EP3600902A1

EP3600902A1 - Security devices

Info

Publication number: EP3600902A1
Application number: EP18714053.8A
Authority: EP
Inventors: Adam Lister
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2017-03-24
Filing date: 2018-03-16
Publication date: 2020-02-05
Also published as: WO2018172750A1; GB2564076A; GB201704662D0

Abstract

A security device comprises an array of elements (12) of a first image (I1) arranged across an array area in accordance with a pattern which is periodic at least in a first dimension, the first image elements being spaced from one another by regions (14). A second image (l2) underlies the array of elements (12) of the first image, the second image extending continuously across the array area. Elements of the second image (l2) are exposed through the regions (14) between the first image elements (12), such that the elements of both images can be viewed from the same side of the image array, and wherein the security device is one of a set of such security devices each of which has the same first image and a different second image.

Description

SECURITY DEVICES

This invention relates to security devices. 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. Methods of manufacturing security devices are also disclosed.

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. 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.

The present invention is mainly, but not exclusively, concerned with lenticular devices. Lenticular devices which do not rely upon magnification, synthetic or otherwise, typically comprise an array of focusing elements, typically cylindrical lenses, that overlies a corresponding array of image elements, 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-2011/051669, WO-A-201 1051670, 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 British patent application numbers GB-A-2539389, WO-A-2015/01 1493 and WO-A- 2015/011494. Lenticular devices have the advantage that different images can be displayed at different viewing angles, giving rise to the possibility of animation and other striking visual effects which are not possible using the moire magnifier or integral imaging techniques.

Security devices such as lenticular devices, as well as others such as barrier strip type devices (which utilise a masking grid in place of focusing elements) depend for their success significantly on the resolution with which the image array 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 element 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 element needs to be even finer in order to fit all of the image elements 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 element must have a width of 5 microns or less. The same is true for many security devices which do not make use of focusing elements, e.g. barrier stripe 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 m and 50 μητ For example, with gravure or wet lithographic printing it is possible to achieve line widths down to about 15 μητ Methods such as these are limited to the formation of single-colour image elements, since it is not possible to achieve the high registration required between different workings of a multi-coloured print. In the case of a lenticular device for example, the various interlaced image elements must all be defined on a single print master (e.g. a gravure or lithographic cylinder) and transferred to the substrate in a single working, hence in a single colour. The various images displayed by the resulting security device will therefore be monotone, or at most duotone if the so-formed image elements are placed against a background of a different colour.

The devices described above are normally produced in a "generic" format. In other words, many identical devices are produced but because of the difficulty of producing them, they can act as strong security devices which are difficult to counterfeit. A problem, however, is that there is a desire to customise these devices by providing a personalised image amongst the images that are viewable as the device is tilted. This is difficult because of the very small dimensions required for the individual image elements which can only be produced using certain printing techniques which are not suitable to be rapidly varied from one device to another. These conventional techniques include lithographic and flexographic printing. Inkjet printing is a suitable technique for printing different images from device to device but cannot typically achieve dimensions of less than about 80 or 90 m which is unacceptable when the requirement is 50 microns or less as explained above.

In accordance with the present application, a security device comprises:

an array of elements of a first image arranged across an array area in accordance with a pattern which is periodic at least in a first dimension, the first image elements being spaced from one another by regions; and

a second image underlying the array of elements of the first image, the second image extending continuously across the array area;

wherein elements of the second image are exposed through the regions between the first image elements, such that the elements of both images can be viewed from the same side of the image array, and wherein the security device is one of a set of such security devices each of which has the same first image and a different second image. We have realised that instead of trying to interleave a unique or personalised image within the primary array of elements, it is possible to provide that image as a continuous second image extending fully across the array area with the first image elements acting as a mask, the second image being viewable through the spaces between the first image elements. This then enables the second image to be provided using conventional personalised printing techniques such as laser printing, inkjet printing and D2T2 (dye diffusion thermal transfer) printing.

The first image can be provided by simply printing the array of elements using conventional non-variable image techniques such as intaglio printing, flexographic printing, gravure printing or lithographic printing. Other methods include laser printing, inkjet printing, dye diffusion thermal transfer ("D2T2") printing or letterpress printing and high print resolution of the sort required in previous image array manufacturing methods is not a necessity (although sufficiently high resolution to achieve an image of acceptable appearance to the human eye is of course still desirable). Moreover, the first image can be multi-coloured, i.e. comprising at least two colours, more preferably at least three colours, but most preferably being a "full colour" image such as a RGB, RGBK or CMYK print.

Specific high resolution printing techniques can also be employed including 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 m to 3 μητ Further suitable techniques are described in WO-A-2017/081447.

In a similar manner, the second image can be single or multi-coloured while both first and second images may be screened or half-toned images. The first and second images are typically provided on a substrate and may be implemented in various different ways. In particular it should be noted that the images need not be provided directly on the surface of the substrate. There may instead be one or more pre-existing layers located on the substrate surface, on top of which the image(s) are provided. The first substrate may also be monolithic or could be multi-layered. In some embodiments, the first substrate will be at least semi-transparent (i.e. visually clear, with low optical scattering, and preferably colourless but may carry a tint), but in other cases this is not necessary and the substrate could be translucent or even opaque. If the optical density of the first image elements by themselves is sufficiently high, the second image array (variable image) can be placed over the first image without diminishing the appearance of the first image elements. However in practice the first image elements may transmit light to such a degree that the underlying second image may be visible therethrough, or otherwise affect the appearance of the first image elements in an undesirable manner. Therefore, in particularly preferred embodiments, a masking layer is provided under the first image elements.

The masking layer, which is typically present only under the first image elements and being absent in the intervening regions, comprises a material which is preferably substantially opaque to visible light (e.g. advantageously having an optical density in the range of 2 to 3 (optical density is a logarithmic ratio and hence dimensionless), such as a metal or metal alloy layer, or a pigmented masking coat such as a binder containing substantially opaque particles e.g. aluminium oxide particles. The optical density values given refer to optical density when measured on a transmission densitometer, with an aperture area equivalent to that of a circle with a 1 mm diameter. A suitable transmission densitometer is the MacBeth TD932. Providing the masking layer under each first image element helps to block any light from an underlying surface so that the appearance of the first image elements is not influenced by what lies beneath. Preferably the masking layer is of uniform appearance (e.g. colour) across the array area, and most preferably reflects substantially while light (as would be the case from a white material or silver-coloured metal) so as not to change the appearance of the first image elements. Use of a metal or metal alloy as the masking layer also provides the advantage that its surface will be reflective and so enhance the visibility of the overlying first image elements in reflected light.

The pattern will be configured in accordance with the visual effect desired to be generated by the security device of which the image array is to form part. In some preferred embodiments, the proportion of the array formed by image elements is between 40% and 60%, preferably between 45% and 55%, most preferably around 50%. This is particularly desirable where the image array is to form part of a lenticular device since then the first image will be displayed over a corresponding proportion of viewing angles (i.e. around half), and not at others, resulting in an even switching effect.

In a first preferred implementation the pattern defined by the array of elements of the first image is a line pattern, periodic in the first dimension which is perpendicular to the direction of the lines, the line pattern preferably being of straight parallel lines, and the width of the lines preferably being substantially equal to the spacing between the lines. A line pattern such as this would be particularly appropriate where the image array is to be used in a one- dimensional lenticular device for example, or in a Venetian blind device. In a second preferred implementation the pattern is a grid pattern, periodic in the first dimension and in a second dimension, wherein the grid pattern is preferably arranged on an orthogonal or hexagonal grid, the grid pattern preferably being of dots arranged according to the grid, most preferably square, rectangular, circular or polygonal dots. A grid pattern such as this would be particularly appropriate where the image array is to be used in a two-dimensional lenticular device for example, or two-dimensional Venetian blind device. In a particularly preferred example, the grid pattern is a checkerboard pattern.

Typically, the elements and/or the regions of the pattern are 50 microns or less in at least one dimension, preferably 30 microns or less, most preferably 20 microns or less. The second image may either be in contact with the first image elements or spaced from the first image elements by 15 microns or less, preferably 10 microns or less, still preferably 5 microns or less.

Typically, the array of elements of the first image are provided on a first surface of a substrate and the second image is provided on a second surface of the substrate, the substrate being at least semi-transparent. In this case, the substrate is made to be as thin as possible so that the two image planes are closely adjacent one another. The security device may be provided as a barrier strip device but preferably includes a focussing element array overlapping the array area, wherein the image array and focussing element array are configured to co-operate to generate an optically variable effect so as to form a lenticular device. Thus, the focusing element array is 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 regions therebetween in dependence on the viewing angle, whereby depending on the viewing angle the array of focusing elements directs light from either the array of first image elements or from the regions therebetween, such that as the device is tilted the first image is displayed by the first image elements in combination at a first range of viewing angles and not at a second range of viewing angles when the second image is visible.

In such lenticular devices, the focusing element array is preferably registered to the image array at least in terms of orientation and optionally also in terms of translation. The latter is not required unless it is desired to ensure a particular one of the images is displayed at particular viewing angles.

Typically, at least the first image elements are located approximately in the focal plane of the focusing element array, and the second image elements are preferably also located approximately in the focal plane of the focusing element array.

The focal length of each focusing element is typically substantially the same, preferably to within +/- 10 microns, more preferably +/- 5 microns, for all viewing angles along the direction(s) in which it is capable of focussing light.

We also provide in accordance with the invention a set of such security devices that differ from one another due to the differences between the second images.

According to a further aspect, we provide a set of security articles or security documents, each member of the set having a security device according to the invention, the second images of the security devices being different. In preferred examples, the security articles or documents differ from one another at least by virtue of a pattern or other indicia unique to the security article or document within the set and wherein the second image of each security device is in the form of the corresponding unique pattern or indicia. Examples of typical unique identifiers include serial numbers and biometric data such as a photo image, portrait, date of birth or fingerprint. The unique identifier may be a PI D key which is a product key, also known as a software key, for a computer program, where the key is required to activate or authenticate the product. Typically, the security articles are chosen from security threads, strips, foils, inserts, transfer elements, labels or patches. These articles can then be adhered to or form an integral part of a security document or the like.

Examples of security documents include banknotes, cheques, passports, identity cards, driver's licences, certificates of authenticity, fiscal stamps or other documents for securing value or personal identity. Thus, a sequence of banknotes of the same denomination and series but with unique serial numbers could be provided with security devices in which the second image is in the form of the serial number. In the case of passports, identity cards and the like which relate to the holder of the document, the second image can form unique information relating to the holder of the document such as biometric data including a photo image, fingerprint, signature and the like.

The security device could be manufactured directly on the substrate of the security document or on one or more other substrates which are applied to or incorporated into the document. For example, in a document with a transparent (e.g. polymer) substrate, such as a polymer banknote, the image array could be formed on one side of the document substrate, or on another substrate which is then laminated to it, and the focusing element array could be applied to the other side of the document substrate. In a document with a conventional paper substrate the security element could be formed on a thread, stripe or patch and incorporated into or onto the document, e.g. as a windowed thread or via hot stamping or adhesive.

We also provide a method of manufacturing a security device comprising:

providing an array of elements of a first image arranged across an array area in accordance with a pattern which is periodic at least in a first dimension, the first image elements being spaced from one another by regions; and

providing a second image underlying the array of elements of the first image, the second image extending continuously across the array area; wherein elements of the second image are exposed through the regions between the first image elements, such that the elements of both images can be viewed from the same side of the image array, and wherein the security device is one of a set of such security devices each of which has the same first image and a different second image.

As explained above, this method provides a very convenient and commercially valuable way of manufacturing not only a single security device but, particularly advantageously, a set of such security devices each of which has been provided with its own second image or, in other words, personalised.

Thus, in preferred aspects, the method comprises manufacturing a set of security devices in which a different second image is provided to each security device, the security devices otherwise being identical.

These security devices can then be applied on or into respective security articles or security documents and indeed such articles or documents could form part of the security devices. Examples of security devices, and methods of manufacture thereof, in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a lenticular device, in (a) perspective view, (b) cross-section, and (c) plan view from two different viewing angles;

Figure 2 shows steps of a first embodiment of a method of manufacturing an image array and incorporation into a security device;

Figures 3(a) to (e) show stages in the manufacture of an exemplary image array and security device in accordance with the method of Figure 2;

Figures 4(a) and (b) show two alternative security devices which may be formed via the method of Figure 2;

Figure 5 shows steps of a second embodiment of a method of manufacturing an image array and optional incorporation into a security device;

Figures 6(a) to (g) show stages in the manufacture of an exemplary image array and security device in accordance with the method of Figure 5; Figure 7 shows steps of a third embodiment of a method of manufacturing an image array and optional incorporation into a security device; and

Figures 8(a) to (g) show stages in the manufacture of an exemplary image array and security device in accordance with the method of Figure 7.

Figure 1 depicts a lenticular device 1 according to an example of the invention prior to the provision of a second image. A transparent substrate 2 (which more generally may be at least semi-transparent) is provided on one surface with an array of focussing elements 5, here in the form of cylindrical lenses, and on the other surface with an image array 10. The image array comprises first image elements 12, each of which carries a (different) portion of a corresponding first image \ i , whilst the size and shape of each first image element 12 is substantially identical. The first image elements 12 are spaced by regions 14 in which no image element is present in this example, i.e. gaps. The image elements 12 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 5, which here is along the y-axis. The lateral extent of the pattern (including its elements 12 and regions 14) is referred to as the array area.

As shown best in the cross-section of Figure 1 (b), the image array 10 and focussing element array have substantially the same periodicity as one another in the x-axis direction, such that one first image element 12 and one region 14 lies under each lens 5. In this case, as is preferred, the width w of each element 12 is approximately half that of the lens pitch p, as is the space s between each adjacent pair of elements 12 (corresponding to the width of the regions 14). Thus approximately 50% of the array area carries first image elements 12 and the other 50% corresponds to regions 14. In this example, the image array 10 is registered to the lens array 5 in the x-axis direction (i.e. in the arrays' direction of periodicity) such that a first image element 12 lies under the left half of each lens and a region 14 lies under the right half. However, registration between the lens array 5 and the image array 10 in the periodic dimension is not essential. When the device 1 is viewed by a first observer d from a first viewing angle, each lens 5 will direct light from its underlying first image element 12 to the observer, with the result that the device as a whole exhibits the complete first image \ i across the array area, as illustrated in the left diagram of Figure 1 (c). In this example, the first image is a multi-coloured sun-shaped symbol on a white background. When the device is tilted so that it is viewed by second observer 0₂ from a second viewing angle, now each lens 5 directs light from its underlying blank region 14 to the observer. As such, prior to the provision of a personalised second image, the whole array area now appears blank, as shown in the right diagram of Figure 1 (c), which effectively constitutes a temporary second image l₂. Hence, as the device 1 is tilted back and forth between the positions of observer d and observer 0₂, the appearance of the device switches between first image and second image l₂, which in this case gives the effect of first image \ i "flashing" on and off.

In order to achieve an acceptably low thickness (t) of the security device 1 (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 p of the lenses must also be around the same order of magnitude (e.g. 70 microns or 40 microns). Therefore the width w of the first image elements is preferably no more than half such dimensions, e.g. 35 microns or less.

Figure 2 shows steps of a first embodiment of a method by which the image array 10 can be formed. Figures 3(a) to (e) depict the corresponding process stages for an exemplary array. In a first step S100, a release substance 18 is applied to a first surface of a first substrate 19 (optionally after treating the substrate 19 to enhance ink adhesion, e.g. with a primer layer or by corona treatment), which in this example is a transparent substrate (e.g. PET) although this is not essential as explained below. As shown in Figure 3(a), the release substance 18 is selectively applied according to a pattern such that the release substance is present in regions P₂ of the pattern and absent in between, defining elements Pi . The pattern according to which the release substance 18 is applied ultimately defines the size, shape and position of the image elements in the image array and so, preferably, an application technique capable of high resolution is utilised. In preferred examples, the elements Pi and or the regions P₂ of the pattern are 50 microns or less in at least one dimension, preferably 30 microns or less, most preferably 20 microns or less. This at least one dimension may correspond to the line width of an elongate element, the (narrowest) side-to-side dimension of a circular or polygonal element, or the line weight of a microimage such as a letter or number, for example. Printing methods are preferred, such as intaglio, gravure, wet lithographic printing or dry lithographic printing, with which it is possible to achieve line widths down to around 30 microns. For still finer line widths, flexographic or letterpress type printing techniques are preferred, such as those disclosed in WO-A-2015/044671 . It is not necessary to apply the release substance 18 in a heavy coat weight and indeed a low coat weight may be preferred, as long as good coverage of the regions P₂ is obtained. This is beneficial since high coat weights can reduce the attainable print resolution, due to increased ink spreading under gravity. Preferred coat weights for the release substance are therefore in the range 0.5 grams per square metre (gsm) to 2 gsm, most preferably around 1 gsm. The nature of the release substance 18 itself will be described further below. A wetting agent such as ethanol may be added to the release substance to promote formation of the desired pattern regions.

Next, in step S1 10, a first image \ i is printed over the patterned release substance 18, as shown in Figure 3(b). Any printing technique can be used to apply the first image and high resolution is not a requirement (though still desirable). For example, the first image could be applied by a digital printing method such as laser printing or inkjet, or by techniques including lithographic printing, gravure printing, letterpress printing or flexographic printing. The first image may be formed of one or more inks, an "ink" being a substance comprising a binder (typically polymeric) carrying a visible colourant such as a dye, pigment or reflective (e.g. metallic or optically variable) particles. Most preferably the first image may be a multi-coloured image, i.e. including at least two and more preferably at least three colours forming the first image layer itself, all on top of the release substance 18. The term "colour" used here and throughout this disclosure includes achromatic "colours" such as black, grey and white, as well as hues such as red, green, blue etc. , and also metallic "colours" such as silver, gold, bronze etc. For example, the first image could be a full colour image such as a RGB or CMYK image and may be highly complex, e.g. a photographic image such as a portrait. The first image \ i may therefore be laid down in more than one print working using conventional multi-pass printing techniques. The different workings need only be registered to one another to the extent necessary to form a multi-coloured image which is acceptable to the human eye and need not take into account the need for accurately registered image elements, formed later.

When a set of security devices are being manufactured, the first image will be the same for each device, i.e. a generic image.

In the next step S120, the array area is processed so as to remove the patterned release substance 18 from the substrate 19, which will also lift off those portions of the first image \ i overlying the release substance regions. The result is an image array 10 as shown in Figure 3(c). Only those portions of the first image \ i located in the pattern elements Pi remain, as first image elements 12. Between the first image elements 12 are regions 14, which here are transparent gaps. The manner in which the area is processed in step S120 to remove the release substance will depend on the nature of the release substance selected. In particularly preferred examples, the release substance comprises a soluble material, most preferably water soluble, and the processing involves washing the array area with an appropriate solvent such as water. In this case the release substance 18 should preferably be highly soluble whilst also preferably having the ability to film-form into a dry, tack-free layer suitable for printing on. (It is not essential for the release substance 18 to be tack free since the first image could be printed directly over the release substance in the next step of an in-line process without any intermediate storage step which might otherwise involve winding the substrate web up on a reel). The efficiency of removal may be enhanced by also heating the solvent and/or applying mechanical removal means such as brushing or agitation. Examples of suitable soluble release substances include polyacrylic acid (PAA), polyvinyl alcohol (PVA), starch, carboxymethyl cellulose, polyethylene oxide, polyvinyl pyrolidinone, gelatine, pectin, guar gum, or gum Arabic. Alternatively, the release substance could comprise an oil (e.g. a low molecular weight oil) or a wax. Materials such as these may prevent or reduce retention of the overlying layer(s) onto the substrate, e.g. by degassing upon subsequent printing, or changing the surface energy of the substrate so as to promote reticulation and prevent proper adhesion of the print. In such cases the processing step S120 may be substantially passive, requiring little further intervention to remove the release material from the regions 14, e.g. simply wiping any loose material away. In other cases such an oil or wax based release substance may be activated by heating.

Where the release substance 18 is soluble, e.g. water-soluble, the ink(s) from which the first image is formed should of course not be significantly dissolvable by the same solvent as that in which the release substance dissolves. For example, where the release substance 18 is water-soluble, water-soluble inks (such as common ink jet inks) should not be used for the first image. However, inks based on other solvents and also radiation-curable inks (including for ink jet) may still be used.

In all cases it may be preferable for the release substance 18 to include a filler, such as pigments. This roughens the surface of the release substrate and assists in preventing layers deposited on top (such as first image ) from forming a contiguous film, which may hinder removal of the release substrate. Suitable fillers include aerosols or chalk-like pigments. Removal of the release substance may also be promoted by forming the first image as a half-toned or screened print such that it includes discontinuities. It is also desirable that the ink forming the first image layer should break cleanly along the boundaries between elements Pi and regions P₂ of the pattern, and this can be assisted by selecting an ink with a high filler (e.g. pigment) loading. Lithographic inks are particularly preferred for this purpose. Whilst the first image should be allowed to dry to an extent before step S120 is performed, it may be desirable not to allow complete drying so to avoid the first image layer forming a contiguous film.

A particularly preferred option for printing the first image \ i is to use radiation- curable (e.g. UV-curable) ink(s). In this case, the one or more inks will be printed in accordance with the first image and then step S120 will be performed to remove the regions overlying release substance 18. Subsequently, the remaining portions of the first image (the first image elements) are cured by exposure to appropriate radiation (UV) to fix the image elements. This has been found to achieve cleaner demarcation between the retained image elements and the intervening regions as compared with thermally drying inks for which the extent of drying at any point is inherently ill-defined and may vary from one location to another. The resulting structure shown in Figure 3(c) constitutes a complete image array 10 which is then used to form a security device in the manner described below.

A second image l₂, which is continuous across the array area, is provided in step S130. This is a personalised or unique image that will differ from device to device. Examples include unique serial numbers, for example of banknotes and other security instruments, or data unique to the holder of an article, such as a passport or identity card, on or in which the security device is provided. The second image will typically be the same as a unique image appearing elsewhere on the security document or article with which the security device is associated.

This may be implemented in various different ways. In a preferred approach, the second image l₂ is applied (e.g. printed) onto the second surface of the first substrate, as shown in Figure 3(d) (note that this step could equally be performed before step S100 or at any point thereafter). The second image l₂ is exposed in the regions 14 between the first image elements 12, such that both images are visible from the same side of the arrangement (the upper side as depicted in the Figures). Thus once a focusing element array 5 is provided on top of the image array 10 (step S140, Figure 3(e)), the device will appear to switch between the first and second images as the assembly is tilted. The focussing element array 5 can be applied for example by laminating the image array 10 to a transparent layer 2 carrying the lenses. In this example the transparent layer 2 is itself a transparent adhesive layer. It should be noted that the various components of the device are not shown to scale in the Figures and in practice the two image layers and l₂ will lie close to one another, preferably approximately in the focal plane of the lenses.

The form of the second image could be determined by reference to a store of second images or in some cases by reading a unique image on a security document or article and then recording that as the second image. The security device would then be affixed to the corresponding security document or article.

The second image l₂ can likewise be formed by any convenient personalisation technique such as ink-jet, laser printing, or D2T2. Importantly, no registration between the first and second images is required. Again, it is preferred that the second image l₂ is multi-coloured and it may be formed in multiple print workings. The security device will therefore display two different multi-coloured images as the device is tilted, one of which is a personalised image unique to that security device.

Substantially the same result can be arrived at in various different ways. For example, the order of steps S130 and S140 may be reversed as denoted by the different arrows shown in Figure 2. Hence, the image array 10 depicted in Figure 3(c) (carrying only the first image ) may be provided with a focusing element array 5 directly, forming a lenticular device of the sort shown in Figure 1 and then the second image is applied.

Figure 4 shows two alternative constructions of security device which may be formed using the above-described method. In these cases the first substrate 19 is itself used to space the focusing element array from the image array, i.e. acting as substrate 2 shown in Figure 1 ). The substrate 19 could therefore be a layer of a card construction or the substrate of a (polymer) banknote. In the Figure 4(a) example, after performing steps S100, S1 10 and S120 on the first surface of substrate 19, resulting in first image elements 12, the second image l₂ is then printed directly over the first image elements 12 on the same surface of substrate 19. The second image effectively fills in the regions 14 between the first image elements 12, so as to form second image element. A focusing element array 5 is applied to the second surface of the substrate 19 and both sets of image elements are viewed through the substrate 19 in use.

The Figure 4(b) example is substantially the same except that rather than print the second image l₂ over the first image elements 12, it is formed on a second substrate 20 and then laminated over the first image elements (using a transparent adhesive, not shown). Again, the second image l₂ is exposed in the regions 14 between the first image elements 12.

The Figure 4 examples are more preferable than those of Figure 3 since in Figure 3 the two image arrays are separated by the substrate 19 and hence are not at the same position with respect to the lenses unlike Figure 4 where effectively they are in the same plane. The thickness of the substrate 19 may be as low as 6 microns but this will have some effect on the device, albeit acceptable in some devices. In all cases, the focusing element array 5 can be provided before or after the second image is applied.

For best results using the method of Figure 2, the first image \ i should be applied with a high optical density, most preferably being substantially opaque, in order that when a second image l₂ is provided underneath it does not affect the appearance of the first image elements and to avoid the second image being visible at all viewing angles. If the first image elements were significantly light- transmissive, the second image would be visible to an extent through the first image elements which would diminish the resulting visual effect. In practice, it can be difficult to achieve sufficiently high print density and this also places limits on the ink compositions and printing techniques that can be used to form the first image \ i . A second embodiment of a method of manufacturing an image array which addresses this issue is shown in Figure 5, with corresponding manufacturing stages for an exemplary device being shown in Figures 6(a) to (f). Steps labelled with like numbers are the same as described with respect to the first embodiment. Hence, in the first step S100, a release substance 18 is applied to a first substrate in accordance with a pattern, as shown in Figure 6(a). Next, in step S106, a masking layer 17 is applied continuously across the array area, over the release substance 18. The masking layer is preferably substantially opaque to visible light, and most preferably is a metal or metal alloy layer, such as aluminium, copper, bronze, chromium or nickel, although any other high optical density material could be used such as a binder containing substantially opaque particles, e.g. aluminium oxide or titanium dioxide.

A first image \ i is then printed over the masking layer 17 (step 1 10), using any convenient printing technique as described previously. Again, most preferably the first image is multi-coloured.

Next, step S120 is performed whereby the array area is processed to remove the release substance 18. Since the masking layer 17 overlies the release substance 18, this also results in removal of the masking layer 17 (and the first image) from the regions 14 of the substrate to which the release substance had been applied. Again the nature of the removal process will depend on the type of release substance 18 used, as described above. The resulting image array 10 is shown in Figure 6(d) and it will be seen that portions of the masking layer 17 are retained only directly under the first image elements 12. A personalised second image l₂ is provided (step S130), either by direct application to the other side of substrate 19 as shown in Figure 6(e) or by applying the image array to another surface. The masking layer portions 17 are located between the first and second images such that, even if the first image elements 12 have low optical density, the underlying second image l₂ is substantially obscured (and preferably entirely hidden) by the masking layer locally. As such the appearance of the first image elements 12 is not affected by the underlying second image l₂. It will be appreciated that to avoid the masking layer itself affecting the appearance of the first image elements, the masking layer is preferably of uniform appearance and most preferably reflects substantially white light. A metal or metal alloy layer such as aluminium or nickel is particularly preferred for this purpose and also provides the additional advantage of high reflectivity so as to enhance the visibility of the first image.

The image array 10 is combined with a focussing element array 5 (step S140) to form a security device 1 as shown in Figure 6(f). As before this can be performed before or after the second image is provided.

Figure 6(g) illustrates a structure similar to that shown in Figure 6(f) but in addition indicates in pictorial form the lines of elements of the first image array \ i and the second image array l₂ as viewed at their respective viewing angles. As can be seen, the second image l₂ is formed from a photo image and the first image from a scene \ i .

Depending on the type of release substance 18 selected for use in the Figure 5 method, difficulties can be encountered in removing the release substance in step S120. For example, if the masking layer 17 is a metal or metal alloy layer, this can act as a barrier to the ingress of solvent fluid during washing and impede it from reaching the release substance 18. It has been found that such difficulties can be alleviated by providing the release substance with fillers such as pigments which result in a roughened surface and act to prevent the masking material forming a contiguous film over the release substance in regions 14. This enables fluid to permeate more successfully through the masking layer and thereby assists in the removal step. An alternative method according to a third embodiment which avoids this difficulty is shown in Figure 7, with corresponding manufacturing stages being depicted for an exemplary image array in Figures 8(a) to (g). Again, method steps which are the same as those described previously are denoted using like numbers. In this case, the masking layer 17 is applied to the substrate 19 first (step S106), continuously across the array area as shown in Figure 8(a). Next, step S100 is performed to apply release substance 18 onto the masking layer 17 in accordance with a desired pattern of elements Pi and regions P₂, as shown in Figure 8(b). The first image \ i is then printed over the patterned release substance 18 (step S1 10) using any technique as discussed previously. Next, the removal step S120 now comprises two stages. First, in step S122, the release substance 18 is removed together with the overlying portions on image in the same manner as previously described, the particulars of which will depend on the type of release substance in use. The resulting structure is shown in Figure 8(d) and it will be seen that the masking layer 17 is now exposed in the regions 14 between the first image elements 12. Next, in step S124, the exposed portions of the masking layer 17 are removed using the first image elements 14 to protect the unexposed portions of the masking layer 17. For example, where the masking layer 17 comprises a metal or alloy layer, the exposed portions may be removed by etching with the first image elements 15 acting as an etch resist. The resulting image array 10 is shown in Figure 8(e) which it will be seen is the same as in Figure 6(d). A personalised second image l₂ and focussing element array 5 is then provided as before. The method according to the third embodiment has the advantage that the release substance 18 is not covered by the masking layer 17 and therefore ingress of solvent is not impeded. However, the ink forming the first image does require suitable protective properties, e.g. to act as a resist against etchant. Lithographic inks and toners used in laser printing have been found suitable in this regard, amongst other examples. The use of a radiation-curable ink (as mentioned earlier) is particularly preferred in this embodiment since the ink can be made highly resistant to etchant upon curing.

Claims

1. A security device comprising:

wherein elements of the second image are exposed through the regions between the first image elements, such that the elements of both images can be viewed from the same side of the image array, and wherein the security device is one of a set of such security devices each of which has the same first image and a different second image.

2. A security device according to claim 1 , further comprising a masking layer between the first image and the second image, the masking layer being present only under the first image elements and being absent in the intervening regions.

3. A security device according to claim 1 or 2, wherein the first image and/or the second image is/are multi-coloured.

4. A security device according to claim 1 , 2 or 3, wherein the first image and/or the second image is a screened or half-toned image.

5. A security device according to at least claim 2, wherein the masking layer is substantially opaque to visible light, preferably having an optical density in the range 2 to 3.

6. A security device according to at least claim 2, wherein the masking layer is a metal or metal alloy layer, or a pigmented coating.

7. A device according to any of the preceding claims, wherein the first image has been printed by laser printing, inkjet printing, lithographic printing, gravure printing, intaglio printing, lithographic printing, flexographic printing, letterpress or dye diffusion thermal transfer printing.

8. A device according to any of the preceding claims, wherein the pattern is a line pattern, periodic in the first dimension which is perpendicular to the direction of the lines, the line pattern preferably being of straight parallel lines, and the width of the lines preferably being substantially equal to the spacing between the lines.

9. A device according to any of the preceding claims, wherein the pattern is a grid pattern, periodic in the first dimension and in a second dimension, wherein the grid pattern is preferably arranged on an orthogonal or hexagonal grid, the grid pattern preferably being of dots arranged according to the grid, most preferably square, rectangular, circular or polygonal dots, or wherein the grid pattern is a checkerboard pattern.

10. A device according to any of the preceding claims, wherein the second image has been provided by one of laser printing, inkjet printing and D2T2 printing.

11. A device according to any of the preceding claims, wherein the elements and/or the regions of the pattern are 50 microns or less in at least one dimension, preferably 30 microns or less, most preferably 20 microns or less.

12. A security device according to any of the preceding claims, wherein the second image is either in contact with the first image elements or is spaced from the first image elements by 15 microns or less, preferably 10 microns or less, still preferably 5 microns or less.

13. A security device according to any of claims 1 to 12, wherein the array of elements of the first image are provided on a first surface of a substrate and the second image is provided on a second surface of the substrate, the substrate being at least semi-transparent.

14. A security device according to any of the preceding claims, further comprising:

a focussing element array overlapping the array area;

wherein the image array and focussing element array are configured to co- operate to generate an optically variable effect.

15. A security device according to claim 14, wherein the periodicity of the focusing element array is substantially equal to or a multiple of that of the pattern, at least in the first dimension.

16. A security device according to claim 14 or 15, wherein the focusing element array is configured such that each focusing element can direct light from a respective one of the first image elements or from a respective one of second image elements in dependence on the viewing angle, whereby depending on the viewing angle the array of focusing elements directs light from either the array of first image elements or from the second image elements, such that as the device is tilted the first image is displayed by the first image elements in combination at a first range of viewing angles and the second image is displayed by the second image elements in combination at a second range of viewing angles.

17. A security device according to any of claims 14 to 16, wherein at least the first image elements are located approximately in the focal plane of the focusing element array, and the second image elements are preferably also located approximately in the focal plane of the focusing element array.

18. A security device according to any of claims 14 to 17, wherein the focal length of each focussing element is substantially the same, preferably to within +/- 10 microns, more preferably +/- 5 microns, for all viewing angles along the direction(s) in which it is capable of focussing light.

19. A security device according to any of the preceding claims, wherein the second image defines a unique identifier.

20. A security device according to claim 19, wherein the second image defines one of a serial number, a PID key and biometric data.

21. A security device according to claim 20, wherein the biometric data comprises one of a photo image such as a portrait, date of birth and a fingerprint.

22. A set of security devices according to any of the preceding claims, the security devices differing from one another at least due to the differences between the second images.

23. A set of security articles each comprising a security device according to any of the preceding claims, the second images of the security devices being different.

24. A set of security articles according to claim 23, wherein the security articles differ from one another at least by virtue of a pattern or other indicia unique to the security article within the set and wherein the second image of each security device is in the form of the corresponding unique pattern or indicia.

25. A set of security articles according to claim 23 or claim 24, wherein the set is chosen from security threads, strips, foils, inserts, transfer elements, labels or patches.

26. A set of security documents each comprising a security device according to any of the preceding claims, the second images of the security devices being different.

27. A set of security documents according to claim 26, wherein the security documents differ from one another at least by virtue of a pattern or other indicia unique to the security document within the set and wherein the second image of each security device is in the form of the corresponding unique pattern or indicia.

28. A set of security documents according to claim 27, wherein the set is chosen from banknotes, cheques, passports, identity cards, driver's licences, certificates of authenticity, labels, fiscal stamps or other documents for securing value or personal identity.

29. A method of manufacturing a security device, the method comprising: providing an array of elements of a first image arranged across an array area in accordance with a pattern which is periodic at least in a first dimension, the first image elements being spaced from one another by regions; and

providing a second image underlying the array of elements of the first image, the second image extending continuously across the array area;

30. A method according to claim 29, further comprising providing a masking layer between the first image and the second image, the masking layer being present only under the first image elements and being absent in the intervening regions.

31. A method according to claim 29 or 30, wherein the first image and/or the second image is/are multi-coloured.

32. A method according to any of claims 29 to 31 , wherein the first image and/or the second image is a screened or half-toned image.

33. A method according to at least claim 30, wherein the masking layer is substantially opaque to visible light, preferably having an optical density in the range 2 to 3.

34. A method according to at least claim 30, wherein the masking layer is a metal or metal alloy layer, or a pigmented coating.

35. A method according to any of claims 29 to 34, wherein the first image is printed by laser printing, inkjet printing, lithographic printing, gravure printing, intaglio printing, lithographic printing, flexographic printing, letterpress or dye diffusion thermal transfer printing.

36. A method according to any of claims 29 to 35, wherein the pattern is a line pattern, periodic in the first dimension which is perpendicular to the direction of the lines, the line pattern preferably being of straight parallel lines, and the width of the lines preferably being substantially equal to the spacing between the lines.

37. A method according to any of claims 29 to 36, wherein the pattern is a grid pattern, periodic in the first dimension and in a second dimension, wherein the grid pattern is preferably arranged on an orthogonal or hexagonal grid, the grid pattern preferably being of dots arranged according to the grid, most preferably square, rectangular, circular or polygonal dots, or wherein the grid pattern is a checkerboard pattern.

38. A method according to any of claims 30 to 38, wherein the second image is provided by one of laser printing, inkjet printing and D2T2 printing.

39. A method according to any of claims 29 to 38, wherein the elements and/or the regions of the pattern are 50 microns or less in at least one dimension, preferably 30 microns or less, most preferably 20 microns or less.

40. A method according to any of claims 29 to 39, wherein the second image is either in contact with the first image elements or is spaced from the first image elements by 15 microns or less, preferably 10 microns or less, still preferably 5 microns or less.

41. A method according to any of claims 29 to 40, wherein the array of elements of the first image are provided on a first surface of a substrate and the second image is provided on a second surface of the substrate, the substrate being at least semi-transparent.

42. A method according to any of claims 29 to 41 , further comprising:

providing a focussing element array overlapping the array area;

wherein the image array and focussing element array are configured to cooperate to generate an optically variable effect.

43. A method according to claim 42, wherein the periodicity of the focusing element array is substantially equal to or a multiple of that of the pattern, at least in the first dimension.

44. A method according to claim 42 or 43, wherein the focusing element array is configured such that each focusing element can direct light from a respective one of the first image elements or from a respective one of second image elements in dependence on the viewing angle whereby depending on the viewing angle the array of focusing elements directs light from either the array of first image elements or from the second image elements, such that as the device is tilted the first image is displayed by the first image elements in combination at a first range of viewing angles and the second image is displayed by the second image elements in combination at a second range of viewing angles.

45. A method according to any of claims 42 to 44, wherein at least the first image elements are located approximately in the focal plane of the focusing element array, and the second image elements are preferably also located approximately in the focal plane of the focusing element array.

46. A method according to any of claims 42 to 45, wherein the focal length of each focussing element is substantially the same, preferably to within +/- 10 microns, more preferably +/- 5 microns, for all viewing angles along the direction(s) in which it is capable of focussing light.

47. A method according to any of claims 29 to 46, wherein the second image defines a unique identifier.

48. A method according to claim 47, wherein the second image defines one of a serial number, a PID key and biometric data.

49. A method according to claim 48, wherein the biometric data comprises one of a photo image such as a portrait, date of birth and a fingerprint.

50. A method according to any of claims 29 to 49 of manufacturing a set of security devices having the same first image and different second images.

51. A method according to claim 50, wherein each security device is incorporated in or on a respective security article.

52. A method according to claim 51 , wherein the security articles differ from one another at least by virtue of a pattern or other indicia unique to the security article within the set and wherein the second image of each security device is in the form of the corresponding unique pattern or indicia.

53. A method according to claim 51 or claim 52, wherein the set of security articles is chosen from security threads, strips, foils, inserts, transfer elements, labels or patches.

54. A method according to any of claims 29 to 49, wherein each security device is incorporated in or on a respective security document.

55. A method according to claim 54, wherein the security documents differ from one another at least by virtue of a pattern or other indicia unique to the security document within the set and wherein the second image of each security device is in the form of the corresponding unique pattern or indicia.

56. A method according to claim 54 or claim 55, wherein the set of security documents is chosen from banknotes, cheques, passports, identity cards, driver's licences, certificates of authenticity, fiscal stamps, labels or other documents for securing value or personal identity.