AU2016100288B4 - A security device including a zero order covert image - Google Patents

Abstract

Abstract A security device for a security document is provided comprising a substrate, the security device being provided on or within the substrate. The security device includes an image area formed from a plurality of diffractive devices each having a defined diffraction efficiency, wherein the defined diffraction efficiencies of the diffractive devices vary across the security device so that an image formed within the image area is not discernible in the zero order when illuminated by unpolarised light, but is discernible in the zero order when the image area is illuminated by light passed through a first incident light polariser and the zero order reflected light is passed through a second polariser which is crossed at right angles with the first incident light polariser.

Description

A SECURITY DEVICE INCLUDING A ZERO ORDER COVERT IMAGE Technical Field [0001] The invention relates generally to security documents in which optical security devices are used as an anti-counterfeiting measure, and in particular to the configuration of such optical security devices.

Background of Invention [0002] Security devices are applied to security documents or similar articles, such as identity cards, passports, credit cards, bank notes, cheques and the like and may take the form of diffraction gratings and similar optically detectable microstructures. Such security devices are difficult to falsify or modify, and are easily damaged or destroyed by any attempts to tamper with the document. Often security devices are designed to be overt features of the document, such that they are observable with the naked eye. This type of public or primary security device enables members of the public to perform some degree of authentication of the document, without the use of any additional viewing apparatus.

[0003] The ever increasing sophistication of counterfeiting operations requires continuous improvement in the design of security devices for protecting documents against forgery. Whilst it is difficult for a counterfeiter to reproduce the exact optical effect of security features such as the overt features described, forgeries that produce an optical effect sufficiently similar to deceive a casual observer are readily produced. Moreover, members of the public are typically not skilled in detecting the minor variations produced by the counterfeit optical effects.

[0004] The use of polarisation effects to produce secondary security features in the form of hidden or covert images has not been successfully exploited. Such covert images rely on surface plasmon generation effects produced by linearly polarised light striking metallic diffraction gratings at an angle to the grating elements. See the paper entitled “Polarisation Conversion through the Excitation of Surface Plasmons on a Metallic Grating” by Bryan-Brown, Hutley and Sambles in the Journal of Modern Optics Volume 37, Issue 7 in 1990 and “Optical excitation of surface plasmons: An introduction” by J.R. Sambles et al (Contemporary Physics, 1991, Vol. 32., No. 3, pages 173 - 183).

[0005] US Patent No. 6,522,399 B1 by Sambles and Lawrence relates to a signature recognition system for providing articles with distinctive signatures and means for verifying those signatures. More specifically, directing polarised electromagnetic radiation to a suitably proportioned diffraction grating under strict conditions reveals unique identification codes for users of personal credit or security cards. The strict conditions require that each diffraction grating exhibits a periodic wave surface profile having a depth-to-pitch ratio δ of between 0.1 and 0.5. Moreover, the source of polarized electromagnetic radiation must have a wavelength λ such that the pitch G of the periodic wave surface profile of the diffraction gratings is comparable to an integer multiple n of that wavelength. Finally, the source of polarized electromagnetic radiation must be directed to the surface of the gratings at a plane of incidence substantially normal to the plane of the surface of the diffraction grating and at an angle of approximately 45° azimuth to the alignment of the grooves on the surface.

[0006] It would be desirable to provide improved security documents including secondary security features for authentication. Primary security features provide a first level of security which is observable in the absence of additional viewing apparatus, while the secondary security features provide an additional level of security to enable the security document to be authenticated more reliably if the necessary additional viewing apparatus is available. More particularly, it would be desirable to provide secondary security features for authentication which do not require strict conditions to enable their observation.

[0007] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Definitions

Security Document or Token [0008] As used herein the term security document includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

[0009] The invention is particularly, but not exclusively, applicable to security documents such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied. The diffraction gratings and optically variable devices described herein may also have application in other products, such as packaging.

Substrate [0010] As used herein, the term substrate refers to the base material from which the security document or token is formed. The base material may be paper or other fibrous material such as cellulose; a plastic or polymeric material including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

[0011] The use of plastic or polymeric materials in the manufacture of security documents pioneered in Australia has been very successful because polymeric banknotes are more durable than their paper counterparts and can also incorporate new security devices and features. One particularly successful security feature in polymeric banknotes produced for Australia and other countries has been a transparent area or “window”.

Transparent Windows and Half Windows [0012] As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.

[0013] A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.

[0014] A partly transparent or translucent area, hereinafter referred to as a “halfwindow,” may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the “half-window” is not fully transparent, but allows some light to pass through without allowing objects to be viewed clearly through the half-window.

[0015] Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.

Opacifying Layers [0016] One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT<L0 where L0 is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.

Security Device or Feature [0017] As used herein the term security device or feature includes any one of a large number of security devices, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or tampering. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and may take a wide variety of forms, such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed features, including relief structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Embossable Radiation Curable Ink [0018] The term embossable radiation curable ink used herein refers to any ink, lacquer or other coating which may be applied to the substrate in a printing process, and which can be embossed while soft to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place before the radiation curable ink is embossed, but it is possible for the curing process to take place either after embossing or at substantially the same time as the embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink maybe cured by other forms of radiation, such as electron beams or X-rays.

[0019] The radiation curable ink is preferably a transparent or translucent ink formed from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as sub-wavelength gratings, transmissive diffractive gratings and lens structures.

[0020] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating, [0021] Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation curable embossable coatings maybe based on other compounds, e.g. nitro-cellulose.

[0022] The radiation curable inks and lacquers used herein have been found to be particularly suitable for embossing microstructures, including diffractive structures such as diffraction gratings and holograms, and microlenses and lens arrays. However, they may also be embossed with larger relief structures, such as nondiffractive optically variable devices.

[0023] The ink is preferably embossed and cured by ultraviolet (UV) radiation at substantially the same time. In a particularly preferred embodiment, the radiation curable ink is applied and embossed at substantially the same time in a Gravure printing process.

[0024] Preferably, in order to be suitable for Gravure printing, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 150 centipoise. The viscosity may be determined by measuring the time to drain the lacquer from a Zahn Cup #2. A sample which drains in 20 seconds has a viscosity of 30 centipoise, and a sample which drains in 63 seconds has a viscosity of 150 centipoise.

[0025] With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the adhesion of the embossed structure formed by the ink to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethylene imine. The primer layer may also include a crosslinker, for example a multi-functional isocyanate. Examples of other primers suitable for use in the invention include: hydroxyl terminated polymers; hydroxyl terminated polyester based co-polymers; cross-linked or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; zirconium complexes; aluminium acetyl acetone; melamines; and carbodi-imides.

Summary of Invention [0026] According to an aspect of the present invention, there is provided a security device for a security document comprising a substrate, the security device being provided on or within the substrate, the security device including an image area formed from a plurality of diffractive devices each having a defined diffraction efficiency, wherein the defined diffraction efficiencies of the diffractive devices vary across the security device so that an image formed within the image area is not discernible in the zero order when illuminated by unpolarised light, but is discernible in the zero order when the image area is illuminated by light passed through a first incident light polariser and the zero order reflected light is passed through a second polariser which is crossed at right angles with the first incident light polariser.

[0027] In an embodiment, each diffractive device forms a pixel. The security device may comprise a pixelated array.

[0028] The defined diffraction efficiencies of the diffractive devices may vary across the security device in accordance with a predetermined algorithm.

[0029] The diffractive devices may take a variety of forms. According to one embodiment, the diffractive devices includes a plurality of curvilinear grating elements. Each curvilinear grating element may be spaced from an adjacent curvilinear grating element in accordance with a substantially fixed spacing with respect to at least one point along the curvilinear grating element. Alternately, the curvilinear grating elements are spaced from one another in accordance with variable spacings.

[0030] A curvature of the curvilinear grating elements may vary between the pixels forming the security device. In another form, of the invention, spacings between the curvilinear grating elements vary between the pixels forming the security device.

[0031] A curvature of the curvilinear grating elements may vary between pixels forming the security device in a horizontal plane. Alternately, a depth of the curvilinear grating elements varies between pixels forming the security device in a vertical plane.

[0032] In another embodiment, each diffractive device includes a plurality of parallel grating elements. An orientation of the parallel grating elements may vary between the pixels forming the security device.

[0033] The parallel grating elements may be spaced from one another such that the spacing between the parallel grating elements varies across each pixel. That is, the diffraction efficiency or observed brightness can be varied by varying the spacing between grating elements within a pixel, from say the bottom of the pixel to the top of the pixel. The variation in spacing should be continuous.

[0034] In another form of the invention, a depth of the parallel grating elements varies between pixels forming the security device in a vertical plane.

[0035] According to one embodiment, the security device has a plane in which the diffractive devices lie and the image formed within the image area which is discernible in the zero order varies in from a positive tone image to a negative tone image and vice versa when the security device is rotated about an axis that is perpendicular to the plane of the security device. That is, a positive to negative image flip in which pixels in particular regions of the image area change from high brightness to low brightness and vice versa. This happens because grooves that were previously aligned with the polarisation direction of the incident light are now aligned at right angles to the polarisation direction and vice versa.

[0036] The security device may be formed from an embossed radiation curable ink. In a preferred embodiment, the substrate includes at least one region of transparent or translucent plastics material forming a window area. The security device may be integrated into the window area.

[0037] In a particular form of the invention, the security document is a banknote, with the security device located on the substrate of the banknote.

[0038] According to another aspect of the present invention, there is provided a method for revealing a covert image provided in an image area formed in a security device for a security document comprising a substrate, the security device being provided on or within the substrate, the method including: providing a first incident light polariser through which incident light is passed to provide a source of polarised light; illuminating a security device including an image area formed from a plurality of diffractive devices each having a defined diffraction efficiency, wherein the defined diffraction efficiencies of the diffractive devices vary across the security device; and providing a second polariser to be crossed at right angles with the first incident light polariser such that zero order reflected light passes through the second polariser; wherein an image formed within the image area is discernible in the zero order when the image area is illuminated by polarised light and the zero order reflected light is passed through the second polariser which is crossed at right angles with the first incident light polariser, but is not discernible in the zero order when illuminated by unpolarised light.

Brief Description of Drawings [0039] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be understood that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings: [0040] Figure 1 is a schematic showing zero order image observation of a security device in unpolarised light.

[0041] Figure 2 is a schematic showing zero order image observation of a security device between crossed polarisers (AXB) according to an embodiment of the present invention.

[0042] Figures 3A to 3D show photographically, an example of a diffractive optically variable device as observed from different viewing angles in both polarised and unpolarised light, together with the underlying optical microstructure shown in Figure 3E.

[0043] Figures 4A to 4D show photographically, an example of another diffractive optically variable device as observed from different viewing angles in both polarised and unpolarised light.

[0044] Figure 5A is a chart showing the zero order diffraction efficiency for a relatively high efficiency grating array.

[0045] Figure 5B is a chart showing the zero order diffraction efficiency for a relatively low efficiency grating array.

[0046] Figures 6A, 6B and 6C show three exemplary of curvilinear pixel grating structures for implementing the diffractive optically variable device as shown for example in Figures 4A to 4D.

[0047] Figures 7A, 7B and 7C show three alternate examples of pixel grating structures for implementing a diffractive optically variable device having a covert image observable in the zero order under cross polarisers.

[0048] Figure 8 is a flowchart showing a method for revealing a covert image provided in an image area formed in a security device according to an embodiment of the present invention.

Detailed Description [0049] Referring firstly to Figure 1, there is shown schematically, an arrangement for observation of a security device 100 in unpolarised light. The diffraction grating 110 has a generally periodic structure comprised of a number of grating elements (not shown). Each grating element is substantially identical and may constitute a buried grating, i.e. a groove, a surface relief, or any other suitable optically detectable microstructure. The spacing between adjacent grating elements influences the angle at which incident light 120 is diffracted by the diffraction grating 110.

[0050] A diffraction grating 110 is referred to as a zero order diffraction grating, when it produces optical effects only in the zero order reflection under illumination by light at given wavelength. In Figure 1, the zero order reflected light 130 is indicated as Iro(x,y), whilst the diffracted orders are shown as positive orders 140 as n = +1; n = +2 and negative orders 150 as n = -1; n = -2.

[0051] If In = In(x,y) and Ir0(x,y)is the zero order component of the reflected scattered light at the point x,y then: i(x,y) = ir0(x,y) + ii(x,y) + i-i(x,y) + i2(x,y) + i-2(Xy) + i3(x,y) + i-3(x,y) +...... where I is the constant incident light intensity. This can be written as: / = /r0(x,y) +Xn=i^jn*0/n(x,y) Equation (1) where the index M is the number of diffracted orders, and no light absorption into the grating is assumed such that the total outgoing light intensity must equal the total incoming light intensity.

[0052] The visibility V0(x,y) of the reflected zero order image is given by the visibility equation:

Equation (2) where Imax and Imin are the maximum and minimum values of the zero order reflected intensity across the grating respectively, i.e. Imax is the maximum value reached by Iro(x<y) across the grating and Imin is the minimum value reached by Ir0(x,y) across the grating. It follows therefore, that the image visibility will be high only if there are large differences between Imax and Imin across the surface of the diffraction grating 110.

[0053] Based on the foregoing principles, the present invention proposes a security device 100 configured so that the diffraction efficiencies of the diffractive gratings 110 that make up the security device vary across the security device. This results in an image formed within an image area of the security device which is not visible to the naked eye in the zero order when illuminated by unpolarised light, but is visible in the zero order when illuminated by incident light 120 that is passed through an incident light polariser 160, and when the zero order reflected light 130 is passed through a second polariser 170 which is crossed at right angles with the incident light polariser. This proposed arrangement is shown schematically in Figure 2.

[0054] Detailed zero order covert images can be provided by arranging the diffraction grating nth order diffraction efficiencies In(x,y) such that they vary across the security device as a function of the (x,y) coordinates, i.e. In=ln(x,y), where ln(x,y) varies by significant values across the diffraction grating, particularly, where n=+l or n=-l. Variation of the diffraction efficiencies across the security device may be defined in accordance with a predetermined algorithm.

[0055] Moreover, the diffraction grating is arranged to have a relatively low diffraction efficiency with respect to zero order scattered light, so that the zero order image is relatively uniform in appearance when observed in unpolarised incident light.

[0056] Referring now to Figures 3A to 3D there is shown a series of images that may be observed as a result of different viewing angles, i.e. a positive or negative order respectively in the case of Figures 3A and 3B, for an exemplary diffractive optically variable device. The diffractive image shown in Figure 3A is an example of a colour (shown in greyscale) diffractive image that is observed from a positive order viewing angle. Figure 3B shows the same diffractive image observed from a negative order viewing angle. In the case of Figures 3A and 3B, the diffractive image is observed in unpolarised light. In Figures 3C and 3D, the diffractive image of Figures 3A and 3B is observed from the zero order viewing angle. Figure 3C shows the zero order image observed in unpolarised light which gives rise to an undiscernible image, and Figure 3D shows the same zero order image observed in polarised light by means of crossed polarisers to give rise to a discernible greyscale image. That is the greyscale image discernible in the zero order under cross polarisers is undiscernible or hidden/covert in unpolarised light. Figure 3E shows a more detailed view of an exemplary diffractive optically variable device illustrating variations in the orientation of the diffraction gratings as will be described in more detail with reference to Figures 7A, 7B and 7C.

[0057] Referring now to Figures 4A to 4D there is shown another example of a series of images observed as a result of different viewing angles. Again, Figure 4A shows a diffractive image (observed in colour but shown in greyscale) observed from a positive order viewing angle. Figure 4B shows the same diffractive image observed from a negative order viewing angle. In both cases, the diffractive image is observed in unpolarised light. In Figures 4C and 4D, the same diffractive image is observed from the zero order viewing angle. Figure 4C shows the zero order image as observed in unpolarised light resulting in an undiscernible image, and Figure 4D shows the same zero order image observed in polarised light by means of crossed polarisers (in accordance with the arrangement described with reference to Figure 2), resulting in a discernible greyscale image. In the case of Figures 4A to 4D, examples of the diffraction grating structures used to construct the diffractive optically variable device are described in more detail with reference to Figures 6A to 6C.

[0058] The proposed security device relies on the surface plasmon generation effects produced by linearly polarised light incident upon a metallised diffraction grating at an angle to the grating elements. The reflected intensity of the P-polarised incident light is a function of the angle between the polarisation vector of the incident light and the grating elements. Maximum P-polarised reflected intensity occurs when the polarisation vector is parallel with the grating elements and minimum intensity occurs when the incident light strikes the grating elements at right angles. At angles in between these two extremes the reflected P-polarised intensity is correspondingly related, P-polarised reflected intensity smoothly reducing from a maximum to a minimum as the angle with respect to the grating elements increases from 0 to 90 degrees.

[0059] Referring now to Figure 5A, there is shown the zero order diffraction efficiency (or reflected intensity) for a comparatively high efficiency diffraction grating. When a security device is illuminated with unpolarised incident light and the zero order light is observed without a polariser, the visibility in the zero order reflectance V0(x,y), as provided by Equation (2), will only be observable with the naked eye where there are large differences in the maximum and minimum values of the zero order diffraction efficiency across the diffraction grating forming part of the security device.

Equation (3) [0060] In the case where there is little variation in the zero order efficiency, the background zero order reflectance will tend to dominate the scattered light and thereby conceal any variations in reflectance due to diffraction effects. This scenario, as shown in Figure 5B, produces an almost constant variation in visibility which consequently conceals, or at least masks any variations in diffraction efficiency.

[0061] If however, the zero order reflectance is observed through cross polarisers, in accordance with the configuration exemplified in Figure 2, the situation is different. In this case, the maximum and minimum values of the zero order reflected intensity are given as:

Equation (4) [0062] If the background reflected undiffracted light IB is relatively large then the zero order image visibility is very low and the image is concealed (or effectively swamped) by the large amount of directly reflected light.

[0063] If the incident light is polarised and zero order reflectance is again observed with the naked eye. The corresponding expressions to Equation (4) becomes:

Equation (5) where the superscript "P" indicates that the incident light is now polarised and the values of the various parameters will differ from the unpolarised case. The visibility will still be very low however, as in the unpolarised case.

[0064] However, by observing the zero order reflectance using a second polariser crossed at right angles with the incident light polariser, as shown in configuration exemplified in Figure 2, a significant change will take place as the background reflected light IB is largely cancelled out by the second polariser and the visibility equation becomes:

Equation (6) since IBP * 0 if the polarisers are of high efficiency.

[0065] Such a security device can be created by use of a pixel based structure or a pixelated array, comprising a number of diffraction pixels. Each diffraction pixel comprises a plurality of diffractive devices. The image area of the security device is defined by the arrangement of the diffraction pixels. The dimensions of the each diffraction pixel may be up to of 500pm by 500pm, but will typically be in the range of 30 x 30pm to 60 x 60pm .

[0066] Observing a low visibility zero order covert image which satisfies the requisite conditions, using the described cross polariser technique effectively converts the covert image to a high visibility overt image. That is, provided that significant variations exists in the diffraction efficiency or maximum and minimum intensities across the security device.

[0067] Providing significant variation in the diffraction efficiency of the diffraction gratings across the security device can be achieved in a number of ways. One is to vary the curvature of the grating elements from point to point across the image area. This option is exemplified in Figures 6A to 6C, wherein the curvature of the diffraction gratings is observably reducing from Figure 6A to Figure 6C. In an alternative embodiment, the depth of the diffraction grating is varied in the vertical plane to achieve a similar variation in the diffraction efficiency across the image area.

[0068] One particular way of achieving the requisite variations in the diffraction efficiencies of the diffractive devices across the security device is to provide curvilinear diffraction gratings having substantially variable spacing between the curvilinear grating elements within each diffraction pixel. Where the spacings between the grating elements are substantially fixed, the spacings and/or the curvature of the curvilinear diffraction grating elements is varied between the diffraction pixels forming the diffractive device. Alternatively, the spacing between the grating elements within each diffraction pixel may be varied.

[0069] The curvature of the curvilinear diffraction grating elements may vary between pixels, in the horizontal plane, i.e. the plane in which the diffractive device lies, or alternatively, may vary in the vertical plane, being perpendicular to the plane in which the diffractive device lies. In this sense, the height and/or depth of the grating elements is varied.

[0070] In the example shown in Figure 6A, the curvature of the diffraction grating is varied within the pixel. The same is true of Figures 6B and 6C, although it is evident that the curvature of the diffraction grating is reducing from Figure 6A to 6C when viewed in succession.

[0071] Referring now to Figures 7A to 7C, an alternate means to provide sufficient variation in the diffraction efficiency of the diffraction gratings across the security device is to provide parallel diffraction gratings (i.e. straight line gratings), and to vary the orientation of the parallel diffraction gratings within or between pixels forming the image area. In the illustrated case, the spacing between adjacent grating elements is fixed, whilst the orientation of the grating elements varies within the pixel and between the pixels.

[0072] Where the zero order covert images are produced by observation of diffractive devices composed of a pixelated array, wherein individual pixels comprise a plurality of curvilinear grating elements, the image that is discernible in the zero order will vary when the security device is rotated about an axis perpendicular to the plane in which the diffraction grating lies. This phenomena arises since the diffraction efficiency of the different orders is also a function of the angle of orientation of the incident light polarisation vector with respect to the diffraction grating elements.

[0073] Finally referring to Figure 8, there is shown a flowchart illustrating a method 800 for revealing a covert image which is observable in the zero order when illuminated by polarised light when the zero order reflected light is passed through a second polariser crossed at right angles with a first incident light polariser, but not observable in the zero order when illuminated by unpolarised light. The method involves firstly at step 810, providing a first incident light polariser through which incident light is passed to provide a source of polarised light. Secondly, at step 820, a security device including an image area formed from a plurality of diffractive devices each having a defined diffraction efficiency, wherein the defined diffraction efficiencies of the diffractive device vary across the security device is illuminated by the source of polarised light. Thirdly, at step 830 a second polariser is provided to be crossed at right angles with the first incident light polariser, such that zero order reflected light passes through the second polariser. And finally, at step 840, the zero order image is observed.

[0074] The security device described provides an improved secondary security feature which can be advantageously applied to security documents, such as identity cards, passports, credit cards, bank notes, cheques and the like for the purposes of document authentication. This secondary security feature provides an additional level of security to an primary security feature which will typically be present, to permit more reliable authentication of the security document where a polarised light source is available.

[0075] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0076] While the invention has been described in conjunction with a limited number of embodiments, it will be appreciated by those skilled in the art that many alternative, modifications and variations in light of the foregoing description are possible. Accordingly, the present invention is intended to embrace all such alternative, modifications and variations as may fall within the spirit and scope of the invention as disclosed.

[0077] The present application may be used as a basis or priority in respect of one or more future applications and the claims of any such future application may be directed to any one feature or combination of features that are described in the present application. Any such future application may include one or more of the following claims, which are given by way of example and are non-limiting in regard to what may be claimed in any future application.

Claims (5)

1. The claims defining the invention are as follows:

   1. A security device for a security document comprising a substrate, the security device being provided on or within the substrate, the security device including an image area formed from a plurality of diffractive devices each having a defined diffraction efficiency, wherein the defined diffraction efficiencies of the diffractive devices vary across the security device so that an image formed within the image area is not discernible in the zero order when illuminated by unpolarised light, but is discernible in the zero order when the image area is illuminated by light passed through a first incident light polariser and the zero order reflected light is passed through a second polariser which is crossed at right angles with the first incident light polariser.
2. 2. A security device according to claim 1, wherein each diffractive device forms a pixel and the security device comprises a pixelated array.
3. 3. A security device according to claim 1 or 2, wherein the defined diffraction efficiencies of the diffractive devices vary across the security device in accordance with a predetermined algorithm.
4. 4. A security device according to any one of claims 1 to 3, wherein each diffractive device includes a plurality of curvilinear grating elements.
5. 5. A security device according to any one of claims 1 to 4, wherein each diffractive device includes a plurality of parallel grating elements.