exhibited by the colour filter, comprising a plurality of laterally-offset regions which vary in colour and/or tone from one region to another; for each region of the non-projection image, providing a relief structure segment based on the colour and/or tone of the region; and arranging the relief structure segments according to the arrangement of regions of the non-projection image.

"Providing" the relief structure segments could involve reallocating segments of a pre-designed caustic relief configured to display a certain projected image and/or creating a relief for each segment, which is designed to redirect the light transmitted through that region to the desired position on a screen to thereby build up a caustic image. Once all the relief segments have been determined and arranged, the resulting structure can then be physically made, e.g. by etching the design into a suitable surface from which the relief structure can be cast.

In accordance with a second aspect of the invention, a security document is provided, comprising: a first transparent window region in which is disposed a first security device comprising a transparent, refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and laterally offset from the relief structure, a second security device which exhibits an image when viewed in transmitted light, the image exhibited by the second security device being the same as or complementary to the caustic image.

The first security device could be a security device in accordance with the first aspect of the invention but this is not essential. The second aspect of the invention provides the advantage that both the visual effects generated by the first security device on one hand and by the supplementary security feature on the other can be observed in the same viewing mode, preferably simultaneously, by the observer. By arranging the caustic image and the image exhibited by the supplementary security feature to be the same as one another or at least complementary, authenticity of the security document can be checked by comparing the two images against one another For example, the images will be complementary if one provides a missing element of the other (e.g. "E" and "10"), or if both provide related information content (e.g. "QEII" and a portrait of the Queen).

Preferably, the second security device comprises a watermark, a pseudo-watermark or a transmissive optically variable device such as a lenticular device or moire magnifier Advantageously, the first and second security devices abut one another or are positioned sufficiently close to one another such that the caustic image and the image exhibited by the second security device can be viewed simultaneously by one observer.

In preferred examples, the caustic image and the image exhibited by the second security device have the same information content as one another, or one provides an element missing in the other.

Depending on the nature of the second security device, it may or may not be disposed in a window region of the security document. For example, if the second security device is a watermark or a pseudo-watermark, this will typically be placed in a non-window region of the security document, or potentially in a half-window region. However, in other cases it may be desirable to place the second security device in a window region. In such cases the second security device may be disposed in the first transparent window region or in a second transparent window region of the security document.

In accordance with a third aspect of the invention, a security device comprises: a substrate; and a reflective or refractive light-redirecting relief structure on a first side of the substrate, the relief structure comprising a first area provided with a caustic relief configured to redirect light from a light source to thereby project a caustic image; wherein the relief structure further comprises a second area laterally offset from the first area, the second area being provided with a non-caustic surface, and the first area of the substrate having a periphery shaped to define a periphery image.

In this aspect, a colour filter of the sort provided in the first aspect of the invention may or may not be provided. The security level is enhanced by forming the periphery of the caustic relief to exhibit recognisable image information. The caustic relief and hence the first area will be distinguishable from the second area due to the different nature of the two relief structures, especially upon changing the viewing angle, and so the peripheral image will be visible to an observer at least under close inspection. For example, the non-caustic surface in the second area may be flat relative to the caustic relief, or could carry a matte or scattering surface structure, or a set of facets such as a blaze grating which appears bright at some viewing angles and dark at others. In all cases the difference between the relief structures in the two areas will enable the periphery image to be perceived at least at certain viewing angles.

The periphery image is preferably a human-intelligible image and optionally comprises any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, or a scene such as of a landscape, an architectural structure, a person, animal or plant. Of course, the image will be in silhouette form.

Most preferably the peripheral image will be different from the caustic image, in terms of its information content. For instance, the peripheral image could be an outline of a country and the caustic image could be the relevant currency symbol for that country. If the security device is also provided with a colour filter, any non-projected image displayed by the colour filter will also preferably be different from the peripheral image.

In accordance with a fourth aspect of the invention, a security device is provided, comprising: a substrate; a relief structure on the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a first light source at a first illumination angle to thereby project a first caustic image and to redirect light from a second light source at a second, different, illumination angle to thereby project a second caustic image, the first caustic image being different from the second caustic image.

By configuring the relief structure in this way, the security device exhibits a "switching" effect upon changing the direction of illumination between the first and second illumination angles. When the device is illuminated from the first illumination angle, the relief structure projects the first caustic image whereas when the device is illumination from the second illumination angle, the relief structure projects the second caustic image, which is different from the first. Hence the projected image appears to change depending on the illumination angle. Preferably, the relief structure is further configured such that the first caustic image is not projected when the security device is illuminated by the second light source at the second illumination angle and the second caustic image is not projected when the security device is illuminated by the first light source at the first illumination angle. This creates the sharpest "switch" effect since only one of the first and second caustic images will be visible at the first and second illumination angles respectively. It will be appreciated that the relief structure could be configured to project three or more different caustic images, each at a different respective illumination angle. In this way it is possible to create even more complex projected effects, such as morphing, zooming or animation effects.

Advantageously, the relief structure comprises a first sector configured to project the first caustic image under illumination from a first range of illumination angles which includes the first illumination angle, and a second sector configured to project the second caustic image under illumination from a second range of illumination angles which includes the second illumination angle, the first and second sectors being laterally offset from one another. Conventionally, when creating a caustic with a single projected image, the designer would normally aim to maximise useability by minimising the sensitivity to the incoming illumination angle. For example, the caustic structure may be designed to operate over a wide range of projection angles, e.g. from -45 degrees to +45 degrees either side of the substrate normal. This is achieved by minimising the deflection required on any part of the caustic which means the projected image will simply shift when the illumination angle is shifted. However, in the present aspect of the invention it is desirable to create a projected image which is more sensitive to illumination angle.

This can be achieved by incorporating a greater range of deflection angles into the caustic relief structure (e.g. elements of the left hand side of the projected image being made up light from both the left side and right side of the caustic). The rate of change of deflection vs angle of incidence will then vary depending on the degree of deflection leading to a more illumination-angle-selective projected image. One such caustic relief can then be provided in a first sector of the relief structure, and a second one in a second sector thereof. Each sector will then be configured to generate the corresponding caustic image only when illuminated over the designated range of illumination angles. When the sector is illuminated from an angle outside that range, it will not form a caustic image, e.g. instead scattering the incident light. If the device is adapted to project three or more caustic images, it will be provided with a corresponding three or more sectors.

The laterally offset sectors of the relief structure can be arranged in various different ways relative to one another, on a macroscopic or microscopic scale. For example, the first and second sectors may abut one another or may be laterally spaced from one another. The sectors could for instance be arranged concentrically with one another (e.g. as a series of annular areas), or as tessellating tiles, or as distinct areas each having a peripheral image (e.g. forming numbers or letters). In some particularly preferred embodiments, the first and second sectors each comprise a plurality of sub-sectors, the first sub-sectors preferably being interlaced with the second sub-sectors in one or two dimensions, wherein still preferably the first sub-sectors are arranged to alternate periodically with the second sub-sectors on one or two dimensions. This is advantageous since, to the observer, the relief structure can appear to form a continuous area with no obvious distinction between the first and second sectors, making it particularly surprising that two or more different caustic images can be projected.

Preferably, the first and second ranges of illumination angles do not overlap.

However it is also possible to arrange for there to be some partial overlap of the ranges, at which illumination angles both the first and second caustic images are projected. This may be desirable where an animation effect is to be displayed, for example, since it gives rise to the impression of a smooth transition between images.

Desirably, the first and second ranges of illumination angles should be sufficiently small to allow for the first and second caustic images to be displayed distinctly upon tilting the device relative to the light source. In preferred examples, each range has a magnitude of 60 degrees or less, more preferably 30 degrees or less.

The first and second caustic images can be of any sort and will differ from one another at least in terms of their information content. For instance, the first and second caustic images may each comprise any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure, a person, animal or plant.

Preferably, the relief structure is configured to project the first and second caustic images towards substantially the same position relative to the security device when illuminated from the first and second illumination angles respectively. For instance, when the security device is held in a fixed position and illuminated with light from the first illumination angle, it will project the first caustic image onto a certain point of a screen fixed at a distance from the device. When the light source is changed to the second illumination angle, the relief structure preferably projects the second caustic image onto the same point on the screen. In this way, the two caustic images will appear to replace one another at the same position on the screen.

As described with reference to the first aspect of the invention, the relief surface is preferably segmented, the segments being distributed such that areas of the relief structure and the portions of the first or second caustic image which each area generates have different lateral arrangements relative to one another in the relief structure and in the caustic images, respectively. However it is also possible to utilise a non-redistributed caustic structure in each sector if preferred.

The security device of the fourth aspect may or may not be used in conjunction with a colour filter, such as that provided in the first aspect of the invention.

However, preferably the security device further comprises a colour filter, the colour filter being configured to overlap in use at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby the first and/or second caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light. The colour filter can be implemented in any of the ways described above with respect to the first aspect.

Preferably, the light transmitted by the colour filter varies in colour and/or tone between laterally offset regions of the colour filter, most preferably such that the first and second caustic images exhibit different colour(s) from one another when the security device is illuminated with white light from the first and second illumination angles respectively. Desirably, the light transmitted by the colour filter varies in colour and/or tone between first and second laterally offset regions of the colour filter which correspond to the first and second sectors of the relief structure, respectively. The colour filter can be configured to display a non-projected image (which is preferably different from one or both of the caustic images) using the same principles as explained above.

Preferably the relief structure is configured to project a real caustic image which can be viewed on a projection screen, or to project a virtual caustic image which can be viewed directly by the naked eye. Advantageously the relief structure is a refractive light-redirecting relief structure, the relief structure and the substrate being transparent.

The fourth aspect of the invention further provides a security article comprising a security device according to the fourth aspect, the security article preferably being a security thread, strip, patch, label, foil or insert.

Also provided is a security document comprising a security device according to the fourth aspect, or a security article as just described, the security document preferably being a banknote, a passport, an identity document, a driver's licence, a certificate, a visa or a stamp.

The security device is preferably arranged in one or more windows of the security document, which may be defined by gaps in an opacifying layer. In some preferred embodiments, the first and second sectors of the relief structure are located in a window region of the security document. By placing the two sectors in one and the same window, this further hides the fact that two caustic structures are present. However in other embodiments, the first sector may be located in a first window region of a security document and the second sector may be located in a second window region of the security document, the first and second window regions being spaced from one another by a non-transparent region of the security document.

The fourth aspect of the invention further provides a method of manufacturing a security device, comprising: providing a substrate; forming a relief structure on the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a first light source at a first illumination angle to thereby project a first caustic image and to redirect light from a second light source at a second, different, illumination angle to thereby project a second caustic image, the first caustic image being different from the second caustic image.

Preferably the relief structure is formed by applying an embossing layer comprising one or more curable materials to the substrate or to a casting relief on the surface of a casting tool, the casting relief defining the relief structure therein, bringing the casting tool into contact with the substrate with the embossing layer therebetween to thereby form the relief structure into the surface of the embossing layer, and curing the embossing layer to retain the relief structure.

The method can be adapted to provide the security device with any of the preferred features described above.

If a colour filter is provided, this can be implemented using any of the preferred techniques described with reference to the first aspect.

Examples of security devices in accordance with the present invention, and methods for their manufacture, will now be described with reference to the accompanying drawings, in which: Figure 1(a) schematically depicts a first embodiment of a security device in cross-sectional view, Figures 1(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figures 2(a) to (g) schematically depict seven alternative embodiments of security devices in cross-sectional view; Figure 3(a) schematically depicts a second embodiment of a security device in cross-sectional view, Figures 3(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figure 4(a) schematically depicts a third embodiment of a security device in cross-sectional view, Figures 4(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figure 5(a) schematically depicts a fourth embodiment of a security device in cross-sectional view, Figures 5(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figure 6(a) schematically depicts a fifth embodiment of a security device in cross-sectional view, Figures 4(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figure 7(a) schematically depicts a sixth embodiment of a security device in cross-sectional view, Figures 7(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figure 8(a) schematically depicts a seventh embodiment of a security device in cross-sectional view, Figures 8(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figure 9(a) schematically depicts a eighth embodiment of a security device in cross-sectional view under illumination by a first light source, Figures 9(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device; Figure 10(a) schematically depicts the eighth embodiment of a security device in cross-sectional view under illumination by a second flight source, Figure 10(b) showing parts of the colour filter intercepted by light from the second light source, and Figures 10(c) and (d) respectively showing the surface relief and the caustic image projected by the security device; Figure 11(a) schematically depicts a ninth embodiment of a security device in cross-sectional view, Figures 11(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figure 12(a) schematically depicts a tenth embodiment of a security device in cross-sectional view, Figures 12(b), (c) and (d) respectively showing schematic plan views of the colour filter, surface relief and caustic image projected by the security device when illuminated by a light source; Figure 13 shows an embodiment of a security document provided with an exemplary security device in accordance with the invention, illuminated by an exemplary light source, and the corresponding projected caustic image; Figures 14 to 19 show six further embodiments of security devices in accordance with the invention, (a) in cross-section, (b) under reflected light, (c) in transmitted light and (d) projecting the caustic image; Figures 20 to 28 show nine further embodiments of security devices in accordance with the invention (a) in cross-section, (b) under reflected light and (c) the caustic image projected by the security device; Figure 29(a) schematically depicts in cross-section a first example of apparatus which may be used to manufacture security devices in accordance with embodiments of the invention, Figure 29(b) showing a perspective view of the corresponding security devices at various stages of manufacture; Figure 30 schematically depicts in cross-section a second example of apparatus which may be used to manufacture security devices in accordance with embodiments of the invention; Figure 31 schematically depicts an embodiment of a security document in accordance with the present invention (a) in plan view, and (b) in cross section along the line X-X'; Figure 32 schematically depicts another embodiment of a security document in accordance with the present invention (a) in plan view, and (b) in cross section along the line X-X'; Figure 33 shows a comparative example of a security device when illuminated by a first light source (a) in cross-section, (b) under reflected light and (c) the caustic image projected by the security device; Figure 34 shows the comparative example of Figure 33 when illuminated by a second light source (a) in cross-section, (b) under reflected light and (c) the caustic image projected by the security device; Figure 35 shows another comparative example of a security device when illuminated by a first light source (a) in cross-section, (b) under reflected light and (c) the caustic image projected by the security device; Figure 36 shows the comparative example of Figure 35 when illuminated by a second light source (a) in cross-section, (b) under reflected light and (c) the caustic image projected by the security device; Figures 37 and 38 shows two further comparative examples of security device when illuminated by a first light source and a second light source respectively, (a) in cross-section, (b) under reflected light and (c) the caustic image projected by the security device; Figure 39 shows a further embodiment of a security device in accordance with the invention when illuminated by first and second light sources (a) in cross-section, (b) under reflected light and (c) the caustic image projected by the security device; Figures 40(a) to (d) schematically show four alternative arrangements of the relief structure in the Figure 39 embodiment, in plan view; Figure 41 schematically shows another embodiment of a security device in accordance with the invention in cross section (a) when illuminated by a first light source and (b) when illuminated by a second light source; Figures 42, 43 and 44 show three exemplary security documents carrying security devices in accordance with embodiments of the present invention (a) in plan view, and (b) in cross-section; and Figure 45 illustrates a further embodiment of an security document carrying a security device in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section.

The ensuing description will focus on preferred examples of security devices in which the caustic relief is transparent and the caustic image is generated by the mechanism of refraction. However, all described embodiments can be converted into reflective devices by applying a reflection enhancing material to the caustic relief, such as a vapour-deposited metal layer or a metallic ink, in which case the caustic image will be formed as a result of reflection. Likewise, in the embodiments presented, the caustic image is a real image, which can be observed by projecting the image onto a surface, such as a wall, screen or worksurface. However, all described embodiments can be converted to instead generate virtual caustic images which can be observed directly by the naked eye. Details of the mechanisms involved in forming caustic images by refraction and by reflection are set out in WO-A-2019/063778, WO-A-2019/063779 and WO-A- 2020/070304. WO-A-2020/070299 explains how to convert a relief which generates a real caustic image into one which generates a virtual caustic image. These documents also disclose methods for designing a surface relief configured to project a certain caustic image upon illumination, which can be used to create the relief structures used in embodiments of the present invention.

A first embodiment is depicted in Figure 1. The security device 10 comprises a substrate 12, a relief structure 20 which is configured to project a caustic image Cl when illuminated by a light source L, and a colour filter 30 overlapping at least part (preferably all) of the relief structure 20. The relief structure 20 forms the caustic image Cl by redirecting the incident light rays as a result of either refraction or reflection, so as to form bright and dark spots which together define the desired image. In this embodiment, the relief structure 20 is carried in the surface of an embossing layer 14 which is disposed on first surface 12a of the substrate 12, and the colour filter 30 is embodied as a colour filter layer 18 disposed on the second surface 12b of the substrate 12. However, the relief structure 20 and colour filter 30 could be implemented in various other ways as will be explained with reference to Figure 2 below. The overall thickness of the security device 10 (comprising the substrate 12, caustic relief structure 20 and colour filter 30) in the direction normal to its plane is preferably 1 mm or less, more preferably 500 microns or less, still preferably 150 microns or less, most preferably 100 microns or less.

The colour filter 30 comprises at least one material 32a which is semi-transparent and transmits only a subset of the visible spectrum corresponding to a non-white colour Ci. In this example, the colour filter consists of a single such material 32a, covering the whole of the colour filter which here is rectangular, as shown in Figure 1(b) which is a plan view of the colour filter 30. When the security device is viewed under reflected light, this will be the reflective appearance R of the device. The relief structure 20 is configured to generate a caustic image Cl of the currency symbol "8". To achieve this, the relief structure will comprise some area(s) 21a which redirect light towards, and hence contribute to the bright portion Pi of the caustic image, forming the symbol "8". Optionally, the relief structure may also comprise some area(s) 21b with a non-caustic surface which do not contribute to the bright portion P1 of the caustic image -for instance these areas 21b may redirect light away from the image and/or scatter light so as to ensure the background to bright portion Pi appears dark. In some cases, the area(s) 21b will be flat relative to the area(s) 21a. It should be noted that while Figure 1(c), which is a plan view of the relief structure, shows the area 21a as having a lateral shape approximately corresponding to that of the caustic image, this is schematic and will not necessarily be the case, although it may be preferred. In other cases, the lateral shape of area 21a may be selected to corresponding to an image (referred to as a "periphery image") which is different from the caustic image. For instance, the outline of area 21a could correspond to the shape of a country while the caustic image Cl could display the currency symbol of that country.

Since (at least some of) the light rays forming the caustic image have passed through colour filter 30, the colour Ci exhibited by the bright portion P1 will depend on the colour transmitted by the material 32a forming the colour filter 31. For example, if material 32a transmits red light, then when the security device 10 is illuminated by a white light source L, the bright portion Pi of caustic image Cl will appear largely red. In this particular example, since the colour filter does not cover all of the area 21a, some of the redirected light will remain white and so the edges of the bright portion Pi may appear pink (as a result of the mixing of red and white light), resulting in a multi-tonal caustic image. Of course, any number of different tones can be provided in the image by arranging for different proportions of red and white light to be mixed in different parts of the image by the caustic relief 20.

If a mono-chromatic image is preferred, the colour filter 30 can be extended to cover the whole of area 21a.

A multi-tonal caustic image Cl can also be achieved in the case where the colour filter 30 is of uniform colour/tone and does cover the whole area 21a, through design of the caustic relief. The caustic relief can be configured to give rise to variations of intensity of the light forming the caustic image, which will appear as a corresponding tonal variation across the image. Hence the colour filter 30 does not need to be specially configured to achieve a multi-tonal caustic image if the caustic relief is designed to provide this. (Although such a multi-tonal caustic image can alternatively be achieved through design of the colour filter as explained below).

In an example, if the security device 10 is illuminated from "infinity", the relief structure 20 and projected image are each 15mm wide, and the projected image Cl is 70mm from the device 10, then the largest deflection required (from one extremity of the surface relief to the opposite extremity of the caustic image) is 12 degrees. In turn, this requires an angle of the surface relief (e.g. a microprism) out of the surface plane of 23 degrees. If the surface relief 20 incorporated such a microprism 70 microns long with an angle of 23 degrees, it would protrude from the document On the y-axis) by about 30 microns. Hence the surface relief 20 typically has a depth profile (i.e. from the top of the protrusions to the base of the intervening depressions) of this order of magnitude or less. This is acceptably thin for a security device.

The security device 10 is of course suitable for inspection by the naked eye, with a user being able to determine the authenticity of the device by illuminating it with a suitable light source (such as a torch on a smart phone, or a desk lamp) and observing the projected caustic image Cl. This can if necessary be compared against a reference image, potentially provided on the same security document or separately. However, the security device 10 can equally be inspected by machine. Security documents are often inspected by machines as part of an automated process (e.g. banknote and cheque acceptors, passport e-gate inspection). This inspection often includes imaging of the document and comparison to a template which allows for the caustic element to be validated. The caustic relief will be illuminated by a controlled light source and the transmitted or reflected light will fall onto an electronic sensor. The illumination will be distorted by the caustic element either to form an image that would be the same as seen by the public or to form a distorted image (if there is not sufficient distance between the caustic and the sensor) which in either case will be characteristic of the caustic device and therefore useable for document validation. In some cases the sensor will be a linescan camera and the image will be built up from sequential scans as the document is moved in front of the linescan camera. The sensor can work in the visible spectrum but could equally work with infra-red light so long as the caustic element refracts or reflects light in infrared as well as the visible spectrum.

As noted above, the relief structure 20 can be implemented in various different ways with the same or similar end result. In the example shown in Figure 1(a), the relief structure 20 is formed in an embossing layer 14, which may be a monolithic or multi-layer embossing lacquer, the relief being applied into the surface thereof using heat and/or pressure, or more preferably comprises a cast-cured resin layer shaped to carry the desired relief. Exemplary cast-cure methods will be described below. However, it is also possible to emboss the relief structure 20 directly into the surface of substrate 12, as shown for example in Figure 2(a), provided the substrate is formed of a suitable, typically polymeric material.

Likewise, the colour filter 30 can be embodied in various different ways. In the Figure 1(a) embodiment, the colour filter is provided in the form of a colour filter layer 18. However, it is also possible to provide the colour filter 30 integrally with the substrate 12 as shown in Figure 2(a). For example, the substrate material may carry an appropriately coloured tint throughout (hence the substrate material is material 32 of the colour filter 30). This implementation is particularly well adapted to embodiments in which the colour filter comprises a single, uniform colour over the whole extent of the caustic image. However, it is possible to achieve more complex coloured patterns by forming the substrate 12 as a multi-layer structure and including therewithin an arrangement of appropriate coloured inserts before laminating. It should be noted that the colour filter 30 can be provided integrally with the substrate 12 whether the relief structure 20 is formed in the first surface 12a of the substrate 12 (Figure 2(a)) or in an embossing layer 14 applied thereto (Figure 2(b)).

Alternatively still, if the relief structure is formed in an embossing layer 14, the colour filter 12 could be provided integrally with the embossing layer, as shown in Figure 2(c). The embossing layer 14 can be formed of one or more materials (laterally offset from one another) which transmit the designed colour(s) of light.

A particularly preferred method for producing such an embossing layer by cast-cure techniques will be described below.

Alternative locations for a colour filter layer 18 (of the sort provided in the Figure 1 embodiment) are also possible. For example, the colour filter layer 18 could be located on the same side of the substrate 12 as the relief structure 20, either between the relief structure 20 and the substrate 12 (Figure 2(d)) or on the relief structure 20 itself (Figures 2(e) and 2(f)). In the latter case it is important to maintain the contours of the relief structure 20 either at the interface between the embossing layer 14 and colour filter layer 18 and/or at the opposite surface of the colour filter layer 18. The latter option can be achieved by arranging the colour filter layer 18 to follow the contours of the surface relief structure on both of its sides, as shown in Figure 2(e). Alternatively, as shown in Figure 2(f), the colour filter layer 18 need not conform to the contours of the relief structure 20 if there is a sufficient difference in refractive index between the materials in contact at the interface between embossing layer 14 and colour filter layer 18. This maintains the necessary optical surface shape at the interface.

Combinations of any of the above configurations can also be used. For example, it may be convenient to provide one colour of the colour filter in the embossing layer 14 and another, different, colour of the colour filter in a colour filter layer 18. This can be achieved by forming the appropriate region(s) of each component of a suitably coloured material and arranging those regions to have the necessary positions relative to one another, so that the colour filter 30 is formed by the two components 14, 18 in combination with one another. Another example of a possible combination is shown in Figure 2(g), where the colour filter 30 is distributed across three colour filter layers 18a, 18b, 18c which have different locations in the device structure. Colour filter 18a is located on second surface 12b of the substrate, is formed of a first colour filter material and hence has a first transmitted colour (e.g. red). Colour filter layer 18b is located on the first surface 12b of the substrate, between the substrate 12 and the embossing layer 14, and is formed of a second colour filter material and hence has a second transmitted colour (e.g. yellow). Colour filter layer 18c is located on the relief structure 20 and is formed of a third colour filter material and hence has a third transmitted colour (e.g. blue). The layers 18a, 18b, 18c in this case at least partially overlap one another although in other cases they could fully overlap or not overlap. The various overlaps in this case produce four distinct regions ft... R4 of the colour layer 30 in each of which the transmitted colour is different. In first region R1, colour filter layers 18a and 18c overlap, resulting in a mixed colour (e.g. purple) being transmitted. In region R2, only colour filter layer 18a is present so its colour determines the colour of transmitted light (e.g. red). In region R3 colour filter layers 18a and 18b are present resulting in a different mixed colour (e.g. orange) being transmitted. In region R4, only colour filter layer 18b is present so it determines the transmitted colour (e.g. yellow). Of course, in other cases one of more of the colour filter layers 18a, 18b, 18c could have a more complex design and be formed of more than one material.

Generally, in whichever way the colour filter 30 is implemented, it is preferable for the colour filter 30 to be on the light source side of the caustic relief 20, so that the light passes through the colour filter, and takes on the colour thereof, before it is redirected by the relief structure 20. This is particularly important where the colour filter 30 comprises multiple regions of different colour as discussed below, since otherwise, if the colour filter is located such that the redirected light rays pass through it, unintended mixing of the various colours may occur. Hence, assuming each of the security devices 10 shown in Figures 2(a) to (f) are illuminated by a light source arranged facing the second side 12b of the substrate, and project a caustic image towards the region of space on the first side 12a of the substrate, the arrangements shown in Figure 2(a), (b), (c) and (d) are preferred for this reason over those in Figures 2(e) and (f). However it should be noted that the relief structure 20 could be configured to project in the opposite direction (i.e. with the light source on the first side 12a of the substrate), in which case the Figure 2(e) or (f) arrangement would be preferred.

It is assumed in the examples described so far that the security device is transparent and that the caustic relief 20 operates on refraction. However as noted above a reflective-mode device could be performed by applying a reflection enhancing layer to the relief structure 20 (not shown in Figure 2). In this case, the light source and the caustic image will be on the same side of the device as one another. As such it is desirable for the colour filter 30 to be in close contact with the relief structure 20 so that light rays only pass through it when they are in the immediate vicinity of the relief structure 20 and inadvertent colour mixing is minimised. Hence in this case the arrangements of Figures 2(c) or 2(e) are preferred.

In most preferred embodiments, the substrate 12 will be at least semi-transparent (i.e. optically clear, with low optical scattering, but may carry a coloured tint as noted above). For example, the substrate 12 may comprise one or more plastics materials such as BOPP, polyethylene, polycarbonate, PET or the like. However, where the security device is a reflective device, it is also possible to use a non-transparent substrate 12, such as paper, in which case the configuration of Figure 2(e) would be utilised, with the reflective layer (not shown) inserted at the interface between embossing layer 14 and colour filter layer 18 (both the light source and the caustic image being located to the first side 12a of the substrate 12).

For simplicity, all of the embodiments disclosed below will be described with the relief structure 20 and the colour filter 30 being implemented in the same way as in the Figure 1 example. However it will be appreciated that any of the alternative configurations just explained could be applied to any of the ensuing embodiments, unless otherwise stated below.

In the Figure 1 embodiment, the colour filter 30 comprises a single material 32a and hence the transmitted colour is uniform across the filter. More complex visual effects, and a higher security level, can be achieved by forming the colour filter 30 of more than one material in respective laterally offset regions of the filter, so that the colour of light transmitted by the filter varies across its extent. Alternatively or in addition the thickness of the material(s) forming the filter could be varied from one region to another, to vary the tone of the transmitted colour, as will be exemplified below. The regions could be arranged without reference to the caustic image, so that different portions of the caustic image exhibit different colours (or tones) without regard for the image content. However, it is preferable to provide at least a coarse correlation between the colour filter and the image, so that certain portions of the caustic image can be consistently allocated desired corresponding colours (or tones).

Figure 3 shows a third embodiment of a security device 10 in which this is the case. The construction of the security device 10 is generally the same as already discussed with reference to Figure 1. However, in this embodiment the colour filter 30 comprises two laterally offset regions 31a, 31b each formed of a different transparent or semi-transparent material 32a, 32b respectively. At least one of the materials 32a, 32b transmits only a sub-set of visible wavelength and hence light from a white light source L transmitted through it is non-white. The other material may also transmit a non-white colour or could be coloudess and hence transmit white light (assuming the light source L is a white light source). The first region 31a, comprising first material 32a which transmits first colour Ci, has the form of a rectangle and makes up the left half of colour filter 30, while second region 31b, comprising second material 32 which transmits second colour 02, forms another rectangle making up the right half of the filter. Thus when viewed in reflected light, the security device will display a two-coloured rectangle as shown in Figure 3(b).

The relief structure 20 is configured to project a caustic image Cl which here is of the number "10". In this example, a first area 21a of the relief structure 20 which contributes to a first portion Pi of the image, conveying the digit "1", is generally in the left half of the relief structure 20. Similarly, a second area 21b of the relief structure 20 which contributes to a second portion P2 of the image, conveying the digit "0", is generally in the right half of the relief structure 20. Other areas 21c, which do not contribute to either portion Pi, P2 of the image may also be provided.

As mentioned in relation to Figure 1, while Figure 3(c) shows the areas 21a, 21b as having shapes similar to that of the corresponding image portions, this may not be the case in practice. The areas 21a, 21b of the relief structure which contribute to each respective portion H, P2 of the caustic image can be identified, for instance, for a given relief structure 20 by ray tracing techniques. These could be performed using a computer model of the relief structure 20 and appropriate ray tracing software. Alternatively, the areas 21 could be identified manually by applying colour(s) to various parts of the relief structure 20 and observing where those colours appear in the caustic image. Alternatively still, the correlation between the areas 21 and the portions of the caustic image could be known from the process according to which the relief structure 20 was generated.

Since the first region 31a of the colour filter 30 overlaps first area 21a of the relief structure, the first portion Pi of the caustic image Cl will exhibit the first colour Ci. Likewise, since the second region 31b of the colour filter 30 overlaps second area 21b of the relief structure, the second portion P2 of the caustic image Cl will exhibit the second colour 02. Of course, any number of image portions could be coloured in this way by providing appropriate coloured materials in corresponding regions of the colour filter. Particularly strong visual effects can be achieved if each image portion corresponds to a distinct element of the caustic image, as is the case in the Figure 3 example since each portion corresponds to the whole of a bright part of the image (here a single digit "1" or "0"), and is separated from other elements of the image. However, the portions could together form a continuous bright part of the image in multiple colours, if desired -for instance the first portion could correspond to the top half of both digits "1" and "0", and the second portion could provide the lower half of both digits "1" and "0". This could be achieved by rotating the colour filter through 90 degrees.

In the example shown, the first and second portions P1, P2, of the caustic image Cl (corresponding to the digit "1" and the digit "0" respectively) will appear in different colours Ci, C2. For instance, if the first material 32a transmits blue light, and the second material 32b transmits yellow light, the "1" will appear blue while the "0" will appear yellow, i.e. the caustic image is multi-coloured. Alternatively, a two-colour image can be provided if one of the materials 32a, 32b is colourless.

For instance, if the first material 32a transmits red light, and the second material 32b transmits white light, the "1" will appear red while the "0" will appear white. Alternatively, a white portions of an image can be achieved by omitting the colour filter 30 in the appropriate region entirely (at least where the colour filter is provided by a colour filter layer).

Figure 4 shows an alternative embodiment in which the colour filter 30 comprises a single material 32a, the thickness of which varies between regions 31a, 31b of the colour filter. In all other respects, the security device is the same as that of the Figure 3 embodiment and the same description applies. Thus in the first region 31a, the colour filter 30 is formed of a first material 32a' having a first thickness On the direction normal to the substrate plane) and in the second region 31b, the colour filter 30 is formed of the same first material 32a" having a second, different thickness. In this case, the material is thicker in the first region 31a than in the second region 31b. The material is of a sort which absorbs selected wavelengths in order to generate its transmitted colour. As such, the two regions 31a, 32b transmit the same colour of light but different tones (shades), Ti, T2. The different tones are also visible in the non-projected image R displayed directly by the colour filter 30. The projected caustic image Cl now appears wholly in the first colour C1, but the first portion P1 of the image exhibits a first tone T1 which is darker than a second tone 12 exhibited by the second portion P2 of the image. It will be appreciated that while the Figure 4 embodiment utilises a single material, the same principle can be applied to embodiments having two or more materials forming the colour filter, in order to achieve both colour and tonal variation in the non-projection image.

Figure 5 shows a further embodiment based on that of Figure 4, like reference numerals denoting like features. In this case, the colour filter 30 comprises three regions 31a, 31b and 31c each having a different thickness of the first material 32a', 32a" and 32a7 In the third region 31c, the thickness of the first material 32a" is such that, locally, it is substantially opaque to the incident light L. As such, light striking the third region 31c does not contribute to the caustic image Cl and the third region 31a can be considered to constitute a masking component. The masking component will be visible in the non-projected image R On this case, taking the form of an annular shape as shown in Figure 5(b), but will not be visible in the projected caustic image Cl. Of course, the caustic relief structure 20 will need to be designed taking into account the presence and extent of the masking component, so that the desired caustic image Cl is still achieved. As such, the particular shape of the relief structure 20 in the Figure 5 embodiment will be different (if only in minor respects) from that of the Figure 4 embodiment, in order to achieve identical caustic images Cl.

In the embodiments described so far, there is a direct correspondence between the colour(s) and/or tone(s) transmitted by the material(s) making up the colour filter 30 and those exhibited by the caustic image. However, this is not necessarily the case and strong security effects can be achieved by arranging the colour(s) exhibited by the caustic image to be different from, e.g. fewer than, those apparent from the colour filter itself. An example of this is shown in Figure 6, which embodiment is a modification of the Figure 3 embodiment. The relief structure 20 has the same configuration as in Figure 3 and generates a caustic image exhibiting the number "10". In the colour filter 30, however, the first and second materials 32a, 32b are arranged as a series of alternating regions 31a, 31b, such that parts of the first area 21a of the relief structure 20 are overlapped by the first material 32a and others by second material 32b. Likewise, parts of the second area 21b of the relief structure 20 are overlapped by the first material 32a and others by second material 32b. In other words, each area 21a, 21b of the relief structure is overlapped by a respective set of (plural) materials. In this case the two sets of materials each consist of the first material 32a and the second material 32b, and are hence the same as one another (but in other embodiments the sets could be different from one another).

As a result, the first and second transmitted colours Ci, 02 are mixed by the caustic relief and both the first and second portions Pi, P2 of the caustic image appear in a mixed colour 03. For example, if the first material 32a transmits blue light, and the second material 32b transmits yellow light, both portions of the caustic image may appear green (i.e. the caustic image is monochromatic). Of course, different shades of the mixed colour could be achieved by providing different proportions of the two materials 32a, 32b overlapping the different respective areas 21a, 21b of the relief structure. It is also possible to achieve white as the mixed colour, if the colour filter includes appropriate red, green and blue transmitting regions (or equivalent) which are then mixed by the relief structure.

Figure 7 shows a further embodiment which is a variant of that described with respect to Figure 6. The Figure 7 embodiment is identical to that of Figure 6 except for the addition of a masking component 40. Here, the masking component 40 has the same effect as that disclosed in relation to Figure 5 above. However, rather than being formed from regions of the colour filter 30 having increased thickness, in this case the masking component 40 is formed as a separate layer. For instance the masking component 40 could be provided by a separate print of a suitable substantially opaque ink, or as a patterned layer of metal or another substantially opaque material. The masking component could lie over the colour filter 30 as shown or could take any other position within the layer structure of the device 10, e.g. between the colour filter layer 18 and the substrate 12, or between substrate 12 and embossing layer 14. The masking layer overlaps a sub-part of the relief structure 20, which will no longer contribute to the caustic image Cl since light is no longer transmitted through the device at that position. The masking component 40 is preferably provided in the form of a pattern or image, e.g. alphanumeric text. In this example, the masking component forms the word "TEN". The masking component 40 will be visible when the security device is viewed directly On reflection or transmission) and therefore contributes to any non-projected image displayed by the colour filter. As such, the design of the masking component 40 may be chosen to complement or enhance the non-projected image. The pattern formed by masking component 40 will not be visible in the projected caustic image Cl, the relief structure 20 having been designed so as to account for the presence of the masking component 40.

In the examples described so far, the correlation between the regions 31 of the colour filter 30 and the areas 21 of the relief structure 20 is coarse, with the result that the reflective appearance of the colour filter is typically of a set of coloured stripes or blocks. To further enhance the visual effect of the security device, it is desirable to configure the colour layer so as to convey a non-projected image when viewed directly in reflected or transmitted light, as a result of the arrangement of different materials in their respective regions or as a result of a variation in the thickness of one material from region to region. Where the caustic relief structure 20 is a continuous relief structure, such as may be formed by the methods disclosed in any of WO-A-2019/063779, WO-A-2020/070304 and WO-A-2020/070299, the relative positions of the areas 21 in the relief structure will typically correspond, at least approximately, to the relative positions of the portions P of the caustic image which those areas contribute to. As a result, the necessary arrangement of the regions 31 in the colour filter will also broadly correspond to what is seen in the caustic image.

This is the case in the embodiment shown in Figure 8. Once again, the construction of the device is generally the same as discussed with respect to Figure 1. The relief structure 20 is configured to generate a caustic image Cl which here depicts a flower as shown in Figure 8(d). A first portion Pi of the image, corresponding to the petals of the flower, is generated by a first area 21a of the relief structure 20, shown in Figure 8(c), which occupies an approximately square area at the centre top of the structure (corresponding to the position of the first portion Pi in the image Cl). A second portion P2 of the image, corresponding to the stem and leaf of the flower, is generated by a second area 21b of the relief structure 20, occupying a chevron-shaped part of the relief structure 20 below the first area 21a. Thus the positional relationship of the first and second areas 21a, 21b relative to each other is substantially the same as that of the first and second image portions Pi, P2. In the colour filter 30, shown in Figure 8(b), first and second regions 31a, 31b are designated corresponding to and overlapping the first and second areas 21a, 21b of the relief structure 20, each being formed by a material transmitting the desired colour for that portion of the image. For example, the first material 32a forming first region 31a may be purple while the second material 32b forming second region 31b may be green, with the result that the caustic image will exhibit a corresponding image in which the petals of the flower (first portion Pi) are purple, while the stem and leaves are green. Hence the non-projected image (here termed reflected image RI) is a version of the caustic image Cl, in that the colours have the same general relative arrangement.

In this case and in the examples which follow (Figures 9 to 13), the materials forming the colour filter are each assumed to have the same reflected and transmitted colour, with the result that any non-projected image will have the same appearance in reflected and transmitted light. As such, the non-projected image is described in terms of its reflected appearance only (and hence may be referred to as the reflection or reflected image RI) but this will be the same as its transmitted appearance. All of these embodiments can instead be implemented using colour filter materials which have different reflection and transmission colours. Further embodiments which make particular use of such materials will be given below.

The security level can be significantly enhanced by decoupling the non-projected appearance of the colour filter 30 from that of the caustic image Cl. That is, the arrangement of material(s) in the colour filter is independent of the eventual projected caustic image. This enables the non-projected image (if any) to be different from the caustic image. This can be achieved by use of a segmented relief structure 20, the segments 25 of which are laterally distributed relative to one another such that the areas 21 which contribute to respective portions P of the caustic image do not have the same relative positions in the relief structure 20 as to the portions Pin the caustic image Cl. The corresponding regions 31 of the colour filter, the arrangement of which is determined by that of areas 21 of the relief structure, will therefore also have different relative positions compared to the portions P in the caustic image Cl, with the result that the dependence of one on the other is removed.

Figure 9 shows an embodiment which makes use of this principle. The construction of the security device 10 is the same as described with reference to Figure 1 aside from the modifications explained here. The relief structure 20 comprises an array of segments 251, 252,... 25n. Whilst, for clarity, Figure 9(a) illustrates the segments as being demarcated from each other through the thickness of embossing layer 14, in practice the segment boundaries may exist only in the design of the surface relief 20 itself. Each segment 25 contributes light to (only) one portion P of the caustic image Cl, but each portion P of the caustic image Cl may be generated by one or many segments 25. The colour filter 30 is arranged according to a corresponding array of cells 351, 352,... 35n1, 35,1. Each cell 35 overlaps a single one of the segments 25 and has substantially the same size and shape thereof. It should be noted that whilst all the segments 25 (and hence all the cells 35) in this example have the same size and shape as one another, this is not essential.

A segmented relief structure 20 can be derived in a number of ways. In some preferred examples, the process may begin with a computer model of a non-segmented (continuous) relief structure which has been designed to project a certain caustic image, such as those described with reference to Figures 1 and 3 to 8. The non-segmented relief structure can then be divided into segments. In one example, this could be performed by dividing the relief structure into an arbitrary array of segments, e.g. with the same size and shape as one another, without reference to the contours of the relief structure. For instance, the segments used for redistribution of the caustic element are preferably arranged as a regular grid, e.g. square or rectangular segments (although the segments' shapes can be adapted at boundaries of the non-projected image to give smoother curves in the non-projected image). It can then be determined to which portion Pi of the caustic image each segment 25i primarily redirects light, in which case that segment 25i is designated as part of the corresponding area 21. This can be achieved using ray tracing software or any of the other techniques for identifying areas of the relief structure already mentioned above. Alternatively, the various areas 21 of the relief structure 20 could be identified before division into segments takes place and then the segments can be defined with reference to the areas, e.g. by drawing segment boundaries around each part of the relief structure which redirects light primarily to a single portion P of the image. In this case, the segments are unlikely to have the same shape and size as one another. In other preferred examples, a segmented relief structure 20 can be derived directly without being based on a non-segmented relief structure. For example, the generation process can begin with a desired grid defining the segment boundaries. Each position in the grid can then be allocated a certain relief, forming a segment of the eventual relief structure, configured to redirect incident light to a certain portion P of the image.

In the Figure 9 example, the segments which contribute to first portion Pi of the caustic image are interspersed in a periodic manner with those which contribute to the second portion P2 of the caustic image Cl. Hence, as shown best in Figure 9(b), the first segment 251 provides part of first area 21a of the relief structure, while the second segment 252 provides part of second area 21b of the relief structure, and so on. In this example, the periodicity is in two dimensions (i.e. along both the x and z-axes) although in other examples it could be one dimensional (i.e. along either the x-axis or the z-axis). The cells 35 of the colour filter 30 are arranged accordingly in a periodic pattern, with all of the cells (such as cell 351) which overlap segments providing first area 21a being formed of a first material 32a, and collectively forming a first (discontinuous) region of the colour filter. Likewise, all of the cells (such as cell 352) which overlap segments providing second area 21b are formed of a second material 32b and collectively forming a second (discontinuous) region of the colour filter 30. Hence in reflected light, the colour filter 30 appears either as a periodic pattern of different materials or, if the cells are sufficiently small such that they cannot be individually resolved by the naked eye, as a uniform area of mixed colour.

When illuminated by light source L, each segment 25, of the relief structure redirects light of the colour transmitted by corresponding cell 35; towards a respective portion P of the caustic image to thereby recreate the image. It will be appreciated that the distribution of the segments 25 does not significantly change the caustic image, since the generation of the bright and dark spots is more heavily dependent on the angles to which the various light rays are directed, rather than the specific position within the relief structure from which each light ray emanates. In this example, since all of the cells 35 corresponding to the segments 25 which make up first area 21a of the relief structure are provided with the same first material 32a, the first portion P1 of the caustic image will exhibit the transmitted colour of material 32a -for instance, in this case the petals of the flower in caustic image Cl may appear purple. Similarly, since all of the cells 35 corresponding the segments 25 which make up second area 21b of the relief structure are provided with the same second material 32b, the second portion P2 of the caustic image will exhibit the transmitted colour of material 32b -for instance, in this case the stem and leaf of the flower in caustic image Cl may appear green.

More generally, the colour exhibited by each portion P of the caustic image will depend on the colour(s) transmitted by a set of material(s) provided in the cells 35 of the colour filter 30 corresponding to the relevant segments 25 of the relief structure 20. In the present example, each set comprises a single material.

However, one or other of the sets could comprise multiple materials. For instance, instead of forming all of the cells (such as cell 352) which overlap segments forming part of second area 21b of a green second material 32b, half of those cells could be formed of a yellow material and the other half of a blue material. The yellow and blue transmitted colours will be mixed by the caustic relief with the result that the second portion P2 of the caustic image once again appears green. It is also possible to perform colour mixing by providing more than one material within a single cell 35 of the colour filter, e.g. if the individual regions can be defined at dimensions smaller than the cells. For instance, the colour filter could be formed as a pixel array where a pixel is the smallest unit and each pixel can only comprise a single material. Each cell may comprise one pixel, or may comprise multiple pixels (e.g. a 4x4 group of pixels). All of the pixels formed of the same material make up one region of the colour filter, which may be continuous or non-continuous depending on the arrangement of pixels.

In the Figure 9 embodiment, the arrangement of the segments 25 (and hence of the cells 35) is periodic. This has particular advantages as will be explained below. However, it will be apparent that the segments 25 could be arranged laterally in any desired order, including at random (or pseudo-random) positions across the relief structure 20. In this case, the arrangement of cells 35 in the colour filter 30 will also be random (or pseudo-random), which if the cells are sufficiently small can lead to the reflective appearance of the colour filter being substantially uniform (e.g. a mixed colour) with no discernible image information.

This helps to conceal the presence of the security feature, and the projection of a well-defined image Cl with distinct colours in different portions thereof is particularly unexpected. The colour filter 30 could, for instance, take the form of a regular grid of lines or pixels in a selection of colours such as RGB or CMYK.

Since RGB and CMYK palettes can be used to achieve any colour, such implementations have the advantage that the same design of colour filter could be used for any device, with the caustic relief determining which colour(s) are displayed and where in the caustic image.

Periodic arrangements of the segments and cells, such as that shown in Figure 9, can also be used to achieve particularly good optically variable effects in which the colour(s) of the caustic image change or switch upon tilting of the device relative to the light source. In order for any optically variable effect to be achieved, there must be a spacing between the colour filter 30 and the relief structure 20. In the Figure 9 embodiment, this spacing is provided by the substrate 12 and the body of embossing layer 14, and has a combined thickness t. However, suitable spacings can also be achieved using the alternative device constructions shown in Figures 2(b) and 2(d), for example. Due to the parallax effect, the light ray incident on a particular location of the relief structure 20 will pass through a different position on the colour filter 30 depending on the tilt angle of the device relative to the incident light. If the colour transmitted by the colour filter 30 varies between those different positions, the colour contributed to the caustic image by that location of the relief structure will also change.

Figure 10 illustrates this effect using the same exemplary security device 10 as already discussed with respect to Figure 9. In Figure 10(a), the light source (designated L') has been moved along the x-axis relative to the security device (which is equivalent to tilting the security device about the z-axis) and the resulting light rays are shown in solid lines. The light rays from the original position of the light source (designated (L)) are shown in dashed lines for comparison (and are identical to what is shown in Figure 9(a)). It will be seen that each segment 25 no longer receives light having passed through the overlapping cell 35, but rather light which has passed through the adjacent cell 35-1. For example, the light which passes through cell 351 is then redirected by segment 252 towards second portion P2 of the caustic image, and so on. This is illustrated in Figure 10(b) which is not a plan view of the colour filter 30 but rather shows the effective colour filter 30' which will modify the light coming from light source L' for each segment of the relief structure 20.

Thus, the first segment 251 of the relief structure will receive light having passed through cell 350, which contains second material 32b, while the second segment 252 of the relief structure will receive light having passed through cell 351, which contains first material 32a (as also shown in Figure 9(b)). As a result, the colour of light redirected by all of the interleaved segments 25 which form part of first area 21a and thus contribute to the first portion P1 of the caustic image (the flower petals, in this example) is now determined by the second material 32b. Likewise, the colour of light redirected by all of the interleaved segments 25 which form part of second area 21b and thus contribute to the second portion P2 of the caustic image (the stem and leaf, in this example) is now determined by the first material 32a. Therefore, if the first material 32a transmits purple light while the second material 32b transmits green light, in the caustic image Cl', the flower petals will now appear green while the stem and leaf of the flower appear purple. When the security device 10 is tilted relative to the light source between the position shown in Figure 9(a) and that shown in Figure 10(a), the caustic image will therefore appear to switch colours. There will also be a small positional displacement Ax between the position of the caustic image Cl seen when the security device is normal to the incident light (Figure 9) and that of the caustic image Cl' seen when the security device is tilted relative to the incident light (Figure 10), as shown best in Figure 10(d), where the position of original caustic image Cl is shown in dashed lines for comparison.

It will be appreciated that it is not essential to arrange the segments and cells in a periodic manner in order to attain an optically variable effect on tilting -indeed, it is not essential for the relief structure to be segmented. Any embodiment in which there is a spacing between the colour filter 30 and the relief structure 20 and the colour transmitted by the colour filter 30 varies across the device may give rise to some colour change in the caustic image upon tilting. However, in many cases this will be minimal. For example, in the Figure 3 embodiment, if the security device were tilted relative to light source L in the same manner as shown in Figure 10, a small amount of the light forming second portion P2 of the caustic image may pass through first material 32a instead of second material 32b. This may slightly alter the colour in which the second portion P2 appears but the change may be difficult to detect. It would require a very large tilt angle for the majority of light forming second portion P2 to pass through first material 32a and hence for the image to significantly change colour. By arranging the relief structure 20 in periodic segments, and the colour filter 30 accordingly, the amount of tilt required for all of the light contributing to a certain portion of the image to change colour is significantly reduced. Best results are achieved where the spacing t between the relief structure 20 and the colour filter 30 is greater than or equal to the lateral width w of the cells. In order that the thickness of the security device can be kept small, in preferred cases the cells 35 (and hence the segments 25) have a lateral width w of 200 microns or less, preferably 100 microns or less, more preferably 50 microns or less.

While in the embodiment shown in Figures 9 and 10, there are only two portions P1, P2 of the caustic image, and hence two interleaved areas 21a and 21b in the relief structure 20, it will be appreciated that any number of image portions and corresponding areas of the relief structure could be provided and interleaved in a periodic manner. The periodicity could be in two dimensions (as exemplified in Figures 9 and 10), in which case an optically variable effect can be achieved when the device is tilted in either dimension, or in only one dimension, in which case the optically variable effect may be exhibited only when the device is tilted about one axis. For example, if the periodicity is only in the x-axis, the device would need to be tilted about the z-axis to perceive the optically variable effect.

While in the preceding examples, the colour filter 30 appears either uniform or patterned when viewed in reflected light (due to the periodic, random or pseudorandom arrangement of materials forming cells 35), it is also possible to configure the device such that a recognisable (human-intelligible) image is exhibited by the colour filter. This results in a strong security effect since the non-projected image (reflected image RI) can be different from, e.g. conveying different information content from, the caustic image Cl, which is unexpected. The only constraint is that the reflected image RI needs to transmit appropriate colour(s), and appropriate proportions of those colours, to enable the caustic image to exhibit the desired colours. It will be appreciated that this still does not require an exact corresponding between the selection of colours transmitted by the colour filter 30 and those which appear in the caustic image Cl for all the reasons outlined above. However, this may be preferred. It should be noted that the non-projected image could be single-colour, multi-tonal or multi-coloured. For instance, if the regions of the colour filter vary only in thickness of the material forming the colour filter, the non-projected image will appear in a single colour but multiple tones. However, if the colour filter comprises different coloured materials in different regions, the non-projected image can be multi-coloured.

Figure 11 shows an embodiment of a security device 10 which exhibits a reflection image RI which is different from the caustic image Cl it projects when illuminated by a light source L. The general construction of the security device 10 is the same as discussed with reference to Figure 9, with the relief structure being defined according to an array of segments 251, 252, ... 25,_1, 25,, and the colour filter 30 comprising a corresponding array of overlapping cells 351, 352, ... 35,.

Applying the same principles as described with reference to Figure 9, the segments 25 are distributed across the surface relief 20. However, rather than being arranged periodically, in this case the segments 25 are arranged in accordance with a desired reflection image RI to be exhibited by the device. Since each segment 25 will be allocated a cell 35 with a certain colour (or colours), depending on the desired colour which the portion of the caustic image Cl which that segment contributes to is to display, the segments can be positioned based on their allocated cell colour, to form the desired reflection image.

For example, in the Figure 11 embodiment, the reflection image RI is a square of a first material 32a, transmitting first colour Ci, surrounded by a rectangular frame in a second material 32b, transmitting second colour 02. In contrast, the caustic image Cl comprises the letters "AB", with a first portion H of the image (the letter "A") exhibiting the first colour Ci, and a second portion P2 of the image (the letter "B") exhibiting the second colour C2. This is achieved by placing all of the segments 25 of the relief structure 20 which contribute to the first portion Pi of the caustic image into a square area at the centre of the relief structure, which segments collectively form first area 21a (shown in Figure 11(c)). All of the segments 25 which contribute to the second portion P2 of the caustic image are placed in a rectangular area surrounding the square, collectively forming second area 21b. The corresponding cells 35 of the colour filter 30 are allocated the necessary materials, resulting in the above-described reflection image RI shown in Figure 11(b). Thus when the security device 10 is viewed under reflected white light, the reflection image RI will be visible (e.g. a red square surrounded by a blue rectangle), whereas when the security device 10 is illuminated by white light (typically in transmitted mode), the caustic image Cl will be projected (e.g. the letters "AB" where the "A" is red and the "B" is blue). It should be noted that whilst the reflected colour of the or each material forming the colour filter 30 will most often be the same as its transmitted colour, this is not always the case -for instance if the colour filter comprises an interference layer material, the reflected and transmitted colours thereof will typically be different.

In the above example, the set of materials provided in cells overlapping the first area 21a of the relief structure 20 is a set of one material (first material 32a) and so the colour of the first portion P1 of the caustic image corresponds to that transmitted by the first material 32a. Likewise, the second portion P2 of the caustic image has the colour transmitted by second material 32b. However, more generally, each portion P of the caustic image may have its colour determined by a set of one or more materials forming the colour filter which overlap the areas of the relief structure which contribute to that portion P. If the set comprises multiple materials with different transmitted colours, the portion P will exhibit a mixed colour An example is shown in Figure 12. Here, the construction of the security device is much the same as that of Figure 11. However, the colour filter 30 (and hence the reflection image RI) is modified, with the previously blue rectangular background being replaced by two regions 31b, 31c each having the shape of a square bracket. Region 31b is formed of material 32b which transmits colour C2 (e.g. blue), while region 31c is formed of material 32c which transmits colour 03 (e.g. yellow). Hence, as before the caustic image Cl displays its first portion P1 (e.g. letter "A") in colour Ci corresponding to the transmitted colour of material 32a (e.g. red), but now the second portion P2 (e.g. letter "B") appears in a different colour C4 which is a mixture of the colours transmitted by materials 32b and 32c (e.g. green).

The segments 25 of the relief structure may abut one another, as shown in the preceding embodiments, or may be spaced from one another (not shown). In the latter case, it may be advantageous to space the segments 25 from one another with light-scattering relief structures disposed between them, which redirect incoming light such that it does not significantly contribute to the caustic image.

This acts as a "buffer zone" around the segment so that if the alignment of the cells 35 with the segments 25 is not exact, light passing through an adjacent cell may be intercepted by the light-scattering zone and removed from the system such that the colour of the caustic image is as intended. This makes the security device 10 more tolerant of misregistrafion between the colour filter 30 and the relief structure 20. This would also reduce any optical variability in the colours displayed by the caustic image, as may be desirable in some embodiments. It should be noted that scattering zones need not be provided between every segment 25. For example, scattering zones provided only at interfaces between different areas 21a, 21b of the relief will typically be sufficient.

It will be appreciated that the security device 10 can be configured to display any desired combination of reflection image RI and caustic image Cl, by extending the above principles as necessary. Desired colours of the caustic image can be achieved either by providing materials with matching transmission colours in the colour filter and/or by mixing transmitted colours as necessary. If the caustic image Cl requires a white portion, this can be achieved in one of three ways: providing a colourless material in the appropriate region of the colour filter 30; providing materials which collectively transmit substantially all wavelengths of the visible spectrum (e.g. red, green and blue) in the region and mixing their transmitted colours to form white; or by omitting the colour filter 30 entirely in the region.

In general terms, devices of this sort can, in one embodiment, be designed by: providing (e.g. drawing or selecting) a non-projection image to be exhibited by the colour filter, comprising a plurality of laterally-offset regions which vary in colour and/or tone from one region to another; for each region of the non-projection image, providing a relief structure segment based on the colour and/or tone of the region; and arranging the relief structure segments according to the arrangement of regions of the non-projection image. Thus the non-projected image is selected first and the relief structure is then designed to account for the desired non-projected image and place the projected light rays in the correct positions to achieve the required arrangement of colours in the caustic image. "Providing" the relief structure segments could involve reallocating segments of a pre-designed caustic relief configured to display a certain projected image and/or creating a relief for each segment, which is designed to redirect the light transmitted through that region to the desired position on a screen to thereby build up a caustic image.

Once all the relief segments have been determined and arranged, the resulting structure can then be physically made, e.g. by etching the design into a suitable surface from which the relief structure can be cast.

An example of a security device 10 configured to display more complex images is depicted in Figure 13. The security device is shown illuminated by a preferred light source L, namely the torch of a smart phone or other mobile device 90. Here the security device 10 is carried on a security article 5 in the form of a strip applied to a security document 100 such as a banknote. The strip 5 is positioned over a transparent window region of the security document 100 (not visible in Figure 13).

The security device 10 could occupy the whole strips, or just a portion thereof as shown here (in which case the portion will be aligned with the transparent window region). Under reflected light, the strip 5 displays a reflection image RI, corresponding to the colour filter 30, which in this example exhibits an array of the number "20" with both digits in red. Each number "20" has a thick white outline region. The remainder of the colour filter is blue and provides a background to the array. This image can be continued across the whole of strip 5 (as shown), even if only a portion of it overlaps the relief structure 20 (not visible in Figure 13) to thereby form security device 10. The relief structure is configured to project a caustic image Cl which here comprises a flag, such as the Union Jack. The areas of the relief structure which generate the red "cross" elements of the flag (first portion Pi) are aligned with the red "20s" of the colour filter image RI, so that these elements of the caustic image appear red. The areas of the relief structure which generate the white "cross" elements of the flag (second portion P2) are aligned with the white outline regions surrounding each "20" of the colour filter image RI, so that these elements of the caustic image appear white. The areas of the relief structure which generate the blue background elements of the flag (third portion P3) are aligned with the blue background regions surrounding of the colour filter image RI, so that these elements of the caustic image appear blue.

In one embodiment, the following steps are involved in creating the security device: 1 Define non-projection image RI of colour filter 30 -in the Figure 13 embodiment this corresponds to defining the shape of the "20"s and their contoured outlines, as well as their placement on the background. More generally, the non-projection image RI can be complex, e.g. a rectangular array of dots that when viewed appear as an image, or simplistic, e.g. a macro shape on a square background.

Define projected caustic image Cl -in the Figure 13 embodiment this corresponds to defining the shape of the Union Jack flag. Again, this image could take any form. It will be appreciated that steps 1 and 2 could take place in either order.

Define colour placement within non-projected image RI on colour filter 30 -in the Figure 13 embodiment, the "20"s are defined as red, their outline as clear (colourless) and the background is defined as blue.

Define colour placement within projected caustic image Cl -in the Figure 13 embodiment this will be as per the Union Jack flag. In generally it will be appreciated that the colours can only be chosen based on colours/mixing of colours available in colour filter 30. Again, the order of steps 3 and 4 could be reversed.

Map/ray trace projection colours in caustic image Cl to corresponding colours in non-projection image on colour filter 30. This involves tracing the rays from the projected image back to the device. In performing this step, it is necessary to factor in parameters such as projection distance etc. The ray trace defines the positional location of specific parts of the overall surface relief structure. There could be multiple location options for the trace back to the colour filter; e.g. blue flag to blue background has many options, red flag to red "20" less, and clear flag to clear outline less again. Define caustic structure on device to produce ray trace -a relief structure is designed which will generate the desired mapping between the non-projection image and caustic image. This might not be the optimum structure placement for best caustic efficiency (brightest/sharpest projection). Optionally, an iterative process might be carried out to improve the caustic efficiency. For instance, in a further step the relief structure may be adjusted and/or repositioned with alternative mapping to improve the efficiency of device.

It will be appreciated that the above process will typically take place using a computer model or other software-based technique. Once the caustic structure has been defined it can be physically formed, e.g. by cast-cure, in accordance with the designed surface structure. As mentioned previously, in the above embodiments it has been assumed that the material(s) forming the colour filter 30 have the same colour(s) in reflection and in transmission. However, in variants thereof, one or more of the materials forming the colour filter could be of a type which exhibits different colours in reflection and in transmission, such as a plasmonic material (e.g. plasmonic particles or a plasmonic structure), or an interference material (e.g. interference pigments, a thin film structure or liquid crystal). In this case, the reflection images RI in each of the above embodiments (i.e. the appearance of the non-projected image exhibited by the colour filter 30 in reflected light) will differ in colour from those described above. The appearance of one or more different colours in the reflected image as compared with those seen when the non-projected image is viewed in transmitted light and with those seen in the caustic image provides a strong security effect.

Some embodiments which make particular use of this principle will now be described with reference to Figures 14 to 19. In each case, the security device 10 is depicted as having the same construction as explained with reference to Figure 1 but again any of the alternative constructions shown in Figure 2 could be employed instead. In these embodiments, the security device 10 is shown disposed on a transparent substrate 12 which is equipped with external layers 15 on either side (such as opacifying layers), which are omitted across the area of the substrate 12 where the device 10 is located, e.g. forming a window region.

In Figure 14, the colour filter 30 is formed of a single semi-transparent material 32a which exhibits different colours in reflected and transmitted light, such as a plasmonic material or an interference material. For instance, the material 32a may appear gold in reflected white light and blue in transmitted white light. As shown best in Figure 14(b), under reflected white light the non-projected image NPIR therefore appears as uniform area with no information content, which is gold in colour However, when the device is viewed directly in transmission against a white light source, the non-protected image NPIT now appears uniformly blue.

When the caustic image Cl is projected, e.g. onto screen S as shown in Figure 14(d), it appears in the transmitted colour of the material 32a, e.g. blue In this example, the caustic image Cl is of the number "20".

Figure 15 shows a more complex embodiment in which the colour filter 30 is formed of two materials 32a, 32b in respective regions. Each of the materials 32a, 32b exhibits a different colour in reflection and in transmission, and in this case the two materials also contrast with one another under both viewing conditions. For example, material 32a may appear gold in reflected white light and blue in transmitted white light, whilst material 32b may appear green in reflected white light and red in transmitted white light. The regions may be aligned with the caustic relief structure such that certain portions of the caustic images appear in certain colours, as explained above with reference to Figure 3 for example. Hence in this example, in reflected white light the non-projected image NPIR appears as a circle with a gold left half and green right half, whereas in transmitted white light the non-projected image NPIT has the same image content, but the left half of the circle is blue and the right half is red. In the projected caustic image Cl, the left portion (digit "2") is blue while the right portion (digit "0") is red.

The embodiment of Figure 16 expands on the same principle by adding a third material 32c to the colour filter 30, which again exhibits different colours in reflection and transmission. In this example the third material 32c appears orange in reflected white light and green in transmitted white light. This transmitted colour is the same as the reflective colour of second material 32b. As shown in Figure 16(b), under reflected white light the non-projected image NPIR is a circle with a gold left half and two quadrants in the right half which appear green (top) and orange (bottom). When viewed in transmitted white light, the non-projected image NPIT has the same image content but the colours are different. Now, the left half is blue while the quadrants appear red (top) and green (bottom). In the caustic image Cl, the left portion ("2") appears blue as in the previous embodiment while the top of the right portion ("0") is red and the bottom is green.

In Figure 17, a surprising effect is achieved by using a first material 32a which appears different colours in reflection and transmission (e.g. gold under reflected white light and blue in transmitted white light), and a second material 32b which has the same colour under both viewing conditions. The colour of the second material 32b is the same as one of the colours exhibited by the first material. For instance, in this case the second material appears gold in both reflected white light and transmitted white light. Thus, when viewed in reflection under white light the non-projected image NPIR appears uniformly gold and, if the colours of the materials are sufficiently closely matched, it may appear to contain no information content. However, when viewed in transmitted white light, the non-projected image NPIT now reveals a left half which is blue and a right half which is gold.

Likewise, the caustic image Cl appears with a blue left portion ("2") and a gold right portion ("0").

More complex designs can also be created. In Figure 18, the colour filter is formed of at least three materials 32a, 32b, 32c arranged to convey when viewed directly a graphic such as a portrait or (as shown here) a landscape. In this example, each of the three materials exhibits different colours in reflection and transmission (e.g. gold/blue; orange/green; and green/red) although standard materials could also be provided. In this example when viewed under reflected white light, the non-projected image exhibits a complex graphic of a sunset scene, in which the sky is yellow, the sun green and the cactus orange. When viewed in transmitted white light, the non-projected image NPIT shows the same image but in different colours with the sky now appearing blue, the sun red and the cactus green. The projected caustic image Cl is a much simpler graphic, here the number "20", with different portions thereof appearing blue, red and green.

In Figure 19, the principle is reversed such that the non-projected image is less complex than the caustic image. Again, the colour filter 30 is formed of the three materials 32a, 32b, 32c but arranged in a simple block formation. Thus, under reflected white light the non-projected image NPIR exhibits a circle with a gold left half, and quadrants in the right half which are green (top) and orange (bottom). In white transmitted light, the non-projected image NPR-has the same formation but the left half of the circle is blue and the quadrants are red (top) and green (bottom).

The projected caustic image Cl is a complex graphic such as the sunset scene shown more clearly in Figure 18, with the sky appearing blue, the sun appearing red and the cactus appearing green.

As mentioned above, the relief structure 20 and the colour filter 30 may or may not have the same lateral extent as one another. Figures 20 and 21 show two embodiments in which the colour filter extends beyond the bounds of the relief structure 30. In Figure 20, the security device 10 is formed in a window region 101 of a security document bounded by opacifying layer 15 arranged on transparent substrate 12. The caustic relief structure 20a is formed in an embossing layer 14 disposed on a first surface of the substrate and the colour filter 30 is formed in a colour filter layer 18 disposed on the second surface. Of course, the colour filter 30 could be implemented in any of the alternative ways described above in relation to Figure 2. The colour filter 30 covers a circular area occupying all or the majority of the window region 101 whereas the relief structure 20 is present only in a smaller concentric circular area. Thus the colour filter 30 extends laterally beyond the relief structure 20a in all directions. As shown in Figure 20(a) the incident light impinging on the relief structure 20a is redirected to form a projected caustic image Cl on screen S in the above-described manner. Meanwhile, light incident on the non-caustic areas 20c passes straight through the device 10, forming a faint halo H effect around the caustic image Cl, which here is a "2" symbol. In this case, since the colour filter 10 is of uniform colour, the caustic image Cl and halo H will have the same colour as one another. However, the colour intensity of the halo H will typically be less than that of the caustic image Cl since it is formed by unfocussed light. The Figure 21 embodiment is the same as that shown in Figure 20 except here the colour filter is formed of a first material 32a in a circular area corresponding to that of the caustic relief area 20a, and a second material 32b in an annular area surrounding material 32a and corresponding to (at least part of) the non-caustic area 20c. The first and second materials transmit different colours 01, Cz. As such the caustic image Cl appears in a first colour (e.g. green) while the surround halo H appears in a second colour (e.g. red).

Whilst in most embodiments described above, the relief structure has been formed as a single continuous caustic relief area, this is not essential. In other cases it may be desirable to provide the relief structure in the form of two or more caustic relief areas which are spaced by a non-caustic surface (which may be a flat area of the surface of the substrate). Figures 22 to 26 show some examples in which this is the case. In the Figure 22 embodiment, as shown in Figure 22(a), the relief structure comprises two caustic relief areas 20a, 20b each formed in a patch of embossing layer 14 which are spaced from one another by non-caustic areas 20c in which the embossing layer 14 is absent. The caustic relief areas 20a, 20b are configured as previously described to together project a caustic image Cl which here is the symbol "E". A colour filter 30 is provided in the form of a colour filter layer 18 although as before this could take any of the alternative implementations described with reference to Figure 2. In this case the colour filter is of uniform colour and extends over the whole window region 101 in which the security device 10 is disposed. As such the non-projected image is a uniform colour block filling the whole window 101 as shown by the linear shading in Figure 22(b). VVhen illuminated, the device 10 projects a caustic image Cl onto screen S, which is accompanied by faint halo lines H in the same colour as the caustic image Cl.

The Figure 23 embodiment is substantially the same as the Figure 22 embodiment, except that here the colour filter 30 is not provided across the whole window region 101 but rather only in alignment with the two caustic relief areas 20a, 20b. As such the non-projected image appears to have the same shape as the caustic relief areas 20a and 20b, appearing as two coloured areas within the window 101 separated by a colourless area corresponding to the non-caustic region 20c. The security device projects a coloured caustic image Cl as before.

However in this case there may be no halo effect since the light passed by the non-caustic region 20c is not coloured.

The multiple caustic relief areas 20a, 20b can be provided within one and the same window region 101 as just shown. However, in other cases it may be desirable to distribute the caustic relief areas across different window regions. Preferably the windows should be sufficiently close to one another that they can be illuminated simultaneously with substantially the same illumination angle. This is exemplified in Figures 24, 25 and 26. In the Figure 24 embodiment, the security device 10 comprises a first caustic relief structure area 20a located in a first window region 101a and a second caustic relief structure area 20b located in a second window region 101b. A colour filter 30 is provided in both window regions 101a, 101b and in this case it has a uniform colour in both regions. The two window regions 101a, 101b are separated from one another by a non-transparent part of the substrate in which the opacifying layers 15 are present. The first and second caustic relief structure areas 20a, 20b in this case are each configured to form a respective portion P1, P2 of the caustic image projected onto screen S when the device 10 is illuminated. For instance, here the caustic image Cl is the number "10", formed of a first portion P1 which is the digit "1", and a section portion P2 which is the digit "0". In this case since the colour filter 30 is of the same transmissive colour in both windows 101a, 101b, both portions Pi, P2 appear in the same colour Ci (e.g. red). Figure 25 shows a variant in which the colour filter 30 is formed of a first material 32a in window 101a and a second material 32b in window 101b, the first and second materials having different transmissive colours 01, 02. The security device 10 is otherwise the same as in Figure 24. As such, the caustic image now appears in two colours, with the first portion P1 appearing in a first colour Ci (e.g. red) and the second portion P2 in a second colour 02 (e.g. blue).

It will be appreciated that it is not necessary for the first and second caustic relief areas 20a, 20b to generate distinct portions of the caustic image from one another. Rather, each area 20a, 20b can contribute to any part of the caustic image. In the Figure 26 embodiment, the construction is generally the same as in Figure 25, with the colour filter being formed of first and second materials 32a, 32b in first and second window regions 101a, 101b respectively. However the caustic relief in areas 20a, 20b has been modified such that now the first caustic relief area 20a provides light to parts of both the first and second portions A1, P2 of the caustic image Cl, and likewise so does the second caustic relief 20b. The result is that the colours of the transmitted light becomes mixed and the two portions Pi, P2 of the caustic image both appear in a third colour C3 (e.g. purple) which is a mixture of the colours transmitted by materials 32a, 32b.

It has been mentioned above that the relief structure periphery may or may not relate to the caustic image. The security level can be enhanced by arranging for the periphery of the caustic relief 20 to itself convey image information, which is preferably different from that of the caustic image. It should be noted that this concept could be deployed with or without a colour filter forming part of the device 10. Figures 27 and 28 provide examples of this. The relief structure preferably comprises at least one area of caustic relief 20a and at least one area 20c of non-caustic surface, which may be relatively flat or could carry a matte or scattering structure, for example. The embossing layer 14 (if one is used) may or may not be present in the non-caustic surface 20c. The lateral extent of the caustic relief area 20a will be discernible to a user when the security device is directly examined because it will possess different reflection characteristics from the non-caustic surface adjacent it. This will be particularly apparent upon changing the viewing or illumination angle (e.g. by tilting the device). Thus, as shown in Figure 27, here the caustic relief area 20a has a periphery forming the shape of the digit "5", referred to as periphery image Pl. However, the caustic image Cl projected by the structure 20a under illumination is different, here taking the form of the digit "10". This unexpected result provides a strong security effect, and can be achieved (for example) using a redistributed caustic image of the sort described above. In this case, since there is no colour filter in this embodiment, the caustic relief sill be designed based on the desired periphery image. Figure 28 shows a variant in which a colour filter 30 is also provided -otherwise, the security device 10 is the same as in Figure 27. Now, the caustic image Cl will appear in a colour Ci determined by the colour filter 30. If the colour filter 30 is of uniform colour (as shown), the caustic relief can be designed based on the periphery image PI only as before. However, if a more complex colour filter 30 is provided the caustic relief will be designed taking the arrangement of colours into account, as described above.

The periphery image can be of any form, comprising for example any of: alphanumeric text (as in the above examples), a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, or a scene such as of a landscape, an architectural structure (e.g. a building or bridge), a person, animal or plant. Of course, the information content will need to be conveyable in the form of a silhouette.

Suitable preferred apparatus, materials and methods for forming the relief structures disclosed herein are described in WO-A-2018/153840 and WO-A2017/009616. In particular, if the caustic relief structure 20 is to be formed in an embossing layer 14, this can be achieved using the in-line casting devices detailed in WO-A-2018/153840 (e.g. that designated 80 in Figure 4 thereof), using an embossing tool 85 carrying an appropriately designed micro-optical structure from which can be cast the desired relief structure shape. Similarly, the cast-curing apparatuses and methods disclosed in section 2.1 of WO-A-2017/009616 (e.g. in Figures 4 to 8 thereof) can also be used to form the presently disclosed relief structures, by replacing the relief 225 carried on casting tool 220 with an appropriate relief from which can be cast the desired shapes. In particular it will be noted that whilst WO-A-2017/009616 describes the use of the apparatus to form focussing elements, the same apparatus can be used to form any desired relief structure by appropriate reconfiguration the relief 225, including that envisaged herein.

Whichever casting apparatus is used, the curable material(s) from which the caustic relief structure 20 is cast may be applied either directly to the tool carrying the desired relief shape (e.g. to the embossing tool 85 of WO-A-2018/153840 or to the casting tool 220 of WO-A-2017/009616), or the curable material(s) may be applied directly to the substrate on which the relief structure is to be formed, and then brought into contact with the tool (e.g. by impressing the tool onto the deposited curable material). Both options are described in the aforementioned documents. Preferably, the latter option is employed and the curable material(s) are applied to the substrate by printing (e.g. screen printing as detailed in WO-A2018/153840), before being formed into the desired relief structure. If the former option is employed, it should be noted that there is preferably no wiping of the casting tool surface relief between applying the curable material to it, and bringing it into contact with the substrate, so that a base layer of curable material remains connecting the protrusions of the relief structure together on the substrate (the base layer will be of much lesser height than the protrusions).

Suitable curable materials are disclosed in WO-A-2017/009616, section 2.1. UV-curable materials are most preferred. Curing of the material(s) preferably takes place while the casting tool is in contact with the curable material, against the substrate.

Due to the nature of the cast-cure process, the resulting relief structure will typically include a base layer of material on top of the substrate, connecting the protrusions of the relief at their base. In many cases this base layer is integral with the relief structure and formed of the same curable material(s), resulting from either the shape of the casting relief and/or the manner in which the curable material is pressed between the substrate and the casting tool during processing. An example of such a base layer and its formation is disclosed in WO-A- 2017/009619, Figure 8. It is also possible to provide (alternatively or in addition) a base layer in the form of a pedestal layer, applied in a preceding step. Apparatus and methods for providing such a pedestal layer are disclosed in WO-A2017/09620, Figures 8 to 12. The colour filter 30 of the presently disclosed security device could be provided integrally with such a pedestal layer.

WO-A-2018/153840 and WO-A-2017/009616 also disclose print stations, which are disposed in line with the above-described casting apparatus. Print stations such as these are suitable for applying a colour filter layer 18, if used, to the same side of the substrate as the cast relief structure, or to the opposite side. The apparatus disclosed in WO-A-2018/153840 can achieve particularly high registration between such cast relief structures and the printed colour filter layer Suitable substrates 12 on which the disclosed security devices can be formed are disclosed in WO-A-2017/009616, section 1, and apparatus/methods for applying opacifying layers thereto in section 4, including the formation of window regions. Preferably, the opacifying layers are applied before formation of the presently disclosed security devices on the substrate. For instance, the sheet material supplied to the apparatus of WO-A-2018/153840 may comprise a polymer substrate of the sort disclosed in WO-A-2017/009616, already provided with one or more opacifying layers.

Depending on how it is implemented, the colour filter 30 may be formed of various different types of material(s) 32. For example, if the colour filter 30 is integral with a cast-cured embossing layer 14 it will need to be formed of one or more curable materials, whereas if the colour filter 30 is applied as a separate colour filter layer it could be formed of material(s) such as inks, or deposited layers/structures such as an thin film interference layer structure, or plasmonic pillars or holes. In all cases, the material(s) forming the colour filter 30 need to be at least semi-transparent (i.e. optically clear, with low or zero optical scattering), in order to avoid redirection of the light rays and preserve the caustic image Cl. Hence, where colour is imparted to the material by way of an additive, this is most preferably an absorbent dye and/or very finely ground pigment, of which examples will be given below. Preferred particle sizes are less than 500 nm, more preferably equal to or less than 100 nm. The dye and/or pigment may be disposed in a suitable binder, such as an ink binder or a curable resin, as appropriate for the form in which the colour filter 30 is to be provided. Such dyes and/or pigments can also be used as a tint in the substrate 12.

Examples of suitable pigments which can be used in materials forming the colour filter 30 include the following products set out in the following table, which are available from BASF Group: BASF Product code Particle size (pm) Microlithe Yellow 1061 K 0.100 Microlithe Yellow 2040 K 0.080 Microlithe Red 3630 K 0.049 Microlithe Magenta 4330 K 0.054 Microlith® Magenta 4535 K 0.061 Microlithe Blue 7080 K 0.039 Microlith0 Black 0066 K 0.078 The BASF Microlithe products comprise high-grade organic pigments (plus carbon black and titanium dioxide) predispersed in a vinyl chloride/vinyl acetate copolymer binder. Each of the above pigments will exhibit the same colour whether it is viewed under reflected light or in transmission. The colour of the pigment is indicated in the product code.

Examples of suitable dyes which can be used in the materials forming the colour filter include the Orasole and Neptune ranges of products available from BTC Europe Speciality Chemicals. Orasol® dyes are solvent-soluble dyes comprising metal-complex dyes in powder form. They are readily soluble in a large number of organic solvents. They give rise to intense colours with high transparency and high chromaticity. Neptune dyes are metal-free solvent dyes with high purity and very low insoluble fractions. Many colours are available in each range. Again, these materials will each exhibit the same colour under reflected and transmitted light viewing conditions.

Suitable materials which exhibit different colours in reflected and transmitted light include those which select wavelengths for transmission / reflection as a result of plasmonic interactions or interference.

Plasmonic materials include plasmonic particles, such as nanoplatelets, which will typically be disbursed in a binder and can be applied for example by printing, as well as plasmonic structures such as nanopillars or nanoholes. Examples of suitable plasmonic particles include silver nanoplatelets as supplied by BASF. A specific example can be found in WO-A-2020083794 where silver nanoplatelets with an average diameter in the range of 20 to 70nm and an average thickness of 5 to 30nm are used in a printing ink and will transmit a red or magenta colour whilst reflecting metallic green. AU-B-2014256335 discloses further examples of suitable materials which can be used to provide a broader range of colours.

Examples of plasmonic structures, such as nanopillars or nanoholes, which could be used to form the colour filter 30 described herein can be found in WO-A2019/180460. Plasmonic structures such as these would typically be formed in a separate process and then applied to the opposite side of the substrate 12 from the caustic relief 20 either as an applied film or a transfer structure.

Thin film interference structures comprise repeating layers of different refractive indices; examples can include purely dielectric stacks (metal oxide or polymer) or those composed of alternate dielectric and metallic layers. Thin film interference structures are also known as Bragg stacks or 1D photonic crystals. Such structures include two or more closely spaced interfaces, at least one of which partially reflects and partially transmits incident light, i.e. the amplitude of the incident light is split. The transmitted portion is reflected at the second or subsequent interfaces and interferes with the portion reflected from the first or earlier interfaces, leading to constructive interference of some wavelengths and destructive interference of others, and hence a characteristic colour which varies with viewing angle. Moreover, the material transmits certain wavelengths and reflects others, leading to different colours in reflection and transmission. Examples of such structures are disclosed in WO-A-2017149284. In the present invention, the colour filter 30 can be formed by depositing appropriate thin layers of materials to form an interference structure over the desired region(s).

Alternatively, such interference structures can be broken into fragments and used as interference pigments in a binder, for application by printing or coating techniques. Examples of such materials can be found in US-A-4434010 and EP- A-0657297. Liquid crystal materials such as those disclosed in US-A- 20040155221 and EP-A-0657297 can also be used.

Two particularly preferred methods for manufacturing security devices of the sod disclosed herein will now be described with reference to Figures 29 and 30. The apparatus and method discussed with reference to Figure 29 is suitable for manufacturing a security device 10 with the construction shown in Figure 2(c), the colour filter 30 being integral with an embossing layer 14 in a surface of which the relief structure 20 is carried. Figure 29(a) shows the apparatus in schematic cross-section, while Figure 29(b) shows the result of each manufacturing step on the substrate 12 in perspective view. In this example, the relief structure 20 is formed by cast-cure, and the embossing layer 14 is formed of two different curable at least semi-transparent materials 32a, 32b, each of which transmits a different colour of light therethrough. A transparent substrate 12, in the form of a sheet or web, is conveyed through the apparatus 50 in the machine direction MD by a transport assembly (not shown). A first material 32a is applied to the substrate 12 using a first application module 51a which here comprises a patterned print cylinder 54a which is supplied with the curable material 32a from a doctor chamber 52a via an intermediate roller 53a. For example, the components shown could form part of a gravure printing system. Other printing techniques such as lithographic, flexographic, screen printing or offset printing could also be used.

Print processes such as these are preferred since the curable material 32a can then be laid down on the substrate only in first regions 31a thereof, the size, shape and location of which can be selected by control of the print process, e.g. through appropriate configuration of the pattern on cylinder 54a. The curable material 32a is applied to the substrate 12 in an uncured (or at least not fully cured) state and therefore may be fluid or a formable solid. The substrate 12 is then conveyed to a second application module 51b at which a second curable material 32b is applied to second regions 31b of the substrate using the same process, resulting in the formation of complete colour filter 30.

The substrate 12 is then conveyed to a casting module 55 which here comprises a casting tool 56 in the form of a cylinder carrying a surface relief 57 defining the shape of the caustic relief structure 20 which is to be cast into the curable materials 32a, 32b (collectively forming the embossing layer 14 in this example). As each region of curable material comes into contact with the cylinder 56, the curable materials 32a, 32b fill corresponding parts of the surface relief 57, forming the surface of the curable materials into the shape defined by the relief. The cylinder 56 will typically be configured such that the surface relief 57 is only provided at regions corresponding to the shapes and positions of the applied patches of curable material forming colour filters 30. Accurate registration between the application modules 51a, 51b and the casting module 55 is necessary in order that the appropriate areas of the caustic relief structure 20 are accurately placed on the correct region of curable material. This can be achieved using standard registration methods between two in-line application units. For example, the unit applying the curable materials (e.g. coloured resins) may also apply registration markers which will be detected by the casting unit and adjustments made accordingly.

Having been formed into the correct surface relief structure 20, the curable materials 32a, 32b are cured by exposure to appropriate curing energy E such as UV radiation from a source 58. This preferably takes place while the curable materials are in contact with the surface relief 57 although if the materials are already sufficiently viscous this could be performed after separation. In the example shown, the source 58 is positioned above the substrate 12, e.g. inside cylinder 56 if the cylinder is formed from a suitable transparent material such as quartz, although curing could instead be performed through the substrate 12.

The result of the Figure 29 process is a security device 10 with a colour filter 30 integral with the embossing layer 14 in which the relief structure 20 is formed, and comprising multiple different materials such that the transmitted colour varies across the device. This can be adapted to implement any of the embodiments described above (although the device will not be optically variable) through appropriate configuration of the application modules 51a, 51b such that the materials 32a, 32b are applied according to the desired layout of the colour filter 30, and of the casting module 55 so as to impart the appropriate relief structure into the materials. Highly accurate registration between the materials 32 and the relief structure 20 (e.g. to ±75pm) can be achieved since both are applied in the same in-line process. Of course, third and optionally additional materials can also be incorporated by providing corresponding application modules as necessary.

Figure 30 shows exemplary apparatus 60 in an alternative preferred manufacturing method, which results in a security device 10 having the construction shown in Figure 1. In this case the relief structure 20 is again formed by cast-curing. This could be done using the same process described with reference to Figure 29, but preferably with a single (e.g. colourless) curable material 69 into which the relief structure 20 is formed. However, Figure 30 shows a variant in which, rather than apply the curable material 69 to the substrate 12, it is applied instead to the surface of the casting cylinder 66, directly on to surface relief 67. Again this is preferably done in a patterned manner, using a print cylinder 64 to transfer the curable material 69 from a chamber 62 only onto the appropriate areas on the casting cylinder 66 to form each relief structure 20. Upon contact with the first surface 12a of substrate 12, the areas of curable material 69 affix to the substrate 12 and curing preferably takes place at this stage to ensure strong bonding. Since the colour filter 30 is not integral with the relief structure 20 in this embodiment, it can be applied by a printing process which preferably takes place simultaneously as the relief structure 20 is cast onto substrate 12, at the same position along the machine direction MD. Figure 30 shows a print module 70 arranged so as to achieve this, comprising a print cylinder 71 configured to transfer material(s) 32a, 32b, arranged on its surface 72 in accordance with the desired colour filter 30, onto the second surface 12b of substrate 12. The inking system by which the materials 32a, 32b are transferred to print cylinder 71 are standard and hence not shown here. In this example, the substrate 12 passes through a nip formed by the print cylinder 71 and casting tool 66 and it is at the nip that both components are applied to the substrate. Alternatively, simultaneous application of the two components can be achieved using an arrangement such as that shown in Figure 30(b) of WO-A-2017/009616. Simultaneous application of the colour filter layer and relief structure is preferred since in this way exact registration can be achieved, there being no deformation or slippage of the substrate 12 between their application.

Figure 31 shows an embodiment of a security document 100 in accordance with the invention, in plan view and cross section along the line X-X'. In this example, the security document is a banknote but it could instead take the form of a passport page, a cheque, a certificate, a bank card, a licence, an identity document or similar. A security device 10 is provided in a transparent window region 101 of the document 100, which may be a security device 10 as described in any of the preceding embodiments or could be a conventional caustic security device without a colour filter, such as those described in any of WO-A-2019/063778, WO-A2019/063779, WO-A-2020/070304 and WO-A-2020/070299. Also provided on the security document 100 is a supplemental security feature 150 which exhibits an image when viewed in transmitted light, such as a watermark, a pseudo-watermark or a transmissive optically variable device such as a moire magnifier or lenticular device. The supplemental security feature 150 may or may not be disposed in a window region of the document 100. The security document may optionally be provided with other security features, such as applied security strip 140.

The security device 10 is configured to project a caustic image Cl in the same manner as previously described (with or without colouration). The caustic image Cl is designed to be the same as or complementary to the image exhibited by supplemental security feature 150 when viewed in transmitted light. For example, both images may be identical to one another or may convey the same image content -e.g. both images may be of the number "10", in the same or different typefaces. Alternatively one image may complement the other either conceptually or physically -e.g. the caustic image Cl could project the text "0E11" while the supplemental security feature 150 shows a portrait of the Queen, or one image could provide a missing part of the other. By arranging for the two images to have related content in these ways, the security level of the document as a whole is enhanced since the user can check one image against the other to ensure the document is authentic. The need to provide both related images presents an extra challenge to the would-be counterfeiter. Preferably the caustic security device 10 and the supplemental security feature 150 are located sufficiently close to one another that their images can be viewed by a user simultaneously.

In this example, the security document is based on a transparent polymer document substrate 112, such as BOPP, which acts as the substrate 12 for the security device 10. The document substrate 112 is provided with opacifying layers 115 over the majority of its surface to act as a background for printing. The opacifying layers 115 are omitted in window region 101 so that this remains transparent. The security device 10 is formed in the window region 101 by provision of a caustic surface relief 20 (e.g. by cast cure as described above) and optionally a colour filter 30, shown here in the form of a colour filter layer applied to the opposite side of the substrate. Hence in this example the security device is of the same construction as shown in Figure 1 but as noted above any of the constructions shown in Figure 2 are also possible, and in this embodiment the colour filter 30 may be omitted due to the additional security provided by supplemental security feature 150.

Preferably, the security device 10 in the Figure 31 embodiment is a security device including a colour filter 30, such as any of those described with reference to the preceding embodiments. In this case it is preferred that the image displayed by the supplemental security feature 150 when viewed in transmitted light exhibits the same one or more colours as those exhibited by the caustic image projected by security device 10.

The supplemental security feature 150 exemplified in Figure 31(b) is a pseudo-watermark, formed by applying one or more of the opacifying layers 115 according to a pattern such that the optical density of the opacifying layers in combination varies in accordance with a desired image. Hence when the document is viewed in transmitted light, the image appears as a result of contrast between light and dark portions of the multilayer substrate construction. The image may be multi-tonal if desired. Examples of techniques for forming pseudo-watermarks such as this can be found in WO-A-2017/055823. Alternative supplementary security features 150 include moire magnifiers and lenticular devices, such as those disclosed in WO-A-2017/009616. If the security document 100 were instead of a traditional paper substrate construction (in which case the security device 10 may be incorporated via a security article as will be described below), the supplementary security feature 150 could be a traditional watermark, caused by varying the density of paper fibres across the area.

Figure 32 shows another embodiment of a security document 100 in accordance with the invention, in plan view and cross section along the line X-X'. Again, the security document 100 is a banknote in this example. Here, the security document is provided with a security device 10 in accordance with any of the preceding embodiments, formed as a security device assembly comprising a caustic relief structure 20 which is located in a laterally offset position from colour filter 30, but which can be brought into overlapping relation by folding the document 100 along the line Y-Y'. Hence, the caustic relief structure 20 is provided in a first window region 101 of the document 100 while the colour filter 30 is provided in a second window region 102 of the document 100. As shown in Figure 32(b), the two window regions 101, 102 are formed by omission of opacifying layers 115 in the relevant areas of transparent document substrate 112, in the same manner as previously described. Again, the relief structure 20 can be formed in any of ways previously mentioned but here is exemplified as carried in a cast-cured embossing layer on a first side of the substrate 112. Similarly the colour filter 30 can be embodied integrally with the substrate 112 (not shown) or as a colour filter layer (as shown), in which case it can be located on either surface of the substrate.

When the document 100 is folded along line Y-Y' so that corner 105a meets corner 105b, and corner 105c meets corner 105d, the colour filter 30 will overlap caustic relief structure 20, such that when the folded assembly is illuminated by an appropriate light source L, a coloured caustic image will be projected as previously described. Any of the embodiments disclosed herein can be implemented in this way.

Another aspect of the invention will now be exemplified with reference to Figures 33 to 41 hereto. For many caustic devices, it is desirable for the projected image to be generated across a wide range of illumination angles, so that a precise illumination geometry is not required to view the intended caustic image. This may be the case in any of the above embodiments for instance. Hence, the designer would normally aim to maximise useability by minimising the sensitivity to the incoming illumination angle. This is achieved by minimising the deflection required on any part of the caustic relief, which means the projected image will simply shift when the illumination angle is shifted (as shown in Figure 10 for instance). Figures 33 and 34 depict a security device of this sort, which is a comparative example not encompassed by the present claims. As before, the security device 10 comprises a relief structure 20 arranged on a substrate, preferably in a window region of a security document. When the device is illuminated by a first light source Li, at an illumination angle which falls within the device's operational range 0, a caustic image Cl will be projected and can be displayed on a suitably positioned screen S. Provided the light source Li is positioned such that the illumination angle falls with range 0, the caustic image Cl will be projected although its position on the screen S may vary in the x and/or z axes. The operational illumination range 0 is designed to be wide (e.g. -45 to +45 around normal incidence so a range 8 of 90 degrees) so that the device generates the expected caustic image Cl over a wide range of illumination angles. Only when a light source L2 at a large angular displacement is used, such that the illumination angle is no longer within range 0 (as shown in Figure 34) will the caustic image Cl no longer be generated. Now, the relief structure is unable to focus the incident light which is simply scattered.

It is also possible to design caustic relief structures which have a narrow operational illumination range O. A comparative example of a security device 10 illustrating this is shown in Figures 35 and 36. The construction of the security device 10 is generally the same as in Figures 33 and 34, except that here the relief structure 20 is designed so that it will only generate a caustic image when illuminated at an angle within a relatively narrow range 8 (as shown in Figure 35).

For instance, the magnitude of the range 0 may be 60 degrees or less, more preferably 30 degrees or less. VVhen illuminated outside the range 8, the caustic relief will scatter the light and no caustic image Cl will be projected (as shown in Figure 36). To achieve this narrower operational illumination angle range 0, a greater spread of deflection angles may be incorporated into the caustic structure (e.g. elements of the left hand side of the projected image being made up light from both the left side and right side of the caustic relief). The rate of change of deflection vs angle of incidence will then vary depending on the degree of deflection leading to a more illumination-angle-selective projected image.

This principle is made use of in embodiments of the present invention by configuring the relief structure to project a first caustic image when illuminated from a first illumination angle, and a second (different) caustic image when illuminated from a second (different) illumination angle. This is achieved by combining two caustic reliefs into the relief structure, each configured to operate over a different operational illumination angle range a Figure 37 and 38 show two different security devices (which each are comparative examples) to help illustrate this. In Figure 37, the security device 10' comprises a first caustic relief structure 20' which is configured to project a first caustic image Cli when illuminated from a first illumination angle Li which falls within a first illumination angle range 01. In this case, the first caustic image Cli is of the numeral "10". The first illumination angle range 91 is preferably offset from the substrate, e.g. in the -x direction. In Figure 38, the security device 10" comprises a second caustic relief structure 20" which is configured to project a second caustic image 012 when illuminated from a second illumination angle L2 which falls within a second illumination angle range 92. In this case, the second caustic image 012 is of the currency symbol "E". The second illumination angle range 02 is preferably offset from the substrate, e.g. in the +x direction. Each of the security devices shown in Figures 37 and 38 will project a single caustic image. The Figure 37 device will display its image Cli under illumination from a light source Li within the first angle range 91 (and not elsewhere), while the Figure 38 device will display its image Cl2 under illumination from a light source L2 within the second angle range 02 (and not elsewhere).

Figure 39 shows a security device 10 in accordance with an embodiment of the present invention, which comprises the security devices 10' and 10" of Figures 37 and 38 respectively in combination. Thus the security device 10 shown in Figure 39 comprises a relief structure 20 which comprises both caustic structures 20' and 20". As such, when the security device 10 is illuminated by a first light source L1 at an illumination angle within the first range 01, the first caustic image C11 will be projected (here the number "10"), whereas when the security device 10 is illuminated by a second light source L2 at an illumination angle within the second range 02, the second caustic image 0I2 will be projected (here the symbol "E"). Thus as the illumination angle is changed between L1 and L2 (e.g. by tilting the device 10 or moving the light source), the displayed caustic image will appear to switch between Cli and 012. If the security device 10 is illuminated from both angles Li and L2 simultaneously (as shown in Figure 39), both caustic images CI and 0I2 will be projected simultaneously. Preferably, as shown, the relief structure is configured such that the first and second caustic images Cli, 0I2 are both projected towards substantially the same position on screen S, so that they are superimposed (i.e. at least partially overlapping) if both are projected simultaneously.

The relief structure 20 can be configured in various different ways to incorporate the two caustic reliefs 20' and 20". Typically, each caustic relief will occupy a respective sector of the relief structure, the sectors being laterally offset from one another (e.g. abutting one another or spaced from one another). Figures 40(a) to (d) show some examples. Here the first and second sectors (corresponding to first and second caustic structures 20', 20") are denoted using different colours but it will be appreciated that this will not be the case in practice (unless a colour filter designed to provide each structure with a different colour is also provided).

In the Figure 40(a) embodiment, the first sector 20' is an annular region and the second sector 20" is a concentric circular region. In the Figure 40(b) embodiment, the relief structure 20 is divided into two semi-circular sectors 20' and 20". Many different macroscopic arrangements of the sectors 20' and 20" such as these can be envisaged.

In the Figure 40(c) embodiment, each of the sectors is provided as a plurality of sub-sectors, here taking the form of line elements or "slices". The first sub-sectors 20' (all of which contribute to the first caustic image Cli) are interleaved with the second sub-sectors 20" in the x-direction, preferably in an alternating, periodic manner In the Figure 40(d) embodiment, again each sector is provided as a plurality of sub-sectors which here are square or rectangular The first sub-sectors 20' are interleaved with the second sub-sectors 20" in two dimensions (along the x-and z-axes), forming a "checkerboard" pattern. Interleaved arrangements such as those shown in Figures 40(c) and 40(d) are preferred since it is more difficult for an observer to tell that the relief structure 20 incorporates more than one caustic relief.

The relief structure may be a redistributed relief structure or a non-redistributed relief structure as described previously. Generally, segmented relief structures will be preferred in the present embodiment since the segmentation of the structures lends itself well to combining of first and second caustic relief sectors as just described. The security device may or may not be provided with a colour filter, such as those disclosed with reference to any of the previous embodiments.

While in the Figure 39 embodiment, the first and second sectors 20', 20" of the relief structure are provided in one window 101 of the security document, this is not essential. Figures 41(a) and (b) show another embodiment of a security device 10 in accordance with the present invention, in which the first and second sectors 20', 20" of the relief structure are provided in separate window regions 101a, 101b of the security document, separated from one another by a non-transparent region formed by the presence of opacifying layers 15.

Figure 41(a) shows the security device 10 illuminated by a first light source Li at a first illumination angle. It will be seen that the caustic relief in the first sector 20' redirects the incident light to project first caustic image Cli onto screen S. Meanwhile the light incident on the second sector 20" of the relief structure is scattered. As such only the first caustic image Cli is exhibited. When the illumination angle is changed to [2, as shown in Figure 41(b), now the first sector 20' of the relief structure scatters the incident light while the second sector 20" projects the second caustic image 012. By changing the illumination angle between Li and L2, the caustic image displayed by the device can therefore be switched.

In the example shown, the first and second caustic images Cli, Cl2 are projected to different locations on screen S. If this is not desired, the relief structure could be adjusted to project both of the caustic images towards the same position, e.g. between the two windows 101a, 101b.

Figures 41(a) and (b) also illustrate an optional colour filter 30 which may be provided. The colour filter 30 could take any of the forms described in the preceding embodiments. For instance, the colour filter 30 could be of uniform colour and tone across the whole device. However in a preferred implementation, the colour filter defines laterally offset regions 31a, 31b thereof which correspond to the different sectors 20', 20" of the relief structure. The regions 31a, 31b may be configured to transmit light of different colours and/or tones. This leads to the first and second caustic images C11, Cl2 appearing in different colours/tones from one another thereby further enhancing the security level.

In the embodiments of Figures 39 to 41, the security device has been configured to project first and second caustic images which are different from one another.

The same principle can be extended to provide for three or more caustic images to be projected at different illumination angles. This can be arranged to provide for more complex visual effects such as morphing, zooming or animation. Each caustic image could take any form, e.g. alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure (e.g. a building or bridge), a person, animal or plant.

Security devices of the sorts described above can be incorporated into or applied to any product for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into documents of value such as banknotes, passports, driving licences, cheques, identification cards etc. The security device can either be formed directly on the security document (e.g. on a polymer substrate forming the basis of the security document) or may be supplied as part of a security article, such as a security thread or patch, which can then be applied to or incorporated into such a document.

Such security articles can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 6mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.

The security article may be incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate at at least one window of the document. Methods of incorporating security elements in such a manner are described in EP-A-1 141480 and WO-A-03054297. In the method described in EP-A-1 141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

Base substrates suitable for making security substrates for security documents may be formed from any conventional materials, including paper and polymer.

Techniques are known in the art for forming substantially transparent regions in each of these types of substrate. For example, WO-A-8300659 describes a polymer banknote formed from a transparent substrate comprising an opacifying coating on both sides of the substrate. The opacifying coating is omitted in localised regions on both sides of the substrate to form a transparent region. In this case the transparent substrate can be an integral part of the security device or a separate security device can be applied to the transparent substrate of the document. WO-A-0039391 describes a method of making a transparent region in a paper substrate. Other methods for forming transparent regions in paper substrates are described in EP-A-723501, EP-A-724519, WO-A-03054297 and EP-A-1398174.

The security article may also be applied to one side of a paper substrate, optionally so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.

Examples of such documents of value and techniques for incorporating a security device will now be described with reference to Figures 42 to 45. In all of these examples the provision of the colour filter 30 is optional, since it may be omitted if the security device is of the sort described with respect to Figures 39 to 41 above.

Figure 42 depicts an exemplary document of value 100, here in the form of a banknote. Figure 42a shows the banknote in plan view whilst Figure 42b shows a cross-section of the same banknote along the lines X-X'. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 112. Two opacifying layers 115 are applied to either side of the transparent substrate 112, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 112.

The opacifying layers 115 are omitted across a selected region 101 forming a window within which a security device 10 is located. In Figure 42(b), the security device is disposed within window 101, with a relief structure 20 arranged on one surface of the transparent substrate 112, and a colour filter 30 in the form of a colour filter layer 30 on the other surface. Of course, the security device 10 could instead have any of the alternative constructions shown in Figure 2. The relief structure 20 and/or colour filter layer 30 could be manufactured on separate respective substrates which are then laminated to the document substrate 112 in the window region, or could be manufactured directly on the document substrate 112.

If the security device 10 is one operating on reflection rather than refraction, it need not be located in a window region 101 but could instead be located on an opaque portion of the document or in a "half-window" region, in which one of the opacifying layers is continued over all or part of the device 10. Depending on the opacity of the opacifying layers, the half-window region will tend to appear translucent relative to surrounding areas in which opacifying layers are provided on both sides.

In Figure 43 the banknote 100 is a conventional paper-based banknote provided with a security article 120 in the form of a security thread, which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 110 lie on either side of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread 120 in window regions 101 of the banknote. Alternatively the window regions 101 may for example be formed by abrading the surfaces of the paper in these regions after insertion of the thread. The security device 10 is formed on the thread 120, which comprises a transparent substrate with a relief structure 20 provided on one side and colour filter 30 provided on the other (again, the alternative constructions of Figure 2 are available). Windows 101 reveal parts of the device, which may be formed continuously along the thread. Alternatively several security devices could be spaced from each other along the thread, with different or identical images displayed by each.

In Figure 44, the banknote 100 is again a conventional paper-based banknote, provided with a strip element or insert 125. The strip 125 is based on a transparent substrate and is inserted between two plies of paper 110a, 110b. The security device 10 is formed by a relief structure 20 on one side of the strip substrate, and colour filter 30 on the other (again, the alternative constructions of Figure 2 are available). The paper plies 110a, 110b are apertured across region 101 to reveal the security device 10, which in this case may be present across the whole of the strip 125 or could be localised within the aperture region 101.

A further embodiment is shown in Figure 45 where Figures 45(a) and (b) show the front and rear sides of the document 100 respectively, and Figure 45(c) is a cross section along line Z-Z'. Security article 130 is a strip or band comprising a security device 10 according to any of the embodiments described above. The security article 130 is formed into a security document 100 comprising a fibrous substrate 110, using a method described in EP-A-1 141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (Figure 45(a)) and exposed in one or more windows 101 on the opposite side of the document (Figure 45(b)). Again, the security device is formed on the strip 125, which comprises a transparent substrate with a relief structure 20 formed on one surface and a colour filter 30 as previously described on the other (again, the alternative constructions of Figure 2 are available). Alternatively a similar construction can be achieved by providing paper 110 with an aperture 101 and adhering the strip element 125 onto one side of the paper 110 across the aperture 101. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting. The arrangement shown in Figure 45 is particularly suitable for implementing the Figure 13 embodiment, for example.

In still further embodiments, if the security device comprises a reflective caustic relief structure, the complete security device could be formed entirely on one surface of a security document which could be transparent, translucent or opaque, e.g. a paper banknote irrespective of any window region.

In general when applying a security article such as a strip or patch carrying the security device to a document, it is preferable to bond the article to the document substrate in such a manner which avoids contact between the relief structure surface, which is utilised in generating the desired optical effects, and the adhesive, since such contact can render the relief structure inoperative. For example, the adhesive could be applied to the relief structure as a pattern that leaves an intended windowed zone of the relief structure uncoated, with the strip or patch then being applied in register (in the machine direction of the substrate) so the uncoated lens region registers with the substrate hole or window.

The security device of the current invention can be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.

For example, it is possible to include detectable materials such as taggants in the layer forming the caustic relief structure 20 (e.g. embossing layer 14 or substrate 12). Examples of suitable taggants include any luminescent, fluorescent or phosphorescent, or a material which exhibits Raman scattering, for example. Exemplary phosphors can be any compound that is capable of emitting IRradiation upon excitation with light. Suitable examples of phosphors include, but are not limited to, phosphors that comprises one or more ions capable of emitting IR radiation at one or more wavelengths, such as transition metal-ions including Ti-, Fe-, Ni-, Co-and Cr-ions and lanthanide-ions including Dy-, Nd-, Er-, Pr-, Tm, Ho-, Yb-and Sm-ions. The exciting light can be directly absorbed by an!Remitting ion. Acceptable phosphors also include those that use energy transfer to transfer absorbed energy of the exciting light to the one or more 1R-emitting ions such as phosphors comprising sensitizers for absorption (e.g. transition metal-ions and lanthanide-ions), or that use host lattice absorption or charge transfer absorption. Acceptable infrared emitting phosphors include Er-doped yttrium aluminium garnet, Nd-doped yttrium aluminium garnet, or Cr-doped yttrium aluminium garnet.

Another type of machine readable material that can be used is a direct bandgap semiconductor, for example a group 1I-VI (e.g. ZnO, ZnS, ZnSe, CdS, CdTe, CdSe etc) or a group II-V (eg GaN, GaAs, AIN, I nN etc) semiconductor can show strong luminescence. Another alternative is nanostructured materials (e.g. such as metallic, semiconductor and dielectric materials and combinations thereof), which can show many different types of luminescence such as fluorescence, phosphorescence, elastic and inelastic scattering.

A particularly preferred substance suitable is Er-Yb-KGd(P03)4 (also known as Er-Yb-KGP). Er-Yb-KGP strongly absorbs in the infra-red portion of the electromagnetic spectrum between about 960 nm and 990 nm. This substance can thus be regarded as having a waveband for absorption with a width of about 30 nm, and the predetermined input radiation for a security print medium incorporating it can be defined as radiation that falls within this waveband. After being excited by the predetermined input radiation, Er-Yb-KGP emits radiation across a range of wavelengths. The emission is also in the infra-red portion of the electromagnetic spectrum and is strongest between about 1520 nm and 1560 nm. The predetermined output radiation to be detected when authenticating a security device incorporating this substance can be regarded as that falling within the output waveband which has a width of about 40 nm. The wavebands of the input and output radiation of Er-Yb-KGP are thus relatively narrow. This is advantageous.

Typically the machine readable substance may take the form of particles, pigments or a dye which can be incorporated into a material such as curable material forming embossing layer 14. The size of the particles or pigments should preferably be sufficiently small and/or their concentration should be sufficiently low to avoid introducing optical scatter.

Advantages of providing taggants such as these in a relief structure, such as the present caustic structure 20, are disclosed in our British patent application no. 30 2014325.1.

Additional optically variable devices or materials can be included in the security device such as thin film interference elements, liquid crystal material and photonic crystal materials. Such materials may be in the form of filmic layers or as pigmented materials suitable for application by printing. If these materials are transparent they may be included in the same region of the device as the security feature of the current invention or alternatively and if they are opaque may be positioned in a separate laterally spaced region of the device.

Aspects and examples of the invention are set out in the following numbered clauses: Clause 1. A security device, comprising: a substrate; a relief structure on a first side of the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and a colour filter, the colour filter being configured to overlap in use at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby the caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light.

Clause 2. A security device according to clause 1, wherein in use the colour filter overlaps substantially all of the relief structure.

Clause 3. A security device according to clause 1 or clause 2, wherein the security device is configured such that the caustic image projected by the security device when illuminated with white light exhibits a multi-tonal and/or multicoloured appearance.

Clause 4. A security device according to any of the preceding clauses, wherein the light transmitted by the colour filter varies in colour and/or tone between laterally offset regions of the colour filter Clause 5. A security device according to any of the preceding clauses, wherein the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of one of the at least semi-transparent materials, or an overlapping combination of two or more of the at least semi-transparent materials, whereby the colour of light transmitted by the colour filter varies across the colour filter in accordance with the regions.

Clause 6. A security device according to clause 5, wherein the colour filter comprises a colourless transparent material forming one or more of the regions and a semi-transparent material transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour forming another one or more of the regions.

Clause 7. A security device according to clause 5 or 6, wherein the colour filter comprises at least two different semi-transparent materials each transmitting only a subset of visible light wavelengths corresponding to a different respective non-white colour, each forming one or more different respective regions of the colour filter.

Clause 8. A security device according to any of clauses 3 to 7, wherein a first group of one or more of the regions of the colour filter, which overlap one or more first areas of the relief structure configured to generate a first portion of the caustic image, are formed of a first set of one or more at-least semi-transparent materials, such that the first portion of the caustic image exhibits a first colour which is determined by the colour(s) transmitted by the first set of materials, the first portion of the caustic image preferably being a first distinct element of the caustic image.

Clause 9. A security device according to clause 8, wherein a second group of one or more of the regions of the colour filter, which overlap one or more second areas of the relief structure configured to generate a second portion of the caustic image, are formed of a second set of one or more at-least semi-transparent materials, such that the second portion of the caustic image exhibits a second colour which is determined by the colour(s) transmitted by the second set of materials, the second colour being different from the first colour, and the second portion of the caustic image preferably being a second distinct element of the caustic image.

Clause 10. A security device according to clause 8 or 9, wherein each of the first and/or second set of materials comprises a single at least semi-transparent material, each of the first and/or second colours being the colour transmitted by the respective single at least semi-transparent material.

Clause 11. A security device according to clause 8 or 9, wherein each of the first and/or second set of materials comprises a plurality of different at least semitransparent materials, each forming different region(s) of the colour filter, whereby each of the first and/or second colours is a colour arising from the mixture of colours transmitted by the materials in the respective set.

Clause 12. A security device according to any of clauses 4 to 11, wherein the colour filter comprises a semi-transparent material which varies in thickness between regions of the colour filter, the tone of the transmitted light depending on the thickness of the semi-transparent material.

Clause 13. A security device according to any of the preceding clauses, wherein areas of the relief structure and the portions of the caustic image which each area generates have substantially the same lateral arrangement relative to one another in the relief structure and in the caustic image, respectively.

Clause 14. A security device according to clause 13 when dependent on any of clauses 4 to 12, wherein the colour filter exhibits a non-projected image arising due to the arrangement of regions, the non-projected image being a version of the caustic image, the non-projected image preferably being a human-intelligible image and optionally comprising any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure, a person, animal or plant..

Clause 15. A security device according to any of clauses 1 to 12, wherein the relief surface is segmented, the segments being distributed such that areas of the relief structure and the portions of the caustic image which each area generates have different lateral arrangements relative to one another in the relief structure and in the caustic image, respectively.

Clause 16. A security device according to clause 15, wherein the relief structure is configured such that each segment thereof redirects light towards a single portion of the caustic image, each portion of the caustic image receiving light from one or multiple segments, and the regions of the colour filter are arranged in cells, each cell of the colour filter overlapping a single segment of the relief structure and each cell comprising one or more regions, preferably a single region, of the colour filter, such that the colour of light contributed to the caustic image by a segment is determined by the colour(s) transmitted by the material(s) forming the region(s) in the corresponding cell of the colour filter.

Clause 17. A security device according to clause 16, wherein the segments of the relief structure are arranged laterally relative to one another such that the colour filter exhibits a non-projected image arising due to the corresponding arrangement of cells, the non-projected image being different from the caustic image, the non-projected image preferably being a human-intelligible image and optionally comprising any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure, a person, animal or plant..

Clause 18. A security device according to clause 16, wherein the lateral arrangement of the segments of the relief structure relative to one another is random or pseudo-random such that the colour filter exhibits a substantially uniform colour across the colour filter, arising due to the corresponding random or pseudo-random arrangement of cells.

Clause 19. A security device according to any of clauses 16 to 18, wherein the segments of the relief structure and the cells of the colour filter are configured such that at least some of the colours transmitted by the materials in the colour filter are mixed in the caustic image, whereby the number of colours exhibited by a non-projected image of the colour filter is greater than the number of colours exhibited by the caustic image.

Clause 20. A security device according to any of clauses 16 to 19, wherein segments of the relief structure which contribute to a first portion of the caustic image are arranged to alternate periodically with segments of the relief structure which contribute to a second portion of the caustic image, and the cells of the colour filter exhibit a corresponding periodic pattern.

Clause 21. A security device according to any of the preceding clauses, wherein the relief structure and the colour filter are spaced from one another in the direction normal to the plane of the security device, the colour filter preferably being provided on the second side of the substrate and the substrate being transparent, such that upon tilting of the security device relative to the light source, the colour(s) exhibited by the caustic image change and/or a different portion of the caustic image displays a non-white colour.

Clause 22. A security device according to clauses 20 and 21, wherein the ratio of the spacing between the relief structure and the colour filter to the lateral width of the cells in the colour filter is such that, on tilting, the first and second portions of the caustic image appear to switch colours.

Clause 23. A security device according to clause 22, wherein the spacing between the relief structure and the colour filter is greater than or equal to the lateral width of the cells.

Clause 24. A security device according to clause 22 or 23, wherein the cells have a lateral width of 200 microns or less, preferably 100 microns or less, more preferably 50 microns or less.

Clause 25. A security device according to any of clauses 15 to 24, wherein at least some of the segments are laterally spaced from one another in the relief structure by scattering areas of the relief structure which substantially do not contribute to the caustic image.

Clause 26. A security device according to any of the preceding clauses, wherein at least one of the materials forming the colour filter transmits only a first subset of visible light wavelengths corresponding to a first non-white colour and reflects only a second subset of visible wavelengths corresponding to a second non-white colour, such that the colour exhibited by the material is different when viewed under reflected light and in transmitted light.

Clause 27. A security device according to clause 26, wherein the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of one of the at least semi-transparent materials, or an overlapping combination of two or more of the at least semi-transparent materials, and the colour filter exhibits a non-projected image arising due to the arrangement of regions, one or more colours in the non-projected image being different when viewed under reflected light and in transmitted light.

Clause 28. A security device according to any of the preceding clauses, wherein the colour filter is located on the same side of the relief structure as that on which the light source is positioned to project the caustic image.

Clause 29. A security device according to any of the preceding clauses, wherein the relief structure comprises a first area provided with a caustic relief configured to redirect light from a light source to thereby project the caustic image, and a second area laterally offset from the first area, the second area being provided with a non-caustic surface, and the first area of the substrate having a periphery shaped to define a periphery image, the periphery image preferably being a human-intelligible image and optionally comprising any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure, a person, animal or plant.

Clause 30. A security device according to clause 29, wherein the periphery image has different information content from the caustic image.

Clause 31. A security device according to any of the preceding clauses, wherein the colour filter is fixed in overlapping relation to the surface relief Clause 32. A security device according to clause 31, wherein the colour filter is integral with the substrate, the substrate being formed of the one or more at least semi-transparent materials.

Clause 33. A security device according to clause 31, wherein the relief structure is formed in the surface of an embossing layer applied to the substrate, the colour filter being integral with the embossing layer, the embossing layer being formed of the one or more at least semi-transparent materials.

Clause 34. A security device according to clause 31, wherein the colour filter comprises a colour filter layer formed of the one or more at least semi-transparent materials, the colour filter layer being disposed on the first or second surface of the substrate, or on the surface relief.

Clause 35. A security device according to any of clauses 1 to 30, wherein the colour filter is provided on the substrate at a first location and the surface relief is provided on the substrate at a second location, the substrate being transparent and foldable such that the colour filter can be positioned so as to overlap the surface relief in use.

Clause 36. A security device according to any of the preceding clauses, wherein the colour filter and the relief structure are registered to one another, preferably to within ±250pm, more preferably ±150pm, still preferably ±75pm.

Clause 37. A security device according to any of the preceding clauses, wherein the colour filter is a printed colour filter, preferably formed by offset printing, gravure printing, lithographic printing, flexographic printing or screen printing.

Clause 38. A security device according to any of the preceding clauses, wherein the or each material transmitting only a subset of visible light wavelengths corresponding to a respective colour comprises any of: an absorptive dye, an absorptive pigment, a plasmonic material, an interference layer structure, an interference layer pigment or a liquid crystal material.

Clause 39. A security device according to any of the preceding clauses, further comprising a masking component configured to overlap in use a sub-part of the relief structure, the masking component comprising one or more materials which are substantially opaque to visible light.

Clause 40. A security device according to any of the preceding clauses, wherein the relief structure is configured to project a real caustic image which can be viewed on a projection screen, or to project a virtual caustic image which can be viewed directly by the naked eye.

Clause 41. A security device according to any of the preceding clauses, wherein the relief structure is a refractive light-redirecting relief structure, the relief structure and the substrate being transparent.

Clause 42. A security article comprising a security device according to any of the preceding clauses, the security article preferably being a security thread, strip, patch, label, foil or insert.

Clause 43. A security document comprising a security device according to any of clauses 1 to 41, or a security article according to clause 42, the security document preferably being a banknote, a passport, an identity document, a driver's licence, a certificate, a visa or a stamp.

Clause 44. A security document according to clause 43, wherein the security device is located in at least one first transparent window region of the security document.

Clause 45. A security document according to clause 43 or 44, further comprising a supplemental security feature which exhibits an image when viewed in transmitted light, the image of the supplemental security feature being the same as or complementary to the caustic image and/or exhibiting the same non-white colour(s) as the caustic image.

Clause 46. A security document according to clause 45, wherein the supplemental security feature is a watermark, pseudo-watermark, or transmissive optically variable device such as a lenticular device or moire magnifier.

Clause 47. A security document according to clause 45 or 46 when dependent on clause 44, wherein the supplemental security feature is located in a non-window region, or in one of the at least one first transparent window region(s) or in a second transparent window region of the security document.

Clause 48. A method of manufacturing a security device, comprising: providing a substrate; forming a relief structure on a first side of the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and providing a colour filter, the colour filter being configured to overlap in use at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby at least part of the caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light.

Clause 49. A method according to clause 43, wherein the relief structure is formed by applying an embossing layer comprising one or more curable materials to the first side of the substrate or to a casting relief on the surface of a casting tool, the casting relief defining the relief structure therein, bringing the casting tool into contact with the substrate with the embossing layer therebetween to thereby form the relief structure into the surface of the embossing layer, and curing the embossing layer to retain the relief structure.

Clause 50. A method according to clause 49, wherein the colour filter is integral with the embossing layer, the curable material(s) of the embossing layer comprising the one or more at least semi-transparent materials.

Clause 51. A method according to clause 48 or 49, wherein the colour filter is provided by applying a colour filter layer formed of the one or more at least semitransparent materials, the colour filter layer being applied to the first or second surface of the substrate, or onto the surface relief.

Clause 52. A method according to clause 51, wherein the colour filter layer is applied by printing, preferably offset printing, gravure printing, lithographic printing, flexographic printing or screen printing.

Clause 53. A method according to clause 51 or 52, wherein the colour filter layer and the relief structure are applied to the substrate on the same processing line, while the substrate is being conveyed along the processing line in the machine direction.

Clause 54. A method according to clause 53, wherein the colour filter layer is applied to the second surface of the substrate, the colour filter layer and the relief structure being applied simultaneously to opposite surfaces of the substrate at the same position along the machine direction.

Clause 55. A method according to any of clauses 48 to 54, wherein the steps of forming the relief structure and providing the colour filter are performed in register with one another, preferably to within ±250pm, more preferably ±150pm, still preferably ±75pm.

Clause 56. A method according to clause 48 or clause 49, wherein the colour filter is provided integrally with the substrate.

Clause 57. A method according to any of clauses 48 to 56 wherein the security device is configured such that the caustic image projected by the security device when illuminated with white light exhibits a multi-tonal and/or multicoloured appearance.

Clause 58. A method according to any of clauses 48 to 57, wherein the light transmitted by the colour filter varies in colour and/or tone between laterally offset regions of the colour filter.

Clause 59. A method according to any of clauses 48 to 58, wherein the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of one of the at least semi-transparent materials, or an overlapping combination of two or more of the at least semi-transparent materials, whereby the colour of light transmitted by the colour filter varies across the colour filter in accordance with the regions.

Clause 60. A method according to clause 59, wherein a first group of one or more of the regions of the colour filter, which overlap one or more first areas of the relief structure configured to generate a first portion of the caustic image, are formed of a first set of one or more at least semi-transparent materials, such that the first portion of the caustic image exhibits a first colour which is determined by the colour(s) transmitted by the first set of materials, the first portion of the caustic image preferably being a first distinct element of the caustic image.

Clause 61. A method according to clause 60, wherein the colour filter is created by: selecting a first portion of the caustic image and a first colour for the selected first portion; identifying the one or more first areas of the relief structure which generate the selected first portion, preferably by ray tracing; designating the one or more regions of the colour filter which overlap the identified first area(s) as the first group, and assigning the first set of one or more at least semi-transparent materials to the designated region(s), wherein the first set of one or more at least semi-transparent materials transmit in combination the first colour.

Clause 62. A method according to any of clauses 48 to 61, further comprising, prior to forming the relief structure: designing the relief structure by: providing a non-projection image to be exhibited by the colour filter comprising a plurality of laterally-offset regions which vary in colour and/or tone from one region to another; for each region of the non-projection image, providing a relief structure segment based on the colour and/or tone of the region; and arranging the relief structure segments according to the arrangement of regions of the non-projection image.

Clause 63. A method according to any of clauses 48 to 62, adapted to provide the security device with any of the features of clauses 2 to 41.

Clause 64. A security document, comprising: a first transparent window region in which is disposed a first security device comprising a transparent, refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and laterally offset from the relief structure, a second security device which exhibits an image when viewed in transmitted light, the image exhibited by the 5 second security device being the same as or complementary to the caustic image.

Clause 65. A security document according to clause 64, wherein the second security device comprises a watermark, a pseudo-watermark or a transmissive optically variable device such as a lenticular device or moire magnifier.

Clause 66. A security document according to clause 64 or 65, wherein the first and second security devices abut one another or are positioned sufficiently close to one another such that the caustic image and the image exhibited by the second security device can be viewed simultaneously by one observer.

Clause 67. A security document according to any of clauses 64 to 66, wherein the caustic image and the image exhibited by the second security device have the same information content as one another, or one provides an element missing in the other.

Clause 68. A security document according to any of clauses 64 to 67, wherein the second security device is disposed in the first transparent window region or in a second transparent window region of the security document.

Clause 69. A security device, comprising: a substrate; a relief structure on the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a first light source at a first illumination angle to thereby project a first caustic image and to redirect light from a second light source at a second, different, illumination angle to thereby project a second caustic image, the first caustic image being different from the second caustic image.

Clause 70. A security device according to clause 69, wherein the relief structure is further configured such that the first caustic image is not projected when the security device is illuminated by the second light source at the second illumination angle and the second caustic image is not projected when the security device is illuminated by the first light source at the first illumination angle.

Clause 71. A security device according to clause 69 or clause 70, wherein the relief structure comprises a first sector configured to project the first caustic image under illumination from a first range of illumination angles which includes the first illumination angle, and a second sector configured to project the second caustic image under illumination from a second range of illumination angles which includes the second illumination angle, the first and second sectors being laterally offset from one another.

Clause 72. A security device according to clause 71, wherein the first and second sectors abut one another or are laterally spaced from one another.

Clause 73. A security device according to clause 71 or 72, wherein the first and second sectors each comprise a plurality of sub-sectors, the first sub-sectors preferably being interlaced with the second sub-sectors in one or two dimensions, wherein still preferably the first sub-sectors are arranged to alternate periodically with the second sub-sectors on one or two dimensions.

Clause 74. A security device according to any of clauses 71 to 73, wherein the first and second ranges of illumination angles do not overlap.

Clause 75. A security device according to any of clauses 71 to 74, wherein the first and second ranges of illumination angles each have a magnitude of 60 degrees or less, preferably 30 degrees or less.

Clause 76. A security device according to any of clauses 69 to 75, wherein the first and second caustic images each comprise any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure, a person, animal or plant.

Clause 77. A security device according to any of clauses 69 to 76, wherein the relief structure is configured to project the first and second caustic images towards substantially the same position relative to the security device when illuminated from the first and second illumination angles respectively.

Clause 78. A security device according to any of clauses 69 to 77, wherein the relief surface is segmented, the segments being distributed such that areas of the relief structure and the portions of the first or second caustic image which each area generates have different lateral arrangements relative to one another in the relief structure and in the caustic images, respectively.

Clause 79. A security device according to any of clauses 69 to 78, further comprising a colour filter, the colour filter being configured to overlap in use at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby the first and/or second caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light.

Clause 80. A security device according to clause 79, wherein the light transmitted by the colour filter varies in colour and/or tone between laterally offset regions of the colour filter, preferably such that the first and second caustic images exhibit different colour(s) from one another when the security device is illuminated with white light from the first and second illumination angles respectively.

Clause 81. A security device according to clause 80 and 71, wherein the light transmitted by the colour filter varies in colour and/or tone between first and second laterally offset regions of the colour filter which correspond to the first and second sectors of the relief structure, respectively.

Clause 82. A security device according to any of clauses 69 to 81, wherein the relief structure is configured to project a real caustic image which can be viewed on a projection screen, or to project a virtual caustic image which can be viewed directly by the naked eye.

Clause 83. A security device according to any of clauses 69 to 82, wherein the relief structure is a refractive light-redirecting relief structure, the relief structure and the substrate being transparent.

Clause 84. A security article comprising a security device according to any of clauses 69 to 83, the security article preferably being a security thread, strip, patch, label, foil or insert.

Clause 85. A security document comprising a security device according to any of clauses 69 to 83, or a security article according to clause 84, the security document preferably being a banknote, a passport, an identity document, a driver's licence, a certificate, a visa or a stamp.

Clause 86. A security document according to clause 85 wherein the relief structure comprises a first sector configured to project the first caustic image under illumination from a first range of illumination angles which includes the first illumination angle, and a second sector configured to project the second caustic image under illumination from a second range of illumination angles which includes the second illumination angle, the first and second sectors being laterally offset from one another, and the first and second sectors are located in a window region of the security document.

Clause 87. A security document according to clause 85 wherein the relief structure comprises a first sector configured to project the first caustic image under illumination from a first range of illumination angles which includes the first illumination angle, and a second sector configured to project the second caustic image under illumination from a second range of illumination angles which includes the second illumination angle, the first and second sectors being laterally offset from one another, wherein the first sector is located in a first window region of a security document and the second sector is located in a second window region of the security document, the first and second window regions being spaced from one another by a non-transparent region of the security document.

Clause 88. A method of manufacturing a security device, comprising: providing a substrate; forming a relief structure on the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a first light source at a first illumination angle to thereby project a first caustic image and to redirect light from a second light source at a second, different, illumination angle to thereby project a second caustic image, the first caustic image being different from the second caustic image.

Clause 89. A method according to clause 88, wherein the relief structure is formed by applying an embossing layer comprising one or more curable materials to the substrate or to a casting relief on the surface of a casting tool, the casting relief defining the relief structure therein, bringing the casting tool into contact with the substrate with the embossing layer therebetween to thereby form the relief structure into the surface of the embossing layer, and curing the embossing layer to retain the relief structure.

Clause 90. A method according to clause 88 or 89, adapted to provide the security device with any of the features of clauses 70 to 87.

Claims

CLAIMS1. A security device, comprising: a substrate; a relief structure on a first side of the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and a colour filter, the colour filter being configured to overlap in use at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby the caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light.
2. A security device according to claim 1, wherein the security device is configured such that the caustic image projected by the security device when illuminated with white light exhibits a multi-tonal and/or multi-coloured appearance.
3. A security device according to any of the preceding claims, wherein the light transmitted by the colour filter varies in colour and/or tone between laterally offset regions of the colour filter.
4. A security device according to any of the preceding claims, wherein the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of one of the at least semi-transparent materials, or an overlapping combination of two or more of the at least semi-transparent materials, whereby the colour of light transmitted by the colour filter varies across the colour filter in accordance with the regions.
5. A security device according to claim 4, wherein the colour filter comprises a colourless transparent material forming one or more of the regions and a semi-transparent material transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour forming another one or more of the regions.
6. A security device according to claim 4 or 5, wherein the colour filter comprises at least two different semi-transparent materials each transmitting only a subset of visible light wavelengths corresponding to a different respective nonwhite colour, each forming one or more different respective regions of the colour filter.
7. A security device according to any of claims 2 to 6, wherein a first group of one or more of the regions of the colour filter, which overlap one or more first areas of the relief structure configured to generate a first portion of the caustic image, are formed of a first set of one or more at-least semi-transparent materials, such that the first portion of the caustic image exhibits a first colour which is determined by the colour(s) transmitted by the first set of materials, the first portion of the caustic image preferably being a first distinct element of the caustic image.
8. A security device according to claim 7, wherein a second group of one or more of the regions of the colour filter, which overlap one or more second areas of the relief structure configured to generate a second portion of the caustic image, are formed of a second set of one or more at-least semi-transparent materials, such that the second portion of the caustic image exhibits a second colour which is determined by the colour(s) transmitted by the second set of materials, the second colour being different from the first colour, and the second portion of the caustic image preferably being a second distinct element of the caustic image.
9. A security device according to claim 7 or 8, wherein each of the first and/or second set of materials comprises a single at least semi-transparent material, each of the first and/or second colours being the colour transmitted by the respective single at least semi-transparent material.
10. A security device according to claim 7 or 8, wherein each of the first and/or second set of materials comprises a plurality of different at least semi-transparent materials, each forming different region(s) of the colour filter, whereby each of the first and/or second colours is a colour arising from the mixture of colours transmitted by the materials in the respective set.
11. A security device according to any of claims 3 to 10, wherein the colour filter comprises a semi-transparent material which varies in thickness between regions of the colour filter, the tone of the transmitted light depending on the thickness of the semi-transparent material.
12. A security device according to any of claims 1 to 11, wherein the relief surface is segmented, the segments being distributed such that areas of the relief structure and the portions of the caustic image which each area generates have different lateral arrangements relative to one another in the relief structure and in the caustic image, respectively.
13. A security device according to claim 12, wherein the relief structure is configured such that each segment thereof redirects light towards a single portion of the caustic image, each portion of the caustic image receiving light from one or multiple segments, and the regions of the colour filter are arranged in cells, each cell of the colour filter overlapping a single segment of the relief structure and each cell comprising one or more regions, preferably a single region, of the colour filter, such that the colour of light contributed to the caustic image by a segment is determined by the colour(s) transmitted by the material(s) forming the region(s) in the corresponding cell of the colour filter.
14. A security device according to claim 13, wherein the segments of the relief structure are arranged laterally relative to one another such that the colour filter exhibits a non-projected image arising due to the corresponding arrangement of cells, the non-projected image being different from the caustic image, the non-projected image preferably being a human-intelligible image and optionally comprising any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure, a person, animal or plant..
15. A security device according to claim 13, wherein the lateral arrangement of the segments of the relief structure relative to one another is random or pseudorandom such that the colour filter exhibits a substantially uniform colour across the colour filter, arising due to the corresponding random or pseudo-random arrangement of cells.
16. A security device according to any of claims 13 to 15, wherein the segments of the relief structure and the cells of the colour filter are configured such that at least some of the colours transmitted by the materials in the colour filter are mixed in the caustic image, whereby the number of colours exhibited by a non-projected image of the colour filter is greater than the number of colours exhibited by the caustic image.
17. A security device according to any of claims 13 to 17, wherein segments of the relief structure which contribute to a first portion of the caustic image are arranged to alternate periodically with segments of the relief structure which contribute to a second portion of the caustic image, and the cells of the colour filter exhibit a corresponding periodic pattern.
18. A security device according to any of the preceding claims, wherein the relief structure and the colour filter are spaced from one another in the direction normal to the plane of the security device, the colour filter preferably being provided on the second side of the substrate and the substrate being transparent, such that upon tilting of the security device relative to the light source, the colour(s) exhibited by the caustic image change and/or a different portion of the caustic image displays a non-white colour
19. A security device according to claims 17 and 18, wherein the ratio of the spacing between the relief structure and the colour filter to the lateral width of the cells in the colour filter is such that, on tilting, the first and second portions of the caustic image appear to switch colours.
20. A security device according to any of the preceding claims, wherein at least one of the materials forming the colour filter transmits only a first subset of visible light wavelengths corresponding to a first non-white colour and reflects only a second subset of visible wavelengths corresponding to a second non-white colour, such that the colour exhibited by the material is different when viewed under reflected light and in transmitted light.
21. A security device according to claim 20, wherein the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of one of the at least semi-transparent materials, or an overlapping combination of two or more of the at least semi-transparent materials, and the colour filter exhibits a non-projected image arising due to the arrangement of regions, one or more colours in the non-projected image being different when viewed under reflected light and in transmitted light.
22. A security device according to any of the preceding claims, wherein the relief structure comprises a first area provided with a caustic relief configured to redirect light from a light source to thereby project the caustic image, and a second area laterally offset from the first area, the second area being provided with a non-caustic surface, and the first area of the substrate having a periphery shaped to define a periphery image, the periphery image preferably being a human-intelligible image and optionally comprising any of: alphanumeric text, a typographic symbol, a currency symbol, a geometric shape, a logo, a flag, a map, a country outline, a portrait, a drawing, a photographic image, or a scene such as of a landscape, an architectural structure, a person, animal or plant.
23. A security device according to claim 22, wherein the periphery image has different information content from the caustic image.
24. A security device according to any of the preceding claims, wherein the colour filter is fixed in overlapping relation to the surface relief.
25. A security device according to claim 24, wherein the colour filter is integral with the substrate, the substrate being formed of the one or more at least semi-transparent materials.
26. A security device according to claim 24, wherein the relief structure is formed in the surface of an embossing layer applied to the substrate, the colour filter being integral with the embossing layer, the embossing layer being formed of the one or more at least semi-transparent materials.
27. A security device according to claim 24, wherein the colour filter comprises a colour filter layer formed of the one or more at least semi-transparent materials, the colour filter layer being disposed on the first or second surface of the substrate, or on the surface relief.
28. A security device according to any of claims 1 to 23, wherein the colour filter is provided on the substrate at a first location and the surface relief is provided on the substrate at a second location, the substrate being transparent and foldable such that the colour filter can be positioned so as to overlap the surface relief in use.
29. A security device according to any of the preceding claims, wherein the colour filter and the relief structure are registered to one another, preferably to within ±250pm, more preferably ±150pm, still preferably ±75pm.
30. A security device according to any of the preceding claims, wherein the colour filter is a printed colour filter, preferably formed by offset printing, gravure printing, lithographic printing, flexographic printing or screen printing.
31. A security device according to any of the preceding claims, wherein the or each material transmitting only a subset of visible light wavelengths corresponding to a respective colour comprises any of: an absorptive dye, an absorptive pigment, a plasmonic material, an interference layer structure, an interference layer pigment or a liquid crystal material.
32. A security device according to any of the preceding claims, further comprising a masking component configured to overlap in use a sub-part of the relief structure, the masking component comprising one or more materials which are substantially opaque to visible light.
33. A security article comprising a security device according to any of the preceding claims, the security article preferably being a security thread, strip, patch, label, foil or insert.
34. A security document comprising a security device according to any of claims 1 to 32, or a security article according to claim 33, the security document preferably being a banknote, a passport, an identity document, a driver's licence, a certificate, a visa or a stamp.
35. A security document according to claim 34, wherein the security device is located in at least one first transparent window region of the security document
36. A security document according to claim 34 or 35, further comprising a supplemental security feature which exhibits an image when viewed in transmitted light, the image of the supplemental security feature being the same as or complementary to the caustic image and/or exhibiting the same non-white colour(s) as the caustic image.
37. A security document according to claim 36, wherein the supplemental security feature is a watermark, pseudo-watermark, or transmissive optically variable device such as a lenticular device or moire magnifier
38. A security document according to claim 36 or 37 when dependent on claim 44, wherein the supplemental security feature is located in a non-window region, or in one of the at least one first transparent window region(s) or in a second transparent window region of the security document.
39. A method of manufacturing a security device, comprising: providing a substrate; forming a relief structure on a first side of the substrate, the relief structure being a reflective or refractive light-redirecting relief structure configured to redirect light from a light source to thereby project a caustic image; and providing a colour filter, the colour filter being configured to overlap in use at least part of the relief structure, the colour filter comprising one or more at least semi-transparent materials, at least one of the materials transmitting only a subset of visible light wavelengths corresponding to a respective non-white colour, whereby at least part of the caustic image projected by the security device exhibits one or more colour(s) when the security device is illuminated with white light.
40. A method according to claim 39, wherein the relief structure is formed by applying an embossing layer comprising one or more curable materials to the first side of the substrate or to a casting relief on the surface of a casting tool, the casting relief defining the relief structure therein, bringing the casting tool into contact with the substrate with the embossing layer therebetween to thereby form the relief structure into the surface of the embossing layer, and curing the embossing layer to retain the relief structure.
41. A method according to claim 40, wherein the colour filter is integral with the embossing layer, the curable material(s) of the embossing layer comprising the one or more at least semi-transparent materials.
42. A method according to claim 39 or 40, wherein the colour filter is provided by applying a colour filter layer formed of the one or more at least semi-transparent materials, the colour filter layer being applied to the first or second surface of the substrate, or onto the surface relief
43. A method according to claim 42, wherein the colour filter layer is applied by printing, preferably offset printing, gravure printing, lithographic printing, flexographic printing or screen printing.
44. A method according to claim 42 or 43, wherein the colour filter layer and the relief structure are applied to the substrate on the same processing line, while the substrate is being conveyed along the processing line in the machine direction.
45. A method according to claim 44, wherein the colour filter layer is applied to the second surface of the substrate, the colour filter layer and the relief structure being applied simultaneously to opposite surfaces of the substrate at the same position along the machine direction.
46. A method according to any of claims 39 to 45, wherein the steps of forming the relief structure and providing the colour filter are performed in register with one another, preferably to within ±250pm, more preferably ±150pm, still preferably ±75pm.
47. A method according to claim 39 or claim 40, wherein the colour filter is provided integrally with the substrate.
48. A method according to any of claims 39 to 47 wherein the security device is configured such that the caustic image projected by the security device when illuminated with white light exhibits a multi-tonal and/or multi-coloured appearance.
49. A method according to any of claims 39 to 48, wherein the light transmitted by the colour filter varies in colour and/or tone between laterally offset regions of the colour filter.
50. A method according to any of claims 39 to 49, wherein the colour filter comprises a plurality of different at least semi-transparent materials arranged in respective laterally offset regions, each region of the colour filter being formed of one of the at least semi-transparent materials, or an overlapping combination of two or more of the at least semi-transparent materials, whereby the colour of light transmitted by the colour filter varies across the colour filter in accordance with the regions.
51. A method according to claim 50, wherein a first group of one or more of the regions of the colour filter, which overlap one or more first areas of the relief structure configured to generate a first portion of the caustic image, are formed of a first set of one or more at least semi-transparent materials, such that the first portion of the caustic image exhibits a first colour which is determined by the colour(s) transmitted by the first set of materials, the first portion of the caustic image preferably being a first distinct element of the caustic image.
52. A method according to claim 51, wherein the colour filter is created by: selecting a first portion of the caustic image and a first colour for the selected first portion; identifying the one or more first areas of the relief structure which generate the selected first portion, preferably by ray tracing; designating the one or more regions of the colour filter which overlap the identified first area(s) as the first group, and assigning the first set of one or more at least semi-transparent materials to the designated region(s), wherein the first set of one or more at least semi-transparent materials transmit in combination the first colour.
53. A method according to any of claims 39 to 52, further comprising, prior to forming the relief structure: designing the relief structure by: providing a non-projection image to be exhibited by the colour filter comprising a plurality of laterally-offset regions which vary in colour and/or tone from one region to another; for each region of the non-projection image, providing a relief structure segment based on the colour and/or tone of the region; and arranging the relief structure segments according to the arrangement of regions of the non-projection image.
54. A method according to any of claims 39 to 53, adapted to provide the security device with any of the features of claims 2 to 33.