Security devices and methods of manufacture thereof

Abstract

A security device 100, comprises a substrate (10, Fig.2); an array 20 of viewing elements (21, Fig.2); and an image layer 30 overlapping with the array of viewing elements. The image layer comprises a

Description

FIELD OF THE INVENTION This invention relates to security devices that may be used, for example, on documents of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps, and other secure documents, in order to confirm their authenticity. Methods of manufacturing such security devices are also disclosed. BACKGROUND Articles of value, and particularly documents of value such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and / or changes to any data contained therein. Typically, such objects are provided with a number of visible security devices for checking the authenticity of the object. By “security device” we mean a feature which is not possible to reproduce accurately by taking a visible light copy, e.g. through the use of standardly available photocopying or scanning equipment. One class of security devices are lenticular devices, which make use of focussing elements (such as lenses) to produce an optically variable effect, meaning that the appearance of the device is different at different angles of view and / or illumination. Such devices are particularly effective as security devices since direct copies (e.g. photocopies) will not produce the optically variable effect and hence can be readily distinguished from genuine devices. In lenticular devices, an array of viewing elements, typically cylindrical lenses, overlies an image layer having a corresponding array of image elements, each of which depicts only a portion of an image which is to be displayed. Image elements from two or more different images are interleaved and, when viewed through the array of viewing elements, at each viewing angle, only selected image elements will be directed towards the viewer. In this way, different composite images can be viewed at different angles. Some examples of lenticular devices are described in US-A-4892336, WO-A-2011 / 051669, WO-A-2011051670 and US-B-6856462. Lenticular devices have the advantage that different images can be displayed at different viewing angles, giving rise to the possibility of animation and other striking visual effects that allow for simple authentication of a device that is simultaneously difficult to counterfeit. To enhance the security of lenticular security devices, it is desirable for one or more of the exhibited images to be multi-tonal or multi-coloured. This increases the complexity and recognition of the images, simultaneously increasing both the ease of authentication and the difficulty of counterfeit. However, it can be difficult to achieve the desired level of tonal control within an image due to the size (e.g. line width) of printed image elements increasing following printing, in some cases by up to 200%. This so-called “dot gain” is particularly problematic for devices exhibiting a large number of different images, which requires smaller individual image elements. It is also difficult to achieve the necessary registration of the image elements having different tones or colours within the required tolerances of the image layer. Additionally, it is desirable to increase the ease with which security devices may be integrated into a document or article carrying the device. Consequently, improvements to security devices, and in particular lenticular security devices, are constantly sought in order to stay ahead of would-be counterfeiters. SUMMARY OF INVENTION In accordance with a first aspect of the invention, there is provided a (e.g. lenticular) security device, comprising: a substrate; an array of viewing elements disposed in or on the substrate; and an image layer disposed in or on the substrate and overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, wherein the set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; wherein at least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein the image layer comprises at least one region that is fully contained within the lateral area of the first image and overlaps with a plurality of the viewing elements, said at least one region having a first image tone ratio that is the number of first image channel positions occupied by first image elements to the number of first image channel positions within the region; and wherein: (i) the image layer comprises first and second regions having different first image tone ratios; and / or (ii) the second image channel is occupied by a corresponding set of second image elements formed of image layer material that together defines a second image, and within said at least one region of the image layer, the first image tone ratio is different from a second image tone ratio that is the number of second image channel positions occupied by second image elements to the number of second image channel positions within the region. The security device of the present invention incudes an image layer in which at least one region has a first image tone ratio that is the number of first image channel positions occupied by first image elements to the number of image channel positions within the region. By controlling the first image tone ratio across the image layer, the perceived tone of the exhibited first image can advantageously be controlled, for example to provide first and second regions of the first image having different perceived tones, or to control the relative perceived tone of two or more images that are exhibited by the device upon a change in viewing angle. The (e.g. at least partial) occupation of the image channel positions by image elements may be described as “selective” occupation of the image channel positions by image elements, with the selective occupation enabling enhanced tonal control of the perceived image(s) exhibited by the device as described above. The security device according to the first aspect of the invention is typically a lenticular security device. Herein, by “different tone”, we mean regions of an image exhibited by the device that are perceived to perceptibly differ in “brightness” or “darkness” under cursory inspection by the naked human eye. As discussed, at least the first image channel is (e.g. “selectively”) occupied by a corresponding set of first image elements formed of image layer material that together defines a first image. The first image is perceived by a viewer of the device at the corresponding viewing angle. At least some of the first image channel positions are not occupied by first image elements. In other words, the present invention advantageously comprises first image channel positions in which the corresponding image element is not present, in other words forming a “blank” or “off’ channel for that position. Each region overlaps with a plurality of viewing elements. The human eye averages the visual effect of the image elements over the corresponding plurality of viewing elements to perceive the image. Consequently, the inclusion of “off’ image channel positions means that the human eye will perceive the corresponding region of the image exhibited by the device to have a different tone compared to a region in which each image channel position is occupied (e.g. “on”). The present invention provides a different technique for exhibiting tonal control in comparison to conventional methods of controlling the size (e.g. line width) of the individual image elements. This advantageously reduces the problems associated with dot gain, and means that the images exhibited by devices according to the present invention may exhibit an increased number of different tones compared to conventional devices. The image layer comprises at least one region that is fully contained within the lateral area of the first image, said at least one region having a first image tone ratio that is the number of first image channel positions occupied by first image elements to the number of first image channel positions within the region. In other words, the first image tone ratio is a ratio of the number of “on” first image channels (i.e. occupied by a first image element) to the number of image channel positions available within the region. Each region is fully contained within the lateral area of (e.g. within the perimeter of) of the first image. In other words, each region forms a part of the image perceived by the viewer. Thus, each region has at least one “on” image channel such that it corresponds to a region of the image perceived when viewing the device. Typically, each region is perceived to have a uniform tone across its domain. A first image tone ratio of 1:1 (i.e. each first image channel position within the region being occupied) will exhibit the highest contrast / saturation (e.g. “darkest” tone), whereas a first image tone ratio of 1:n where n>1 (i.e. one first image channel position being occupied to every (n-1) first image channels being unoccupied, or “off’) will exhibit a lower contrast / saturation (e.g. “lighter” tone). In general, the first image tone ratio is 1 :n, where typically 1<n<30, more typically 1 <n<10, and even more typically 1 <n<5. In the “1 :n” nomenclature, it is noted that n need not be an integer. The image layer may comprise first and second regions having different first image tone ratios. In this way, the resulting first image exhibited by the device will be perceived to have first and second regions (each being fully contained within the lateral area of the first image) of different tone corresponding to the respective first image tone ratios. This advantageously enables the device to exhibit a multi-tonal first image when viewed at the first range of viewing angles. Typically, both of the first and second regions comprise image channel positions unoccupied by first image elements. The second image channel may be a “blank” image channel that does not comprise any image elements and consequently does not exhibit any image. This results in a device that exhibits an optically variable “on / off switch” effect upon tilting, switching between a “blank” appearance when viewed at the second range of viewing angles, and a multi-tonal first image when viewed at the first range of viewing angles. Alternatively, the second image channel may contain a set of second image elements formed of image layer material that together defines a second image (different from the first image) that is exhibited at the second range of viewing angles. As discussed, the image layer may comprise first and second regions having different first image tone ratios. Alternatively or in addition, the second image channel may be occupied by a corresponding set of second image elements formed of image layer material that together defines a second image, and within said at least one region of the image layer, the first image tone ratio is different from a second image tone ratio that is the number of second image channel positions occupied by second image elements to the number of second image channel positions within the region. Such devices may advantageously be used to reduce differences in perceived colour saturation between the first and second images. For example, a first image formed of image layer material having a first colour that has a relatively greater colour saturation (e.g. red ink) may have a lower image tone ratio than a second image formed of image layer material having a second colour that has a relatively lower colour saturation (e.g. yellow ink). In such embodiments, typically, the first image tone ratio is different from the second image tone ratio across the whole domain of the image layer. Each image element is formed of image layer material (e.g. ink), whereby the image layer material in combination defines an image. Herein, the term “image” refers to the graphical form of the image as perceived by a viewer of the device. Preferably, at least one image exhibited by the device is in the form of indicia or an indicium, preferably one or more geometric shapes, letters, logos, currency signs or other symbols. Preferably, in the first aspect of the invention, each first image element is formed of the same image layer material (e.g. ink). Thus, in embodiments, the image layer material of each first image element has substantially the same colour. However, in some embodiments it is envisaged that the image elements forming an image may be formed from different image layer materials. Similarly, in embodiments, each second image element may be formed of the same image layer material (typically different from the image layer material forming the first image elements). Typically, the image layer material forming a particular image may be a single ink. In this way, efficient control of the tone of the image(s) exhibited by the device may be achieved. Preferably, the at least one region comprises a plurality of repeating zones each exhibiting the first image tone ratio. This provides a convenient way of providing the first image tone ratio across the at least one region. The term “repeating” here refers to the configuration of the “on” and “off’ first image channel positions within each zone, rather than the specific form of the image layer material. This may be considered a “periodic” arrangement of “on” and “off’ first image channel positions. Thus, each repeating zone exhibits the same first tone ratio. Typically, both of the first and second regions comprise a plurality of repeating zones each exhibiting the respective first image tone ratio. In embodiments, the zones within both the first and second regions each comprise first image channel positions not occupied by first image elements. In otherwords, in such embodiments, the repeating zones within both of the first and second regions each comprise “off’, or “blank”, first image channel positions. In some embodiments, the zones may have a predetermined size (e.g. corresponding to a predetermined number of viewing elements). The predetermined size may be based on the number of different image tone ratios (and therefore the number of different perceived tone levels) desired to be achieved. In embodiments in which the second image channel is occupied by a corresponding set of second image elements, preferably, said at least one region comprises a plurality of repeating zones each exhibiting the first image tone ratio and the second image tone ratio. In such embodiments, the at least one region may extend fully over the lateral areas of both images. In other words, in such embodiments, the first image tone ratio is uniform across the first image layer, and the second image tone ratio (which is different from the first image tone ratio) is uniform across second image. Typically, within the at least one region, each of the zones is laterally contiguous. As discussed, the present invention utilises the eye averaging the perceived visual effect over multiple viewing elements of the device. Thus, the at least one region overlaps with a plurality of the viewing elements. It will be appreciated that the size of the at least one region will depend on the image(s) being exhibited. Typically, each region has a minimum dimension of 300pm, preferably 500pm, more preferably 1mm. In this way, the human eye will be capable of perceiving the resulting differences in tone due to differences in image tone ratio. Typically, the minimum dimension of each region is along the direction of the interlacing of the first and second image channel positions. In some embodiments, the image layer may comprise a plurality of regions that define an intermediate section, the intermediate section being located between two regions of the image layer having different first image tone ratios, wherein each region within the intermediate section comprises a different first image tone ratio so as to exhibit a perceived (e.g. gradual) change in tone between the two regions across the intermediate section. The inclusion of such an intermediate section advantageously provides an increase in the complexity of the image by exhibiting a gradual change in tone between two regions, rather than an abrupt change. This may also be used to achieved three-dimensional effects. In this way, the security level of the device may be increased. In order to achieve a gradual change in tone, the first image tone ratios of the regions forming the intermediate section preferably differ in a successive manner from a “darker” tone to a “lighter” tone, or vice-versa. For example, (e.g. contiguous) regions of the intermediate section may exhibit successive first image tone ratios of 4:5, 3:5, and 2:5, thereby forming an intermediate section between a first region exhibiting a first image tone ratio of 1:1 and a second region exhibiting a first image tone ratio of 1:5. In accordance with a second aspect of the invention, there is provided a (e.g. lenticular) security device comprising: a substrate; an array of viewing elements disposed in or on the substrate; and an image layer disposed in or on the substrate and overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, wherein the set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; wherein at least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein a first subset of the first image elements are formed of image layer material exhibiting a first colour, and a second subset of the first image elements are formed of image layer material exhibiting a second colour different from the first colour; and the image layer comprises at least a first colour region that overlaps with a plurality of the viewing elements, the first colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a first colour ratio, whereby the first colour region is perceived to have a resultant colour due to a combination of the first and second colours. As with the first aspect of the invention, a security device according to the second aspect of the invention utilises the fact that the human eye averages the visual effect of the image elements over a plurality of viewing elements. In this way, the viewer perceives at least a first colour region of the exhibited image to have a resultant colour that is due to a combination of the first and second colours of the respective first and second subsets of the first image elements. This advantageously increases the ease with which a device exhibiting a complex multi-coloured image may be manufactured. For example, in cases where the image layer material is an ink, fewer inks of different colour need to be printed, as the present invention utilises colour mixing to achieve additional colours. The security device according to the second aspect of the invention is typically a lenticular security device. The first colour region comprises first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a first colour ratio. In other words, the first colour ratio is the ratio of the number of first image elements of the first subset to the number of first image elements of the second subset, within the region. Typically, the first colour region is fully contained within the lateral area of the first image. The first colour region comprises both first image elements of the first subset and first image elements of the second subset. In this way, the resultant colour is different from the first and second colours. Typically, the first colour region is perceived to have a uniform resultant colour. Although the first image exhibited by the device is typically a multi-coloured image, this need not be the case. For example, in embodiments the first colour ratio may be uniform across the device such that the resulting first image has a uniform resultant colour. The use of colour mixing according to embodiments of the invention may be particularly advantageous in scenarios where the substrate of the security device is a substrate of a security document, such as a banknote, carrying the device. For example, print works applied to the banknote substrate outside the lateral confines of the security device itself (e.g. lithographic works) may be applied using first and second inks of different colour. The first image elements of the security device image layer may then be printed in the same printing step and exhibit a (different) resultant colour(s) resulting from colour mixing of the first and second colours. This advantageously reduces the number of printing steps and number of different inks required to form the security document carrying the device. This advantageously increases the ease with which security devices may be integrated into a document or article carrying the device. Herein, by “different colour”, we mean colours that have a perceptible difference when viewed by the naked human eye. Typically, two colours will be perceived to be different if the Euclidean distance AE* between them in CIELAB colour space (i.e. the CIE 1976 L*a*b* colour space) is greater than 2.3, preferably greater than 3, more preferably greater than 5, even more preferably greaterthan 10. The value of AE* is measured using the formula: A£” = 7(^’)2 + (Aa*)2 + (Ab*)2 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). The difference AE* can be measured using any commercial spectrophotometer, such as those available from Hunterlab of Reston, Virginia, USA. Preferably, the first colour region comprises a plurality of repeating first colour zones, each first colour zone exhibiting the first colour ratio. In a corresponding manner to the first aspect of the invention, the term “repeating” here refers to the arrangement of the first and second subsets of the first image elements, rather than the specific form of the image layer material. Thus, each repeating zone exhibits the same first colour ratio. The use of repeating zones in this manner provides a convenient way to achieve the desired resultant colour across the first colour region. In some embodiments, the colour zones may have a predetermined size (e.g. corresponding to a predetermined number of viewing elements). In preferred embodiments, the image layer further comprises a second colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a second colour ratio; wherein the first and second colour ratios are different whereby the first and second colour regions are perceived to have different colours. In this way, embodiments of the present invention advantageously enable complex multi-coloured images to be exhibited through “colour mixing” of the image elements forming the image. Typically, the second colour region comprises a plurality of repeating second colour zones, each second colour zone exhibiting the second colour ratio. In a similar manner to as has been discussed with reference to the first embodiment of the invention, preferably each colour region has a minimum dimension of 300pm, preferably 500pm, more preferably 1mm. In this way, the human eye is capable of perceiving differences in colour between different colour regions. In some embodiments, the first image elements may comprise, in addition to the first subset and the second subset, a third subset formed of image layer material exhibiting a different colour from both the first and second colours. Such embodiments allow further complex effects such as CMYK colour mixing. In security devices according to the second aspect of the invention, at least the first image channel is occupied by a corresponding set of first image elements. The second image channel may be a “blank” image channel, giving rise to an “on / off switch” optically variable effect. The second image channel may be occupied by image elements defining a second image, different from the first, whereby the security device exhibits an image switch effect upon tilting. In embodiments in which the second image channel exhibits a second image, the second image may utilise colour mixing as described with the first image. Thus, in some embodiments, the second image channel is occupied by a corresponding set of second image elements formed of image layer material that together defines a second image, wherein a first subset of the second image elements are formed of image layer material exhibiting a third colour, and a second subset of the second image elements are formed of image layer material exhibiting a fourth colour different from the third colour, and wherein within at least one colour region of the image layer, the image layer comprises second image channel positions occupied by second image elements of the first subset and second image channel positions occupied by second image elements of the second subset in a second colour ratio, whereby the colour region is perceived to have a resultant colour due to a combination of the third and fourth colours. The third and fourth colours may be the same as the first and second colours, or may be different colours from the first and second colours. In this way, the security device may advantageously exhibit more than one complex, multi-coloured image on tilting that utilises colour mixing through the eye perceiving the visual effect of the image layer across multiple ones of the viewing elements. In some embodiments, the image layer may comprise a plurality of colour regions that define an intermediate colour section, the intermediate colour section being located between two portions of the image layer exhibiting different colours, wherein each colour region within the intermediate colour section comprises a different colour ratio so as to exhibit a perceived (e.g. gradual) change in colour between the two portions across the intermediate colour section. The inclusion of such an intermediate colour section advantageously increases the complexity of the image by exhibiting a gradual change in colour between two portions of the image, rather than an abrupt change. The portions of the image either side of the intermediate colour zone may be regions exhibiting colour mixing, or may be areas of a single colour that do not exhibit colour mixing. In order to achieve the gradual change in colour across the domain of the intermediate colour section, the colour ratios of the (e.g. contiguous) regions defining the intermediate colour section preferably differ in a successive manner. As discussed, a security device according to both the first and second aspects of the invention comprises a first image channel and a second image channel. In some embodiments, the device may comprise only the first image channel and second image channel, and therefore may be referred to as a “two-channel” device. However, the invention also applies to devices which comprise further (e.g. third, fourth etc.) image channels. In general, the invention applies to all n-channel (e.g. lenticular) security devices, where n is equal to or greater than 2. 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 (e.g. 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. Embodiments of the present invention may also be applied to two-dimensional lenticular devices. In such embodiments, the viewing elements are adapted to focus light in at least two (e.g. non-parallel, preferably orthogonal) directions, in which case the viewing elements are preferably spherical or aspherical focussing elements. The pitch of the viewing element array (e.g. array of lenses) is typically in the range of 10pm to 200pm, preferably 20pm to 200pm, more preferably 50pm to 200pm. The pitch of the viewing elements is typically uniform across the array. Although the viewing elements are typically in the form of focussing elements such as lenses, in some embodiments the array of viewing elements may be in the form of a masking grid. In such embodiments, each viewing element typically comprises a substantially opaque region and a substantially transparent region, such that the masking grid comprises a plurality substantially opaque regions spaced by gap regions. The image layer is viewable through the substantially transparent regions. Preferably, in the case where the viewing elements are focussing elements, the image layer is located approximately in the focal plane of the array of focussing elements. The required spacing between the focussing elements and the image layer may be provided by the substrate itself and / or any optical spacing or pedestal layer as is known in the art. In typical embodiments, the substrate is at least semi-transparent (preferably fully transparent), and wherein the array of viewing elements is disposed in or on a first surface of the substrate and the image layer is disposed in or on a second, opposing surface of the substrate. It is noted that the term “on” does not necessarily mean in direct contact; for example, there may be a primer layer positioned between the substrate and the array of viewing elements. It will be appreciated that in such configurations the substrate will need to be at least semitransparent (the term “transparent” herein being used to mean optically clear and non-scattering, although may carry a coloured tint). In this case, the substrate is typically formed of one or more polymer materials, such as BOPP, PET, PE, PC or the like. In alternative embodiments, the viewing elements may be disposed on the same side of the substrate as the image layer, e.g. by incorporating an optical spacing into their design or providing an at least semi-transparent pedestal layer between the viewing elements and the image layer. In such embodiments, the substrate need not be semi-transparent and may be of any type, opaque or otherwise. This includes paper substrates, although polymer-based substrates are preferred. The image layer is preferably provided by a print working, preferably printed by a gravure, intaglio, screen, micro-intaglio, flexographic or (wet or dry) lithographic technique, or by a digital printing technique, for example inkjet or laser printing. With careful design and implementation, such techniques can be used to print image elements with a line width (e.g. in the direction of interlacing) of between 10pm and 100pm. For example, with flexographic or wet lithographic printing it is possible to achieve line widths down to about 5-25pm. In this way, the image elements may be described as “microimage” elements. The image layer is typically formed as a single layer (“image layer”) disposed in or on the substrate. Where provided as a print working, the image layer is preferably formed in a single print working (e.g. one pass of a printing machine). Other (non-print) methods of forming the image layer may be used. For example the image elements may be in the form of, or comprise, metallised or de-metallised regions, filled recesses, laser-marked regions, or (e.g. diffractive) surface relief structures including first order, zero order and sub-wavelength gratings such as plasmonic structures. Other examples include cast or embossed recesses (that may be filled or unfilled with marking material), and / or posts. 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, ora data page fora security document such as a passport. Security articles such as these, carrying the security device, can then be applied to or incorporated in a security document or any other object, e.g. by hot stamping, cold stamping, via adhesive or lamination, or by introduction during papermaking. A fourth aspect of the invention provides a security document comprising a security device or security article as described above, wherein the security document is preferably a banknote, cheque, passport, identity card, driver’s licence, certificate of authenticity, fiscal stamp or other document for securing value or personal identity. The security device can either be formed directly on the security document, in which case the document substrate may act as the substrate of the security device, or could be formed on a security article which is then applied to or incorporated into the security document as described above. In accordance with a fifth aspect of the invention there is provided a method of manufacturing a (e.g. lenticular) security device, comprising: (a) providing a substrate; (b) applying an array of viewing elements to the substrate; and (c) forming an image layer in or on the substrate, the image layer overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, wherein the set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; wherein at least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein the image layer comprises at least one region that is fully contained within the lateral area of the first image and overlaps with a plurality of the viewing elements, said at least one region having a first image tone ratio that is the number of first image channel positions occupied by first image elements to the number of first image channel positions within the region; and wherein: (i) the image layer comprises first and second regions having different first image tone ratios; and / or (ii) the second image channel is occupied by a corresponding set of second image elements formed of image layer material that together defines a second image, and within said at least one region of the image layer, the first image tone ratio is different from a second image tone ratio that is the number of second image channel positions occupied by second image elements to the number of second image channel positions within the region. 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. In preferred embodiments of the fifth aspect, the method may further comprise, before step (c), forming an image layer template by identifying regions of different tone within the first image; and in step (c), forming the image layer in accordance with the image layer template. The image layer template is typically in the form of a computer file. In accordance with a sixth aspect of the invention there is provided a method of manufacturing a (e.g. lenticular) security device, comprising: (a) providing a substrate; (b) applying an array of viewing elements to the substrate; and (c) forming an image layer in or on the substrate, the image layer overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, wherein the set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; wherein at least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein a first subset of the first image elements are formed of image layer material exhibiting a first colour, and a second subset of the first image elements are formed of image layer material exhibiting a second colour different from the first colour; and the image layer comprises at least a first colour region that overlaps with a plurality of the viewing elements, the first colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a first colour ratio, whereby the first colour region is perceived to have a resultant colour due to a combination of the first and second colours. 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 of the sixth aspect, the method may further comprise, before step (c), forming an image layer template by identifying one or more regions of colour mixing within the first image; and in step (c), forming the image layer in accordance with the image layer template. The image layer template is typically in the form of a computer file. In forming the image layer template, the identified regions of colour mixing are used to generate the required colour ratio to be defined by the first colour region of the image layer such that the required resultant colour is exhibited. Typically, the image layer may be formed by a printing technique, preferably a gravure, intaglio, screen, micro-intaglio, flexographic, lithographic or digital technique. Typically, the image layer is formed in a single print working. However, other (non-print) methods of forming the image layer may be used, for example metallisation, de-metallisation, casting and filling recesses, laser marking, and forming (e.g. diffractive) surface relief structures including first order, zero order and sub-wavelength gratings such as plasmonic structures. Other examples include casting or embossing of recesses (that may be filled or unfilled with marking material), and / or posts. The viewing elements (typically focussing elements) can be produced by known means such as embossing or cast-curing, and may be formed directly on the substrate or on a separate substrate from which they are transferred to the device, or which is attached to and then forms part of the device substrate. In some cases the viewing elements may be applied to the substrate by forming (e.g. embossing) the viewing elements into the substrate material itself. The array of viewing elements and the image layer may be provided in either order. In other words, the array of viewing elements may be applied to the substrate before the application of the image layer, or vice-versa. However, in preferred embodiments, the viewing elements (e.g. lenses) are applied to a first side of the substrate and the image layer is applied to a second, opposing, side of the substrate simultaneously at the same location along the substrate. Such simultaneous application of the viewing elements and image layer advantageously provides highly accurate register between the two. BRIEF DESCRIPTION OF DRAWINGS Embodiments of the invention will now be described with reference to the appended drawings, in which:- Figure 1 schematically illustrates a security document carrying a conventional lenticular device as is known in the art; Figure 2 is a cross-sectional view of a conventional lenticular device; Figures 3(a) to 3(f) schematically illustrates a security device according to an embodiment of the present invention; Figures 4(a) to 4(e) schematically illustrate a security device according to a further embodiment of the invention; Figures 5(a) and 5(b) schematically illustrate a security device according to a further embodiment of the invention; Figure 5A schematically illustrates a security device according to a further embodiment of the invention; Figures 6(a) to 6(f) schematically illustrate a security device of a further embodiment of the invention; Figures 7(a) to 7(f) schematically illustrate a security device of a further embodiment of the invention; Figure 8(a) to 8(e) show portions of the security device depicted in Figures 7(a) to 7(f) in more detail; Figure 9 schematically illustrates a security device in accordance with a further embodiment of the invention; Figure 10 schematically illustrates, in cross-section, a device according to a further embodiment of the invention; Figure 11 is a flow diagram setting out the principal steps of a method according to an embodiment of the present invention; Figure 12 is a flow diagram setting out the principal steps of a method according to a further embodiment of the present invention; Figures 13,14 and 15 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 16 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 For clarity of explanation, various figures herein use different shading patterns or colours to schematically illustrate the positions where image elements of different sets of image elements are present. The use of shading does not necessarily indicate the arrangement or colour of the image layer material forming the image elements. Cross-sectional diagrams schematically illustrate the position of the image layer material if it is required to be present at any particular location to form the respective image. Figure 1 schematically illustrates, in plan view, a security document 1000, here in the form of a banknote, carrying a conventional lenticular security device 101 known in the art. Figure 2 schematically illustrates a cross-sectional view of the device 101 along the line Q-Q’. The device 101 comprises a transparent substrate 10, which may or may not be the base substrate of the document. On a first side 10a of the substrate 10 there is disposed an array 20 of cylindrical lenses 21 that extend parallel to each other and into the plane of the page (along the z-dimension). On the opposing side 10b of the substrate, the device 101 comprises an image layer 30 comprising a plurality of image elements formed of image layer material (e.g. ink) that in combination form the images exhibited by the device. The thickness, T, of the substrate 10 substantially corresponds to the focal length of the lenses 21 such that the image layer 30 is formed substantially within the focal plane of the lens array 20. In this example, the image layer 30 is formed as a single image layer. The device of Figure 2 is a two-channel device comprising a set of first image channel positions ii and a set of second image channel positions i2 interleaved with each other periodically along the x-direction. The set of first image channel positions i1 together form a first image channel, and the set of second image channel positions together form a second image channel. As would be understood by the skilled reader, each image channel position of a particular image channel occupies the same relative position under the respective lens. Here, in the view of Figure 2, each first image channel position i1 of the first image channel occupies the “left” half of the respective lens corresponding to viewing angle 91, and each second image channel position i2 of the second image channel occupies the “right” half of the respective lens corresponding to viewing angle 92. In this example, each image channel position is in the form of an elongate line element extending parallel with the direction of elongation of the cylindrical lenses (i.e. along the z-direction). In this way, the image channel positions of the image layer 30 and the array of lenses 20 cooperate with each other such that at a first viewing angle 91, light from the first image channel is directed to the viewer, and at a second viewing angle 92, light from the second image channel is directed to the viewer. In the device 101, both the first and second image channels are occupied by image elements formed of image layer material that in combination defines a respective image. The occupation of each image channel position by a respective image element is represented in the cross section of Figure 2 by shading. The first image channel positions are occupied by image elements that together define image 11 viewable at viewing angle 91, and the second image channel positions are occupied by image elements that together define image I2 viewable at viewing angle 02. Consequently, the device 101 exhibits an “image switch” optically variable effect upon tilting between viewing angles 91 and 92. Figures 3(a) to 3(f) schematically illustrate a lenticular security device 100 according to an embodiment of the invention. Figure 3(a) illustrates the optically variable effect exhibited by the device, which in this example is an “on / off switch” effect between an image 11 exhibited at the first viewing angle 91, and a “blank”, or “off’ view at viewing angle 92. In other words, no image is exhibited at viewing angle 92. The image 11 exhibited by the image elements occupying the first image channel is a multi-tonal image of a turtle. The image layer 30 comprises five regions R1, R2, R3, R4 and R5 (illustrated in Figures 3(b) to 3(f) respectively), each region being located within the lateral area of the image 11. The configuration of the image elements is different in each region such that the regions exhibit different perceived density, or “tone”, as will now be described in further detail. Each of Figures 3(b) to 3(f) illustrates both a cross sectional view through the device 100 (with the substrate omitted for clarity purposes), and a plan view of the image layer 30 showing the arrangement of the image elements in more detail. Here, the diagrams indicate the presence or absence of each image element, rather than necessarily the specific form of each element required to form the image. Each of Figures 3(b) to 3(f) shows a portion of the respective region (R1 to R5) of the image layer 30. We now consider Figure 3(b) which illustrates the region R1 having the darkest tone. As the device is a two-channel device, each lens of the lens array 20 is associated with a first image channel position and a second image channel position. Each second image channel position i2 is blank, thereby exhibiting the “blank” visual effect at viewing angle 92 shown in Figure 3(a). For brevity and clarity, only two second image channel positions i2 are labelled in Figure 3(b). In region R1, each first image channel position is occupied by an image element S1. Therefore, the image tone ratio for region R1 is 1:1. The region R1 may be considered to be comprised of a plurality of repeating zones Z1. The concept of repeating zones will be described in more detail with reference to Figures 3(c) to 3(f). Figure 3(c) schematically illustrates the configuration of the image layer 30 in the second region R2. In this region, a subset of the first image channel positions are unoccupied by first image elements. The unoccupied (or “off’) first image channel positions are shown at U1, with the image elements occupying the first image channel positions shown at S1. Consequently, region R2 will be perceived to have a lighter tone than region R1 due to the presence of the unoccupied first image channel positions across the region. The region R2 is comprised of a plurality of laterally contiguous repeating zones Z2. Each zone Z2 has the same arrangement of occupied (“on”) and unoccupied (“off’) image elements, and in this way are considered to be repeating, even if the specific arrangement of the image layer material in each zone differs (i.e. in accordance with the image). Thus, each zone Z2 exhibits the same image tone ratio, which here is 4:5; in otherwords, four “on” first image channel positions for every one “off’ image channel position. In this embodiment, the lens pitch is 60pm, and each zone overlaps with five lenses, with the “width” D of each zone being 300pm. Therefore, five different tone levels may be achieved, varying between image tone ratios of 1:1 and 1:5. If additional tone levels are desired, a convenient way of achieving this is to increase the size of each zone overlap with more focussing elements. Figures 3(d), 3(e) and 3(f) show, respectively, the arrangement of the image layer for the regions R3, R4 and R5 that contain increasing numbers of “off’ first image channel positions, and therefore exhibit increasingly lightertones. The image tone ratio of region R3 is 3:5, the image tone ratio of region R4 is 2:5, and the image tone ratio of region R5 is 1:5. It is noted that in this embodiment the same image layer material (e.g. the same ink) is used to form each image element of the image layer (although this is not essential). In this way, by forming (e.g. printing) regions of the image layer having different image tone ratios, it is possible to form an image layer that exhibits a multi-tonal image in a single manufacturing step, e.g. a single printing pass. Figures 4(a) to 4(e) schematically illustrate a device 100 according to an embodiment of the invention which exhibits the same optically variable effect as the device of Figure 3, but where the differences in perceived tone across the different regions of the exhibited image are achieved through a different configuration of the image layer 30. As with the device of Figure 3, each second image channel position is “off’ such that the device exhibits and “on / off’ optically variable effect. Region R1 exhibits an image tone ratio of 1:1 in the same manner as that of the Figure 3 device. However, in each of the regions R2 to R5, the image tone ratio is defined by the arrangement of the “on” and “off’ first image channel positions over the entire domain of the respective region, rather than through a plurality of repeating zones as in Figure 3. In other words, the “off’ image channel positions are not uniformly distributed across each region. With each of regions R2, R3 and R4, the image layer 30 comprises sub-regions 32 in which the first image channel positions are occupied by first image elements S1 (“on”), laterally spaced by a sub-region 34 in which each first image channel position is unoccupied by first image elements (“off’). Region R5, shown in Figure 4(e), comprises one subregion 32 in which the first image channel positions are occupied by first image elements S1, laterally adjacent a sub region 34 in which each first image channel position is unoccupied by first image elements. Although the arrangements depicted in Figures 4(b) to 4(e) achieve the desired image tone ratios, it is generally preferred to use a plurality of repeating zones, as illustrated in Figure 3, in order to achieve the desired image tone ratios, as this provides a more uniform distribution of colour across the corresponding region. It is noted that the arrangement of the “on” and “off” channels depicted in Figures 3 and 4 is schematic only, and used to illustrate the concept of the invention. It will be appreciated that in practice the “on” and “off’ channels will be arranged in dependence on the dimensions of the regions in order that the eye perceives a uniform tone across the corresponding region. Figures 5(a) and 5(b) illustrate a lenticular security device 100 according to a further embodiment of the invention. In this embodiment, the device 100 exhibits an “image switch” optically variable effect, with a first image 11 exhibited at the first viewing angle 01, and a second, different, image I2 exhibited at the second viewing angle 92. Here, the first image 11 is in the form of a red numerical character “5”, and the second image I2 is in the form of a yellow diamond. Figure 5(b) schematically illustrates the configuration of the image layer 30 in a similar manner to Figures 3 and 4. Here, the first image channel positions are occupied by first image elements (S1) formed by red ink, and the second image channel positions are occupied by second image elements (S2) formed by yellow ink. In this embodiment, different image tone ratios are used to reduce the difference in relative colour saturation between the two images 11 and I2 (the red ink forming the first image 11 having a greater saturation of colour than the yellow ink defining the second image I2). Thus, the first and second image channels are selectively populated with the corresponding image elements such that the first image channel exhibiting the first image has a lower image tone ratio than the second image channel exhibiting the second image. Figure 5(b) illustrates a portion of the image layer 30 which comprises a plurality of repeating zones Z. As illustrated, the first image channel comprises occupied first image channel positions (S1) and unoccupied first image channel positions (U1) in a first image tone ratio of 3:5. Conversely each second image channel position is occupied by a second image element (S2), thereby exhibiting a colour tone ratio of 1:1. In this way, as the device is viewed and the viewing angle changed, the two images 11 and I2 are perceived to have a reduced difference in colour density than would be the case if both image channels defined a colour tone ratio of 1:1. Figure 5A schematically illustrates an example security device 100 according to an embodiment of the present invention. The device exhibits an “on / off’ optically variable effect, with a multi-tonal image 11 of a pen nib exhibited at viewing angle 01, and a blank appearance exhibited at viewing angle 92 (not shown). In this example, the image layer 30 comprises a plurality of regions R2, R3, R4 having different first image tone ratios so as to exhibit a gradual change in tone between a first region R1 and laterally spaced second region R5, where the regions R1 and R5 exhibit different first image tone ratios (1:1 and 1:5 respectively). In this way, the plurality of (contiguous) regions R2, R3 and R4 define an intermediate section (“IS”) between the first and second regions R1, R5. The gradual change in tone (and, in this case, the accompanying 3D effect) across the intermediate section IS is seen in the image 11. In the example shown in Figure 5A, the first image tone ratio of the intermediate section successively changes from a “4:5” ratio in region R2 to “3:5” ratio in region R3 and finally to a “2:5” ratio in region R4. Figures 6(a) to 6(f) schematically illustrate a security device 200 according to a further embodiment of the invention, in which image elements of different colour are advantageously used to generate complex multi-coloured images through colour mixing. In a similar manner to Figure 3, Figure 6(a) illustrates the optically variable effect exhibited by the device 200, and Figures 6(b) to 6(f) illustrates the configuration of the image layer 30 providing the perceived effect. As shown in Figure 6(a), the device 200 exhibits an “on / off’ optically variable effect, with a multi-coloured image 11 being exhibited at the first viewing angle 91, and a blank appearance (no image) being exhibited at the second viewing angle 92. Thus, each second image channel position of the image layer is blank, or “off’. The first image channel positions are occupied with first image segments of different colours in order to generate multiple regions of the image that are perceived to have different colours. In this example, a first subset of the first image elements (S1 -1) have a red colour (e.g. formed by red ink), and a second subset of the first image elements (S1-2) have a yellow colour (e.g. formed by a yellow ink). The image layer 30 comprises four colour regions R1, R2, R3 and R4, each being located within the lateral area of the image, having different colour ratios of red and yellow image elements and therefore exhibiting colour mixing. The image layer 30 also comprises an area A1 (shown in Figure 6(b)) in which only image elements of the first subset (S1 -1) are present, and therefore does not exhibit colour mixing. Thus, the area A1 appears red. The configuration of the image layer 30 in each of the colour regions R1 to R4 that exhibit different resultant colours due to different colour ratios will now be described in further detail with reference to Figured 6(c) to 6(f). As shown in Figure 6(c), the first colour region R1 of the image layer 30 comprises a plurality of repeating colour zones Z1 each containing five first image channel positions. In each colour zone Z1, four of the first image channel positions are occupied by image elements S1-1 of the first subset, and one image channel position is occupied by an image element S1-2 of the second subset. This provides a colour ratio of 4:1 across the region R1. Consequently, the colour region R1 is perceived to exhibit a resultant colour due to the combination of the (red) first subset and (yellow) second subset of the first image elements. In this example, each colour zone has a dimension, D, in the direction of the interlacing, of 300pm. As shown in Figure 6(d), colour region R2 comprises a plurality of colour zones Z2 in which the colour ratio between the firstand second subsets of the first image elements is 3:2. In colour region R3, illustrated in Figure 6(e), the colour ratio in each of the colour zones Z3 is 2:3, and in colour region R4, the colour ratio in each of the colour zones Z4 is 1:4. Therefore, through the use of colour mixing of two colours (here red and yellow) in different ratios, the image 11 exhibited by the device is perceived to comprise a plurality of regions of different colour. In some alternative embodiments, the colour mixing effect provided by the invention may be used to generate an image 11 exhibiting a single uniform colour across its domain. For example, in some embodiments, colour region R2 (exhibiting a colour ratio of 3:2) may correspond to the entire image, so that the image 11 is perceived as a single uniform resultant colour due to colour mixing between the red and yellow inks. Such embodiments may be advantageous if the image layer of the device is desired to be formed in the same manufacturing step as print workings outside of the lateral area of the image layer, which print workings are formed using red and yellow inks. Figures 7(a) to 7(f) illustrate a security device 200 similar to the one described in Figure 6, but which exhibits an image switch effect upon tilting. The first image channel is substantially as described with reference to Figure 6 (thereby exhibiting the same multi-coloured image 11 at viewing angle 01), but here the second image channel is also occupied by image elements defining a second multi-coloured image I2 that is exhibited at viewing angle 92. In a first area A1 of the image layer (shown in Figure 7(b)), each of the first image channel positions is occupied by a first subset of first image elements (defined by red ink), and each of the second image channel positions is occupied by a first subset of second image elements (defined by yellow ink). This arrangement is shown more clearly in Figure 8(a), where the first subset of the first image elements are labelled S1-1, and the first subset of the second image elements are labelled S2-1. In the first colour region R1 of the image layer 30 that exhibits colour mixing (illustrated in Figure 7(c)), both of the first and second image channels contain image elements of both the respective first and second subsets, and the region comprises a plurality of repeating zones Z1. Figure 8(b) more clearly shows the configuration of the image elements within a zone Z1. The first image channel positions are occupied by first image elements of the first subset (S1 -1) defined by red ink, and first image elements of the second subset (labelled S1-2) which are defined by yellow ink. Similarly, the second image channel positions are occupied by second image elements of the first subset (S2-1) defined by yellow ink, and second image elements of the second subset (labelled S2-2) which are defined by red ink. In the first colour region R1, both the first and second image channels exhibit a colour ratio of 4:1. Colour region R2 of the image layer (illustrated in Figure 7(c)) comprises a plurality of repeating zones Z2 (a single repeating zone Z2 is shown in more detail in Figure 8(c)) in which both the first and second image channels exhibit a colour ratio of 3:2 Colour region R3 of the image layer (illustrated in Figure 7(d)) comprises a plurality of repeating zones Z3 (shown in more detail in Figure 8(d)) in which both the first and second image channels exhibit a colour ratio of 2:3. In colour region R4 (shown in Figure 7(e)), which comprises a plurality of repeating zones Z4 (shown in more detail in Figure 8(e), each of the first and second image channels exhibits a colour ratio of 1:4. In the example embodiment of Figure 7, the first subset of the first image elements are formed of the same (red) ink as the second subset of the second image elements, and the second subset of the first image elements are formed of the same (yellow) ink as the first subset of the second image elements. Furthermore, in each region of the image layer, the first and second image channels exhibit the same colour ratios. This produces a distinctive “inverse colour” effect when switching the viewing angle from 61 to 62 that is achieved when using only two inks. However, this symmetry between the first and second image channels is not essential, and in further embodiments, the colour(s) and colour ratios exhibited by the second image channel may differ from those of the first image channel. It is also envisaged that the second image may differ from the first image in graphical form as well as colour. Thus far, we have considered colour mixing within an image channel between first and second colours. In other words, the image elements occupying a particular image channel and defining an image comprise a first subset of the image elements exhibiting a first colour, and a second subset of the image elements exhibiting a second colour. It is envisaged that the image elements occupying a particular image channel may comprise more than two subsets of different colour, to thereby exhibit a muti-colour image utilising more complex colour mixing effects such as CMYK colour mixing. In such cases, the colour ratio within a particular region of the image layer would represent the relative ratios of the three (or more) subsets of image elements. For example, in an RGB system, a region of the first image layer which is desired to exhibit a yellow colour would have a colour ratio of 1:1:0. Figure 9 schematically illustrates an example security device 200 according to a further embodiment of the present invention. The device is similar to the device described in relation to Figure 6, exhibiting an “on / off’ optically variable effect between a multi-coloured image of a turtle 11 at viewing angle 01, and a blank appearance at the second viewing angle 92 (not shown). In this example, the image layer 30 comprises a plurality of colour regions that are used to exhibit a gradual change in colour between two areas of single colour (labelled A1 and A2), rather than an abrupt change between two colours. Figure 9 illustrates a portion of the image layer 30 exhibiting such a gradual change between a first area (A1) in which each first image channel position is occupied by (red) image elements of a first subset S1-1 and a second area (A2) in which each first image channel position is occupied by (yellow) image elements of a second subset S1-2. In between the areas A1 and A2, the image layer comprises a plurality of colour regions R1, R2, R3, R4, in which the colour ratio successively changes from a “4:1” ratio in colour region R1 (i.e. a greater mix of red to yellow) to a “1:4” ratio in colour region R4 (i.e. a greater mix of yellow to red). Here, colour region R1 defines a colour ratio of 4:1, colour region R2 defines a colour ratio of 3:2, colour region R3 defines a colour ratio of 2:3, and colour region R4 defines a colour ratio of 1:4. In this way, the eye perceives a smooth change in colourfrom red to yellow across the colour regions R1 to R4. The plurality of colour regions R1 to R4 may be seen to define an “intermediate colour section” (“ICS”) of the image layer. The visual effect of the ICS is shown in the image 11 illustrated in Figure 9, which illustrates two such intermediate colour sections. By gradually “blending” the colours between the two areas A1 and A2 (rather than exhibiting an abrupt change in colour between the two areas), the perceived image exhibits additional complexity which increases the difficulty of counterfeit. In the example of Figure 9 the two areas A1 and A2 either side of the intermediate colour section ICS only contain image elements of one subset (area A1 comprising only (red) image elements of the first subset and area A2 comprising only (yellow) image element of the second subset), and therefore do not exhibit colour mixing. However, this is not essential, and in other embodiments the intermediate colour section ICS may be located between two regions that display colour mixing. As has been described herein, in preferred embodiments the viewing elements are in the form of focussing elements such as lenses. In alternative embodiments of the invention, the device 100 may instead comprise an array of viewing elements in the form of a masking grid 90 (shown in Figure 10) 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 30 such that light from different image channels is directed to the viewer at different viewing angles, as shown. In the example of Figure 10, at the first viewing angle 01 light from the first image channel positions i1 is directed to the viewer, and at the second viewing angle 92 light from the second image channel positions i2 is directed to the viewer, as shown. Figure 11 is a flowchart outlining the steps of a preferred method 300 of forming a security device according to an embodiment of the invention. The steps of the method will be described in relation to the example device depicted in Figure 3. In step S301, the image(s) to be displayed by the device are provided. In this example, the device 100 to be formed exhibits a single image 11 at the first viewing angle 91, with a “blank” appearance being exhibited at the second viewing angle 92. Thus, the image 11 is provided in step S301. However, in general the method may also be used to manufacture security devices exhibiting two (or more) images in dependence on viewing angle, such as the device described with reference to Figure 5. In step S303, the regions of different tone, or perceived density, within the image(s) are determined. Here, the image 11 contains five regions of different tone as identified in Figures 3(b) to 3(f). In step S305, the identified regions of different tone are used to form an image layer template. The image layer template defines the population of the image channel positions within the regions of the image layer corresponding to the image regions identified in step S303. More specifically, the image layer template defines the image tone ratios of each region of the image layer (R1, R2, R3, R4, R5) corresponding to the identified regions of the image. In preferred embodiments, the template defines repeating zones within each region, with each zone defining the image layer positions to be occupied and unoccupied in order to achieve the required tone ratio. In some embodiments, the zones may have a predetermined size (e.g. corresponding to a predetermined number of viewing elements). The template may typically be in the form of a computer file. At step S307 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 web-based 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 S309 and S311 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. In step S311, the image layer is formed in accordance with the image layer template formed in step S305. As described above, in preferred cases the image layer is provided as a print working, formed by a printing technique, preferably a gravure, intaglio, micro-intaglio, flexographic or lithographic technique, ora 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. 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 S309 and S311 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 S309 and S311 are performed, additional steps could be performed, for example the provision of additional layers or security features on the substrate. Figure 12 is a flowchart outlining the steps of a preferred method 400 of forming a security device according to an embodiment of the invention. The steps of the method will be described in relation to the example device depicted in Figure 6. In step 401, the image(s) to be displayed by the device are provided. In this example, the device 200 to be formed exhibits a single multi-coloured image 11 at the first viewing angle 01, with a “blank” appearance being exhibited at the second viewing angle 92. Thus, the image 11 is provided in step S401. However, the method may also be used to manufacture security devices exhibiting two (or more) images in dependence on viewing angle, such as the device described with reference to Figure 7. In step S403, the region(s) of colour mixing within the image(s) are determined. Here, the image 11 contains four regions which utilise colour mixing, as depicted in Figures 7(c) to 7(f). Although the image 11 of Figure 6 includes a plurality of regions that adopt colour mixing in order to exhibit different colours, as has been discussed herein, in some embodiments the image(s) exhibited by the device may define a single, uniform colour that is achieved by uniform colour mixing across the domain of the image. Such embodiments may be particularly advantageous in use cases where the image layer material (e.g. ink) of different colours used to generate the resultant colour(s) exhibited in the image is used in print work(s) laterally spaced outside of the device. Thus, the image layer and the additional print works laterally separate from the security device may be applied in a single pass. In step S405, the identified region(s) of colour mixing are used to form an image layer template (e.g. in the form of a computer file). The image layer template defines the colour ratio of the first and second subsets of the first image segments of each colour region of the image layer (R1, R2, R3, R4) corresponding to the regions of the image identified in step S303. In preferred embodiments, the template defines repeating colour zones within each colour region, with each colour zone defining the image layer positions to be occupied by the first and second subsets in order to achieve the required colour ratio. In some embodiments, the colour zones may have a predetermined size (e.g. corresponding to a predetermined number of viewing elements). At step S407 a substrate is provided, and in steps S409 and S411 the viewing elements are applied and the image layer formed in accordance with the image layer template. Steps S407, S409 and S411 are performed in the same manner as described above with reference to steps S307, S309 and S311. 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 13 to 16. Figure 13 depicts an exemplary document of value 1500, here in the form of a banknote. Figure 13(a) shows the banknote in plan view whilst Figure 13(b) shows a cross-section of the same banknote along the line X-X' and Figure 13(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 13(b), a security device 100 is disposed within window 1502, with a focusing element array 20 arranged on one surface of the transparent substrate 10, and image layer 30 on the other. Figure 13(c) shows a variation in which a second security device 100’ is also provided on banknote 1500, in a second window 1502’. The arrangement of the second security device 100’ can be reversed so that its optically variable effect is viewable from the opposite side of the security document as that of device 100, if desired. It will be appreciated that, if desired, any or all of the windows 1502, 1502’ could instead be “half-windows”, in which an opacifying layer (e.g. 1505a or 1505b) is continued over all or part of the image layer 30. Depending on the opacity of the opacifying layers, the half-window region will tend to appear translucent relative to surrounding areas in which opacifying layers 1505a and 1505b are provided on both sides. In Figure 14 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 eitherside of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread 1601 in window regions 1602a,b,c of the banknote. Alternatively the window regions 1602a,b,c may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. It should be noted that it is not necessary for the window regions to be “full thickness” windows: the thread 1601 need only be exposed on one surface if preferred. For example, in some embodiments the windows are “half-thickness” windows, and the paper is continuous on the side of the image layer 30 with only the lens array 20 exposed. The security device is formed on the thread 1601, which comprises a transparent substrate, a focusing array 20 provided on one side and an image layer 30 provided on the other. Windows 1602a, 1602b, 1602c reveal parts of the device 100, which may be formed continuously along the thread. (In the illustration, the lens arrays are depicted as being discontinuous between each exposed region of the thread, although in practice typically this will not be the case and the lens arrays (and image layer) will be formed continuously along the thread.) Alternatively several security devices could be spaced from each other along the thread, as in the embodiment depicted, with different or identical images displayed by each. In Figure 15, 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 16 where Figures 16(a) and 16(b) show the front and rear sides of the document 1800 respectively, and Figure 16(c) is a cross section along line Z-Z’. Security article 1803 is a strip or band comprising a security device 100 according to any of the embodiments described above. The security article 1803 is formed into a security document 1800 comprising a fibrous substrate 1805, using a method described in EP-A-1141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (Figure 16(a)) and exposed in one or more windows 1802 on the opposite side of the document (Figure 16(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. Further exemplary embodiments of the present disclosure are set out in the following numbered clauses: Numbered Clause 1. A security device, comprising: a substrate; an array of viewing elements disposed in or on the substrate; and an image layer disposed in or on the substrate and overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, wherein the set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; wherein at least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein the image layer comprises at least one region that is fully contained within the lateral area of the first image and overlaps with a plurality of the viewing elements, said at least one region having a first image tone ratio that is the number of first image channel positions occupied by first image elements to the number of first image channel positions within the region; and wherein: (i) the image layer comprises first and second regions having different first image tone ratios; and / or (ii) the second image channel is occupied by a corresponding set of second image elements formed of image layer material that together defines a second image, and within said at least one region of the image layer, the first image tone ratio is different from a second image tone ratio that is the number of second image channel positions occupied by second image elements to the number of second image channel positions within the region. Numbered Clause 2. The security device of Numbered Clause 1, wherein each first image element is formed of the same image layer material. Numbered Clause 3. The security device of Numbered Clause 1 or Numbered Clause 2, wherein each second image element is formed of the same image layer material. Numbered Clause 4. The security device according to any of the preceding Numbered Clauses, wherein the at least one region comprises a plurality of repeating zones each exhibiting the first image tone ratio. Numbered Clause 5. The security device according to Numbered Clause 4, wherein both of the first and second regions comprise a plurality of repeating zones each exhibiting the respective first image tone ratio. Numbered Clause 6. The security device according to Numbered Clause 5, wherein the zones within both the first and second regions each comprise first image channel positions not occupied by first image elements. Numbered Clause 7. The security device of Numbered Clause 4, wherein the second image channel is occupied by a corresponding set of second image elements, and wherein said at least one region comprises a plurality of repeating zones each exhibiting the first image tone ratio and the second image tone ratio. Numbered Clause 8. The security device according to any of Numbered Clauses 4 to 7, wherein within the at least one region, each of the zones is laterally contiguous. Numbered Clause 9. The security device of any of Numbered Clauses 4 to 8, wherein each region has a minimum dimension of 300pm, preferably 500pm, more preferably 1mm. Numbered Clause 10. The security device of any of the preceding Numbered Clauses, wherein the image layer comprises a plurality of regions that define an intermediate section, the intermediate section being located between two regions of the image layer having different first image tone ratios, wherein each region within the intermediate section comprises a different first image tone ratio so as to exhibit a perceived change in tone between the two regions across the intermediate section. Numbered Clause 11. A security device, comprising: a substrate; an array of viewing elements disposed in or on the substrate; and an image layer disposed in or on the substrate and overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, wherein the set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; wherein at least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein a first subset of the first image elements are formed of image layer material exhibiting a first colour, and a second subset of the first image elements are formed of image layer material exhibiting a second colour different from the first colour; and the image layer comprises at least a first colour region that overlaps with a plurality of the viewing elements, the first colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a first colour ratio, whereby the first colour region is perceived to have a resultant colour due to a combination of the first and second colours. Numbered Clause 12. The security device of Numbered Clause 11, wherein the first colour region comprises a plurality of repeating first colour zones, each first colour zone exhibiting the first colour ratio. Numbered Clause 13. The security device of Numbered Clause 11 or Numbered Clause 12, wherein the image layer further comprises a second colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a second colour ratio; wherein the first and second colour ratios are different whereby the first and second colour regions are perceived to have different colours. Numbered Clause 14. The security device of Numbered Clause 13, wherein the second colour region comprises a plurality of repeating second colour zones, each second colour zone exhibiting the second colour ratio. Numbered Clause 15. The security device of any of Numbered Clauses 12 to 14, wherein each colour region has a minimum dimension of 300pm, preferably 500pm, more preferably 1mm. Numbered Clause 16. The security device of any of Numbered Clauses 11 to 15, wherein the second image channel is occupied by a corresponding set of second image elements formed of image layer material that together defines a second image, wherein a first subset of the second image elements are formed of image layer material exhibiting a third colour, and a second subset of the second image elements are formed of image layer material exhibiting a fourth colour different from the third colour, and wherein within at least one colour region of the image layer, the image layer comprises second image channel positions occupied by second image elements of the first subset and second image channel positions occupied by second image elements of the second subset in a second colour ratio, whereby the colour region is perceived to have a resultant colour due to a combination of the third and fourth colours. Numbered Clause 17. The security device of any of Numbered Clauses 10 to 16, wherein the image layer comprises a plurality of colour regions that define an intermediate colour section, the intermediate colour section being located between two portions of the image layer exhibiting different colours, wherein each colour region within the intermediate colour section comprises a different colour ratio so as to exhibit a perceived change in colour between the two portions across the intermediate colour section. Numbered Clause 18. The security device of any of the preceding Numbered Clauses, wherein at least one image exhibited by the device is in the form of indicia or an indicium, preferably one or more geometric shapes, letters, logos, currency signs or other symbols. Numbered Clause 19. The security device of any of the preceding Numbered Clauses, wherein the viewing elements are focussing elements adapted to focus light in one direction, preferably wherein the focussing elements are cylindrical focussing elements. Numbered Clause 20. The security device of any of the preceding Numbered Clauses, wherein the array of viewing elements comprises an array of lenses. Numbered Clause 21. The security device of any of the preceding Numbered Clauses, wherein the substrate is at least semi-transparent, and wherein the array of viewing elements is disposed in or on a first surface of the substrate and the image layer is disposed in or on a second, opposing surface of the substrate. Numbered Clause 22. The security device of any of the preceding Numbered Clauses, wherein the image layer is a provided as a print working, preferably printed by a gravure, intaglio screen, micro-intaglio, flexographic, lithographic or digital technique. Numbered Clause 23. A security article comprising the security device of any of the preceding Numbered Clauses, wherein the security article is preferably a security thread, strip, foil, insert, transfer element, label, patch, or a data page for a security document. Numbered Clause 24. A security document comprising a security device according to any of Numbered Clauses 1-22, or a security article according to Numbered Clause 23, wherein the security document is preferably a banknote, cheque, passport, identity card, driver’s licence, certificate of authenticity, fiscal stamp, or other document for securing value or personal identity. Numbered Clause 25. A method of manufacturing a security device, comprising: (a) providing a substrate; (b) applying an array of viewing elements to the substrate; and (c) forming an image layer in or on the substrate, the image layer overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, wherein the set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; wherein at least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein the image layer comprises at least one region that is fully contained within the lateral area of the first image and overlaps with a plurality of the viewing elements, said at least one region having a first image tone ratio that is the number of first image channel positions occupied by first image elements to the number of first image channel positions within the region; and wherein: (i) the image layer comprises first and second regions having different first image tone ratios; and / or (ii) the second image channel is occupied by a corresponding set of second image elements formed of image layer material that together defines a second image, and within said at least one region of the image layer, the first image tone ratio is different from a second image tone ratio that is the number of second image channel positions occupied by second image elements to the number of second image channel positions within the region. Numbered Clause 26. The method of Numbered Clause 25, further comprising, before step (c), forming an image layer template by identifying regions of different tone within the first image; and in step (c), forming the image layer in accordance with the image layer template. Numbered Clause 27. A method of manufacturing a security device, comprising: (a) providing a substrate; (b) applying an array of viewing elements to the substrate; and (c) forming an image layer in or on the substrate, the image layer overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, wherein the set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; wherein at least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein a first subset of the first image elements are formed of image layer material exhibiting a first colour, and a second subset of the first image elements are formed of image layer material exhibiting a second colour different from the first colour; and the image layer comprises at least a first colour region that overlaps with a plurality of the viewing elements, the first colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a first colour ratio, whereby the first colour region is perceived to have a resultant colour due to a combination of the first and second colours. Numbered Clause 28. The method of Numbered Clause 27, further comprising, before step (c), forming an image layer template by identifying one or more regions of colour mixing within the first image; and in step (c), forming the image layer in accordance with the image layer template. Numbered Clause 29. The method of any of Numbered Clauses 25 to 28, wherein the image layer is formed by a printing technique, preferably a gravure, intaglio, screen, micro-intaglio, flexographic, lithographic or digital technique. Numbered Clause 30. The method of any of Numbered Clauses 25 to 29, wherein the image is formed in a single print working. Numbered Clause 31. The method of any of Numbered Clauses 25 to 30, 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. Numbered Clause 32. The method of any of Numbered Clause 25 to 31, adapted to produce the security device of any of Numbered Clauses 1 to 22.

Claims

1. A security device, comprising:a substrate;an array of viewing elements disposed in or on the substrate; andan image layer disposed in or on the substrate and overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, whereinthe set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; whereinat least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein a first subset of the first image elements are formed of image layer material exhibiting a first colour, and a second subset of the first image elements are formed of image layer material exhibiting a second colour different from the first colour; andthe image layer comprises at least a first colour region that overlaps with a plurality of the viewing elements, the first colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a first colour ratio, whereby the first colour region is perceived to have a resultant colour due to a combination of the first and second colours.

2. The security device of claim 1, wherein the first colour region comprises a plurality of repeating first colour zones, each first colour zone exhibiting the first colour ratio.

3. The security device of claim 1 or claim 2, wherein the image layer further comprises a second colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a second colour ratio; whereinthe first and second colour ratios are different whereby the first and second colour regions are perceived to have different colours.4 The security device of claim 3, wherein the second colour region comprises a plurality of repeating second colour zones, each second colour zone exhibiting the second colour ratio.

5. The security device of any of claims 2 to 4, wherein each colour region has a minimum dimension of 300pm, preferably 500pm, more preferably 1mm.

6. The security device of any of claims 1 to 5, wherein the second image channel is occupied by a corresponding set of second image elements formed of image layer material that together defines a second image, wherein a first subset of the second image elements are formed of image layer material exhibiting a third colour, and a second subset of the second image elements are formed of image layer material exhibiting a fourth colour different from the third colour, and wherein within at least one colour region of the image layer, the image layer comprises second image channel positions occupied by second image elements of the first subset and second image channel positions occupied by second image elements of the second subset in a second colour ratio, whereby the colour region is perceived to have a resultant colour due to a combination of the third and fourth colours.

7. The security device of any of claims 1 to 6, wherein the image layer comprises a plurality of colour regions that define an intermediate colour section, the intermediate colour section being located between two portions of the image layer exhibiting different colours, wherein each colour region within theintermediate colour section comprises a different colour ratio so as to exhibit a perceived change in colour between the two portions across the intermediate colour section.

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

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

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

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

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

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

14. A security document comprising a security device according to any of claims 1-12, or a security article according to claim 13, wherein the security document is preferably a banknote, cheque, passport, identity card, driver’s licence, certificate of authenticity, fiscal stamp, or other document for securing value or personal identity.

15. A method of manufacturing a security device, comprising:(a) providing a substrate;(b) applying an array of viewing elements to the substrate; and(c) forming an image layer in or on the substrate, the image layer overlapping with the array of viewing elements, the image layer comprising a first image channel and a second image channel, the first image channel formed of a set of first image channel positions each associated with a corresponding viewing element and the second image channel formed of a set of second image channel positions each associated with a corresponding viewing element, whereinthe set of first image channel positions and the set of second image channel positions are interlaced with each other, whereby at a first range of viewing angles, light from the first image channel is directed to the viewer, and at a second range of viewing angles, light from the second image channel is directed to the viewer; whereinat least the first image channel is occupied by a corresponding set of first image elements formed of image layer material that together defines a first image; wherein a first subset of the first image elements are formed of image layer material exhibiting a first colour, and a second subset of the first image elements are formed of image layer material exhibiting a second colour different from the first colour; andthe image layer comprises at least a first colour region that overlaps with a plurality of the viewing elements, the first colour region comprising first image channel positions occupied by first image elements of the first subset and first image channel positions occupied by first image elements of the second subset in a first colour ratio, whereby the first colour region is perceived to have a resultant colour due to a combination of the first and second colours.

16. The method of claim 15, further comprising, before step (c), forming an image layer template by identifying one or more regions of colour mixing within the first image; andin step (c), forming the image layer in accordance with the image layer template.

17. The method of any of claims 15 to 16, wherein the image layer is formed by a printing technique, preferably a gravure, intaglio, screen, micro-intaglio, flexographic, lithographic or digital technique.5 18. The method of any of claims 15 to 17, wherein the image is formed in asingle print working.

19. The method of any of claims 15 to 18, wherein the viewing elements are applied to a first side of the substrate and the image layer is applied to a second, 10 opposing side of the substrate simultaneously at the same location along the substrate.

20. The method of any of claims 15 to 19, adapted to produce the security device of any of claims 1 to 12.A

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