WO2025125815A1

WO2025125815A1 - Security devices and methods of manufacture thereof

Info

Publication number: WO2025125815A1
Application number: PCT/GB2024/053104
Authority: WO
Inventors: John Godfrey; Rebecca LOCKE
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2023-12-15
Filing date: 2024-12-12
Publication date: 2025-06-19
Anticipated expiration: 2026-06-15
Also published as: WO2025125813A1; WO2025125814A1

Abstract

A security device is disclosed. The security device comprises a substrate; an array of elongate 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, each set of image segments in combination defining a respective image and forming an image channel; wherein each viewing element defines an optical footprint in the image layer corresponding to the lateral area of the viewing element, and wherein for each optical footprint within the image layer: the image segments are arranged as an array having a plurality of first rows each comprising image segments of a first subset of the plurality of image channels and a plurality of second rows each comprising image segments of a second subset of the image channels different from the first subset; wherein each of the first and second rows is aligned with a first direction that is substantially perpendicular to the direction of elongation of the viewing elements, and wherein within a row the respective image segments are arranged so as to cooperate with the respective viewing element such that upon tilting the device about a second direction that is parallel with the direction of elongation of the viewing element, light from different image channels within a row is selectively directed to the viewer by the viewing element in dependence on viewing angle; and further wherein the plurality of first rows and the plurality of second rows are interlaced with each other along the second direction, whereby when viewing the device, the image segments cooperate with the array of viewing elements such that when tilting the device about the second direction, the images of different image channels are selectively exhibited to the viewer in dependence on viewing angle, and for each of a plurality of different viewing angles of the device, light from an image channel of the first subset of image channels and light from an image channel of the second subset of image channels is directed to the viewer simultaneously. Methods of manufacturing such security devices are also disclosed.

Description

SECURITY DEVICES AND METHODS OF MANUFACTURE THEREOF

FIELD OF THE INVENTION

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.

BACKGROUND

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 is not possible to reproduce accurately by taking a visible light copy, e.g. through the use of standardly available photocopying or scanning equipment.

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.

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.

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 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.

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

SUMMARY OF INVENTION

In accordance with a first aspect of the invention there is provided a (e.g. lenticular) security device, comprising: a substrate; an array of elongate 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, each set of image segments in combination defining a respective image and forming an image channel; wherein each viewing element defines an optical footprint in the image layer corresponding to the lateral area of the viewing element, and wherein for each optical footprint within the image layer: the image segments are arranged as an array having a plurality of first rows each comprising (e.g. only) image segments of a first subset of the plurality of image channels and a plurality of second rows each comprising (e.g. only) image segments of a second subset of the image channels different from the first subset; wherein each of the first and second rows is aligned with a first direction that is substantially perpendicular to the direction of elongation of the viewing elements, and wherein within a row the respective image segments are arranged so as to cooperate with the respective viewing element such that upon tilting the device about a second direction that is parallel with the direction of elongation of the viewing element, light from different image channels within a row is selectively directed to the viewer by the viewing element in dependence on viewing angle; and further wherein the plurality of first rows and the plurality of second rows are interlaced with each other along the second direction, whereby when viewing the device, the image segments cooperate with the array of viewing elements such that when tilting the device about the second direction, the images of different image channels are selectively exhibited to the viewer in dependence on viewing angle, and for each of a plurality of different viewing angles of the device, light from an image channel of the first subset of image channels and light from an image channel of the second subset of image channels is directed to the viewer simultaneously. In this way, the security device according to the present invention advantageously increases the ease with which the number of (different) images exhibited by the device can be increased. In other words, for a given resolution (e.g. line width) of the image segments, the number of images exhibited by the device is increased in comparison with a standard device. This is achieved by arranging the image segments of the image layer as a plurality of interlaced first and second rows such that for each of a plurality of different viewing angles of the device (preferably for any one viewing angle of the device), light from both the first rows and the second rows is sampled by each viewing element and directed to the viewer.

As discussed, for each of a plurality of different viewing angles of the device, light from an image channel of the first subset of image channels and light from an image channel of the second subset of image channels is directed to the viewer simultaneously. In this way, for each of a plurality of different viewing angles of the device (preferably for any one viewing angle of the device), the viewer perceives (e.g. at least a portion of) the corresponding images simultaneously.

The security device is preferably a lenticular security device.

The interlacing of the first rows and the second rows 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 (e.g. “brightness”) of the corresponding images exhibited by the device. However, the inventors have realised that this reduction in perceived density is offset by the advantageous increase in complexity of visual effect exhibited by the device due to the increase in the number of image channels.

As discussed, each viewing element defines an optical footprint in the image layer corresponding to the lateral area of the viewing element. Stated differently, the optical footprint may be considered as the projection of the viewing element onto the image layer when viewed from a direction that is normal to the image layer. In this way, the image layer may be seen to comprise a plurality of (contiguous) optical footprints, each corresponding to a respective viewing element of the array. The elongate viewing elements are preferably in the form of elongate focussing elements. Such elongate focussing elements are typically adapted to focus light in one dimension (e.g. focussing light to a line). Typically, the elongate focussing elements are cylindrical lenses. The pitch of the viewing elements is typically in the range of 10 pm to 200 pm, preferably 20 pm to 200 pm, more preferably 50 pm to 200 pm.

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.

Although the viewing elements are typically in the form of focussing elements (e.g. elongate 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.

As discussed, the image layer comprises a plurality of sets of image segments, each set of image segments in combination defining a respective image and forming an image channel. It will be appreciated by the skilled reader that the relative position of each image segment of a set of image segments under the respective viewing elements preferably remains consistent across the image layer, dependent on the desired viewing angle for that image channel. Preferably, each image segment of an image channel has substantially the same relative position along the first direction with respect to each viewing element. In other words, image segments of a particular image channel preferably have the same relative positions in each optical footprint of the image layer. Typically, each 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 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.

The image layer material of different sets of image segments (different image channels) may be the same image layer material (e.g. having the same colour): for example, the image layer material of at least two of the plurality of image segments in different rows may have the same colour such that the respective different images that are perceived simultaneously at a particular viewing angle have the same colour. The image layer material of different sets of image segments may differ such that different images (e.g. perceived simultaneously) have different colours. In some cases, at least one of the images exhibited by a set of image segments may be a multicoloured image utilising a plurality of different coloured image layer materials (e.g. different coloured inks) within the image channel.

Preferably, the image layer material of each set of image segments is one or more (e.g. coloured) inks, typically provided as a print working.

Typically, each image segment 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 image segment has a dimension (e.g. a “length” along the direction of elongation of the viewing elements) of less than 150pm, more preferably less than 100pm and even more preferably less than 70pm.

In general, the images exhibited by the device 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 being perceived within a viewing plane). For example, if two images do not overlap, the viewer (e.g. simultaneously) perceives the full outline of each image laterally separate from each other. On the other hand, if two images 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. As each image displayed by the device is different, there will only be partial overlap, rather than full overlap, between any two images.

The image segments are arranged as an array having a plurality of first rows each comprising image segments of a first subset of the plurality of image channels and a plurality of second rows each comprising image segments of a second subset of the image channels different from the first subset. Preferably, each image segment within the first and second rows corresponds to a different image channel. However, it is envisaged that the first and second rows may contain image elements from the same image channel, as long as the sets of image elements in the first and second rows differ. In this way, for at least one viewing angle, light from at least two different image channels from different rows is directed to the viewer.

As outlined above, a device according to the present invention advantageously increases the ease with which the number of (different) images exhibited by the device can be increased. Although not essential, preferably, the image channels define an animation sequence (e.g. as the viewing angle is changed). Increasing the number of image channels exhibited by the device leads to an enhanced (e.g. smoother transition between frames) animation effect that is straightforward to authenticate and yet more difficult to counterfeit. In this way, the present invention advantageously provides a device with increased security level. Preferably, the animation sequence comprises any of: a lateral movement, a rotation, an expansion or contraction. The animation may comprise a change in colour.

Typically, each first row comprises image segments from two or more primary image channels, and each second row comprises image segments from at least one secondary image channel. As will be described in more detail herein, dependent on the arrangement of the image segments of the image layer, the primary images may typically be perceived with increased perceived density (e.g. brightness) relative to the secondary images. In some embodiments, the primary images may exhibit an animation sequence in isolation, with the secondary images providing “intermediate” image frames between the primary images to enhance (or “smooth”) the animation.

In embodiments, the first rows and the second rows may comprise the same number of image segments. For example, each first row within an optical footprint may comprise exactly 3 image segments arranged along the first direction, and each second row within an optical footprint may comprise exactly 3 image segments arranged along the first direction. However, it is envisaged that different arrangements of the image segments within different rows may be employed, dependent on the secure visual effect that is desired. In general, typically, each first row comprises image segments from M image channels, and each second row comprises image segments from m image channels, and wherein M>=m, preferably wherein M=m or /W=m+1 .

In some embodiments, in at least a region of the image layer (in some embodiments across the whole domain of the image layer), the image segments of the second rows are laterally aligned with the image segments of the first rows along the first direction. Herein, the lateral alignment refers to the relative positions of the image segments along the first direction. In some embodiments, in at least a region of the image layer (in some embodiments across the whole domain of the image layer), the image segments of the second rows are laterally offset from the image segments of the first rows along the first direction. Laterally offsetting the image segments of different rows can advantageously provide complex visual effects due to the varying proportions of image channels that are exhibited at particular viewing angles of the device. This may help to smooth the transition between image frames of an animation, for example. It is envisaged that in some embodiments, the image layer may comprise region(s) in which the image segments of different rows are aligned, and region(s) in which the image segments of different rows are offset.

In embodiments, in at least a region of the image layer (in some embodiments across the whole domain of the image layer), a width of the image segments in the second rows is different from a width of the image segments in the first rows. Here, the term “width” refers to the dimension of the image segments along the first dimension. Varying the width of the image segments (and therefore their lateral area within the image layer) can advantageously control the perceived density of the exhibited images of the corresponding image channels. Image channels having different widths may also be perceived for different ranges of viewing angles as the device is tilted (e.g. an image channel with a relatively larger width will be replayed, or “on”, for longer than an image channel with relatively smaller width). In this way, embodiments in which the width of the image segments differs may provide additional complex visual effects. In some embodiments, in at least a region of the image layer (in some embodiments across the whole domain of the image layer), a width of the image segments varies within a row.

As discussed above, varying the lateral area of the image segments within the image layer may be used to advantageously control the perceived density of the images exhibited by the device. A convenient way in which the relative perceived density of the image channels can be controlled is through variation of the length of the image segments of different rows, wherein for a given line width, an image channel defined by image segments of greater length will appear relatively brighter. In embodiments, each image segment has a length along the second direction, and wherein a ratio of the length of the image segments in the first rows to the length of the image segments in the second rows is between 1 :1 and 10:1 , preferably between 2:1 and 10:1. For example, in the case where the first rows contain image segments of primary images and the second rows contain image segments of secondary images, the ratio may be 2:1 or greater such that the primary images are perceived with relatively increased density. In some embodiments, the ratio of the length of the image segments in the first rows to the length of the image segments in the second rows is substantially the same across the image layer. However, in some embodiments, the image layer may comprise a first zone and a second zone, wherein the ratio of the length of the image segments in the first rows to the length of the image segments in the second rows in the first zone is different from the ratio of the length of the image segments in the first rows to the length of the image segments in the second rows in the second zone. In this way, the relative perceived density of the exhibited images may vary across the device, giving rise to further complex visual effects that advantageously increase the security level of the device. Such a variation in brightness may provide tonal effects in accordance with an image, for example.

Typically, a length of the image segments is equal to the length of the corresponding row.

In embodiments, for each optical footprint, the array of image segments further comprises a third row comprising image segments of a third subset of the image channels different from the first and second subsets. In general, it is envisaged that for each optical footprint, the array of image segments comprises two or more rows comprising image segments of respective (different) subsets of the image channels.

Typically, the arrangement of image segments within each optical footprint across at least a portion of the image layer (typically across the entire image layer) is substantially the same. It is noted that the term “arrangement” here refers to the relative positioning of image segments within each optical footprint, rather than the specific form of the image segments themselves (e.g. the positioning of the image material within each image segment). Portion(s) of the image layer in which the optical footprints have substantially the same arrangement of image segments may be referred to as having a repeating arrangement of optical footprints. Such a repeating arrangement may increase manufacturing ease of the image layer. In some embodiments, the image layer comprises a plurality of first optical footprints each having image segments of a first group of image channels and a plurality of second optical footprints each having image segments of a second group of image channels different from the first group, wherein the arrangement of the first optical footprints and the second optical footprints is such that for each of a plurality of different viewing angles of the device (preferably for any one viewing angle of the device), light from an image channel of the first group and light from an image channel of the second group is directed to the viewer from different viewing elements simultaneously.

By including a plurality of first optical footprints and a plurality of second optical footprints, the first group of image channels is associated with a corresponding first group of viewing elements, and the second group of image channels is associated with a second group of viewing elements. In this way, when viewing the device, for each of a plurality of different viewing angles, the viewer simultaneously perceives at least a portion of the images from both the first and second groups of image channels, “originating” from different viewing elements. This approach advantageously enables additional image channels to be exhibited by the device and consequently provides further complexity of the exhibited visual effect and a corresponding increase in security level of the device.

Typically, the first optical footprints are interlaced with the second optical footprints along the first direction (substantially perpendicular to the direction of elongation of the viewing elements). The ratio of the number of first optical footprints to second optical footprints may be controlled in order that the images of the first and second groups of image channels are perceived with the desired relative perceived density. Typically, the ratio of first optical footprints to second optical footprints is between 1 :1 and 20:1 , preferably between 2:1 and 20:1.

Preferably, at least one 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 may comprise any of: a lateral movement, a rotation, an expansion or contraction of the indicia or indicum.

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 semitransparent (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.

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. In some examples, the image layer may comprise image segments formed by different mechanisms, which may be used to control the perceived image density across the device in addition to or alternatively to adjusting the lateral size of the image segments.

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”.

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.

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.

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.

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.

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, ora 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.

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.

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

(a) providing a substrate;

(b) applying an array of elongate 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, each set of image segments in combination defining a respective image and forming an image channel; wherein each viewing element defines an optical footprint in the image layer corresponding to the lateral area of the viewing element, and wherein for each optical footprint within the image layer: the image segments are arranged as an array having a plurality of first rows each comprising image segments of a first subset of the plurality of image channels and a plurality of second rows each comprising image segments of a second subset of the image channels different from the first subset; wherein each of the first and second rows is aligned with a first direction that is substantially perpendicular to the direction of elongation of the viewing elements, and wherein within a row the respective image segments are arranged so as to cooperate with the respective viewing element such that upon tilting the device about a second direction that is parallel with the direction of elongation of the viewing element, light from different image channels within a row is selectively directed to the viewer by the viewing element in dependence on viewing angle; and further wherein the plurality of first rows and the plurality of second rows are interlaced with each other along the second direction, whereby when viewing the device, the image segments cooperate with the array of viewing elements such that when tilting the device about the second direction, the images of different image channels are selectively exhibited to the viewer in dependence on viewing angle, and for each of a plurality of different viewing angles of the device, light from an image channel of the first subset of image channels and light from an image channel of the second subset of image channels is directed to the viewer simultaneously. 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.

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.

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. BRIEF DESCRIPTION OF DRAWINGS

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

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

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

Figure 3 is a schematic perspective view of a single lens forming part of a lens array of a conventional device;

Figure 4 is a schematic perspective view of a portion of a security device 100 according an embodiment of the invention;

Figures 5(a) and 5(b) schematically illustrate images exhibited by devices according to embodiments of the invention;

Figure 6 schematically illustrates a device according to an embodiment of the invention;

Figure 7 schematically illustrates a portion of a device according to an embodiment of the invention;

Figures 8 and 9(a) to (c) illustrate portions of further devices according to the invention;

Figure 10(a) schematically illustrates a device according to an embodiment of the invention; Figure 10(b) is a magnified view of a portion of the device, and Figure 10(c) illustrates the visual effect perceived by a viewer of the device;

Figure 11 schematically illustrates a device according to a further embodiment of the invention;

Figure 11 A schematically illustrates a device according to a further embodiment of the invention;

Figures 12, 13 and 14 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;

Figure 15 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; Figures 16A to 161 illustrate different examples of relief structures which may be used to form the image layer in embodiments of the invention; and

Figure 17(a) is a schematic perspective view of a portion of a security device according a further embodiment of the invention, and Figure 17(b) illustrates the images exhibited by the device.

DETAILED DESCRIPTION

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.

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 interlaced 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. The image layer comprises a first set of image segments and a second set of image segments i₂ that are interleaved with each other periodically along the x- direction (i.e. perpendicular to the direction of elongation of the cylindrical lenses). 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. 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-dimension). Thus, the image layer 30 lies within the x-z plane.

Figure 3 is a schematic perspective view of a single lens 21 forming part of the lens array 20 of a conventional device similar to the one depicted in Figure 1 and 2, but in this case selectively exhibiting three images in dependence on viewing angle (i.e. a three-channel device). The lens 21 defines an optical footprint 23, shown in dashed outline, in the image layer 30. The optical footprint 23 corresponds to the lateral area of the image layer 30 covered by the lens 21. Stated differently, the optical footprint 30 is the projection of the lens 21 onto the image layer 30 when viewed from a direction that is normal to the image layer 30. In this way, the image layer 30 may be seen to comprise a plurality of (contiguous) optical footprints 23, each corresponding to a respective lens 21 of the lens array 20.

In the conventional device 101 , each optical footprint 23 contains an image segment i1 from the first image channel, an image segment i2 from the second image channel, and an image segment i3 from the third image channel, positioned laterally separate from each other along the x-dimension. The arrangement of the image segments is repeated in each optical footprint of the image layer, such that image segments within the same image channel occupy the same relative position under the associated 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 segments themselves (e.g. the positioning of ink within each image segment). This means that at a given viewing angle of the device, each lens 21 “selects” light from the same image channel such that the viewer perceives a single image corresponding to that image channel. As the device is tilted (about a tilt axis that is parallel with the z-axis) and the viewing angle changes, the viewer perceives the images of the different image channels.

Thus, in conventional devices, the line 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 difficulties 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.

Figure 4 is a schematic perspective view of a portion of a security device 100 according to an embodiment of the invention. The structure of the device 100 is similar to that of the device 101 described with reference to Figures 1 and 2, but the arrangement of the image layer 30 fundamentally differs, as shall now be described. Figure 4 illustrates a portion of the device corresponding to a single cylindrical lens 21 of the lens array and the arrangement of the image layer 30 in the corresponding optical footprint 23 defined by the lens.

In this example, the device comprises six sets of image segments i1 , i2, i3, i4, i5, and i6, defining six image channels respectively. Within the image layer 30, each optical footprint 23 contains an image segment from each image channel, arranged as two rows R1 and R2 (a “row” extending along the direction substantially perpendicular to the direction of elongation of the cylindrical lenses 21). The first row R1 comprises only image segments from a first subset of the image channels, in this case image segments i1 , i2 and i3 from the first, second and third image channels respectively. The second row R2 comprises only image segments from a second subset (different from the first) of the image channels, in this case image segments i4, i5 and i6 from the fourth, fifth and sixth image channels respectively. The image segments i4, i5 and i6 are laterally aligned with image segments i1 , i2 and i3 respectively. For simplicity, only one first row R1 and one second row R2 are shown in Figure 4. However, in practice, a plurality of first rows R1 and a plurality of second rows R2 are interlaced with each other along the z dimension.

Consequently, as the device is tilted and the viewing angle varied, each lens 21 directs light from two image channels to the viewer simultaneously. More specifically, at viewing angle 01 , a viewer of the device will perceive images 11 and I4 of the first and fourth image channels substantially simultaneously; at viewing angle 02, the viewer will perceive images I2 and I5 of the second and fifth image channels substantially simultaneously; and at viewing angle 03, the viewer will perceive images I3 and I6 of the third and sixth image channels respectively. It is noted that the images of each image channel differ from each other, and in general may or may not overlap with each other. The reference to “overlap” of images herein refers to the perception of the “macro” images by the viewer, e.g. in a viewing plane.

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 line width of the image segments as in conventional devices. It is noted that in comparison with the device 101 depicted in Figure 3, for a given lens diameter D, the present invention enables additional image channels to be exhibited without a requirement for a reduction in the width of an image segment. The inclusion of additional image channels provided by the invention has particular benefit in lenticular devices that exhibit an animation sequence, as an increased number of image frames may provide an enhanced animation effect (e.g. a smoother transition between image frames) upon a change of viewing angle. This advantageously enables complex visual effects to be generated that are difficult to counterfeit, thereby increasing the security level of the device.

As described above, in general the “macro” images exhibited by the device may or may not overlap within the viewing plane. Figure 5(a) schematically illustrates an example in which the images exhibited by the device 100 of Figure 4 do not overlap. Figure 5(a) illustrates each image 11 to I6 as defined by the respective image channel, as observed by a viewer of the device. It will be appreciated that Figure 5(a) illustrates each image in a single figure, rather than what would be observed by a viewer when viewing the device in practice. Here, as the images do not overlap, the observer perceives the full outline of each image (in the example in the form of circle sectors) laterally separate from, or contiguous with, each other. This in contrast to the scenario depicted in Figure 5(b) in which there is overlap between the different images.

We now consider what a viewer of the device 100 of Figure 4 will perceive upon tilting the device, assuming that the images exhibited by the image channels correspond to those seen in Figure 5(a). As described above, due to the arrangement of the image segments within the image layer 30, at viewing angle 01 , the viewer perceives the images 11 and I4 substantially simultaneously. As the device is tilted from 01 to 02 to 03, the viewer observes images I2 and I5 substantially simultaneously, and then images I3 and I6 together. In this way, as the device is tilted, the viewer observes a rotational animation effect, with the sectors depicted in Figure 5(a) appearing to move from one position to the next in a clockwise manner. Without the image channels I4, I5 and I6 (i.e. as in the conventional device illustrated in Figure 3), the viewer would perceive the image changing from 11 to I2 to I3 only, which would appear discontinuous. The inclusion of the additional image channels I4, I5 and I6 advantageously “smooths” the animation effect between the images 11 , 12 and I3. In this way, the image channels 11 to I3 of the first row may be termed “primary” image channels, and the image channels I4 to I6 may be termed “secondary”, or “intermediate” image channels. Similarly, a device exhibiting overlapping images as in Figure 5(b) exhibits a smoother animation effect due to the increase in the number of image channels compared to a conventional device. Here, the overlap of the images may provide an enhanced animation effect (e.g. smoother transition between image frames) and/or further complex visual effects such as colour mixing in the regions of image overlap.

Figure 6 schematically illustrates a device 100 according to an embodiment of the invention. The device 100 of Figure 6 is a 6-channel device with the image segments of the image channels arranged into a plurality of first (R1) and second (R2) rows that are interlaced with each other along the z dimension (i.e. along the direction of elongation of the associated cylindrical lens 21) in the same manner as described above with reference to Figure 4. Thus, the image segments of the image layer are interlaced with each other along two orthogonal dimensions. The dashed lines in Figure 6 indicate that further rows R1 and R2 are included within the image layer.

Figure 6 illustrates, in dashed outline, the optical footprint 23 of a first cylindrical lens 21a of the lens array 20. As can be seen from the figure, the optical footprints of the further lenses 21b, 21c, 21 d of the array have the same arrangement of image segments as the optical footprint corresponding to lens 23a. Thus, the arrangement of image segments within each optical footprint may be described as a repeating arrangement across the image layer 30 (along the x-dimension of the image layer).

In the example of Figure 6, each individual image segment occupies the same lateral area within the image layer 30. Consequently, this means that each image will be exhibited with substantially the same perceived density (e.g. “brightness”). In some embodiments, the areal size of each image segment may be varied to advantageously control the relative brightness at which each image is perceived. A convenient manner in which this may be performed whilst maintaining a constant line width is by varying the length (along the z-dimension) of the image segments. As can be seen in Figure 6, each image segment within the first rows R1 has a length L1 , and each image segment within the second rows R2 has a length L2. In this example, the ratio L1 :L2 is 1 :1. However, it may be desired that the images 11 , I2 and I3 have a greater perceived density relative to images I4, I5 and I6. This may be the case, for example, if images 11 , I2 and I3 represent “primary” images of an animation sequence, and images I4, I5 and I6 represent “secondary”, or “intermediate”, images that are used to smooth the transition between the primary images as the device is tilted, as described above with reference to Figures 5(a) and 5(b). In such case, the ratio of L1 :L2 may be increased, for example to 3:2 or 2:1 or greater. In general, it is envisaged that the ratio of L1 : L2 may be in the range of 1 : 1 to 10: 1 .

The ratio of the perceived density of the images between image channels (i.e. relative image segment lateral size) may be constant across the image layer, or may vary in different regions of the image layer, for example in accordance with the images exhibited by the device.

Figure 7 schematically illustrates a lens 21 and its associated optical footprint of a device 100 according to a further embodiment of the invention. In the examples described in relation to Figures 4 and 5, each optical footprint contained a plurality of first rows R1 comprising exactly three image segments, and one or more second rows R2 comprising exactly three (different) image segments. In the device 100 of Figure 7, within each optical footprint, the image layer 30 comprises four rows R1 , R2, R3 and R4, each containing image segments of a different subset of the image channels exhibited by the device. Here, each row contains exactly three image segments such that the device is a 24 channel device and at each viewing angle 01 , 02, 03, the device simultaneously exhibits four images to a viewer. As indicated in the figure, the sequence of rows R1 , R2, R3, R4 repeats along the z axis such that the rows are interlaced with each other along the length of the lens 21.

In the examples discussed thus far, the number of image segments within the different rows of an optical footprint has been the same (e.g. three image segments per row), and the image segments have been laterally aligned with each other. Further complex optical effects may be exhibited by varying the numbers of image segments per row and/or the lateral offset of the image segments within different rows, as will now be described with reference to Figures 8 to 10. Figure 8 schematically illustrates a portion of a device 100 according to a further embodiment of the invention. As before each lens defines an optical footprint 23 within the image layer 30, with each optical footprint comprising a first subset of image segments arranged as a first row and a second subset of image segments arranged as a second row. In the device depicted in Figure 8, the first row R1 contains image segments i1 , i2 and i3 from first, second and third image channels respectively. The second row R2 contains a different number of image segments from the first row; in this case two image segments i4 and i5 forming parts of image channels I4 and I5 respectively. The image segments i4 and I5 have the same line width as image segments i1 to i3, and are laterally offset along the x-axis from the image segments of the first row R1 .

Due to the lateral offset of the image segments i4 and i5 in the second row compared to the image segments of the first row, the image channel I4 will be exhibited at both viewing angles 01 and 02, and similarly image channel I5 will be exhibited at viewing angles 02 and 03. In this way, the additional image channels I4 and I5 may advantageously enhance an animation effect between images 11 , I2 and I3 upon tilting of the device, smoothing the transition between “primary” images 11 , I2 and I3.

Figures 9(a) to 9(c) schematically illustrate further examples of how the arrangement of the additional image channels in the second rows R2 may be adjusted to achieve further complex effects. Each of Figures 9(a) to 9(b) depicts a single lens 21 of the device and the arrangement of the image segments within the associated optical footprint. In Figure 9(a), the additional image segments in row R2 have a larger line width than the image segments in row R1. In Figure 9(b), the additional image segments in row R2 have a smaller line width than the image segments of the first row R1 . In Figure 9(c), the second row R2 comprises additional image segments of varying line width. As described above, adjusting the line width may be used to control the relative perceived density of the images exhibited by the device. Furthermore, adjustment of the line width can be used to control the range of viewing angles over which an image segment is perceived, in other words for how long an image channel will be replayed (or “on”) as the device is tilted. For example, in Figure 9(a), the image segments in row R2 will be “on” for a longer time than the image segments in row R1 as the device is tilted. Conversely, in the device of Figure 9(b), the channels in row R2 will be “on” for less time than the R1 channels. In this way, including image segments with different line widths may be used to provide further complex visual effects to the device.

Figure 10(a) schematically illustrates a device 100 according to an embodiment of the invention in which the image segments of different rows are offset from each other along the x-dimension. The device 100 is a 7-channel device, with each optical footprint containing first (R1), second (R2) and third (R3) rows that are interlaced with each other along the direction of elongation of the cylindrical lenses (i.e. along the z-dimension). As more clearly seen in the magnified portion of the image layer shown in Figure 10(b), each first row R1 contains three image segments i1 , i2 and i3, each second row R2 contains two image segments i4 and i5 offset from those of the first row R1 , and each third row i3 contains two image segments i6 and i7 offset from those of the second row R2. The image channels of the device exhibit a rotational animation effect upon tilting of the device, as schematically illustrated in Figure 10(c).

Figure 10(c) illustrates each image (11 to I7) exhibited by the device in one figure. Each individual image is in the form of a substantially rectangular shape, with different images having different colours and rotational positions within the viewing plane. Thus, in practice, as the device is tilted from viewing angle 01 to 02 to 03, the viewer perceives a rotational animation effect of the rectangular graphic moving from one position to the next and changing colour. The images 11 , I2 and I3 may be considered to be the primary images, with images I4 to I7 being “secondary images” that smooth the transition effect between the primary images, thereby enhancing the animation and increasing the security level of the device. We now consider the visual effect perceived at viewing angle 01 . Here, the viewer will see image 11 , together with parts of images I4 and I6. As the device is tilted and the viewer observes the device at viewing angle 02, the primary image changes from image 11 to I2, but the device still exhibits portions of the intermediate image frames I4 and I6. This has the advantage of enhancing (e.g. smoothing) the animation transition from image 11 to image I2. Similarly, at viewing angle 02, the observer further perceived portions of images I5 and I7 due to the offset nature of the image segments within the optical footprint. This enhances the transition from image I2 to I3 as the device is tilted from viewing angle 02 to 03. The colours of the images may also be chosen to aid the smoothing of the animation effect as the images change colour in addition to appearing to rotate as the device is tilted.

Typically, the arrangement of the image segments within each optical footprint is the same across the image layer. In otherwords, the image channels represented by the image segments within each optical footprint (and their relative arrangement) is the same. Figure 11 schematically illustrates a security device 100 according to a further embodiment of the invention in which the image layer comprises optical footprints having different arrangements of image segments. The image layer 30 comprises a first optical footprint 23a that is associated with lens 21a. First optical footprint 23a comprises a first group of image segments i1 to i6 corresponding to respective image channels 11 to I6, arranged across interlaced first and second rows R1 , R2 as previously described. The image layer also comprises a second optical footprint 23b associated with second lens 21 b that comprises a second group of image segments i7 to i12 corresponding to respective image channels I7 to 112.

Consider viewing the device 100 at a first viewing angle 01. The first lens 21a direct light from image segments i1 and i4 of the first optical footprint 23a to the viewer substantially simultaneously, in the same manner as described for the preceding embodiments. Similarly, the second lens 21b will direct light from image segments i7 and i10 of the second optical footprint 23b. Thus, when viewing the device as a whole at the first viewing angle 01 , the viewer will simultaneously perceive image segments i1 , i4, i7 and i10, and therefore simultaneously perceive four images. This effect is due to a combination of different images being simultaneously exhibited by the same lens, and additionally different images being simultaneously exhibited by different lenses of the array.

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 11 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 optical footprints. Additionally, it is also envisaged that the image segments within different rows of an optical footprint may have different lengths and/or line widths, and/or may be offset from each other as has previously been described.

The ratio of the number of first optical footprints 23a to the number of second optical footprints 23b may be adjusted in order to control the relative perceived density of the image channels. In the example of Figure 11 , the ratio of first 23a to second 23b optical footprints is 1 :1. However, if for example the image segments i7 to i12 of the second optical footprints 23b correspond to “secondary” images desired to have a relatively lower perceived density or brightness, the ratio of first optical footprints 23a to second optical footprints 23b may be increased, for example to 3:2, 2:1 or greater. In general, the ratio may be between 1 :1 (no relative change in perceived density) and 20:1.

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 11 A) that comprises substantially opaque regions 93 spaced by substantially transparent regions 95 (e.g. defined by gaps between the opaque regions). Each viewing element (shown at 97) comprises a gap region 95 positioned substantially centrally over the corresponding optical footprint (shown at 98). In the cross-sectional view of Figure 11 A, the image segments of the optical footprints extend into the plane of the page.

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.

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

The security article may be incorporated into 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.

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

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

Examples of documents of value and techniques for incorporating a security device will now be described with reference to Figures 12 to 15.

Figure 12 depicts an exemplary document of value 1500, here in the form of a banknote. Figure 12(a) shows the banknote in plan view whilst Figure 12(b) shows a cross-section of the same banknote along the line X-X' and Figure 12(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.

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 12(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 12(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.

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.

In Figure 13 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.

In Figure 14, 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.

A further embodiment is shown in Figure 15 where Figures 15(a) and 15(b) show the front and rear sides of the document 1800 respectively, and Figure 15(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 15(a)) and exposed in one or more windows 1802 on the opposite side of the document (Figure 15(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.

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.

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.

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.

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

The 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.

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. 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 16A-16I. Thus, Figure 16A 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 16B 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.

In another approach, the relief structures can be in the form of diffraction gratings (Figure 16C) or moth eye I fine pitch gratings (Figure 16D). 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. Such diffraction gratings for moth eye I fine pitch gratings can also be located on recesses or bumps such as those of Figures 16A and 16B, as shown in Figures 16E and 16F respectively.

Figure 16G illustrates the use of a simple scattering structure providing an achromatic effect.

Further, in some cases the recesses of Figure 16A could be provided with an ink or the debossed regions or bumps in Figure 16B could be provided with an ink. The latter is shown in Figure 16H 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 161 illustrates the use of an Aztec structure.

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 17(a) and 17(b).

Figure 17(a) is a schematic perspective view of a portion of a security device 100 similar to that described with reference to Figure 4. As with Figure 4, Figure 17(a) illustrates a portion of the device corresponding to a single cylindrical lens 21 and the arrangement of the image layer 30 in the corresponding optical footprint 23 defined by the lens.

As with the device of Figure 4, the device depicted in Figure 17(a) comprises six sets of image segments i1 , i2, i3, i4, i5 and i6, defining six image channels respectively. In the device of Figure 17(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 of the first row R1 each comprise a luminescent ink that exhibits a visible colour under both visible light illumination and 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 of the second row R2 each comprise a luminescent ink that is substantially transparent and colourless in visible light and exhibits a visible colour under UV illumination. As with Figure 4, for simplicity, only one first row R1 and one second row R2 are shown in Figure 17(a). However, in practice, a plurality of first rows R1 and a plurality of second rows R2 are interlaced with each other along the z dimension.

The visual effect when the device is viewed under both visible light and under UV illumination is schematically shown in Figure 17(b). When the device is viewed under visible light, the luminescent ink forming image segments i4, i5 and i6 exhibits a transparent, colourless appearance and consequently as the device is tilted from viewing angle 01 to 03, the device exhibits images 11 , I2 and I3 in sequence. As schematically shown in Figure 17(b)(i), this provides a discontinuous change in the rotational animation effect from image 11 to I2 to I3. 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, 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, as schematically shown in Figure 17(b)(ii). (It will be appreciated that Figures 17(b)(i) and 17(b)(ii) illustrate each image in a single figure, rather than what would be observed when tilting the device in practice.)

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.

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

1 . A security device, comprising: a substrate; an array of elongate 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, each set of image segments in combination defining a respective image and forming an image channel; wherein each viewing element defines an optical footprint in the image layer corresponding to the lateral area of the viewing element, and wherein for each optical footprint within the image layer: the image segments are arranged as an array having a plurality of first rows each comprising image segments of a first subset of the plurality of image channels and a plurality of second rows each comprising image segments of a second subset of the image channels different from the first subset; wherein each of the first and second rows is aligned with a first direction that is substantially perpendicular to the direction of elongation of the viewing elements, and wherein within a row the respective image segments are arranged so as to cooperate with the respective viewing element such that upon tilting the device about a second direction that is parallel with the direction of elongation of the viewing element, light from different image channels within a row is selectively directed to the viewer by the viewing element in dependence on viewing angle; and further wherein the plurality of first rows and the plurality of second rows are interlaced with each other along the second direction, whereby when viewing the device, the image segments cooperate with the array of viewing elements such that when tilting the device about the second direction, the images of different image channels are selectively exhibited to the viewer in dependence on viewing angle, and for each of a plurality of different viewing angles of the device, light from an image channel of the first subset of image channels and light from an image channel of the second subset of image channels is directed to the viewer simultaneously.

2. The security device of claim 1 , wherein the image channels define an animation sequence.

3. The security device of claim 1 or claim 2, wherein each first row comprises image segments from two or more primary image channels, and each second row comprises image segments from at least one secondary image channel.

4. The security device of claim 1 or claim 2, wherein each first row comprises image segments from M image channels, and each second row comprises image segments from m image channels, and wherein M>=m, preferably wherein M=m or M=m+'\ .

5. The security device of any of the preceding claims, wherein in at least a region of the image layer, the image segments of the second rows are laterally aligned with the image segments of the first rows along the first direction.

6. The security device of any of the preceding claims, wherein in at least a region of the image layer, the image segments of the second rows are laterally offset from the image segments of the first rows along the first direction.

7. The security device of any of the preceding claims, wherein in at least a region of the image layer, a width of the image segments in the second rows is different from a width of the image segments in the first rows.

8. The security device of any of the preceding claims, wherein in at least a region of the image layer, a width of the image segments varies within a row.

9. The security device of any of the preceding claims, wherein each image segment has a length along the second direction, and wherein a ratio of the length of the image segments in the first rows to the length of the image segments in the second rows is between 1 :1 and 10:1 , preferably between 2:1 and 10:1.

10. The security device of claim 9, wherein the ratio of the length of the image segments in the first rows to the length of the image segments in the second rows is substantially the same across the image layer.

11 . The security device of claim 9, wherein the image layer comprises a first zone and a second zone, wherein the ratio of the length of the image segments in the first rows to the length of the image segments in the second rows in the first zone is different from the ratio of the length of the image segments in the first rows to the length of the image segments in the second rows in the second zone.

12. The security device of any of the preceding claims, wherein each image segment of an image channel has substantially the same relative position along the first direction with respect to each viewing element.

13. The security device of any of the preceding claims, wherein for each optical footprint, the array of image segments further comprises a third row comprising image segments of a third subset of the image channels different from the first and second subsets.

14. The security device of any of any of the preceding claims, wherein the arrangement of image segments within each optical footprint across at least a portion of the image layer is substantially the same.

15. The security device of any of claims 1 to 13, wherein the image layer comprises a plurality of first optical footprints each having image segments of a first group of image channels and a plurality of second optical footprints each having image segments of a second group of image channels different from the first group, wherein the arrangement of the first optical footprints and the second optical footprints is such that for each of a plurality of different viewing angles of the device, light from an image channel of the first group and light from an image channel of the second group is directed to the viewer from different viewing elements simultaneously.

16. The security device of claim 15, wherein the ratio of first optical footprints to second optical footprints is between 1 :1 and 20:1 , preferably between 2:1 and 20:1.

17. 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.

18. The security device of any of the preceding claims, wherein the viewing elements are focussing elements, preferably wherein the viewing elements are cylindrical 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 provided in or on a first surface of the substrate and the image layer is provided 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 luminescent visible 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 passport.

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 elongate 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, each set of image segments in combination defining a respective image and forming an image channel; wherein each viewing element defines an optical footprint in the image layer corresponding to the lateral area of the viewing element, and wherein for each optical footprint within the image layer: the image segments are arranged as an array having a plurality of first rows each comprising image segments of a first subset of the plurality of image channels and a plurality of second rows each comprising image segments of a second subset of the image channels different from the first subset; wherein each of the first and second rows is aligned with a first direction that is substantially perpendicular to the direction of elongation of the viewing elements, and wherein within a row the respective image segments are arranged so as to cooperate with the respective viewing element such that upon tilting the device about a second direction that is parallel with the direction of elongation of the viewing element, light from different image channels within a row is selectively directed to the viewer by the viewing element in dependence on viewing angle; and further wherein the plurality of first rows and the plurality of second rows are interlaced with each other along the second direction, whereby when viewing the device, the image segments cooperate with the array of viewing elements such that when tilting the device about the second direction, the images of different image channels are selectively exhibited to the viewer in dependence on viewing angle, and for each of a plurality of different viewing angles of the device, light from an image channel of the first subset of image channels and light from an image channel of the second subset of image channels 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 26 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 22.