WO2019145691A1

WO2019145691A1 - Security device and methods of manufacture thereof

Info

Publication number: WO2019145691A1
Application number: PCT/GB2019/050156
Authority: WO
Inventors: Adam Lister
Original assignee: De La Rue International Limited
Priority date: 2018-01-23
Filing date: 2019-01-21
Publication date: 2019-08-01
Also published as: GB201801104D0; GB2572938A

Abstract

An optical security device comprising: a colour layer defining an array of pixels each of which comprises at least two regions of different colours; a liquid crystal layer overlapping the colour layer and comprising birefringent regions defining at least a first image to be exhibited by the pixels of the optical security device; and a polariser layer overlapping the liquid crystal layer and having a primary axis, the primary axis defining a secondary axis lying in the plane of the polariser layer and perpendicular to the primary axis, the polariser layer configured such that incident light having a polarisation parallel to said primary axis is transmitted while the transmission of incident light having a polarisation component parallel to said secondary axis is inhibited; wherein at least the first image is exhibited when the liquid crystal layer is positioned between a source of linearly polarised light and the polariser layer.

Description

SECURITY DEVICE AND METHODS OF MANUFACTURE THEREOF

FIELD OF THE INVENTION

This invention relates to optical security devices suitable for establishing the authenticity of objects of value, particularly security documents, and their methods of manufacture.

BACKGROUND OF THE INVENTION

Articles of value, and particularly documents of value such as banknotes, cheques, passports, identity cards, driver’s licences, credit cards, certificates of authenticity, fiscal stamps and the like 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 and documents are provided with security devices to confirm the authenticity of the object.

In particular, articles of value are commonly provided with visible security devices which allow the holder to immediately assess the veracity of the document. Examples of such security devices include complex printed patterns, security inks and structural features, such as holograms, lenticular devices and moire interference devices. Other known security devices include watermarks, embossings, perforations and the use of iridescent (e.g. 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 traditional reproduction techniques such as photocopying.

Equally, objects and documents may also be provided with covert security devices which exhibit effects which are not visible to the naked eye under natural light. These devices may include magnetic materials or luminescent and fluorescent inks which are only visible under certain conditions (e.g. under infrared or UV light). However, as counterfeiting methods become more sophisticated, there is a constant need to develop new security devices with more complex overt and covert effects in order to stay ahead of would-be counterfeiters.

Furthermore, as security devices have become increasingly complex, the methods required to produce security devices and security documents have also increased in complexity. Consequently, known methods of producing security devices and security documents are slow, expensive and require large and complicated machinery.

Therefore, there is a need for security devices which may be manufactured quickly, easily and accurately.

There is also the problem that covert security devices require specialist equipment in order to create the conditions under which an article of value may be authenticated.

These and other problems are addressed by the present invention.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there is provided an optical security device comprising: a colour layer defining an array of pixels each of which comprises at least two regions of different colours; a liquid crystal layer overlapping the colour layer and comprising birefringent regions defining at least a first image to be exhibited by the pixels of the optical security device; and a polariser layer overlapping the liquid crystal layer and having a primary axis, the primary axis defining a secondary axis lying in the plane of the polariser layer and perpendicular to the primary axis, the polariser layer configured such that incident light having a polarisation parallel to said primary axis is transmitted while the transmission of incident light having a polarisation component parallel to said secondary axis is inhibited; wherein at least the first image is exhibited when the liquid crystal layer is positioned between a source of linearly polarised light and the polariser layer.

The present invention provides an optical security device which may be authenticated using the polarised light emitted by LCD based displays, such as laptop screens, computer monitors, televisions, and phone screens, thereby obviating the need for specialist authentication equipment. When illuminated with such polarised light an image will be revealed, thereby authenticating an article of value on or in which the optical security device has been provided. The image could also be personalized for use in identification documents, such as passports and the like.

The polarisation of the light is at least partially rotated by the birefringent regions of the liquid crystal layer, thereby enabling light initially aligned with the secondary axis of the polariser layer to be at least partially transmitted through said polariser layer. The birefringent regions of the liquid crystal layer therefore define areas of the optical security device through which light aligned with the secondary axis may be transmitted, while in other regions such light is blocked. As the liquid crystal layer overlaps the colour layer, the birefringent regions define how much light passes through each pixel of the colour layer. Therefore, by adjusting the proportions of light transmitted by each pixel of the colour layer, a colour image may be displayed by the optical security device.

The present invention incorporates both optical security devices suitable for use in transmission and those suitable for use in reflection. When used in transmission, incident linearly polarised light passes through the liquid crystal layer first before passing through the polariser layer, thereby enabling a viewer on the polariser side of the device to view the image. When used in reflection, the light is reflected after passing through the polariser, and travels back through the polariser layer and the liquid crystal layer, such that a viewer on the liquid crystal side of the device may view the image.

Throughout this specification, the term“visible colour” means a colour which can be seen by the naked human eye. This includes achromatic hues such as black, grey, white, silver etc., as well as chromatics such as red, blue, yellow, green, brown etc.

The term sub-pixel used in this specification refers to the portion of a pixel of the device corresponding to a single colour region of the colour layer, and the colour of a sub-pixel will be the colour of this colour region. References throughout this specification to the curing of sub-pixels of a given colour will be understood to refer to curing regions of a liquid crystal layer corresponding to regions of a colour layer having said given colour. Similarly, references to the birefringent regions of a sub-pixel or sub-pixels will be understood to refer to the birefringent regions of the liquid crystal layer corresponding to said sub-pixel or sub-pixels, while references to the non-birefringent regions of a sub-pixel or sub-pixels will be understood to refer to the non-birefringent regions of the liquid crystal layer corresponding to said sub-pixel or sub-pixels.

Throughout this specification, the term “light” refers to both visible light (see below) and non-visible light outside the visible spectrum, such as infra-red and ultraviolet radiation. “Visible light” refers to light having a wavelength within the visible spectrum, which is approximately 400 to 750nm, and to any sub range of the visible spectrum, including white light containing substantially all the visible wavelengths in more or less even proportion. The ultra-violet spectrum typically comprises wavelengths from about 200nm to about 400nm, and the infra-red spectrum typically comprises wavelengths from about 750nm to 1 mm. By“red light” we mean light having a wavelength of between 620 and 750nm, by“green light” we mean light having a wavelength of between 495 and 570 nm, and by “blue light” we mean light having a wavelength of between 450 and 495nm.

The thickness of the liquid crystal layer may be such that the birefringent regions of the liquid crystal layer act as half-wave plates when illuminated by a given frequency of green light. However, all that is necessary for an image to be exhibited is for the light to no longer be linearly polarised along the primary axis of the polariser layer. Therefore, a pure half wave plate or region is not essential. A different thickness of the liquid crystal layer will still enable the effect, and equally the thickness of the liquid crystal layer could be chosen such that the birefringent regions act as half-wave plates for different colours of light.

In order for a colour image to be displayed, the pixels of the colour layer will typically comprise red, green, and blue regions. Nevertheless, fewer colour regions could be used if a full colour image is not required, and equally more colour regions could be provided if so desired. These regions will preferably appear in equal proportions, but if the proportions are different then a full colour image is still possible by suitably varying the corresponding birefringent regions of the liquid crystal layer.

The colour layer may also comprise cyan, magenta, yellow, and black (CMYK) regions. Such a colour layer is particularly suitable when the device is designed to be viewed in reflection. It is of course possible to omit one or more of these regions, and the black regions in particular are not essential for a full colour image to be exhibited. Equally, when the device is designed to be viewed in reflection, any other subtractive colour model may be used. For example the colour layer may comprise red, yellow, and blue (RYB) regions. Additive colour models, such as the red, green, and blue regions described above, are also suitable when the device is intended to be viewed in reflection.

The first image may comprise at least one item of information comprising any of: indicia, alphanumeric text, a letter or number, a symbol, a portrait, a logo or another graphic, but the first image is not limited to these.

In accordance with a second aspect of the present invention, there is provided a method of producing an optical security device comprising:

(a) producing a liquid crystal layer which comprises birefringent regions and non-birefringent regions defining at least a first image to be exhibited by the optical security device by:

(a1 ) providing a mask comprising gap regions, the gap regions in the mask defining the birefringent regions of the liquid crystal layer;

(a2) providing a liquid crystal layer overlapping the mask; (a3) illuminating the liquid crystal layer with light, wherein the mask is positioned between the liquid crystal layer and the source of said light, and wherein uncured regions of the liquid crystal layer are birefringent, thereby curing the birefringent regions of the liquid crystal layer corresponding to the gap regions of the mask; and

(a4) heating the liquid crystal layer such that uncured regions of the liquid crystal layer lose substantially all birefringence, and illuminating said liquid crystal layer with light, wherein the mask is not positioned between the device and the source of said light, thereby curing the non- birefringent regions of the liquid crystal layer; and

(b) providing the liquid crystal layer overlapping a colour layer which defines an array of pixels each of which comprises at least two regions of different colours such that the birefringent regions define at least a first image to be exhibited by the pixels of the optical security device.

It is difficult to print a liquid crystal to the fine dimensions required for exhibiting a colour image, and the method of this aspect of the present invention overcomes this problem.

Rather than limiting the resolution of colour images to that achievable by printing of liquid crystal molecules, this aspect of the present invention instead makes use of the higher resolutions achievable by the available demetalisation technology and by using photoinitiators in the liquid crystal layer to cure the liquid crystal molecules with light.

Liquid crystal molecules that have polymerisable groups attached to them (for example acrylate groups, which polymerise via a free-radical mechanism occurring at the double bond, or epoxy groups, which polymerise via a ring opening mechanism) can be fixed into their current configuration by curing with light. Photoinitiators in the liquid crystal layer interact with the light to create reactive species, for example free radicals or ions. These reactive species activate the polymerisable groups, thereby causing bonds to be formed between the polymerisable groups on the molecules, and it is these bonds that maintain the configuration of the liquid crystal molecules. Therefore, when a mask is positioned between the source of the curing light and the liquid crystal, any pattern exhibited by the mask will be fixed into the liquid crystal layer. When the liquid crystal molecules in the liquid crystal layer are substantially aligned, these cured regions will be birefringent. By using demetalisation to produce said mask, the high resolution possible with demetalisation patterns may be used to produce a corresponding high resolution in an image defined by the birefringent regions of a liquid crystal layer.

The next step of the method then fixes the remaining regions of the liquid crystal layer into a configuration whereby those regions are substantially non- birefringent. Therefore, on heating, the uncured regions of the liquid crystal layer will lose substantially all of their birefringence, and this state can then be fixed by curing with light. A mask is typically not necessary during this step as the birefringent regions have already been cured.

The light source used in one or both of steps (a3) and (a4) preferably emits both visible and ultraviolet frequencies of light, although a light source could equally be selected that emits only visible light or only ultraviolet light.

The liquid crystal molecules of the liquid crystal layer will preferably have one or both of a nematic phase and a smectic phase, and in step (a3) the uncured regions of the liquid crystal layer will preferably be in the nematic phase or the smectic phase.

In accordance with a third aspect of the present invention, there is also provided a method of producing an optical security device comprising:

(a1 ) providing a mask comprising gap regions, the gap regions in the mask defining the non-birefringent regions of the liquid crystal layer;

(a2) providing a liquid crystal layer overlapping the mask;

(a3) heating the liquid crystal layer such that the uncured regions of the liquid crystal layer lose substantially all birefringence, and illuminating the liquid crystal layer with light, wherein the mask is positioned between the liquid crystal layer and the source of light, thereby curing the non- birefringent regions of the liquid crystal layer corresponding to the gap regions of the mask; and

(a4) illuminating said liquid crystal layer with light, wherein the mask is not positioned between the device and the source of said light, and wherein uncured regions of the liquid crystal layer are birefringent, thereby curing the birefringent regions of the device; and

The third aspect of the invention is an alternative to the second aspect in which the non-birefringent regions are cured first. According to this aspect, the liquid crystal layer is heated, and the inverse mask to the one used in the second aspect of the invention is positioned between the liquid crystal layer and the source of curing light. The liquid crystal layer is then cooled and the liquid crystal molecules re-aligned before illumination of the entire layer with light. This achieves all the benefits of the second aspect of the invention.

The light source used in one or both of steps (a3) and (a4) preferably emits both visible and ultraviolet frequencies of light.

The liquid crystal molecules of the liquid crystal layer will preferably have one or both of a nematic phase and a smectic phase, and in step (a4) the uncured regions of the liquid crystal layer will preferably be in the nematic phase or the smectic phase.

In some embodiments of the second or third aspects of the invention, an alignment layer is used to orient the liquid crystal molecules when curing the birefringent regions of the liquid crystal layer. The interaction between the liquid crystal molecules and the surface of such an alignment layer, which can arise in a number of ways, including through p-p stacking, causes the optic axis of the birefringent regions to lie substantially within the plane of the liquid crystal layer, thereby enabling the liquid crystal layer to rotate the polarisation of light incident on these regions. The molecules will usually align with grooves in the surface of the alignment layer, and the direction of the alignment can be influenced by the coating direction of the alignment layer or by rubbing the surface of the alignment of felt. Instead of an alignment layer, another method of aligning the liquid crystal molecules could be used, such as an electric field or a magnetic field. Furthermore, it is not necessary that the optic axis lie exactly in the plane of the liquid crystal layer as an image will still be visible when the device is illuminated with polarised white light if the polarisation of incoming light is rotated by less than 90 degrees.

In embodiments of the second or third aspects of the invention in which an alignment layer is used to align the liquid crystal molecules, the alignment layer may be positioned between the mask and the liquid crystal layer, such that the alignment layer functions as a release layer. In such cases, the alignment layer may comprise a polyvinyl alcohol or a polyimide. In other embodiments a separate release layer may be provided between the mask and the alignment layer.

Liquid crystal molecules will typically have a temperature at which they transfer from a phase in which the liquid crystal molecules are substantially aligned, and the liquid crystal layer is consequently birefringent, to an isotropic phase, in which the liquid crystal molecules are not unaligned and the liquid crystal layer is consequently non-birefringent. Therefore, to ensure the non-birefringent regions lose substantially all birefringence, a step of heating the crystal layer preferably comprises heating the liquid crystal layer above the temperature of the phase transition between the nematic phase and the isotropic phase. This temperature depends on the blend of materials used in the liquid crystal layer, with a preferable blend having a transition temperature of around 70°C. Alternatively, the non-birefringent regions may be dissolved using a suitable solvent, such as MEK. In accordance with a fourth aspect of the present invention, there is provided a method of producing an optical security device comprising:

(a) producing an intermediate device by providing a liquid crystal layer overlapping a colour layer, the colour layer defining an array of pixels each of which comprises at least two regions of different colours; and

(b) for each colour of the colour layer, producing birefringent regions in the corresponding regions of the liquid crystal layer by:

(b1 ) providing a mask comprising gap regions overlapping the array of pixels of the colour layer, the gap regions in the mask defining the birefringent regions of the liquid crystal layer, wherein the colour layer is positioned between the liquid crystal layer and the mask; and

illuminating the liquid crystal layer with light, wherein the colour layer and the mask are positioned between the liquid crystal layer and the first source of said light, and wherein uncured regions of the liquid crystal layer are birefringent, thereby curing the birefringent regions of the liquid crystal layer corresponding to the gap regions of the mask and to the colour of the colour layer; and

(b2) heating the liquid crystal layer such that uncured regions of the liquid crystal layer lose substantially all birefringence and illuminating said liquid crystal layer with light, wherein the colour layer is positioned between the liquid crystal layer and the source of said light and the mask is not positioned between the device and said source of said light so as to cure the non-birefringent regions of the liquid crystal layer corresponding to the colour of the colour layer.

In some embodiments of the fourth aspect of the invention, the light source used in one or both of steps (b1 ) and (b2) emits both visible and ultraviolet frequencies of light. In such embodiments of the invention, the intensity of the light used to illuminate the liquid crystal layer may be greater when curing regions of the liquid crystal layer corresponding to colours of a longer wavelength than when curing regions of the liquid crystal layer corresponding to colours of a shorter wavelength. This has benefits both when the photoinitiators in the liquid crystal layer decompose under visible light and when the photoinitiators decompose under ultraviolet light. In the first instance, as the output spectrum of an ultraviolet light source will include a higher proportion of blue light than red light, to take a specific example, when curing regions of the liquid crystal layer corresponding to blue regions of the colour layer it is not necessary to use as high an intensity of ultraviolet light as when curing regions of the liquid crystal layer corresponding to red regions of the colour layer. In the second instance, this feature is beneficial as regions of the liquid crystal layer corresponding to colours of a longer wavelength will transmit a smaller proportion of ultraviolet light than regions of the liquid crystal layer corresponding to colours of a shorter wavelength. Nevertheless, an intensity of light suitable for curing regions of the liquid crystal layer corresponding to red regions of the colour layer will also be suitable for curing regions of the liquid crystal layer corresponding to blue regions of the colour layer, and if necessary the light could be attenuated using one or more filters.

The method of the fourth aspect of the invention could involve providing, for at least one colour of the colour layer, a first light filter between the mask and the source of light in step (b1 ) and a second light filter between the colour layer and the source of light in step (b2), wherein said first and second light filters transmit light having substantially the same colour as said colour of the colour layer. In preferred embodiments, one or both of the light filters used in embodiments of the fourth aspect of the invention could be high pass frequency filters which block light having a frequency below the frequency of the colour region corresponding to the region of the liquid crystal layer currently being cured. It is equally possible for one or both of the light filters to be band pass frequency filters configured to transmit only light having a colour substantially the same as the regions of the colour layer or low pass frequency filters. When these filters are high pass frequency filters, the first and second light filters may be omitted when curing the birefringent regions corresponding to the coloured regions which transmit the longest wavelength of light.

The birefringent regions may be produced in order of increasing wavelength of light transmitted by the corresponding coloured regions of the colour layer. This feature is especially beneficial when the light used to cure the birefringent regions is emitted by an ultraviolet light source or in embodiments where first and second light filters have been provided between a light source and the device during curing.

An alignment layer may be used in step (b1 ) to orient the optic axis of each uncured birefringent region of the liquid crystal layer such that the optic axes of the uncured birefringent regions are substantially parallel to the surface of the liquid crystal layer, or alternatively this orientation could be achieved using an electric or a magnetic field.

The liquid crystal molecules of the liquid crystal layer will preferably have one or both of a nematic phase and a smectic phase, and in step (b1 ) the uncured regions of the liquid crystal layer will preferably be in the nematic phase or the smectic phase.

A benefit of the fourth aspect of the invention is that the liquid crystal layer does not need to be registered to a colour layer after the corresponding birefringent regions have been cured, but rather each step of the method ensures that the pattern of birefringent and non-birefringent regions of the liquid crystal layer is produced in register with the corresponding coloured regions of the colour layer. The different colour regions of the colour may be used to filter out the light incident on the colour layer such that it only reaches those regions of the liquid crystal layer corresponding to regions of the colour layer which are either to be cured in the current step of the method or which have already been cured. As such the curing of the liquid crystal layer in the fourth aspect of the invention is self-registering to the colour layer.

In accordance with a fifth aspect of the present invention, there is provided a method of producing an optical security device comprising:

(b) for each colour of the colour layer, producing birefringent regions in the corresponding regions of the liquid crystal layer by: (b1 ) providing a mask comprising gap regions overlapping the array of pixels of the colour layer, the gap regions in the mask defining the non- birefringent regions of the liquid crystal layer, wherein the colour layer is positioned between the liquid crystal layer and the mask; and

heating the liquid crystal layer such that uncured regions of the liquid crystal layer lose substantially all birefringence and illuminating the liquid crystal layer with light, wherein the colour layer and the mask are positioned between the liquid crystal layer and the first source of said light thereby curing the non-birefringent regions of the liquid crystal layer corresponding to the gap regions of the mask and to the colour of the colour layer; and

(b2) illuminating said liquid crystal layer with light, wherein the colour layer is positioned between the liquid crystal layer and the source of said light and the mask is not positioned between the device and said source of said light, and wherein uncured regions of the liquid crystal layer are birefringent, so as to cure the birefringent regions of the liquid crystal layer corresponding to the colour of the colour layer.

Similarly to the second and third aspects, the fifth aspect of the invention is an alternative to the fourth aspect in which the non-birefringent regions corresponding to a given colour of the colour layer are cured before the birefringent regions of corresponding to the given colour of the colour layer. This achieves all the benefits of the fourth aspect of the invention.

The liquid crystal molecules of the liquid crystal layer will preferably have one or both of a nematic phase and a smectic phase, and in step (b2) the uncured regions of the liquid crystal layer will preferably be in the nematic phase or the smectic phase.

The methods of the fourth and fifth aspects of the invention could be performed with the optical security device remaining in situ throughout the process and the various masks, light filters, and sources of light being positioned around the device as needed. Alternatively, the device could be conveyed between different stations corresponding to the steps of the method. In this latter case, the mask could be provided around a rotary conveying means.

The fourth and fifth aspects of the invention also achieve all of the benefits of the second and third aspects of the invention.

The optical security device of the first aspect of the invention may be produced by the methods of the second, third, fourth, or fifth aspects of the invention. The optical security device of the first aspect could also be produced by a different method, and the methods of the second, third, fourth, or fifth aspects of the invention could be used to produce optical security devices different to those described as embodiments of the first aspect of the invention.

BRIEF DESCRIPTION OF THE FIGURES

Some examples of security devices and methods according to the invention will now be described with reference to the accompanying drawings, in which:

Figure 1A shows a plan view of an opaque banknote carrying an embodiment of an optical security device according to the invention, where the optical security device is shown as it would appear when illuminated with unpolarised light;

Figure 1 B shows the banknote of Figure 1A, where the optical security device is shown as it would appear when illuminated with linearly polarised light;

Figure 1 C shows a cross-section of a banknote as shown in Figures 1A and 1 B along the line X-X’, wherein the optical security device has been provided over a transparent window of the banknote;

Figure 1 D shows a similar banknote to that shown in Figure 1 C, wherein the polariser layer of the optical security device 100 has been provided on the other side of the transparent window of the banknote to the other layers of the device; Figure 1 E shows a cross-section of a banknote as shown in Figures 1A and 1 B along the line X-X’, wherein the optical security device has been provided over an aperture of the banknote;

Figure 1 F shows a similar banknote to that shown in Figure 1 E, wherein the polariser layer of the optical security device 100 has been provided on the other side of the aperture to the other layers of the device;

Figure 1 G shows a cross-section of a banknote as shown in Figures 1A and 1 B along the line X-X’, wherein the optical security device has been provided as a patch onto the banknote;

Figure 2A shows the pixels of an optical security device according to an embodiment of the invention;

Figure 2B shows a single pixel of an optical security device according to an embodiment of the invention;

Figure 3A is a cross-section through one pixel of an embodiment of the invention in which the pixels each comprise three colour regions and which a first image is exhibited when viewed in transmission or in reflection;

Figure 3B is a cross section through a specific example of a single pixel of an optical security device according to the invention;

Figure 4A shows an example of an optical security device configured to exhibit an effect when viewed in reflection;

Figure 4B shows another example of an optical security device configured to exhibit an effect when viewed in reflection, wherein the effect may be seen if the device is illuminated with unpolarised or circularly polarised light;

Figures 5 to 13 show the steps of a method for producing an optical security device according to the invention; Figures 14 and 15 shows steps of an alternative method to that shown in Figures 5 to 12;

Figure 16A shows another method for producing an optical security device according to the invention;

Figure 16B shows a stage of the method shown in Figure 16A in more detail;

Figure 16C shows an exploded view of a portion A-A’ of Figure 16B;

Figure 17A shows a plan view of an opaque banknote carrying optical security devices according to the invention which have been incorporated on a security strip, where the optical security devices are shown as they would appear when illuminated with unpolarised light;

Figure 17B shows the banknote of Figure 17A, where the optical security device is shown as it would appear when illuminated with linearly polarised light;

Figure 17C shows a cross-section of a bank note as shown in Figures 17A and 17B along the line Y-Y’, wherein the optical security devices have been provided over transparent windows of the banknote;

Figure 17D shows a cross-section of a bank note as shown in Figures 17A and 17B along the line Y-Y’, wherein the optical security devices have been provided over apertures of the banknote; and

Figure 17E shows a cross-section of a bank note as shown in Figures 17A and 17B along the line Y-Y’, wherein the security strip has been applied to a banknote.

DETAILED DESCRIPTION OF THE FIGURES The following describes in detail optical security devices for incorporation into security documents, and methods for the production thereof, such as are embodied by the present invention.

Figure 1A shows a banknote 1 incorporating an optical security device 100 according to an embodiment of the present invention. In the figure, the device is being viewed in unpolarised light and optical security device 100 is not displaying an image. However, when illuminated with linearly polarised light, such as is emitted by commonly available liquid crystal displays, the optical security device 100 exhibits an image 1000, as shown in Figure 1 B.

According to different embodiments of the invention, the device 100 could be positioned over a window in banknote 1 , such that the optical effect exhibited by the device may be viewed in transmission, or alternatively banknote 1 could be opaque, such that the optical effect exhibited by the device may be viewed in reflection. Examples of some of these embodiments are shown in figures 1 C to 1G.

A window in a banknote can take the form of a gap in the opacifying layers of a polymer banknote, or as an aperture in an opaque banknote. Figure 1C shows the first of these cases, with optical security device 100 provide on one side of polymer substrate 2 and gaps 4 in opacifying layers 3 enabling the device to be viewed in transmission. As will be described herein, the optical security device has a layered structure including a colour layer 11 , a polariser layer 12, and a liquid crystal layer 13. As such it is possible for one or more of these layers to be provided on one side of a polymer banknote and the one more other layers to be provided on the other side. In particular, the polariser layer 12 of the optical security devices of the present invention need not be registered to the other layers of the device and the polariser layer 12 is therefore especially suitable for positioning on the reverse side of a banknote to the other layers of the device. An example of such a banknote is shown in Figure 1 D.

Figures 1 C shows the device 100 positioned with the colour layer 1 1 proximal to the substrate 2, with the liquid crystal layer 13 distal to the substrate 2, and with the polariser layer 12 positioned between layers 1 1 and 13. A similar windowed banknote is shown in Figure 1 D, but in which the polariser layer 12 of device 100 is positioned on the other side of substrate 2 to layers 1 1 and 13. Many other configurations of the layers of the device are, of course, possible, and it is also possible to include additional layers in addition to layers 1 1 , 12, and 13.

The alternative example of providing an aperture in an opaque banknote is shown in Figure 1 E. Banknote 1 could be a traditional paper banknote, or it could equally be a polymer banknote with opacifying layers. Figure 1 E shows optical security device 100 provided on one side of the aperture but, as with a window in a polymer banknote, the layers of the device could be provided on different sides of the aperture, as shown in Figure 1 F.

When the optical security device is to be viewed in reflection, it is preferably provided as a patch on a security document, such as in Figure 1G where optical security device 100’ is provided on banknote 1. In such embodiments of the invention, one or both of the optical security device 100’ and banknote 1 will typically include a reflection enhancing layer 14’ positioned between the banknote and colour layer 1 1’, polariser layer 12’, liquid crystal layer 13’, and any other layers of the optical security device 100’.

An optical security device according to an embodiment of the present invention is also suitable for use on security documents other than banknotes, and banknote 1 could therefore be replaced by a cheque, passport, identity card, driver’s license, credit card, certificate of authenticity, fiscal stamp, or any other security document.

As shown in figure 1 B, the optical security device 100 exhibits an image 1000 when illuminated with polarised light. In order for this image to be exhibited in colour, embodiments of the present invention provide a pixel structure in a colour layer of the device 100, an example of which is shown in Figure 2A. The composition of a single pixel 1004 of the image is shown in Figure 2B. In the embodiment shown, each pixel comprises three regions, or sub-pixels, 1001 , 1002 and 1003 of different colours, for example red, green, and blue. When illuminated with unpolarised light, the colour regions of the colour layer uniformly transmit light, such that each pixel 1004 of the optical security device 100 appears white. The optical security device is configured such that the proportion of incident polarised light transmitted by each colour region may be varied to determine the overall colour exhibited by each individual pixel. Therefore, the overall effect of the array of pixels will be to display a colour image when illuminated with polarised light.

Figure 3A shows a cross-section of a pixel of an optical security device according to a first embodiment of the present invention. A colour layer 1 1 is provided on a polariser layer 12, which in turn is provided on a liquid crystal layer 13.

Liquid crystal layer 13 comprises non-birefringent regions 131 r, 131 g, and 131 b and birefringent regions 132r, 132g, and 132b. Birefringence in liquid crystal layer 13 arises because the liquid crystal molecules in each of birefringent regions 132r, 132g, and 132b are substantially aligned along a common axis. This defines an optic axis for each birefringent region. Linearly polarised light with a polarisation parallel to this optic axis will experience a different refractive index n_e to light with a polarisation perpendicular to this optic axis, which will experience a refractive index n₀. As a result, when linearly polarised light is incident on one of birefringent regions 132r, 132g, or 132b, the polarisation component perpendicular to the optic axis (the“ordinary ray”) will travel through the layer at a different speed to the polarisation component parallel to the optic axis (the“extraordinary ray”), thereby causing the polarisation of the incident light to be altered.

The respective optic axes of the different birefringent regions need not all be parallel; all that is required is that each birefringent region has an optic axis lying substantially along a direction within the liquid crystal layer. In contrast, the liquid crystal molecules in the non-birefringent regions 131 r, 131 g, and 131 b are not aligned, and these regions of the liquid crystal layer are therefore not optically active. As such, the polarisation of incident light is unchanged when passing through the non-birefringent regions 131 r, 131 g, and 131 b. Therefore, the polarisation of light incident on the polariser layer 12 will be different depending on which of regions 131 r, 132r, 131 g, 132g, 131 b, and 132b the light passed through.

The polariser layer 12 is configured such that incident light having a polarisation parallel to a primary axis will be transmitted. The primary axis of the polariser layer 12 defines a secondary axis which lies in the plane of the polariser layer and which is perpendicular to the primary axis, and incident light having a polarisation component parallel to this secondary axis will be inhibited.

As mentioned above, the polarisation of light incident on the polariser layer will be different when said light has first passed through one of non-birefringent regions 131 r, 131 g, or 131 b than when said light has first passed through one of birefringent regions 132r, 132g, or 132b. Therefore, although light incident on the liquid crystal layer 13 and which is linearly polarised along the secondary axis will be blocked by the polariser in regions corresponding to non-birefringent regions 131 r, 131 g, and 131 b, at least a portion of this light will be transmitted in the regions of the polariser corresponding to birefringent regions 132r, 132g, and 132b because the polarisation has been altered. By carefully selecting which regions are to be birefringent, the liquid crystal layer 13 and polariser layer 12 may thereby exhibit an image when the liquid crystal layer is positioned between the polariser layer and a source of linearly polarised light. The effect will be strongest when the linearly polarised light is polarised along either the primary axis or the secondary axis, with the two respective images in each case being the inverse of each other.

In the embodiment shown in Figure 3A, colour layer 1 1 comprises three regions of different colours 11 1 , 112 and 1 13, respectively corresponding to red, green, and blue regions. Colour regions 1 11 , 1 12 and 1 13 are provided in register with non-birefringent regions 131 r, 131 g, and 131 b and birefringent regions 132r, 132g, and 132b of liquid crystal layer 13, such that the relative sizes of the regions 131 r, 132r, 131 g, 132g, 131 b, and 132b determine the proportion of the incident light transmitted by each colour region 11 1 , 1 12 and 1 13, thereby determining the proportions of red, green, and blue light transmitted by the pixel 10.

This mechanism is best understood by considering the specific example of a pixel configured to transmit light having an 8-bit colour value of R; 127, G;255, B;0, which corresponds to a light green, when the device is illuminated with white light polarised along the primary axis of the polariser layer. The 8-bit colour value gives the amount of light of each colour transmitted by a pixel of the device, with a value of 0 indicating that no light of that colour is transmitted, increasing up to a value of 255 indicating that all incident light of that colour is transmitted. Figure 3B shows a schematic example of such a pixel 20. The portion of the liquid crystal layer 23 corresponding to red colour region 211 comprises non-birefringent region 231 r and birefringent region 232r in a 1 : 1 ratio; the portion of the liquid crystal layer 23 corresponding to green colour region 212 comprises a continuous birefringent region 232g extending across the width of the green colour region 212; and the portion of the liquid crystal layer 23 corresponding to blue colour region 213 comprises a continuous non- birefringent region 231 b extending across the width of the blue colour region 213.

Although Figure 3B shows the portion of the liquid crystal layer 23 corresponding to red colour region 21 1 having a single birefringent region 232r and a single non-birefringent region 231 r, any number of birefringent and non-birefringent regions could be provided in any combination, so long as the ratio of the total area of non-birefringent regions to birefringent regions is 1 : 1. This is the case more generally, and the ratio of birefringent to non-birefringent regions corresponding to a given colour region will determine the proportion of incident light that passes through that colour region, when the incident light is polarised along the primary axis. The converse is true when the incident light is polarised along the secondary axis, with the proportion of light transmitted then given by the ratio of the non-birefringent to birefringent regions, and the specific pixel shown in Figure 3B would transmit light having a colour value of R;127, G;0, B; 255 when illuminated with light polarised along the secondary axis of the polariser layer. The layers of the device are shown as being in direct contact with each other, but it is equally possible for intermediate layers to be provided. One example of an intermediate layer would be to provide an image to be displayed when the device is viewed in unpolarised light. Another possibility is for the substrate of a polymer banknote to be provided between two of the layers of an optical security device according to an embodiment of the present invention. Any other transparent substrate of a security document would also be suitable for incorporation between the layers of an optical security device according to an embodiment of the present invention, and equally an opaque substrate having an aperture could also be provided between the layers of an optical security device according to an embodiment of the invention, provided the aperture at least partially overlapped the optical security device.

In other embodiments of the invention, the three layers of the optical security device could be provided in any order, the effect being visible when incident linearly polarised light passes through the liquid crystal layer 13 before passing through the polariser layer 12. Colour layer 1 1 could also be positioned between the polariser layer 12 and liquid crystal layer 13 or below both of layers 12 and 13.

The optical security devices shown in Figures 3A and 3B have been described for an image to be viewed in transmission, but these devices are equally suitable for use in reflection, as the fact that incident light passes through the liquid crystal and polariser layers twice does not impact the ability of the device to exhibit an image. An example of such a device 10’ configured to be viewed in reflection is shown in Figure 4A. In addition to colour layer 11’, polariser layer 12’ and liquid crystal layer 13’, this device includes a reflection enhancing layer 14’, which could comprise a metal such as copper or aluminium, or could comprise a transparent high refractive index material. In embodiments of the present invention, a high refractive index will typically mean a refractive index greater than 2, a good example being zinc sulphide, which has a refractive index of 2.37. Typically, the polariser layer 12’ will be positioned between the liquid crystal layer 13’ and the reflection enhancing layer 14’, such that the first pass through layers 1 1’, 12’, and 13’ is the same as when the device is viewed in transmission, with incident polarised light passing through liquid crystal layer 13’ before the polariser layer 12’. The light incident on the reflective surface 14’ will therefore be linearly polarised along the primary axis of the polariser layer. The well-known Fresnel equations describe the behaviour of polarised light upon reflection at an interface. Importantly for the purposes of the present invention, the polarisation of linearly polarised light is unchanged by reflection at an interface, and the reflected light therefore passes back through the polariser 12’ substantially without attenuation. The image exhibited is not altered by the second pass through the liquid crystal layer as this merely rotates the polarisation in the birefringent regions, which is not noticeable by the naked human eye.

An alternative arrangement of a device configured to be viewed in reflection would be to position the liquid crystal layer 13” between the polariser layer 12” and the reflection enhancing layer 14”, as shown in Figure 4B. Incident light passes through the polariser layer 12” before the liquid crystal layer 13”, is reflected at the interface with the reflection enhancing layer 14”, and then passes through the liquid crystal layer 13” once more before reaching the polariser layer 12”. As the light will have passed through the liquid crystal layer 13” before reaching the polariser layer 12” the second time, the polarisation state of the light will be altered if it passed through a birefringent region of the liquid crystal layer 13” and the polariser layer 12” will therefore attenuate this light. An image defined by the birefringent regions of the liquid crystal layer will therefore be visible when the optical security device is arranged as in Figure 4B, even when the device is illuminated with unpolarised or circularly polarised light.

A first method of producing the optical security devices shown in Figures 3A and 3B is shown in Figures 5 to 14.

The first step of the method is to provide a mask comprising gap regions 33. These gap regions 33 will define the birefringent regions of the liquid crystal layer, while the non-gap regions 34 should be substantially opaque to the frequencies of light that will be used to cure the liquid crystal layer. Figure 5 shows an example of such a mask 32 provided on a substrate 31. In some embodiments of the invention, the mask 32 may not be provided in direct contact with the substrate 31 and there may be one or more transparent intermediate layers. The substrate 31 may, in some embodiments, be used as a carrier layer for the finished optical security device, and is preferably substantially transparent at least to the frequencies of light that will be used to cure the liquid crystal layer. A typical example of such a substrate is a PPT carrier layer.

One means of providing such a mask 32 is to first provide a continuous metal layer 30 onto the substrate 31 , as shown in Figure 6, and then to apply an etchant substance 301 to the metal layer in regions corresponding to the gap regions, as shown in Figure 7, in order to remove the metal in these regions.

In some embodiments, the metal of the mask will be soluble in alkaline conditions and this etchant used will be alkaline. An alkaline etchant is particularly suitable when the metal of the mask comprises aluminium or chromium, or alloys thereof. Iron and copper may also be etched under alkaline conditions, but these will dissolve much more slowly than the metals mentioned above.

In other embodiments, the mask comprises a metal soluble in acidic conditions, for example copper or chromium, or alloys thereof.

Another means of providing a mask 32 is shown in Figures 8 and 9. First a soluble ink 302 is provided (Figure 8) onto the substrate 31 in regions corresponding to the gap regions. A continuous metal layer 30 is then provided (Figure 9) onto the substrate over the soluble ink, such that metal is provided in regions corresponding to both the gap regions 33 and the non-gap regions 34. Finally, a solvent is used to remove the soluble ink along with any metal provided thereon, leaving the mask 32 on substrate 31. Both means of producing a mask allow for a high resolution image to be defined by the gap regions of the mask.

In both cases, the metal may be provided onto the substrate by vacuum deposition, encompassing sputtering, resistive boat evaporation, or electron beam evaporation, or the metal may be provided by chemical vapour deposition.

An alignment layer 41 is then provided over the mask 32, followed by a liquid crystal layer 42, as shown in Figure 10. The alignment layer is preferably of a thickness that the liquid crystal layer is not in contact with the metal regions of the mask 32. The liquid crystal molecules of the liquid crystal layer are thermotropic, meaning that the liquid crystals exhibit a number of different phases as the temperature is changed. In particular, the liquid crystals preferably have a nematic phase in which the long axes of the liquid crystal molecules are aligned. Another possibility is for the liquid crystals to exhibit a smectic phase, which is itself typically separated into a number of “mesophases”.

In the smectic phase, the liquid crystals separate into distinct layers, with the liquid crystals aligning within each of these layers, and the direction of this alignment will depend on which of the smectic mesophases the liquid crystals are in. In the smectic-A and smectic-B mesophases the long axes of the liquid crystal molecules are oriented with the normal to the smectic layers, while in the smectic-C and smectic-C* mesophases the liquid crystal molecules in each layer are oriented at a constant tilt angle from the normal to that layer. In the smectic- C mesophases this orientation axis is the same for all layers, while in the smectic-C* mesophases this orientation axis rotates from one layer to the next.

In this embodiment, an alignment layer 41 has been provided between the mask 32 and the liquid crystal layer 42. In order for the optical security device of the present invention to properly exhibit an image when illuminated with linearly polarized light, the respective optic axes of the birefringent regions of the liquid crystal layer need to be substantially along a direction within the liquid crystal layer. The surface of the alignment layer 41 interacts with the liquid crystal molecules such that the long axes of the liquid crystal molecules closest to the alignment layer are substantially parallel to the surface of this alignment layer. When the liquid crystals are in the nematic phase, the other liquid crystal molecules align with the molecules closest to the alignment layer 42 such that substantially all of the liquid crystal molecules have their long axes aligned substantially parallel to the surface of the alignment layer 42. This alignment between the liquid crystal molecules means that it is sufficient for the alignment layer to induce the alignment of just a single layer of liquid crystal molecules. One particular advantage of using an alignment layer to align the liquid crystals is that this alignment layer may also function as a release layer. Nevertheless, alternatives to an alignment layer may be used, for example an electric field or a magnetic field, along with a conventional release layer, such as one comprising wax, that does not induce alignment of the liquid crystal molecules. Furthermore, a separate release layer could, in some embodiments, be provided between the alignment layer 42 and the mask 32.

The alignment step is similar when the liquid crystals are in the smectic phase, with the smectic layers forming perpendicularly to the surface of the alignment layer. In the smectic-A and smectic-B mesophases this will result in the birefringent regions having optic axes parallel to the surface of the alignment layer, while in the smectic-C and smectic-C* mesophases the respective optic axes of the birefringent regions will be tilted at an angle from the surface of the alignment layer. This tilting of the optic axes in the smectic-C and smectic-C* mesophases will not unduly affect the birefringence of the birefringent regions.

Once the liquid crystal molecules are aligned, the liquid crystal layer 42 is illuminated with light 44 through layer 31 , such that the mask 32 is positioned between light source 43 and the liquid crystal layer 42, such that only some regions of the liquid crystal layer 42 are illuminated. This ensures that only the regions 422 of the liquid crystal layer 42 corresponding to the birefringent regions are cured, leaving the other regions 421 of the liquid crystal layer 42 uncured. Although regions 421 and 422 are shown in Figure 10 separated by dotted lines, these regions are indistinguishable prior to curing of both regions. The liquid crystal molecules used in the present method have polymerisable groups between which bonds may be formed, one example being acrylate groups, another being epoxy groups, and the liquid crystal layer also comprises photoinitiators. One example of a suitable liquid crystal molecule is 1 ,4-bis-[4-(3- acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene, which transitions to the isotropic phase at around 120°C. The photoinitiators are configured such that they interact with certain frequencies of light to create reactive species, such as free radicals and ions, which cause links to be formed between the polymerisable groups. The illumination therefore fixes (or “cures”) the configuration of the liquid crystal molecules in regions 422, thereby fixing the birefringence of these regions. The frequency of the light 44 used to illuminate the liquid crystal layer will depend on the frequency or frequencies at which the photoinitiators decompose into reactive species. Typically this will occur in the ultraviolet spectrum, but photoinitiators have also been developed which form reactive species under exposure to visible light, such as carboxylated camphorquinone and derivatives of thioxanthone dicarboxamide. As such, light 44 will typically be in the ultraviolet spectrum, but could also be in the visible spectrum. Equally, liquid crystal layer 42 could comprise more than one type of photoinitiator that decompose at different frequencies, in which case it will be preferable for light 44 to comprise a range of frequencies across the visible and ultraviolet light spectrums, and indeed most available light sources will light in a characteristic spectrum across such a range. Of course, in such embodiments many possible sources of light could be used.

In the present embodiment, this step is performed with the liquid crystal layer at a temperature, typically around 70°C or higher, such that the liquid crystals are in the nematic phase or the smectic phase. As noted earlier, a particularly preferable blend of materials for use in the liquid crystal layer transitions to the isotropic phase at around 70°C, but this temperature can be higher as with 1 ,4- bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene. The liquid crystals may be applied at a different temperature, at which they are not in the nematic or smectic phase, and then brought to one of these phases by either cooling or heating the device. There are many suitable methods of applying the liquid crystals, including gravure printing, meyer bar coating, knife coating, and slot die coating. The cooling or heating of the device may be active or passive, or involve a combination of both.

So as to not unduly inhibit the light used to cure the regions of the liquid crystal layer, the alignment layer 41 is preferably substantially transparent to at least the frequencies at which the photoinitiators decompose. Of course, in embodiments of the invention in which the liquid crystal layer 42 comprises a variety of photoinitiators that decompose at different frequencies, it is not essential that the alignment layer 41 is transparent to the all of said frequencies.

Having cured the birefringent regions of the liquid crystal layer, the next step of the method is to also cure the non-birefringent regions. This is depicted in Figure 1 1. As described above, the liquid crystals of the liquid crystal layer 42 are thermotropic, and as such there exists a temperature above which the liquid crystals of the liquid crystal layer 42 will transition to an isotropic phase, in which the liquid crystal molecules are not aligned. In this isotropic phase, the liquid crystals will lose substantially all of their birefringence.

Therefore, in order to produce the non-birefringent regions of the liquid crystal layer, heat 52 is applied to the liquid crystal layer 42. This causes liquid crystal molecules in the uncured regions 421 of the liquid crystal layer 42 to become substantially unaligned and thereby to lose substantially all of their birefringence. As the birefringent regions 422 of the liquid crystal layer have already been cured, the liquid crystal molecules in these regions remain aligned when the liquid crystal layer 42 is heated.

A light source 53, which is positioned such that mask 32 does not lie between source 53 and liquid crystal layer 42, is then used to illuminate liquid crystal layer 42 with light 54, thereby curing regions 421 and ensuring that these regions remain non-birefringent upon cooling. As when curing the birefringent regions of the device, light 54 will typically be in the ultraviolet spectrum, but could also be in the visible light spectrum, while most available light sources 43 will emit light across a range of frequencies of the electromagnetic spectrum. The heat 52 could also be applied during the curing step itself to ensure that the liquid crystal molecules do not re-align before they have fully cured. The duration of this heating will depend on the rate of cooling of the liquid crystal layer as compared with the rate of curing.

Liquid crystal layer 42 now comprises non-birefringent regions 421 and birefringent regions 422. When provided with polariser layer 62, the specific arrangement of these regions will define an image to be exhibited when the device is illuminated by polarised light, as has been explained above. In order for this image to be in colour, regions 421 and 422 are provided in register with the pixels of a colour layer 61. It is not necessary to achieve a perfect register as any misalignment will merely lead to a colour shift in the image exhibited by the device. In order to achieve a full colour image the colour layer 61 requires at least three different colour regions, and in this embodiment three colour regions 61 1 , 612 and 613 have been provided respectively corresponding to red, green, and blue. The registering of the liquid crystal layer to the colour layer is typically achieved by providing a camera system along with tensioning and guiding systems for the layers of the device. The camera system provides feedback to ensure the liquid crystal layer is registered to the colour layer. Once the liquid crystal layer 42 has been provided in register with the colour layer 61 , along with the polariser layer 62, the three layers are laminated together to leave a finished optical security device 60 provided on a substrate 31.

In the embodiment shown in Figure 12, the polariser layer 62 has been provided in between the colour layer 61 and the liquid crystal layer 42, but the method could equally be used to provide a finished optical security device in which a colour layer is provided in between a liquid crystal layer and a polariser layer.

The substrate 31 could function as a carrier layer for the finished device 60 when the device is to be applied as a patch to a security document. As shown in Figure 13, in such embodiments a pressure or heat sensitive adhesive 130 is applied in a layer to the other side of the device to the substrate 31. The adhesive layer 130 will typically be continuous, but could also be provided discontinuously, for example with a gap region corresponding to a window over which the device 60 is to be provided. When the substrate 31 is to function as a carrier layer, alignment layer 41 will typically also function as a release layer, although a separate release layer could be provided between the alignment layer 41 and the mask 32, as described above. In these embodiments of the invention the alignment layer would remain when the device is attached to a security document using the adhesive and the carrier is removed.

The skilled person will, of course, readily understand that the mask 32 is removed along with the substrate 31 when device 60 is attached to a security document.

Of course, there is no need for a carrier layer or adhesive when, as described above, the substrate of a security document is provided between the layers of the optical security device 60, thereby supporting the layers of the optical security device 60.

In the embodiment of Figures 5 to 14, the birefringent regions of the liquid crystal layer are cured first, before heating and curing the non-birefringent regions of the liquid crystal layer. However, it is of course possible to first cure the non- birefringent regions, and Figures 14 and 15 show steps of a method by which this may be achieved.

The step of the alternative method shown in Figure 14 involves providing a mask 32’ which is the inverse to that shown in Figures 5 to 14, with the gap regions 34’ of mask 32’ corresponding to the non-gap 34 regions of mask 32, and the non- gap regions 33’ of mask 32’ corresponding to the gap regions 33 of mask 32. This mask may of course be produced using the same demetalisation techniques discussed in relation to mask 32, and is therefore shown on substrate 31. As with Figure 10, a release layer 41 has been provided between mask 32’ and a liquid crystal layer 42, and this release layer may also act as an alignment layer.

Whereas in the step shown in Figure 10 the liquid crystal layer is illuminated with light while the liquid crystal molecules are aligned, in Figure 14 the liquid crystal layer 42 is heated 52 such that all regions of the liquid crystal layer lose substantially all birefringence. Light 44 is then used to illuminate the liquid crystal layer 42 such that the mask 32’ is positioned between the light source 43 and the liquid crystal layer 42, thereby curing the non-birefringent regions 421 of the liquid crystal layer 42. As with the heating shown in Figure 1 1 , heat may be applied during as well as prior to the curing of the non-birefringent regions.

The liquid crystal layer is allowed to cool, which could in some embodiments of the invention encompass active cooling, causing the uncured regions of the liquid crystal layer to transition back into the nematic or the smectic phase. If an alignment layer has been used, the liquid crystal molecules will align such that the optic axes are respectively substantially along directions within the liquid crystal layer 13. As the birefringent regions are now separated by the cured non-birefringent regions, the optic axes in the different birefringent regions may not be parallel, but as explained above this does not affect the functioning of the invention.

Once the liquid crystal layer has cooled and the liquid crystal molecules in the birefringent regions have aligned, the device is illuminated with light 54, as depicted in Figure 15, such that the mask 32’ is not positioned between the source 53 of light 54 and liquid crystal layer 42, thereby curing the birefringent regions of the liquid crystal layer.

The next step is to provide a colour layer and a polariser layer, and these may be provided in the same manner as described in relation to Figure 12, along with an adhesive to produce a device substantially similar to those shown in Figures 13 and 14.

A device produced according to this alternative method will exhibit substantially the same effect as one produced by the method described in relation to Figures 5 to 14.

The present invention also provides for a method of producing an optical security device which does not require a separate step of registering a colour layer to the liquid crystal layer. An embodiment of this method will now be described in relation to Figures 16A, 16B, and 16C.

Rather than starting by first curing a liquid crystal layer, this method starts with a layered device 170 including both a liquid crystal layer 171 and a colour layer 174, and then makes use of the colour layer 174 to control selective curing of the regions of the liquid crystal layer 171 corresponding to each individual colour, and is therefore termed a“self-registering” method.

According to embodiments of the self-registering method, the layers of the optical security device 170 are laminated together in the form of an elongate web prior to curing the liquid crystal layer 171 of this device. In addition to liquid crystal layer 171 , device 170 will typically comprise an alignment layer 172, a polariser layer 173, and a colour layer 174. The liquid crystal layer 171 is cured in stages, with regions of the liquid crystal layer 171 corresponding to different colours of the colour layer 174 being cured in different stages. These stages are shown in Figure 16A for a device with a colour layer 174 having red, green, and blue colour regions 1741 , 1742, and 1743. Each stage involves curing the birefringent regions of the liquid crystal layer 171 corresponding to a given colour of the colour layer at a first station, before curing the non-birefringent regions of the liquid crystal layer corresponding to said given colour at a second station. In the present embodiment, the blue sub-pixels are cured first, in stage 177b, and the web is conveyed on to stages 177g and 177r for curing of the green and red sub-pixels, respectively, by means of rotary conveying means 1700b, 1700g, and 1700r. Additional support means 1700a are also shown in Figure 16A for supporting the web between the conveying means 1700b, 1700g, 1700r, and the skilled person will understand that numerous different arrangements of these are possible. As such, the device web 170 will typically be provided on a carrier layer 176 suitable for use on these rotary conveying means, with a release layer 175 provided between the device and the carrier layer 176. Stage 177b is shown in more detail in Figure 16B, and an analogous setup to that shown will typically be used to cure other colours of sub-pixel. Figure 16C shows further detail of the device web 170, mask 1701 b, and rotary conveying means 1700b. Not shown in Figure 16C is that the pixels of the device form a two dimensional array.

At the first station 1771 b, a mask 1701 b is positioned between a source 1704b of light 1703b and the device web 170, such that the colour layer 174 is between light source 1704b and liquid crystal layer 171. The light impinging on the liquid crystal layer 171 is selected such that it only cures regions of the liquid crystal layer corresponding to the blue regions of the colour layer. As shown in the figure, this is typically done by providing a filter 1702b, but it could also be achieved through the choice of light source 1704b or by altering the intensity of light 1703b. The combination of the mask 1701 b, light filter 1702b, and light source 1704b acts such that only the birefringent regions 1712b of the blue sub- pixels are cured, and in particular the filter 1702b blocks red and green frequencies of light from reaching regions of the liquid crystal layer corresponding to red or green sub-pixels. Light is also blocked by the mask so as to ensure only the birefringent regions of the blue sub-pixels are cured.

Once the birefringent regions 1712b of the blue sub-pixels have been cured, the device web is moved to the next station 1772b where the device is heated 1705b from the upper side of the device, that is the side of the device distal to the colour layer, such that the uncured regions of the liquid crystal layer 171 lose substantially all birefringence. This typically involves heating at least part of the liquid crystal layer 171 above the temperature of the phase transition between the nematic phase and the isotropic phase. In most embodiments the heating step will include uncured regions of the liquid crystal layer 171 corresponding to green and red sub-pixels, although of course embodiments in which only the uncured regions of the blue sub-pixels are heated are equally suitable for producing a device according to the present invention. The liquid crystal layer 171 is then illuminated with light 1707b, wherein the colour layer 174 and a light filter 1706b are positioned between the liquid crystal layer 171 and the source 1708b of light 1707b, such that the non-birefringent regions 171 1 b of the blue sub-pixels are cured. The light filter 1706b and light source 1708b will typically be substantially identical to light filter 1702b and light source 1704b. In Figure 16B the heating 1705b is shown earlier in the path of the device web 170 than light filter 1706b, such that the liquid crystal layer 171 is first heated and then cured. However, the heat 1705b could be provided in a region at least partially overlapping the curing region, such that heat is applied during as well as prior to the curing of the non-birefringent regions. Similarly, the device could be heated from the lower side of the device additionally or alternatively to being heated from the upper side.

Next, the device web is conveyed onto station 1771 g of stage 177g for curing the birefringent regions 1712g of the green sub-pixels. A mask 1701 g is positioned between a source 1704g of light 1703g and the device web 170, such that the colour layer 174 is between the light source 1704g and liquid crystal layer 171. As the blue sub-pixels have been cured by this point, it does not matter if the light incident on the device web 170 passes through the blue regions 1743 of the colour layer 174, and the specific combination of light filter 1702g and light source 1704g will be selected based on the requirement that the birefringent regions 1712g of the green sub-pixels be cured without inadvertently curing any regions of the red sub-pixels. In particular, light filter 1702g will block red frequencies of light but need not necessarily block blue frequencies of light, while the mask ensures that only the birefringent regions of the green sub-pixels are cured.

To cure the non-birefringent regions 1711 g of the green sub-pixels, the device web is moved to station 1772g where the device is heated 1705g from the upper side of the device such that the uncured regions of the liquid crystal layer 171 lose substantially all birefringence. The liquid crystal layer 171 is then illuminated with light 1707g, wherein the colour layer 174 and a light filter 1706g are positioned between the liquid crystal layer 171 and the source 1708g of light 1707g, such that the non-birefringent regions 1711 g of the green sub-pixels are cured. The light filter 1706g and light source 1708g will typically be substantially identical to light filter 1702g and light source 1704g.

After this, the device web is conveyed onto station 1771 r of stage 177r for curing the birefringent regions 1712r of the red sub-pixels. A mask 1701 r is positioned between a source 1704r of light 1703r and the device web 170, such that the colour layer 174 is between the light source 1704r and liquid crystal layer 171. As the green and blue sub-pixels have been cured by this point, it does not matter if the light incident on the device web 170 reaches regions of the liquid crystal layer corresponding to green or blue sub-pixels. As such it is not necessary to block any particular frequencies of light, and a light filter need not be provided.

Finally, to cure the non-birefringent regions 1711 r of the red sub-pixels, the device web is moved to station 1772r where the device is heated 1705r from the upper side of the device such that the uncured regions of the liquid crystal layer 171 lose substantially all birefringence. The liquid crystal layer 171 is then illuminated with light 1707r such that the non-birefringent regions 171 1 r of the red sub-pixels are cured.

As noted above, light source 1702b and light filter 1704b, used when curing birefringent regions 1712b of the blue sub-pixels, are chosen such that light incident on the colour layer does not reach regions of the liquid crystal layer corresponding to red or green sub-pixels. This is achieved by controlling both the frequencies of light incident on the colour layer 174 and the intensity of such incident light, and in preferred embodiments of the invention involves using a light source 1702b that emits both visible and ultraviolet frequencies of light in combination with a light filter 1704b which blocks all greed, red, and infrared light but allows through blue and ultraviolet light.

The above described combination of light source 1702b and light filter 1704b is particularly preferable as it is suitable for use with a variety of photoinitiators that decompose at different frequencies. The light filter 1702b ensures that photoinitiators that decompose under visible light are only activated in regions corresponding to the blue sub-pixels, while also allowing the use of photoinitiators that decompose under ultraviolet light. The use of ultraviolet light to cure the liquid crystal layer is possible as, although the blue colour regions 1743 of colour layer 174 are designed to only allow through blue light, there will be a degree of“leakage” of ultraviolet frequencies owing to the non-ideal nature of these blue colour regions. This effect will be more pronounced in the blue regions 1743 than in the green regions 1742 and red regions 1741 , and so an intensity of light 1703b may be chosen for which only a negligible intensity of ultraviolet light leaks through the red and green regions 1741 and 1742 of the colour layer 174 but for which an intensity sufficient to induce curing leaks through the blue regions 1743 of the colour layer 174. Light 1707b of substantially the same intensity as light 1703b would then be used to cure non- birefringent regions 171 1 b.

Similar considerations apply for selecting light source 1702g and light filter 1704g used to cure birefringent regions 1712g of the green sub-pixels. Another high-pass frequency filter is selected to block red and infrared frequencies of light while allowing through green, blue and ultraviolet frequencies of light. As the proportion of ultraviolet light leaked by the green regions 1742 of the colour layer 174 is less than the proportion leaked by the blue regions 1743 but more than the proportion leaked by the red regions 1741 , the intensity of light 1703g and 1707g will be greater than the intensity of light 1703b and 1707b.

As both the green and blue sub-pixels have been cured already, when curing the red sub-pixels the only requirement is that a non-negligible intensity of red and ultraviolet frequencies of light pass through the red regions 1741 of the colour layer 174. A high intensity of light 1703r and 1707r may therefore be used without a colour filter.

By using high pass frequency filters, numerous light sources may be used to cure the liquid crystal layer. This could include using blue, green, and red light to cure the blue, green, and red sub-pixels respectively, or alternatively ultraviolet light of different intensities could be used, with weak ultraviolet used to cure the blue sub-pixels and progressively more intense ultraviolet light used to cure the green then red sub-pixels. Therefore, a wide range of photoinitiators are suitable for use in this method.

In a particularly preferable embodiment, curing light sources emitting light from across the visible and ultraviolet spectra are used. When curing the birefringent regions of the blue sub-pixels, the light filter 1702b will let through the blue and higher frequencies of the light 1703b emitted by light source 1704b. This light then impinges on the colour layer 174. The blue frequencies of light will be blocked by the red and green regions of the colour layer, while the intensity of the light source 1702b is chosen such that the intensity of the ultraviolet light leaked by the green and red regions of the colour layer is negligible relative to the intensity required to cure the liquid crystal molecules. Typically, light source 1708b and the light sources used in stages 177g and 177r emit the same frequencies of light as source 1704b, and so the light impinging on the liquid crystal layer at station 1772b has the same frequency distribution as at station 1771 b. Different filters are used in stage 177g to stage 177b, so green light also impinges on the colour layer when curing the green sub-pixels. By increasing the intensity of the light, a non-negligible intensity of light will leak through the green regions of the colour layer, although the intensity will be sufficiently low that the red sub-pixels remain substantially uncured. Finally, when curing the red sub-pixels, the light filter may be omitted and a high intensity of light used.

One alternative to the above described embodiment is to forgo the use of ultraviolet light to cure the liquid crystal layer either by using band pass filters which allow through light of the same colour as the sub-pixels being cured in each of stages 177b, 177g, and 177r along with a visible light source, or by using blue, green, and red light sources for stages 177b, 177g, and 177r, respectively. This has the benefit that the different coloured sub-pixels could be cured in any order, but it does not allow for as wide a variety of light sources to be used.

Low pass frequency filters could equally be used, provided that the red sub- pixels were cured first, followed by the green then blue sub-pixels.

In embodiments where the red, green, and blue sub-pixels are cured in a different order to that shown in Figure 16A, light filters are usually provided for at least the first two stages. One particular example of where this is not necessary is when blue, green, and red light sources are used to cure the blue, green, and red sub-pixels, respectively. As shown in Figure 16A, the masks 1701 b, 1701 g, and 1701 r will typically be provided around rotary conveying means 1700b, 1700g, and 1700r.

Nevertheless, it is possible for the masks and the conveying means to be separate. It is also possible for the curing of both the birefringent regions and non-birefringent regions to occur with the device in situ and the various light filters, masks, and light sources being moved around the device. In such embodiments, a carrier layer may not be necessary.

In embodiments where the masks are not provided on the rotary conveying means, the light filters could be positioned between their respective masks and the device web, rather than between their respective masks and light sources as shown in Figure 16A.

The masks 1701 b, 1701 g, and 1701 r serve the same purpose as the mask 32 in the embodiments described in relation to Figures 5 to 15 by ensuring that only respective birefringent regions 1712b, 1712g, and 1712r are cured, although as only one colour of sub-pixel is cured at a time the specific structure of the masks will be different. Whereas the mask shown in Figure 5 has gap regions corresponding to the birefringent regions across the whole liquid crystal layer, the gap regions of mask 1701 b shown in Figure 16C only define the birefringent regions 1712b of the blue sub-pixels, while similarly gap regions of masks 1701 g and 1701 r only define the birefringent regions 1712g and 1712r of the green and red sub-pixels. The masks do not need to be registered to the colour layer 174 itself, as shifting the masks will not lead to a visible change at the macroscopic scale of the image exhibited by the device when illuminated with polarised white light. This can be understood by considering just the blue sub-pixels. In the embodiment shown in Figure 16C, the mask 1701 b includes the pattern of birefringent 1712b and non-birefringent 171 1 b regions to be cured into each of the blue sub-pixels three times, with two thirds of this mask obscured by red or green colour regions. Shifting the mask 1701 b would simply cause the blue portion of the image to be shifted by a corresponding amount, as this would just cause a different portion of the mask to be obscured by red and green colour regions. Methods known in the art may be used to align the masks 1701 b, 1701 g, and 1701 r with each other, such that the different colours of the macroscopic colour image are provided in register with each other.

As shown in Figure 16C, the optical security device will typically also include an alignment layer 172, which forms part of the final device. As with the alignment layer 41 described earlier, this serves to ensure that the optic axes of the birefringent regions of the device are substantially along a direction within the layer 172 by ensuring that long axes of the liquid crystal molecules are substantially parallel to the surface of the alignment layer 172 when the liquid crystal molecules are in the nematic or the smectic phase. As the sub-pixels are cured colour-by-colour, the optic axes of the different birefringent regions of the liquid crystal layer may not be parallel, but as described above this does not affect the operation of the device. This alignment layer does not need to serve as a release layer as it forms a part of the final device, and indeed it is preferable that it does not allow the liquid crystal layer to be easily separated from the device. The alignment layer is preferably substantially transparent to visible light, in order that an image be viewed using linearly polarised visible light, and also to the frequencies of light necessary to cure the liquid crystal layer. The alignment layer could be positioned on the reverse side of the liquid crystal layer to that shown in Figure 16C, and in such cases the alignment layer would not need to be transparent to the frequency of light used to cure the liquid crystal layer 171 , or to visible light when the device is configured for viewing in reflection. Alignment layer 172 could of course be omitted if magnetic or electric fields are to be used to align the liquid crystals.

In Figure 16C, the polariser layer 173 is provided between the alignment layer and the colour layer. The polariser layer 173 has a primary axis wherein incident light having a polarisation parallel to this axis is transmitted. The primary axis defines a secondary axis which is perpendicular to the primary axis and parallel to the plane of the polariser layer, and incident light having a polarisation component parallel to this secondary axis will be inhibited. The polarising effect of the polariser layer 173 does not affect the self-registering method, and as such the polariser layer 173 could be positioned anywhere within the device. As with the method shown in Figures 14 and 15, the method could also be modified such that the non-birefringent regions 171 1 b of the blue sub-pixels are cured prior to curing the birefringent regions 1712b. In such embodiments, heating would be provided in a region adjacent to or overlapping the mask, and the mask used would be the inverse to that shown in Figure 16C. Stages 177g and 177r could be similarly modified.

Once all regions of the liquid crystal layer 171 have been cured, a heat or pressure sensitive adhesive is typically applied on the uppermost layer of the device, which in the embodiment shown is the liquid crystal layer. The device web may then be cut to size and applied to a security document or article of value and the carrier layer removed.

Although the above method is described in relation to a device having red, green, and blue sub-pixels, it is suitable for a device with different combinations of sub-pixels. The skilled person will readily understand how the light sources, light sources, and masks used would need to be adapted for such alternative devices.

The above method may also be adapted such that the non-birefringent regions of each colour of sub-pixel of the device are cured first. Analogously to the methods shown in Figures 14 and 15, this is achieved by inverting the masks used in stations 1771 b, 1771 g, and 1771 r and by heating the device web in stations 1771 b, 1771 g, and 1771 r rather than stations 1772b, 1772g, and 1772r.

Figures 1A to 1G show a single device 100 applied to a banknote, as discussed above, while Figures 17A to 17E depict another example of a banknote, to which a security device 181 in the form of a security thread or security strip has been applied. Three optical security devices 180 according to embodiments of the present invention are carried on strip 181 and are arranged in a line on banknote 18. As shown in Figure 17A, the images defined by the birefringent and non- birefringent regions of the liquid crystal layers of the optical security devices 180 are not visible when the device is viewed under unpolarised light. However, when illuminated with linearly polarised light an image 1800 is revealed, as shown in Figure 17B. The devices may be configured such that the images 1800 are visible when the devices are viewed in transmission, or when they are viewed in reflection. The three devices may, in some embodiments, exhibit different images, the same image, or a sequence of images, and equally one or more of the devices may be configured to be viewed in transmission while the other devices are configured for viewing in reflection. Figures 17C to 17E show cross sections of the device along the line Y-Y’ according to alternative ways of incorporating the security strip 181 in the banknote 18.

Figure 17C depicts the security thread or strip 181 incorporated within the banknote 18. In the case of a paper banknote, the security thread or strip 181 may be incorporated within the substrate’s structure during the paper making process using well-known techniques. To form the windows, the paper may be removed locally after completion of the paper making process, e.g. by abrasion, leaving opacifying regions 17A and 17B. Alternatively, the paper making process could be designed so as to omit paper in the desired window regions. Equally, the security thread or strip 181 could be incorporated into the substrate of a polymer banknote using techniques known in the art, with the opacifying layers being provided in regions 17A and 17B to leave windows through which devices 180 may be viewed.

An alternative arrangement is shown in Figure 17D, in which the security thread or strip 181 carrying the security devices 180 is applied to one side of banknote 18, for example using an adhesive. The windows take the form of apertures in the banknote 18, which may exist prior to the application of strip 181 or may alternatively be formed afterwards, for example by abrasion. Gaps have been shown between the edges of the apertures and the devices 180, but the devices 180 could also be in contact with the edges of the apertures, or the apertures could be smaller than the devices, with the banknote 18 overlapping the devices 180. It is equally possible, when the strip 181 is substantially transparent, for the banknote to be provided on the other side of the strip 181 , with the optical security devices 180 provided opposite to the apertures. The banknote could be formed from a paper substrate or from a polymer substrate with opacifying layers. Figure 17E shows another alternative in which the security thread or strip 181 has been applied to a banknote 18, such that the banknote is on the reverse side of strip 181 to the devices 180. The strip 181 could be applied using adhesive. The banknote 18 will typically be opaque, so the security devices 180 shown in Figure 17E will typically be configured for viewing in reflection, but if the banknote is constructed from a polymer substrate with opacifying layers, windows could be left in the opacifying layers through which the devices 180 might be viewed, and the devices 180 could therefore also be configured for viewing in transmission.

Although Figures 17A to 17E have been described in terms of a security strip 181 applied to a banknote 18, the security strip 181 could equally be applied to any other security document in the same fashion as has been described in relation to application to a banknote.

Claims

1. An optical security device comprising:

a colour layer defining an array of pixels each of which comprises at least two regions of different colours;

a liquid crystal layer overlapping the colour layer and comprising birefringent regions defining at least a first image to be exhibited by the pixels of the optical security device; and

a polariser layer overlapping the liquid crystal layer and having a primary axis, the primary axis defining a secondary axis lying in the plane of the polariser layer and perpendicular to the primary axis, the polariser layer configured such that incident light having a polarisation parallel to said primary axis is transmitted while the transmission of incident light having a polarisation component parallel to said secondary axis is inhibited;

wherein at least the first image is exhibited when the liquid crystal layer is positioned between a source of linearly polarised light and the polariser layer.

2. An optical security device according to claim 1 , wherein the thickness of the liquid crystal layer is such that the birefringent regions of the liquid crystal layer act as half-wave plates when illuminated by a given frequency of green light.

3. An optical security device according to claim 1 or claim 2, wherein the pixels of the colour layer comprise red, green, and blue regions.

4. An optical security device according to claim 1 or claim 2, wherein the pixels of the colour layer comprise cyan, magenta, yellow, and black (CMYK) regions.

5. An optical security device according to any preceding claim, wherein the first image comprises at least one item of information comprising any of: indicia, alphanumeric text, a letter or number, a symbol, a portrait, a logo or another graphic.

6. A method of producing an optical security device comprising: (a) producing a liquid crystal layer which comprises birefringent regions and non-birefringent regions defining at least a first image to be exhibited by the optical security device by:

(a2) providing a liquid crystal layer overlapping the mask;

(a3) illuminating the liquid crystal layer with light, wherein the mask is positioned between the liquid crystal layer and the source of said light, and wherein uncured regions of the liquid crystal layer are birefringent, thereby curing the birefringent regions of the liquid crystal layer corresponding to the gap regions of the mask; and

7. A method according to claim 6, wherein the light source used in one or both of steps (a3) and (a4) emits both visible and ultraviolet frequencies of light.

8. A method according to claim 6 or claim 7, wherein an alignment layer is used in step (a3) to orient the optic axis of each uncured birefringent region of the liquid crystal layer such that the optic axes of the uncured birefringent regions are substantially parallel to the surface of the liquid crystal layer.

9. A method according to claim 8, wherein the alignment layer is positioned between the mask and the liquid crystal layer, such that the alignment layer functions as a release layer.

10. A method according to claim 8 or claim 9, wherein the alignment layer comprises a polyvinyl alcohol or a polyimide.

11. A method according to any of claims 6 to 1 1 , wherein step (a4) comprises heating the liquid crystal layer above the temperature of the phase transition to the isotropic phase.

12. A method of producing an optical security device comprising:

(a2) providing a liquid crystal layer overlapping the mask;

(a3) heating the liquid crystal layer such that the uncured regions of the liquid crystal layer lose substantially all birefringence, and illuminating the liquid crystal layer with light, wherein the mask is positioned between the liquid crystal layer and the source of said light, thereby curing the non- birefringent regions of the liquid crystal layer corresponding to the gap regions of the mask; and

13. A method according to claim 12, wherein the light source used in one or both of steps (a3) and (a4) emits both visible and ultraviolet frequencies of light.

14. A method according to claim 12 or claim 13, wherein an alignment layer is used in step (a4) to orient the optic axis of each uncured birefringent region of the liquid crystal layer such that the optic axes of the uncured birefringent regions are substantially parallel to the surface of the liquid crystal layer.

15. A method according to claim 14, wherein the alignment layer is positioned between the mask and the liquid crystal layer, such that the alignment layer functions as a release layer.

16. A method according to claim 14 or claim 15 wherein the alignment layer comprises a polyvinyl alcohol or a polyimide.

17. A method according to any of claims 12 to 16, wherein step (a3) comprises heating the liquid crystal layer above the temperature of the phase transition to the isotropic phase.

18. A method according to any of claims 6 to 17, wherein the thickness of the liquid crystal layer is such that the birefringent regions of the liquid crystal layer act as half-wave plates when illuminated by a given frequency of green light.

19. A method according to any of claims 6 to 18, wherein the pixels of the colour layer comprise red, green, and blue regions.

20. A method according to any of claims 6 to 19, wherein the pixels of the colour layer comprise cyan, magenta, yellow, and black (CMYK) regions.

21. A method according to any of claims 6 to 20, wherein the first image comprises at least one item of information comprising any of: indicia, alphanumeric text, a letter or number, a symbol, a portrait, a logo or another graphic.

22. A method according to any of claims 6 to 21 , wherein the liquid crystal layer comprises photoinitiators.

23. A method of producing an optical security device comprising:

(a) producing an intermediate device by providing a liquid crystal layer overlapping a colour layer, the colour layer defining an array of pixels each of which comprises at least two regions of different colours; and (b) for each colour of the colour layer, producing birefringent regions in the corresponding regions of the liquid crystal layer by:

illuminating the liquid crystal layer with light, wherein the colour layer and the mask are positioned between the liquid crystal layer and the source of said light, and wherein uncured regions of the liquid crystal layer are birefringent, thereby curing the birefringent regions of the liquid crystal layer corresponding to the gap regions of the mask and to the colour of the colour layer; and

24. A method according to claim 23, wherein the light source used in one or both of steps (b1 ) and (b2) emits both visible and ultraviolet frequencies of light.

25. A method according to claim 24, wherein the intensity of the light used to illuminate the liquid crystal layer is greater when curing regions of the liquid crystal layer corresponding to colours of a longer wavelength than when curing regions of the liquid crystal layer corresponding to colours of a shorter wavelength.

26. A method according to any of claims 23 to 25, wherein for at least one colour of the colour layer:

in step (b1 ) a first light filter is provided between the mask and the source of light; in step (b2) a second light filter is provided between the colour layer and the source of light; and

said first and second light filters transmit light having substantially the same colour as said colour of the colour layer.

27. A method according to claim 26 wherein, when producing the birefringent regions corresponding to the coloured regions which transmit the longest wavelength of light, the first and second light filters are not provided.

28. A method according to any of claims 23 to 27, wherein the birefringent regions are produced in order of increasing wavelength of light transmitted by the corresponding coloured regions of the colour layer.

29. A method according to any of claims 23 to 28, wherein an alignment layer is used in step (b1) to orient the optic axis of each uncured birefringent region of the liquid crystal layer such that the optic axes of the uncured birefringent regions are substantially parallel to the surface of the liquid crystal layer.

30. A method according to any of claims 23 to 29, wherein step (b2) comprises heating the liquid crystal layer above the temperature of the phase transition to the isotropic phase.

31. A method of producing an optical security device comprising:

(b1 ) providing a mask comprising gap regions overlapping the array of pixels of the colour layer, the gap regions in the mask defining the non- birefringent regions of the liquid crystal layer, wherein the colour layer is positioned between the liquid crystal layer and the mask; and

heating the liquid crystal layer such that uncured regions of the liquid crystal layer lose substantially all birefringence and illuminating the liquid crystal layer with light, wherein the colour layer and the mask are positioned between the liquid crystal layer and the source of said light thereby curing the non-birefringent regions of the liquid crystal layer corresponding to the gap regions of the mask and to the colour of the colour layer; and

32. A method according to claim 31 , wherein the light source used in one or both of steps (b1 ) and (b2) emits both visible and ultraviolet frequencies of light.

33. A method according to claim 32, wherein the intensity of the light emitted by the ultraviolet light source is greater when curing regions of the liquid crystal layer corresponding to colours of a longer wavelength than when curing regions of the liquid crystal layer corresponding to colours of a shorter wavelength.

34. A method according to any of claims 31 to 33, wherein for at least one colour of the colour layer:

in step (b1 ) a first light filter is provided between the mask and the source of light;

in step (b2) a second light filter is provided between the colour layer and the source of light; and

35. The method of claim 34 wherein, when producing the birefringent regions corresponding to the coloured regions which transmit the longest wavelength of light, the first and second light filters are not provided.

36. A method according to any of claims 31 to 35, wherein the birefringent regions are produced in order of increasing wavelength of light transmitted by the corresponding coloured regions of the colour layer.

37. A method according to any of claims 31 to 36, wherein an alignment layer is used in step (b2) to orient the optic axis of each uncured birefringent region of the liquid crystal layer such that the optic axes of the uncured birefringent regions are substantially parallel to the surface of the liquid crystal layer.

38. A method according to any of claims 31 to 37, wherein step (b1 ) comprises heating the liquid crystal layer above the temperature of the phase transition to the isotropic phase.

39. A method according to any of claims 23 to 38, wherein the thickness of the liquid crystal layer is such that the birefringent regions of the liquid crystal layer act as half-wave plates when illuminated by a given frequency of green light.

40. A method according to any of claims 23 to 39, wherein the pixels of the colour layer comprise red, green, and blue regions.

41. A method according to any of claims 23 to 40, wherein the first image comprises at least one item of information comprising any of: indicia, alphanumeric text, a letter or number, a symbol, a portrait, a logo or another graphic.

42. A method according to any of claims 23 to 41 , wherein the mask is provided around a rotary conveying means.

43. A method of manufacturing a plurality of security devices, wherein each security device is manufactured according to the method of any of claims 6 to 42 and wherein the first pattern is varied for each of said plurality of security devices.

44. A security device manufactured in accordance with the method of any of claims 6 to 42.

45. A security article comprising a security device according to any of claims 1 to 5 or manufactured in accordance with any of claims 6 to 42, and wherein the security article is preferably a security thread, strip, foil, insert, transfer element, label or patch.

46. A security document comprising a security device according to any of claims 1 to 5 or claim 44, or a security article according to claim 45, wherein the security document is preferably a banknote, cheque, passport, identity card, driver’s licence, certificate of authenticity, fiscal stamp or other document for securing value or personal identity.

47. A transfer assembly suitable for use in a method according to any of claims 6 to 42.