GB2578117A

GB2578117A - Security devices and methods for their manufacture

Info

Publication number: GB2578117A
Application number: GB1816843.5A
Authority: GB
Inventors: Lister Adam
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2018-10-16
Filing date: 2018-10-16
Publication date: 2020-04-22
Anticipated expiration: 2038-10-16
Also published as: GB2578117B; WO2020079402A1; GB201816843D0

Abstract

A security device comprises at least one cholesteric liquid crystal (LC) layer 3 disposed on a substrate 2, the cholesteric LC layer collectively exhibiting an image I detectable by the naked eye under non-polarised visible light illumination. The cholesteric LC layer has a respective pattern P of one or more first regions 4 of the layer in which the cholesteric LC is ordered and thereby exhibits an optically variable visible colour, alongside one or more second regions 5 of the layer in which the cholesteric LC is disordered and thereby is substantially transparent and colourless in the visible spectrum. The combination of the one or more respective patterns forms the image. There is also provided a security device component comprising a plurality of curable cholesteric LC layers (3a-c, Fig, 2b) disposed on the substrate, each formed of a different radiation curable cholesteric LC material and exhibiting an optically variable visible colour, wherein when viewed at a predetermined viewing angle, the visible colour exhibited by each one of the cholesteric LC layers is different from that of the other cholesteric LC layer(s). Some of the different radiation-curable cholesteric LC materials comprise respective photoinitiators which are responsive to different wavelengths of radiation.

Description

Intellectual Property Office Application No. GII1816843.5 RTM Date:16 April 2019 The following terms are registered trade marks and should be read as such wherever they occur in this document: Paliocolor Intellectual Property Office is an operating name of the Patent Office www.gov.uk /ipo

SECURITY DEVICES AND METHODS FOR THEIR MANUFACTURE

This invention relates to methods of manufacturing security devices, and security devices themselves. Security devices are used for example on documents of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents, in order to confirm their authenticity.

Articles of value, and particularly documents of value such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such objects are provided with a number of visible security devices for checking the authenticity of the object. By "security device" we mean a feature which it is not possible to reproduce accurately by taking a visible light copy, e.g. through the use of standardly available photocopying or scanning equipment. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, venetian blind devices, lenticular devices, moire interference devices and moire magnification devices, each of which generates a secure visual effect. Other known security devices include holograms, watermarks, embossings, perforations and the use of colour-shifting or luminescent / fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult, or impossible, to copy using available reproduction techniques such as photocopying. Security devices exhibiting non-visible effects such as magnetic materials may also be employed.

One class of security devices are those which produce an optically variable effect, meaning that the appearance of the device is different at different angles of view. Such devices are particularly effective since direct copies (e.g. photocopies) will not produce the optically variable effect and hence can be readily distinguished from genuine devices. Optically variable effects can be generated based on various different mechanisms, including holograms and other diffractive devices, moire interference and other mechanisms relying on parallax such as venetian blind devices, and also devices which make use of focusing elements such as lenses, including moire magnifier devices, integral imaging devices and so-called lenticular devices. Another type of optically variable effect is the colour shifting or iridescence effect exhibited by certain materials such as interference films and pigments, mica flakes, pearlescent pigments and the like. Materials such as these exhibit different colours and/or brightness levels depending on the viewing angle.

Another type of material of which some varieties can exhibit a visible, optically variable colour shifting effect is liquid crystal (LC). In particular cholesteric liquid crystals are formed by adding a chiral dopant to a nematic liquid crystal host material, which results in the LC molecules being arranged in Bragg layers, according to a helix. When the cholesteric liquid crystal is viewed in white light at different angles, the colour in which it appears changes due to the layers reflecting and transmitting different wavelengths of the incident light. The particular colour which is exhibited at each viewing angle depends on the pitch of the helices. Some examples of such materials, and security devices incorporating them, are described in WO-A-2008/043981. The same document proposes forming a spatially patterned LC effect by providing a partial layer of a light absorbing material under a continuous layer of LC material, or by providing the LC material itself in a partial form.

New security devices are constantly being sought in order to stay ahead of would-be counterfeiters.

In accordance with the present invention, a security device comprises at least one cholesteric liquid crystal layer disposed on a substrate, the at least one cholesteric liquid crystal layer collectively exhibiting an image detectable by the naked eye under non-polarised visible light illumination, the or each cholesteric liquid crystal layer having a respective pattern of one or more first regions of the layer in which the cholesteric liquid crystal is ordered and thereby exhibits an optically variable visible colour, alongside one or more second regions of the layer in which the cholesteric liquid crystal is disordered and thereby is substantially transparent and colourless in the visible spectrum, the combination of the one or more respective patterns forming the image.

Since the or each layer of cholesteric liquid crystal material (also referred to herein for brevity as an "LC layer") is configured to have first regions thereof in which the LC molecules are ordered (i.e. arranged according to a helix, as previously described) and second regions thereof in which the LC molecules are disordered (preferably isotropic), a visible pattern is displayed within the layer itself. The ordered, first regions of the layer appear visibly coloured, the particular colour being displayed varying according to the viewing angle (but being the same across all the first regions at any one viewing angle), whilst the disordered, second regions of the layer beside them (and viewable simultaneously with the first regions) appear substantially transparent and colourless (at all viewing angles). This does away with the need for achieving spatial patterning of the optically variable effect either via provision of an additional light absorbing layer or via selective application of the LC material itself, which can instead be applied in a continuous, all-over form. However, it should be noted that the present LC layer could additionally be applied pattern-wise and/or be provided with a light-absorbing layer to produce further effects if desired. Examples of suitable cholesteric liquid crystal materials will be given below.

The described arrangement of first and second regions can be formed by any suitable method but preferred examples will be given below. It should be appreciated that the arrangement of first and second regions is fixed in the security device: that is, in the first region(s) the LC molecules are permanently fixed in their ordered state, and in the second region(s) the LC molecules are permanently fixed in their disordered state. The shapes, sizes and positions of the first and second regions relative to the LC layer are also permanently fixed.

By the at least one cholesteric liquid crystal layer being disposed "on" the substrate, it is meant that the LC layers are carried by the substrate, but it is not essential that the said layer (or any of them, if there are more than one) is directly in contact with the substrate. Rather, one or more intermediate layers might exist between the substrate and the LC layer(s), such as a primer layer, an adhesive layer and/or a print layer. However, in other cases there could be direct contact, e.g. if the substrate is primed for application of the LC layer(s) by corona treatment. Further, it is irrelevant whether the or each LC layer is "above" or "below" the substrate; both are considered "on" the substrate. Indeed, as discussed below, where more than one LC layer is provided, they could be disposed on either or both sides of the substrate, and there may be additional layers between them. It is also the case that whilst the substrate will be a self-supporting layer which provides structural support to the security device, there may be other such layers incorporated in the security device also.

As stated above, the at least one LC layer collectively exhibits an image which is detectable by the naked eye under non-polarised visible light illumination. By "detectable" it is meant that the human eye can sense the difference between the optically variable visible colour exhibited by the first region(s) and the lack of colour exhibited by the second region(s), and no equipment such as polarisers is required to reveal the image. However, this does not mean that the image need be of a scale which is resolvable to the naked eye (although this is preferred in some embodiments) -it may require magnification for the viewer to be able to distinguish the first region(s) from the second region(s) alongside. It should also be understood that while if there is a single LC layer then its pattern will constitute the full image, where there are multiple LC layers, the image will be formed by the multiple corresponding patterns in combination with one another.

Whilst the security device could comprise a single LC layer of the sort described, in preferred embodiments, the at least one cholesteric liquid crystal layer comprises a plurality of cholesteric liquid crystal layers. In this case, each LC layer is formed of a different cholesteric liquid crystal material and exhibits an optically variable visible colour, and at least when the security device is viewed at a predetermined viewing angle (preferably along the normal to the substrate), the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) in the one or more first regions thereof. As a result, at least when the security device is viewed along the predetermined viewing angle (which is preferably its normal, i.e. perpendicular to the plane of the substrate), the various LC layers (each of which is optically variable) will exhibit different visible colours from one another. This enables more complex visual effects to be created and thus increases the security level of the device. It should be noted that the degree to which the different colours are actually viewable alongside one another to an observer will depend on the degree to which the patterns of the respective LC layers overlap, if at all. For example, if there are two LC layers with matching patterns which exactly overlap, then the different respective colours of the two LC layers visible from the predetermined viewing position will be combined at all points and only a single, mixed colour will be visible from the first regions of the security device as a whole even though each layer still exhibits a different visible colour itself. If the patterns do not match or do not exactly overlap, then the resulting security device will appear multi-coloured (i.e. at least two colours will be simultaneously visible alongside one another) since the colour of one LC layer will be visible at some points, that of the other LC layer at some other points, and potentially a third mixed colour at some further points should there be any overlapping.

Since each the colour of each LC layer will varying with viewing angle, in the above scenario it is possible that two or more of the LC layers will colour match at certain viewing angle(s), though not at the predetermined viewing angle. However, in more preferred implementations, at every viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) in the one or more first regions thereof at the same viewing angle. That is, none of the LC layers will colour match one another at any viewing angle. In this way, a device which appears multi-coloured at one viewing angle will still appear multi-coloured at every other viewing angle (although the selection of colours displayed may change). It should be noted that whilst this requirement applies to the whole recited plurality of LC layers, it is possible that one or more additional LC layers could be provided to which this requirement does not apply.

The colours displayed by each LC layer, and the corresponding angles at which they are visible, can be controlled through configuration of the respective LC materials themselves. In preferred embodiments, each of the different cholesteric liquid crystal materials comprises liquid crystal molecules arranged in accordance with a helix, the pitch of the helix being different in each of the different cholesteric liquid crystal materials thereby giving rise to the different respective visible colours exhibited by the cholesteric liquid crystal layers when viewed at the predetermined viewing angle. The pitch of the helix can in turn be controlled by varying the type and/or amount of chiral dopant added to the nematic LC to form the cholesteric LC. Examples of suitable cholesteric liquid crystal materials, including dopant concentrations, will be given below.

The plurality of LC layers could be configured to exhibit any selection of colours. However, advantageously, the plurality of cholesteric liquid crystal layers include a first cholesteric liquid crystal layer of which the first region(s) appear red under white light illumination at the predetermined viewing angle, a second cholesteric liquid crystal layer of which the first region(s) appear green under white light illumination at the predetermined viewing angle, and a third cholesteric liquid crystal layer of which the first region(s) appear blue under white light illumination at the predetermined viewing angle. In this way, at the predetermined viewing angle, the image that is displayed by the device can be a full colour RGB image. Again, whether a multi-coloured RGB image is actually displayed will depend on the degree of overlap between the patterns, but preferably the patterns are configured such that a multi-coloured RGB image is displayed at least at the predetermined viewing angle.

It is not essential that any of the LC layers overlap. For instance, the various LC layers could be laterally offset from one another, e.g. abutting one another or spaced from one another along the substrate. If the layers are on the same side of the substrate and/or the substrate is transparent (at least in places) a multi-coloured effect can still be achieved. Nonetheless, in preferred embodiments, at least some, preferably all, of the plurality of cholesteric liquid crystal layers (and their respective patterns) at least partially overlap one another. This enables the creation of particularly complex designs since the underlying pattern(s) will be viewed through the overlying LC layer(s), which will themselves be patterned. Some parts of the underlying pattern(s) may therefore be viewed through coloured first regions of the overlying layer(s) and hence produce a mixed colour, whereas others will be viewed through transparent second regions of the overlying layer(s) and hence their inherent colour will be visible.

As previously mentioned, the LC layers can be disposed on either or both sides of the substrate. In some preferred embodiments, at least some, preferably all, of the plurality of cholesteric liquid crystal layers are disposed on a first side of the substrate. This ensures that all of those layers can be visualised from the same side of the security device irrespective of the nature of the substrate. In addition or as an alternative, it is also preferred if at least an area of the substrate is transparent, at least one of the plurality of cholesteric liquid crystal layers is disposed on a first side of the substrate in the transparent area and at least one of the plurality of cholesteric liquid crystal layers is disposed on a second side of the substrate, preferably in the transparent area. This can be used to produce particular visual effects and/or can be taken advantage of to introduce manufacturing benefits as will be discussed below. Providing a transparent area of the substrate ensures that the LC layers on both sides can still be viewed from one side of the device. In particularly preferred embodiments the substrate may be transparent all-over.

Preferably, the respective patterns of the plurality of cholesteric liquid crystal layers are registered to one another. Methods by which this can be achieved will be discussed below. Register means that in a plurality of the said security devices, the respective patterns of the LC layers will have substantially the same position relative to one another in all of the security devices.

The LC layers could be provided directly adjacent one another. However, more preferably the security device further comprises a visually transparent spacer layer between each of the plurality of cholesteric liquid crystal layers. Such a spacer layer acts to keep different overlapping LC materials separate and prevent mixing during manufacture. The spacer layer(s) could be of similar composition and construction to the substrate (e.g. a plastic foil), but preferably they are each thinner than the substrate to avoid adding unnecessary thickness to the security device.

Advantageously, the security device further comprises at least one visually transparent protective layer, the at least one cholesteric liquid crystal layer being located between the visually transparent protective layer(s) and the substrate. That is, the or each protective layer will be located over the outermost LC layer on each side of the substrate. The protective layer(s) help to protect the LC materials from damage, particularly during manufacture, and may be of a similar construction to the aforementioned spacer layers.

In especially preferred embodiments, the or each cholesteric liquid crystal layer is a cured radiation-curable material comprising a photoinitiator. For example, each LC layer may comprise a cross-linked polymer which fixes the LC molecules in position. This has particular benefits for preferred manufacturing techniques, as discussed below. Where the security device comprises a plurality of LC layers, it is preferable that at least some of the cholesteric liquid crystal layers comprise respective different photoinitiators responsive to different wavelengths of radiation. The advantages of this will be explained below with reference to preferred manufacturing options.

In all embodiments comprising one or more cured LC layers, it is also preferably that the security device further comprises a radiation-absorbent layer configured to absorb radiation wavelength(s) to which one or more of the photoinitiator(s) are responsive (preferably all such wavelengths). The radiation-absorbent layer could be an additional layer, or its functionality could be combined into one of the existing layers. Hence, in especially preferred embodiments, the radiation-absorbing layer is the substrate. It could also be combined into one or more of the separator layers, if provided. The radiation-absorbent layer may nonetheless be visibly transparent. For example, the or each wavelength to which the LC layer(s) are responsive may lie in the UV spectrum (e.g. between 10 nm and 390 nm wavelength). It is especially advantageous to provide a radiation-absorbent layer where there is more than one LC layer, and in this case it is particularly advantageous if at least one of the LC layers is arranged on each side of the radiation-absorber layer. This provides manufacturing advantages, discussed below.

Advantageously, the security device further comprises a visible light absorbing layer, such as a black or other dark-coloured layer. When an LC layer is viewed with such a layer behind it (from the perspective of the viewer), the radiation absorbent layer suppresses light other than that reflected by the LC layer, which might otherwise overwhelm the optically variable visible colour exhibited by the LC layer, thereby making the effect more readily visible (as discussed further in WO-A-2008/043981). The visible light absorbing layer preferably absorbs the majority (preferably all) of the wavelengths in the visible spectrum, e.g. 390 nm to 700 nm. As in the case of the radiation-absorbent layer, the visible light absorbing layer can be an extra layer additional to those already mentioned, or could be combined into one or more of them. Hence in other preferred embodiments, the visible light absorbing layer may be the substrate (which could also be the radiation-absorbing layer). Since the visible light absorbing layer will be substantially visibly opaque, it is preferred that all of the LC layers contributing to the image will be located on the same side of the visible light absorbing layer. However, other LC layers (potentially forming a second image) could be located on the other side.

As already mentioned, the image could be of any scale and might be configured in a number of different ways to achieve different security effects. In some preferred embodiments, the image is a macro-scale image such as a portrait, passport photograph, logo, fingerprint, iris scan, alphanumeric character(s) or other graphic resolvable by the naked eye, preferably a full colour macro-scale image. In this case, each pattern provided by the respective LC layer(s) may correspond to a different colour component of the image. For instance, the pattern of a first LC layer could contribute pixels of a first colour, that of a second LC layer pixels of a second colour, and so on, resulting in a multi-coloured image formed by the combination of the pixels. The pixels themselves may or may not be resolvable by the naked eye. It will be appreciated that the particular colour displayed by each pattern will vary with viewing angle and so the resulting image will also change in colour (e.g. between its intended colour at one viewing angle and various false colour versions thereof at other viewing angles) upon tilting the security device.

In other embodiments, the image may be a microscopic image, such as microtext or an array of microimages or other image elements, not resolvable by the naked eye in the absence of magnification and/or filtering. Again, if there is more than one LC layer, the pattern exhibited by each will contribute to the image so in the aforementioned examples, the individual microtext, microimages or image elements could be multi-coloured, or the colour of the various elements could vary across the array (e.g. from one element to the next).

In particularly preferred embodiments, the image is an image array and the security device further comprises a viewing component configured to cooperate with the image array to generate an optically variable effect. For instance, the viewing component may preferably comprise an array of focusing elements or a viewing screen. In such cases, the resulting security device could be a moire magnifier, an integral imager or a lenticular device.

Moir6 magnifier devices (examples of which are described in EP-A-1695121, WO-A-94/27254, WO-A-2011/107782 and W02011/107783) make use of an array of focusing elements (such as lenses or mirrors) and a corresponding array of microimages, wherein the pitches of the focusing elements and the array of microimages and/or their relative locations are mismatched with the array of focusing elements such that a magnified version of the microimages is generated due to the moire effect. Each microimage is a complete, miniature version of the image which is ultimately observed, and the array of focusing elements acts to select and magnify a small portion of each underlying microimage, which portions are combined by the human eye such that the whole, magnified image is visualised. This mechanism is sometimes referred to as "synthetic magnification". The magnified array appears to move relative to the device upon tilting and can be configured to appear above or below the surface of the device itself. The degree of magnification depends, inter alia, on the degree of pitch mismatch and/or angular mismatch between the focusing element array and the microimage array.

Integral imaging devices are similar to moire magnifier devices in that an array of microimages is provided under a corresponding array of lenses, each microimage being a miniature version of the image to be displayed. However here there is no mismatch between the lenses and the microimages. Instead a visual effect is created by arranging for each microimage to be a view of the same object but from a different viewpoint. When the device is tilted, different ones of the images are magnified by the lenses such that the impression of a three-dimensional image is given.

"Hybrid" devices also exist which combine features of moire magnification devices with those of integral imaging devices. In a "pure" moire magnification device, the microimages forming the array will generally be identical to one another. Likewise in a "pure" integral imaging device there will be no mismatch between the arrays, as described above. A "hybrid" moire magnification / integral imaging device utilises an array of microimages which differ slightly from one another, showing different views of an object, as in an integral imaging device. However, as in a moire magnification device there is a mismatch between the focusing element array and the microimage array, resulting in a synthetically magnified version of the microimage array, due to the moire effect, the magnified microimages having a three-dimensional appearance. Since the visual effect is a result of the moire effect, such hybrid devices are considered a subset of moire magnification devices for the purposes of the present disclosure. In general, therefore, the microimages provided in a moire magnification device should be substantially identical in the sense that they are either exactly the same as one another (pure moire magnifiers) or show the same object/scene but from different viewpoints (hybrid devices).

Moir6 magnifiers, integral imaging devices and hybrid devices can all be configured to operate in just one dimension (e.g. utilising cylindrical lenses) or in two dimensions (e.g. comprising a 2D array of spherical or aspherical lenses).

Lenticular devices on the other hand do not rely upon magnification, synthetic or otherwise. An array of focusing elements, typically cylindrical lenses, overlies a corresponding array of image sections, or "slices", each of which depicts only a portion of an image which is to be displayed. Image slices from two or more different images are interleaved and, when viewed through the focusing elements, at each viewing angle, only selected image slices will be directed towards the viewer. In this way, different composite images can be viewed at different angles. However it should be appreciated that no magnification typically takes place and the resulting image which is observed will be of substantially the same size as that to which the underlying image slices are formed. Some examples of lenticular devices are described in US-A-4892336, WO-A2011/051669, WO-A-2011051670, WO-A-2012/027779 and US-B-6856462. More recently, two-dimensional lenticular devices have also been developed and examples of these are disclosed in W02015/011493 and W02015/011494 Lenticular devices have the advantage that different images can be displayed at different viewing angles, giving rise to the possibility of animation and other striking visual effects which are not possible using the moire magnifier or integral imaging techniques.

The present invention further provides a method of making a security device, 25 comprising: (a) applying a radiation-curable cholesteric liquid crystal material on a first surface of a substrate to form a cholesteric liquid crystal layer across at least a portion thereof, such that the cholesteric liquid crystal is ordered and exhibits an optically variable visible colour across the portion; (b) exposing one or more first regions of the portion, selected in accordance with a pattern, to radiation to thereby at least partially cure the first region(s) and fix the cholesteric liquid crystal in its ordered state therein; (c) heating the radiation-curable cholesteric liquid crystal layer across the portion to an elevated temperature so as to disorder the cholesteric liquid crystal in one or more selected second regions of the portion, the one or more selected second regions corresponding to those parts of the portion not exposed to radiation in step (b), whereupon the second region(s) become substantially transparent and visibly colourless; and (d) at the elevated temperature, exposing at least the second region(s) of the radiation-curable cholesteric liquid crystal layer to radiation to thereby at least partially cure the second region(s) and fix the cholesteric liquid crystal in its disordered state therein; whereby a pattern of first region(s) exhibiting the optically variable visible colour alongside substantially transparent and colourless second region(s) is formed, the pattern forming all or part of an image detectable by the naked eye under non-polarised visible light illumination.

The result of this method is a security device of the sort already described above and having the same attendant benefits. The method provides wide design freedom for the pattern, constrained only by the manner in which the exposure in step (b) is carried out which may comprise irradiation by a laser beam or exposure to a radiation source through a mask, for instance, as described further below. Hence high resolution patterns of any configuration can be readily achieved.

Terminology already defined above with respect to the security device has the same meaning in connection with the method. In particular, applying the LC material "on" the first surface of the substrate again does not require direct contact between the LC material and the substrate.

In relation to step (a), it has been found that good alignment of the liquid crystal molecules is automatic upon application of the LC material to many substrate materials, such as PET. However, if necessary, extra steps may be taken to achieve alignment.

As in the case of the above-described security device, while a single LC layer may be provided, more complex effects can be achieved where multiple LC layers are present. This can be achieved in different ways.

In a first preferred approach, the method further comprises: repeating steps (a) to (d) one or more times using different radiation-curable cholesteric liquid crystal material(s) on respective substrate(s), resulting in a plurality of cholesteric liquid crystal layers each disposed on a respective substrate and each exhibiting a respective pattern of first region(s) in an optically variable visible colour, wherein at least when the security device is viewed at a predetermined viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) in the one or more first regions thereof, wherein the predetermined viewing angle is preferably the normal to the substrate; and then affixing the plurality of cholesteric liquid crystal layers and respective substrates together, the image being formed by the plurality of respective patterns in combination, wherein preferably the respective patterns are different from one another.

In this way, the respective patterns can be formed separately and then combined with one another. This allows all the LC layers to be responsive to the same wavelength of radiation and hence the same or an identical radiation source to be used in step (b) for all of the patterns, if desired. It should be noted that the respective patterns could be formed (i.e. steps (a) to (d) repeated) sequentially (in which case the same apparatus can be used to produce each patterned LC layer, one after the other), or simultaneously (if more than one set of equipment is available). The respective substrates on which the patterns are formed could be separate substrates from the outset or could be different portions of a common substrate which is then cut up so that they can be affixed to one another. The affixing could be through the use of one or more adhesive layers, or if the LC layers are not fully cured during step (b) and/or step (d), the intrinsic tackiness of the LC material could be utilised to join the patterns together. For instance, a partially-cured LC layer could be assembled in contact with the substrate of a different LC layer and then the assembly fully cured to fix them together.

It will be appreciated that in this scenario, there will be multiple substrates which may or may not have the same characteristics as one another. Preferably one or more, still preferably all, of the substrates are visibly transparent.

Advantageously, the plurality of cholesteric liquid crystal layers and respective substrates are affixed to one another such that at least some, preferably all of, the respective patterns at least partially overlap one another. As previously described, in this way complex designs can be achieved since underlying pattern(s) will be viewed through the overlying pattern(s).

Preferably, the plurality of cholesteric liquid crystal layers and respective substrates are affixed to one another in a registered manner. This can be achieved for instance by including registration marks in one or more of the patterns and aligning them when the patterns are affixed to one another.

Advantageously, the plurality of cholesteric liquid crystal layers and respective substrates are affixed to one another in a stacked manner with the substrates lying in between the cholesteric liquid crystal layers. In this manner the substrates perform the function of the separator layers mentioned previously.

In a second preferred approach, step (a) comprises applying a plurality of different radiation-curable cholesteric liquid crystal materials to first and/or second surface(s) of the substrate to form a corresponding plurality of cholesteric liquid crystal layers across at least a portion of the substrate, such that the cholesteric liquid crystal of each layer is ordered and exhibits an optically variable visible colour, wherein at least when the security device is viewed along a predetermined viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s), wherein the predetermined viewing angle is preferably the normal to the substrate.

This approach enables the security device to be assembled prior to patterning the LC layers, which makes the technique particularly well adapted for use in circumstances requiring individualisation of the security device since the assembly can be supplied in a complete form to a merchant or other office at which the patterns are applied to the LC layers in the desired form.

As in the case of the security device already described, the various LC layers could be overlapping or non-overlapping. However, preferably, in step (a) the plurality of different radiation-curable cholesteric liquid crystal materials are applied so as to at least partially overlap one another. As previously described this enables the formation of more complex effects in which one pattern is viewed through another in the finished device.

The overlapping LC materials could be applied directly on top of one another. However, this risks intermingling of the different LC materials which is undesirable. Therefore, in preferred embodiments, step (a) further comprises applying a separator layer between each of the overlapping cholesteric liquid crystal layers, the or each separator layer preferably being visibly transparent. As previously described, the separator layer(s) can be of similar construction to the substrate (e.g. a plastic foil) but are preferably each thinner than the substrate.

It is particularly advantageous if at least some of the different radiation-curable cholesteric liquid crystal layers (particularly those overlapping one another) comprise respective photoinitiators which are responsive to different wavelengths of radiation. This enables each of those layers to be exposed in step (b) in accordance with a different respective pattern at the appropriate wavelength, whilst not affecting the other LC layers. For instance, where two overlapping radiation-curable LC layers are provided, the first LC layer being responsive to a first radiation wavelength Ai and the second LC layer being responsive to a second (different) radiation wavelength A2, the assembly can be exposed to the first radiation wavelength Al in accordance with a first pattern, which will at least partially cure the relevant regions of the first LC layer but be transmitted through the second LC layer without effect. The assembly can also be exposed to the second radiation wavelength A2 in accordance with a second pattern, which will at least partially cure the relevant regions of the second LC layer but be transmitted through the first LC layer without effect. It should be noted that each of the LC layers could be responsive to a plurality of radiation wavelengths (i.e. a waveband), which may or may not overlap those of other layers, but there must be at least one wavelength to which the first LC layer will respond and not the second LC layer, and vice versa.

Hence, preferably, step (b) comprises exposing the plurality of overlapping cholesteric liquid crystal layers to a corresponding plurality of different wavelengths of radiation, each in accordance with a respective pattern of selected first regions, the image being formed by the plurality of respective patterns in combination, wherein preferably the respective patterns are different from one another. The various exposures to different wavelengths could take place simultaneously or sequentially.

The curing of the second regions of the various LC layers (i.e. step (d)) could also take place in a series of sequential exposures at each of the relevant wavelengths. However, preferably, step (d) comprises simultaneously exposing the plurality of overlapping cholesteric liquid crystal layers to a corresponding plurality of different wavelengths of radiation, preferably over the whole portion of the substrate. It should be noted that this means all of the wavelengths are delivered simultaneously, preferably using a broadband radiation source which emits all of the relevant wavelengths. It does not necessarily require all of the second regions to be exposed simultaneously, although this is preferred.

Similarly whilst in this step it is only essential to expose the second regions, the first regions may also be exposed (for a second time). Thus the whole of the portion of the substrate can be exposed to a radiation source in this step, which can be used to finish curing the LC layers.

If all of the LC layers are responsive to different radiation wavelengths, each one can be patterned individually using the corresponding wavelength without affecting the other layer(s). However, this may limit the number of LC layers that can be provided if only a limited number of different photoinitiators and/or radiation sources are provided. Therefore it is preferable that the method further comprises providing a radiation-absorbent layer which is configured to absorb radiation wavelength(s) to which one or more of the radiation-curable cholesteric liquid crystal materials are responsive, wherein preferably the radiation-absorbing layer is the substrate. As previously discussed, the radiation-absorbing layer can be an additional layer beyond those already mentioned or its functionality can be incorporated into one of the existing layers, such as the substrate. The provision of such a layer will block the passage of certain radiation wavelengths and therefore can allow a certain LC layer to be patterned whilst protecting others which may be responsive to the same wavelength.

In particularly preferred embodiments, each of the cholesteric liquid crystal layers provided on one side of the radiation-absorbent layer are responsive to different radiation wavelengths from one another. That is, if there is more than one LC layer provided on the first side of the radiation-absorbent layer, all of those LC layers on the first side will be responsive to different respective radiation wavelengths, and preferably the same applies to the second side of the radiation-absorbent layer also. In this way, all of the LC layers on one side of the radiation-absorbent layer can be patterned individually. It is further preferable that at least one of the cholesteric liquid crystal layers is provided on each side of the radiation-absorbent layer and in step (b) the respective liquid crystal layers are exposed from different corresponding sides of the radiation-absorbent layer. In this case the LC layer(s) provided on one side of the radiation-absorbent layer can be responsive to the same radiation wavelength(s) as the LC layer(s) provided on the other side of the radiation-absorbent layer.

For instance, four LC layers could be provided, two on each side of the radiation absorbent layer. The two LC layers on the first side could be responsive to wavelengths Al and A2 respectively, and likewise the two LC layers on the second side could be responsive to wavelengths Al and A2 respectively. Each LC layer can still be patterned individually by exposing the first side to wavelengths Al and A2 in accordance with first and second patterns respectively, and the second side to wavelengths Al and A2 in accordance with third and fourth patterns respectively.

The following preferred features apply to both the first and second preferred methods: Advantageously, in step (a), the or each radiation-curable cholesteric liquid crystal material is applied by printing or coating, preferably gravure printing or reverse slot die printing. In general it is preferred if the or each LC layer is an all-over, continuous layer (e.g. flood coated) but this is not essential and one or more of the LC layers could be selectively applied on only parts of the substrate if desired. This could be used to introduce a further level of complexity to the security device.

Preferably, in step (a), the or each radiation-curable cholesteric liquid crystal material is applied as a solution thereof and then solvent is dried off leaving the material in a tacky state. This assists in allowing the LC material to change state (from ordered to disordered) upon heating. Generally, the LC molecules will automatically be ordered (i.e. arranged in layers in a accordance with a helix, as described in more detail elsewhere herein) as a result of the application of the LC material to the substrate. The material "self-orders" as the solvent is driven off, and the nematic layers self-order due to the interactions of the molecules.

For instance, it has been found that good alignment of the liquid crystal molecules is automatic upon application of the LC material to substrates including PET. However, if necessary extra steps may be taken to achieve alignment.

As alluded to above, the exposure to patterned radiation in step (b) can take place in a variety of ways. Advantageously, the or each cholesteric liquid crystal layer is exposed to radiation by directing a beam of radiation, preferably a laser beam, over the selected first region(s) or by exposure to a radiation source via a mask which defines the selected first region(s) in accordance with the pattern. The former option has the particular advantage that the pattern design is digital and can be changed on the fly through control of the radiation beam since no "master" plate is required. This is well adapted to scenarios in which the image to be displayed by the security device is personalised or individualised, for example (such as a passport photo or a serial number). The latter option is better suited to scenarios in which many identical security devices are to be formed, or a continuous device of a repeating pattern, as may be found on a security thread for instance. In this case it is desirable that the mask moves alongside the substrate during the exposure so that the patterning can be carried out continuously during a sheet-fed or web-based process and does not require the substrate to be stationary, so as to improve through-put.

In step (b), the first region(s) of the cholesteric liquid crystal material are preferably sufficiently cured such that they exhibit substantially no colour change upon heating. It is not essential that full curing is achieved in this step (although this may be preferred). If the first regions are only partially cured in step (b), the cure may be completed in step (d) or an optional additional curing step.

As already discussed in relation to the finished security device, it is only necessary that the respective LC layers exhibit different colours from one another at a predetermined viewing angle. However, it is preferred that, at every viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) in the one or more first regions thereof at the same viewing angle. Again, this can be achieved through control of the LC compositions and typically each of the different cholesteric liquid crystal materials applied in step (a) comprises liquid crystal molecules arranged in accordance with a helix, the pitch of the helix being different in each of the different cholesteric liquid crystal materials thereby giving rise to the different respective visible colours exhibited by the cholesteric liquid crystal layers when viewed at the predetermined viewing angle.

In especially preferred embodiments, the plurality of cholesteric liquid crystal layers include a first cholesteric liquid crystal layer of which the first region(s) appear red under white light illumination at the predetermined viewing angle, a second cholesteric liquid crystal layer of which the first region(s) appear green under white light illumination at a predetermined viewing angle, and a third cholesteric liquid crystal layer of which the first region(s) appear blue under white light illumination at a predetermined viewing angle. This results in a full colour RGB image at least at the predetermined viewing angle.

The same considerations apply regarding the image resulting from the method as discussed above in relation to the finished security device. Thus, in some cases the image may be a macro-scale image such as a portrait, passport photograph, logo, fingerprint, iris scan, alphanumeric character(s) or other graphic resolvable by the naked eye, preferably a full colour macro-scale image.

In other cases, the image may be a microscopic image, such as microtext or an array of microimages or other image elements, not resolvable by the naked eye in the absence of magnification and/or filtering.

In especially preferred embodiments, the image is an image array and the method further comprises providing a viewing component configured to cooperate with the image array to generate an optically variable effect, the viewing component preferably comprising an array of focusing elements or a viewing screen. This can be used to make moire magnifiers, integral imaging devices or lenticular devices as discussed above.

The present invention also provides a security device made in accordance with the above method.

Also provided by the present invention is a security device component comprising a plurality of curable cholesteric liquid crystal layers disposed on a substrate, each formed of a different radiation-curable cholesteric liquid crystal material and exhibiting an optically variable visible colour, wherein at least when the security device component is viewed at a predetermined viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that of the other cholesteric liquid crystal layer(s), at least some of the plurality of curable cholesteric liquid crystal layers at least partially overlapping one another, wherein at least some of the different radiation-curable cholesteric liquid crystal materials comprise respective photoinitiators which are responsive to different wavelengths of radiation.

This security device component is a precursor product from which a security device of the sort described above can be made. The security device component comprises the assembled LC layers, not yet cured, which can then be patterned using the techniques described above in relation to the second preferred approach for making a security device. Hence a security device component of this sort, or a batch of them, can be supplied blank to a distributor, such as a passport issuing office, which can be equipped with suitable exposure equipment, just as a laser connected to a control system. Each security device component can then be patterned and cured so as to exhibit the desired image by the distributor.

It should be noted that whilst each of the LC layers by itself will exhibit a different visible colour at the preferred viewing angle, these different colours may not be visible from the security device component since the various colours may be mixed by overlapping of the layers. For instance, if there are three LC layers, all entirely overlapping one another, and at the predetermined angle one appears red, another green and the third blue, the combination will appear white or off-white at the predetermined viewing angle. However, once the layers have been exposed to patterned radiation, heated and then exposed again, the various colours will be revealed to form the image.

The same preferred features discussed above in relation to the security device apply equally to the security device component.

The invention further provides a security article comprising a security device or a security device component each as disclosed above, the security article preferably comprising a security thread, strip, insert, patch or foil.

Also provided is a security document comprising a security device, a security device component or a security article, each as disclosed above, the security document preferably comprising a banknote, passport, identification card, bank card, visa, certificate, stamp or cheque.

Examples of security devices and methods for their manufacture, as well as security device components, will now be described with reference to the accompanying drawings, in which: Figure 1 shows a first embodiment of a security device in accordance with the present invention, (a) in plan view, and (b) and (c) showing two alternative cross-sections thereof; Figure 2 shows a second embodiment of a security device in accordance with the present invention, (a) in plan view and (b), (c) and (d) showing three alternative cross-sections thereof; Figure 3 shows a third embodiment of a security device in accordance with the present invention, (a) in plan view, (b) in cross-section and (c) illustrating the three patterns making up the image displayed by the security device; Figure 4 is a photograph showing a further embodiment of a security device in accordance with the present invention arranged on a security document, in oblique plan view; Figure 5 schematically shows the arrangement of liquid crystal molecules in an ordered region of an exemplary cholesteric liquid crystal material; Figure 6 is a flowchart showing steps of a first embodiment of a method for making a security device in accordance with the present invention; Figure 7(a) to (d) show an exemplary security device during stages of its manufacture in accordance with the method of Figure 6; Figure 8 schematically depicts exemplary apparatus for manufacturing a security device in accordance with an embodiment of the present invention; Figure 9 is a flowchart showing steps of a second embodiment of a method for making a security device in accordance with the present invention; Figures 10(a) to (f) show an exemplary security device during stages of its manufacture in accordance with the method of Figure 9; Figures 11 and 12 show two further embodiments of security devices in accordance with the present invention; Figure 13(a), (b) and (c) show another embodiment of a security device made in accordance with the present invention, (a) in perspective view, (b) in cross-section and (c) in plan view from two different viewing angles; Figure 14 shows another embodiment of a security device in accordance with the present invention, (a) in cross-section, (b) showing the image array thereof alone, and (c) showing the appearance of the assembled security device in plan view; Figures 15, 16 and 17 show three exemplary security documents carrying security devices in accordance with embodiments of the present invention, (a) in plan view and (b) in cross-section; and Figure 18 illustrates a further embodiment of a security document carrying a security device in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section.

A first embodiment of a security device 1 in accordance with the present invention is shown in plan view in Figure 1(a). In this example the security element 1 is of an elongate rectangular configuration and therefore might be suitable for example for use on a security thread, but the same principles can be applied to any form of security article such as patches, strips, folds or the like, or the security device can be incorporated directly onto or into a security document as will be described further below.

The security device 1 exhibits an image I which here is formed of a single pattern P comprises the repeating letters "DLR". Each of the letters is formed by a first region 4 of the security device which appears visibly coloured and can be visually distinguished from the surrounding background formed by second regions 5, which is substantially transparent and colourless. Each of the first regions 4 displays the same visible colour at any one viewing angle, but upon tilting, the visible colour of all of the first regions 4 appears to change, as will be described further below. It should be noted that the image I can be visualised by the human eye in standard, non-polarised visible light illumination, without the need for any equipment such as polarisers. However, depending on the scale of the image, magnifying means may be necessary for the naked eye to resolve the imagery. For example, the letters shown could be in the form of microtext.

Figure 1(b) shows a cross-section through the security device 1 along the line X-X' in a first of two alternative constructions. Here it will be seen that the security device comprises a substrate 2 and a cholesteric liquid crystal layer 3 (hereinafter a "LC layer") thereon. The substrate 2 may comprise for example a polymer foil such as PET, polyamide, polypropylene (preferably BOPP), polyethylene, polycarbonate or the like and is preferably visually transparent although alternative options are feasible as discussed below. It is also possible for an area of the substrate 2 to be transparent whilst another laterally offset area may be opaque or otherwise non-transparent. An example of a suitable cholesteric liquid crystal material which could be used to form layer 3 is a mixture of Paliocolor LC 242 and Paliocolor LC 756, both available from BASF SE (www.basf.com), and discussed further below.

The LC layer 3 is carried on a first surface 2a of the substrate 2 although it will be noted that this may not involve direct contact. For example, one or more intermediate layers such as a primer layer might exist between the substrate 2 and the LC layer 3. The pattern P is defined within the LC layer 3 and comprises a number of first regions 4 alongside second regions 5. In the first regions 4, the cholesteric liquid crystal layer 3 is ordered and arranged according to a spiral or helix. A schematic diagram illustrating such an arrangement is shown in Figure which depicts seven exemplary planes of liquid crystal molecules within the material, labelled 90 to 96. It will be seen that the orientation of the molecules within each respective layer rotates about the Z axis through 360° between layer and layer 96. The structure imparts an optically variable quality to the colour displayed in the first regions 4 with the result that a first observer 01 viewing the security device at a first viewing angle (shown here as the normal to the device) will perceive the first regions 4 as appearing in one colour, whereas a second observer 02 viewing the security device from a different viewing angle (here depicted as a), will perceive the same regions 4 as appearing in a different colour. Thus, the visible colour of each first region 4 varies upon tilting the device. The particular colour which will be displayed at each viewing angle and the degree to which it changes upon tilting is determined by the length of the helix shown in Figure 5 along the Z axis. This in turn can be controlled by appropriate selection of the composition of the LC layer 3.

In the second regions 5, the LC molecules are not arranged according to such a helix and are therefore disordered. Most preferably, they are arranged isotropically: that is, with substantially the same average distribution in all directions. As a result, the second regions 5 do not exhibit any significant visible colour or optically variable effect and are substantially colourless and transparent to the naked eye. Hence, the security device 1 as a whole appears to display the letters DLR in an optically variable visible colour against a transparent background. It should be noted that the letters DLR and their optically variable effect can be viewed from either side of the security device. Of course, if only an area of the substrate 2 were transparent, the letters could be viewed within that area from either side, but only from one side (that of observers 01 and 02) outside that area.

It should be noted that the arrangement of LC molecules throughout layer 3 is permanently fixed. Thus, the shape, size and position of the first and second regions 4, 5 cannot change.

Figure 1(c) shows the same security device 1 in cross-section with an alternative construction. The substrate 2 and LC layer 3 are the same as already described in relation to Figures 1(a) and 1(b). The only additions are layers 7 and 8. Layer 7 is a transparent protective layer applied to the outermost side of the LC layer 3. As described below, this is useful to protect the LC layer 3 from damage, particularly during manufacture. The protective layer 7 can comprise for example a transparent polymer foil or a layer of lacquer. Layer 8 is a visible light absorbing layer which may optionally be provided to improve the visibility of the optically variable effect exhibited by the first regions 4 of the LC layer 3. The visible light absorbing layer 8 may for example be a layer of dark-coloured ink (e.g. black) or a suitably dyed polymer foil. The visible light absorbing layer 8 absorbs transmitted light and stray reflections which might otherwise overwhelm the colour exhibited by the liquid crystals. As such, in this variant the security device 1 can only be viewed from its upper side (i.e. through protective layer 7) as the image I will not be visible through visible light absorbing layer 8. The letters DLR will now appear against a dark-coloured background corresponding to the colour of visible light absorbing layer 8. It should be noted that, as an alternative, layer 8 could be omitted from the security device 1 itself and the (transparent) device could be temporarily held against a black background or similar to ease viewing by the observer.

Another embodiment of a security device 1 will now be described with reference to Figure 2. Here, a plurality of LC layers are provided in order to allow more complex visual effects to be achieved. Figure 2(a) shows the security device 1 in plan view and here it will be seen that the image I comprises three interlocking circles labelled 4a, 4b and 4c. Each circle appears in a different respective colour and the overlapping portions 4' and 4" appear in two further colours formed by the mixing of the colours from the two adjacent circles. The various colours are visible simultaneously alongside one another and hence the image appears multi-coloured. Since each of the circles is formed by a different LC layer (each of the sort described above with reference to Figure 1), the particular visible colour it displays will vary with viewing angle. However, at at least one predetermined viewing angle, and preferably at all viewing angles, the three circles 4a, 4b and 4c will appear in different colours from one another.

A first possible construction of the security device 1 is shown in cross-section in Figure 2(b). Three overlapping LC layers 3a, 3b and 3c are disposed on first surface 2a of substrate 2. Each of the LC layers exhibits a corresponding pattern P1, P2, P3 formed of a first region 4a in the form of a circle, surrounded by a second region 5a which provides the background. The three circular first regions 4a, 4b, 4c are laterally offset from one another as shown and in each case the second region 5a, 5b, 5c extends across the remainder of the layer. In each of the first regions 4a, 4b, 4c, the LC molecules are ordered and arranged according to a helix as already described. As such, each first region 4a, 4b, 4c displays an optically variable visible colour and the combination thereof results in the image I visible in plan view.

Each of the LC layers 3a, 3b, 3c comprises a different cholesteric liquid crystal material and thus the visible colour displayed by each one of the first regions 4a, 4b, 4c is different, at least when the combination of layers is viewed at a predetermined viewing angle. Most preferably, this predetermined viewing angle is or includes the normal to the device so that the image appears multi-coloured from the normal viewing angle. More preferably, the colours of the three layers do not match one another at any viewing angle so that the image always appears multi-coloured irrespective of the tilt angle. Nonetheless, it will be appreciated that since the colour displayed by each region changes upon tilting the arrangement of the colours will not be the same at all viewing angles. For example, when viewed from the normal, the first regions 4a, 4b, 4c may appear red, green and blue respectively. However, when viewed from a different angle, the region 4a may appear blue, the region 4b may appear red and the region 4c may appear green.

The inner LC layers 3a and 3b will be viewed through the outermost LC layer 3c. Thus, some parts of layers 3a and 3b will be visible without modification through the transparent and colourless second region 5c of layer 3c. However, other parts of LC layers 3a and 3b will be viewed through the visibly coloured first region 4c of layer 3c. This results in the mixed colour region 4'. Similarly, the appearance of LC layer 3a will also be modified by LC layer 3b. The mixed colours visible in overlapping regions 4' and 4" will also vary upon tilting the device. Since the substrate 2 is transparent in this example, the image I can be viewed from either side of the device as indicated by observers 01 and 02 in Figure 2(b).

Figure 2(c) shows an alternative construction in cross-section which will produce the same visual effect as that already described. Here, the only difference relative to the construction shown in Figure 2(b) is that two of the LC layers 3a and 3b are provided on the first surface 2a of substrate 2 whilst the third LC layer 3c is provided on the second surface 2b of substrate 2. Again, since the substrate 2 is transparent, the appearance of the device from either side is the same as that shown in Figure 2(a) and discussed above.

Figure 2(d) shows a third alternative construction of the same security device in cross-section. In this case the arrangement of the LC layers 3a, 3b and 3c is the same as that described immediately above with reference to Figure 2(c).

However, in this case a number of additional layers are provided. Firstly, a separator layer 6 is provided between LC layers 3a and 3b. The separator layer 6 is transparent and acts to prevent mixing of the two LC materials. The separator layer 6 could comprise for example a polymer foil, e.g. PET or polyamide. The separator layer 6 could be of similar construction to the substrate 2, but is preferably thinner: for instance the substrate 2 might be around 12 microns thick whilst the separator layer 6 might be around 6 microns thick. Typically each LC layer 3a, 3b, 3c is around 1 to 2 microns thick. The layers 7a and 7b are protective layers located on the outer surfaces of the security device in order to protect the LC layers from damage. As previously described, the protective layers can also be polymer foils or could be formed by lacquers. Again, both are preferably transparent.

It should be noted that whilst in Figures 2(b), (c) and (d) the LC layers are described as fully overlapping one another, this is not essential. In other cases, the layers may only partially overlap one another and it is also possible for two or more different LC materials to be positioned alongside one another on the same surface.

Figure 3 shows a further embodiment of a security device 1 in accordance with the present invention. As shown in the plan view of Figure 3(a), here the image I which is formed of three overlapping patterns (P1, P2, P3) is a full colour image such as a passport photo. The security device is constructed in much the same way as discussed with respect to the Figure 2 embodiment and a schematic cross-section thereof is shown in Figure 3(b). Here, three LC layers 3a, 3b and 3c are provided overlapping one another on substrate 2, and in this case, all three layers are located on the same surface thereof as in the Figure 2(a) example. The separator layers 6a and 6b are located between the respective LC layers. A protective layer 7 is provided over the top of the outermost LC layer 3 and down the sides of the structure. Optionally, a visible light absorbing layer 8 may be provided on the opposite surface of substrate 2 but in other cases, a separate dark background could be used to help visualise the image if necessary.

As in previous embodiments, each of the LC layers 3a, 3b, 3c exhibits a visible pattern P1, P2, P3 formed of visibly coloured first regions 4a, 4b, 4c (which are also optically variable) alongside transparent colourless second regions 5a, 5b, 5c.

In this case, each of the patterns corresponds to a separate colour component of the desired image I. For example, the patterns P1, P2, P3, shown in Figure 3(c) could correspond to the red, green and blue channels of the image I, respectively. The three LC layers 3a, 3b, 3c have cholesteric liquid crystal compositions which in the ordered first regions 4a, 4b, 4c will display different respective colours from one another at least when the security device is viewed from the predetermined angle, and preferably at all viewing angles. Thus, for example, the first regions 4a of LC layer 3a may exhibit a red colour when viewed at the normal, the first regions 4b of LC layer 3b may exhibit a green colour when viewed at the normal and the first regions 4c of liquid crystal 3c may exhibit a blue colour when viewed from the normal. As explained above, the visible colour displayed by the pattern of each layer will change upon viewing angle and so the portrait I may appear in false colour versions thereof at some viewing angle ranges. This provides a distinctive and easily testable security effect.

Figure 4 is a photograph showing a further embodiment of a security device formed according to the same principles described with reference to Figure 3.

Here, the security device 1 is in the form of a strip displaying a full colour photographic style image of four butterflies against a background of grass and flowers. The particular set of colours displayed varies according to the viewing angle. The security device 1 is depicted affixed to the surface of a security document 100, here a banknote.

As mentioned above, the particular visible colour which is displayed by a cholesteric liquid crystal material in an ordered state at any particular viewing angle is determined by the dimensions of the helical arrangement of LC molecules, shown schematically in Figure 5. To achieve the multi-coloured effects described above, the respective LC layers are provided with cholesteric liquid crystal materials of different composition from one another which therefore have different helical dimensions. The dimensions of the helix and therefore the colours displayed at each viewing angle can be controlled or tuned by varying the composition of the material and in particular through the amount of chiral dopant added to the material.

Examples of suitable cholesteric liquid crystal materials which can be used in any of the embodiments disclosed here in (for example, from which each of LC layers 3a, 3b and 3c in Figure 2 or 3 can be made) include mixtures of Paliocolor LC 242 and Paliocolor LC 756, both available from BASF SE (see www.bastcorn), in different respective ratios. Paliocolor LC 242 is an example of a nematic liquid crystal, with the chemical name 4-[[[4-[(1-0xo-2-propeny1)-oxy]butoxy]carbonyl]oxypbenzoic acid 2-methyl-1,4-phenylene ester. Its molecular formula is C371-138014; and molecular weight 704.67. Paliocolor LC 756 is a chiral dopant, with the chemical name 1,4:3,6-Dianhydro-D-glucitol bis[4-[[4- [[[4-[(1-oxo-2-propeny1)-oxy]butoxy]carbonyl]oxy]benzoyl]oxy]benzoate]. Its molecular formula is C501-146020 and its molecular weight is 966,89 When the two materials are combined, the resulting chalestenc liquid crystal material exhibits an optically variable visible colour as already described. The particular colour which is exhibited at a certain viewing angle depends on the concentration of Pailocolour LC 756 in the composition. Table I lists some exemplary ratios of Paliocolor LC 242 and LC 756; and the corresponding resulting reflection colour (in terms of wavelength), when viewed aong the normal to the substrate:

Table

Paliocolor LC 242 Paliocolor LC 756 Reflection wavelength (weight %) (weight %) (nm, before curing) 96.89 3.11 745 96.42 3.58 682 95.53 4.47 523 95.24 4.76 490 94.95 50.5 457 It will be noted that the reflection wavelengths indicated are before curing.

Curing tends to cause a slight shrinkage of the material and a resulting shortening of the helix, which in turn causes a shift of the reflection wavelengths towards shorter wavelengths (10 to 30 nm depending on the wavelength). However, the addition of a photoinitiator (e.g. 2 to 3.5 weight %) to facilitate curing tends to have the opposite effect, slightly increasing the reflection wavelengths, and so the two effects cancel one another out to an extent. Examples of suitable photoinitiators which can be added to the materials include any of: 4,4'-Bis(dimethylamino)benzophenone (peak responsivity is around 365 nm); Benzophenone (around 260 nm); 2,2'Dimethoxy-2-phenylacetophenone (around 260 nm); or Camphorquinone (around 476 nm). The various components can be mixed together either using solvents and/or heat.

The above-described security devices can be manufactured using any available techniques which result in the structures shown. Some preferred methods by 25 which the security devices can be manufactured will now be described with reference to Figures 6 to 12.

A first embodiment of a method for manufacturing a security device is shown in the form of a flowchart in Figure 6 and corresponding manufacturing stages of an exemplary device in cross-section in Figure 7(a) to (d).

In a first step S101, a curable LC layer 3 comprising a cholesteric liquid crystal material is applied onto a substrate 2. The resulting structure is shown in Figure 7(a). At this stage, the liquid crystal molecules are arranged in an ordered, helical formation across the whole layer 3 and therefore display an optically variable colour across the whole substrate (or the portion thereof to which the layer 3 has been applied, if this is less than the whole substrate). The layer 3 can be applied by any suitable printing or coating means and may be an all-over layer or could be provided in some parts and not in others, potentially in accordance with a further pattern as desired. The LC layer 3 is typically applied in the form of a solution and is then dried (with or without the application of heat) to drive off the solvent, leaving the uncured LC material in a tacky state.

Generally it has been found that the LC molecules automatically become ordered upon application to appropriate substrate surfaces (e.g PET). However, if necessary extra steps may be taken to achieve alignment. Some application techniques have also been found to produce better alignment, including gravure printing and reverse slot die printing. In general, coating processes which use high shear forces are appropriate including all types of roll or reverse roll coaters, as well as a knife coater or knife reverse roll coater.

In the next step S103, the LC layer 3 is exposed to curing radiation R at a wavelength A to which the curable LC layer 3 is responsive. The LC layer 3 is selectively exposed to the radiation R in accordance with a pattern P so that only first regions 4 of the LC layer 3 are irradiated. The first regions 4 are at least partially cured (preferably fully cured) by the irradiation, thereby fixing the LC molecules in their ordered positions in the first region 4. The remainder of the layer 3 in which the LC material remains uncured forms a second region 5.

The patterned exposure can be carried out in a number of ways. In the example depicted, a mask 25 is utilised which comprises opaque regions and transparent regions defining the pattern P. In practice, such a mask could be supported on the circumference of a curing cylinder for example. The LC layer 3 is then exposed to a suitable radiation source through the mask 25. Alternatively, the first region 4 could be exposed using a directed radiation beam such as a laser beam, controlled by appropriate equipment in accordance with a pattern stored in memory or created "on the fly" by software.

Once the first region 4 is sufficiently cured so that the LC molecules therein will not lose their order upon heating (this can be tested by observing the first region 4 during the next step and checking that it does not change or lose its colour), in the next step S105 the structure is heated by exposure to a heat source H to an elevated temperature. The elevated temperature is sufficiently high that the LC molecules in the unexposed second region 5 become disordered. Most preferably, the elevated temperature is above the LC material's isotropic point and the LC molecules in the second region 5 become isotropically arranged. For instance, heating to about 80 degrees C has been found to achieve the necessary results in the LC materials mentioned herein.

With the assembly held at the elevated temperature, in step S107 the second region 5 of the LC layer 3 is then exposed to curing radiation R, again of the wavelength A, so as to be at least partially cured. In this step the entire LC layer 3 can, if desired, be exposed to the radiation R in which case the cure of the first region can be completed if necessary. It will be appreciated that, whilst in the present example, the two radiation steps are shown as being carried out at the same wavelength A, this is not essential and all that is necessary is that the LC materials 3 is responsive to the wavelength(s) used in both steps S103 and S107 respectively. Thus the same or different exposure apparatus could be used to carry out the two steps. If the LC layer 3 is not fully cured by the end of step S107, a further final curing step may be carried out later.

The result of the method described in relation to Figures 6 and 7 is a security device of the same sort described above with reference to Figures 1(a) and 1(b).

Additional layers such as protective layers and/or visible light absorbing layers can be added before or after the exposure and heating steps are carried out.

Two preferred approaches for forming security devices with multiple LC layers will now be described.

In a first preferred approach, of which an example is shown in Figure 8, the various LC layers are patterned and cured separately from one another and finally affixed to one another to form the security device. Thus, the process already described with reference to Figures 6 and 7 is effectively repeated for each one of the LC layers and the resulting structures are then joined together. The patterning of the respective LC layers can be carried out sequentially or in parallel. Figure 8 shows exemplary apparatus for implementing the latter option. Here, three substrates 2, 2' and 2" are conveyed on three parallel transport paths. An application station 10, 10' and 10" on each one, such as a print cylinder, is used to apply a respective LC material 3a, 3b and 3c onto the surface of each substrate 2, 2' and 2". As previously mentioned, there may be one or more intermediate layers between the substrate and the LC material which are not shown. Each LC layer 3a, 3b and 3c is then subject to a patterned exposure to curing radiation R which takes place at patterning stations 20, 20' and 20". In this example, each of the patterning stations comprises a laser operating at the same wavelength A and all of the curable materials 3a, 3b, 3c are responsive to that wavelength A. Patterning station 20 exposes first regions 4a of the LC layer 3a to radiation R in accordance with a first pattern P1.

Patterning station 20' exposes first regions 4b of the LC layer 3b to curing radiation R in accordance with a second pattern P2. The third patterning station 20" exposes first regions 4c of the LC layer 3c to radiation R in accordance with a third pattern P3. It will be appreciated that the various patterns P1, P2, P3 can be the same or different from one another depending on the nature of the image to be formed.

Next, the three LC layers 3a, 3b and 3c are each heated to an elevated temperature by heating units 30, 30' and 30" respectively. As previously described, this causes the LC molecules in the unexposed second regions 5a, 5b, 5c to become disordered and thus transparent and colourless. The LC layers 3a, 3b and 3c are then cured once again at curing units 40, 40' and 40" to fix the LC molecules in their disordered state in the second regions 5a, 5b, 5c resulting in the three patterned LC layers. The curing units 40, 40', 40" may simply comprise appropriate radiation sources since no patterning is necessary at this stage.

The three transport paths then converge bringing the layers together, preferably with their respective substrates 2, 2' and 2" arranged to alternate between the LC layers as shown. Thus, in this example, the substrates 2 and 2' perform the same function as the separator layers 6a and 6b mentioned in previous embodiments. If necessary, an optional final curing station 50 may be provided to complete the curing of one or more of the LC layers 3. This could be used for example to affix the layers together using any remaining inherent tackiness of the partially cured LC materials 3c and 3b to form a bond between them and the adjacent substrates 2 and 2'. Alternatively or in addition, adhesive could be introduced at the appropriate locations as the substrates are brought together. This will be necessary for instance if the LC layers are fully cured at the point at which they are brought together. The result is a security device of the sort shown in Figure 3b. A protective layer 7 and/or a visible light absorbing layer 8 can be added if desired.

Advantageously, the various patterned LC layers 3a, 3b, 3c will be affixed to one another in such a manner that their respective patterns P1, P2, P3 are registered to one another. If necessary, this can be achieved by providing registration marks as part of the respective patterns. A camera system can then be used to view and identify the registration marks and adjust the respective transport systems to align them before affixing.

In the example shown, the three patterning processes are carried out in parallel on separate substrates. However, this is not essential and if for example only one set of patterning apparatus 10, 20, 30, 40 is available, the various LC materials could be laid down sequentially, potentially on the same substrate, and the patterns formed one after the other. If the patterns of the different materials are formed on the same substrate they will need to be cut into lengths before they can be affixed in the overlapping manner shown. It should also be noted that whilst in the example shown the three LC materials are all responsive to the same radiation wavelength A, this is not essential and each of the curing stations 20, 40, 20', 40', 20" and 40" could be configured to deliver the appropriate different wavelengths. If a final cure is carried out as indicated at 50, this unit may emit all of the relevant curing wavelengths.

A second preferred approach for manufacturing a security device having multiple patterned LC layers will now be described with reference to Figures 9 and 10. Figure 9 is a flowchart depicting steps of the method and Figure 10 shows an exemplary security device at corresponding manufacturing stages. In a first step S201, a plurality of curable cholesteric liquid crystal layers is applied to a substrate, using any of the techniques described above. An example of the resulting structure is shown in Figure 10(a), which here comprises three LC layers 3a, 3b and 3c arranged on a first surface of substrate 2. The LC layers may have separator layers 6a and 6b between them and a protective layer 7 may also be applied at this point. The various LC layers 3a, 3b and 3c are each of different composition such that, as applied (with the LC molecules in their ordered state), at least at a predetermined viewing angle each one exhibits a different colour. It should be noted that these different colours may not be viewable from the assembly depicted in Figure 10(a) since each one is wholly overlapped by the others and therefore a single mixed colour will be visible across the whole component. For example, if LC layer 3a appears red, LC layer 3b appears green and LC layer 3c appears blue (all at the predetermined viewing angle) then the resulting structure may appear white at this stage.

The separator layers 6a and 6b are of particular use in this manufacturing technique since at this stage the LC layers 3a, 3b and 3c will be uncured and hence tacky. Without the separator layers 6a and 6b it is likely that there would be some mixing of the LC materials at the interfaces between them and hence the separator layers serve to prevent this. Similarly, without the protective layer 7, the outermost LC layer 3c in its tacky state will be particularly prone to damage. In addition to the different optical characteristics of the three respective LC layers 3a, 3b, 3c, in this embodiment the respective LC layers are also responsive to different wavelengths of curing radiation. This can be achieved by providing each of the curable materials with a different photo initiator which responds to a different wavelength. Of course in practice each curable material may be responsive to a waveband of radiation wavelengths and there may be some overlap between the wavebands of the different layers but importantly, for each layer there will be at least one wavelength which it will be responsive to and the other LC layers will not. The protective layer 7 and the separator layers 6a, 6b are transmissive to all the relevant curing wavelengths.

In the next step S203, the structure is exposed to patterned curing radiation R of three different wavelengths. This can be carried out simultaneously or as a sequence of sub-steps. The latter case will be shown here for clarity. Thus, in sub-step S203a, the assembly is exposed to a first wavelength Al (e.g. 345 nm) of curing radiation in accordance with a first pattern P1. Of the LC layers present, only LC layer 3a is responsive to radiation Al and therefore only regions of LC layer 3a become at least partially cured. The radiation Al is transmitted through the other layers without effect. In the next sub-step S203b, the assembly is exposed to a second wavelength of curing radiation A2 (e.g. 375 nm), in accordance with a second pattern P2. Only LC layer 3b is responsive to wavelength A2 and thus regions of that layer only are partially cured. Finally, in sub-step S203c, the assembly is exposed to a third wavelength of curing radiation A3 (e.g. 415 nm) in accordance with a third pattern P3. Only LC layer 3c is responsive to wavelength A3 and hence only regions thereof are partially cured.

It will be appreciated that the said exposure steps can be repeated for any number of LC layers and corresponding wavelengths.

Once all of the LC layers have been exposed to the respective patterns, the assembly is heated to an elevated temperature which as before is preferably above the isotropic point(s) of the plurality of LC layers 3a, 3b and 3c. As a result, in the uncured second regions 5a, 5b and 5c of each layer, the LC molecules become disordered and the layers become colourless and transparent. The result is shown in Figure 10(e).

Finally, in step S207 at the elevated temperature, the assembly is exposed to curing radiation which includes the wavelengths Al, A2 and A3 so as to cure all of the remaining LC material and thus fix the disordered LC molecules in place in the second regions 5a, 5b, 5c. The result is again a security device as shown in Figure 3(b).

In order to ensure accurate reproduction of the desired image, if the exposure sub-steps S203a, b and c are carried out sequentially, it is preferred that they are registered to one another to ensure correct relative positioning of the three patterns P1, P2, P3. Simultaneous exposure to the multiple wavelengths could be carried out using multiple laser beams, each of different wavelengths, or by providing different radiation sources and corresponding patterned masks on both sides of the substrate. It is also possible to use an array of radiation sources (e.g. LEDs) which are arranged in accordance with the various patterns.

Whilst in some cases the entire method of Figure 9 could be carried out by the same manufacturer, potentially in a continuous in-line process, this is not essential. Indeed, in some cases it will be preferable to form the complete assembly of LC layers on a substrate with any desired separator layers protective layers and/or light absorbing layers, and store the assembly in a blank form (without any patterning) for later use. The completed assembly is referred to as a security device component 70 and this is what is shown in Figure 10(a) (albeit without all of the optional components). For example, a security device assembly 70 of this sort could be provided blank by a manufacturer to a distribution centre such as a passport issuing office, at which location the remainder of the steps S203, S205 and S207 would be carried out. This is particularly suitable where the image I to be formed is personalised or individualised, e.g. taking the form of a passport photo or incorporating unique information such as a serial number or bibliographic information relating to the holder.

In the technique described above in relation to Figures 9 and 10, each of the LC layers 3a, 3b and 3c must be responsive to a different wavelength of curing radiation in order that they can be individually patterned. In practice, this can limit the number of layers which can be incorporated into the device since only a limited number of different wavelength radiation sources and/or photoinitiators may be available. As such, additional steps can be taken in order to use more than one LC layer which is responsive to the same wavelength of curing radiation. Examples of such embodiments are shown in Figures 11 and 12. In both cases, a radiation absorbing layer is provided which blocks the transmission of at least one of the relevant curing wavelengths (which may typically lie in the UV spectrum for example). By inserting such a radiation absorbing layer between two LC layers responsive to the same radiation wavelength, each one can be patterned individually from respective sides of the assembly. The radiation absorbing layer can be an extra layer provided in addition to those already mentioned, or its functionality could be incorporated into one or more of the layers already described.

In the Figure 11 embodiment, the substrate 2 itself forms the radiation absorbing layer and this can be achieved by forming the substrate 2 of a suitable material which is absorbent to the relevant wavelength(s). For instance, a UV absorbent additive may be added to a polymer forming the substrate 2. In this example, the substrate 2 is absorbent to at least the wavelength Al. First and second LC layers 3a and 3b are provided overlapping one another on first surface 2a of the substrate 2, whilst a third LC layer 3c is provided on the second surface 2b of substrate 2. A separator layer 6a is located between LC layers 3a and 3b and protective layers 7a and 7b are provided on the outermost surfaces of the LC layers 3b and 3c. To pattern the LC layers, curing radiation is applied from both sides of the substrate 2. This can be carried out sequentially or simultaneously.

In this example, a first radiation step indicated by R1 is carried out from the side of the first surface 2a of the substrate 2 during which LC layer 3a is exposed to a first pattern P1 of radiation wavelength Al and LC layer 3b is exposed to a second pattern P2 at a second radiation wavelength A2. In a second exposure step indicated by R2 from the other side of the substrate, the third LC layer 3c is exposed to a third pattern P3 at the first radiation wavelength Al (the same as that to which LC layer 3a is responsive). Thus only two different curing radiation wavelengths Aland A2 are needed in order to form independent patterns in three different LC layers 3a, 3b and 3c.

It should be noted that in this example while the substrate 2 is absorbent to the curing radiation wavelength Al (and optionally to all curing wavelengths) it is visibly transparent. As a result, a structure such as that shown in Figure 2(d) and having the appearance shown in Figure 2(a) can be formed by this method with the image I being visible from either side of the resulting device.

Figure 12 shows a further embodiment of a security device 1 with a similar construction. Here, again the substrate 2 acts as a radiation absorbing layer which in this case blocks at least three radiation wavelengths A1, A2 and A3. The layer is also in this case visually opaque and therefore doubles as a visible light absorbing layer improving the optically variable effect of the finished device. On the first surface 2a of the substrate 2, three overlapping LC layers 3a, 3b and 3c are provided, with separator layers 6a and 6b between them and a protective layer 7a overlying the stack. A similar sequence of three LC layers 3d, 3e and 3f is provided on the second surface 2b of the substrate with separator layers 6d and 6e between them and another protective layer 7b over the top. As in the example described with reference to Figures 9 and 10, each of the three LC layers 3a, 3b and 3c is responsive to a different curing radiation wavelength A1, A2 and A3 respectively. Likewise, the LC layers 3d, 3e and 3f are responsive to the same three wavelengths, respectively. The two sets of LC layers, one on each side of the substrate 2, can then be patterned using the same methods described in steps S203, S205 and S207 of Figure 9. Layers 3a, 3b and 3c are patterned in a first radiation step R1 from the side of the first surface 2a of the substrate, and the other LC layers 3d, 3e and 3f are patterned from the other side in a second radiation step R2.

In this case since the substrate 2 is opaque (e.g. black) the images formed on either side of the substrate are decoupled from one another and thus may be the same or different. Only one of the images is viewable from each side of the device.

In the examples described so far, the images formed have tended to be macroscale images which are visible to the naked eye without magnification.

For example, the images might be portraits, logos, alphanumeric text, passport photos or the like. However, this is not essential and the same techniques can be used to form microscale images which require spatial filtering and/or magnification in order to be resolvable to the human eye. The nature of the displayed image and its scale can be controlled straightforwardly by appropriate configuration of the exposure steps. Thus, in further preferred examples, the image may be microscopic, such as micro text or an image array used in devices such as moire magnifiers, lenticular devices, integral images and the like, the mechanisms of which were described previously.

Figure 13 depicts a further embodiment of a security device 1, which here is a lenticular device. A transparent substrate 2 is provided on one surface with an array of focussing elements 75, here in the form of cylindrical lenses, and on the other surface with an image array 76 formed of one or more patterned LC layers as described above. In this case, as shown best in Figure 16(b), two overlapping LC layers 3a and 3b are provided each carrying a respective pattern of first regions P1, P2. In this example, the size and shape of each first region in P2 of layer 3b is substantially identical, taking the form of elongate image strips lying substantially parallel to the axial direction of the focussing elements 75, which here is along the y-axis. The lateral extent of the array 76 (including the patterns P1 and P2) is referred to as the array area.

As shown best in the cross-section of Figure 13(b), the pattern formed in LC layer 3b and the focussing element array 75 have substantially the same periodicity as one another in the x-axis direction, such that one first region and one second region of pattern P2 lies under each lens 21. In this case, as is preferred, the width of each region is approximately half that of the lens pitch.

Thus approximately 50% of the array area carries optically variable coloured first regions and the other 50% corresponds to transparent second regions of layer 3b. In this example, the image array is registered to the lens array 75 in the x-axis direction (i.e. in the arrays' direction of periodicity) such that a coloured first region lies under the left half of each lens and a transparent second region lies under the right half. However, registration between the lens array 75 and the image array in the periodic dimension is not essential.

The other pattern P1 in LC layer 3a can take any form, including that of a complex image such as a photograph. Additional LC layers can be provided to provide further colours.

When the device is viewed by a first observer 01 from a first viewing angle, each lens 75a will direct light from its underlying coloured first region of pattern P2 to the observer, with the result that the device as a whole appears uniformly coloured (the colour varying upon tilting, e.g. about the x-axis), corresponding to the appearance of the optically variable first regions of LC layer 3b, as shown in Figure 13(c)(ii). This is referred to more generally as (first) image 11 since this amounts to a first image channel of the lenticular device. When the device is tilted so that it is viewed by second observer 02 from a second viewing angle, now each lens 75a directs light from the transparent second regions to the observer. As such the whole device will now appear to display the appearance of the LC layer 3a, which in this case carries a star shaped image as shown in Figure 13(c)(i) which constitutes a (second) image 12. Hence, as the security device is tilted back and forth between the positions of observer 01 and observer 02, the appearance of the device switches between image 1, and image 12.

In order to achieve an acceptably low thickness of the security device (e.g. around 70 microns or less where the device is to be formed on a transparent document substrate, such as a polymer banknote, or around 40 microns or less where the device is to be formed on a thread, foil or patch), the pitch of the lenses must also be around the same order of magnitude (e.g. 70 microns or 40 microns). Therefore the width of the pattern elements is preferably no more than half such dimensions, e.g. 35 microns or less.

Two-dimensional lenticular devices can also be formed, in which the optically variable effect is displayed as the device is tilted in either of two directions, preferably orthogonal directions. The focusing elements in this case will be spherical or aspherical, and arranged on a corresponding orthogonal grid, registered to the image array in terms of orientation but not necessarily in terms of translational position along the x or y-axes. If the pitch of the focussing elements is the same as that of the image array in both the x and y directions, the footprint of one focussing element will contain a 2 by 2 array of pattern elements. From an off-axis starting position, as the device is tilted left-right, the displayed image will switch as the different pattern elements are directed to the viewer, and likewise the same switch will be exhibited as the device is tilted up-down. If the pitch of the focusing elements is twice that of the image array, the image will switch multiple times as the device is tilted in any one direction.

Lenticular devices can also be formed in which the two or more images (or "channels") displayed by the device at different angles do not correspond exclusively to the first regions of an LC layer on one hand and the second regions of the LC layer on the other. Rather, both region types are used in combination to define sections of two or more images, interleaved with one another in a periodic manner. Thus, in an example the first regions may correspond to the black portions of a first image and those of a second image, whilst the second regions may provide the white portions of the same images, or vice versa. Of course the images need not be black and white but could be defined by any other pair of colours with sufficient contrast. Sections of the first and second images are interleaved with one another in a manner akin to the pattern of lines shown in Figure 13. When the device is tilted the two images will be displayed over different ranges of angles giving rise to a switching effect. More than two images could be interleaved in this way in order to achieve a wide range of animation, morphing, zooming effects etc. Another embodiment of a security device 1 incorporating an image array formed using the above methods will now be described with reference to Figure 14. In this case the security device is a moire magnifier, comprising an image array 86 formed using the methods described above defining an array of microimages and an overlapping focussing element array 85 with a pitch or rotational mismatch as necessary to achieve the moire effect. A cross-section through the device is shown in Figure 14(a) and it will be seen that focussing element array 85 is provided on the first side 2a of substrate 2, while the image array 86 formed by LC layers 3a and 3b is provided on the second surface 2b. In this example, the different LC materials 3a, 3b are not overlapping one another but rather are provided in laterally offset locations alongside one another on the same side of the substrate 86. Each has been patterned using the techniques described above to form an array of microimages.

Figure 14(b) depicts pad of the image element array 86 as it would appear without the overlapping focusing element array 85, i.e. the non-magnified microimage array (but shown at a greatly increased scale for clarity). In contrast, Figure 14(c) depicts the appearance of the same portion of the completed security device 1, i.e. the magnified microimages, seen when viewed with the overlapping focussing element array, at one viewing angle.

In this example, the microimage array 86 is formed using the methods described above and has a cross section corresponding substantially to that shown in Figure 1(b), but comprising two different LC materials in different portions of the device (this is not essential: a single LC material could be used instead). Figure 14(b) shows the patterned LC layers 3a, 3b and it will be seen that the first regions 4a, 4b form a regular array of microimages which here each convey the digit "5". In this case all of the microimages are of identical shape and size. The transparent second regions 5a, 5b form a contiguous, uniform background surrounding the microimages. Since the two LC materials are of different colour (at a predetermined viewing angle), the microimages in one half of the device appear in a first colour (here represented as black), whilst those in the other half appear in a second colour (here represented as grey).

Figure 14(c) shows the completed security device 30, i.e. the image element array 86 shown in Figure 14(b) plus an overlapping focusing element array 85, from a first viewing angle which here is approximately normal to the plane of the device 1. It should be noted that the security device is depicted at the same scale as used in Figure 14(b): the apparent enlargement is the effect of the focusing element array 85 now included. The moire effect acts to magnify the microimage array such that magnified versions of the microimages are displayed. In this example just two of the magnified microimages are shown. In practice, the size of the enlarged images and their orientation relative to the device will depend on the degree of mismatch between the focussing element array. This will be fixed once the focusing element array is joined to the image array. In this example, the first magnified microimage is formed from microimages all within the left half of the device and hence appears black whilst the second magnified microimage is from microimages all within the right half of the device and hence appears grey. Upon tilting the magnified microimages may appear to change colour since their position relative to the device will change and they may cross into the other half of the device where the microimages are of a different colour.

Security devices of the sorts described above are suitable for forming on security articles such as threads, stripes, patches, foils and the like which can then be incorporated into or applied onto security documents such as banknotes and examples of this will be provided further below. In such cases, the image may be manufactured on a first substrate, using the methods discussed above, and then transferred onto or affixed into a document substrate, optionally using a transparent adhesive. This may be achieved by foil stamping, for example. Alternatively, the security device can also be constructed directly on security documents which are formed of a suitable document substrate, such as polymer banknotes. For example, the image could be formed directly on the document substrate by applying the LC layer(s) to the surface of the document substrate (optionally across selected portions only), and performing the above-described method on the document substrate to form an image thereon. Whether the image is formed directly on the document substrate or applied in the form of a security article, if desired, a focusing element array can be applied to the opposite side of the document substrate, e.g. by transfer, embossing or cast-curing, before or after the image is applied, to form security devices such as those shown in Figures 13 and 14.

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

Such security articles can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 6mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.

The security article may be incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate at at least one window of the document. Methods of incorporating security elements in such a manner are described in EP-A-1141480 and WO-A-03054297. In the method described in EP-A-1141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

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

The security device may also be applied to one side of a paper substrate, optionally so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.

Examples of such documents of value and techniques for incorporating a security device will now be described with reference to Figures 15 to 18.

Figure 15 depicts an exemplary document of value 100, here in the form of a banknote. Figure 15a shows the banknote in plan view whilst Figure 15b shows a cross-section of the same banknote along the lines Q-Q'. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 101. Two opacifying layers 102a and 102b are applied to either side of the transparent substrate 101, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 101.

The opacifying layers 102a and 102b are omitted across a selected region 105 forming a window within which a security device is located. In Figure 15(b), the security device 1 is disposed within window 105, with one or more LC layers arranged on the surface(s) of substrate 101 (e.g. as in any of Figures 1 to 3 above). It will be appreciated that, if desired, the window 105 could instead be a "half-window", in which one of the opacifying layers (e.g. 102a or 102b) is continued over all or part of the window 105. Depending on the opacity of the opacifying layers, the half-window region will tend to appear translucent relative to surrounding areas in which opacifying layers 102a, 102b are provided on both sides.

In Figure 16 the banknote 100 is a conventional paper-based banknote provided with a security article 110 in the form of a security thread, which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 112 lie on either side of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread 110 in window regions 111 of the banknote. Alternatively the window regions 111 may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. It should be noted that it is not necessary for the window regions 111 to be "full thickness" windows: the thread 110 need only be exposed on one surface if preferred. The security device 1 is formed on the thread 110, which comprises a substrate carrying one or more patterned LC layers thereon. Windows 111 reveal parts of the device, which may be formed continuously along the thread. Alternatively several security devices could be spaced from each other along the thread, with different or identical images displayed by each.

In Figure 17, the banknote 100 is again a conventional paper-based banknote, provided with a strip element or insert 115. The strip 115 is based on a transparent substrate and is inserted between two plies of paper 117a and 117b.

The exemplary security device here is formed by partially overlapping LC layers disposed on each side of the strip substrate. The paper plies 117a and 117b are apertured across region 116 to reveal the security device, which in this case may be present across the whole of the strip 115 or could be localised within the aperture region 116. It should be noted that the ply 117b need not be apertured and could be continuous across the security device.

A further embodiment is shown in Figure 18 where Figures 18(a) and (b) show the front and rear sides of the document 100 respectively, and Figure 18(c) is a cross section along line Q-Q'. Security article 120 is a strip or band comprising a security device according to any of the embodiments described above. The security article 120 is formed into a security document 100 comprising a fibrous substrate 125, using a method described in EP-A-1141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (Figure 18(a)) and exposed in one or more windows 121 on the opposite side of the document (Figure 18(b)). Again, the security device is formed on the strip 120, which comprises a substrate with one or more patterned LC layers disposed thereon.

Alternatively a similar construction can be achieved by providing paper 120 with an aperture 121 and adhering the strip element 120 onto one side of the paper across the aperture 121. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting.

In still further embodiments, a complete security device could be formed entirely on one surface of a security document which could be transparent, translucent or opaque, e.g. a paper banknote irrespective of any window region, or a passport page. For example a security device 1 such as that shown in Figure 3 or a security device component 70 such as that shown in Figure 10(a) can be affixed to the surface of the document substrate, e.g. by adhesive or hot or cold stamping. If the document substrate is opaque the security device will of course only be visible from one side of the document.

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

The presence of a visible light absorbing layer 8 in the security device can be used to conceal the presence of a machine readable dark magnetic layer, or the visible light absorbing layer 8 itself could be magnetic (in whole or in part). When a magnetic material is incorporated into the device the magnetic material can be applied in any design but common examples include the use of magnetic tramlines or the use of magnetic blocks to form a coded structure. Suitable magnetic materials include iron oxide pigments (Fe2O3 or Fe304), barium or strontium ferrites, iron, nickel, cobalt and alloys of these. In this context the term "alloy" includes materials such as Nickel:Cobalt, Iron:Aluminium:Nickel:Cobalt and the like. Flake Nickel materials can be used; in addition Iron flake materials are suitable. Typical nickel flakes have lateral dimensions in the range 5-50 microns and a thickness less than 2 microns. Typical iron flakes have lateral dimensions in the range 10-30 microns and a thickness less than 2 microns.

In an alternative machine-readable embodiment a transparent magnetic layer can be incorporated at any position within the device structure. Suitable transparent magnetic layers containing a distribution of particles of a magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent are described in W003091953 and W003091952.

Claims

CLAIMS1. A security device comprising at least one cholesteric liquid crystal layer disposed on a substrate, the at least one cholesteric liquid crystal layer collectively exhibiting an image detectable by the naked eye under non-polarised visible light illumination, the or each cholesteric liquid crystal layer having a respective pattern of one or more first regions of the layer in which the cholesteric liquid crystal is ordered and thereby exhibits an optically variable visible colour, alongside one or more second regions of the layer in which the cholesteric liquid crystal is disordered and thereby is substantially transparent and colourless in the visible spectrum, the combination of the one or more respective patterns forming the image.
2. A security device according to claim 1, wherein the at least one cholesteric liquid crystal layer comprises a plurality of cholesteric liquid crystal layers, each formed of a different cholesteric liquid crystal material and exhibiting an optically variable visible colour, wherein at least when the security device is viewed at a predetermined viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) in the one or more first regions thereof, wherein the predetermined viewing angle is preferably the normal to the substrate.
3. A security device according to claim 2, wherein at every viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) in the one or more first regions thereof at the same viewing angle.
4. A security device according to claim 2 or 3, wherein each of the different cholesteric liquid crystal materials comprises liquid crystal molecules arranged in accordance with a helix, the pitch of the helix being different in each of the different cholesteric liquid crystal materials thereby giving rise to the different respective visible colours exhibited by the cholesteric liquid crystal layers when viewed at the predetermined viewing angle.
5. A security device according to any of claims 2 to 4, wherein the plurality of cholesteric liquid crystal layers include a first cholesteric liquid crystal layer of which the first region(s) appear red under white light illumination at the predetermined viewing angle, a second cholesteric liquid crystal layer of which the first region(s) appear green under white light illumination at the predetermined viewing angle, and a third cholesteric liquid crystal layer of which the first region(s) appear blue under white light illumination at the predetermined viewing angle.
6. A security device according to any of claims 2 to 5, wherein at least some, preferably all, of the plurality of cholesteric liquid crystal layers at least partially overlap one another.
7. A security device according to any of claims 2 to 6, wherein at least some, preferably all, of the plurality of cholesteric liquid crystal layers are disposed on a first side of the substrate.
8. A security device according to any of claims 2 to 7, wherein at least an area of the substrate is transparent, at least one of the plurality of cholesteric liquid crystal layers is disposed on a first side of the substrate in the transparent area and at least one of the plurality of cholesteric liquid crystal layers is disposed on a second side of the substrate, preferably in the transparent area.
9. A security device according to any of claims 2 to 8, wherein the respective patterns of the plurality of cholesteric liquid crystal layers are registered to one another.
10. A security device according to any of claims 2 to 9, further comprising a visually transparent spacer layer between each of the plurality of cholesteric liquid crystal layers.
11. A security device according to any of the preceding claims, further comprising at least one visually transparent protective layer, the at least one cholesteric liquid crystal layer being located between the visually transparent protective layer(s) and the substrate.
12. A security device according to any of the preceding claims, wherein the or each cholesteric liquid crystal layer is a cured radiation-curable material comprising a photoinitiator.
13. A security device according to claim 12 when dependent on at least claim 2, wherein at least some of the cholesteric liquid crystal layers comprise respective different photoinitiators responsive to different wavelengths of radiation.
14. A security device according to claim 12 or 13, further comprising a radiation-absorbent layer configured to absorb radiation wavelength(s) to which one or more of the photoinitiator(s) are responsive, wherein preferably the radiation-absorbing layer is the substrate.
15. A security device according to any of the preceding claims, further comprising a visible light absorbing layer, wherein preferably the visible light absorbing layer is the substrate.
16. A security device according to any of the preceding claims, wherein the image is a macro-scale image such as a portrait, passport photograph, logo, fingerprint, iris scan, alphanumeric character(s) or other graphic resolvable by the naked eye, preferably a full colour macro-scale image.
17. A security device according to any of claims 1 to 15, wherein the image is a microscopic image, such as microtext or an array of microimages or other image elements, not resolvable by the naked eye in the absence of magnification and/or filtering.
18. A security device according to any of the preceding claims wherein the image is an image array and the security device further comprises a viewing component configured to cooperate with the image array to generate an optically variable effect, the viewing component preferably comprising an array of focusing elements or a viewing screen.
19. A method of making a security device, comprising: (a) applying a radiation-curable cholesteric liquid crystal material on a first surface of a substrate to form a cholesteric liquid crystal layer across at least a portion thereof, such that the cholesteric liquid crystal is ordered and exhibits an optically variable visible colour across the portion; (b) exposing one or more first regions of the portion, selected in accordance with a pattern, to radiation to thereby at least partially cure the first region(s) and fix the cholesteric liquid crystal in its ordered state therein; (c) heating the radiation-curable cholesteric liquid crystal layer across the portion to an elevated temperature so as to disorder the cholesteric liquid crystal in one or more selected second regions of the portion, the one or more selected second regions corresponding to those parts of the portion not exposed to radiation in step (b), whereupon the second region(s) become substantially transparent and visibly colourless; and (d) at the elevated temperature, exposing at least the second region(s) of the radiation-curable cholesteric liquid crystal layer to radiation to thereby at least partially cure the second region(s) and fix the cholesteric liquid crystal in its disordered state therein; whereby a pattern of first region(s) exhibiting the optically variable visible colour alongside substantially transparent and colourless second region(s) is formed, the pattern forming all or part of an image detectable by the naked eye under non-polarised visible light illumination.
20. A method according to claim 19, further comprising: repeating steps (a) to (d) one or more times using different radiation-curable cholesteric liquid crystal material(s) on respective substrate(s), resulting in a plurality of cholesteric liquid crystal layers each disposed on a respective substrate and each exhibiting a respective pattern of first region(s) in an optically variable visible colour, wherein at least when the security device is viewed at a predetermined viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) in the one or more first regions thereof, wherein the predetermined viewing angle is preferably the normal to the substrate; and then affixing the plurality of cholesteric liquid crystal layers and respective substrates together, the image being formed by the plurality of respective patterns in combination, wherein preferably the respective patterns are different from one another.
21. A method according to claim 20, wherein the plurality of cholesteric liquid crystal layers and respective substrates are affixed to one another such that at least some, preferably all of, the respective patterns at least partially overlap one another.
22. A method according to claim 20 or claim 21, wherein the plurality of cholesteric liquid crystal layers and respective substrates are affixed to one another in a registered manner.
23. A method according to any of claims 20 to 22, wherein the plurality of cholesteric liquid crystal layers and respective substrates are affixed to one another in a stacked manner with the substrates lying in between the cholesteric liquid crystal layers.
24. A method according to any of claims 20 to 23, wherein one or more, preferably all, of the substrates are substantially visibly transparent.
25. A method according to claim 19, wherein step (a) comprises applying a plurality of different radiation-curable cholesteric liquid crystal materials to first and/or second surface(s) of the substrate to form a corresponding plurality of cholesteric liquid crystal layers across at least a portion of the substrate, such that the cholesteric liquid crystal of each layer is ordered and exhibits an optically variable visible colour, wherein at least when the security device is viewed along a predetermined viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s), wherein the predetermined viewing angle is preferably the normal to the substrate.
26. A method according to claim 25, wherein in step (a) the plurality of different radiation-curable cholesteric liquid crystal materials are applied so as to at least partially overlap one another.
27. A method according to claim 26, wherein step (a) further comprises applying a separator layer between each of the overlapping cholesteric liquid crystal layers, the or each separator layer preferably being visibly transparent.
28. A method according to any of claims 25 to 27, wherein at least some of the different radiation-curable cholesteric liquid crystal layers comprise respective photoinitiators which are responsive to different wavelengths of 20 radiation.
29. A method according to claim 28, wherein step (b) comprises exposing the plurality of overlapping cholesteric liquid crystal layers to a corresponding plurality of different wavelengths of radiation, each in accordance with a respective pattern of selected first regions, the image being formed by the plurality of respective patterns in combination, wherein preferably the respective patterns are different from one another.
30. A method according to any of claims 25 to 29, wherein step (d) comprises simultaneously exposing the plurality of overlapping cholesteric liquid crystal layers to a corresponding plurality of different wavelengths of radiation, preferably over the whole portion of the substrate.
31. A method according to any of claims 25 to 30, further comprising providing a radiation-absorbent layer which is configured to absorb radiation wavelength(s) to which one or more of the radiation-curable cholesteric liquid crystal materials are responsive, wherein preferably the radiation-absorbing layer is the substrate.
32. A method according to claim 31, wherein each of the cholesteric liquid crystal layers provided on one side of the radiation-absorbent layer are responsive to different radiation wavelengths from one another.
33. A method according to claim 31 or 32, wherein at least one of the cholesteric liquid crystal layers is provided on each side of the radiation-absorbent layer and in step (b) the respective liquid crystal layers are exposed from different corresponding sides of the radiation-absorbent layer.
34. A method according to any of claims 19 to 33, wherein in step (a), the or each radiation-curable cholesteric liquid crystal material is applied by printing or coating, preferably gravure printing or reverse slot die printing.
35. A method according to any of claims 19 to 34, wherein in step (a), the or each radiation-curable cholesteric liquid crystal material is applied as a solution thereof and then solvent is dried off leaving the material in a tacky state.
36. A method according to any of claims 19 to 35, wherein in step (b), the or each cholesteric liquid crystal layer is exposed to radiation by directing a beam of radiation, preferably a laser beam, over the selected first region(s) or by exposure to a radiation source via a mask which defines the selected first region(s) in accordance with the pattern.
37. A method according to any of claims 19 to 36, wherein in step (b), the first region(s) of the cholesteric liquid crystal material are sufficiently cured such that they exhibit substantially no colour change upon heating, preferably fully cured.
38. A method according to any of claims 19 to 37, wherein at every viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) in the one or more first regions thereof at the same viewing angle.
39. A method according to any of claims 20 to 38, wherein each of the different cholesteric liquid crystal materials applied in step (a) comprises liquid crystal molecules arranged in accordance with a helix, the pitch of the helix being different in each of the different cholesteric liquid crystal materials thereby giving rise to the different respective visible colours exhibited by the cholesteric liquid crystal layers when viewed at the predetermined viewing angle.
40. A method according to any of claims 20 to 39, wherein the plurality of cholesteric liquid crystal layers include a first cholesteric liquid crystal layer of which the first region(s) appear red under white light illumination at the predetermined viewing angle, a second cholesteric liquid crystal layer of which the first region(s) appear green under white light illumination at a predetermined viewing angle, and a third cholesteric liquid crystal layer of which the first region(s) appear blue under white light illumination at a predetermined viewing angle.
41. A method according to any of claims 19 to 40, wherein the image is a macro-scale image such as a portrait, passport photograph, logo, fingerprint, iris scan, alphanumeric character(s) or other graphic resolvable by the naked eye, preferably a full colour macro-scale image.
42. A method according to any of claims 19 to 40, wherein the image is a microscopic image, such as microtext or an array of microimages or other image elements, not resolvable by the naked eye in the absence of magnification and/or filtering.
43. A method according to any of claims 19 to 42, wherein the image is an image array and the method further comprises providing a viewing component configured to cooperate with the image array to generate an optically variable effect, the viewing component preferably comprising an array of focusing elements or a viewing screen.
44. A security device made in accordance with any of claims 19 to 43.
45. A security device component comprising a plurality of curable cholesteric liquid crystal layers disposed on a substrate, each formed of a different radiation-curable cholesteric liquid crystal material and exhibiting an optically variable visible colour, wherein at least when the security device component is viewed at a predetermined viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that of the other cholesteric liquid crystal layer(s), at least some of the plurality of curable cholesteric liquid crystal layers at least partially overlapping one another, wherein at least some of the different radiation-curable cholesteric liquid crystal materials comprise respective photoinitiators which are responsive to different wavelengths of radiation.
46. A security device component according to claim 45, wherein the predetermined viewing angle is the normal to the substrate.
47. A security device component according to claim 45 or 46, wherein at every viewing angle, the visible colour exhibited by each one of the cholesteric liquid crystal layers is different from that exhibited by each of the other cholesteric liquid crystal layer(s) at the same viewing angle.
48. A security device component according to any of claims 45 to 47, wherein each of the different cholesteric liquid crystal materials comprises liquid crystal molecules arranged in accordance with a helix, the pitch of the helix being different in each of the different cholesteric liquid crystal materials thereby giving rise to the different respective visible colours exhibited by the cholesteric liquid crystal layers when viewed at the predetermined viewing angle.
49. A security device component according to any of claims 45 to 48 further comprising a separator layer between each of the overlapping cholesteric liquid crystal layers, the or each separator layer preferably being visibly transparent. 5
50. A security device component according to any of claims 45 to 49, further comprising a radiation-absorbent layer which is configured to absorb radiation wavelength(s) to which one or more of the radiation-curable cholesteric liquid crystal materials are responsive, wherein preferably the radiation-absorbing layer is the substrate.
51. A security device component according to claim 50, wherein each of the cholesteric liquid crystal layers disposed on one side of the radiation-absorbent layer are responsive to different radiation wavelengths from one another.
52. A security device component according to claim 50 or 51, wherein at least one of the cholesteric liquid crystal layers is disposed on each side of the radiation-absorbent layer.
53. A security article comprising a security device according to any of claims 1 to 18 or 44, or a security device component according to any of claims 45 to 52, the security article preferably comprising a security thread, strip, insert, patch or foil.
54. A security document comprising a security device according to any of claims 1 to 19 or 44, a security device component according to any of claims 45 to 52 or a security article according to claim 53, the security document preferably comprising a banknote, passport, identification card, bank card, visa, certificate, stamp or cheque.