Security devices and methods of manufacture thereof

Abstract

A security device is disclosed. The security device comprises: a substrate; an array of viewing elements disposed in or on the substrate; and an image layer disposed in or on the substrate and overlapping with the array of viewing elements, the image layer comprising a plurality of sets of image segments interlaced with each other, wherein each set of image segments in combination defines a respective image; wherein the array of viewing elements and the image layer cooperate with each other such that the device selectively exhibits the images in dependence on viewing angle; and wherein the image layer comprises a repeating arrangement of the plurality of sets of image segments, wherein the repeating arrangement has a repeat dimension that is larger than a pitch of the array of viewing elements such that for each of a plurality of different viewing angles of the device, a first subset of the viewing elements directs light from one of the plurality of sets of image segments to the viewer and a second subset of the viewing elements directs light from a different one of the plurality of sets of image segments to the viewer; whereby for each of a plurality of different viewing angles of the device, light from at least two different sets of image segments is directed to the viewer simultaneously. A method of manufacturing such security devices is also disclosed.

Description

[0001] SECURITY DEVICES AND METHODS OF MANUFACTURE THEREOF

[0002] FIELD OF THE INVENTION

[0003] This invention relates to security devices that may be 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 such security devices are also disclosed.

[0004] BACKGROUND

[0005] 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.

[0006] One class of security devices are lenticular devices, which make use of focussing elements (such as lenses) to produce an optically variable effect, meaning that the appearance of the device is different at different angles of view and / or illumination. Such devices are particularly effective as security devices since direct copies (e.g. photocopies) will not produce the optically variable effect and hence can be readily distinguished from genuine devices.

[0007] In lenticular devices, an array of viewing elements, typically cylindrical lenses, overlies an image layer having a corresponding array of image segments, each of which depicts only a portion of an image which is to be displayed. Image segments from two or more different images are interleaved and, when viewed through the array of viewing elements, at each viewing angle, only selected image segments will be directed towards the viewer. In this way, different composite images can be viewed at different angles. Some examples of lenticular devices are described in US-A-4892336, WO-A-2011 / 051669, WO-A-2011051670 and US-B-6856462. 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 that allow for simple authentication of a device that is simultaneously difficult to counterfeit.

[0008] A problem with lenticular devices is that the number of images displayed by the device is dependent on the number of image segments that are positioned under each focussing element. For an “n-channel” device that exhibits n images in dependence on viewing angle, n different image segments corresponding to the n images are laterally positioned under each lens such that as the viewing angle changes, each lens directs light from a particular one of the image segments to the viewer. As the number of images to be displayed increases, the lateral width of each image segment is required to be reduced in order to accommodate the desired number of image segments under each lens. It would be desirable to increase the number of images displayed by a lenticular device so as to increase the complexity of the exhibited optically variable effect (such as an animation sequence) and increase the security level. However, as the number of image channels, n, increases, it becomes more difficult to achieve the required printing resolution and registration requirements.

[0009] Consequently, improvements to security devices, and in particular lenticular security devices, are constantly sought in order to stay ahead of would-be counterfeiters.

[0010] SUMMARY OF INVENTION

[0011] In accordance with a first aspect of the invention there is provided a (e.g. lenticular) security device, comprising: a substrate; an array of viewing elements disposed in or on the substrate; and an image layer disposed in or on the substrate and overlapping with the array of viewing elements, the image layer comprising a plurality of sets of image segments interlaced with each other, wherein each set of image segments in combination defines a respective image; wherein the array of viewing elements and the image layer cooperate with each other such that the device selectively exhibits the images in dependence on viewing angle; and wherein the image layer comprises a repeating arrangement of the plurality of sets of image segments, wherein the repeating arrangement has a repeat dimension that is larger than a pitch of the array of viewing elements such that for each of a plurality of different viewing angles of the device, a first subset of the viewing elements directs light from one of the plurality of sets of image segments to the viewer and a second subset of the viewing elements directs light from a different one of the plurality of sets of image segments to the viewer; whereby for each of a plurality of different viewing angles of the device, light from at least two different sets of image segments is directed to the viewer simultaneously.

[0012] The array of viewing elements and the image layer cooperate with each other such that the device selectively exhibits the different images in dependence on viewing angle. In other words, as the device is tilted so as to change the viewing angle, different images are exhibited by the device. The security device is preferably a lenticular security device.

[0013] The security device according to the present invention advantageously increases the ease with which the number of (different) images exhibited by the device as the viewing angle changes can be increased. This is achieved by utilising the fact that the human eye processes the visual effect exhibited by the device over multiple viewing elements. The image layer of the device comprises a repeating arrangement (e.g. “periodic arrangement”) of the sets of image segments that has a repeat dimension larger than the pitch of the viewing element array. In this way, the device is configured such that for each of a plurality of different viewing angles (preferably for any particular viewing angle), a first subset of the viewing elements directs light from one of the plurality of sets of image segments to the viewer and a second subset of the viewing elements (different from the first subset) directs light from a different one of the plurality of sets of image segments. In this way, for each of a plurality of different viewing angles of the device, the viewer perceives at least a portion of each of the corresponding images simultaneously.

[0014] In other words, for each of a plurality of different viewing angles, the first subset of viewing elements exhibits (at least a portion of) a first image to the viewer but not a second image, and simultaneously the second subset of viewing elements exhibits (at least a portion of) the second image but not the first image. Thus, the observer perceives two different images simultaneously, “originating” from different subsets of the viewing elements.

[0015] Arranging the interlacing of the sets of image segments across multiple viewing elements as in the present invention is a fundamentally different approach from conventional lenticular devices in which each viewing element is associated with an image segment for each of the exhibited images, whereby at any given viewing angle, the device directs light from only one set of image segments to the viewer such that only one image is perceived. In other words, in conventional devices, a repeat dimension of the image segment interlacing is equal to the pitch of the viewing element array, in contrast to the present invention in which the repeat dimension is larger than the pitch of the array of viewing elements.

[0016] Typically the repeat dimension (e.g. repeat distance) is larger than the pitch of the viewing element array by a factor of at least 2.

[0017] Each set of image segment comprises image layer material (e.g. ink) that defines the respective image. Thus, each set of image segments typically comprises regions absent of image layer material. Each image segment, particularly in the case of devices in which the viewing elements are in the form of “one dimensional” cylindrical lenses (described further herein) may be described as an “image strip”. Herein, the term “image” as perceived by a viewer of the device refers to the graphical form of the image formed by the image layer material. It is noted that the term “repeating arrangement” here refers to the relative positioning of image segments with respect to the viewing elements, rather than the specific form of the image segments (e.g. the form of the image layer material within the image segments) themselves.

[0018] Typically, the repeating arrangement comprises a repeating arrangement of regions, each region laterally overlapping with a plurality of viewing elements; wherein each region has a first zone and a second zone, the first zone being configured to cooperate with the first subset of viewing elements and the second zone being configured to cooperate with the second subset of viewing elements; and wherein for each region, the sets of image segments and / or the arrangement thereof in the first zone is different from the sets of image segments and / or the arrangement thereof in the second zone.

[0019] Typically, the plurality of regions are laterally contiguous. Similarly, the first and second subsets of viewing elements are typically laterally contiguous. The first and second subsets of viewing elements are typically interleaved with each other in accordance with the arrangement of the zones.

[0020] The repeating arrangement of regions (and therefore repeating arrangement of zones) repeats across at least a part of (and in some cases the entire) image layer. Thus, in some embodiments a region may be considered to be a repeating unit, or “unit cell” of the image layer. It will be appreciated that a region may comprise three or more zones dependent on the desired arrangement and number of images to be exhibited by the device.

[0021] Typically, each region has a dimension less than 500 pm. Preferably, each region has a dimension such that it is not individually resolvable by the naked human eye. It is generally accepted that the lower limit of human vision resolution is of the order of 150pm at typical viewing distances of the device (~30cm). Therefore, preferably each region has a dimension less than 150pm, more preferably less than 100pm and even more preferably less than 70pm.

[0022] Thus, at a particular viewing angle of the device, the viewer perceives an image originating from the image segments of the first zone of each region (via the first subset of viewing elements), and simultaneously an image originating from the image segments of the second zone of each region (via the second subset of viewing elements). Typically, for each zone, a repeat dimension of the image segments corresponds to a single viewing element. In other words, the image segments and the corresponding subset of viewing elements (e.g. lenses) cooperate such that at a particular nthviewing angle (with n being the number of different image segments within the zone), only one image is exhibited to the viewer. Typically, within each region, the first and second zones are laterally contiguous.

[0023] For each region, the sets of image segments in the first zone may be different from the sets of image segments in the second zone. For example, as will be described further herein, the first zone may comprise image segments that each form part of “primary” images, and the second zone may comprise image segments that each form part of “secondary” images, with each image being different. In some embodiments, the image segments in the first zone may have a different arrangement from the image segments in the second zone. In such examples, the image segments in each zone may be constituent parts of the same images, but the arrangement of the image segments (e.g. the positioning of each image segment with respect to the respective viewing element) in each zone differs such that for the same viewing angle, the viewer perceives different images from the first zone and second zone.

[0024] Typically, the number of different sets of image segments within the first zones is the same as the number of different sets of image segments within the second zones. However, this is not necessarily the case, and in some embodiments different zones may comprise different numbers of sets of image segments. Typically, for each set of image segments, the image segments within corresponding zones have the same relative position with respect to a viewing element, dependent on the desired viewing angle for the image segments.

[0025] Typically, the plurality of sets of image segments comprises two or more sets of primary image segments, each set of primary image segments in combination defining a respective primary image and at least one set of secondary image segments, the at least one set of secondary image segments in combination defining a respective secondary image. In embodiments, the primary images may be perceived with increased perceived density (e.g. “brightness”) relative to the secondary images.

[0026] In some embodiments, the first zones each comprise only image segments of each of the two or more sets of primary image segments, and the second zones each comprise only image segments of each of the at least one set of secondary image segments, wherein the first zones are configured to cooperate with the first subset of viewing elements such that the device selectively exhibits the two or more primary images dependent on viewing angle and the second zones are configured to cooperate with the second subset of viewing elements such that the device selectively exhibits the secondary image(s) dependent on viewing angle, whereby for each of a plurality of viewing angles of the device, the viewer perceives (e.g. at least a portion of) a primary image and an secondary image simultaneously.

[0027] Thus, in such embodiments, at a particular viewing angle the observer perceives a primary image resulting from the cooperation of the first zones with the corresponding first subset of viewing elements, and simultaneously a secondary image resulting from the cooperation of the second zones with the corresponding second subset of viewing elements.

[0028] Such an arrangement in which the first zone comprises only primary image segments and the second zone comprises only secondary image segments provides a convenient way to structure the image layer such that the primary and secondary images are exhibited. However, in some alternative embodiments, at least one of the first and second zones may comprise image segments from both the primary and secondary sets of image segments. In other words, in general, the primary and secondary image segments may be “freely distributed” within a region (as long as the relative positions of each image segment within a set of image segments remains consistent with respect to the viewing elements) as the eye will average the visual effect across the plurality of viewing associated with the region.

[0029] Although not essential, preferably the primary images and the secondary image(s) define an animation sequence. Advantageously, by increasing the number of images displayed by the device in comparison to conventional lenticular devices, such an animation sequence, exhibited as the device is tilted, may be enhanced due to the increased number of image frames (e.g. a “smoother” transition between image frames). This provides an enhanced optical effect that is easy to authenticate and yet difficult to counterfeit. Typically, the primary images exhibit an animation sequence in isolation, with the secondary images providing “intermediate” image frames between the primary images to enhance, or “smooth” the animation. Preferably, the animation sequence comprises any of: a lateral movement, a rotation, an expansion or contraction. The animation may comprise a change in colour. It will be appreciated that the individual images of the animation sequence will need to be sufficiently similar (e.g. exhibiting small changes in spatial position) in order for the eye to perceive the images frames as an animation.

[0030] The primary and secondary images in general may or may not laterally overlap. Herein, the term “laterally overlap” refers to the overlap of the images when the device is viewed (e.g. overlap of the “macro” images in a viewing plane). For example, if a primary and secondary image do not laterally overlap, the viewer (e.g. simultaneously) perceives the full outline of each image laterally separate from each other. On the other hand, if a primary and secondary image do overlap, the viewer will perceive a region in the resulting image from viewing both images simultaneously where the first and second images laterally overlap, as well as a region where only the first image is visible, and a region where only the second image is visible. Each image displayed by the device is different (in other words the images defined by each set of image segments in combination are different from each other). Therefore, there will only be partial overlap, rather than full overlap, between any two images. Herein, two images are different if they occupy different spatial positions (even if they have the same graphical form). Images may also differ in colour and / or graphical form.

[0031] In some preferred embodiments, at least one of the primary images overlaps with at least one of the secondary image(s). Such embodiments may advantageously smooth the animation effect provided by the primary and secondary images.

[0032] Typically, the number of sets of primary image segments is M, and the number of sets of secondary image segments is m, wherein M>m, preferably either M=m or M=m+1.

[0033] As discussed above, each region laterally overlaps with a plurality of viewing elements. Preferably, each region overlaps with (e.g. each region is associated with) N viewing elements, wherein N is in the range of 2 to 25, preferably in the range of 2 to 10, even more preferably in the range of 2 to 6. In other words, preferably, the repeating arrangement of the plurality of sets of image segments has a repeat dimension that is larger than a pitch of the array of viewing elements by a factor in the range of 2 to 25, preferably by a factor in the range of 2 to 10, even more preferably by a factor in the range of 2 to 6.

[0034] Advantageously, the ratios of the image segments within each region may be adjusted to control the visual effect exhibited to the observer. For example, the ratio of the primary image segments to the secondary image segments may be greater than 1 :1 such that the primary images appear with increased perceived density, or brightness, in comparison to the secondary images. Typically, N is an integer, although this is not essential and in some embodiments N may not be an integer, for example if the viewing element array and image segments are out of synchronization.

[0035] Preferably, for each region, for each viewing angle of the device, the ratio of primary image segments to secondary image segments is between 1 :1 and 20:1 , preferably between 2:1 and 10:1. In other words, within a region, for each relative position with respect to a viewing element (e.g. corresponding to a viewing angle of the device), the ratio of primary image segments to secondary image segments is between 1 : 1 and 20: 1 , preferably between 2: 1 and 10:1. For embodiments in which the first zones comprise only image segments of the primary image channels, and the second zones comprise only image segments of the secondary image channels, each first zone is associated with x viewing elements of the array of viewing elements, and each second zone is associated with y viewing elements, and wherein a ratio of x / y is between 1 :1 and 20:1 , preferably between 2:1 and 10:1.

[0036] For each viewing angle, the ratio of primary image segments to secondary image segments within a region may be the same for all regions across the image layer. However, in some embodiments, for each viewing angle of the device, the ratio of primary image segments to secondary image segments may be different for different regions across the image layer. Thus, the image layer may comprise different repeating arrangements of regions in different areas of the device. This advantageously allows complex effects to be generated across the domain of the device, for example to control the relative perceived density of the primary and secondary images, and / or the amount of perceived animation in different areas of the device.

[0037] Typically, the variation in the ratio of primary image segments to secondary image segments may be in accordance with one or more images exhibited by the device.

[0038] As has been discussed, the present invention utilises the fact that the human eye will perceive the visual effect exhibited by a device across multiple lenses of the lens array, in order to simultaneously exhibit two or more different images to the viewer that “originate” from different subsets of the lens array. In some embodiments, the number of image channels that the device exhibits may be further increased by additionally configuring the image layer such that for a particular viewing angle, each individual lens exhibits more than one image segment contemporaneously. Hence, in some embodiments, the viewing elements are elongate viewing elements; and the image layer comprises a plurality of first regions and a plurality of second regions, wherein the image segments and / or the arrangement thereof in the first regions is different from the image segments and / or the arrangement thereof in the second regions, wherein the first regions and second regions are interlaced with each other along the direction of elongation of the viewing elements, such that at least at one viewing angle, the viewer perceives (e.g. at least a portion of) at least two (e.g. different) images from the same subset of viewing elements, simultaneously.

[0039] In this way, for a given viewing angle of the device, the viewer will simultaneously perceive different images from respective different subsets of the viewing element array, as well as different images from an individual subset of viewing elements (e.g. light from different image segments is directed to the viewer from a common viewing element). Therefore, such embodiments advantageously enable additional image channels to be included within the device, for example to further enhance (e.g. “smooth”) an animation sequence that is exhibited by the device upon tilting.

[0040] In such embodiments, the viewing elements are elongate viewing elements, with the plurality of first regions interlaced with the second regions along the direction of elongation of the viewing elements. The elongate viewing elements are typically cylindrical lenses adapted to focus light in one dimension (e.g. focussing light to a line). Thus, for any given viewing angle of the device, due to the interlacing of the first and second regions along the direction of elongation of the lenses, each lens will direct light from both the first and second regions to the viewer, thereby exhibiting the corresponding (different) images simultaneously. The interlacing of the first and second regions along the direction of elongation of the viewing elements means that the lateral area occupied by each image segment within the image layer is reduced, thereby reducing the perceived density of the corresponding images exhibited by the device. This may be acceptable in trade-off for the increased number of image channels exhibited by the device and the advantages that this provides. In some embodiments, the relative brightness of the images exhibited by the device may be controlled by the ratio of the length of the first regions (and therefore the lateral area of the image segments within the first regions) to the length of the second regions (and therefore the lateral area of the image segments within the second regions) along the direction of elongation of the viewing elements. Typically, a ratio of a length of the first regions along the direction of elongation of the viewing elements to a length of the second regions along the direction of elongation of the viewing elements is between 1 :1 and 10:1 , preferably between 2:1 and 10:1.

[0041] Typically, the number of image segments within the first regions will be the same as the number of image segments within the second regions. However, this is not essential and the number of image segments within the first and second regions may differ. A difference in the number of image segments arranged within the first and second regions may alternatively or additionally be used to control the relative perceived density of the exhibited images.

[0042] Preferably, at least one image exhibited by the device (typically each image exhibited by the device) is in the form of (e.g. may consist of) indicia or an indicium, preferably one or more geometric shapes, letters, logos, currency signs or other symbols. Where the optically variable effect exhibited is in the form of an animation, the animation sequence the animation sequence may comprise any of: a lateral movement, a rotation, an expansion or contraction of the indicia or indicum.

[0043] In the present invention, the array of viewing elements may take various forms. In preferred embodiments, the array of viewing elements comprises an array of focussing elements. The focussing elements may be adapted to focus light in one dimension, in which case the focussing elements are preferably (e.g. elongate) cylindrical focussing elements. In such embodiments, the image segments are typically elongate image segments. Embodiments of the present invention may also be applied to two-dimensional lenticular devices. In such embodiments, the viewing elements are adapted to focus light in at least two (e.g. non-parallel, preferably orthogonal) directions, in which case the viewing elements are preferably spherical or aspherical focussing elements.

[0044] The pitch of the viewing element array (e.g. array of lenses) is typically in the range of 10 pm to 200 pm, preferably 20 pm to 200 pm, more preferably 50 pm to 200 pm. The pitch of the viewing elements is typically uniform across the array.

[0045] Although the viewing elements are typically in the form of focussing elements such as lenses, in some embodiments the array of viewing elements may be in the form of a masking grid. In such embodiments, each viewing element typically comprises a substantially opaque region and a substantially transparent region, such that the masking grid comprises a plurality substantially opaque regions spaced by gap regions. The image layer is viewable through the substantially transparent regions.

[0046] Preferably, in the case where the viewing elements are focussing elements, the image layer is located approximately in the focal plane of the array of focussing elements. The required spacing between the focussing elements and the image layer may be provided by the substrate itself and / or any optical spacing or pedestal layer as is known in the art.

[0047] In typical embodiments, the substrate is at least semi-transparent (preferably fully transparent), and wherein the array of viewing elements is disposed in or on a first surface of the substrate and the image layer is disposed in or on a second, opposing surface of the substrate. It is noted that the term “on” does not necessarily mean in direct contact; for example, there may be a primer layer positioned between the substrate and the array of viewing elements. It will be appreciated that in such configurations the substrate will need to be at least semi- transparent (the term “transparent” herein being used to mean optically clear and non-scattering, although may carry a coloured tint). In this case, the substrate is typically formed of one or more polymer materials, such as BOPP, PET, PE, PC or the like. In alternative embodiments, the viewing elements may be disposed on the same side of the substrate as the image layer, e.g. by incorporating an optical spacing into their design or providing an at least semi-transparent pedestal layer between the viewing elements and the image layer. In such embodiments, the substrate need not be semi-transparent and may be of any type, opaque or otherwise. This includes paper substrates, although polymer-based substrates are preferred.

[0048] The image layer is preferably provided by a print working, preferably printed by a gravure, intaglio, screen, micro-intaglio, flexographic or (wet or dry) lithographic technique, or by a digital printing technique, for example inkjet or laser printing. With careful design and implementation, such techniques can be used to print image segments with a line width (e.g. in the direction of interlacing) of between 10pm and 100pm. For example, with flexographic or wet lithographic printing it is possible to achieve line widths down to about 5-25pm. In this way, the image segments may be described as “microimage” segments. The image layer is typically formed as a single layer (“image layer”) disposed in or on the substrate. Where provided as a print working, the image layer is preferably formed in a single print working (e.g. one pass of a printing machine). Other (non-print) methods of forming the image layer may be used. For example the image segments may be in the form of, or comprise, metallised or de-metallised regions, filled recesses, laser-marked regions, or (e.g. diffractive) surface relief structures including first order, zero order and sub-wavelength gratings such as plasmonic structures. Other examples include cast or embossed recesses (that may be filled or unfilled with marking material), and / or posts.

[0049] In some embodiments the image layer may comprise image segments formed from different mechanisms. It is envisaged that in such embodiments, the use of different image layer formation mechanisms may be used to provide differences in perceived density of the images in different areas of the device. The security device of the invention may be intended for viewing only under visible light illumination, in which case each of the images defined by the respective sets of image segments is perceivable by the viewer under visible light illumination. In some embodiments, the device may utilise luminescent material(s) (which term includes materials or substances having fluorescent or phosphorescent properties) in the image layer to generate complex visual effects to further enhance the security level of the device. For example, in some embodiments each of the plurality of sets of image segments may comprise a luminescent material which luminesces in response to irradiation at at least one excitation wavelength, whereby each of the plurality of sets of image segments exhibits a luminescent visible colour when illuminated with an excitation illumination condition that comprises illumination with the at least one excitation wavelength. Such luminescent materials respond visibly to irradiation at a certain wavelength or range of wavelengths outside the visible spectrum (typically within the ultraviolet, UV, region of the electromagnetic spectrum), typically by emitting light of a particular colour characteristic of the material in question. Typically, the at least one excitation wavelength is at least one wavelength within the ultra-violet (UV) part of the electromagnetic spectrum. In such cases the excitation illumination condition may be referred to for brevity as illumination under “UV light” or “UV illumination”.

[0050] In this way, when the device is viewed under the excitation illumination condition and light from at least two different sets of image segments is directed to the viewer simultaneously (at a particular viewing angle), the viewer perceives at least a portion of each of the corresponding (luminescing) images simultaneously. Such an effect under the excitation illumination condition may advantageously increase the security level of the device.

[0051] In such embodiments the image layer material is preferably an ink comprising the luminescent material. The luminescent material may be the same for each set of image segments, or may be different for different sets of image segments (e.g. exhibiting different colours under specified illumination conditions). The image layer material for any particular set of image segments may exhibit different visual appearances under different illumination conditions. For example, under a first illumination condition that that comprises illumination with visible light in the absence of the at least one excitation wavelength the image layer material of a set of image segments may exhibit a first non-luminescent visible colour, and under the excitation illumination condition the image layer material may exhibit a luminescent visible colour different from the non-luminescent visible colour. This may advantageously be used to achieve different effects under different illumination conditions. Herein, “different” colours or appearances are those which are visibly different to the naked eye without a close inspection.

[0052] The term “non-luminescent visible colour” simply refers to the colour (e.g. chromatics such as red, blue, yellow, green, brown etc.) exhibited when the device is illuminated with visible light in the absence of the at least one excitation wavelength. Similarly, the term “luminescent visible colour” refers to a colour exhibited when the device is illuminated with the excitation wavelength (e.g. viewing the device under illumination by a UV lamp). “Visible light” refers to light having a wavelength within the visible spectrum, which is approximately 400 to 750nm. It is most preferable that the visible light is white light, i.e. contains substantially all the visible wavelengths in more or less even proportion. The first illumination condition “comprising illumination with visible light in the absence of the at least one excitation wavelength” may also be referred to herein for brevity as “visible light”, “visible light only” or “non-UV light”. The ultraviolet spectrum typically comprises wavelengths from about 200nm to about 400nm.

[0053] In some embodiments, the image layer material of at least one of the sets of image segments is substantially colourless when illuminated with a first illumination condition that comprises illumination with visible light in the absence of the at least one excitation wavelength. Such embodiments provide a striking visual effect where, under illumination with the excitation illumination condition (e.g. under a UV lamp), the number of image channels is perceived to increase compared to viewing under the first illumination condition. Typically, the sets of image segments having image layer material that is substantially colourless under the first illumination condition are image segments of the second zone. For example, in some embodiments, each set of image segments in the second zone may be substantially colourless under the first illumination condition. A second aspect of the invention provides a security article comprising the security device as described above, wherein the security article is preferably a security thread, strip, foil, insert, transfer element, label, patch, or a data page for a security document such as a passport. Security articles such as these, carrying the security device, can then be applied to or incorporated in a security document or any other object, e.g. by hot stamping, cold stamping, via adhesive or lamination, or by introduction during papermaking.

[0054] A third aspect of the invention provides a security document comprising a security device or security article as described above, wherein the security document is preferably a banknote, cheque, passport, identity card, driver’s licence, certificate of authenticity, fiscal stamp or other document for securing value or personal identity. The security device can either be formed directly on the security document, in which case the document substrate may act as the substrate of the security device, or could be formed on a security article which is then applied to or incorporated into the security document as described above.

[0055] In accordance with a fourth aspect of the invention, there is provided a method of manufacturing a (e.g. lenticular) security device, comprising:

[0056] (a) providing a substrate;

[0057] (b) applying an array of viewing elements to the substrate; and

[0058] (c) forming an image layer in or on the substrate, the image layer overlapping with the array of viewing elements, the image layer comprising a plurality of sets of image segments interlaced with each other, wherein each set of image segments in combination defines a respective image; wherein the array of viewing elements and the image layer cooperate with each other such that the device selectively exhibits the images in dependence on viewing angle; and wherein the image layer comprises a repeating arrangement of the plurality of sets of image segments, wherein the repeating arrangement has a repeat dimension that is larger than a pitch of the array of viewing elements such that for each of a plurality of different viewing angles of the device, a first subset of the viewing elements directs light from one of the plurality of sets of image segments to the viewer and a second subset of the viewing elements directs light from a different one of the plurality of sets of image segments to the viewer; whereby for each of a plurality of different viewing angles of the device, light from at least two different sets of image segments is directed to the viewer simultaneously.

[0059] The result of the method of the fourth aspect is a security device of the sort already described above in relation to the first aspect of the invention, with all the advantages discussed. Any of the preferred features described above could be provided via appropriate adaptation of the method.

[0060] Typically, the image layer may be formed by a printing technique, preferably a gravure, intaglio, screen, micro-intaglio, flexographic, lithographic or digital technique. Typically, the image layer is formed in a single print working. However, other (non-print) methods of forming the image layer may be used, for example metallisation, de-metallisation, casting and filling recesses, laser marking, and forming (e.g. diffractive) surface relief structures including first order, zero order and sub-wavelength gratings such as plasmonic structures. Other examples include casting or embossing of recesses (that may be filled or unfilled with marking material), and / or posts.

[0061] The viewing elements (typically focussing elements) can be produced by known means such as embossing or cast-curing, and may be formed directly on the substrate or on a separate substrate from which they are transferred to the device, or which is attached to and then forms part of the device substrate. In some cases the viewing elements may be applied to the substrate by forming (e.g. embossing) the viewing elements into the substrate material itself. The array of viewing elements and the image layer may be provided in either order. In other words, the array of viewing elements may be applied to the substrate before the application of the image layer, or vice-versa. However, in preferred embodiments, the viewing elements (e.g. lenses) are applied to a first side of the substrate and the image layer is applied to a second, opposing, side of the substrate simultaneously at the same location along the substrate. Such simultaneous application of the viewing elements and image layer advantageously provides highly accurate register between the two.

[0062] BRIEF DESCRIPTION OF DRAWINGS

[0063] Embodiments of the invention will now be described with reference to the appended drawings, in which:-

[0064] Figure 1 schematically illustrates a security document carrying a conventional lenticular device as is known in the art;

[0065] Figure 2 is a cross-sectional view of a conventional lenticular device;

[0066] Figure 3(a) schematically illustrates a security device according to an embodiment of the invention, together with the images perceived at different viewing angles;

[0067] Figure 3(b) schematically illustrates the images exhibited by the device of Figure 3(a);

[0068] Figure 3(c) is a schematic view of a portion of the security device of Figure 3(a);

[0069] Figure 4(a) schematically illustrates a security device according to a further embodiment of the invention, and Figure 4(b) illustrates the images exhibited by the device;

[0070] Figures 5(a) and 5(b) illustrate the optically variable effect exhibited by a security device according to a further embodiment of the invention;

[0071] Figure 6(a) schematically illustrates a conventional lenticular device;

[0072] Figure 6(b) schematically illustrates, in cross-section, a security device according to an embodiment of the invention, and Figure 6(c) illustrates the images exhibited by the device;

[0073] Figures 7(a) to 7(c) schematically illustrate a security device according to a further embodiment of the invention, together with the optically variable effect exhibited by the device; Figures 8(a) and 8(b) schematically illustrate a further embodiment of the invention;

[0074] Figure 9 schematically illustrates a further embodiment of the invention;

[0075] Figure 10 schematically illustrates a further embodiment of the invention;

[0076] Figure 10A schematically illustrates a further embodiment of the invention;

[0077] Figures 11 , 12 and 13 show three exemplary security documents carrying security devices made in accordance with embodiments of the present invention (a) in plan view, and (b) / (c) in cross-section;

[0078] Figure 14 illustrates a further embodiment of a security document carrying a security device made in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section;

[0079] Figures 15A to 151 illustrate different examples of relief structures which may be used to form the image layer in embodiments of the invention; and

[0080] Figure 16(a) schematically illustrates, in cross-section, a security device according to an embodiment of the invention, and Figure 16(b) illustrates the images exhibited by the device.

[0081] DETAILED DESCRIPTION

[0082] For clarity of explanation, various figures herein use different shading patterns to schematically illustrate image segments of different sets of image segments (different image channels). The use of shading does not necessarily indicate the arrangement or colour of the image layer material present within the image segments. Cross-sectional diagrams schematically illustrate the position of the image layer material if it is required to be present at any particular location to form the respective image.

[0083] Figure 1 schematically illustrates, in plan view, a security document 1000, here in the form of a banknote, carrying a conventional lenticular security device 101 known in the art. Figure 2 illustrates a cross-sectional view of the device 101 along the line Q-Q’. The device 101 comprises a transparent substrate 10, which may or may not be the base substrate of the document. On a first side 10a of the substrate 10 there is disposed an array 20 of cylindrical lenses 21 that extend parallel to each other and into the plane of the page (along the z-dimension). On the opposing side 10b of the substrate, the device 101 comprises an image layer 30 comprising a plurality of image segments comprising image layer material (e.g. ink) that in combination form the images exhibited by the device. The thickness, T, of the substrate 10 substantially corresponds to the focal length of the lenses 21 such that the image layer 30 is formed substantially within the focal plane of the lens array 20. In this example, the image layer 30 is formed as a single image layer.

[0084] The image layer comprises a first set of image segments and a second set of image segments i2that are interleaved with each other periodically along the x- direction. The first image segments together define the first image 11 that is viewable at a first viewing angle 01 , and the second image segments together define a second image I2 that is viewable at a second viewing angle 02. In this way, each set of image segments defines an image channel, such that in this example the device is a two-channel lenticular device. As would be understood by the skilled reader, each image segment of a particular image channel occupies the same relative position under the respective lens. Here, in the view of Figure 2, each image segment i1 of the first image channel occupies the “left” half of the respective lens corresponding to viewing angle 01 , and each image segment i2 of the second image channel occupies the “right” half of the respective lens corresponding to viewing angle 02. In this example, each image segment is in the form of an elongate line element extending parallel with the direction of elongation of the cylindrical lenses (i.e. along the z-direction).

[0085] Figure 3(a) schematically illustrates a security device 100 according to an embodiment of the invention. The security device is schematically shown in crosssection, together with schematic illustrations of the different images exhibited to a viewer at different viewing angles 01 and 02 of the device. As with the conventional device 101 shown in Figure 2, the device 100 comprises an array 20 of cylindrical lenses and an image layer 30 comprising a plurality of interleaved image segments i1 , i2, and i3. The device further comprises a transparent substrate positioned between the lens array and the image layer, although this is omitted for clarity purposes here. The first set of image segments i1 (as a first image channel) in combination define image 11 , the second set of image segments i2 (as a second image channel) in combination define image I2, and the third set of image segments i3 (as a third “image channel”) in combination define image I3.

[0086] In a conventional “three channel” lenticular device that selectively exhibits three images dependent on viewing angle, each individual lens 21 is associated with an image segment from each image channel. In other words, the interlacing arrangement of the image segments of the image layer has a repeat dimension corresponding to the pitch of the lens array, (here shown as the diameter, D, of each individual lens). It is noted that the term “repeating arrangement” here refers to the relative positioning of image segments with respect to the lenses, rather than the specific form of the image layer material within the image segments. This means that at a given viewing angle of the device, each lens “selects” light from the same image channel such that the viewer views a single image corresponding to that image channel. As the device is tilted and the viewing angle changes, the viewer perceives the images of the different image channels.

[0087] Thus, in conventional devices, the width (along the x-axis) of each image segment is a function of the lens diameter, D, and the number of image channels, n. Typically, in a conventional device, the width of each image segment is given by D / n. This causes problems when attempting to increase the number of image channels of a device, as a reduction of the width of the image segments causes difficulty with printing resolution and registration.

[0088] In the device 100 according to the invention, the image layer is arranged as a repeating plurality of contiguously arranged regions, R, whereby the interlacing arrangement of the image segments has a repeat dimension, r, along the direction of interlacing (along the x-axis) that extends over a plurality of the lenses. In the device 100 of Figure 3(a), each region, R, extends over a set of four lenses. This is in contrast to conventional devices, such as the one shown in Figure 2, in which the repeat dimension of the image segment interlacing is substantially equal to the lens diameter D. This is clearly shown in Figure 3(c), which is a schematic view of the device 100 illustrating the lens array 20 in cross section and the image layer 30 in plan view. As shown in Figure 3(c), a (repeating) region R comprises a first zone Z1 that is laterally positioned so as to cooperate with a first subset X1 of lenses (21a), and a second zone Z2 that is laterally positioned so as to cooperate with a second subset X2 of lenses (21 b). The first zone Z1 comprises only image segments i1 and i2 of the first and second image channels, and the second zone Z2 comprises only image segments i3 of the third image channel. The image layer comprises a plurality of regions in a repeating arrangement along the x dimension.

[0089] For each zone, a repeat distance of the image segments corresponds to a single focussing element. In other words, for each zone, the image segments and the corresponding subset of lenses cooperate such that at a particular nthviewing angle (with n being the number of different image channels within the zone), only one image is exhibited to the viewer.

[0090] Therefore, at the first viewing angle 01 , each lens 21 a of the first subset X1 selects light from image segments i1 of the first image channel, and each lens 21 b of the second subset X2 selects light from image segments i3 of the third image channel. Similarly, at the second viewing angle 02, each lens 21a of the first subset X1 selects light from image segments i2 of the second image channel, and each lens 21b of the second subset selects light from image segments i3 of the third image channel. Consequently, when viewing the device at the first viewing angle 01 , the viewer perceives both the first and third images simultaneously, and at the second viewing angle 02, the viewer perceives both the second and third images simultaneously. In this way, at any given viewing angle of the device, the device exhibits two images (i.e. two image channels) simultaneously.

[0091] This effect of viewing two images simultaneously is schematically shown in Figure 3(a), which may be further understood with reference to Figure 3(b) which illustrates the “macro” images 11 , I2 and I3 as observed in isolation. Image 11 is comprised of a red rectangle, and image I2 is comprised of a violet rectangle. The two images 11 and I2 do not laterally overlap. The third image I3 is comprised of a light purple rectangle (an “intermediate” colour of the red and violet rectangles of the first and second images), and laterally overlaps with both the first and second images. Due to the lateral overlap of the images (in the viewing plane), at the first viewing angle 01 , the viewer sees a combined (“resultant”) image that comprises different sections due to the overlap of the two images. Similarly, at viewing angle 02, the viewer sees a combined image that comprises different sections due to the overlap of the two images.

[0092] Consequently, the device of the present invention advantageously allows an increased number of images to be exhibited by the device without the requirement to reduce the width of the image segments as in conventional devices. As illustrated in Figures 3(a) and 3(c), although the device exhibits three images, each lens is associated with at most two image segments. Furthermore, due to the overlap of the third image with the first and second image - and in this example the complementary colour relationship - a smooth transition between the first and second images is exhibited upon a change of viewing angle. This allows complex visual effects to be generated, as will be explained herein.

[0093] In the example shown in Figures 3(a) to 3(c), within each region R, the ratio of the number of first image segments to the number of third image segments is 3:1. Similarly, the ratio of the number of second image segments to the number of third image segments is also 3:1 (with the ratio of first image segments to second image segments being 1 :1). Therefore, at each viewing angle, although the viewer will perceive two images simultaneously, the relative perceived density (or “brightness”) of the first / second images will be greater than that of the third image. Here, the first and second images may be termed “primary” images, and the third image may be termed a “secondary” image, or “intermediate” image, as it “smooths” the transition between the first and second images.

[0094] Figure 4(a) schematically illustrates a device 100 according to a further embodiment of the invention. In a similar manner to Figure 3(c), Figure 4(a) depicts the lens array 20 in cross sectional view, and the image layer 30 in plan view for ease of visualisation. In this example, the device contains five image channels, with image segments of each channel labelled i1 , i2, i3, i4, i5 respectively. As before, the image layer comprises a plurality of repeating regions R (one region being shown in Figure 4(a)). Within each region, a first zone Z1 is arranged to cooperate with a first subset of the focussing lenses X1 , and a second zone Z2 that is contiguous with zone Z1 is arranged to cooperate with a second subset of the focussing lenses X2.

[0095] The first zone Z1 comprises only image segments i1 , i2, i3 from image channels 1 , 2, and 3. These image segments are arranged such that each lens of the subset X1 is associated with an image segment from each channel whereby at a first viewing angle 01 , light from the image segments i1 is directed to the viewer; at a second viewing angle 02, light from the image segments i2 is directed to the viewer; and at a third viewing angle 03, light from the image segments i3 is directed to the viewer. The second zone Z2 comprises only image segments i4, i5 from image channels 4 and 5. These image segments are arranged such that each lens of the second subset X2 is associated with an image segment from each of the fourth and fifth image channels. Considering the second zones Z2 and the corresponding subset of lenses X2 in isolation, at a first viewing angle only image I4 is exhibited, and at a second viewing angle, only image I5 is exhibited.

[0096] The individual “macro” images exhibited by the different image channels are illustrated in Figure 4(b). Each image comprises a coloured rectangle. Image 1 (a red rectangle), image 2 (a yellow rectangle) and image 3 (a blue / purple rectangle) do not laterally overlap. Image 4 (an orange rectangle) laterally overlaps with the rectangles of image 1 and image 2. In this example, the orange colour is used as a mixture of the red and yellow colours of images 1 and 3. Image 5 (a green rectangle) laterally overlaps with the rectangles of image 2 and image 3. In this example, the green colour is used as a mixture of the yellow and blue / purple colours of images 2 and 3. Here, the non-overlapping images 1 , 2 and 3 are primary images, and the images 4 and 5 are secondary, “intermediate” images. We now consider the visual effect perceived when viewing the device at the three viewing angles 01 , 02 and 03. At viewing angle 01 , the first subset of lenses X1 will direct light from the image segments i1 of the first image channel to the viewer, and the second subset of lenses X2 will direct light from the image segments i4 of the fourth image channel. As the eye averages the visual effect across the array of lenses, the viewer will simultaneously perceive images 11 and I4, with image 11 appearing with higher perceived density due to the greater amount of light received from the image segments i1 (due to the ratio of the image channels across the regions R). At viewing angle 02, the viewer will perceive images I2, I4 and I5 simultaneously, with image I2 appearing with the greatest perceived density. At viewing angle 03, the viewer will perceive images I3 and I5 simultaneously, with image I3 exhibiting the greatest perceived density. Consequently, as the device is tilted from viewing angle 01 to 02 to 03, the viewer will perceive a smooth change of image from primary images 11 to I2 to I3, with the image transition smoothed due to the presence and appearance of the intermediate image channels 4 and 5.

[0097] Figures 5(a) and 5(b) schematically illustrate an example 5-channel embodiment of the invention similar in structure to the device of Figure 4, but wherein the images depict concentric circles rather than rectangles. Figure 5(a) illustrates each “macro” image 11 , I2, I3, I4 and I5 in isolation, and Figure 5(b) schematically shows the combined images perceived by a viewer observing the device at viewing angles 01 , 02 and 03 as the device is tilted. Specifically, at viewing angle 01 , the viewer perceives a combination of images 11 and I4, at viewing angle 02 the viewer perceives a combination of images I2, I4 and I5, and at viewing angle 03, the viewer perceives a combination of images I3 and I5.

[0098] In the embodiments discussed thus far, the optically variable effect exhibited by the device has comprised a lateral movement or an expansion / contraction animation effect. Embodiments of the present invention may exhibit an enhanced rotational animation effect, as will now be described with reference to Figures 6(a) to 6(c). Figure 6(a) schematically illustrates a conventional three-channel lenticular device that exhibits a rotational animation effect upon tilting. As shown in Figure 6(a), as the viewing angle is changed from 01 to 02 to 03, the viewer perceives a logo appearing to rotate clockwise whilst changing colour (in this example from red to yellow to blue / purple). The rotation may appear to “jump” from one frame to the next as the animation sequence only contains three frames. However, as previously explained, attempting to include additional image channels within a conventional lenticular device in order to “smooth” the animation typically causes difficulties with printing resolution and registration as it is difficult to accurately print and register large numbers of image segments under each lens.

[0099] Figures 6(b) and 6(c) schematically illustrate a device 100 according to an embodiment of the invention, which effectively constitutes a six channel device. In this way, the rotational animation sequence between the “primary” image frames 11 , 12 and 13 is smoothed, providing an enhanced visual effect to the viewer that is easy to authenticate and yet difficult to counterfeit.

[0100] Figure 6(c) shows the individual images exhibited by the six channels of the device 100. Primary images 11 , I2, and I3 are the same images exhibited by the conventional device of Figure 6(a). The image layer of the device comprises three additional “intermediate” channels, which respectively exhibit images I4, 15 and I6. Images I4, I5 and I6 each depict the same logo as the “primary” images 11 , I2 and I3, but have different spatial positions within the rotational animation sequence. As can be seen in Figure 6(c), image I4 laterally overlaps with images 11 and I2 and has a colour that is a combination of the colours of images 11 and I2. Similarly, image I5 laterally overlaps with images I2 and I3 and has a colour that is a combination of the colours of images I2 and I3. Image I6 laterally overlaps with images I3 and 11 and has a colour that is a combination of the colours of images I3 and 11.

[0101] As seen in Figure 6(b), the image layer 30 of the device is arranged such that each repeating region R comprises a first zone Z1 containing only image segments i1 , i2 and i3 of the “primary” image channels, and a second zone Z2 that contains only image segments i4, i5 and i6 of the “intermediate” image channels. In this example the ratio of the primary image channels to the intermediate image channels is 3:1. Consequently, and as schematically shown in Figure 6(b), at viewing angle 01 the viewer simultaneously perceives images 11 and I4 (with the primary image 11 relaying with increased perceived density relative to I4); at viewing angle 02 the viewer simultaneously perceived images I2 and I5; and at viewing angle 03 the viewer simultaneously perceives images I3 and I6.

[0102] In this way, as the device is tilted, the viewer experiences an enhanced animation effect in comparison to the conventional device of Figure 6(a), advantageously without the requirement to increase the number of image segments under an individual lens. It is noted that although in this embodiment the additional “intermediate” images overlap with the “primary” images, this is not an essential requirement to achieve an enhanced animation effect (e.g. greater number of image frames).

[0103] Thus far we have considered embodiments in which the zones within a region, R, of the image layer comprise image segments of different image channels. In other words, “secondary” or “intermediate” image channels have been included within the image layer in addition to the “primary” image channels. Figures 7(a) to 7(c) illustrate a device 100 according to a further embodiment of the invention in which the repeating region R comprises first and second zones as before, but in which the arrangement of the image segments is different in the first and second zones. As can be seen in Figure 7(a), within the region R, the first zone Z1 contains image segments i1 , i2 and i3 of first, second and third image channels. The images 11 ,

[0104] 12 and I3 exhibited by the three image channels are the same as images 11 , 12 and

[0105] 13 of the Figure 6 device, and are shown in Figure 7(b). The second zone Z2 contains the image segments from the same three image channels, but having a different arrangement (i.e. a different ordering of the image segments with respect to the lenses). Thus, at the first viewing angle 01 , the viewer simultaneously perceives images 11 and I3, at the second viewing angle 02 the viewer simultaneously perceives images I2 and 11 , and at the third viewing angle 03 the viewer simultaneously perceives images I3 and I2. This effect provides a less abrupt change in the images 11 , I2 and I3 as the viewing angle changes from 01 to 02 to 03, as schematically shown in Figure 7(c).

[0106] In this example, due to the ratio of the number of lenses in the first and second zones being 2:1 , at each viewing angle the images formed from the image segments of the first zone are perceived with increased perceived density in comparison with the images formed from the image segments of the second zone.

[0107] In the embodiments discussed so far, each zone has contained only image segments from primary image channels, or image segments from secondary image channels. However, as the present invention utilises the viewer “averaging” the visual effect exhibited by the device across the lens array such that more than one image is simultaneously perceived at any one viewing angle, this need not be the case and the image segments of primary and secondary image channels may be more freely distributed across a region R of the image layer. This concept will now be described with reference to Figures 8(a) and 8(b).

[0108] Figure 8(a) schematically shows, in cross section, a device 100 according to a further embodiment of the present invention. As in previous figures, the transparent substrate located between the lens array 20 and the image layer 30 has been omitted for clarity. The device 100 is a six channel device containing three primary image channels i1 , i2, i3 and three secondary image channels i4, i5, i6, and exhibits the same optically variable effect as the device of Figure 6(b) as the viewing angle is varied. However, the arrangement of the image layer 30 differs from that of Figure 6(b).

[0109] As more clearly shown in Figure 8(b), a repeating region R of the image layer comprises four zones Z1 , Z2, Z3, Z4. Each zone is configured to cooperate with a respective subset of lenses X1 , X2, X3, X4. Zone Z1 comprises only image segments from the primary image channels i1 , i2, i3. Each of zones Z2 to Z4 contain image segments from both primary and secondary image channels. Zone Z2 comprises an image segment from the primary image channels i2 and i3 together with an image segment from the secondary image channel i4. Zone Z3 comprises an image segment from the primary image channels i1 and i3 together with an image segment from the secondary image channel i5. Zone Z4 contains image segments from the primary image channels i1 and i2, together with an image segment from the secondary image channel i6. The relative positions of image segments from the same image channel with respect to the lenses is the same across the region.

[0110] As the eye averages the effect over the domain of the region, the same optically variable effect as shown in Figure 6(b) is displayed. In particular, at the first viewing angle 01 , the subsets X1 , X3 and X4 direct light from the first image channel to the viewer and the subset X2 directs light from the fourth image channel to the viewer such that images 11 and I4 are simultaneously exhibited. Similarly, at the second viewing angle 02, the subsets X1 , X2 and X4 each direct light from the second image channel to the viewer and the subset X3 directs light from the fifth image channel such that images I2 and I5 are simultaneously perceived. At the third viewing angle 03 the subsets X1 , X2 and X3 each direct light from the third image channel to the viewer, and the subset X4 directs light from the sixth image channel to the viewer whereby images I3 and I6 are simultaneously exhibited to the viewer.

[0111] The ratio of the number of primary image channels to secondary image channels that are exhibited at a particular viewing angle may be varied in order to generate further complex visual effects that increase the difficulty of counterfeit. Figure 9 illustrates an example device 100 according to an embodiment of the invention in which the ratios of primary image channels to secondary image channels are different for different viewing angles. The region R contains three zones, with the first zone Z1 comprising only image segments from image channels 1 , 2 and 3; zone Z2 comprising only image segments from image channels 4, 5 and 6, and zone Z3 comprising only image segments from image channels 4, 2 and 3.

[0112] At the first viewing angle 01 , the viewer perceives images from image channels 1 and 4 simultaneously, with the ratio of the number of image segments i1 to i4 being 3:2. At viewing angle 02, the viewer perceives images from image channels

[0113] 2 and 5 simultaneously, with the ratio of the number of image segments i2 to i5 being 4:1 . At viewing angle 03, the viewer perceives images from image channels

[0114] 3 and 6 simultaneously, with the ratio of the number of image segments i3 to i6 being 4:1 . The difference in the ratio between the number of image segments of different image channels being exhibited at a particular viewing angle can be used to control the contrast between the images that are simultaneously exhibited at that viewing angle.

[0115] Figure 10 schematically illustrates a security device 100 according to a further embodiment of the present invention. The lenses of the lens array 20 are elongate lenses that are adapted to focus light in one direction. In this example, the lenses are elongate cylindrical lenses. As in the previous embodiments, the image layer 30 comprises a plurality of repeating regions. Here, the image layer comprises a plurality of first regions R1 and a plurality of second regions R2. The first regions R1 each comprise primary image segments i1 , i2, i3 (constituting a first zone) that cooperate with a first subset of the lenses X1 , and secondary image segments i4, i5, i6 (constituting a second zone) that cooperate with a second subset of the lenses X2. The second regions R2 each comprise primary image segments i7, i8, i9 (constituting a first zone of the second regions) that cooperate with the first subset of lenses X1 , and secondary image segments ilO, i11 , i12 that cooperate with the second subset of lenses X2.

[0116] As can be seen in Figure 10, the first regions R1 are interlaced with the second regions R2 along the direction of elongation of the lenses. Consequently, each lateral position of the image layer under a lens comprises image segments from different images (different image channels). In this way, for a given viewing angle of the device, each lens will direct light from different image channels to the viewer due to the interlacing of the first regions and the second regions along the elongation direction of the lenses, whereby the viewer will perceive the images of the different image channels simultaneously. Consider viewing the device 100 at a first viewing angle 01. Due to the arrangement of the image segments in region R1 , the first subset of lenses X1 will direct light from the image segments i1 to the viewer, and the second subset of lenses X2 will direct light from the image segments i4 to the viewer, such that the images of the first and fourth image channels are perceived simultaneously. This uses the same mechanism as in the previously described embodiments. In the present embodiment, additionally, at the first viewing angle 01 , the second regions R2 will direct light from image segments i7 and i10 to the viewer (via lens subsets X1 and X2 respectively), such that the images of the seventh and tenth image channels are simultaneously perceived by the viewer. Therefore, when viewing the device as a whole, due to the interlacing of the first regions R1 with the second regions R2 along the direction of interlacing, at the first viewing angle 01 the viewer will simultaneously perceive image segments i1 , i4, i7 and i10, and therefore simultaneously perceive four images.

[0117] Similarly at the second viewing angle 02 the device will simultaneously exhibit image segments i2, i5, i8 and i11 ; and at the third viewing angle 03 the device will simultaneously exhibit image segments i3, i6, i9 and i 12. In this way the device acts as a 12 channel device which is capable of exhibiting highly complex optically variable effects, thereby providing a high security level. Although the device shown in Figure 10 comprises 12 different image channels, it is envisaged that fewer or greater numbers of channels may be included within the device dependent on the arrangement of the image segments within the first and second regions.

[0118] It will be appreciated that by interlacing the first regions R1 with the second regions R2, the print density (e.g. the amount of ink per unit area) for each image channel is reduced, thereby lowering the brightness of each image exhibited by the device. This reduction in image brightness may be acceptable in trade off for the increased complexity of the optically variable effect as a consequence of the increased number of image channels. In the example device shown in Figure 10, each image segment has the same dimension (“length”) along the direction of elongation of the lenses. In some embodiments, the relative perceived density between the exhibited images may be controlled by adjustment of the ratio of the image segment lengths of the first region to the image segment lengths of the second region. Typically, this ratio is between 1 :1 (as shown in Figure 10) and 10:1. The relative brightness between different image channels may also be controlled dependent on the number of image channels within each region. For example, if the first regions R1 comprised only four image channels compared to the six image channels each second region R2, the contrast / brightness of the images exhibited by the first regions will be relatively greater as a consequence of the larger print area for each image channel.

[0119] As has been described herein, in preferred embodiments the viewing elements are in the form of focussing elements such as lenses. In alternative embodiments of the invention, the device 100 may instead comprise an array of viewing elements in the form of a masking grid 90 (shown in Figure 10A) that comprises substantially opaque regions 93 spaced by substantially transparent regions 95 (e.g. defined by gaps between the opaque regions). The transparent regions 95 cooperate with the image layer such that light from different sets of image segments is directed to the viewer at different viewing angles, as shown. In the example of Figure 10A, the image layer 30 has substantially the same arrangement as that shown in Figure 3(a), whereby at the first viewing angle 01 the viewer perceives the images 11 and I3 simultaneously, and at the second viewing angle 02 the viewer perceives the image I2 and I3 simultaneously.

[0120] 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 layer and / or the complete security device can either be formed directly on the security document (preferably using the methods described in WO-A- 2018 / 153840 and WO-A-2017 / 009616), 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.

[0121] 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.

[0122] The security article may be incorporated into or on the surface of a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate at at least one window of the document. Methods of incorporating security elements in such a manner are described in EP-A-1141480 and WO-A-03054297. In the method described in EP-A-1141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

[0123] 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.

[0124] 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 .

[0125] Examples of documents of value and techniques for incorporating a security device will now be described with reference to Figures 11 to 14.

[0126] Figure 11 depicts an exemplary document of value 1500, here in the form of a banknote. Figure 11 (a) shows the banknote in plan view whilst Figure 11 (b) shows a cross-section of the same banknote along the line X-X' and Figure 11 (c) shows a cross-section through a variation of the banknote. In this case, the banknote is a polymer (or hybrid polymer / paper) banknote, having a transparent substrate 10. Two opacifying layers 1505a and 1505b are applied to either side of the transparent substrate 10, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 10.

[0127] The opacifying layers 1505a and 1505b are omitted across selected regions 1502 (and 1502’), each of which forms a window within which a security device 100, 100’ is located. In Figure 11 (b), a security device 100 is disposed within window 1502, with a focusing element array 20 arranged on one surface of the transparent substrate 10, and image layer 30 on the other. Figure 11 (c) shows a variation in which a second security device 100’ is also provided on banknote 1500, in a second window 1502’. The arrangement of the second security device 100’ can be reversed so that its optically variable effect is viewable from the opposite side of the security document as that of device 100, if desired.

[0128] It will be appreciated that, if desired, any or all of the windows 1502, 1502’ could instead be “half-windows”, in which an opacifying layer (e.g. 1505a or 1505b) is continued over all or part of the image layer 30. 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 1505a and 1505b are provided on both sides.

[0129] In Figure 12 the banknote 1600 is a conventional paper-based banknote provided with a security article 1601 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 1605a and 1605b 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 1601 in window regions 1602a,b,c of the banknote. Alternatively the window regions 1602a,b,c 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 to be “full thickness” windows: the thread 1601 need only be exposed on one surface if preferred. For example, in some embodiments the windows are “half-thickness” windows, and the paper is continuous on the side of the image layer 30 with only the lens array 20 exposed. The security device is formed on the thread 1601 , which comprises a transparent substrate, a focusing array 20 provided on one side and an image layer 30 provided on the other. Windows 1602a, 1602b, 1602c reveal parts of the device 100, which may be formed continuously along the thread. (In the illustration, the lens arrays are depicted as being discontinuous between each exposed region of the thread, although in practice typically this will not be the case and the lens arrays (and image layer) will be formed continuously along the thread. Alternatively several security devices could be spaced from each other along the thread, as in the embodiment depicted, with different or identical images displayed by each.

[0130] In Figure 13, the banknote 1700 is again a conventional paper-based banknote, provided with a strip element or insert 1703. The strip 1703 is based on a transparent substrate and is inserted between two plies of paper 1705a and 1705b. The security device 100 is formed by an array of focusing features provided by a lens array 20 on one side of the strip substrate 1703, and an image layer 30 on the other. The paper plies 1705a and 1705b are apertured across region 1702 to reveal the security device 100, which in this case may be present across the whole of the strip 1703 or could be localised within the aperture region 1702. It should be noted that the ply 1705b need not be apertured and could be continuous across the security device.

[0131] A further embodiment is shown in Figure 14 where Figures 14(a) and 14(b) show the front and rear sides of the document 1800 respectively, and Figure 14(c) is a cross section along line Z-Z’. Security article 1803 is a strip or band comprising a security device 100 according to any of the embodiments described above. The security article 1803 is formed into a security document 1800 comprising a fibrous substrate 1805, using a method described in EP-A-1141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (Figure 14(a)) and exposed in one or more windows 1802 on the opposite side of the document (Figure 14(b)). Again, the security device 100 is formed on the strip 1803, which comprises a transparent substrate with a lens array 20 formed on one surface and a co-operating image layer 30 as previously described on the other.

[0132] Alternatively a similar construction can be achieved by providing paper 1800 with an aperture 1802 and adhering the strip element 1803 onto one side of the paper 1800 across the aperture 1802. 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 100 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 layer 30 can be affixed to the surface of the substrate, e.g. applying it directly thereto, or by forming it on another film which is then adhered to the substrate 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.

[0133] 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 preferably 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.

[0134] The security device of the current invention can be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials. The inclusion of such detectable materials in the image layer in particular may provide additional secure visual effects.

[0135] 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.

[0136] The security device may comprise a metallic layer laterally spaced from the security feature of the current invention. The presence of a metallic layer can be used to conceal the presence of a machine readable dark magnetic layer. When a magnetic material is incorporated into the device the magnetic material can be applied in any design but common examples include the use of magnetic tramlines or the use of magnetic blocks to form a coded structure. Suitable magnetic materials include iron oxide pigments (Fe2O3 or Fe3O4), 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.

[0137] In an alternative machine-readable embodiment a transparent magnetic layer can be incorporated at any position within the device structure. Suitable transparent magnetic layers containing a distribution of particles of a magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent are described in W003091953 and W003091952.

[0138] Negative or positive indicia may be created in the metallic layer or any suitable opaque layer. One way to produce partially metallised / demetallised films in which no metal is present in controlled and clearly defined areas, is to selectively demetallise regions using a resist and etch technique such as is described in US- B-4652015. Other techniques for achieving similar effects are for example aluminium can be vacuum deposited through a mask, or aluminium can be selectively removed from a composite strip of a plastic carrier and aluminium using an excimer laser. The metallic regions may be alternatively provided by printing a metal effect ink having a metallic appearance such as Metalstar® inks sold by Eckart. As noted previously, while print workings are a preferred kind of structure suitable for providing the image layer, in embodiments of the invention, relief structures can also be utilised. It will be appreciated that where more than one structure exhibiting different respective colours are required, these could be embodied as different parts of a single relief structure with correspondingly different properties. A variety of different relief structures suitable for forming image segments in implementations of the present invention are shown in Figures 15A-15I. Thus, Figure 15A illustrates image regions of the image segments (IM) in the form of embossed or recessed regions while the non-embossed portions correspond to the non-imaged regions of the segments (Nl). Figure 15B illustrates image regions of the segments in the form of debossed lines or bumps. A coloured marking material (e.g. ink or resin) could be applied into the embossed portions in order to provide the image segments with a desired colour, as described in WO- A-2005052650.

[0139] In another approach, the relief structures can be in the form of diffraction gratings (Figure 15C) or moth eye I fine pitch gratings (Figure 15D). Where the image segments are formed by diffraction gratings, then different portions of an image (within one image segment or in different segments) can be formed by gratings with different characteristics. A preferred method for writing such a grating would be to use electron beam writing techniques or dot matrix techniques.

[0140] Such diffraction gratings for moth eye I fine pitch gratings can also be located on recesses or bumps such as those of Figures 15A and 15B, as shown in Figures 15E and 15F respectively.

[0141] Figure 15G illustrates the use of a simple scattering structure providing an achromatic effect.

[0142] Further, in some cases the recesses of Figure 15A could be provided with an ink or the debossed regions or bumps in Figure 15B could be provided with an ink. The latter is shown in Figure 15H where ink layers 1910 are provided on the bumps 1900. Thus each image segment could be created by forming appropriate raised regions or bumps in a resin layer provided on a transparent substrate. This could be achieved for example by cast curing or embossing. A coloured ink is then transferred onto the raised regions typically using a lithographic, flexographic or gravure process. Figure 151 illustrates the use of an Aztec structure.

[0143] As briefly discussed above, the use of materials that react to an external stimulus can be utilised to generate striking optical effects. One example of such a material that may be used in this manner is a luminescent ink, as will now be described with reference to Figures 16(a) and 16(b).

[0144] Figure 16(a) illustrates a cross-sectional view of a security device 100 similar to that described with reference to Figure 6(a). As with the device of Figure 6(a), the image layer 30 is arranged such that each repeating region R comprises a first zone Z1 containing only image segments i1 , i2, and i3, and a second zone Z2 that contains only image segments i4, i5 and i6. Here, the ratio of the “primary” image channels i1 , i2 and i3 to the “secondary” image channels is 3:1 , although it will be appreciated that other ratios may be used. In the device of Figure 16(a), each image segment is printed using a luminescent ink that exhibits a luminescent visible colour under UV illumination. In particular, the image segments i1 , i2 and i3 each comprise a luminescent ink that exhibits a visible colour in both visible light and under UV illumination. The colours exhibited by the luminescent inks under the different illumination conditions may be the same or may differ. On the other hand, the image segments i4, i5 and i6 each comprise a luminescent ink that is substantially transparent and colourless in visible light and exhibits a visible colour under UV illumination.

[0145] The visual effect when viewed under both visible light and under UV illumination is schematically shown in Figure 16(b). When the device is viewed under visible light, it effectively acts as a 3-channel lenticular device as the luminescent ink forming image segments i4, i5 and i6 exhibits a transparent, colourless appearance. As the device is tilted from viewing angle 01 to 03, the device exhibits images 11 , I2 and I3 in sequence, providing the animation effect shown in Figure 16(b)(i). However, when the device is viewed under UV light (e.g. when viewing the device under a UV lamp in a dark room), the luminescent inks forming each of the sets of image segments i1 to i6 exhibit respective luminescent visible colours (which may or may not differ) and consequently each of the image channels is perceived as the device is tilted. Consequently, as shown in Figure 16(b)(ii), under UV illumination, at viewing angle 01 the viewer simultaneously perceives images 11 and I4; at viewing angle 02 the viewer simultaneously perceived images I2 and I5; and at viewing angle 03 the viewer simultaneously perceives images I3 and I6. Thus, under UV illumination, the device exhibits an enhanced, “smoother” (greater number of image frames) animation effect upon tilting, compared to when viewed under visible light in the absence of UV illumination.

[0146] This change in animation effect under different lighting conditions provides a complex optically variable effect that is particularly difficult to counterfeit. However, it is also envisaged that in some embodiments where luminescent inks are used, each of the luminescent inks may exhibit a visible colour under visible light as well as under UV illumination. In such embodiments, the simultaneous replay of at least two images will be perceived when viewed at a particular viewing angle in both visible light and under UV illumination. However, the colours exhibited by the image channels may differ from one illumination condition to the other.

[0147] Examples of suitable ink formulae that may be used in in embodiments of the present invention that exhibit different effects under different lighting conditions may be found in W02004 / 050376, WO2018 / 206936 and WO2024 / 180326.

Claims

CLAIMS1 . A security device, comprising: a substrate; an array of viewing elements disposed in or on the substrate; and an image layer disposed in or on the substrate and overlapping with the array of viewing elements, the image layer comprising a plurality of sets of image segments interlaced with each other, wherein each set of image segments in combination defines a respective image; wherein the array of viewing elements and the image layer cooperate with each other such that the device selectively exhibits the images in dependence on viewing angle; and wherein the image layer comprises a repeating arrangement of the plurality of sets of image segments, wherein the repeating arrangement has a repeat dimension that is larger than a pitch of the array of viewing elements such that for each of a plurality of different viewing angles of the device, a first subset of the viewing elements directs light from one of the plurality of sets of image segments to the viewer and a second subset of the viewing elements directs light from a different one of the plurality of sets of image segments to the viewer; whereby for each of a plurality of different viewing angles of the device, light from at least two different sets of image segments is directed to the viewer simultaneously.2 The security device of claim 1 , wherein the repeating arrangement comprises a repeating arrangement of regions, each region laterally overlapping with a plurality of viewing elements; wherein each region has a first zone and a second zone, the first zone being configured to cooperate with the first subset of viewing elements and the second zone being configured to cooperate with the second subset of viewing elements; and wherein for each region,the sets of image segments and / or the arrangement thereof in the first zone is different from the sets of image segments and / or the arrangement thereof in the second zone.

3. The security device of claim 2, wherein within the plurality of regions are laterally contiguous.

4. The security device of any of the preceding claims, wherein the plurality of sets of image segments comprises two or more sets of primary image segments, each set of primary image segments in combination defining a respective primary image and at least one set of secondary image segments, the at least one set of secondary image segments in combination defining a respective secondary image.

5. The security device of claim 4 when dependent on claim 2 or claim 3, wherein the first zones each comprise only image segments of each of the two or more sets of primary image segments, and the second zones each comprise only image segments of each of the at least one set of secondary image segments, wherein the first zones are configured to cooperate with the first subset of viewing elements such that the device selectively exhibits the two or more primary images dependent on viewing angle and the second zones are configured to cooperate with the second subset of viewing elements such that the device selectively exhibits the secondary image(s) dependent on viewing angle, whereby for each of a plurality of viewing angles of the device, the viewer perceives a primary image and an secondary image simultaneously.

6. The security device of claim 4 when dependent on claim 2 or claim 3, wherein at least one of the first and second zones comprises image segments from both the primary and secondary sets of image segments.

7. The security device of any of claims 4 to 6, wherein the primary images and the secondary image(s) define an animation sequence.

8. The security device of claim 7, wherein the animation sequence comprises any of: a lateral movement, a rotation, an expansion or contraction.

9. The security device of claim 7 or claim 8, wherein at least one of the primary images overlaps with at least one of the secondary image(s).

10. The security device of any of claim 2 to 9, wherein each region overlaps with N viewing elements, wherein N is in the range of 2 to 25, preferably in the range of 2 to 10.11 . The security device of any of claims 2 to 10, wherein for each region, for each viewing angle of the device, the ratio of primary image segments to secondary image segments is between 1 :1 and 20:1 , preferably between 2:1 and 20:1.

12. The security device of claim 10, wherein for each viewing angle of the device, the ratio of primary image segments to secondary image segments is different for different regions across the image layer.

13. The security device of any of claims 2 to 12, wherein the viewing elements are elongate viewing elements; and the image layer comprises a plurality of first regions and a plurality of second regions, wherein the image segments and / or the arrangement thereof in the first regions is different from the image segments and / or the arrangement thereof in the second regions, wherein the first regions and second regions are interlaced with each other along the direction of elongation of the viewing elements, such that at least at one viewing angle, the viewer perceives at least two images from the same subset of viewing elements, simultaneously.

14. The security device of claim 13, wherein a ratio of a length of the first regions along the direction of elongation of the viewing elements to a length of the second regions along the direction of elongation of the viewing elements is between 1 :1 and 10:1 , preferably between 2:1 and 10:1.

15. The security device of any of the preceding claims, wherein at least one image exhibited by the device is in the form of indicia or an indicium, preferably one or more geometric shapes, letters, logos, currency signs or other symbols.

16. The security device of any of the preceding claims, wherein the viewing elements are focussing elements adapted to focus light in one direction, preferably wherein the focussing elements are cylindrical focussing elements.

17. The security device of claim 16, wherein the image segments are elongate image segments.

18. The security device of any of the preceding claims, wherein the array of viewing elements comprises an array of lenses.

19. The security device of any of the preceding claims, wherein the substrate is at least semi-transparent, and wherein the array of viewing elements is disposed in or on a first surface of the substrate and the image layer is disposed in or on a second, opposing surface of the substrate.

20. The security device of any of the preceding claims, wherein the image layer is a provided as a print working, preferably printed by a gravure, intaglio screen, micro-intaglio, flexographic, lithographic or digital technique.21 . The security device of any of the preceding claims, wherein each of the plurality of sets of image segments comprises a luminescent material which luminesces in response to irradiation at at least one excitation wavelength, whereby each of the plurality of sets of image segments exhibits a luminescentvisible colour when illuminated with an excitation illumination condition that comprises illumination with the at least one excitation wavelength.

22. The security device of claim 21 , wherein the image layer material of at least one of the sets of image segments is substantially colourless when illuminated with a first illumination condition that comprises illumination with visible light in the absence of the at least one excitation wavelength.

23. A security article comprising the security device of any of the preceding claims, wherein the security article is preferably a security thread, strip, foil, insert, transfer element, label, patch, or a data page for a security document.

24. A security document comprising a security device according to any of claims 1-22, or a security article according to claim 23, wherein the security document is preferably a banknote, cheque, passport, identity card, driver’s licence, certificate of authenticity, fiscal stamp, or other document for securing value or personal identity.

25. A method of manufacturing a security device, comprising:(a) providing a substrate;(b) applying an array of viewing elements to the substrate; and(c) forming an image layer in or on the substrate, the image layer overlapping with the array of viewing elements, the image layer comprising a plurality of sets of image segments interlaced with each other, wherein each set of image segments in combination defines a respective image; wherein the array of viewing elements and the image layer cooperate with each other such that the device selectively exhibits the images in dependence on viewing angle; and wherein the image layer comprises a repeating arrangement of the plurality of sets of image segments, wherein the repeating arrangement has a repeat dimension that is larger than a pitch of the array of viewing elements such that for each of a plurality of different viewing angles of the device, a first subset of the viewing elements directs light from one of the plurality of sets of image segments to theviewer and a second subset of the viewing elements directs light from a different one of the plurality of sets of image segments to the viewer; whereby for each of a plurality of different viewing angles of the device, light from at least two different sets of image segments is directed to the viewer simultaneously.

26. The method of claim 25, wherein the image layer is formed by a printing technique, preferably a gravure, intaglio, screen, micro-intaglio, flexographic, lithographic or digital technique.

27. The method of claim 25 or claim 26, wherein the image layer is formed in a single print working.

28. The method of any of claims 25 to 27, wherein the viewing elements are applied to a first side of the substrate and the image layer is applied to a second, opposing side of the substrate simultaneously at the same location along the substrate.

29. The method of any of claims 25 to 28, adapted to produce the security device of any of claims 1 to 24.

Images