EP4565434A1

EP4565434A1 - Security devices and methods of manufacture thereof

Info

Publication number: EP4565434A1
Application number: EP23753939.0A
Authority: EP
Inventors: John Godfrey
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2022-08-03
Filing date: 2023-07-26
Publication date: 2025-06-11
Also published as: AU2023318141A1; WO2024028574A1; GB2621154A; GB2621154B; GB202211330D0; MA71682A

Abstract

A security device is disclosed, comprising: a substrate; an array of viewing elements disposed on the substrate; and an image layer disposed in or on the substrate, wherein the image layer comprises at least first and second sets of image segments, the first set of image segments comprising image layer material that together defines a first image and the second set of image segments comprising image layer material that together defines a second image, the first set of image segments and the second set of image segments being interleaved with each other; and wherein the array of viewing elements and the image layer cooperate with each other such that at a first range of viewing angles, light from the first set of image segments is directed to a viewer such that the first image is exhibited; and at a second range of viewing angles, light from the second set of image segments is directed to the viewer such that the second image is exhibited; wherein the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region, and wherein the arrangement of the image layer material of the image segments and/or the colour density of the image segments within a region is dependent upon the number of sets of image segments having image segments present within that region, and is different for different regions such that when viewing the device within the first range of viewing angles the first image is perceived to have a uniform tone, and when viewing the device within the second range of viewing angles the second image is perceived to have a uniform tone. Methods of manufacturing such security devices are also disclosed.

Description

SECURITY DEVICES AND METHODS OF MANUFACTURE THEREOF

FIELD OF THE INVENTION

The present invention relates to security devices which exhibit an optically variable effect. Security devices are used for example on documents of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents, in order to confirm their authenticity. Methods of manufacturing security devices are also disclosed.

BACKGROUND TO THE INVENTION

Articles of value, and particularly documents of value such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such objects are provided with a number of visible security devices for checking the authenticity of the object. By “security device” we mean a feature which it is not possible to reproduce accurately by taking a visible light copy, e.g. through the use of standardly available photocopying or scanning equipment.

One class of security devices are lenticular devices, which make use of focussing elements (such as lenses) or a masking grid 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 or a masking grid, 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. Typically, no magnification takes place and the resulting image which is observed will be of substantially the same size as that to which the underlying image segments are formed. Some examples of lenticular devices are described in US-A-4892336, WO-A-2011/051669, WO-A-2011051670 and US-B-6856462. More recently, two-dimensional lenticular devices have also been developed and examples of these are disclosed in WO2015/011493 and WO2015/011494. Lenticular devices have the advantage that different images can be displayed at different viewing angles, giving rise to the possibility of animation and other striking visual effects which are not possible using the moire magnifier or integral imaging techniques.

In an ideal lenticular device, each viewing element would direct light from only one set of image segments (an “image channel”) to the observer at a particular viewing angle. In reality however, as a lenticular device typically comprises a large number of viewing elements (e.g. lenses), at a particular viewing angle of the device, the observer views each lens at a slightly different relative angle. As such, across the extent of the device, each lens will direct light from slightly different respective portions of the array of image segments.

Consequently, when viewing the device at a viewing angle corresponding to a particular image channel, portions of adjacent image channels will also be visible. This is a particular problem in areas of the device where the macro images exhibited by the different channels overlap on the image layer, as this causes differences in contrast - or “tone” - to be perceived in the exhibited image. These contrast artefacts in the exhibited optically variable image can undesirably reduce the clarity of the information conveyed by the device, and reduce the security level in the case of security devices.

There is therefore a requirement to provide security devices that overcome the above issues.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided a security device, comprising: a substrate; an array of viewing elements disposed on the substrate; and an image layer disposed in or on the substrate, wherein the image layer comprises at least first and second sets of image segments, the first set of image segments comprising image layer material that together defines a first image and the second set of image segments comprising image layer material that together defines a second image, the first set of image segments and the second set of image segments being interleaved with each other; and wherein the array of viewing elements and the image layer cooperate with each other such that at a first range of viewing angles, light from the first set of image segments is directed to a viewer such that the first image is exhibited; and at a second range of viewing angles, light from the second set of image segments is directed to the viewer such that the second image is exhibited; wherein the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region, and wherein the arrangement of the image layer material of the image segments and/or the colour density of the image segments within a region is dependent upon the number of sets of image segments having image segments present within that region, and is different for different regions such that when viewing the device within the first range of viewing angles the first image is perceived to have a uniform tone, and when viewing the device within the second range of viewing angles the second image is perceived to have a uniform tone.

The inventors have realised that by using different arrangements of the image layer material of the image segments and/or different colour densities of the image segments in different regions of the image layer, each of the first and second images exhibited by the device may be perceived by the viewer to have a (e.g. substantially) uniform tone. This contrasts with conventional devices which, as explained above, typically exhibit images that are perceived to display regions of different tone (or “contrast”).

In this way, the (e.g. lenticular) security device of the present invention advantageously provides an increase in security level compared to conventional devices, as the would-be counterfeiter not only needs to replicate the optically variable effect exhibited by the device, but also the perceived uniform tone of each of the first and second images. Furthermore, the perceived uniform tone of each image increases the clarity of the information exhibited by the device, therefore providing a secure device that is straightforward to authenticate and yet highly difficult to replicate.

As discussed above, the arrangement of the image layer material of the image segments and/or the colour density of the image segments is configured such that the first image is perceived to have a uniform tone and the second image is perceived to have a uniform tone. Typically, the first image is perceived to have substantially the same uniform tone as the second image (and any other images, if present). By “uniform tone”, we mean that each portion of an image exhibited by the device is perceived to have the same tone (e.g. portions of the image do not perceptibly differ in “brightness” or “darkness”) under cursory inspection by the naked human eye (although there may not be an exact match under close examination). Stated differently, each of the first and second images is perceived to have a uniform contrast.

For example, in preferred embodiments, two regions of an image will be perceived to exhibit the same tone (and the image therefore exhibiting a uniform tone) if the Euclidean distance AE*ab between them in Cl ELAB colour space (i.e. the CIE 1976 L*a*b* colour space) is less than 5, more preferably less than 3, still preferably less than 2.3. The value of AE*ab is measured using the formula: where AL*, Aa* and Ab* are the distance between the two regions along the L*, a* and b* axes respectively (see “Digital Color Imaging Handbook” (1.7.2 ed.) by G. Sharma (2003), CRC Press, ISBN 0-8493-0900-X, pages 30 to 32). In other words, the naked eye does not perceive a difference in tone between the two regions. The difference AE*ab can be measured using any commercial spectrophotometer, such as those available from Hunterlab of Reston, Virginia, USA. It will be appreciated that the AE*ab value will need to be measured from the completed device (i.e. via the focussing elements), at the appropriate viewing angle.

As discussed, the image layer comprises at least first and second sets of image segments. Each set of image segments may be referred to as an “image channel”. In some embodiments, the image layer may comprise only the first and second sets of image segments, and thereby exhibit exactly two images (i.e. the first image and the second image). In other words, the device may have exactly two (e.g. interleaved) image channels, and may be referred to as a “two- channel device”. However, the invention applies to devices in which the image layer comprises further (e.g. third, fourth etc.) sets of interleaved image segments, whereby such devices exhibit three or more images in dependence on viewing angle. In general, the invention applies to all n-channel lenticular devices, where n is equal to or greater than 2.

As set out above, the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region. This may be viewed as each region being defined based on the number of the first and second (and more, where present) images overlapping in that region of the device (here, “overlapping” refers to the macro images defined by the respective image segments overlapping). For example, a first region may be defined by image segments of only one set of image segments being present (e.g. no image overlap), and a second (different) region may be defined by image segments of two image channels being present (e.g. two images overlapping). As will be described further herein, the arrangement of the image layer material of the image segments and/or the colour density of the image segments is different for different regions so as to generate the desired perceived uniform tones of the images. It is noted that the arrangement of the image layer material of the image segments and/or the colour density of the image segments is different for regions with different numbers of sets of image segments having image segments present (e.g. not all regions of the image layer are necessarily considered to be “different” or have different arrangements of the image layer material of the image segments and/or colour density of the image segments). For example, in a three or more channel device, there may be multiple regions having image segments from exactly two sets of image segments, and these regions (which may be considered to be regions of the same “type”) would typically have the same arrangement of image layer material of the image segments and/or colour density of the image segments in order to generate the perceived uniform tone of the images.

As discussed, the arrangement of the image layer material in the different regions of the image layer may be configured in order to provide the perceived uniform tone. In embodiments, each region of the image layer has substantially the same ratio of the area of image layer material to the area absent of image layer material. Stated differently, the “line density” within each of the regions of the image layer is substantially the same. Such embodiments find particular application when the colour density of the image segments is uniform across the image layer (e.g. the same image layer material and thickness thereof is used for each image segment).

A convenient way of varying the arrangement of the image layer material of the image segments in the different regions of the image layer is to introduce gap regions into the image segments. In other words, such an image segment will be comprised of region(s) of image layer material spaced by gap region(s). Thus, in some embodiments, within at least one (preferably all) of the regions of the image layer in which image segments from more than one set of image segments are present, at least some of the image segments, preferably all of the image segments, each comprise at least one gap region that is defined by an absence of image layer material. Thus, the gap regions may be configured such that each region of the image layer has substantially the same ratio of the area of image layer material to the area absent of image layer material.

Preferably, in such embodiments, within a region of the image layer in which the image segments comprise gap regions, a ratio of the area of image layer material to the area of the gap region(s) of each image segment is substantially the same. Thus, for example, image segments of the first set of image segments will typically have the same “gap ratio” as image segments of the second set of image segments within the same region.

Preferably, in embodiments in which gap regions are introduced into image segments, image segments located within the region(s) of the image layer in which only one set of image segments has image segment present do not comprise any gap regions.

Typically, within a region of the image layer in which the image segments comprise gap regions, the ratio of the area of the image layer material to the area of the gap regions within an image segment is dependent on the number of sets of image segments having image segments present within that region. The ratio may be defined as 1/N where N is the number of sets of image segments (e.g. image channels) having image segments present in that region. For example, in a region having first and second image segments present (i.e. N=2), the ratio of the area of the image layer material to the area of the gap regions is typically 1/2. In a region of a three-channel device having segments from three channels present (i.e. N=3), the ratio is 1/3.

The arrangement of the image layer material is configured such that the images exhibited by the device are perceived to have a uniform tone to the naked eye. Preferably therefore, each gap region has a dimension such that it is not individually resolvable by the naked human eye. In this way, the images will appear to have a unform tone when viewing the device. 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 gap region has a dimension less than 150pm, more preferably less than 100pm and even more preferably less than 70pm.

Furthermore, it is preferred that the gap regions do not interact with the array of viewing elements, for example to create any moire effects that would detract from the information being displayed by the device. Preferably therefore, the dimensions, pitches, and/or orientations (e.g. skews) of the gap regions are configured such that the gap regions do not interact with the array of viewing elements to generate an optically variable effect (e.g. moire effects).

Typically, the first set of image segments and the second set of image segments are interleaved within each other periodically along at least a first direction, and wherein the image segments are elongate segments extending along a second direction that is preferably orthogonal to the first direction. The first and second directions are non-parallel. The image layer may therefore be described as comprising an array of image segments. As will be explained further herein, the security device may be a “one-dimensional” lenticular device, meaning that it exhibits optical variability in one dimension, or a “two-dimensional” lenticular device, meaning that is exhibits optical variability in two non-parallel (preferably orthogonal) directions. In preferred embodiments of one-dimensional devices, the first image segments and the second image segments are elongate segments extending along the second direction (i.e. preferably orthogonal to the direction of interlacing). For example, the image segments may be rectilinear line elements with a varying length depending on the image in question. Such image segments may typically have line widths (e.g. in the direction of interleaving) of between 1 pm and 100pm, preferably between 1 pm and 80pm, more preferably between 1 pm and 60pm. This may be referred to as a “line structure” of the image layer. Typically, the line segments will extend from one periphery of the image to the opposite periphery along the second direction. Thus, unless the opposite peripheries of the image are parallel to one another, the line segments will not all be identical to one another as they will have different lengths.

The gap regions may be configured in a variety of different ways in order to achieve the effect that the first and second images are each perceived to have a uniform tone. In some embodiments, the first set of image segments and the second set of image segments are interleaved with each other periodically along a first direction, and wherein the gap region(s) of an image segment are configured such that the image layer material of the respective image segment is discontinuous along a second direction that is preferably substantially orthogonal to the first direction. In one dimensional devices, such gaps may be referred to as being arranged horizontally to the line structure. In some preferred embodiments, the gap regions of laterally adjacent image segments may be laterally offset from each other along the second direction, preferably wherein the gap regions are substantially completely offset from each other along the second direction. Although not essential, offsetting the gap regions in this way advantageously provides a more even distribution of the image layer material, helping to ensure a uniform tone of the perceived images and avoid undesirable optically variable (e.g. moire) interaction of the gap regions with the viewing elements.

In some embodiments, the first set of image segments and the second set of image segments may be interleaved with each other periodically along at least a first direction, and wherein the gap region(s) of an image segment are configured such that the image layer material of the respective image segment is substantially continuous along the second direction. In one dimensional devices, such gaps may be referred to as being arranged parallel to the line structure.

The gap regions of an image segment are typically arranged in a periodic manner (e.g. having a constant or repeating size and pitch). However, it is envisaged that in some embodiments, the gap regions of an image segment may be arranged in an aperiodic, or a random or pseudo-random manner (e.g. by a dithering arrangement) as long as the ratio of the area of image layer material to the gap regions meets the requirements for the first and second images to be exhibited with a perceived uniform tone as discussed above.

As discussed above, a convenient way of varying the arrangement of the image layer material of the image segments in the different regions of the image layer is to introduce gap regions into the image segments so as to vary the areal coverage ratio of the image layer material within different regions of the image layer. Alternatively or in addition, the colour density of the image segments (e.g. the amount of light absorbed by the image segments carrying a colour) may differ between the different regions in order to generate the perceived visual effect of each of the first and second images having a uniform tone. Accordingly, in some embodiments, the colour density of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than the colour density of the image segments within regions in which more sets of image segments have image segments present. For example, the colour density of the image segments in a region where N=1 will be greater than the colour density of the image segments in a region where N=2. Similarly to as discussed above in relation to the use of gap regions, the relationship may be described as 1/N relative to the region having image segments of the greatest colour density. Thus, for example, in a region where N=2, the colour density of the image segments will preferably be 1 that of a region where N=1.

One way in which the colour density of the image segments may be varied is by varying the thickness of the image layer material (e.g. the height of the image layer material relative to the substrate). If the image layer material is semitransparent then image segments of greater height will have a higher optical density than image segments of lower height, and therefore a greater colour density. Therefore, the height of the image segments may differ in different regions of the image layer, such that the first and second images exhibited by the device each have a uniform tone. Thus, in preferred embodiments, a thickness of the image layer material of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than a thickness of the image layer material of the image segments within regions in which more sets of image segments have image segments present.

The image layer material will typically will carry a coloured tint (i.e. it is at least partially transparent and exhibits a visible colour). This is typically provided by using a dyed material, although low concentrations of pigment may also confer a coloured tint. Typically, in embodiments in which the height of the image layer material is varied, the concentration of any dye or pigment carried by the image layer material is uniform across the image layer.

Typically, each of the at least first and second sets of image segments is formed of the same image layer material (e.g. any colour carried by the image layer material, and the concentration of the dye or pigment forming conferring that colour, is the same).

In some embodiments in which the colour density of the image segments is varied, a colourant concentration of the image layer material of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than a colourant concentration of the image layer material of the image segments within regions in which more sets of image segments have image segments present. In such embodiments, the image layer material carries a coloured tint, with the colour density of the image segments varying as a function of the colourant concentration carried by the image layer material. For example, tinted image segments that comprise a relatively higher concentration of dye or pigment will appear relatively darker (higher colour density) for the same volume of image layer material than image segments having a relatively lower concentration of dye or pigment. In embodiments in which the colourant concentration of the image layer material is varied, the height of the image layer material forming the image segments typically remains substantially constant across the image layer (i.e. the height of each image segment is substantially the same).

Preferably, within a region of the image layer, the colour density of each image segment is substantially the same.

For the purposes of this specification, image layer materials having different colourant concentrations (e.g. as a result of different concentrations of pigments or dyes) are considered to be different image layer materials.

Typically, in embodiments in which the colour density of the image segments differs between different regions of the image layer, within regions of the image layer in which two or more sets of image segments have image segments present, the image segments do not comprise any gap regions. However, it is envisaged that in some embodiments, a combination of gap region variations and colour density variations (e.g. of the regions of image layer material) may be used across the different regions of the image layer in order that the exhibited first and second image are each perceived to have a substantially uniform tone.

Preferably, each of the at least first and second sets of image segments has the same colour. In this specification, the term “colour” is used primarily to refer to the hue of the image layer material - for example, in Cl ELAB colour space (i.e. the CIE 1976 L*a*b* colour space), here we treat all points in the colour space having the same values of a* and b* as one “colour” but representing different tones of that colour depending on the value of L* (the brightness/darkness axis). For example, “light blue” and “dark blue” are different tones of the same colour (blue). Note that the term “colour” includes achromatic hues such as black, grey, white, silver etc., as well as chromatics such as red, blue, yellow, green, brown etc.

As has been discussed above in relation to the use of gap region(s) within the image layer, the invention is applicable to devices that exhibit more than two images (i.e. three or more channel lenticular devices). In such cases, the colour density variation in the different regions of the image layer (e.g. by variations in the thickness and/or colourant concentration of the image layer material) may be configured accordingly in order that each image exhibited by the device is perceived to have a uniform tone.

In some embodiments, the image layer may further comprise a static zone having static zone elements comprising image layer material, and wherein the arrangement and/or colour density of the static zone elements is configured such that the static zone is perceived to have a uniform tone that is substantially the same as the uniform tone of the first and/or second image. The static zone is “static” in the sense that it is not optically variable; that is, its appearance remains the same at all viewing angles (provided the illumination conditions do not change). This remains the case whether or not the viewing elements extend over the static region (which will typically be the case) or not. The static zone may typically complement, or provide further information to, the first and second images for example. The uniform tone of the static zone is configured to be substantially the same as the uniform tone of the first and/or second image. Thus, in preferred embodiments, each of the static zone, the first image, the second image, and any further image(s) present are perceived to have substantially the same uniform tone. Thus in preferred embodiments, a ratio of the area of image layer material to the area absent of image layer material in the static zone is substantially equal to a ratio of the area of image layer material to the area absent of image layer material in each of the regions comprising image segments. Thus, the static zone elements may be spaced by a plurality of gap regions in order to provide the desired areal ratio, in a similar manner to that described above. For example, the static zone elements may be configured as a screened or half-tone working in order to provide the desired perceived tone. In such embodiments, the dimensions, pitches, and/or orientations of the halftone/screen elements is preferably such that they do not interact with any overlapping viewing element to generate an optically variable effect.

Although in preferred embodiments the arrangement of the static zone elements is configured to generate a uniform tone substantially the same as that of the exhibited image(s), alternatively or in addition, the colour density of the static zone elements may be configured in order to generate the desired uniform tone. For example, in the same manner as described above, the thickness and/or colourant concentration of the image layer material of the static zone elements may be configured accordingly.

The present invention may be applied to devices in which the first image segments and the second image segments are formed of image layer material of different colours. Therefore, in accordance with a second aspect of the invention there is provided a security device, comprising: a substrate; an array of viewing elements disposed on the substrate; and an image layer disposed in or on the substrate, wherein the image layer comprises at least first and second sets of image segments, the first set of image segments comprising image layer material of a first colour that together defines a first image, and the second set of image segments comprising image layer material of a second colour different from the first colour that together defines a second image, the first set of image segments and the second set of image segments being interleaved with each other; and wherein the array of viewing elements and the image layer cooperate with each other such that at a first range of viewing angles, light from the set of first image segments is directed to a viewer such that the first image is exhibited; and at a second range of viewing angles, light from the set of second image segments is directed to the viewer such that the second image is exhibited; wherein the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region, wherein the arrangement of the image layer material of the image segments and/or the colour density of the image segments is different for different regions; and each region of the image layer has substantially the same ratio of the area of image layer material to the area absent of image layer material; and/or the colour density of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than the colour density of the image segments within regions in which more sets of image segments have image segments present.

Conventional (e.g. lenticular) devices that exhibit images of different colours (for example a blue “1” changing to a red “2” upon change in viewing angle) will typically suffer from regions in the exhibited images where the two colours “mix”. For example, in the case of a blue “1” and a red “2”, the regions that comprise image segments from both sets of image segments will exhibit a purple hue rather than the desired red or blue. By controlling the configuration of the image layer material in the different regions of the image layer as discussed above (through the area ratios of image layer material and/or the colour density), the undesirable “colour mixing” effect will be reduced. It will be appreciated that due to the presence of image segments having two different colours, the tone of each image will not be completely uniform (e.g. the eye will perceive differences due to colour mixing); however, there will be reduced tonal variation across each image as compared to state-of-the-art devices. This provides for a more secure security device as this effect will be more difficult to counterfeit. In preferred embodiments of the second aspect of the invention, within at least one (preferably all) of the regions of the image layer in which image segments from more than one set of image segments are present, at least some of the image segments, preferably all of the image segments, each comprise at least one gap region that is defined by an absence of image layer material. As described above in relation to the first aspect of the invention, introducing gap region(s) into the image layer material provides a convenient way of controlling the area ratio of the image layer material in the different region(s) of the image layer in order to produce the desired effect to the naked human eye.

Typically, within a region of the image layer, a ratio of the area of image layer material to the area of the gap region(s) of each image segment is substantially the same. However, in some embodiments, within a region of the image layer comprising image segments of more than one of image segments, a ratio of the area of image layer material to the area of the gap region(s) of the image segments of different sets of image segments may be different. This is particularly the case if one colour is darker (“dominant”) than the other colour(s), in which case the image segments of the darker colour (e.g. having a lower L* value in Cl Elab colour space) may have a greater ratio of the area of gap regions in order to reduce the tonal variations across each image.

Preferably, each gap region has a dimension such that it is not discernible to the naked human eye, preferably wherein each gap region has a dimension less than 150pm, more preferably less than 100pm and even more preferably less than 70pm. Furthermore, preferably, the dimensions, pitches, and/or orientations of the gap regions are configured such that the gap regions do not interact with the array of viewing elements in order to generate an optically variable effect.

In some preferred embodiments of the second aspect of the invention, the first set of image segments and the second set of image segments are interleaved with each other periodically along at least a first direction, and wherein the gap region(s) of an image segment are configured such that the image layer material of the respective image segment is discontinuous along a second direction that is preferably substantially orthogonal to the first direction, preferably wherein the gap regions of laterally adjacent image segments are laterally offset from each other along the second direction, more preferably wherein the gaps are substantially completely offset from each other along the second direction. Although not essential, offsetting the gap regions in this way helps to reduce the amount of colour variation that is observed in each exhibited image.

Alternatively or in addition to varying the areal coverage of the image layer material, in some embodiments the colour density of the image segments may be varied in different regions of the device. As discussed above in relation to the first aspect of the invention, a variation in colour density may be achieved by varying the thickness of the image layer material and/or the colourant concentration carried by the image layer material.

In the present invention, the array of viewing elements may take various forms. In preferred embodiments, the array of viewing elements comprises an array of focussing elements, preferably lenses. The focussing elements may be adapted to focus light in one dimension, in which case the focussing elements are preferably (e.g. elongate) cylindrical focussing elements, or adapted to focus light in at least two (e.g. non-parallel, preferably orthogonal) directions, in which case the focussing elements are preferably spherical or aspherical focussing elements.

As has been discussed above, the device may be a “one-dimensional” lenticular device, meaning that it exhibits optical variability when the viewing angle is changed in one direction only (rotation about a direction perpendicular to the direction of interleaving). In such devices, the focussing elements are typically cylindrical focussing elements, or a line of spherical/aspherical focussing element extending along a direction perpendicular to the direction of interleaving. The present invention may also be applied to two-dimensional lenticular devices (which exhibit optical variability in two directions). In such cases, the focussing features need to have a two-dimensional periodicity and be able to redirect light in two directions accordingly (e.g. spherical or aspherical focussing elements). The 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 typical embodiments, the array of viewing elements is provided on a first surface of the substrate and the image layer is provided on a second, opposing surface of the substrate. It will be appreciated that in such configurations the substrate will need to be at least semi-transparent (i.e. optically clear and nonscattering, 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 building 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 need not be semi-transparent and may be of any type, opaque or otherwise. This includes paper substrates, although polymer-based substrates are preferred.

In embodiments in which array of viewing elements are in the form of focussing elements, the image layer is preferably 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 discussed above.

Although the viewing elements are typically in the form of focussing elements such as lenses, in some embodiments the array of viewing elements may be in the form of a masking grid.

Preferably, the first and second images are each in the form of an indica or indicum, preferably one or more geometric shapes, letters, logos, currency signs or other symbols. In some embodiments, each individual image that is exhibited by the device may be a part of an animation sequence (in such cases the device typically has three of more image channels). Preferably, the first and second images (and any further images, if present) are different.

The image layer (comprising the set of first image segments and the set of second image segments) can be formed in various different ways. In particularly preferred embodiments, the image layer is 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. By a “print working” it is meant any structure of ink or another marking material (“image layer material”) laid down on a surface. The image layer material could be laid down selectively in a pattern or all-over and then patterned by removing or masking certain portions of the material. The image layer material may be curable or non-curable. Using techniques such as gravure, intaglio, flexographic or (wet or dry) lithographic printing, the achievable resolution is affected by several factors, including the viscosity, wettability and chemistry of the ink, as well as the surface energy, unevenness and wicking ability of the substrate, all of which lead to ink spreading. With careful design and implementation, such techniques can be used to print pattern elements with a line width of between 25 pm and 100 pm. For example, with flexographic or wet lithographic printing it is possible to achieve line widths down to about 10-25 pm.

Micro-intaglio printing can achieve a higher resolution. Examples of this technique are disclosed in WO-A-2014/070079, US-A-2009/0297805, WO-A- 2011/102800, and WO-A-2017/009616 (Figures 12 to 15).

In alternative embodiments, the image layer may comprise any of:

• A laser marking and/or laser ablation;

• A relief structure configured to generate structural colour, preferably a diffractive or plasmonic relief structure;

• A relief structure carrying a marking material in the recesses thereof or on the elevations thereof; or

• A demetallised metal or metal alloy layer. Techniques such as these may allow for higher resolution image segments (including gaps, where present) in the image layer. For example, forming the image layer as a relief structure carrying a marking material in the recesses thereof can be achieved by the same means used in the so-called Unison Motion™ product by Nanoventions Holdings LLC, as mentioned for example in WO-A-2005052650. This involves creating pattern elements (“icon elements”) as recesses in a substrate surface before spreading ink over the surface and then scraping off excess ink with a doctor blade. The resulting inked recesses can be produced with line widths of the order of 2 pm to 3 pm. Other reliefbased methods for forming image segments which can be used in the present invention are disclosed in WO-A-2017/009616, section 3.2.

Relief structures of this sort, or of a sort which generates structural colour, can be provided by embossing or cast-curing, described further below. If the relief structure is to generate structural colour, e.g. by diffraction or plasmonic effects, it may be necessary to provide a reflection enhancing layer on the relief structure which follows its contours, e.g. a vapour deposited metal or metal alloy layer, or a metallic ink.

A third aspect of the invention provides a security article comprising the security device as described above, wherein the security article is preferably a security thread, strip, foil, insert, transfer element, label, patch, or a data page for a security document. 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. Examples will be provided below.

A fourth aspect of the invention further provides a security document comprising a security device as described above, wherein the security document is preferably a banknote, cheque, passport, identity care, 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 fifth aspect of the invention there is provided a method of manufacturing a security device, comprising:

(a) providing a substrate;

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

(c) forming an image layer in or on the substrate; wherein the image layer comprises at least first and second sets of image segments, the first set of image segments comprising image layer material that together defines a first image and the second set of image segments comprising image layer material that together defines a second image, the first set of image segments and the second set of image segments being interleaved with each other; and wherein the array of viewing elements and the image layer cooperate with each other such that at a first range of viewing angles, light from the first set of image segments is directed to a viewer such that the first image is exhibited; and at a second range of viewing angles, light from the second set of image segments is directed to the viewer such that the second image is exhibited; wherein the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region, and wherein the arrangement of the image layer material of the image segments and/or the colour density of the image segments within a region is dependent upon the number of sets of image segments having image segments present within that region, and is different for different regions such that when viewing the device within the first range of viewing angles the first image is perceived to have a uniform tone, and when viewing the device within the second range of viewing angles the second image is perceived to have a uniform tone..

The result of the method of the fifth 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. A sixth aspect of the invention provides a method of manufacturing a security device, comprising:

(a) providing a substrate;

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

(c) applying an image layer in or on the substrate, wherein the image layer comprises at least first and second sets of image segments, the first set of image segments comprising image layer material of a first colour that together defines a first image, and the second set of image segments comprising image layer material of a second colour different from the first colour that together defines a second image, the first set of image segments and the second set of image segments being interleaved with each other; and wherein the array of viewing elements and the image layer cooperate with each other such that at a first range of viewing angles, light from the set of first image segments is directed to a viewer such that the first image is exhibited; and at a second range of viewing angles, light from the set of second image segments is directed to the viewer such that the second image is exhibited; wherein the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region, wherein the arrangement of the image layer material of the image segments and/or the colour density of the image segments is different for different regions; and each region of the image layer has substantially the same ratio of the area of image layer material to the area absent of image layer material; and/or the colour density of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than the colour density of the image segments within regions in which more sets of image segments have image segments present.

The result of the method of the sixth aspect is a security device of the sort already described above in relation to the second 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. In preferred embodiments, the method of either the fifth or sixth aspect of the invention may further comprise, before step (c), generating an image layer template by: identifying at least one non-overlap region where the first image does not overlap with the second image; identifying at least one overlap region where the first image overlaps with the second image; and in step (c), forming the image layer in accordance with the image layer template.

The identification of at least one non-overlap region and at least one overlap region refers to identification of overlap (or not) of the macro first and second images, i.e. the images defined by the respective first and second sets of image segments. Two images are deemed to overlap if the periphery of an image is laterally contained within (“overlaps with”) the periphery of the other image. Once the regions of overlap and non-overlap have been identified, the image layer is formed in accordance with the image layer template. For example, for identified regions of non-overlap, the corresponding region of the image layer will comprise image segments from only the set of image segments defining the (non-overlapping) image. Similarly, for an identified region where the first and second images overlap, the corresponding region of the image layer will comprise image segments from both the first and second sets of image segment.

It is noted that for complex images, there are typically a plurality of discrete regions where the macro images do not overlap, and a plurality of discrete regions where the macro images do overlap.

As described above, in preferred cases, the image layer is a print working, formed by a printing technique, preferably a gravure, intaglio, screen, microintaglio, flexographic, lithographic or digital technique. Preferably (e.g. for embodiments in which each of the first and second sets of image segments is formed of the same image layer material), the image layer is formed in a single print working. In embodiments in which the heights of the image layer material are varied across the image layer, this may be achieved in a single print working by appropriate design of a printing or casting plate for example. In other embodiments, the image layer may be formed by two or more print workings (for example to achieve variations in height), although due to the difficulty in achieving the desired register, a single print working is typically preferred.

In alternative embodiments, the image layer may be formed by any of: a laser marking and/or laser ablation; a relief structure configured to generate structural colour, preferably a diffractive or plasmonic relief structure; a relief structure carrying a marking material in the recesses thereof or on the elevations thereof; or a demetallized metal or metal alloy layer.

Preferably, each of the at least first and second sets of image segments is formed of the same image layer material.

As described above, the viewing elements may be in the form of focussing elements such as lenses, or a masking grid. Suitable apparatus, materials and methods for forming relief structures such as lenses (or image segments, in some embodiments) are described in WO-A-2018/153840 and WO-A- 2017/009616. In particular, focussing elements can be formed by the in-line casting devices detailed in WO-A-2018/153840 (e.g. that designated 80 in Figure 4 thereof), using an embossing tool 85 carrying an appropriately designed micro-optical structure from which can be cast the desired shape. Similarly, the cast-curing apparatuses and methods disclosed in section 2.1 of WO-A- 2017/009616 (e.g. in Figures 4 to 8 thereof) can also be used to form the presently disclosed focussing elements.

Whichever casting apparatus is used, the curable material(s) from which the relief structure is cast may be applied either directly to the tool carrying the desired relief shape (e.g. to the embossing tool 85 of WO-A-2018/153840 or to the casting tool 220 of WO-A-2017/009616), or the curable material(s) may be applied directly to the substrate on which the relief structure is to be formed, and then brought into contact with the tool (e.g. by impressing the tool onto the deposited curable material). Both options are described in the aforementioned documents. Preferably, the latter option is employed and the curable material(s) are applied to the substrate by screen printing as detailed in WO-A- 2018/153840, before being formed into the desired relief structure.

Suitable curable materials are disclosed in WO-A-2017/009616, section 2.1. UV-curable materials are most preferred. Curing of the material(s) preferably takes place while the casting tool is in contact with the curable material, against the substrate.

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 THE DRAWINGS

Example embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates, in plan view, a security document carrying a lenticular security device;

Figure 2(a) illustrates a cross-sectional view of a conventional lenticular device, and Figure 2(b) illustrates a desired optically variable effect exhibited by the device;

Figure 3 schematically illustrates how the images exhibited by a conventional lenticular device are perceived;

Figure 4(a) schematically illustrates the ideal theoretical scenario when viewing a lenticular device, and Figure 4(b) schematically illustrates the realistic situation;

Figure 5(a) is a schematic cross-sectional view of a lenticular security device according to an embodiment of the invention, and Figure 5(b) illustrates a portion of the image layer thereof;

Figure 6 is a magnified view of the portion of the image layer illustrated in Figure 5(b); Figure 7 schematically illustrates a portion of the image layer 30 of a device according to an embodiment of the invention;

Figures 8(a) and 8(b) are magnified plan views of portions of an image layer of a device according to an embodiment of the invention;

Figure 9(a) illustrates an image layer according to a conventional device and Figures 9(b) to 9(d) summarise different ways of implementing the image layer of a one-dimensional lenticular device according to the invention;

Figure 10(a) illustrates an image layer according to a conventional device and Figure 10(b) illustrates a way of implementing an image layer of a two- dimensional lenticular device according to the invention;

Figure 11 illustrates an example of four images h, l₂, h, k that may be exhibited by a two-dimensional lenticular device according to the invention;

Figure 12 illustrates a portion of an image layer of a two-dimensional lenticular device according to an embodiment of the invention;

Figures 13(a) and 13(b) illustrate the provision of a static region in accordance with an embodiment of the invention;

Figure 14 schematically illustrates an image layer implementing image segments of different colour density, according to an embodiment of the invention;

Figures 15(a) to 15(c) schematically illustrates different ways in which the colour density may be varied within the image layer;

Figures 16 and 17 illustrate an embodiments of the invention in which the images exhibited by the lenticular device are different colours;

Figure 18 is a flowchart outlining the main steps of a method according to an embodiment of the invention;

Figures 19(a) to 19(b) illustrates various steps of the a method according to an embodiment of the invention;

Figure 20 is a schematic cross-sectional view of a security device according to an embodiment of the invention;

Figures 21 , 22 and 23 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; and

Figure 24 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. DETAILED DESCRIPTION OF THE DRAWINGS

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. Figure 2(a) illustrates a cross-sectional view of the device 101 along the line Q- Q’. In this example, which is used to illustrate the background to the invention and the problem that it addresses, the device 101 is a one-dimensional lenticular device, in that it exhibits an optically variable effect upon a change in viewing angle in one dimension. Here when tilting the device about the y-axis, an observer O observes a change in image (from a “1” to a “2” to a “3”) dependent on viewing angle, as schematically shown in Figure 2(b). Herein, it is noted that each image is defined by the periphery of its graphical form (in other words the background is not part of the image).

Referring to Figure 2(a), the device 101 comprises a substrate 10. 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 (i.e. along the y-axis). On the opposing side 10b of the substrate, the device 101 comprises an image layer 30 comprising a plurality of image segments that 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 is formed substantially within the focal plane of the lens array 20. In this example, the image layer 30 comprises a first set of image segments Si , a second of second image segments S2, and a third set of image segments S3 that are interleaved with each other periodically along the x-direction. The first image segments together define the first image 11 (in the form of the alphanumeric character “1”), the second image segments together define the second image l₂ (in the form of the alphanumeric character “2”) and the third image segments together define the third image l₃ (in the form of the alphanumeric character “3”). In this way, each set of image segments defines an image channel, such that in this example the device is a three-channel lenticular device. In this onedimensional example, each image segment is in the form of an elongate line element extending parallel with the direction of elongation of the cylindrical lenses (i.e. along the y-direction). The arrangements of the image segments and the array of lenses 21 are configured such that each lens corresponds to a segment of each channel. At a first viewing position P1 (corresponding to a particular viewing angle of the device 101), each lens 21 focusses light from the image segments Si of the first image channel such that the first image is exhibited to the observer. Similarly, at a second viewing position P2, each lens 22 focusses light from the image segments l₂ of the second image channel, and at the third viewing position P3, each lens 22 focusses light from the image segments l₃ of the third image channel.

The exhibited images h, l₂ and l₃ illustrated in Figure 2(b) illustrate the desired situation in which each image displays a substantially uniform tone: in other words, no regions that are perceptively “brighter” or “darker” than other regions within the image. However, Figure 3 illustrates the actual impression that is typically observed when viewing conventional lenticular security devices. As can be clearly seen, the observer perceives each image to have regions of different tone (or “contrast”). For example, as schematically shown in Figure 3(a), the image of the character “1” comprises regions of perceived relatively darker tone (3), regions of “intermediate” tone (2) and regions of perceived relatively lighter tone (1).

Figure 3(b) illustrates a magnified plan view of a portion of the image layer 30 of such a conventional device 101 (not necessarily corresponding exactly to the portion shown on image B). The image layer 30 comprises different regions defined by the number of image channels having image segments present: a primary region Ri where only the first image segments Si of the first image channel are present; a secondary region R₂ in which both the first image segments Si and the second image segments S₂ are present (but no third image segments S₃), and a tertiary region R₃ in which the first and second image segments Si and S₂ are present as well as third image segments S₃. As can be seen, due to the nature of the interlacing of the image channels, the areal coverage ratio of the image layer material 35 in each region differs, with the primary region Ri exhibiting the lowest areal coverage ratio (or “line density”); the secondary region R₂ exhibiting a medium areal coverage ratio; and the tertiary region R₃ exhibiting the greatest areal coverage ratio. This variation in the ratio of areal coverage of the image segments in the different regions of the image layer 30 causes the tonal variation in the exhibited image, as will now be described with reference to Figure 4.

Figure 4(a) illustrates the ideal theoretical scenario when viewing such a lenticular device. Here, each lens selectively focusses light from the same image channel to the viewer, and so the variation in areal coverage of the image layer material across the image layer does not matter and each displayed image conveys a uniform tone. On the other hand, Figure 4(b) schematically illustrates the realistic situation, where the observer “sees” a slightly different relative position on each lens 21 such that the light is focussed from slightly different positions on the image layer relative to each lens. Although light reaching the observer originates predominantly from the second image channel (in this example), for some lenses, light from the first and third image channels will also be perceived by the observer. Consequently, light originating from the primary, secondary and tertiary regions of the image layer will be perceived to have different tones due to the different amounts of image layer material present in the different channels.

This effect is exacerbated by the fact that the left and right eyes of the viewer will each “see” different relative positions on each lens. Furthermore, there may be non-active regions between the individual lenses that do not provide a focussing effect (e.g. due to tolerances in the manufacturing of the lenses), which further causes a difference in the tonal perception of the primary, secondary and tertiary regions of the image layer.

Figure 5(a) schematically illustrates, in cross-section, a security device 100 according to a first embodiment of the invention. For clarity of explanation, the device 100 is a two-channel lenticular device that exhibits an “image switch” on a change in viewing angle, from a first image h (the alphanumeric character “1”) to a second image l₂ (the alphanumeric character “2”). Thus, the image layer of the device comprises a first set of image segments defining the “1”, and a second set of image segments defining the “2”. The general structure of the device 100 is identical to the device 101 described in Figures 2 to 4 with the exception of the configuration of the image layer 30, as will now be discussed.

Figure 5(b) shows a magnified view of the image layer 30 in a similar manner to Figure 3(b). As can be seen in Figure 5(b), the image layer 30 comprises a primary region Ri which comprises image segments S2 of the second set of image segments only, and a secondary region R₂ which contains image segments Si , S₂ of both the first and second sets of image segments. However, unlike the secondary region R₂ of the image layer 30 of the conventional service shown in Figure 3, the image segments Si , S₂ of the secondary region in Figure 5 each comprise a plurality of gap regions 40. In other words, each image segment Si, S₂ in the secondary region R₂ is formed of areas of image layer material 35 spaced by gap regions 40 that are defined by an absence of image layer material. The gap regions 40 are configured such that the ratio of the area of image layer material to the area absent of image layer material in the primary region R1 is substantially equal to the ratio of the area of image layer material to the area absent of image layer material in the secondary region R₂. In this way, the line density in each of the primary and secondary regions is substantially the same, and consequently each of the exhibited images h, l₂ is perceived to have a uniform tone when viewed, even allowing for the imperfections described above in relation to Figure 4.

Figure 6 is a magnified view of the portion of the image layer 30 shown in Figure 5(b). Referring first to the primary region R1, the image segments S₂ are formed as uniform rectilinear line elements extending substantially perpendicular to the direction of interleaving, as is conventionally the case for one-dimensional lenticular devices. In the secondary region R₂, the peripheries of two immediately adjacent image segments Si and S₂ have been shown for clarity (although these outlines will not be present in the device in practice). Each image segment Si, S₂ in the secondary region R₂ comprises a plurality of gap regions 40 that are formed “horizontally” to the line structure such that the image layer material 35 is discontinuous in the direction orthogonal to the interleaving. For the two-channel device of this embodiment, the areal coverage ratio of image layer material in the primary region Ri is 50%. Consequently, in the secondary region R₂, the dimensions of the gap regions 40 are substantially equal to the dimensions of the areas of image layer material 35 such that the areal coverage of image layer material 35 in the secondary region R₂ is also 50%. The ratio of the area of the image layer material to the area of the gap regions of the first image segments Si is equal to that of the second image segments S₂ within the secondary region, such that each exhibited image is perceived to have the same uniform tone.

The gap regions 40 have dimensions such that they are not readily discernible to the naked human eye. In other words, when the device is viewed at typical viewing distances (~30cm), the gap regions are not readily observed and consequently the image segments in the secondary region appear to be uniform. Therefore, the dimensions of the gap regions 40 (e.g. a “length” along the direction of the line structure: here along the y-axis) are preferably less than 150pm, more preferably less than 100pm and even more preferably less than 70pm.

Furthermore, it is desired that the gap regions 40 that have been introduced into the image segments do not interact with the lens array in order to generate an optically variable effect. Therefore, the dimensions, pitch and/or orientation (e.g. skew) of the gap regions is chosen so as not to create such an optically variable effect in combination with the lenses (e.g. moire banding). In the embodiment illustrated in Figure 6, in the secondary region R₂, the gap regions 40 of the first image segments Si are completely laterally offset from the gap regions 40 of the second image segments S₂ along the y-axis. In this way, both the areas of image layer material 35 and the areas absent of image layer material are discontinuous along the direction of interlacing (x-axis). This effectively minimises the dimensions of the gap regions such that they are not perceptible to the naked eye or generate any optically variable effect in combination with the lens array 20.

Figure 7 schematically illustrates a portion of the image layer 30 of a device according to an embodiment of the invention which exhibits three different images (h, l₂, l₃) upon a change of viewing angle (a “three channel” lenticular device). Figure 7(a) illustrates a portion of the image layer 30 in an embodiment in which gap regions are introduced into the line structure with no offset, and Figure 7(b) illustrates an embodiment in which the gap regions are offset. As before, the magnified view is schematic and does not necessarily correspond to the exact portion shown on the macro image.

Figure 8(a) is a magnified view of the portion of the image layer 30 shown in Figure 7(a). As discussed, in this embodiment the device is a three channel device, and the arrangement of the macro images is such that in the example of Figure 8(a) the image layer comprises a primary region R1 in which only first image segments Si are present; a secondary region R₂ in which both first image segments Si and second image segments S₂ are present (but not the third image segments S₃), and a tertiary region R₃ in which image segments Si, S₂ and S₃ from each set of image segments are present. The areal coverage ratio of the image layer material 35 in the primary region R1 is 33.3% (as the second and third image channels are “empty”), and therefore the gap regions 40 in the secondary and tertiary regions R₂, R₃ are configured to generate substantially the same 33.3% areal coverage ratio of image layer material in these regions. Therefore, the areal ratio of the image layer material to the gap regions in each of the image segments Si , S₂ within the secondary region R₂ is 50% (1/N were N=2), and in the tertiary region R₃ it is 33.3% (1/N where N=3). In this example, the areal ratio is implemented by varying the length the gap regions in the direction of the line structure (e.g. along the y-axis). Hence, in the secondary region R₂, the gap regions 40 within an image segment have the same length as the portions of image layer material 35 within that image segment. In the image segments of the tertiary region R₃, the gap regions 40 have a length that is twice that of the portions of image layer material 35. The peripheries of example image segments in both the secondary and tertiary regions are shown to highlight the arrangement of the gap regions 40 within the individual image segments, but it will be appreciated that no such outline of each image segment will be present in practice. In the image layer 30 shown in Figure 8(a), the gap regions 40 of the laterally adjacent image segments (e.g. the segments Si and S2 illustrated in region R₂; and the segments Si, S₂ and S₃ illustrated in region R₃) are not laterally offset. This results in areas absent of image layer material within each of the secondary and tertiary regions that are continuous along the direction of interlacing (e.g. along the x-axis). Figure 8(b) illustrates a preferred embodiment in which the gap regions of the laterally adjacent image segments on the secondary and tertiary regions are laterally offset along the direction substantially orthogonal to the direction of interlacing. Such offsetting of the gap regions in this way reduces the cumulative continuous dimensions of the regions absent of image layer material and advantageously provides a more even distribution of image layer material within the secondary and tertiary regions.

It will be appreciated that regions R1, R₂ and R₃ of the image layer correspond to differing levels of overlap between the macro images displayed by the device (e.g. the macro characters “1”, “2” and “3”). Region R1 corresponds to areas where no images overlap (but where an image is present); region R₂ corresponds to areas where two of the three images overlap, and region R₃ corresponds to areas where all three images overlap. Therefore, the number and form of the regions of the image layer will depend on the complexity and difference between the graphical forms of the images.

Figures 9 summarises different embodiments in which the arrangement of the image layer material may be configured in order that the exhibited images by the device are each perceived to have a uniform tone. As a comparison, Figure 9(a) schematically illustrates a conventional way of interleaving image segments in a one-dimensional three channel lenticular device in each of the primary, secondary and tertiary regions of the image layer. As has been discussed, the areal coverage ratio of image layer material in each region increases (from 33.3% in the primary region to 100% in the tertiary region), which causes undesirable variations in tone across the exhibited images. In each of Figures 9(b) to 9(d), gap regions 40 are introduced into the image segments in order that the areal coverage ratio of the image layer material in each region is substantially the same. In Figure 9(b), the gap regions 40 are introduced such that the image layer material is discontinuous along a direction substantially orthogonal to the direction of interlacing. This may be referred to as introducing gap regions that are perpendicular to the line structure. Figure 9(c) illustrates a variation on this embodiment in which the gap regions of laterally adjacent image segments are laterally offset along the direction substantially orthogonal to the direction of interlacing. In Figure 9(d), the gap regions 40 are introduced such that the image layer material 35 is substantially continuous along the direction substantially orthogonal to the direction of interlacing. This may be referred to as introducing gap regions that are parallel with the line structure.

Other arrangements of gap regions and the image layer material in the different regions of the image layer are envisaged, as long as the areal coverage ratio of the image layer material remains substantially the same for each region. For example, the gap regions may be introduced in an aperiodic or substantially random manner such as in accordance with a dithering arrangement. It is also noted that the areal coverage of the image layer material is preferably the same for each image segment within a region in order to exhibit uniform tones of the images.

The corresponding configurations of image layer material may be applied to two channel devices, as well as devices comprising four or more image channels.

Thus far, we have considered one-dimensional lenticular devices; that is, devices that exhibit an optically variable effect upon a change of viewing angle in one direction. The present invention also extends to two-dimensional lenticular devices that exhibit an optically variable effect upon a change of viewing angle in two (typically orthogonal) directions. In such devices, the viewing elements are adapted to focus light in two (preferably orthogonal) directions and are typically spherical or aspherical lenses. Example two-dimensional devices according to the invention are schematically described with reference to Figures 10 to 12. Figure 10(a) schematically illustrates the way in which image segments for a conventional four-channel, two-dimensional, lenticular device may be arranged. As with the one-dimensional embodiments described above, the areal coverage ratio of the image layer material increases with the number of image segments present within the region, thereby leading to undesirable tonal variation in the exhibited images. Figure 10(b) schematically illustrates a portion of an image layer 30 according to an embodiment of the invention in which gap regions 40 are introduced into the image segments Si, S₂, S3, S4 in order to maintain substantially the same areal coverage ratio of image layer material in each of the four regions and thereby exhibit images that are perceived to have a uniform tone.

Figure 11 illustrates an example of four images h, l₂, I3, I4 that may be exhibited by such a two-dimensional device, together with their relative overlapping arrangement. Figure 12 schematically illustrates the corresponding image layer for the zone labelled A in Figure 11. As seen in Figure 12, the image layer comprises a primary region R1 in which only image segments of image l₄ are present; a secondary region R₂ in which (only) image segments of both images l₄ and l₃ are present; a tertiary region R₃ in which (only) image segments of image l₄, I3 and l₂ are present, and a quaternary region R₄ in which image segments from all four channels are present.

Figure 13 illustrates an embodiment of the invention in the image layer 30 further comprises a static zone 60. The appearance of the static zone remains substantially the same at all viewing angles, even as the optically variable part of the device changes (e.g. from a “1” to a “2” in Figure 13). In addition to configuring the image layer such that the first and second images h, l₂ each exhibit a uniform tone, it is desirable that the static zone 60 also presents a uniform tone, typically the same as that of the first and second images. In order to achieve this effect, the static zone comprises static zone elements 65 comprising image layer material that are spaced by gap regions 67, such that the areal coverage ratio of the image layer material in the static zone is substantially equal to the areal coverage ratio of the image layer material in each of the primary and secondary regions R1, R₂. Therefore, in the two-channel device shown in Figure 13, the gap regions 67 in the static zone 60 are configured such that the areal coverage ratio of the image layer material in the static zone is 50%. This may be achieved by a halftoning or screening arrangement of the static zone elements 65, for example as illustrated in Figure 13(b).

Figures 13(a) and 13(b) also illustrate primary Ri and secondary R₂ regions of the image layer 30 in accordance with the image channels, as has been discussed above. The percentage areal coverage of the image layer material in each of the primary and secondary regions, and the static zone of the device in Figure 13 is substantially 50%. In this way, each of the first and second image, and the static zone, is perceived to exhibit the same uniform tone.

In the embodiments discussed thus far, we have considered varying the arrangement of the image layer material of the image layer by including gap regions within certain image segments in order to substantially remove tonal variations across the exhibited images. It is noted that in the embodiments described above, typically, the same image layer material is used to form the image segments, and the regions of image layer material have substantially the same colour density across the device such that the visual effects are generated primarily by the arrangement of the gap regions in the image segments. We now consider embodiments in which, instead of using gap regions (i.e. regions absent of image layer material) introduced into the image segments, the colour density of the image segments is varied in order to generate the perceived uniform tone.

Figure 14 schematically illustrates such an embodiment of the invention. Similarly to Figure 7, Figure 14 shows a magnified plan view of a portion of the image layer 30 which contains a primary region Ri, a secondary region R₂ and a tertiary region R₃ as described above. However, instead of introducing gap regions into the image segments of the secondary and tertiary regions, the colour density of the image segments in each of the secondary and tertiary regions is reduced compared to that of the image segments in the primary region Ri. More specifically, as the number of image channels in which image segments are present in the secondary region R₂ is double that of the primary region Ri, the colour density of the image segments in the secondary region is substantially half that of the image segments present in the primary region. Similarly, the colour density of the image segments in the tertiary region is substantially a third of the colour density of the image segments in the primary region. With each region, each image segment has substantially the same colour density.

In this way, when the observer views the device, he/she perceive each image to have a substantially uniform tone.

A convenient way to vary the colour density of the image segments is to vary their thickness, t, as illustrated in Figure 15(a). Figure 15(a)(i) schematically illustrates a cross sectional view of the device through a portion of a primary region Ri. Here, each image segment Si has a first thickness t1 (e.g. a height relative to the substrate 10) and a corresponding first colour density (CD1). Figure 15(a)(ii) illustrates a schematic cross-sectional view through a portion of a secondary region of the device, in which image segments Si , S₂ from two of the three image channels are present. Here, each image segment Si S₂ has the same thickness t2 that is smaller than t1. The thickness of the image layer material in the secondary region is chosen such that the colour density of the image segments in the secondary region is 1 CD1. Similarly, in the tertiary region shown in Figure 15(a)(iii), each image segment Si , S₂, S₃ has a thickness t3 that is smaller than both t1 and t2. The thickness t3 is chosen such that the colour density of the image segments is ¹/₃ CD1 .

In the embodiment of Figure 15(a), the same (at least semi-transparent) image layer material is used for each image segment of the image layer. Depending on the optical properties of the image layer material, the skilled person would be able to choose suitable thicknesses of the image layer material in order to generate the desired colour density in each region of the image layer. The image layer may be formed in a single print working by a suitable technique, for example by varying the depths of the recesses in the printing or casting plate corresponding to the desired heights of the image segments. Figure 15(b) shows a similar embodiment to that of Figure 15(a) in which the height of the image layer material is varied in different regions of the image layer in order to vary the colour density. In this embodiment, the image layer is formed in multiple print workings in order to generate the increased height in the primary and secondary regions.

Figure 15(c) illustrates a further embodiment in which the colour density of the image segments is varied across the primary, secondary and tertiary regions. In this example, the image layer material in the primary region has a colourant concentration (e.g. a concentration of a pigment or dye carried by the image layer material) that is greater than a colourant concentration of the image layer material forming the image segments in the secondary region. Similarly, the colourant concentration in the secondary region is greater than that in the tertiary region. In this way, the image segments in the primary region have the greatest colour density (and therefore the darkest tone), and the image segments in the tertiary region have the lowest colour density (and therefore the lightest tone). This embodiment has the advantage that each image segment may have the same height (i.e. thickness of image layer material); however, different image layer materials corresponding to the different colourant concentrations are required in order to form the image layer. Such an image layer may be applied in a single print working, for example through appropriate loading of a printing plate, or in multiple print workings in which each region is printed in a respective print working and in appropriate register.

Thus far, we have considered the case where each of the images exhibited by the device has the same colour, with the arrangement of the image layer material of the image segments and/or the colour density of the image segments being configured such that each image is perceived to display a uniform tone. The present invention may also be applied to lenticular devices in which the displayed images have different colours, as will now be explained with reference to Figures 16 and 17. Figure 16(a) illustrates a first image h and a second image l₂ that are selectively displayed in accordance with viewing angle, in a conventional lenticular device. The first image h is in the form of a blue alphanumeric character “1”, and the second image l₂ is in the form of a red alphanumeric character “2”. The exhibited images display regions of different colour due to the eye perceiving a “mixing” of the two colours in the secondary regions R₂ of the image layer in which both red and blue image segments are present (see Figure 16(b)). For example, the first image h is perceived to have blue regions (1a) corresponding to primary regions of the image layer in which only the blue image segments of the first image are present, and purple regions (2) corresponding to the secondary regions of the device in which both red and blue image segments are present. Similarly, the second image l₂ has red regions (1b) corresponding to primary regions in which only the red image segments of the second image are present, and purple regions (2) corresponding to the secondary regions. It is noted that the effect of the darker (“dominant”) colour (e.g. having a lower L* value in Cl Elab colour space) - in this case dark red - has a greater visual effect on the lighter colour - in this case light blue - than vice-versa. Thus, the purple colour mixing in region 2 is more visually apparent in image h than in image I2. Similar visual effects would be experienced in any multi-colour device (with any number of image channels): for example a black image will have a greater visual impact on an interleaved image channel having a lighter colour image than vice- versa. The arrangement of the gap regions in the secondary region R₂ of the image layer can therefore be configured in accordance with the colours of the images, with gap regions within image segments of the darker (“dominant”) image channel having a greater area ratio than in image segments of the lighter image channel.

Figure 17 schematically illustrates a device according to an embodiment of the invention, which has a modified line structure of the image layer 30 as shown in Figure 17(b). In each secondary region R₂ of the image layer, the image segments Si, S₂ corresponding to the respective images h, l₂ comprise gap regions 40 such that the areal coverage ratio of the image layer material in the secondary regions is substantially equal to the areal coverage ratio of image layer material in the primary regions Ri. Each image segment Si, S₂ in the secondary regions has the same ratio of image layer material to gap regions. In this way, the perceived colour mixing of the different colour image channels is substantially reduced relative to the prior art device, therefore increasing the clarity of the information displayed by each image.

Figure 18 is a flowchart outlining the steps of a preferred method of forming a security device according to an embodiment of the invention. The steps of the flowchart will be described with reference to Figure 19.

In step S201 , the images to be displayed by the device are provided. In this example, the device to be formed is a three-channel lenticular device and the images h, l₂ and l₃ are, respectively, in the form of the characters “1”, “2” and “3”, as shown in Figure 19(a)(i).

In step S203, the non-overlap regions of the provided image are determined. Here, by “overlap” we mean the regions in which the macro images overlap each other in the image layer, as illustrated in Figure 19(a)(ii). In this three-channel example, there are regions where an image is present but no images overlap (for example shown at 70), regions where two of the three images overlap (for example shown at 72) and regions where all three images overlap (for example shown at 74). In Step S205, the overlap regions of the three images are determined.

The determination of the non-overlap and overlap regions is illustrated in more detail in Figures 19(b) to 19(d), where the non-overlap and overlap regions are determined for each individual channel. For example, Figure 19(b) illustrates the case where the first image channel (corresponding to image h) is considered. Figure 19(b)(i) illustrates the determined regions 70 where the image 11 does not overlap with either image l₂ or l₃. Figure 19(b)(ii) illustrates the determined regions 72 where the image h overlaps with image l₂ but not image l₃, and Figure 19(b)(iii) illustrates the regions 72 where the image h overlaps with image l₃ but not image l₂. Finally, Figure 19(b)(iii) illustrates the regions 74 where all three images h, l₂ and l₃ overlap with each other. Figures 19(c) and 19(d) illustrates the corresponding determined regions for the second and third image channels respectively. It will be appreciated that there will be some “duplication” of the regions for each channel: for example the determined regions 74 are the same for each channel. The determined regions of overlap and non-overlap are then used to form the image layer as will be explained below.

Referring back to the flowchart of Figure 18, at step S207 a substrate is provided. The substrate could be provided in any form and as part of any suitable process for the manufacture of security devices, for example a webbased or sheet-fed process. As mentioned above, the substrate will typically be transparent (e.g. a polymeric substrate such as BOPP, PET, PE or PC) but in some alternative embodiments could be translucent or opaque (e.g. opacified polymer or paper). Then, in steps S209 and S211 the viewing elements and the image layer are respectively applied to the substrate. As has been discussed herein, the viewing elements are preferably focussing elements in the form of (e.g. cylindrical, spherical or aspherical) lenses, and may be formed using techniques known in the art such as embossing or cast-curing.

The image layer is formed in accordance with the regions of overlap and nonoverlap identified in steps S203 and S205. Taking the first image channel shown in Figure 19(b) as an example, the regions of non-overlap 70 correspond to the primary regions of the image layer in which only image segments of the first image h are present. The regions 72 in Figure 19(b)(ii) correspond to secondary regions of the image layer in which image segments from the first and second images h and l₂ are present, but not the third image. The regions 72 in Figure 19(b)(ii) also correspond to secondary regions of the image layer in which image segments from the first and third images h, l₃ are present, but not the second image. The regions 74 shown in Figure 19(b)(iv) correspond to tertiary regions of the image layer in which image segments from all three image channels are present. The image layer may be formed in accordance with these primary, secondary and tertiary regions as have been described in any of the embodiments herein, for example through the inclusion of gap regions or variation in the colour density. Preferably, the analysis performed in steps S201 to S205 (e.g. to determine the primary, secondary and tertiary regions based on the regions of overlap and non-overlap of the macro images) is used to generate an image layer template (which may for example be in the form of a computer file) defining the gap regions and/or the colour density of the image segments. Typically, in Step S211 , the image layer is formed in accordance with the template.

As described above, in preferred cases the image layer is provided as a print working, formed by a printing technique, preferably a gravure, intaglio, microintaglio, flexographic or lithographic technique, or a digital printing technique such as inkjet printing. However, in other embodiments, the image layer may be formed by any of: a laser marking; forming of a relief structure, preferably by embossing or cast-curing, wherein the relief structure is configured to generate structural colour, preferably a diffractive or plasmonic relief structure; forming of a relief structure, preferably by embossing or cast-curing, and application of a marking material into the recesses thereof or onto the elevations thereof; or demetallisation of a metal or metal alloy layer. As explained previously, suitable apparatus, materials and methods for forming relief structures such as the focussing features, and suitable printing techniques for forming the print workings, disclosed herein are described in WO-A-2018/153840 and WO-A- 2017/009616.

It will be appreciated that the steps S209 and S211 may be performed in any order. For example, the image layer could be applied before the viewing elements, or the viewing elements and image layer could be applied simultaneously. Simultaneous application can achieve highly precise register between the viewing elements and the image segments. Before and/or after steps S209 and S211 are performed, additional steps could be performed, for example the provision of additional layers or security features on the substrate.

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 a masking grid 90 (shown in Figure 20) that comprises substantially opaque regions 93 spaced by substantially transparent regions 95 (e.g. defined by gaps between the opaque regions). The transparent regions 95 cooperate with the image layer such that light from different sets of viewing segments is directed to the viewer upon a change in viewing angle, as schematically shown in Figure 20.

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 21 to 24.

Figure 21 depicts an exemplary document of value 1500, here in the form of a banknote. Figure 21(a) shows the banknote in plan view whilst Figure 21 (b) shows a cross-section of the same banknote along the line X-X' and Figure 21 (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 21(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 (e.g. as in Figure 5(a) above). Figure 21(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 array 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 22 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 1602 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 23, 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 24 where Figures 24(a) and 24(b) show the front and rear sides of the document 1800 respectively, and Figure 24(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, 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 24(a)) and exposed in one or more windows 1802 on the opposite side of the document (Figure 24(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.

Claims

CLAIMS . A security device, comprising: a substrate; an array of viewing elements disposed on the substrate; and an image layer disposed in or on the substrate, wherein the image layer comprises at least first and second sets of image segments, the first set of image segments comprising image layer material that together defines a first image and the second set of image segments comprising image layer material that together defines a second image, the first set of image segments and the second set of image segments being interleaved with each other; and wherein the array of viewing elements and the image layer cooperate with each other such that at a first range of viewing angles, light from the first set of image segments is directed to a viewer such that the first image is exhibited; and at a second range of viewing angles, light from the second set of image segments is directed to the viewer such that the second image is exhibited; wherein the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region, and wherein the arrangement of the image layer material of the image segments and/or the colour density of the image segments within a region is dependent upon the number of sets of image segments having image segments present within that region, and is different for different regions such that when viewing the device within the first range of viewing angles the first image is perceived to have a uniform tone, and when viewing the device within the second range of viewing angles the second image is perceived to have a uniform tone.

2. The security device of claim 1 , wherein each region of the image layer has substantially the same ratio of the area of image layer material to the area absent of image layer material.

3. The security device of claim 1 or claim 2, wherein within at least one of the regions of the image layer in which image segments from more than one set of image segments are present, at least some of the image segments, preferably all of the image segments, each comprise at least one gap region that is defined by an absence of image layer material.

4. The security device of claim 3, wherein within a region of the image layer in which the image segments comprise gap regions, a ratio of the area of image layer material to the area of the gap region(s) of each image segment is substantially the same.

5. The security device of claim 3 or claim 4, wherein within a region of the image layer in which the image segments comprise gap regions, the ratio of the area of the image layer material to the area of the gap regions within an image segment is dependent on the number of sets of image segments having image segments present within that region.

6. The security device of any of claims 3 to 5, wherein each gap region has a dimension such that it is not discernible to the naked human eye, preferably wherein each gap region has a dimension less than 150pm, more preferably less than 100pm and even more preferably less than 70pm.

7. The security device of any of claims 3 to 6, wherein the dimensions, pitches, and/or orientations of the gap regions are configured such that the gap regions do not interact with the array of viewing elements.

8. The security device of any of the preceding claims, wherein the first set of image segments and the second set of image segments are interleaved within each other periodically along at least a first direction, and wherein the image segments are elongate segments extending along a second direction that is preferably orthogonal to the first direction.

9. The security device of any of claims 3 to 8, wherein the first set of image segments and the second set of image segments are interleaved with each other periodically along a first direction, and wherein the gap region(s) of an image segment are configured such that the image layer material of the respective image segment is discontinuous along a second direction that is preferably substantially orthogonal to the first direction.

10. The security device of claim 9, wherein the gap regions of laterally adjacent image segments are laterally offset from each other along the second direction, preferably wherein the gap regions are substantially completely offset from each other along the second direction.

11 . The security device of any of claims 3 to 8, wherein the first set of image segments and the second set of image segments are interleaved with each other periodically along at least a first direction, and wherein the gap region(s) of an image segment are configured such that the image layer material of the respective image segment is substantially continuous along the second direction.

12. The security device of any of the preceding claims, wherein the colour density of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than the colour density of the image segments within regions in which more sets of image segments have image segments present.

13. The security device of claim 12, wherein a thickness of the image layer material of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than a thickness of the image layer material of the image segments within regions in which more sets of image segments have image segments present.

14. The security device of any of the preceding claims, wherein each of the at least first and second sets of image segments is formed of the same image layer material.

15. The security device of claim 12, wherein a colourant concentration of the image layer material of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than a colourant concentration of the image layer material of the image segments within regions in which more sets of image segments have image segments present.

16. The security device of any of claims 12 to 15, wherein within a region of the image layer, the colour density of each image segment is substantially the same.

17. The security device of any of claims 12 to 16, wherein, within regions of the image layer in which two or more sets of image segments have image segments present, the image segments do not comprise any gap regions.

18. The security device of any of the preceding claims, wherein each of the at least first and second sets of image segments has the same colour.

19. The security device of any of the preceding claims, wherein the image layer further comprises a static zone having static zone elements comprising image layer material, and wherein the arrangement and/or colour density of the static zone elements is configured such that the static zone is perceived to have a uniform tone that is substantially the same as the uniform tone of the first and/or second image.

20. The security device of claim 19, wherein a ratio of the area of image layer material to the area absent of image layer material in the static zone is substantially equal to a ratio of the area of image layer material to the area absent of image layer material in each of the regions comprising image segments.

21 . A security device, comprising: a substrate; an array of viewing elements disposed on the substrate; and an image layer disposed in or on the substrate, wherein the image layer comprises at least first and second sets of image segments, the first set of image segments comprising image layer material of a first colour that together defines a first image, and the second set of image segments comprising image layer material of a second colour different from the first colour that together defines a second image, the first set of image segments and the second set of image segments being interleaved with each other; and wherein the array of viewing elements and the image layer cooperate with each other such that at a first range of viewing angles, light from the set of first image segments is directed to a viewer such that the first image is exhibited; and at a second range of viewing angles, light from the set of second image segments is directed to the viewer such that the second image is exhibited; wherein the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region, wherein the arrangement of the image layer material of the image segments and/or the colour density of the image segments is different for different regions; and each region of the image layer has substantially the same ratio of the area of image layer material to the area absent of image layer material; and/or the colour density of the image segments within regions in which fewer sets of image segments have image segments present is relatively greater than the colour density of the image segments within regions in which more sets of image segments have image segments present.

22. The security device of claim 21 , wherein within at least one of the regions of the image layer in which image segments from more than one set of image segments are present, at least some of the image segments, preferably all of the image segments, each comprise at least one gap region that is defined by an absence of image layer material.

23. The security device of claim 21 or claim 22, wherein within a region of the image layer, a ratio of the area of image layer material to the area of the gap region(s) of each image segment is substantially the same.

24. The security device of claim 22 or claim 23, wherein the first set of image segments and the second set of image segments are interleaved with each other periodically along at least a first direction, and wherein the gap region(s) of an image segment are configured such that the image layer material of the respective image segment is discontinuous along a second direction that is preferably substantially orthogonal to the first direction, preferably wherein the gap regions of laterally adjacent image segments are laterally offset from each other along the second direction, more preferably wherein the gaps are substantially completely offset from each other along the second direction.

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

26. The security device of any of the preceding claims, wherein the focussing elements are adapted to focus light in one dimension, in which case the focussing elements are preferably cylindrical focussing elements, or adapted to focus light in at least two directions, in which case the focussing elements are preferably spherical or aspherical focussing elements.

27. The security device of any claim 25 or claim 26, wherein the image layer is located approximately in the focal plane of the array of focussing elements.

28. The security device of any of claims 1 to 24, wherein the array of viewing elements is in the form of a masking grid.

29. The security device of any of the preceding claims, wherein the first and second images are each in the form of indicia or an indicium, preferably one or more geometric shapes, letters, logos, currency signs or other symbols.

30. 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 on a first surface of the substrate and the image layer is provided on a second, opposing surface of the substrate.

31. The security device of any of the preceding claims, wherein the image layer is a print working, preferably printed by a gravure, intaglio, screen, microintaglio, flexographic, lithographic or digital technique.

32. The security device of any of claims 1 to 30, wherein the image layer comprises any of: a laser marking and/or laser ablation; a relief structure configured to generate structural colour, preferably a diffractive or plasmonic relief structure; a relief structure carrying a marking material in the recesses thereof or on the elevations thereof; or a demetallized metal or metal alloy layer.

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

34. A security document comprising a security device according to any of claims 1 to 32, wherein the security document is preferably a banknote, cheque, passport, identity care, driver’s licence, certificate of authenticity, fiscal stamp or other document for securing value or personal identity.

35. A method of manufacturing a security device, comprising:

(a) providing a substrate;

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

(c) forming an image layer in or on the substrate; wherein the image layer comprises at least first and second sets of image segments, the first set of image segments comprising image layer material that together defines a first image and the second set of image segments comprising image layer material that together defines a second image, the first set of image segments and the second set of image segments being interleaved with each other; and wherein the array of viewing elements and the image layer cooperate with each other such that at a first range of viewing angles, light from the first set of image segments is directed to a viewer such that the first image is exhibited; and at a second range of viewing angles, light from the second set of image segments is directed to the viewer such that the second image is exhibited; wherein the image layer comprises a plurality of different regions, each region being defined by the number of sets of image segments having image segments present within that region, and wherein the arrangement of the image layer material of the image segments and/or the colour density of the image segments within a region is dependent upon the number of sets of image segments having image segments present within that region, and is different for different regions such that when viewing the device within the first range of viewing angles the first image is perceived to have a uniform tone, and when viewing the device within the second range of viewing angles the second image is perceived to have a uniform tone.

36. A method of manufacturing a security device, comprising:

(a) providing a substrate;

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

37. The method of claim 35 or claim 36, further comprising, before step (c), generating an image layer template by: identifying at least one non-overlap region where the first image does not overlap with the second image; identifying at least one overlap region where the first image overlaps with the second image; and in step (c), forming the image layer in accordance with the image layer template.

38. The method of any of claims 35 to 37, wherein the image layer is a print working, formed by a printing technique, preferably a gravure, intaglio, screen, micro-intaglio, flexographic, lithographic or digital technique.

39. The method of any of claims 35 to 37, wherein the image layer is formed by any of: a laser marking and/or laser ablation; a relief structure configured to generate structural colour, preferably a diffractive or plasmonic relief structure; a relief structure carrying a marking material in the recesses thereof or on the elevations thereof; or a demetallized metal or metal alloy layer.

40. The method of any of claims 35 or 37 to 39, wherein each of the at least first and second sets of image segments is formed of the same image layer material.

41 . The method of claim 40 when dependent on claim 38, wherein the image layer is formed in a single print working.

42. The method of any of claims 35 to 41 , 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.

43. The method of any of claims 35 to 42, adapted to produce the security device of any of claims 1 to 32.