WO2018172764A1

WO2018172764A1 - Methods of manufacturing security devices and image arrays therefor

Info

Publication number: WO2018172764A1
Application number: PCT/GB2018/050725
Authority: WO
Inventors: Brian William Holmes; John Godfrey
Original assignee: De La Rue International Limited
Priority date: 2017-03-22
Filing date: 2018-03-21
Publication date: 2018-09-27
Also published as: GB2562592B; EP3600908A1; GB2562592A; GB201704495D0; GB201804470D0

Abstract

A method of manufacturing an image array for a security device is provided, comprising the steps of: (a) forming a surface relief structure (12) in the surface of a curable material (11) disposed on a substrate (10), the surface relief structure comprising an arrangement of elevations (12a) and depressions (12b), the elevations defining a pattern corresponding to the desired image array, and curing the curable material to fix the surface relief structure; (b) applying a soluble material (13) to the surface relief structure such that said soluble material is received in the depressions; (c) applying a film material (15) to the surface relief structure such that said film material covers the surface of the cured material, coating the elevations and the soluble material in the depressions, wherein the film material is visually distinguishable from the cured material forming the surface relief structure; and (d) removing the soluble material by exposure to a solvent suitable for removing the soluble material; wherein removal of the soluble material using the solvent causes the removal of the film material from the upper surface of the surface relief structure in regions corresponding to the depressions, and not in regions corresponding to the elevations, the film material thereby forming an image array in accordance with the pattern.

Description

METHODS OF MANUFACTURING SECURITY DEVICES AND IMAGE

ARRAYS THEREFOR

This invention relates to methods of manufacturing image arrays for 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.

Moire magnifier devices (examples of which are described in EP-A-1695121 , WO-A-94/27254, WO-A-201 1/107782 and WO201 1/107783) 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).

Moire 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-A- 201 1/051669, 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 WO2015/01 1493 and WO2015/01 1494 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.

Security devices such as moire magnifiers, integral imaging devices and lenticular devices, as well as others such as Venetian blind type devices (which utilise a masking grid in place of focusing elements) and moire interference devices depend for their success significantly on the resolution with which the image array (defining for example microimages, interleaved image sections or line patterns) can be formed. Since the security device must be thin in order to be incorporated into a document such as a banknote, any focusing elements required must also be thin, which by their nature also limits their lateral dimensions. For example, lenses used in such security elements preferably have a width or diameter of 50 microns or less, e.g. 30 microns. In a lenticular device this leads to the requirement that each image element must have a width which is at most half the lens width. For example, in a "two channel" lenticular switch device which displays only two images (one across a first range of viewing angles and the other across the remaining viewing angles), where the lenses are of 30 micron width, each image section must have a width of 15 microns or less. More complicated lenticular effects such as animation, motion or 3D effects usually require more than two interlaced images and hence each section needs to be even finer in order to fit all of the image sections into the optical footprint of each lens. For instance, in a "six channel" device with six interlaced images, where the lenses are of 30 micron width, each image section must have a width of 5 microns or less.

Similarly high-resolution image elements are also required in moire magnifiers and integral imaging devices since approximately one microimage must be provided for each focusing element and again this means in effect that each microimage must be formed within a small area of e.g. 30 by 30 microns. In order for the microimage to carry any detail, fine linewidths of 5 microns or less are therefore highly desirable.

The same is true for many security devices which do not make use of focusing elements, e.g. Venetian blind devices and moire interference devices which rely on the parallax effect caused when two sets of elements on different planes are viewed in combination from different angles. In order to perceive a change in visual appearance upon tilting over acceptable angles, the aspect ratio of the spacing between the planes (which is limited by the thickness of the device) to the spacing between image elements must be high. This in practice requires the image elements to be formed at high resolution to avoid the need for an overly thick device.

Typical processes used to manufacture image elements for security devices are based on printing and include intaglio, gravure, wet lithographic printing as well as dry lithographic printing. The achievable resolution is limited by several factors, including the viscosity, wettability and chemistry of the ink, as well as the surface energy, unevenness and wicking ability of the substrate, all of which lead to ink spreading. With careful design and implementation, such techniques can be used to print pattern elements with a line width of between 25 pm and 50 pm. For example, with gravure or wet lithographic printing it is possible to achieve line widths down to about 15 pm.

One approach which has been put forward as an alternative to the printing techniques mentioned above is used in the so-called Unison Motion™ product by Nanoventions Holdings LLC, as mentioned for example in WO-A- 2005052650. This involves creating pattern elements ("icon elements") as recesses in a substrate surface before spreading ink over the surface and then scraping off excess ink with a doctor blade. The resulting inked recesses can be produced with line widths of the order of 2 pm to 3 pm. This high resolution produces a very good visual effect, but the process is complex and expensive. Further, limits are placed on the minimum substrate thickness by the requirement to carry recesses in its surface. Alternative methods for producing such high resolution image elements would be highly desirable. The present invention provides a method of manufacturing an image array for a security device, comprising the steps of:

(a) forming a surface relief structure in the surface of a curable material disposed on a substrate, the surface relief structure comprising an arrangement of elevations and depressions, the elevations defining a pattern corresponding to the desired image array, and curing the curable material to fix the surface relief structure;

(b) applying a soluble material to the surface relief structure such that said soluble material is received in the depressions;

(c) applying a film material to the surface relief structure such that said film material covers the surface of the cured material, coating the elevations and the soluble material in the depressions, wherein the film material is visually distinguishable from the cured material forming the surface relief structure; and

(d) removing the soluble material by exposure to a solvent suitable for removing the soluble material;

wherein removal of the soluble material using the solvent causes the removal of the film material from the upper surface of the surface relief structure in regions corresponding to the depressions, and not in regions corresponding to the elevations, the film material thereby forming an image array in accordance with the pattern.

The visibility of the resulting image array results from the visual distinction between the remaining portions of the film material and the intervening parts of the cured material forming the depressions of the surface relief structure. This method has the advantage that the achievable resolution of the image array is not dependent on the accuracy with which the materials ultimately forming the image array (the film material and the curable material) can be applied, but rather on the resolution of the surface relief formed in step (a). Techniques for accurately producing finely structured surface reliefs are known and do not suffer from problems such as ink spreading as encountered during printing processes and the like. The use of a curable material for forming the surface relief therein achieves particularly accurate reproduction of the desired relief structure since the viscosity of the material can be configured to be low during forming so as to conform to the desired contours and subsequently increased to fix the relief.

The depressions of the surface relief structure can be filled with the soluble material using a non-selective application process, and likewise the film material can be applied all over without any initial patterning. The layout of the surface relief structure will then define which portions of the film material are removed by the soluble material and hence the configuration of the finished image array. By making use of the depressions to control the location of the soluble material and using the soluble material to pattern the overlying film material, the image array can be accurately formed with its pattern elements corresponding exactly to the elevations of the surface relief (and the spaces between the pattern elements to the depressions). Pattern elements in the finished array with line widths of 10 microns or less, and even 5 microns or less are achievable.

Any method of forming the curable material to the desired surface relief could be utilised, but cast-curing techniques are preferred. Hence, preferably, step (a) comprises:

(a1 ) providing a casting tool carrying a casting relief structure corresponding to the surface relief structure;

(a2) applying the curable material either to the substrate or to the casting tool;

(a3) forming the curable material with the casting tool;

(a4) curing the curable material so as to retain the surface relief structure therein, in one or more curing steps; and

(a5) before, during or after step (a4), removing the curable material from the casting tool whereby the cured material, formed according to the surface relief structure, is retained on the substrate. It will be noted that the curable material could be applied directly to the substrate and then brought in contact with the casting tool, or directly to the casting tool and then brought in contact with the substrate. It would also be possible to utilise two different curable materials (e.g. of different colours) in different regions of the surface relief structure if desired in order to introduce additional complexity to the image array. In order to enable continuous production of the image array, the casting tool typically comprises a cylinder having the casting relief structure disposed on an outer surface thereof. The cylinder may be transparent to appropriate curing energy such as UV radiation, e.g. formed of quartz.

The substrate could be opaque or translucent but is preferably at least semi- transparent in the visible spectrum. For instance the substrate could be a paper or paper-polymer hybrid substrate, but more preferably comprises one or more polymer materials. Suitable materials include polypropylene (PP) (most preferably bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof. The substrate may be monolithic, e.g. formed from a single one of the above materials, or multi-layered, e.g. having multiple layers of the same type of polymer (optionally with different orientations) or layers of different polymer types. The substrate may be of a type suitable for forming the basis of a security article such as a security thread, strip, patch or transfer foil (typically having thicknesses of between 30 and 70 microns), or of a type suitable for forming the basis of a security document itself, such as a polymer banknote (typically having thicknesses of between 70 and 200 microns). The substrate may include additional layer(s), such as a primer layer underlying the curable material for improving retention thereof. Advantageously, the curable material comprises a curable polymeric material. The curable material could be curable by thermal means, i.e. heating, but preferably is a radiation-curable material, such as a UV-curable material. The visual distinction between the remaining portions of the film material and the cured material can be achieved in a number of different ways. Preferably, the curable material is at least semi-transparent in the visible spectrum. By "at least semi-transparent", here and elsewhere in this disclosure, it is meant that the material is optically clear, i.e. causing substantially no optical scattering such that objects can be viewed therethrough. However, the material may optionally carry a coloured tint. Advantageously, the film material is of a higher optical density than that of the curable material and most preferably is substantially opaque across the visible spectrum. If the curable material is at least semi- transparent then it is preferred that the underlying substrate is also at least semi- transparent to ensure a visual distinction between the retained portions of the film material and the spaces between them where the substrate will be revealed through the cured material. This also enables the image array to be viewed from either side. Alternatively the substrate could be non-transparent (e.g. opaque or translucent) if it has an appearance (e.g. colour) different from the film material, although in this case it will be possible to view the image array from the film material side only, at least in reflected light. Generally, it is desirable for the film material to exhibit a visual contrast relative to the curable material, and in other cases this can be achieved by the film material and the curable material being of different visible colours (in which case it is not essential for them to have different optical densities). For instance, the curable material could be white or another light colour, and the film material could be black or another dark colour. If at least the curable material is translucent, then the image array will be visible from the film material side in reflected light and from either side in transmitted light, and if the curable material is opaque, then the image array will only be visible from the film material side in reflected light (and not in transmission).

The film material could be of various compositions but preferably exhibits a uniform appearance (e.g. colour) across its whole area, at least when viewed from any one viewing angle. In particularly preferred embodiments, the film material is a metal or alloy film comprising at least one metal or alloy, preferably aluminium, copper, nickel or chrome. The use of such materials enables high optical density to be achieved whilst keeping the thickness of the film low. In a particularly preferred embodiment, the film material comprises a multi-layer interference film configured to reflect different wavelength(s) of light at different angles of view. All-dielectric interference layer stacks could be used but metal- dielectric stacks are preferred due to their higher optical density. In still further embodiments, the film material may comprise an ink, preferably a metallic and/or opaque ink. It is also possible for the film material to comprise two or more materials overlapping one another in a multi-layered structure. For instance, the film material could comprise a metal layer overlaid with an ink layer. The ink layer may be configured to provide colour and/or to reduce the intensity of specular reflections from the metal. Depending on the materials involved, it may be desirable to carry out a surface treatment step to improve retention of the soluble material and/or the film material on the surface relief before they are applied. Thus, in preferred embodiments, after step (a) and before step (b), the surface relief structure may be treated to improve retention of materials thereon. This could be achieved in various different ways, such as by corona treatment of the surface relief structure or by the application of a primer layer. In a particularly preferred embodiment, this step may comprise the application of a conformal coating layer comprising a material which promotes adhesion between the soluble material and/or film material on one hand, and the (cured) curable material carrying the surface relief on the other. For instance, such a conformal coating (meaning that both surfaces of the coating conform to the contours of the surface relief structure) could be applied by vapour deposition or by sputtering. The coating could be transparent (i.e. optically clear, but colourless or tinted), or could be translucent or even opaque, provided there is a contrast with the film material.

In step (b), the soluble material could be applied only into the recesses (depressions) of the surface relief, e.g. if the soluble material is of a suitably low viscosity that it flows into those depressions leaving the elevations substantially free of the material. However in preferred examples, step (b) comprises applying the soluble material to the surface relief structure such that the soluble material coats the elevations and is received in the depressions, and then removing the soluble material from the elevations, preferably using a doctor blade, a wiping roller or a squeegee.

Any material which can be dissolved by a suitable solvent (preferably water but alternatively an organic solvent) can be used as the solvable material, but in preferred cases the soluble material comprises a soluble ink. For instance, in a preferred embodiment the soluble ink is a heavily pigmented ink, which can be dissolved by application of a solvent (aqueous or otherwise), thereby impeding adhesion of the film material applied thereto to the surface of the cured material in which the surface relief structure is formed. Preferably, the film material is permeable to a solvent in which the soluble material dissolves. In preferred cases, the soluble material is configured such that, when coated with a thin deposited layer of film material (e.g. metal) the soluble material creates small holes or discontinuities in the film by virtue of the fact that the film (typically 15 to 30nm thick for a metal film) is not thick enough to continuously over coat the pigment grains in the soluble material. When exposed to a suitable solvent (preferably water) the solvent enters through these holes, dissolving the pigment such that the overlying film layer disbands. In order for this mechanism to operate effectively, the pigment grain dimensions in the soluble material are preferably greater than the thickness of the film material and typically in the range 50-500nm. Examples of suitable soluble materials are disclosed in US-A- 5142383, EP-A-1023499 and US-A-3935334. In some cases, the film material may also comprise particles dispersed therethrough, such as pigment particles. This increases the permeability of the film.

The film material can be applied by any convenient technique but preferably in step (c) the film material is applied by vacuum deposition, preferably sputtering, resistive boat evaporation or electron beam evaporation, or chemical vapour deposition. In alternative preferred examples, in step (c), the film material is applied by printing or coating, preferably gravure or slot die printing. In step (d) the die form can be exposed to solvent in a number of alternative ways but in preferred cases the step comprises spraying solvent onto the surface relief structure and/or immersing at least the upper surface of the surface relief structure in a volume of solvent. The nature of the solvent will depend on the type of soluble material selected in step (b) but may comprise water or an organic solvent for instance.

Optionally, the complexity of the image array can be further enhanced by incorporating a static (i.e. optically invariable) macroimage into the array, which does not change appearance on tiling. This can be achieved by, after step (a) and before step (b), applying a graphics layer defining a macroimage to the surface relief structure. The ink or other material forming the graphics layer fills the depressions of the surface relief structure in the area(s) corresponding to the macroimage and thus prevents the ingress of soluble material thereto in step (b). Hence when the film material is applied, in the area(s) of the macroimage it is retained over both the depressions and the elevations and does not form the image elements needed to form an optically variable effect. As a result, these areas form a static macroimage which (if the relief structure is transparent) is visible from both sides of the device. Hence the resulting device comprises both an optically variable effect and a static macroimage, in close connection. If a surface treatment step is performed on the surface relief structure to improve retention of materials, this preferably occurs before the application of the graphics layer defining the macroimage. The graphics layer can be applied by any suitable printing process. The material forming the graphics layer is preferably chosen for good compatibility and adhesion with the surface relief material, and could comprise for example one or more radiation-curable resins (e.g. UV-curable resin), or one or more solvent/aqueous-based resins. The macroimage may be monochromatic or multi-coloured.

The so-formed image array can be directly incorporated into a security device. However in preferred embodiments, the manufacturing method further comprises: (e) applying a cover layer over the surface relief structure, the cover layer covering the remaining portions of the film material on the elevations and filling the depressions. The cover layer can have various functions including protecting the image array from damage and/or preventing the depressions becoming filled with other material such as soil. If the cured material and substrate are each at least semi- transparent, the cover layer could be formed of a non-transparent (e.g. opaque or translucent) material since the image array can be viewed from the side of the substrate. However, it is preferred that the cover layer comprises an at least semi-transparent material such that the image array can be viewed therethrough. In especially preferred embodiments, the cover layer comprises a material having substantially the same refractive index as that of the curable material forming the surface relief. This effectively "indexes out" the surface relief structure at the boundary between the cured material and the cover layer, minimising or eliminating any effect on light impinging on the array between the retained portions of film material and thereby avoiding unwanted reflections. Still preferably the cover layer comprises a material of the same composition as that of the curable material forming the surface relief, thereby ensuring matching refractive indices.

Alternatively or additionally, the cover layer could carry a coloured tint and/or be formed or more than one material with different appearances (e.g. colours) in different laterally offset regions so as to introduce an additional pattern to the image array and hence further increase the security level of the finished device.

The cover layer may preferably be formed of one or more curable materials (which will be the case if it has the same composition as that of the curable material forming the surface relief, for example), in which case the method further includes a step of curing the cover layer material(s) during and/or after its application to the surface relief. In some embodiments where the image array is incorporated into a security device comprising a focussing element array such as lenses (discussed further below), the focussing elements could be formed in the free surface of the cover layer material, e.g. by cast-curing the cover layer material onto the existing surface relief using an appropriate casting tool defining the focussing elements. In further preferred embodiments, the method may further comprise providing an image layer located such that portions of the image layer are exposed between the retained portions of the film material. That is, the image layer will be viewable through the depressions of the cured material in which the surface relief is formed. The cured material will be at least semi-transparent in such embodiments to enable this. Depending on the location of the image layer, it can be provided at various stages during the manufacturing process. For instance if the image layer is located under the curable material on the same surface of the substrate, it will need to be applied prior to applying the curable material on top. Alternatively if the image layer is arranged on the opposite surface of the substrate it could be applied before, during or after any of the other manufacturing steps. If the image layer is provided on top of the cover layer it will need to be applied after steps (a) to (e) have been completed. The image layer could take various forms including complex graphics such as photographic images, alphanumeric text or a uniform block colour. In preferred embodiments the image layer will present a visible contrast against the film material. The image layer may be applied by any convenient technique including printing, coating or lamination.

The arrangement of the elevations and depressions in the surface relief structure will depend on the nature of the desired image array. However as discussed above, high resolution is required and hence small pattern dimensions. Hence in preferred examples, each depression and/or each elevation has a width in the range 0.5 to 5 microns. The depth of each depression may typically be between 1 to 10 microns, more preferably 1 to 5 microns. In some cases it has been found advantageous if the depressions of the surface relief structure have a depth which is greater than their width. Using such high aspect ratio depressions, the likelihood of soluble material being inadvertently removed from the recesses during any wiping stage that might be employed in step (b) is reduced.

The invention further provides an image array manufactured in accordance with the above-described method.

Also provided is a method of manufacturing a security device, comprising:

(i) manufacturing an image array using the method described above; and

(ii) providing a focussing element array overlapping the image array, configured such that the image array is located in a plane spaced from that of the focussing elements by a distance substantially corresponding to a focal length of the focusing elements, whereby the focusing elements exhibit a substantially focussed image of the image array. The focussing elements may comprises lenses or mirrors for example and may be arranged in a regular 1 -dimensional or 2-dimensional array. The focussing elements could comprise cylindrical, spherical or aspherical lenses or mirrors for example. Suitable methods for forming the focussing element array include embossing or cast curing. The focussing element array could be located on the opposite surface of the substrate from the image array, or on the same surface of the substrate over the image array if a suitable optical spacing layer is provided. For instance, the optional cover layer mentioned above could perform this function or an optical spacing could be built into the design of the focussing element array layer.

For example, the security device may be a moire magnifier. Thus, preferably, the image array comprises a microimage array, and the pitches of the focusing element array and of the microimage array and their relative orientations are such that the focusing element array co-operates with the microimage array to generate a magnified version of the microimage array due to the moire effect.

In another case the security device may be an integral imaging device. Hence, preferably, the image array comprises a microimage array, the microimages all depicting the same object from a different viewpoint, and the pitches and orientation of the focusing element array and of the microimage array are the same, such that the focusing element array co-operates with the microimage array to generate a magnified, optically-variable version of the object.

In a still further example, the security device may be a lenticular device. Hence, the image array preferably comprises a set of first image elements comprising portions of a first image, interleaved with a set of second image elements comprising portions of a second image, the focusing element array being configured such that each focusing element can direct light from a respective one of the first image elements or from a respective one of the second image elements therebetween in dependence on the viewing angle, whereby depending on the viewing angle the array of focusing elements directs light from either the set of first image elements or from the second image elements therebetween, such that as the device is tilted, the first image is displayed to the viewer at a first range of viewing angles and the second image is displayed to the viewer at a second, different range of viewing angles.

The invention further provides a security device manufactured using the above described method and a security article comprising such a security device. The security article is preferably a security thread, strip, patch, label or insert. Also provided is a security document comprising a security device or a security article, each manufactured as described above. The security document is preferably a banknote, passport, ID card, licence, cheque, visa, stamp or certificate.

Examples of methods for manufacturing image arrays and security devices, as well as security articles and documents incorporating such devices, will now be described with reference to the accompanying drawings, in which:

Figure 1 is a flow diagram illustrating steps in a method according to a first embodiment of the invention; Figures 2(a) to 2(g) schematically depict selected stages of manufacturing an exemplary image array in accordance with the method of Figure 1 , in cross-section, Figures 2(g') and 2(g") showing two further exemplary image arrays made in accordance with variants of the Figure 1 method;

Figures 3 and 4 schematically show two embodiments of security devices incorporating an image array as formed using the method of Figure 2, in cross- section;

Figure 5 schematically depicts a further embodiment of an image array, in cross-section;

Figure 6 schematically shows an embodiment of a security device incorporating the image array of Figure 5;

Figure 7 illustrates an exemplary manufacturing apparatus arranged to carry out a method in accordance with Figure 2 or Figure 5;

Figures 8, 9 and 10 schematically depict three further embodiments of security devices incorporating image arrays, in cross-section;

Figures 1 1 , 12 and 13 schematically depict three further embodiments of image arrays, in cross-section;

Figure 14(a) illustrates in plan view an exemplary image pattern in accordance with an embodiment of the present invention, Figure 14(b) showing in plan view the appearance of a security device in accordance with an embodiment of the present invention incorporating the image element array of Figure 14(a), at one viewing angle;

Figure 15(a) illustrates an exemplary image pattern in accordance with an embodiment of the invention, and Figure 15(b) shows the appearance of a security device incorporating the image pattern of Figure 15(a);

Figure 16(a) schematically depicts a security device in accordance with a further embodiment of the present invention, Figure 16(b) showing a cross- section through the security device, and Figures 16(c) and (d) showing two exemplary images which may be displayed by the device at different viewing angles;

Figures 17, 18 and 19 show three exemplary articles carrying security devices in accordance with embodiments of the present invention (a) in plan view, and (b) in cross-section; and Figure 20 illustrates a further embodiment of an article carrying a security device in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section. The ensuing description will focus on examples of methods of manufacturing image arrays with high resolution, fine detail in the form of microimage or image element arrays as required for use in security devices such as moire magnifiers, integral imaging devices and lenticular devices (amongst others). Preferred embodiments of such security devices making use of image arrays made in accordance with the described method will then be described below. However it should be appreciated that the disclosed methods of manufacturing image arrays can be used to form any high resolution image pattern, as may be suitable for use in other security devices such as microtext or other micrographics.

A first embodiment of a method of manufacturing an image array will be described with reference to Figures 1 , 2 and 7. Figure 1 is a flow diagram setting out steps of the method, Figures 2(a) to (g) show an exemplary substrate at various stages during the processing thereof in accordance with the method, and Figure 7 illustrates exemplary apparatus 1 for performing the method. In Figure 1 , steps S102a and S102b are optional, as denoted by their dashed lines, Figures 2(g') and 2(g") depicting the resulting structure of the image array at the end of the method should the optional steps S102a, S102b, respectively, be adopted.

First, a (preferably transparent) substrate 10 is provided (step S101 ), as shown in Figure 2(a). The substrate 10 typically comprises at least one polymeric material, such as BOPP, and may be monolithic or multi-layered. The substrate may be of a type suitable for forming the basis of a security article such as a security thread, strip, patch or transfer foil, or of a type suitable for forming the basis of a security document itself, such as a polymer banknote. The substrate may include additional layers, such as a primer layer for improving the retention of subsequent layers thereon. In certain embodiments (described below), the substrate 10 may already carry an image layer on either one of its surfaces, or the method may include a preliminary step of printing or otherwise applying such an image layer to the substrate 10 (not shown in Figures 1 , 2 or 7). Alternatively the substrate could be designed as a transfer film in which case a release layer such as wax may be provided on the top surface of the substrate 10 so that the image pattern produced using the method can later be transferred to another surface.

In the next step S102, a surface relief 12 is formed in a curable material 1 1 applied to a first surface of the substrate 10, and fixed by curing the curable material 1 1 (Figures 2(b) and 2(c)). This is preferably achieved by a cast curing technique of which the Figures illustrate one example whereby, first, a curable material 1 1 is applied to the substrate 10 at an application station 41 (Figure 7), e.g. by slot die coating or gravure printing, as shown in Figure 2(b). The curable material may be applied across the entire substrate area or only to selected regions thereof if it is desired that the finished image array be located in discrete region(s) of the substrate 10. The substrate 10 is then conveyed towards a forming station 42 at which the curable material 1 1 is brought in contact with a casting tool 42a having a casting surface relief corresponding to the desired surface relief structure 12. The curable material is formed by the casting tool and simultaneously and/or subsequently cured so as to fix the surface relief 12 in the curable material 1 1. Figure 7 shows a single source of curing energy 42b located so as to cure the material 1 1 while still in contact with the casting tool, but in practice this may be located after separation of the material 1 1 from the casting tool, or multiple curing energy sources could be provided so as to partially cure the material 1 1 before separation, and complete curing after separation.

The surface relief 12 defines a pattern of elevations (raised portions) and depressions (recesses) which corresponds to the desired arrangement of pattern elements in the finished image array. Thus, preferably, the width of the elevations and/or of the depressions may be 10 microns or less, more preferably between 1 and 5 microns. It has been found advantageous for the depressions to have a high aspect ratio whereby their depth is greater than their width. The lateral layout of the surface relief pattern will depend on the desired image array but may define for example a line pattern (rectilinear or otherwise), a two- dimensional grid of dots, squares or rectangles, an array of microimages (e.g. letters, numbers, logos or other graphics) or more complex arrangements such as interleaved portions of two or more images. Examples will be given below.

The curable material 1 1 is preferably at least semi-transparent, i.e. optically clear but may carry a coloured tint. This is desirable in order to enable viewing of the image array from either side (provided the substrate 10 is either itself transparent or is discarded in the final product). Alternatively, the curable material 1 1 could be translucent or opaque if it is acceptable to view the image array from one side only (in reflected light). In such cases it is desirable that the curable material 1 1 be light in colour, e.g. white. The colour will be provided by one or more pigments or dyes as is known in the art. Additionally or alternatively, the curable material 1 1 may comprise at least one substance which is not visible under illumination within the visible spectrum and emits in the visible spectrum under non-visible illumination, preferably UV or IR. In preferred examples, the curable material comprises any of: luminescent, phosphorescent, fluorescent, magnetic, thermochromic, photochromic, iridescent, metallic, optically variable or pearlescent pigments.

The curable material 1 1 could be a heat-activated curable material, but more preferably it comprises a radiation-curable material. For instance, UV curable polymers employing free radical or cationic UV polymerisation are particularly suitable for use as the curable material 1 1. Examples of free radical systems include photo-crosslinkable acrylate-methacrylate or aromatic vinyl oligomeric resins. Examples of cationic systems include cycloaliphatic epoxides. Hybrid polymer systems can also be employed combining both free radical and cationic UV polymerization. Electron beam curable materials would also be appropriate for use in the presently disclosed methods. Electron beam formulations are similar to UV free radical systems but do not require the presence of free radicals to initiate the curing process. Instead the curing process is initiated by high energy electrons. The nature of curing source(s) 42b will depend on the type of curable material 1 1 selected.

So as to enable continuous production of the image array, it is desirable for the casting tool 42a to be cylindrical as shown, or alternatively arranged on a belt between two support rollers. The casting tool 42a may be transparent to the curing energy (e.g. UV radiation) so that a curing energy source 42b can be located inside the casting tool 42a. For instance, the casting tool 42a may comprise a quartz cylinder having the necessary casting relief structure etched or engraved in its surface.

In the embodiment depicted in the Figures, the curable material 1 1 is applied to the substrate 10 and then brought into contact with the casting tool 42a. However, the same result can be achieved by applying the curable material 1 1 directly to the casting tool and then bringing into contact with the substrate 10 before curing to affix the material 1 1 to the substrate. This can be implemented, for example, by using an inking roller assembly (not shown) comprising a slotted die and a meter roller to apply the curable material 1 1 to the casting tool, such that it coats the elevations and fills the depressions of the casting surface relief.

An example of the resulting surface relief structure 12, formed in curable material 1 1 , is shown in Figure 2(c). The surface relief structure 12 comprises elevations 12a, corresponding to first pattern elements and depressions 12b corresponding to second pattern elements P₂. In this example the surface relief 12 is depicted as having a "square wave" profile whereby the peaks of the elevations and the bases of the depressions are substantially flat, which is preferred but not essential.

Optional steps S102a and S102b will be discussed below.

In the next step (S103), a soluble material 13 is applied into the depressions 12b of the surface relief 12 (only). In the present embodiment this is achieved in a two-step process depicted in Figures 2(d) and 2(e) whereby first the soluble material is applied all over the surface relief 12, filling the depressions 12b and coating the elevations 12a, and then the excess soluble material 13 is removed leaving only those portions inside the depressions 12b. As depicted in Figure 7, the soluble material will be provided at an application station 43, e.g. by slot die coating or printing. The excess material 13 will then be removed by removing means 44 such as a doctor blade, wiping roller or squeegee, which process may also help to complete filling of the depressions 12b. The result is that substantially no soluble material 13 remains on the elevations 12a. Alternatively, it may be possible to achieve the same result in a single step if the dimensions of the depressions 12b permit and the viscosity of the soluble material 13 is sufficiently low so as to flow into the depressions 12b upon application without the need for a removal step. Soluble materials 13 suitable for use in the presently disclosed methods include soluble inks such as a heavily pigmented ink and corresponding solvents include water or organic solvents. Examples of suitable soluble materials are disclosed in US-A-5142383, EP-A-1023499 and US-A-3935334.

Once the soluble material 13 is dry (which may or may not require an active drying step, e.g. heating), in step S104, a film material 15 is applied to the surface relief structure 12, over both the elevations 12a and the depressions 12b, as shown in Figure 2(f). The film material 15 can be of various different types but should be visually distinguishable from the cured material 1 1 forming the surface relief. Preferably, the film material 15 is of higher optical density than that of the cured material 1 1 , and most preferably is substantially visually opaque. Alternatively, the film material 15 may present a visual contrast with the cured material 1 1 , e.g. by virtue of their having different visible colours (hues and/or tones). For instance, the cured material 1 1 may be white (or another light colour) and the film material 15 black (or another dark colour). The film material 15 preferably has a uniform appearance (e.g. colour) across its whole area, at least when viewed from any one viewing angle.

In particularly preferred embodiments, the film material 15 is a metal or alloy layer, such as aluminium, copper, nickel or chrome. Such materials are preferred since a relatively thin layer of the material typically achieves high opacity. In other preferred cases the film material may comprise an interference layer thin-film structure which provides the added benefit of exhibiting a different colour depending on the viewing angle. Metal-dielectric interference layer structures are preferred due to their higher opacity by all-dielectric stacks could also be used. Examples of interference layer structures and methods for their application can be found in US-A-3858977 and US-A-5084351. In still further examples, the film material could comprise an ink, preferably a high optical density ink, and most preferably a substantially opaque ink. For instance the film material could comprise a black ink, or other dark colour, or a reflective ink such as a metallic ink or an optically variable ink (comprising particles of interference layer stacks, as also described in US-A-5084351 ). The film material 15 could also be a multi-layer structure comprising two or more different materials overlapping one another, including any combination of the options mentioned above. For instance the film material 15 could comprise a metal layer and an ink layer overlying the metal layer to introduce colour and/or to reduce specular reflections from the metal.

The film 15 can be applied by any convenient technique(s) suited to the material type, at an appropriate application station 45 (Figure 7). In preferred embodiments, the film material 15 is preferably provided by vacuum deposition, e.g. sputtering, resistive boat evaporation or electron beam evaporation, or chemical vapour deposition. In other case the film material 15 could be applied by a printing or coating technique, such as gravure or slot die. It will be noted that a selective application process whereby the film material 15 is applied in a pattern is not required, although such a technique could be employed if it is desired to form the image array in discrete areas of the substrate only. Where the film material comprises multiple layers, two or more application processes may be employed in sequence to deposit the respective layers of the film 15.

The film material 15 is configured such that it remains permeable to a solvent (typically a fluid) in which the soluble material 5 will dissolve. Typically a metal or alloy film 15 of thickness 20 to 100 microns (preferably 20 to 30 microns) will retain sufficient permeability either due to cracks through the film and/or to boundaries between grains in the microstructure. However in some cases it may be appropriate to enhance the permeability of the film 15 by adding a dispersion of particles such as a pigment to the film material. If an ink or similar material is used to form the film material 15 it will be necessary to ensure the ink is not itself soluble in the same solvent as will be used to remove the soluble material 13. For instance, the soluble material 13 might be selected as a water-soluble ink whilst the film material 15 might be dissolved by organic solvents only. In the next step (S105), the so-formed assembly is then exposed to a solvent at a washing station 46 (Figure 7) which passes through the film 15 and dissolves the soluble material 13 in the recesses 12b. This can be achieved for example by spraying the surface of the substrate with the solvent (e.g. using water jets), or by immersing all or part of the substrate in a volume of the solvent (e.g. by conveying it through a solvent bath). The result, shown in Figure 2(g), is the removal not only of the soluble material 13 but also of the film material 15 in the regions of the depressions 12b, leaving the film material only on the elevations 12a. The film material 15 thus forms an image array 5 made up of first pattern elements Pi in which the film material remains present and second pattern elements P₂ where it is absent, exactly in conformance with the arrangement of elevations 12a and depressions 12b in the surface relief 12.

If both the cured material 1 1 and the substrate 10 are at least semi-transparent, the image array 5 will be visible in reflected light from both sides of the assembly, i.e. both observers d and 0₂ illustrated in Figure 2(g) will be able to view the image array. If the cured material 1 1 and/or substrate 10 are opaque or translucent, the image array will only be visible in reflected light from the side on which the film material 15 is disposed (i.e. to observer d), although if translucent the image array may be visible from either side in transmitted light.

Returning to optional step S102a, depending on the materials in use, it may be desirable to treat the surface of surface relief structure 12 in order to improve the retention of materials subsequently applied to it, such as the soluble material 13 and/or the film material 15. The nature of this treatment step will also vary according to the materials in use. In one preferred example, step S102a may be implemented by Corona treatment of the surface relief structure 12. In this case, the final image array structure will be the same as that depicted in Figure 2(g). However, in other preferred examples, step S102a may involve applying a conformal coating 14 on to the surface relief structure 12, the coating 14 comprising a material which promotes adhesion of the soluble material 13 and/or film material 15 to the (cured) curable material 1 1. Figure 2(g') shows the resulting cross-section of the image array 5 should step 102a be implemented in this way.

The coating 14 could be transparent (i.e. optically clear, but either colourless or tinted), or translucent or opaque, depending on from which side(s) the image array 5 is to be viewed in used. If the image array is to be viewed through the curable material 1 1 then both it and the coating 14 should be transparent. An example of a suitable transparent coating could be a very thin coating of aluminium oxide (e.g. having a thickness of about 30nm and hence an optical density of about 0.04 to 0.5). If the image array is only to be viewed from the opposite side, the coating 14 need not be transparent provided it exhibits a visible contrast with the film material 15. Examples of non-transparent coatings 13 include metal layers, such as titanium, aluminium, gold, copper, silver, zirconium or other metals or metal alloys. Thicker coatings of such materials can be used, with correspondingly higher optical densities. Nonetheless, whatever the material of the coating 14, it is preferred that its thickness is kept as small as possible so as to avoid widening the elevations of the surface relief structure significantly, which would reduce the resolution of the pattern. Conformal coatings (in which both surfaces of the coating conforms to the contours of the surface relief) can be applied by deposition, e.g. evaporation or sputtering for example.

Another optional step (which may be implemented with step S102a, or on its own) is the application of a macroimage, as indicated in step S102b. The resulting image array structure is shown in Figure 2(g"). Here, after formation of the relief structure 12 (and optional surface treatment S102a), a graphics layer 18 defining a macroimage is applied to the surface relief 12. Where the graphics layer 18 is present, the ink(s) or other materials from which it is formed fill the depressions of the relief structure 12 and thereby prevent ingress of the soluble material 13 thereafter. When the film material 15 is applied over the top, it therefore remains in place over both the elevations and the depressions of th relief structure in the area(s) of the macroimage, resulting in an optically invariable (static) image being viewable in those area(s) from both sides of the image array 5 (if the material 1 1 is transparent). The graphics layer 18 is preferably formed of one or more materials with high compatibility (and adhesion) with the (cured) curable material 1 1 and therefore may itself comprise one or more curable resins. Alternatively the graphics layer 18 could be formed of one or more solvent/aqueous-based materials. The graphics layer 18 could be monochromatic or multi-coloured. The macroimage preferably conveys one or more items of information, such as alphanumeric character(s), text, logos, symbols, or the like, and may be used to personalise the image array.

The completed image array 5 shown in any one of Figures 2(g), 2(g') or 2(g") can be incorporated into a security device in a number of ways. Preferably, the image array 5 is combined with an overlapping focussing element array 20, such as an array of lenses or mirrors, to form a security device 1 , of which two examples are shown in Figures 3 and 4 respectively (here, the image array 2(g) is shown as an example, but the variants shown in Figures 2(g') or 2(g") could equally be used instead. In both cases, the film material 15 forming image array is positioned approximately in the focal plane of the focussing elements 20 so that they exhibit a substantially focussed image of the image array, e.g. as in a moire magnifier, integral imager or lenticular device.

In the example of Figure 3, the focussing element array 20 is provided on a second substrate 25, which is at least semi-transparent, e.g. by embossing or cast-curing. The image array is affixed to the opposite surface of the second substrate 25 via a lamination adhesive (not shown). The second substrate 25 provides the necessary optical spacing between the image array 5 and the focussing elements 20. The focussed image can be viewed through the focussing element array 20 and second substrate 25 by observer Oi . Hence in this case, it is not essential for the curable material 1 1 or the substrate 10 to be transparent.

In the example of Figure 4, the focussing element array 20 is provided on the same substrate 10 as that to which the curable material 1 1 is applied, but on the opposite surface. Again this can be achieved by embossing or cast curing and the focussing elements may be applied to the substrate 10 before, during or after the above-described method of forming the image array 5. In this case the substrate 10 and curable material 1 1 provide the necessary optical spacing between the focussing elements and the image array and must therefore be at least semi-transparent. The focussed image will now be visible from the opposite side of the device, by observer 0₂.

Figure 5 shows another embodiment of an image array 5 which is formed via the same method as described with reference to Figure 2 above, followed by an additional step of applying a cover layer 16 over the top of the existing image array structure. Thus the cover layer 16 covers the remaining portions of film material 15 and fills the depressions 12b of the surface relief 12, contacting the cured material 1 1 where the film material 15 is absent. The cover layer 16 can perform various functions including protecting the image array 5 from damage, and preventing soil ingress into the depressions. In some preferred examples, the cover layer 16 will be at least semi-transparent so that the image array 5 remains visible therethrough, whereas in others if the curable material 1 1 and substrate 10 are at least semi-transparent, the cover layer could be translucent or opaque with the image array remaining visible from the opposite side. However, most preferably both the curable material and the cover layer are at least semi-transparent and in order to minimise the visibility of the surface relief 12 in the pattern elements P₂ (corresponding to the depressions 12b), it is preferred that the refractive index of the cover layer material 16 is substantially the same as that of the cured material 1 1 . This "indexes out" the boundary between the two materials such that it has substantially no redirecting effect on light impinging thereon. In particularly preferred examples the cover layer 16 and cured material 11 will be of the same composition to ensure the refractive indices match. Where a cover layer 16 is provided, it is preferable that step 102a (if implemented) is performed by corona treatment instead of applying a conformal coating, but this is not essential and such a coating could be inserted between material 1 1 and cover layer 16. If a transparent appearance is desired, it is preferred that the material of such a coating has as close a refractive index to those of material 11 and cover layer 16 to make the boundary less visible. The cover layer 16 can be applied by any suitable application process including printing or coating and an application station 47 for this purpose is shown in Figure 7, in dashed lines since this step is optional.

The so-formed image array 5 shown in Figure 5 could be incorporated into a security device using either of the approaches shown in Figure 3 and 4, e.g. laminating the construction to a second substrate carrying focussing elements or providing focussing elements on the opposite surface of substrate 10. Alternatively, the focussing elements 20 could be applied directly to the cover layer 16 by embossing or cast curing, as shown in Figure 6. In this case the cover layer 16 provides the additional function of optically spacing the focussing elements and the image array. The cover layer 16 could itself be of a curable material and applied using a cast cure process such as that already described, to form the focussing elements 20 directly in its surface if desired. In many embodiments, the visual distinction between the film material 15 and the curable material 1 1 will be sufficient to achieve the desired visual effect. However, in other cases it may be desirable to provide the image array 5 with an additional image layer 17, as shown in the embodiments of Figures 8, 9 and 10. The image layer 17 will be arranged so that portions of it are visible through the curable material 1 1 whereas other portions will be masked from the viewer by the remaining portions of film material 15. The image layer 17 could be a uniform block colour, or a complex graphic. The latter is particularly suitable for use in a lenticular device where the remaining portions of film material 15 form one image channel and are displayed to the viewer at a first viewing angle, and the portions of the image layer 17 visible therebetween form a second image channel and are displayed to the viewer at a second viewing angle. An example of such a device will be described further below with respect to Figure 16.

The image layer 17 can be provided at various different locations in the security device depending on from which side it is to be viewed. Its location will also determine at what point during the manufacturing process the image layer can be applied. In the Figure 8 embodiment, the image layer 17 is located on the same surface of the substrate 10 as the curable material 1 1 , between the curable material and the substrate. This can be achieved by applying the image layer 17 to the substrate 10 before performing step S102 described above. The resulting image array will be viewed from the side of the film material 15 and so in this example the cover layer 16 will need to be at least semi-transparent.

In the Figure 9 embodiment, the image layer 17 is provided on the opposite surface of the substrate 10 from the curable material 1 1. In this case the image layer 17 could be applied before, during or after the image array manufacturing process. The image array is again viewed from the side of the film material 15 and so in this case the cover layer 16, curable material 1 1 and substrate 10 will all be at least semi-transparent.

In the Figure 10 embodiment, the focussing element array 20 is now provided on the second surface of the substrate 10 and the image layer 17 is located on the outer surface of cover layer 16 not in contact with the surface relief 12. In this case the image array 5 will be viewed through the substrate 10, so the substrate 10, curable material 1 1 and cover layer 16 will all be at least semi-transparent.

While Figures 8, 9 and 10 all depict the provision of a cover layer 16, this is optional and could be omitted. In the case of Figure 10 the image layer 17 would then be applied directly onto the film material 15 and into the depressions 12b. Similarly, whilst Figures 8, 9 and 10 do not depict the inclusion of a conformal coating 14 or a macroimage 18, either or both of these could be provided as described with reference to Figures 2(g') and (g") above.

The curable material 1 1 , coating 14 (if provided) and/or cover layer 16 (if provided) can optionally be utilised to introduce additional complexity to the image array by forming one (or more) of those layers of two or more materials with different optical characteristics (e.g. visible colours). In each case, the two or more materials will be arranged in respective, laterally offset regions which together form the complete layer.

Figures 1 1 , 12 and 13 show examples of image arrays exhibiting additional patterning of this sort. In all three cases the image array has otherwise been constructed as described with reference to Figure 5. In the Figure 1 1 embodiment, the curable material 1 1 comprises a first curable material 1 1a in a first region and a second curable material 1 1 b in a second region. The two regions abut and the surface relief 12 is formed seamlessly across them. This can be achieved by selectively applying the two curable materials either to the substrate or to the casting tool in the above described forming process. The two curable materials have different optical characteristics, e.g. visible colour and/or fluorescent or luminescent responses, resulting in the appearance of the second pattern elements P₂ being different in the respective regions of the image array if the image array is viewed from the film material side. If the image array is viewed through the substrate 10, both the first and second pattern elements will have a different appearance in the first region from that of their counterparts in the second region.

In the Figure 12 embodiment, the cover layer 16 is instead formed of two materials 16a and 16b in respective laterally offset areas. In the example shown these again abut, but here this is not essential and parts of the surface relief could remain uncovered if desired. The two cover layer materials have different optical characteristics, resulting in the appearance of the second pattern elements P₂ being different in the respective regions of the image array if the image array is viewed from the substrate side. If the image array is viewed from the film material side, both the first and second pattern elements will have a different appearance in the first region from that of their counterparts in the second region.

In the Figure 13 embodiment, both the cover layer 16 and the curable material 1 1 are respectively formed of two materials 16a, b and 1 1 a, b. In the example shown the position and extent of the regions in the two layers match, but this is not essential. Further, the optical characteristics of the respective cover layer and curable materials could be the same or different.

An embodiment of a security device incorporating an image array formed using the above methods will now be described with reference to Figures 14(a) and (b). In this case the security device is a moire magnifier, comprising an image array P formed using the methods described above defining an array of microimages and an overlapping focussing element array 20 with a pitch or rotational mismatch as necessary to achieve the moire effect. Figure 14(a) depicts part of the image element array P as it would appear without the overlapping focusing element array, i.e. the non-magnified microimage array (but shown at a greatly increased scale for clarity). In contrast, Figure 14(b) depicts the appearance of the same portion of the completed security device, i.e. the magnified microimages, seen when viewed with the overlapping focussing element array, at one viewing angle. In this example, the microimage array is formed using the methods described above and has a cross section corresponding substantially to that shown in Figure 1 1 . Hence in this case the curable material 1 1 comprises two regions 1 1 a and 1 1 b of different colour, but this is not essential and a single curable material could be used instead. Figure 14(a) shows the patterned metal layer 15 and underlying curable material 1 1 in plan view and it will be seen that the second pattern elements P₂ 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 first pattern elements Pi in which the film material 15 remains present form a contiguous, uniform background surrounding the microimages. Since the curable material 1 1 here has two zones of different colour, the microimages in zone 1 1 a appear in a first colour (here represented as black), whilst those in zone 1 1 b appear in a second colour (here represented as white).

Figure 14(b) shows the completed security device 30, i.e. the image element array P shown in Figure 14(a) plus an overlapping focusing element array 20, from a first viewing angle which here is approximately normal to the plane of the device 30. It should be noted that the security device is depicted at the same scale as used in Figure 14(a): the apparent enlargement is the effect of the focusing element array 20 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 zone 11 a and hence appears black whilst the second magnified microimage is from microimages all within zone 1 1 b and hence appears white. 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 zone of curable material 1 1. In the above example security device, the microimages are all identical to one another, such that the device can be considered a "pure" moire magnifier. However, the same principles can be applied to "hybrid" moire magnifier / integral imaging devices, in which the microimages depict an object or scene from different viewpoints. Such microimages are considered substantially identical to one another for the purposes of the present invention. An example of such a device is shown schematically in Figure 15, where Figure 15(a) shows the unmagnified microimage array, without the effect of focusing elements 21 , and Figure 15(b) shows the appearance of the finished device, i.e. the magnified image. As shown in Figure 15(a), the microimages 31 show an object, here a cube, from different angles. It should be noted that the microimages are formed as lines in which film material 15 is absent (i.e. pattern elements P₂), corresponding to the black lines of the cubes in the Figure, the remainder of the film material being present although this is shown in reverse in the Figure for clarity. The curable material 1 1 here comprises a first region 1 1 a in the form of a single hexagonal zone, which provides a first colour to the demetallised lines, and a second surrounding region with another colour. In the magnified image (Figure 15(b)), the moire effect generates magnified, 3D versions of the cube labelled 34. Those lines of the magnified cubes 34 which coincide with the first region 1 1 a will be of the first colour whilst those portions outside the region 1 1 b will be of a second colour. As the device is tilted the magnified cubes 34 will appear to move across the device and so enter or leave the first colour zone 1 1 a depending on their location and the degree of tilt. This gives the visual impression of the magnified images changing colour as they move across the central portion of the device. This, combined with the 3D appearance of the images, amounts to an effect with significant visual impact.

Figure 16 depicts a further embodiment of a security device 1 , which here is a lenticular device. A transparent substrate 10 is provided on one surface with an array of focussing elements 20, here in the form of cylindrical lenses, and on the other surface with an image array formed of a patterned film material 15 and on a cured material 1 1 carrying a surface relief as described above. The cured material 1 1 is not depicted in Figure 16 for clarity. The image array also comprise an image layer 17 arranged over the retained film material portions and the remainder of the surface relief as shown best in Figure 16(b). The image array comprises first pattern elements and second pattern elements P₂, as previously described. In this example, the size and shape of each first pattern element Pi is substantially identical. The pattern elements in this example are elongate image strips and so the overall pattern of elements is a line pattern, the elongate direction of the lines lying substantially parallel to the axial direction of the focussing elements 20, which here is along the x-axis. The lateral extent of the pattern (including its elements Pi and P₂) is referred to as the array area.

As shown best in the cross-section of Figure 16(b), the pattern formed in film layer 15 and the focussing element array 20 have substantially the same periodicity as one another in the y-axis direction, such that one first pattern element Pi and one second pattern element P₂ lies under each lens 21 . In this case, as is preferred, the width of each element P₂ is approximately half that of the lens pitch. Thus approximately 50% of the array area carries first pattern elements Pi and the other 50% corresponds to second pattern elements P₂. In this example, the image array is registered to the lens array 20 in the y-axis direction (i.e. in the arrays' direction of periodicity) such that a first pattern element Pi lies under the left half of each lens and a second pattern element P₂ lies under the right half. However, registration between the lens array 43 and the image array in the periodic dimension is not essential.

The image layer 17 can take any form, including that of a complex, multicoloured image such as a photograph. When the device is viewed by a first observer d from a first viewing angle, each lens 21 will direct light from its underlying first pattern element Pi to the observer, with the result that the device as a whole appears uniformly coloured, corresponding to the appearance of the film material 15, as shown in Figure 16(d). This is referred to more generally as (first) image

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 21 directs light from the second pattern elements P₂ to the observer. As such the whole device will now appear to display the appearance of the image layer 17, which in this case carries a star shaped image as shown in Figure 16(c) which constitutes a (second) image l₂. Hence, as the security device is tilted back and forth between the positions of observer d and observer 0₂, the appearance of the device switches between image

and image l₂. 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. Examples of patterns suitable for forming image arrays for such devices include forming the first pattern elements Pi as grid patterns of "dots", with periodicity in more than one dimension, e.g. arranged on a hexagonal or orthogonal grid. For instance, the first pattern elements Pi may be square and arranged on an orthogonal grid to form a "checkerboard" pattern with resulting square second pattern elements P₂ in which the image layer 17 is visible. 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.

Similar effects can be achieved with other two dimensional arrays of pattern elements, e.g. using second pattern elements P₂ which are circular rather than square. Any other "dot" shape could alternatively be used, e.g. polygonal. 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 pattern elements on one hand and the second pattern elements on the other. Rather, both pattern elements 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 pattern elements may correspond to the black portions of a first image and those of a second image, whilst the second pattern elements 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 16. 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. In embodiments such as these the curable material 11 and image layer 17 (if provided) preferably each have a uniform appearance (e.g. single colour) across the array as does any cover layer 16 provided resulting in a duo-tone appearance. 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. However the security devices can also be constructed directly on security documents which are formed of a transparent document substrate, such as polymer banknotes. In such cases, the image array may be manufactured on a first substrate, using the method discussed above, and then transferred onto or affixed to one surface of the document substrate, optionally using a transparent adhesive. This may be achieved by foil stamping, for example. Alternatively, the image array could be formed directly on the document substrate by applying the curable material 11 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 array thereon. A focusing element array can be applied to the opposite side of document substrate, e.g. by transfer, embossing or cast-curing, before or after the image array is applied.

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

Such security articles can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-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-1 141480 and WO-A-03054297. In the method described in EP-A-1 141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

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

The security 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 17 to 20.

Figure 17 depicts an exemplary document of value 50, here in the form of a banknote. Figure 17a shows the banknote in plan view whilst Figure 17b shows a cross-section of the same banknote along the lines X-X'. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 51. Two opacifying layers 53 and 54 are applied to either side of the transparent substrate 51 , which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 51.

The opacifying layers 53 and 54 are omitted across a selected region 52 forming a window within which a security device is located. In Figure 17(b), the security device is disposed within window 52, with a focusing element array 48 arranged on one surface of the transparent substrate 51 , and image array formed of portions of film material 15 on the other (e.g. as in any of Figures 2(g), 5 or 1 1 to 13 above). As described in relation to Figure 3 and 4, the image element array 1 1 could be manufactured on a separate substrate which is then laminated to the document substrate 51 (corresponding to substrate 25 in Figure 3) in the window region, or could be manufactured directly on the document substrate 51 by applying the curable material 11 to the document substrate 51 (which here takes the place of substrate 10 shown in Figure 4), at least in the window region 52, and optionally all over the substrate, and then forming a pattern in a film material 15 using the above-described method.

It will be appreciated that, if desired, the window 52 could instead be a "half- window", in which one of the opacifying layers (e.g. 53 or 54) is continued over all or part of the image array 15. 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 53 and 54 are provided on both sides. In Figure 18 the banknote 50 is a conventional paper-based banknote provided with a security article 55 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 56 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 55 in window regions 57 of the banknote. Alternatively the window regions 57 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 57 to be "full thickness" windows: the thread 55 need only be exposed on one surface if preferred. The security device is formed on the thread 55, which comprises a transparent substrate a focusing array 21 provided on one side and an image array 15 provided on the other. Windows 57 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 19, the banknote 50 is again a conventional paper-based banknote, provided with a strip element or insert 58. The strip 58 is based on a transparent substrate and is inserted between two plies of paper 56a and 56b. The security device is formed by a lens array 21 on one side of the strip substrate, and an image array 15 on the other. The paper plies 56a and 56b are apertured across region 59 to reveal the security device, which in this case may be present across the whole of the strip 58 or could be localised within the aperture region 59. It should be noted that the ply 56a need not be apertured and could be continuous across the security device.

A further embodiment is shown in Figure 20 where Figures 29(a) and (b) show the front and rear sides of the document 50 respectively, and Figure 29(c) is a cross section along line Z-Z'. Security article 58 is a strip or band comprising a security device according to any of the embodiments described above. The security article 58 is formed into a security document 50 comprising a fibrous substrate 56, using a method described in EP-A-1 141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (Figure 29(a)) and exposed in one or more windows 59 on the opposite side of the document (Figure 29(b)). Again, the security device is formed on the strip 58, which comprises a transparent substrate with a lens array 21 formed on one surface and a co-operating image array 15 as previously described on the other

Alternatively a similar construction can be achieved by providing paper 56 with an aperture 59 and adhering the strip element 58 onto one side of the paper 56 across the aperture 59. 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. The image array 15 can be affixed to the surface of the substrate, e.g. by adhesive or hot or cold stamping, either together with a corresponding focusing element array 20 or in a separate procedure with the focusing array 20 being applied subsequently.

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

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

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

The presence of a film material 15 such as a metal in the security device can be used to conceal the presence of a machine readable dark magnetic layer, or the film material 15 itself could be magnetic. 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 (Fe₂0₃ 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, lron: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 WO03091953 and WO03091952.

Claims

1. A method of manufacturing an image array for a security device, comprising the steps of:

2. A method according to claim 1 , wherein step (a) comprises:

(a3) forming the curable material with the casting tool;

(a4) curing the curable material so as to retain the surface relief structure therein, in one or more curing steps; and (a5) before, during or after step (a4), removing the curable material from the casting tool whereby the cured material, formed according to the surface relief structure, is retained on the substrate. 3. A method according to claim 1 or claim 2, wherein the curable material is at least semi-transparent in the visible spectrum.

4. A method according to any of the preceding claims, wherein the curable material comprises a curable polymeric material.

5. A method according to any of the preceding claims, wherein the curable material comprises a radiation-curable material, preferably a UV-curable material.

6. A method according to any of the preceding claims, wherein the film material is of a higher optical density than that of the curable material.

7. A method according to any of the preceding claims, wherein the film material exhibits a visual contrast relative to the curable material, the film material and the curable material preferably being of different visible colours.

8. A method according to any of the preceding claims, wherein the film material is substantially opaque across the visible spectrum.

9. A method according to any of the preceding claims, wherein the film material is a metal or alloy film comprising at least one metal or alloy, preferably aluminium, copper, nickel or chrome.

10. A method according to any of claims 1 to 8, wherein the film material comprises a multi-layer interference film configured to reflect different wavelength(s) of light at different angles of view.

1 1. A method according to any of claims 1 to 8, wherein the film material comprises an ink, preferably a metallic and/or opaque ink.

12. A method according to any of the preceding claims, further comprising, after step (a) and before step (b), treating the surface relief structure to improve retention of materials thereon.

13. A method according to any of the preceding claims, wherein step (b) comprises applying the soluble material to the surface relief structure such that the soluble material coats the elevations and is received in the depressions, and then removing the soluble material from the elevations, preferably using a doctor blade, a wiping roller or a squeegee.

14. A method according to any of the preceding claims, wherein the soluble material comprises a water-soluble material.

15. A method according to any of the preceding claims, wherein the film material is permeable to a solvent in which the soluble material dissolves.

16. A method according to any of the preceding claims, wherein the film material comprises particles dispersed therethrough.

17. A method according to any of the preceding claims, wherein in step (c) the film material is applied by vacuum deposition, preferably sputtering, resistive boat evaporation or electron beam evaporation, or chemical vapour deposition.

18. A method according to any of the preceding claims, wherein in step (c), the film material is applied by printing or coating, preferably gravure or slot die printing.

19. A method according to any of the preceding claims, wherein step (d) comprises spraying solvent onto the surface relief structure and/or immersing at least the upper surface of the surface relief structure in a volume of solvent.

20. A method according to any of the preceding claims, further comprising, after step (a) and before step (b), applying a graphics layer defining a macroimage to the surface relief structure.

21. A method according to any of the preceding claims, further comprising:

(e) applying a cover layer over the surface relief structure, the cover layer covering the remaining portions of the film material on the elevations and filling the depressions.

22. A method according to claim 21 , wherein the cover layer comprises an at least semi-transparent material.

23. A method according to claim 21 or claim 22, wherein the cover layer comprises a material having substantially the same refractive index as that of the curable material forming the surface relief.

24. A method according to any of claims 21 to 23, wherein the cover layer comprises a material of the same composition as that of the curable material forming the surface relief.

25. A method according to any of the preceding claims, wherein the depressions of the surface relief structure have a depth which is greater than their width.

26. A method according to any of the preceding claims, wherein each depression has a depth between 1 to 10 microns, more preferably 1 to 5 microns.

27. A method according to any of the preceding claims, wherein each depression has a width in the range 0.5 to 5 microns.

28. A method according to any of the preceding claims, wherein each elevation has a width in the range 0.5 to 5 microns.

29. An image array manufactured in accordance with any of claims 1 to 28.

30. A method of manufacturing a security device comprising:

(i) manufacturing an image array using the method of any of claims 1 to 28; and

(ii) providing a focussing element array overlapping the image array, configured such that the image array is located in a plane spaced from that of the focussing elements by a distance substantially corresponding to a focal length of the focusing elements, whereby the focusing elements exhibit a substantially focussed image of the image array.

31. A method according to claim 30, wherein the image array comprises a microimage array, and the pitches of the focusing element array and of the microimage array and their relative orientations are such that the focusing element array co-operates with the microimage array to generate a magnified version of the microimage array due to the moire effect.

32. A method according to claim 30, wherein the image array comprises a microimage array, the microimages all depicting the same object from a different viewpoint, and the pitches and orientation of the focusing element array and of the microimage array are the same, such that the focusing element array co- operates with the microimage array to generate a magnified, optically-variable version of the object.

33. A method according to claim 30, wherein the image array comprises a set of first image elements comprising portions of a first image, interleaved with a set of second image elements comprising portions of a second image, the focusing element array being configured such that each focusing element can direct light from a respective one of the first image elements or from a respective one of the second image elements therebetween in dependence on the viewing angle, whereby depending on the viewing angle the array of focusing elements directs light from either the set of first image elements or from the second image elements therebetween, such that as the device is tilted, the first image is displayed to the viewer at a first range of viewing angles and the second image is displayed to the viewer at a second, different range of viewing angles.

34. A security device manufactured in accordance with any of claims 30 to 33.

35. A security article comprising a security device according to claim 34, wherein the security articles is preferably a security thread, strip, patch, label or insert.

36. A security document comprising a security device according to claim 34, or a security article according to claim 35, wherein the security document is preferably a banknote, passport, ID card, licence, cheque, visa, stamp or certificate.