EP4284655A1

EP4284655A1 - Security devices and methods of manufacture thereof

Info

Publication number: EP4284655A1
Application number: EP22703953.4A
Authority: EP
Inventors: Lawrence Commander
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2021-01-29
Filing date: 2022-01-28
Publication date: 2023-12-06
Also published as: GB202201145D0; GB202101267D0; GB2605869A; WO2022162380A1

Abstract

A security device comprising: a substrate (107) having an array of focusing features (101) such as lenses disposed on the substrate and arranged periodically, and a structure of a colour which comprises: an active zone (113) overlapping the array of focusing features and disposed in the focal plane thereof, the active zone having a periphery which defines an image, within which image extends an array of image elements (105) spaced from one another, wherein the pitch between the image elements is equal to the pitch between the focusing features. When the viewing angle varies, the tone of the image varies between a minimum tone and a maximum tone. The structure comprises a static zone (111) laterally adjacent the active zone and having a uniform tone that appears to the naked eye the same as the tone of the image at one of the maximum tone and the minimum tone.

Description

SECURITY DEVICES AND METHODS OF MANUFACTURE THEREOF

This invention relates to security devices, for example for use on objects such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other documents of value or personal identity. Methods of manufacturing such security devices are also disclosed.

Objects of value, and particularly security documents such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such objects are provided with a number of visible security devices for checking the authenticity of the object. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, Venetian blind devices, lenticular devices, moire interference devices and moire magnification devices, each of which generates a secure visual effect. Other known security devices include holograms, watermarks, embossings, perforations and the use of colour-shifting or luminescent I fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult, or impossible, to copy using available reproduction techniques such as photocopying. Security devices exhibiting non-visible effects such as magnetic materials may also be employed.

One class of security devices are those which produce an optically variable effect, meaning that the appearance of the device is different at different angles of view. Such devices are particularly effective since direct copies (e.g. photocopies) will not produce the optically variable effect and hence can be readily distinguished from genuine devices. Optically variable effects can be generated based on various different mechanisms, including holograms and other diffractive devices, and also devices which make use of focusing elements such as lenses, including moire magnifier devices and so-called lenticular devices. Moire magnifier devices (examples of which are described in EP-A-1695121 , WO- A-94/27254, US-B-7738175, WO-A-2011/107782 and WO-A-2011/107783) make use of an array of micro-focusing features (such as lenses or mirrors) and a corresponding array of microimage elements, wherein the pitches of the microfocusing features and the array of microimage elements and their relative locations are such that the array of micro-focusing features cooperates with the array of microimage elements to generate a magnified version of the microimage elements due to the moire effect. Each microimage element is a complete, miniature version of the image which is ultimately observed, and the array of focusing features acts to select and magnify a small portion of each underlying microimage element, which portions are combined by the human eye such that the whole, magnified image is visualised. This mechanism is sometimes referred to as “synthetic magnification”.

Lenticular devices on the other hand do not involve synthetic magnification. An array of focusing features, typically cylindrical lenses (although spherical or aspherical lenses can also be used), overlies 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 focusing elements, at each viewing angle, only a selected group of image slices, all from the same image, will be directed towards the viewer. In this way, different composite images can be viewed at different angles - or, if one of the images is “empty”, a single image will seem to appear and disappear upon tiling (i.e. switch “on” and “off’). In both cases it should be appreciated that no magnification typically takes place and the resulting image(s) which are observed will be of substantially the same size as that to which the underlying image slices are formed. Some examples of lenticular devices are described in US-A-4892336, WO-A-2011/051669, WO-A-2011051670, WO-A- 2012/027779 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 which are not possible using the moire magnifier technique.

Security devices such as these depend for their success significantly on the resolution with which the image element array can be formed. Since the security device must be thin in order to be incorporated into a document such as a banknote, any focusing features required to form a lenticular device must also be thin, which by their nature also limits their lateral dimensions. For example, lenses used in such security elements preferably have a width or diameter of 60 microns or less, e.g. 30 microns. (The focal length is usually similar to the width/diameter). In a lenticular device this leads to the requirement that each image element must have a width which is at most half the lens width. For example, in a “two channel” lenticular switch device which displays only two images (one across a first range of viewing angles and the other across the remaining viewing angles), where the lenses are of 30 micron width, each image element must have a width of 15 microns or less (note one of the images could be “empty”). More complicated lenticular effects such as animation, motion or 3D effects usually require more than two interlaced images and hence each element needs to be even finer in order to fit all of the image elements into the optical footprint of each lens. For instance, in a “six channel” device with six interlaced images, where the lenses are of 30 micron width, each image element must have a width of 5 microns or less.

Resolutions such as these are at the limit is what is achievable even with specialist printing techniques and alternative ways of forming image elements, such as providing relief structures. This and other factors lead to lenticular security devices being prone to suffering from “ghosting” or “cross-talk”. Rather than seeing a clean switch from a first image to a second upon tilting to a second viewing position (or equivalently seeing a first image disappear), the first image remains visible at the second viewing position, albeit at reduced intensity (i.e. a fainter tone). In a device in which the first image is intended to appear and disappear, this can significantly reduce the visual effectiveness of the device since the first image may remain visible to some extent at all viewing angles. In a device intended to display different images at different viewing angles, the result can be that both images are visible simultaneously and as a result neither can be viewed clearly. The consequence is a reduction in the security level of the device, because not only is it more difficult for a user to recognise the visual effect and judge whether it is an authentic device, but also it is easier for a poorly-made counterfeit to be mistaken for a genuine device.

In accordance with a first aspect of the invention, a security device is provided comprising: a substrate having a first side and a second side; an array of focusing features disposed on the first or second side of the substrate and arranged periodically along at least a first direction, each focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; and a first structure of a first colour disposed on or in the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone that, when viewed under the constant lighting conditions, appears to the naked eye substantially the same as the tone of the first first-colour image at one of the first-colour maximum tone and the first- colour minimum tone, whereby the tone of the first static zone substantially matches that of the first first-colour image in the first active zone when the viewing angle is such that the first first-colour image appears at said one of the first-colour maximum tone and the first-colour minimum tone.

By providing the (lenticular) security device with a static zone adjacent the active zone in which at least one optically variable image is present (the first first-colour image), and arranging the static zone to exhibit a tone which substantially matches that of the image in either its “on” or its “off” state (i.e. the first-colour maximum tone or the first-colour minimum tone), the detrimental visual effects resulting from ghosting are reduced or eliminated. This is because, from a first viewing position, the image will appear in a tone which contrasts with the background formed by the static zone and hence will be clearly visible. However, at a second viewing position, the image will appear substantially the same in tone as the static zone and hence will be perceived to form a continuation of the static zone - effectively, the first image will seem to have disappeared even though ghosting is occurring. Hence the user perceives a strong and fully demarcated switching effect upon tilting the device.

In some cases it will be more desirable for the tone of the static zone to match the the first-colour minimum tone since the static zone will by definition be relatively pale and unobtrusive. However, the present inventors have found that matching the tone of the static zone to the first-colour maximum tone may in some cases produce better results since at greater colour intensities the human eye is less well able to perceive small differences in the tone and so any unintentional mismatch between the tone of the static zone and the maximum first-colour tone is less likely to be noticed. In particular, if the active and static zones are surrounded by a pale background (as will typically be the case for a document which is mainly white or off-white), any perceptible contrast between the first- colour maximum tone of the active zone and the tone of the static zone(s) will be significantly less than that between the static zone and the background. This will make any mismatch between the tone of the static zone and the first-colour maximum tone less noticeable.

The first active zone is a portion of the first structure. However, by definition, and since the focusing features overlap the first active zone, the same zone of the security device as a whole will be optically active - i.e. exhibit an optically variable effect. There may be further optically active parts of the security device if further structures in other colours are provided as discussed below. The periphery of the first active zone may define only the first first-colour image, in which case its outline will match that of the image, and when viewed via the focussing features in the complete device, the whole of its interior will exhibit a uniform tone at any one viewing angle which changes between a first-colour minimum tone and a first- colour maximum tone upon tilting as described above. However, as explained below, more than one first-colour image could be provided within the first active zone and in this case the periphery may be modified accordingly. If each of the multiple first-colour images are configured to switch on and off at different viewing angles from one another, the whole first active zone will not exhibit uniform behaviour. Rather, parts of the first active zone containing a first image and parts of it containing a second image will simultaneously exhibit different tones from one another at some viewing angles. It is of course the maximum or minimum tone displayed via the focussing features upon varying the viewing angle by the parts of the first active zone containing only the first image which should be matched by the substantially uniform tone in the first static area (although the second image will typically exhibit the same maximum and minimum tones in any case, just at different viewing angles). Examples will be given below. It should be understood that the first static zone, and the substantially uniform tone it displays, is not optically variable. That is, its appearance remains the same at all viewing angles (provided the illumination conditions do not change). This remains the case whether or not the focussing features extend over the static zone (as will typically be the case) or not. The first structure, which provides colour to the first static zone, will typically be arranged as a screen or half-tone in the first static zone, at a scale which appears to present a continuous uniform tone to the naked eye. In this case, on a microscopic scale, the tone will of course be non- uniform (made up of spaced screen elements in the first colour, separated by the absence of that colour) but the screen elements will be too small to be individually resolvable by the naked eye and so the static zone will appear to have a uniform tone at the macroscopic (non-magnified) level. The pitch and orientation of the screen element array will be selected such that the screen elements do not cooperate with any overlapping focussing features to generate an optically variable effect. It will be appreciated that a structure arranged in accordance with a screen or half-tone pattern inherently exhibits a tone that is less than 100% of the tone of the colour of the material that forms (and more than 0%) since the tone perceived by the viewer is a result of the combination of the coloured elements and the absence of colour between them.

In this specification, the term “colour” is used primarily to refer to the hue of an object or image - for example, in CIELAB colour space (i.e. the CIE 1976 L*a*b* colour space), here we treat all points in the colour space having the same values of a* and b* as one “colour” but representing different “tones” of that colour depending on the value of L* (the brightness/darkness axis). For instance, light blue and dark blue are different tones of the same colour (blue). Note also that the term “colour” includes achromatic hues such as black, grey, white, silver etc., as well as chromatics such as red, blue, yellow, green, brown etc. The nature of the first structure will inevitably place a maximum limit of the tone of the first colour which can be exhibited by the security device. For instance, if the first structure is a print working formed by ink of a certain colour (e.g. bright red) having CIELAB colour space co-ordinates L* = Li , a* = ai and b* = bi , only this tone (Li) and tones lighter than it (L*>Li) of the same red hue (a* = ai and b* = bi) can be exhibited by the first first-colour image. Therefore, tones are generally described below relative to the inherent tone of the respective structure (e.g. the tone of a material from which the that structure is formed) unless otherwise indicated. For instance, a solid area of the first structure colour (e.g. a solid area of the bright red ink mentioned above) is described as having a tonal value of 100%. The absence of the colour has a tonal value of 0%. Intermediate values represent intermediate tones of the same colour, the higher the percentage the greater the intensity of the colour. Typically, the first-colour maximum tone exhibited by the device will be less than 100%, and/or the first colour minimum tone exhibited by the device will be greater than 0%. This is the result of ghosting, as described above.

As mentioned already, the substantially uniform tone of the first static zone should appear to the naked eye substantially the same as the tone of the first first-colour image at one of the first-colour maximum tone and the first-colour minimum tone, so that they substantially matches one another. In this specification, “substantially the same” tones are those which appear the same as one another in a cursory inspection (by the naked human eye) although they may not be an exact match under close examination. For example, in preferred embodiments, two tones will be considered substantially the same as one another if the Euclidean distance AE*_ab between them in CIELAB colour space (i.e. the CIE 1976 L*a*b* colour space) is less than 5, more preferably less than 3, still preferably less than 2.3. The value of AE*_ab is measured using the formula:

AE*_ab = V(AL*)² + (Aa*)² + ( b*)² where AL*, Aa* and Ab* are the distance between the two colours 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). Of course, Aa* and Ab* will be zero for two different tones of the same colour. The colour difference AE*_ab can be measured using any commercial spectrophotometer, such as those available from Hunterlab of Reston, Virginia, USA. It will be appreciated that the colour of the first image will need to be measured from the completed device (i.e. via the focussing elements), at the appropriate viewing angle.

It will be clear from the above that the security device is an example of a lenticular device. There is no intentional mismatch between the pitch of the image elements and that of the focussing features, and the orientation of the two arrays is also matched. The first-colour image elements of the first first-colour image may or may not be identical to one another, but they will all have the same position relative to the optical footprint within which the image element falls as one another. The above-described arrangement of the first-colour image elements of the first first- colour image and the regions between them amounts to interlacing of the first first- colour image with at least one as-yet unspecified alternative appearance represented by the regions, which may be blank (in which case the first first-colour image will seem to appear/disappear upon tilting). At least a portion of one first- colour image element of the first first-colour image and of one region is needed in the optical footprint of each focussing feature to enable this, but in practice typically at least a complete one of each will be so located. If the security device is a “two channel” device (i.e. switches between two appearances upon tilting), about 50 % of each optical footprint will typically be occupied by first-colour image element(s) of the first first-colour image, and the other about 50% by region(s) between them. Of course, more channels could be accommodated if desired, in order for the device to exhibit a more complex effect upon tilting. If two or more images are to be displayed by the device (described further below), the image elements corresponding to the additional image(s) can be placed in the regions between the first-colour image elements of the first first-colour image.

The first structure, which provides colour to the security device, can be embodied in a variety of ways and it should be noted that in many cases the first structure will be substantially two-dimensional with no material depth/height. For instance, in many preferred examples, the first structure is a print working comprising a layer of ink (or another marking material), patterned as appropriate. In this case, the first colour will be that of the material forming the first structure (e.g. the ink). However, any structure which exhibits colour could be used, irrespective of the mechanism by which the colour is generated, provided it enables a contrast between the presence of the colour and the absence of the colour to be viewed. More examples will be given below.

Where the term “on” is used in this specification, it should be understood that while this encompasses direct contact between the items in question, the term is not so limited. For example, the focussing features will be considered to be disposed “on” the first or second side of the substrate even if there is an (optional) intermediate layer such as a primer layer or a pedestal layer between the substrate and the focussing features. The term “on” also does not infer any particular orientation of the items (with respect to gravity): an item can be “on” the underneath surface or side surface of another item as well as being “on” its upper surface. Likewise, the term “overlapping” should not be taken to require any specific order or orientation of the overlapping items: an upper item can be “overlapped” by an underneath item and vice versa.

As mentioned above, it may be desirable to increase the complexity of the security device, and hence its security level, by configuring it to exhibit more than one image. This can be achieved through appropriate configuration of the first structure to provide a second image in the same first colour. In this preferred case, the periphery of the first active zone further defines a second first-colour image, within which second first-colour image extends a second array of first- colour image elements, wherein the first-colour image elements of the second first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the second first-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the second first-colour image contains at least a portion of a respective first-colour image element of the second first-colour image and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the second first-colour image, and wherein the pitch between the first-colour image elements of the second first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the second first-colour image appears varies between the first-colour minimum tone and the first-colour maximum tone.

Preferably, the first-colour image elements of the first first-colour image are offset from the first-colour image elements of the second first-colour image along the first direction such that the second first-colour image appears in the first-colour maximum tone at a different viewing angle to that at which the first first-colour image appears at the first-colour maximum tone. Here, “offset” means relative to the focussing elements, so that the first-colour image elements of the first first- colour image will occupy a first portion of each optical footprint, and the first-colour image elements of the second first-colour image will occupy a second (different) portion of each optical footprint, laterally offset from the first portion in the first direction. Note, not every optical footprint will contain both types of image element. Hence, as the device is tilted, the first first-colour image will preferably appear “on” when the second first-colour image appears “off’ and vice versa. If there are more than two first-colour images (e.g. in a device with three or more channels), each image may appear in a different tone at any one viewing angle, to create a more gradual effect. Each first-colour image is preferably different from one another, e.g. in terms of its information content, size and/or orientation.

In some preferred embodiments, the first and second first-colour images will be laterally offset and non-overlapping (e.g. spaced from or abutting one another). However, in other preferred embodiments, the first first-colour image and the second first-colour image at least partially overlap one another such that the optical footprint of each focusing feature in the area across which said images overlap one another contains at least a portion of a respective first-colour image element of each of said images. In other words, in the overlapping area, the image elements of the first and second first-colour images are interlaced, with those from the second first-colour image lying in the regions between the image elements from the first first-colour image. Any number of first-colour images could be interlaced in this way, limited only by the size of the optical footprints and the resolution at which the first structure can be formed.

Where the first and second first-colour images do at least partially overlap, the first structure may be a continuous solid area of the first colour (representing the image elements of the first first-colour image and those of the second first-colour image between them). Of course, this portion of the active area will therefore not appear to change in tone upon tilting of the device. However, it is still presenting a switch from the first first-colour image to the second first-colour image: they just happen to be the same in the overlapping area. The tone of each respective first-colour image can still be judged since this will be visible in the area of that image which does not overlap another first-colour image.

To further enhance the visual impact, complexity and hence security level of the device, it may be desirable to introduce at least a second colour (i.e. one with a different hue from the first colour). This could be in addition to or as an alternative to providing a second first-colour image. Hence, preferably the security device further comprise a second structure of a second colour on or in the substrate, the second structure comprising: a second active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the second active zone having a periphery which defines at least a first second-colour image, within which first second-colour image extends a first array of second-colour image elements, wherein the second-colour image elements of the first second-colour image are spaced from one another at least in the first direction by regions in which no second-colour image elements of the first second-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first second-colour image contains at least a portion of a respective second-colour image element and at least a portion of a region which spaces said second-colour image element from an adjacent second-colour image element of the first second- colour image, and wherein the pitch between the second-colour image elements of the first second-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first second-colour image appears varies between a second-colour minimum tone and a second-colour maximum tone, and a second static zone laterally adjacent the second active zone and configured as an area of a substantially uniform tone that, when viewed under the constant lighting conditions, appears to the naked eye substantially the same as the tone of the first second-colour image at one of the second-colour maximum tone and the second-colour minimum tone, whereby the tone of the second static zone substantially matches that of the first second-colour image in the second active zone when the viewing angle is such that the first second-colour image appears at said one of the second-colour maximum tone and the second-colour minimum tone.

The second structure has all the same characteristics as the first structure, but exhibits a different colour. Typically the second-colour maximum tone is less than 100% and/or the second colour minimum tone is greater than 0%. The first second-colour image could be the same as the first first-colour image (except for its colour) but is preferably different, e.g. in terms of its information content, size and/or orientation. Any number of additional structures in different colours, each having the above characteristics, could be provided. For example, particularly preferred implementations could have three such structures (e.g. red, green and blue structures, or cyan, magenta and yellow structures) or four such structures (e.g. cyan, magenta, yellow and black structures). It is also possible to provide multiple images within any one structure. For example the second structure could additionally define a second second-colour image in the same way as has been described in relation to the first structure providing a second first-colour image. Like the first static zone described above, the second static zone is not optically variable.

The first and second structures could be laterally offset from one another, e.g. spaced from or abutting one another, so that different portions of the device exhibit different colours. However, in preferred embodiments, the first structure and the second structure at least partially overlap one another. In this case, the user may perceive a mixed colour in some areas of the device.

Preferably, the first-colour image elements of the first first-colour image are offset from the second-colour image elements of the first second-colour image along the first direction such that the first first-colour image appears in the first-colour maximum tone at a different viewing angle to that at which the first second-colour image appears at the second-colour maximum tone. As above, “offset” here means relative to the focussing features so that the first-colour image elements of the first first-colour image will occupy a first portion of each optical footprint, and the second-colour image elements of the first second-colour image will occupy a second (different) portion of each optical footprint, laterally offset from the first portion in the first direction. Note, not every optical footprint will contain both types of image element. Hence, as the device is tilted, the first first-colour image will preferably appear “on” when the first second-colour image appears “off’ and vice versa.

In some preferred embodiments, the first first-colour image and the first second- colour image will be laterally offset and non-overlapping (e.g. spaced from or abutting one another). However, in other preferred embodiments, the first first- colour image and the second first-colour image at least partially overlap one another such that the optical footprint of each focusing feature in the area across which said images overlap one another contains at least a portion of a respective image element of each of said images. In other words, in the overlapping area, the image elements of the first first-colour image and of the first second-colour image are interlaced, with those from the first second-colour image lying in the regions between the image elements from the first first-colour image. For instance, the first image elements and the second image elements may be arranged alternately along the first direction. Any number of different colour images could be interlaced in this way, limited only by the size of the optical footprints and the resolution at which the respective structures can be formed.

In cases where the first and second structures at least partially overlap, preferably at least a part of their respective static zones overlap. Here, a user will perceive from the completed device a static tone which is a mix of the selected first-colour tone (matching either the first-colour maximum tone or the first-colour minimum tone) and the selected second-colour tone (matching either the first-colour maximum tone or the first-colour minimum tone). However the individual selected tones provided by each of the first and second structures separately will still act to reduce or eliminate the effects of ghosting in the same manner as explained above: the provision of an extra colour does not diminish this. The area across which the first static zone and the second static zone overlap one another may be referred to, and can be regarded as, a “composite static zone” in which both the first colour and the second colour are present. Preferably, the first and second structures are arranged such that the first static zone at least partially overlaps the second active zone and/or the second static zone at least partially overlaps the first active zone. Specifically, the first static zone will typically continue across those portions of the second active zone which are not overlapped by the first active zone, and vice versa. Preferably, the first and second structures are arranged such that the first active zone at least partially overlaps the second active zone, as described above.

The security device could include a single instance of the first first-colour image. However, preferably the periphery of the first active zone is shaped such that the first first-colour image is repeated a plurality of times across the security device. This has been found to create a particularly strong visual impact and hence to increase the security level, since by showing the same image multiple times simultaneously, a user is better able to make out the information content of that image. This is particularly important if the security device is liable to be held in a non-flat position during inspection (as is often the case in practice) since by providing multiple repeats, the likelihood that at least some of the images will be viewed at the intended angles is increased. If a second first-colour image is provided, the periphery of the first active zone is preferably shaped such that the second first-colour image is repeated a plurality times across the security device. This is particularly desirable where the first and second first-colour images are laterally offset from one another, and preferably non-overlapping. In this way, upon tilting, the device appears to switch between a first pattern of first first-colour images and an adjacent, preferably interwoven second pattern of second first- colour images.

If a second structure in a second colour is provided, preferably the periphery of the second active zone is shaped such that the first second-colour image is repeated a plurality of times across the security device. Again, it is particularly desirable here that the first first-colour image and the first second-colour images are laterally offset from one another, and preferably non-overlapping.

In especially advantageous embodiments, the repeats of the first first-colour image and/or the second first-colour image and/or the first second-colour image are arranged in accordance with a regular pattern, wherein preferably the regular pattern is periodic in at least the first direction (and preferably also in a second orthogonal direction). Such an arrangement has been found to create a particularly strong, easily recognisable and describable visual effect. Most preferably, the repeats of the first first-colour image and/or the second first-colour image and/or the first second-colour image are arranged in a tessellating or pseudo-tessellating pattern. (Note the pattern could include other optically active images too). A “tessellating” pattern is one in which a notional plane is covered without gaps or overlaps by congruent images of one type or a few types. A “pseudo-tessellating” pattern is one which appears, under cursory and quick inspection by a casual observer, to be a tessellating pattern, but in fact there are gaps between some of the images which are only apparent under close inspection. Preferably the area occupied by these gaps are less than the area occupied by the images. For instance in preferred examples, the (optically active) images may occupy at least 50% of the pattern area, preferably at least 60%, more preferably at least 70%, still more preferably at least 80% and most preferably at least 90%. Also, in a pseudo-tessellating pattern, the images will typically only incompletely abut one another. That is, they may contact one another at points or along sections of their boundaries, but any one image will not not share all of its border with its neighbour(s). In other words, there will be portions of the image boundaries which are not common to two or more of the images.

In another particularly advantageous embodiment, the first first-colour image is bounded by a keyline which does not match the appearance of the tone of the first static zone when viewed under the constant lighting conditions at substantially all viewing angles. A keyline is a contrasting outline provided to an image in order to assist user-recognition of the image. Typically the keyline will have a width in the range of 50 to 500 micrometres (pm), preferably 75 to 200 pm. The keyline is static and hence does not appear or disappear upon tilting of the device. In combination with the static zone, the keyline helps to call out the portion of the device where the user should expect to see an appearance change upon tilting and therefore helps to draw attention to the security device. Its static nature, adjacent to the optically variable first first-colour image, can also make it appear three-dimensional to a degree, or at least to sit on a different visual plane from the images. While a keyline can be provided in any of the implementations described above, it has been found particularly advantageous to include a keyline in embodiments where the first first-colour image is repeated a plurality of times across the security device, particularly in a periodic manner, especially if it is in a tessellating or pseudo-tessellating pattern. In such cases, the keyline assists the user in picking out one instance of the first first-colour image (or more, if a keyline is provided to more than one instance of it), and so to recognise the content of the repeating image even where there is no keyline.

A keyline can be formed in various different ways, including applying a print to either surface of the substrate, or by laser engraving. However, most preferably the keyline is integral to the first structure and advantageously the first colour is absent in the keyline. That is, the first structure does not exhibit the first colour along the keyline (although image elements of the first colour will of course still be provided inside the keyline, as before, in order to define the first first-colour image). Corresponding keylines can, if desired, be provided to any second first- colour image and/or to any first second-colour image.

As mentioned above, advantageously, in the first and/or second structure the respective static zone is formed by an array of screen elements, the screen elements preferably being arranged in accordance with a halftone pattern. The pitch and orientation of the pattern will be selected such that there is no cooperation with the focussing features (should they extend over the static zone) and hence the zone remains optically invariable upon tilting. The first-colour tone exhibited by the static zone is thus determined by the proportion of its area covered by parts of the first structure which exhibit the first colour as opposed to those which do not. The substantially uniform tone is typically less than 100% and/or greater than 0%, where 100% is the tone of a solid area of the first colour. In preferred examples, in the first static zone, the substantially uniform (first colour) tone is in the range 5% to 35% or 75% to 100%, preferably 20% to 30% or 80% to 100%, more preferably 20% to 30% or 80% to 90%, where 100% is the tone of a solid area of the first colour. Typically, the first-colour minimum tone will lie in the range 5% to 35% and the first-colour maximum tone will lie in the range 75% to 100% so to achieve matching, the static zone may be provided with a first- colour tone within one of these ranges. Likewise, in the second static zone (if provided), the substantially uniform tone is preferably in the range 5% to 35% or 75% to 100%, preferably 20% to 30% or 80% to 100%, more preferably 20% to 30% or 80% to 90%, where 100% is the tone of a solid area of the second colour.

As indicated above, the first static zone is laterally offset from and adjacent to the first active zone, and the two do not overlap. Preferably the first static zone abuts at least a portion of the periphery of the first active zone defining the first first- colour image or is spaced from it by no more than the width of a keyline (if provided) so that when the tones are matched, the first first-colour image and the first static zone appear substantially continuous. In particularly preferred embodiments, the first static zone surrounds the first active zone and preferably abuts (or nearly abuts, if there is a keyline) the whole periphery, so that the first static zone provides a background to the first-colour image(s).

While the focussing features need only be provided over the active zone(s) of the structure(s), in practice they are typically provided over a wider area and may be present across the whole device. This avoids the need to provide accurate translational register between the focussing features and the structure(s). Thus in preferred embodiments, the array of focusing features extends across at least part of the first static zone and/or, if provided, the second static zone.

The focusing features could take various different forms. In some preferred embodiments, the focussing features comprise elongate focusing features each extending along a (second) direction that is non-parallel to the first direction, preferably perpendicular to the first direction. For instance, these could be formed by cylindrical focussing features, or a line of spherical/aspherical focussing features extending along the second direction. Both options will be suitable where the security device is a one-dimensional lenticular device, meaning that it exhibits optical variability when the viewing angle is changed in one direction only (rotation about the direction perpendicular to the first direction). If the device is a two- dimensional lenticular device (exhibiting optical variability in two directions), the focussing feature array will need to have two-dimensional periodicity and be able to redirect light in two directions accordingly, e.g. spherical or aspherical focussing features. The focussing features can be produced by known means such as embossing or cast-curing, described further below, 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.

It will be appreciated that if the focussing features are disposed on the opposite side of the substrate from the first structure, the substrate will need to be at least semi-transparent (i.e. 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. The focussing features can instead be arranged to focus on the same side of the substrate as that on which they are disposed, e.g. by building an optical spacing into their design or providing an at least semi-transparent pedestal layer between the substrate and the focussing features, in which case the first structure can be provided on the same side of the substrate as the focussing elements and the substrate itself can be of any type, opaque or otherwise. This includes paper substrates although polymer substrates are still preferred.

In preferred embodiments of one-dimensional devices, the first image elements and/or, if provided, the second image elements, are elongate elements extending along a (second) direction non-parallel to the first direction, preferably perpendicular to the first direction. For example, the image elements may be rectilinear line elements with a varying length depending on the image in question. Typically the line elements will extend from one periphery of the image to the opposite periphery along the second direction. Thus, unless the opposite peripheries of the image are parallel to one another, the line elements will not all be identical to one another as they will have different lengths.

The first structure (and, if provided, the second structure) can be formed in various different ways. As mentioned above, in particularly preferred embodiments, each structure is provided by a print working, preferably printed by a gravure, intaglio, micro-intaglio, flexographic or (wet or dry) lithographic technique, or by a digital printing technique, for example inkjet printing. By a “print working” it is meant any structure of ink or another marking material laid down on a surface. The material could be laid down selectively in a pattern or all-over and then patterned by removing or masking certain portions of the material. In cases where multiple structures of different colours are provided, these may ultimately be applied to the device as a single multi-coloured print working which has been collected on an offset cylinder or transfer blanket for example. The individual colour components of that working correspond to the distinct structures described above. Alternatively, each colour structure could be applied as a separate print working (e.g. directly from a dedicated colour print master). The ink or other marking material may be curable or non-curable. Using techniques such as gravure, intaglio, flexographic or (wet or dry) lithographic printing, the achievable resolution is affected by several factors, including the viscosity, wettability and chemistry of the ink, as well as the surface energy, unevenness and wicking ability of the substrate, all of which lead to ink spreading. With careful design and implementation, such techniques can be used to print pattern elements with a line width of between 25 pm and 50 pm. For example, with gravure or wet lithographic printing it is possible to achieve line widths down to about 15 pm.

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

In alternative embodiments, the or each structure could be provided by any of:

• A laser marking;

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

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

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

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

The first aspect of the invention further provides a security article comprising the security device described above, wherein the security article is formed as a security thread, strip, foil, insert, label or patch. Security articles such as these, carrying the security device, can then be applied to or incorporated in a security document or any other object, e.g. by hot stamping, cold stamping, via adhesive or lamination, or by introduction during papermaking. Examples will be provided below.

The first aspect of the invention further provides a security document comprising the security device described above, wherein preferably the security document is selected from banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other documents 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.

The first aspect of the invention further provides a method of manufacturing a security device, the method comprising:

(a) providing a substrate having a first side and a second side;

(b) applying an array of focusing features to the first or second side of the substrate, wherein the focusing features are arranged periodically along at least a first direction, each focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; and

(c) forming a first structure of a first colour on or in the first or second side of the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone that, when viewed under the constant lighting conditions, appears to the naked eye substantially the same as the tone of the first first-colour image at one of the first-colour maximum tone and the first- colour minimum tone, whereby the tone of the first static zone substantially matches that of the first first-colour image in the first active zone when the viewing angle is such that the first first-colour image appears at said one of the first-colour maximum intensity tone and the first-colour minimum tone.

The result of the method is a security device of the sort already described above, with all the attendant benefits. Any of the preferred features described above could be provided via appropriate adaptation of the method.

The tone to be exhibited by the first static zone could be selected in any convenient way which achieves the desired matching. However, in a preferred implementation, the method further comprises, before step (b): providing a test device comprising: a test substrate having a first side and a second side; an array of test focusing features on the first or second side of the test substrate, wherein the test focusing features are arranged periodically along at least a test array direction, each test focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; and a test structure of the first colour on or in the first or second side of the test substrate, the test structure comprising: one or more test active zones overlapping the array of test focusing features and disposed substantially in the focal plane thereof, the or each test active zone having a periphery which defines at least a test image, within which test image extends an array of test image elements, wherein the test image elements of the test image are spaced from one another at least in the test array direction by regions in which no test image elements are present and arranged such that the optical footprint of each test focusing feature overlapping the test image contains at least a portion of a respective test image element and at least a portion of a region which spaces said test image element from an adjacent test image element of the test image, and wherein the pitch between the test image elements of the test image along the test array direction is substantially equal to the pitch between the test focusing features along the test array direction such that, as the viewing angle is varied while the test device is viewed under constant lighting conditions, the tone in which the test image appears varies between the first-colour minimum tone and the first-colour maximum tone, and a plurality of test static zones laterally adjacent the test active zone(s) and each configured as an area of a different respective substantially uniform tone of the first colour; identifying one of the test static zones having a tone that most closely matches the first-colour minimum tone or the first-colour maximum tone when viewed under the constant lighting conditions, the tone of the identified zone being designated the selected tone; and in step (c), producing the first-colour static zone in the selected tone.

In this process, the tone to be exhibited by the first static zone is selected by producing a test device with much the same characteristics as that of the security device to be manufactured, but having multiple test static zones, each exhibiting a different substantially uniform first-colour tone. For example, the test device could include nine test static zones, with respective tones of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% (where 100% corresponds to a solid area of the first colour). Which of the test static zones most closely matches the first- colour minimum tone and the first-colour maximum tone can then be ascertained from the completed test device either by eye or by using a suitable colorimeter. The tone of the identified test static zone is then selected as the tone in which the first static zone of the security device is to be made, in step (c) of the manufacturing method. Of course, any number of test static zones and corresponding test tones could be deployed on the test device. The aim of the test colour measurement is to find a static print halftone value (e.g. 10%) which matches either the “on” of the “off’ state of the active areas of a secure lenticular (when printing both areas with the same ink). To do this, it is preferred to create (in the test device) a print matrix of active areas immediately neighbouring areas of static print each with a uniform halftone value. The print in the active area needs to be representative of the density that would be used in practice.

The active area will by definition vary in intensity with viewing angle but, in addition, the illumination angle will also effect the apparent colour (selectively illuminating either printed or unprinted areas). If the viewing and illumination positions are working co-operatively then the apparent contrast will be enhanced whereas conversely if they are working in opposition then the contrast will be diminished. These two factors affect both any attempt at colour measurement and similarly any visual assessment. To mitigate this problem, it is preferred to create active areas with bands varying through the “on” and “off’ states so that both states can be seen simultaneously at any angle. Colour measurement equipment is not normally set-up to cope with optically variable colours so a visual assessment is usually the preferred method. The preferred method is then to assess which static halftone sufficiently matches the active “on” or “off’ states such that the boundary between the two neighbouring areas is no longer perceptible. To make the assessment more controlled, the sample can be scanned on a desktop scanner so that all areas are captured with uniform illumination. This may require some test steps to align the lenticular sample to the best orientation for the scanner (i.e. that orientation where illumination and the image capture sensor are working cooperatively).

The chosen matches (i.e. halftone value vs active “on” or “off’) should be the same for all devices where the structure is the same (i.e. same lens geometry, materials used and print to lens separation) and the printed linewidth is the same. If these are not varied then the same selected halftone value can be used for all design variants. If desired, a second structure of a second colour can be applied to the substrate, as discussed previously. The tone provided in the second static zone can be determined by forming a test device in the second colour, and selecting the best matching test static zone, in the same manner as described above for the first static zone of the first structure. Preferably, the first structure and the second structure are formed on or in the substrate in register with one another. This can be achieved, for example, by applying the two structures to the substrate in one in-line process. Any number of structures in corresponding colours can be applied (e.g. RGB, CMY, CMYK etc).

It is not essential for the focussing elements and image elements to be translationally registered to one another, although any relative skew is preferably eliminated. Without register along the first direction, the described optically variable effects will still be exhibited although the particular viewing angles at which the various images will appear “on” and “off” will not be controlled. Therefore in other preferred embodiments, the focusing features are applied in register with the image elements of the first and/or second structure, at least along the first direction. One particularly preferred manner in which highly accurate register can be achieved is where the focusing features are applied to the first side of the substrate and the first and/or second structures are applied to the second side of the substrate simultaneously, at the same location along the substrate.

As described above, in preferred cases the first and/or second structure is provided as a print working, formed by a printing technique, preferably a gravure, intaglio, micro-intaglio, flexographic or lithographic technique. However, in other embodiments, the first and/or second structure 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 onon 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 disclosed herein (or image element structures, in some embodiments) are described in WO-A-2018/153840 and WO-A-2017/009616. In particular, the focussing features can be formed by the in-line casting devices detailed in WO-A-2018/153840 (e.g. that designated 80 in Figure 4 thereof), using an embossing tool 85 carrying an appropriately designed micro-optical structure from which can be cast the desired shape. Similarly, the cast-curing apparatuses and methods disclosed in section 2.1 of WO-A-2017/009616 (e.g. in Figures 4 to 8 thereof) can also be used to form the presently disclosed focussing features.

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

Suitable curable materials are disclosed in WO-A-2017/009616, section 2.1 . UV- curable materials are most preferred. Curing of the material(s) preferably takes place while the casting tool is in contact with the curable material, against the substrate. WO-A-2018/153840 and WO-A-2017/009616 also disclose print stations, which may be disposed downstream of the above-described casting apparatus (but alternatively could be located upstream). Print stations such as these are suitable for applying one or more print workings, which may be used to form the first and/or second structures of the present invention, to the same side of the substrate as the focussing features, or to the opposite side. The apparatus disclosed in WO- A-2018/153840 can achieve particularly high registration between such cast focussing features and the printed image elements.

According to a second aspect of the invention, a security device is provided comprising: a substrate having a first side and a second side; an array of focusing features disposed on the first or second side of the substrate and arranged periodically along at least a first direction, each focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; and a first structure of a first colour disposed on or in the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone in the range 5% to 35% or 75% to 100%, where 100% is the tone of a solid area of the first colour.

The security device of the second aspect of the invention has substantially the same construction as that of the first aspect of the invention, however the tone of the first static zone may not match the first-colour minimum tone or the first-colour maximum tone. Nonetheless, since the first-colour minimum tone will typically lie in the range 5% to 35% and the first-colour maximum tone will typically lie in the range 75% to 100%, the provision of a static zone having a uniform tone in one of these ranges will still reduce the detrimental effects of ghosting even if it is not eliminated entirely. Of course, it is still desirable for the tones to match as closely as possible. In preferred cases, the tone of the first static zone lies in the range 20% to 30% or 80% to 100%, more preferably 20% to 30% or 80% to 90%. In any case, typically the first-colour minimum tone is greater than 0% and/or the first colour maximum tone is less than 100%.

Any of the preferred features of the security device according to the first aspect of the invention described above can likewise be provided to the security device according to the second aspect of the invention.

For example, in preferred embodiments, the security device further comprises a second structure of a second colour disposed on or in the substrate, the second structure comprising: a second active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the second active zone having a periphery which defines at least a first second-colour image, within which first second-colour image extends a first array of second-colour image elements, wherein the second-colour image elements of the first second-colour image are spaced from one another at least in the first direction by regions in which no second-colour image elements of the first second-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first second-colour image contains at least a portion of a respective second-colour image element and at least a portion of a region which spaces said second-colour image element from an adjacent second-colour image element of the first second- colour image, and wherein the pitch between the second-colour image elements of the first second-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first second-colour image appears varies between a second-colour minimum tone and a second-colour maximum tone, and a second static zone laterally adjacent the second active zone and configured as an area of a substantially uniform tone in the range 5% to 35% or 75% to 100%, where 100% is the tone of a solid area of the second colour.

As noted above, in some preferred embodiments of the first aspect of the invention, the first and second structures are arranged such that the first static zone at least partially (preferably fully) overlaps the second active zone and/or the second static zone at least partially (preferably fully) overlaps the first active zone. This feature may also be present in some preferred embodiments of the second aspect and in those embodiments provides the same advantages noted above. However, in other preferred embodiments of the second aspect, the first static zone does not overlap the second active zone and the second static zone does not overlap the first active zone. Arranging the first and second static zones such that they do not overlap with the second and first active zones respectively provides an alternative way of reducing the perceptibility of the ghost images that appear when the active zones are viewed in their “off’ states, i.e. when they appear to the viewer at the first-colour minimum tone or the second colourminimum tone. The inventors have observed that while forming the two static zones so as each to overlap the active zone of the other structure does effectively conceal the active zones when viewed in whichever of their “on” or “off’ states (i.e. at their respective maximum or minimum tones) the static zones are configured to (approximately) match, the combination of multiple overlapping static zones can in some cases result in a high coverage of ink across the substrate. This results in the background area (i.e. the parts of the substrate area not occupied by any of the active zones) having a relatively intense colour and can in some cases limit the visual contrast between the “on” or “off” state (whichever is intended to be perceptible to the viewer) and the background.

Arranging the static zones of each of the first and second structures such that they do not overlap with the active zone of the other structure enables the implementation of a different approach to reducing the perceptibility of the ghost images: the inventors have realised that configuring the two static zones such that they together form, where they overlap one another, a colour and tone that is intermediate between the colours and tones of the two ghost images, a low visual contrast between both ghost images and this composite background can be achieved. Hence, in preferred embodiments, the first static zone and the second static zone overlap one another, wherein the overlapping parts of the first static zone and the second static zone together form a composite static zone comprising the first colour and the second colour. Because this composite static zone comprises both the first colour and the second colour, it inevitably does not exactly match the colour of either ghost image. However, it has been found that the two static zones can be configured such that the resulting composite static zone exhibits a sufficiently low visual contrast with each ghost image that the visual effect of the active zones appearing to blend into the background when viewed in their “off’ states can be achieved. In these embodiments, typically the tone of the composite static zone, i.e. that of the first static zone and the second static zone in combination, will be comparable to those of the first-colour and second-colour minimum tones. The background tone will therefore typically be lower than in embodiments where each static zone does overlap the active zone of the other structure, so these embodiments are particularly advantageous where a pale background is desired. In preferred implementations, the tone of the composite static zone is less than or equal to 60%, preferably less than or equal to 40%, more preferably less or equal to than 30%, most preferably less than or equal to 20%. As noted previously, where we specify percentage values for a “tone”, we mean the proportion of the substrate in the area in question on which the colour is present, where a tone of 100% means that the substrate is completely covered by the colour, 0% means that the colour is not at all present, and intermediate values represent partial coverage of the area by the colour (for example by the colour being arranged in accordance with a half-tone pattern). The tone of the composite static zone in this context thus means the proportion of the composite static zone that is covered by either colour: for example, the combined tone being 60% means that 60% of the area of the composite static zone is covered by either the first colour or the second colour and 40% is uncovered; it being 40% means that 40% of the area is covered and 60% is uncovered; it being 30% means that 30% of the area is covered and 70% is uncovered; and so on.

Since the combined tone depends on the proportion of the area covered by the two colours in combination, it will be appreciated that the same combined tone can be achieved by providing the first colour and the second colour in varying proportions (e.g. by varying the size and/or density of dots, in embodiments where the first and second static zones are formed in accordance with half-tone patterns). However, it is preferred that the ratio of the first colour to the second colour in the composite static zone is in the range of 40:60 to 60:40, preferably substantially 50:50. It has been found that the provision of similar quantities of the first and second colours in the composite static zone achieves a close visual match between the composite static zone and each of the first-colour minimum tone and the second-colour minimum tone.

The second aspect of the invention also provides a security article comprising a security device according to the second aspect, wherein the security article is formed as a security thread, strip, foil, insert, label or patch. Security articles such as these, carrying the security device, can then be applied to or incorporated in a security document or any other object, e.g. by hot stamping, cold stamping, via adhesive or lamination, or by introduction during papermaking. Examples will be provided below.

The second aspect of the invention further provides a security document comprising the security device according to the second aspect, wherein preferably the security document is selected from banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other documents 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.

The second aspect of the invention further provides a method of manufacturing a security device, the method comprising:

(a) providing a substrate having a first side and a second side;

(c) forming a first structure of a first colour on or in the first or second side of the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone in the range 5% to 35% or 75% to 100%, where 100% is the tone of a solid area of the first colour.

A third aspect of the invention provides a security device comprising: a substrate having a first side and a second side; an array of focusing features disposed on the first or second side of the substrate and arranged periodically along at least a first direction, each focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; a first structure of a first colour disposed on or in the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and the first structure further comprising a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone; and a second structure of a second colour disposed on or in the substrate, the second structure comprising: a second active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the second active zone having a periphery which defines at least a first second-colour image, within which first second-colour image extends a first array of second-colour image elements, wherein the second-colour image elements of the first second-colour image are spaced from one another at least in the first direction by regions in which no second-colour image elements of the first second-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first second-colour image contains at least a portion of a respective second-colour image element and at least a portion of a region which spaces said second-colour image element from an adjacent second-colour image element of the first second- colour image, and wherein the pitch between the second-colour image elements of the first second-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first second-colour image appears varies between a second-colour minimum tone and a second-colour maximum tone, and the second structure further comprising a second static zone laterally adjacent the second active zone and configured as an area of a substantially uniform tone; wherein the first static zone does not overlap the second active zone and the second static zone does not overlap the first active zone; and wherein the first static zone and the second static zone overlap one another, the overlapping parts of the first static zone and the second static zone together forming a composite static zone comprising the first colour and the second colour, wherein the tone and colour of the composite static zone are between those of the first-colour minimum tone and the second-colour minimum tone.

The presence of the composite static zone in combination with the first and second active zones which are not overlapped by the first and second static zones has the advantages noted above with reference to preferred embodiments of the second aspect of the invention.

In preferred embodiments of the third aspect, the tone and/or colour of the composite static zone are substantially equidistant from those of the first-colour minimum tone and those of the second-colour minimum tone. By the tone and colour being “equidistant” it is meant that the corresponding values in CIE L*a*b* colour space (where L* corresponds to “tone” and a* and b* correspond to the colour) are such that each of the first-colour minimum tone and the second-colour minimum tone is separated from the colour and tone of the composite static zone by the same Euclidean distance AE.

Advantageously, the tone and/or colour of the composite static zone are separated from those of each of the first-colour minimum tone and the second-colour minimum tone in CIE L*a*b* colour space by a Euclidean distance AE less than or equal to 20, preferably less than or equal to 10. These values have been found to produce a sufficiently low visual contrast between the composite static zone and each of the first-colour minimum tone and the second-colour minimum tone that the required visual effect is convincingly achieved.

In preferred implementations, the tone of the composite static zone is in the range of 40:60 to 60:40, preferably substantially 50:50. As noted above with reference to the second aspect of the invention, providing the two colour in these ratios achieves a close visual match between the composite static zone and each of the first-colour minimum tone and the second-colour minimum tone. By “tone of the composite static zone” we mean combined tone of the first colour and the second colour in combination in the composite static zone, which is determined by the proportion of the area in question that is covered by the first and second colours.

Preferably the tone of the composite static zone is less than or equal to 60%, preferably less than or equal to 40%, more preferably less than or equal to 30%, most preferably less than or equal to 20%. This has the advantages noted previously with reference to preferred embodiments of the second aspect that include this feature.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of security devices, articles, documents and methods for the manufacture thereof in accordance with embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a cross-sectional view of an example of a security device in accordance with a first embodiment of the invention;

Figures 2(a) and 2(b) show the appearance of the security device of Figure 1 at different viewing angles, and Figure 2(c) shows a detailed view of a first structure forming part of the same security device;

Figure 3 shows a cross-sectional view of an example of a security device in accordance with a second embodiment of the invention;

Figures 4(a) and 4(b) show the appearance of the security device of Figure 3 at different viewing angles, and Figure 4(c) shows a detailed view of a first structure forming part of the same security device; Figure 5 shows a cross-sectional view of an example of a security device in accordance with a third embodiment of the invention;

Figures 6(a) and 6(b) show the appearance of the security device of Figure 3 at different viewing angles, and Figure 6(c) shows a detailed view of a first structure forming part of the same security device;

Figures 7(a) and 7(b) show an example of a security device in accordance with a fourth embodiment of the invention at two different viewing angles, and Figures 7(c) and 7(d) show an example of a security device in accordance with a fifth embodiment of the invention at two different viewing angles;

Figure 8 is a photograph of a security device in accordance with a sixth embodiment of the invention;

Figure 9 shows a cross-sectional view of an example of a security device in accordance with a seventh embodiment of the invention;

Figures 10(a) and 10(b) show the appearance of the security device of Figure 9 at different viewing angles;

Figure 11 shows a cross-sectional view of an example of a security device in accordance with an eighth embodiment of the invention;

Figures 12(a) and 12(b) show the appearance of the security device of Figure 11 at different viewing angles;

Figure 13 is a flowchart for an exemplary method of manufacturing a security device in accordance with an embodiment of the invention;

Figures 14(a), 14(b) and 14(c) show a test device suitable for use in methods in accordance with embodiments of the invention; Figures 15, 16 and 17 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 18 illustrates a further embodiment of a security document carrying a security device made in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section;

Figures 19(a) to 19(i) illustrate different examples of relief structures which may be used to define image elements and thereby form first structures as may be used in embodiments of the present invention;

Figure 20 shows a further example of a security device in accordance with an embodiment of the invention;

Figures 21(a) and 21 (b) show the appearance of the security device of Figure 20 at different viewing angles; and

Figures 22(a) to 22(c) show the structures of two colours present in the security device of Figure 20.

DETAILED DESCRIPTION

In the discussion that follows, several examples of security devices comprising coloured “structures” of the kind defined above will be presented. As noted above, print workings are the preferred choice of structure, in which case the structure comprise a layer of ink or other marking material, but the invention is not limited to these embodiments. Other suitable kinds of structure for providing colour(s) to the security device include embossed diffractive structures or other microscale structures capable of producing colours, laser-marked structures, and structures obtained by embossing the substrate in order to form depressions and then applying a marking material (e.g. ink or resin) in the depressions (e.g. by coating the embossed substrate then wiping the surface so that only the material in the depressions is retained).

The description of the embodiments below frequently refers the “tone” in which certain parts of the device appear to the viewer, as a result of the colour provided by each structure, either seen via the focussing features or viewed directly from the structure as will be made clear. It will be understood that each structure is binary and provides points where the respective colour is present and points where that colour is absent; nothing in-between. However, through the arrangement of those points the structure can be configured to appear to present intermediate tones of the colour at a macroscopic scale, i.e. to the naked eye. Hence a tone of 100% corresponds to how the part in question would appear to the viewer if it were formed by an area completely coated with the respective colour. A tone of 0% means that the colour is entirely absent in the area in question. Intermediate values correspond to how the area would appears given different levels of coverage of the colour. For example, a halftone pattern can in principle be configured to produce a tone of any value between these extremes by controlling the size and/or density of the elements of the pattern. As explained above, here the term “colour” refers to the hue exhibited by the structure, and different “tones” of the colour refer appearances having the same hue but different lightness/darkness levels. For instance, light blue and dark blue are different tones of the same colour (blue).

Figure 1 shows a cross-sectional view of an example of a security device 100 in accordance with a first embodiment of the invention. The security device includes a planar substrate 107. On a first side 107a of the substrate 107 is an array of focusing features 101 , such as lenses, which are arranged regularly along a first direction X parallel to the plane of the substrate 107. In this example the focusing features 101 are formed as cylindrical lenses which extend parallel to one another, parallel to the plane of the substrate 107, along a second direction Y perpendicular to the first direction X. Each focusing feature 101 has a respective optical footprint of which different parts are directed to the viewer in dependence on the viewing angle, and the array of focusing features 101 defines a focal plane that is spaced from the array along the Z direction. In this case, the thickness of the substrate 107, which is transparent in this example, (i.e. its dimension in the Z direction) is chosen such that the second surface 107b is substantially in this focal plane.

On a second side 107b of the substrate 107 is a first structure of a first colour which comprises an active zone 113 and a static zone 111. Here, the first structure takes the form of a first working of ink or another marking material. The periphery of the active zone 113 defines a first-colour image, which here is of the digit “5”. In the active zone 113, the first working provides an array of first-colour image elements 105, which are arranged periodically along the first direction X, each in the optical footprint of a respective focusing feature, and spaced from one another along the X direction by regions 105’ in which no first-colour image elements 105 are present. This is shown best in Figure 2(c). The pitch between the first-colour image elements 105 along the first direction X is the same as the pitch of the array of focusing features 101 , and each first-colour image element 105 extends along the second direction Y across the full extent of the first-colour image. As a result, one first-colour image element 105 and one of the regions 105’ not occupied by a first-colour image element 105 are present in the optical footprint of each focusing feature 101. Furthermore, because the pitch of the array of first-colour image elements 105 matches that of the array of focusing features 101 , the first-colour image elements 105 are each located in corresponding parts of the optical footprints of the focusing features 101 .

In an idealised scenario, the configuration of the active zone described above would mean that, at one viewing angle, all of the first-colour image elements 105 (and none of the regions that space the image elements 105 from one another) are directed towards an observer 01 viewing the device at that viewing angle. This results in the active zone appearing in an “on” state as a solid, uniform area filled by the first colour in substantially the same tone as would be observed if the focussing features 105 were not present and the active zone were simply completely coated with the first colour (i.e. 100%). Similarly, to an observer 02 viewing the device at a different viewing angle, only the regions 105’ that space the image elements 105 from one another should be visible and the first-colour image defined by the active zone 113 will appear in an “off’ state and as if it does not contain the first colour at all (or in other words, as if the tone of the first colour in the active zone were zero). Hence, as the viewing angle is varied, the active zone should appear to show an image that vanishes and reappears as the active zone changes between the “on” and “off’ states.

As noted above, in reality, devices of this kind include imperfections and other artefacts that diminish the visual contrast between the “on” and “off’ states of the active zone. These can include misalignments of the focusing features 101 and image elements 105, optical defects in the focusing features 101 , light scattering and/or internal reflections within the focusing features, imperfections in the formation of the coloured structure (e.g. due to ink being applied in areas of where the colour should not be present, in the case of a structure formed by a print working), structural defects that cause individual image elements to lie out of the focal plane of the array of focusing features, and variations in the lighting conditions under which the device is viewed (for example the colour, uniformity, directionality and intensity of the lighting). As a result of these imperfections, rather than switching instantaneously between the “on” and “off’ states, the tone in which the active zone appears to the viewer (viewed via the focussing features 101) typically varies between a first-colour maximum tone and a first-colour minimum tone as the viewing angle is varied under constant lighting conditions. The first-colour maximum tone is typically less than 100% (in other words, is less than the tone that would be seen if the active zone were completely covered with the first colour), and may for example have a value of around 90%. The first- colour minimum tone is a non-zero tone due to these effects, and may for example be about 20%. The first colour is therefore still perceptible even when the security device 100 is viewed at viewing angle(s) corresponding to the theoretical “off’ state, resulting in the appearance of a “ghost” image of the first-colour image. The present invention achieves the desired ‘disappearing’ effect by the provision of a static zone 111 as part of the first structure. The static zone is adjacent to the active zone 113 and is configured to appear as an area of substantially uniform tone which, in this example, matches the appearance the first-colour minimum tone when the complete device is viewed under at least one set of constant lighting conditions. Thus, when the first-colour image appears at the first-colour minimum tone (i.e. its “off’ state), the boundary between the active zone 113 and the static zone 111 becomes imperceptible to the observer. As noted previously, the “matching” of the tone of the static zone to the first-colour maximum or minimum tone can be judged objectively based on the Euclidean distance AE*ab between them in CIELAB colour space (i.e. the CIE 1976 L*a*b* colour space). As in this example, it is preferable that the static zone 111 abuts or is closely adjacent to the periphery of the active zone, to help conceal the boundary. It is also preferable that the static zone 111 surrounds the whole periphery of the active zone 113, as in this example, so that all parts of the first image appear to merge with the background at the desired viewing angle. However, it would also be possible to conceal only a part of the image by providing the static zone adjacent/abutting only part of the periphery if desired.

In the embodiment of Figure 1 , the first structure that comprises the active zone 113 and the static zone 111 is formed by a single print working of marking material, e.g. ink, of the first colour. The first-colour image elements 105 are formed as solid strips of the marking material (i.e. each one having a tone of 100%) extending along the second direction Y within the periphery of the active zone 113, and no ink is present between the first-colour image elements 105. In the static zone 111 , the ink is arranged as an array of screen pattern elements 103 arranged to appear to the naked eye as a continuous area of the first-colour minimum tone when viewed under the constant lighting conditions referred to above. While the array of focusing features 101 may optionally extend across the static zone 111 (as indicated by focusing features 101 illustrated by dashed lines), the screen pattern elements 103 of the static zone are arranged so that they do not exhibit any optically variable effects. While in this example the tone of the static zone 111 is matched to the first-colour minimum tone, this embodiment could be modified such that the static zone 111 is matched to the first-colour maximum tone. In that case, the first-colour image will, to the viewer, seem to disappear when the device 100 is viewed at the viewing angle at which it appears at the first-colour maximum tone (i.e. its “on” state).

It should also be noted that while matching of the first-colour minimum or maximum tone is most preferred, in alternative implementations the tones may not be precisely matched. Rather, the static zone could be configured to exhibit a substantially uniform tone in the range 5% to 35% preferably 20% to 30 or in the range 75% to 100%, preferably 80% to 100%. Since the minimum or maximum tone of the first image will typically fall within one of these ranges, this will still reduce the detrimental effects of ghosting even if it is not fully eliminated. This alternative approach can be applied to all embodiments described herein.

Figure 2(a) and 2(b) show the security device 100 of Figure 1 as it would appear to an observer viewing the device 100 under the constant lighting conditions at two different viewing angles. Figure 2(a) shows the device 100 as it appears when viewed at the viewing angle at which the first-colour image defined by the active zone 113 appears at the first-colour maximum tone (for example the viewing angle of the observer 01 shown in Figure 1). The first-colour image defined by the periphery of the active zone 113 is in the shape of a “5” and is strongly perceptible to the viewer at this angle because of the contrast between the first-colour maximum tone and the tone of the static zone 111 (i.e. the first-colour minimum tone). Figure 2(b) shows the security device 100 as it appears to the viewer at a viewing angle at which the first-colour image appears at the first-colour minimum tone (for example the viewing angle of the observer 02 shown in Figure 1). The periphery of the active zone 113 is indicated for illustrative purposes by the dashed line, but this is not perceptible to the viewer because the tone in which the first- colour image appears at this viewing angle matches that of the static zone 111. Thus, as the viewing angle is varied, the first-colour image seems to the viewer to disappear at the viewing angle(s) at which the first-colour image exhibits the first minimum tone.

Figure 2(c) shows a detailed plan view of the layout of the first working of the device of Figures 1-2(b). It can be seen that the first-colour image elements 105 extend parallel to one another along the second direction Y, and are periodically spaced along the first direction X. Also clearly visible here are the regions 105’ which space the first-colour image elements 105 from one another. In the static zone 111 , the screen pattern elements 103 are distributed substantially uniformly -for instance as a halftone pattern - so that, to the naked eye, the static zone 111 appears as an area of substantially uniform tone. The proportion of the static zone covered by the screen elements 103 as compared with the spaces between them will be selected as appropriate to exhibit the desired uniform tone. The pitch and orientation of the screen pattern elements 103 will be selected such that there is no co-operation with any focusing features, and hence no optically variable effect in the static zone 111.

For illustrative purposes, the width of the first-colour image elements 105 and the dimensions of the screen pattern elements 103 is greatly exaggerated in Figure 2(c). In reality, the dimensions of these features are typically on the order of microns and are therefore not individually perceptible to the naked eye. Likewise, the screen pattern has been shown schematically in the Figure as having a coverage of about 50%, which would result in a uniform tone exhibited by the static zone of about 50%. However, in practice, if the static zone is to match the first colour minimum tone, the selected tone will typically be in the range 5% to 35% preferably 20% to 30%. Alternatively, if the static zone is to match the first colour maximum tone, the selected tone will typically be in the range 75% to 100%, preferably 80% to 100%. The screen pattern will be adjusted accordingly.

It should be noted that, while in the embodiment of Figures 1 and 2, the focusing features 101 and structure (print working) are provided on opposite sides of a transparent substrate 107, this is not essential. In other cases, the focusing elements could be designed to include an optical spacing and/or could be located on a transparent pedestal layer provided between first surface 107a of the substrate and the focusing feature array 101. The focal plane could thus be arranged to coincide with the first surface 107a of the substrate 107 in which case the structure (print working) could be located on the same surface of the substrate 107 as the focussing features 101. In this case, the substrate 107 need not be transparent but could also be translucent or opaque. It should further be noted that the focussing elements could be formed as mirrors instead of lenses in which case the visual effects will be viewed from the opposite side of the device 300. These considerations apply to all embodiments.

Figure 3 shows a cross-sectional view of a security device 300 in accordance with a second embodiment of the invention. This device 300 has generally the same construction as that shown in Figures 1 , 2(a) and 2(b), but differs in that the periphery of the static zone defines two first-colour images: a first first-colour image 313a and a second first-colour image 313b. The first first-colour image is the “5” described previously and the second first-colour image is in the shape of a five-pointed star. In this example, the first and second first-colour images are spaced from one another on the substrate 107. Both first-colour images 313a, 313b are formed by first-colour image elements 105 as described above, but the first-colour image elements 105 of the second first-colour image 313b are offset along the first direction X with respect to those of the first first-colour image (and relative to the focusing features 101 ). This results in the second first-colour image 313b appearing at the first-colour maximum tone at a different viewing angle to that at which the first first-colour image 313a appears at that tone. Consequently, as viewing angle at which the device 300 is viewed varies, the first and second first-colour images 313a, 313b appear to alternately appear and disappear. Figures 4(a) and 4(b) show the device 300 as it appears when viewed at two different viewing angles. In Figure 4(a), the first first-colour image 313a is perceptible to the viewer while the second first-colour image 313b is imperceptible against the surrounding static zone 111. At a different viewing angle, shown in Figure 4(b), the second first-colour image 313b appears at the first-colour minimum tone and is hence imperceptible, while the first first-colour image 313a appears at the first-colour maximum tone and can be distinguished from the static zone 111 by the viewer.

Figure 4(c) shows in plan view the structure of the first colour in the security device 300 of Figure 3, again here in the form of a print working. It can be seen that the screen pattern elements 103 which form the static zone 111 are uniformly distributed across the static zone 111 , which results in the static zone 111 appearing to the viewer as an area of substantially uniform tone. Again, while this is schematically depicted with a coverage of about 50% (and hence a tone of about 50%), this will not typically be the case in practice as explained in relation to Figure 2(c). The first-colour image elements 105 of the first image 313a are offset along the first direction X with respect to those of the second image 313b so that the first and second images appear at the first-colour minimum and maximum tones at different viewing angles to one another. This can be seen most clearly if the positions of the image elements in the left-most point of the starshaped second image 313b are compared with those of the first image 313a immediately above and below. Like in the view of Figure 2(c), the size of the screen pattern elements 103 and first-colour image elements 105 is greatly exaggerated in this view for illustrative purposes.

Figures 5, 6(a) and 6(b) show a further example of a security device 600, which is a modified version of the device 300 shown in Figure 3. The first working again contains two first-colour images, a first first-colour image 513a and a second first- colour image 513b. In this example, however, the first-colour images 513a, 513b partially overlap one another. This means that, in the area overlap, the first-colour image elements 105 of the second first-colour image 513b are present in the regions 105’ that space the first-colour image elements 105 of the first first-colour image 513a from one another (and vice versa). The marking material that provides the first colour is therefore present across substantially all of the optical footprint of each focusing feature 101 in the area of overlap between the two images, and as a result, this area of overlap appears to the viewer to be “on” at substantially all viewing angles. Figures 6(a) and 6(b) show the security device 500 at the same viewing angles as are shown in Figures 4(a) and 4(b) respectively.

Figure 6(c) shows in plan view the working that is provided in the security device 500 of Figure 5. If the non-overlapping parts of the two images are compared, it can be seen that the first-colour image elements 105 that form the first first-colour image 513a are offset with respect to those of the second first-colour image 513b along the first direction X. As a result, in the area across which the two first-colour images overlap one another, the first-colour image elements 105 of the second first-colour image 513b are present in the regions that space the first-colour image elements of the first image 513a from one another. It should be appreciated that it is the minimum or maximum tone exhibited by the device in the non-overlapped regions of the first image 513a which needs to be matched by the substantially uniform tone of the static zone 511 . These maximum and minimum tones will be the same as those exhibited by the second image 513b on tilting. Again, whilst this is schematically shown at around 50%, this will typically not be the case in practice.

Figures 7(a) and 7(b) show a further example of a security device 700 in accordance with a fourth embodiment of the invention, at two different viewing angles. This security device 700 is constructed according to the principles illustrated by the foregoing embodiments and has a single working of the first colour which defines a static zone 711 and an active zone. The periphery of the active zone in this embodiment is shaped to define a first first-colour image 713a and a second first-colour image 713b (which is optional), each of which is repeated a plurality of times across the security device 700. The images are each in the shape of a pentagon, the differences between them being that (i) the array of first- colour image elements 105 that defines each repeat of the second first-colour image 713b is offset with respect to those of the first first-colour image 713a, meaning that the repeats of the second first-colour image 713b appear at the first- colour minimum tone at a different viewing angle to those of the first first-colour image 713aa, and (ii) the repeats of the second-first colour image 713b are oriented differently to those of the first first-colour image 713a (because the shape of the second first-colour image 713b is effectively that of the first first-colour image 713a rotated by 180 degrees).

The repeats of the first and second first-colour images 713a, 713b are arranged in a pseudo-tessellating pattern. “Pseudo-tessellating” in this context means that the first and second first-colour images 713a, 713b are able to be arranged in a regular pattern in which the images abut one another, but in which gaps appear between the repeats. Also, in a pseudo-tessellating pattern, the images will typically only incompletely abut one another. That is, they may contact one another at points or along sections of their boundaries, but any one image will not not share all of its border with its neighbour(s). In other words, there will be portions of the image boundaries which are not common to two or more of the images. Preferably the gaps are small enough, and occupy a minor area of the pattern, such that they are not noticeable to the casual observer and at first glance the pattern appears tessellating. For instance in preferred examples, the (optically active) images may occupy at least 50% of the pattern area, preferably at least 60%, more preferably at least 70%, still more preferably at least 80% and most preferably at least 90%. The static zone 711 extends across these gaps so that the repeats of the first and second first-colour images 713a, 713b so that the parts of the periphery of the active zone that shares a boundary with them is not perceptible when the images 713a, 713b appear at the first-colour minimum tone.

Figures 7(c) and 7(d) show a further example of a security device 750 in accordance with a fifth embodiment of the invention, at two different viewing angles. This security device 750 is similar to that of Figures 7(a) and 7(b) in that it includes a single working of a first colour in which the active zone is shaped to define a plurality of repeats of a first first-colour image 763a and a plurality of repeats of a second first-colour image 763b. The first-colour image elements 105 that form the first first-colour images 763a are offset with respect to those of the second first-colour images 763b, so that the second first-colour images 763a appear in the first-colour minimum tone at one viewing angle (shown in Figure 7(c)) and the first first-colour images 763a appear in the first-colour minimum tone at a different viewing angle (shown in Figure 7(d)). In this example, however, the images tessellate with one another so that there are no gaps and no isolated regions of the static zone 761 between the repeats of the images.

Figure 8 is a photograph of a sample security device 800 in accordance with a sixth embodiment of the invention. This security device 800 contains a single colour and its active zone is shaped to define a pseudo-tessellating pattern of first first-colour images 813 and second first-colour images 815 each shaped as pen nibs. A static zone 811 extends across the gaps between the repeats of the images and is formed with a tone that is matched to the first-colour minimum tone of the first and second first-colour images 813, 815. In this example, the first and second first-colour images 813, 815 occupy about 75% of the pattern area, and the gaps about 25%. At the viewing angle at which this image was taken, the first first-colour images 813 are in the “on” state, i.e. appear at a tone that is greater than the first-colour minimum tone and are therefore perceptible, and the second first-colour images are “off”, i.e. appear at a tone that is equal or close to the second-colour minimum tone. Some of the repeats of the first first-colour image in this embodiment are formed with a keyline 820. The keyline 820 is a narrow zone (typically having a width in the range of 50 to 500 pm, preferably 75 to 200 pm), around an instance of the first first-colour image in which the first colour is not perceptible to the viewer at substantially all viewing angles. One way to form such a keyline 820 is to simply omit the first colour from the first working. Alternatively, a keyline 820 can be formed by arranging an additional material such as an opaque ink or resin to prevent the viewer from seeing the first colour in corresponding area.

Figure 9 shows a cross-sectional view of a security device 900 in accordance with a seventh embedment of the invention. This security device 900 differs from the embodiments shown in the preceding Figures in that it includes a second structure of a second colour in addition to a first second of the first colour, each on the second surface 107b of the security device. For example, the second structure could be a patterned layer of ink in a second colour, such as a second working. The first working has a first active zone 913 and a first static zone 911 formed of the first-colour image elements 105 and screen pattern elements 103 of the kind described previously. The second working has a second active zone 917 shaped to define a second-colour image and a second static zone 915. In the second active zone 917, the second working has an array of second-colour image elements 905 arranged periodically along the first direction X with the same pitch as the focusing features 101. In the same way as the first-colour image defined by the first active zone 913 appears to vary between a first-colour minimum tone and a first-colour maximum tone as the viewing angle is varied, the second-colour image, when viewed under the same constant lighting conditions, appears to vary between a second-colour minimum tone and a second-colour maximum tone when. In the second static zone 915, the second working is formed in accordance with a pattern of screen elements 903 (e.g. a halftone pattern), which are arranged to appear to the viewer at the second-colour minimum tone with no substantially no optically variable effect. Like the first static zone 911 , the second static zone 915 could alternatively be configured to appear at the second-colour maximum tone. The area across which the first static zone 911 and the second static zone 915 overlap one another (for example in the left-hand region of Figure 9) can be regarded as a “composite static zone”.

The first static zone 911 extends across the entire second surface 107b, including across the second active zone 917 and second static zone 915, except for the first active zone 913. Similarly, the second static zone 915 extends across the entire second surface 107b except for the area of the second active zone 917. In the parts of the device 900 where the first static zone 911 and second static zone 915 overlap one another, the colour perceived by the viewer is the combination of (i) the first-colour minimum tone and (ii) the second-colour minimum tone. For example, if the first colour is cyan and the second colour is magenta, the area across which the static zones 911 , 915 overlap one another will appear blue. The same composite colour (e.g. blue) will appear to the viewer in the first active zone 915 when the device is viewed at a viewing angle at which the first image appears at the first-colour minimum tone. This is because the second static zone 915 extends across the first active zone 913, and at this viewing angle, the viewer sees the first active zone 913 in a colour that is the combination of the second colour at the second-colour minimum tone (contributed by the screened pattern in the second static zone 915) and the first colour at the first colour minimum tone (contributed by the first-colour image elements 105, which appear at the first- colour minimum tone at this viewing angle) - i.e. the same colour as that seen where the first and second static zones 911 , 915 overlap one another. Similarly, the area across which the second active zone 917 overlaps the first static zone 911 appears in this composite colour when viewed at a viewing angle at which the second active zone 915 appears at the second-colour minimum tone. Thanks to this configuration, the first and second images each appear in different colours when viewed in their respective “on” states but are each concealed when viewed in their respective “off’ states.

Figure 11 shows an example of a security device 1100 that represents a modified version of the device 900 of Figure 9. Like the Figure 9 embodiment, this device 1100 comprises first working of a first colour and a second working of a second colour. The first working has a first active zone 1113, shaped to define a first- colour image in the shape of a “5”, and a first static zone 1111. The second working has a second active zone 1117, shaped to define a second-colour image in the shape of a star, and a second static zone 1115. This embodiment differs from that of Figure 9 in that the first and second active zones 1113, 1117 partially overlap one another. The arrangement of the image elements of the two workings in the area of overlap is similar to that described with reference to the embodiment of Figure 5, in which two arrays of first-colour image elements of a single working overlap one another. In this embodiment, in the area of overlap between the first and second active zones 1113, 1117, the first-colour image elements 105 and the second-colour image elements 905 are arranged alternately along the first direction X such that the second-colour image elements are present in the regions that space the first-colour image elements 105 from one another, as shown best in Figure 12(e). The area of overlap appears different to the background provided by the overlapping static zones 1111 , 1115 at substantially all viewing angles, but the apparent colour of this area varies as the viewing angle is changed.

For example, when the device 1100 is viewed at an angle corresponding to the “off’ state of the first-colour image, the first-colour image appears at the first-colour minimum tone and the second-colour image appears at a tone that is greater than the second-colour minimum tone. The first image is hence concealed, because its tone matches that of the first colour in the surrounding first static zone 1111 , and the second image is perceptible to the viewer in a colour that is the combination of the first-colour minimum tone and the tone of the second colour in which the second-colour image appears. Similarly, when the viewing angle is such that the second-colour image appears at the second-colour minimum tone, it is concealed and the first-colour image is perceptible to the viewer. Figures 12(a) and 12(b) show the appearance of the security device 1100 at two different viewing angles: in Figure 12(a), the viewing angle is such that the second-colour image (shaped as a star) is “off’ and the first-colour image (shaped as a “5”) is perceptible. In Figure 12(b), which represents a different viewing angle, the second-colour image is perceptible and the first-colour image is concealed.

Figure 12(c) shows the layout of the first structure (e.g. print working) of the security device 1100 of this example. This working is identical to that illustrated in Figure 1(c): screen pattern elements 103 form the first static zone 111 and first- colour image elements 105 arranged periodically along the first direction X form the active zone 113, the periphery of which is shaped to define a first-colour image in the shape of a “5”. Figure 12(d) shows the second working, in which the second active zone 1117 has a periphery shaped to define a second-colour image in the shape of a star and contains second-colour image elements 905. Figure 12(e) shows the first and second workings overlapping one another as in the security device 1100. In the area of overlap, the second-colour image elements 905 are arranged in the regions that space the first-colour image elements 105 from one another. Like in the example of Figure 1 (c), the sizes of the first- and second-colour image elements 105, 905 and the screen pattern elements 103, 903 of the static zones in Figures 12(c)-12(e) are greatly exaggerated for illustrative purposes, and in reality the individual elements would not be discernible to the naked eye.

It will of course be appreciated that any number of different structures (e.g. print workings) of different colours could be provided in the device, in order to achieve even more complex multicolour effects. For example, in preferred implementations there may be three structures (e.g. RGB or CMY) or four structures (e.g. CMYK)

The embodiments of Figures 9-12(b) illustrate an important property of devices in accordance with embodiments of the invention comprising two or more structures (e.g. print workings) of different colours. While the presence of additional workings in different colours modifies the colour in which each image appears to the viewer, it does not affect the way in which the contrast between each image and the surrounding parts of the device varies in dependence on the viewing angle. In particular, the presence of other workings does not prevent each image from “disappearing” at viewing angles at which the tone of the image appears in the tone to which the respective static zone is matched.

As noted previously, the invention also provides a method of manufacturing a security device that is suitable for producing devices of the kind described previously. A flowchart depicting the steps of an example of a method in accordance with the invention is shown in Figure 13. The first step S1300 in the method is optional (as indicated by dashed lines) and involves providing a test device, an example of which is shown in Figure 14(a), which is a photograph of a real test device 1400 suitable for use in implementations of the present invention. Note what is shown is the appearance of the complete device, via the focussing features, at a single viewing angle. In this example, the test device comprises a plurality of active zones 1413a, 1413b, 1413c, 1413d and a number of static zones 1411a, 1415a, 1411 b, 1415b, 1411c, 1415c, 1411d and 1415d each arranged in abutment with a respective one of the active zones.

The static zones and active zones in this test device 1400 are formed by a working of a single colour. An array of focusing features extends across the entire device.

The active zones in this example are substantially identical to one another. Each active zone is configured in accordance with the principles described above with reference to Figures 1-8 to define an elongate strip, of which different sections appear “on” and “off” at any one viewing angle, and within the strip, star-shaped regions which appear “on” when the immediately surrounding part of the strip is “off (and vice versa). For example, in the area of the active zone 1413a labelled 1421 , the ‘strip’ part of the active zone is “on” and the star-shaped images within this strip are “off”. In the area labelled 1423, the strip is “off” and the stars are “on”. An enlarged view of this part of the device 1400 is shown in Figure 14(b).

Due to the presence of defects and imperfections of the kind described previously, ghost images of the images in the active zone 1413a are perceptible when the images are in their respective “off” states. For example, the area labelled 1425 in Figure 14(b) is in the “off” state but the boundary of the active zone is still perceptible because of the visual contrast between the tone of the ghost image and that of the static zone 1415a that abuts the active zone 1413a. Each part of the active zone 1413a appears to the viewer in a first-colour minimum tone when in the “off” state and a first-colour maximum tone when in the “on” state when viewed under at least one set of constant lighting conditions.

While is not essential that the shapes of the images defined by the active zone are the same as the images to be formed in the desired security device, the microscopic configuration of the active zone (in particular the width and spacing of the first-colour image elements) and the appearance of the regions between the image elements should be substantially the same as the corresponding properties in the device to be manufactured so that the test device accurately produces the tone of the ghost images expected to be produced by the security device. The construction (i.e. lens geometry, materials used and print-to-lens separation) of the test device should also be the same as to be deployed in the security device.

The static zones are each formed as a uniform strip of a different tone of the colour of the working between 0% and 100%. As explained previously, the static zone of each working in the security device to be manufactured should match either the respective maximum tone (in this case the first-colour maximum tone) or the respective minimum tone (in this case the first-colour minimum tone) of in which the active zone of that working appears as the viewing angle is varied under the constant lighting conditions. For example, the static zone labelled 1411a has a tone of 97% and the static zone labelled 1415a has a tone of 3%. The “off” part of the active zone 1413a in the area labelled 1425 appears darker than the adjacent part of the static zone 1415a, so the first-colour minimum tone must be greater than 3%. In a different area 1427 of the test device 1400, of which an enlarged view is shown in Figure 14(c), it can be seen that the static zone 1415c, which has a tone of 18%, appears visually very similar to an adjacent part of the active zone 1413c that is in the “off” state. Therefore, in this example, 18% could be selected as the tone of the static zone in a working of the colour that was used to produce the test device 1400. This would result in the tone of the static zone matching that of the first-colour minimum tone of the active zone.

As explained previously, when two or more overlapping workings of different colours are present, the tones of the static zones of each working do not need to be modified in order to achieve the required matching. Rather, the tone selected for the static zone of a single-colour device will also achieve the required matching effect when present in a multi-colour device (holding other factors such as the properties of the focusing features constant). Therefore, in order to select the tones required to produce a device with multiple workings, the tone for the static zone of each working can be selected simply based on a respective test device made with a single working of that colour. Using the test device 1400 of Figure 14(a), the tone for the static zone of the security device to be manufactured can be selected in optional step S1301. As noted previously, the selected tone could be matched to either the first-colour minimum tone or the first-colour maximum tone. Step S1301 could involve, for example, identifying the static zone on the test device 1400 which has the minimum Euclidean distance AE*_ab from the tone of the “off’ parts of the static zones, and may be considered to match if AE*_ab is less than a threshold value, for example 5.

Of course, the tone in which the static zone is to be manufactured could be selected in other ways, such as through computer modelling of the designed device to ascertain the expected maximum and/or minimum tones. Alternatively, as mentioned above, in some embodiments, matching is not essential and any tone either in the range 5% to 35% preferably 20% to 30 or in the range 75% to 100%, preferably 80% to 100%, could be selected for the static zone. This may not achieve precise matching but will still reduce the detrimental effects of ghosting.

In step S1302, a substrate is provided. The substrate could be provided in any form and as part of any suitable process for the manufacture of security documents, 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 S1303 and S1304, focusing features and a first structure of a first colour (for example a first working of the first colour) are respectively applied to the substrate. In preferred implementations the first structure (and any additional structures) are formed as print workings of the respective colours, but, as mentioned previously, the structures could be produced in other ways, e.g. as diffractive structures capable of producing colours. Optionally, in order to form a multi-colour device such as those shown in Figures 9-12(b), a second structure could be formed in step S1305. Again, step S1305 can be performed in any order with respect to steps S1303 and S1304. For example, the structure(s) and focusing features could be applied simultaneously. This can achieve highly precise register between the focusing features and structures. Before and/or after steps S1303, S1304 and S1305 are performed, additional steps could be performed, for example the provision of additional layers or security features on the substrate.

The security device produced by this method may be part of a security article, for example a security thread, strip, foil, insert, label or patch. The security article can be incorporated in or applied to security documents and other objects. The security device may also be incorporated in a security document, for example one selected from selected from banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other documents for securing value or personal identity.

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

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 element array (i.e. the one or more structures) and/or the complete security device can either be formed directly on the security document 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 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 15 to 18.

Figure 15 depicts an exemplary document of value 1500, here in the form of a banknote. Figure 15a shows the banknote in plan view whilst Figure 15b shows a cross-section of the same banknote along the line X-X' and Figure 15c 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 1502. Two opacifying layers 1505a and 1505b are applied to either side of the transparent substrate 1502, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 1502. 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 1 , T is located. In Figure 15(b), a security device 1 is disposed within window 1501 , with a focusing feature array 5 arranged on one surface of the transparent substrate 1502, and image element array 10 on the other (e.g. as in Figure 1 above). Figure 15(c) shows a variation in which a second security device 10’ is also provided on banknote 1500, in a second window 1502’. The arrangement of the second security device T can be reversed so that its optically variable effect is viewable from the opposite side of the security document as that of device 1 , if desired.

It will be appreciated that, if desired, any or all of the windows 1502, 1502’ could instead be “half-windows”, in which an opacifying layer (e.g. 1505a or 1505b) is continued over all or part of the image array 10. 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 16 the banknote 1600 is a conventional paper-based banknote provided with a security article 1601 in the form of a security thread, which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 1605a and 1605b lie on either side of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread 1601 in window regions 1602a, b,c of the banknote. Alternatively the window regions 1602a, b,c may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. It should be noted that it is not necessary for the window regions to be “full thickness” windows: the thread 1601 need only be exposed on one surface if preferred. The security device is formed on the thread 1601 , which comprises a transparent substrate a focusing array 5 provided on one side and an image array 10 provided on the other. Windows 1602 reveal parts of the device 1 , 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 arrays) 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 17, 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 1 is formed by an array of focusing features provided by a lens array 5 on one side of the strip substrate 1703, and an image array 10 on the other. The paper plies 1705a and 1705b are apertured across region 1702 to reveal the security device 1 , 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 18 where Figures 18(a) and 18(b) show the front and rear sides of the document 1800 respectively, and Figure 18(c) is a cross section along line Z-Z’. Security article 1803 is a strip or band comprising a security device 1 according to any of the embodiments described above. The security article 1803 is formed into a security document 1800 comprising a fibrous substrate, using a method described in EP-A-1141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (Figure 18(a)) and exposed in one or more windows 1802 on the opposite side of the document (Figure 18(b)). Again, the security device 1 is formed on the strip 1803, which comprises a transparent substrate with a lens array 5 formed on one surface and a co-operating image element array 10 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 1 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 element array 10 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 5 or in a separate procedure with the focusing array 5 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 utilised in generating the desired optical effects and the adhesive, since such contact can render the lenses inoperative. For example, the adhesive could be applied to the lens array(s) as a pattern that leaves an intended windowed zone of the lens array(s) uncoated, with the strip or patch then being applied in register (in the machine direction of the substrate) so the uncoated lens region registers with the substrate hole or window.

The security device of the current invention can be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.

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

The security device may comprise a metallic layer laterally spaced from the security feature of the current invention. The presence of a metallic layer can be used to conceal the presence of a machine readable dark magnetic layer. When a magnetic material is incorporated into the device the magnetic material can be applied in any design but common examples include the use of magnetic tramlines or the use of magnetic blocks to form a coded structure. Suitable magnetic materials include iron oxide pigments (Fe2O3 or Fe3O4), barium or strontium ferrites, iron, nickel, cobalt and alloys of these. In this context the term “alloy” includes materials such as Nickel:Cobalt, lron:Aluminium:Nickel:Cobalt and the like. Flake Nickel materials can be used; in addition Iron flake materials are suitable. Typical nickel flakes have lateral dimensions in the range 5-50 microns and a thickness less than 2 microns. Typical iron flakes have lateral dimensions in the range 10-30 microns and a thickness less than 2 microns.

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

Negative or positive indicia may be created in the metallic layer or any suitable opaque layer. One way to produce partially metallised/demetallised films in which no metal is present in controlled and clearly defined areas, is to selectively demetallise regions using a resist and etch technique such as is described in US- B-4652015. Other techniques for achieving similar effects are for example aluminium can be vacuum deposited through a mask, or aluminium can be selectively removed from a composite strip of a plastic carrier and aluminium using an excimer laser. The metallic regions may be alternatively provided by printing a metal effect ink having a metallic appearance such as Metalstar® inks sold by Eckart.

As noted previously, while print workings are a preferred kind of structure suitable for providing the active and static zones in embodiments of the invention, relief structures can also be utilised. It will be appreciated that where more than one structure exhibiting different respective colours are required, these could be embodied as different parts of a single relief structure with correspondingly different properties. A variety of different relief structures suitable for forming image elements in implementations of the present invention are shown in Figures 19a-19i. Thus, Figure 19a illustrates image regions of the image elements (IM) in the form of embossed or recessed regions while the non-embossed portions correspond to the non-imaged regions of the elements (N I ) , e.g. the regions which space the first-colour image elements from one another in the example of Figure 1. For instance, if the image displayed by the device is a solid, uniform colour block then the whole of each image element will be formed either of an image region (IM) or of a non-image region (Nl). Figure 19b illustrates image regions of the elements in the form of debossed lines or bumps. A coloured marking material (e.g. ink or resin) could be applied into the embossed portions in order to provide the image elements with the required colour, as described in WO-A-2005052650.

In another approach, the relief structures can be in the form of diffraction gratings (Figure 119c) or moth eye I fine pitch gratings (Figure 19d). Where the image elements are formed by diffraction gratings, then different portions of an image (within one image element or in different elements) can be formed by gratings with different characteristics. The difference may be in the pitch of the grating or rotation. This can be used to define the image content of either or both images IA, IB. A preferred method for writing such a grating would be to use electron beam writing techniques or dot matrix techniques. Such diffraction gratings for moth eye I fine pitch gratings can also be located on recesses or bumps such as those of Figures 19a and 19b, as shown in Figures 19e and 19f respectively.

Figure 19g illustrates the use of a simple scattering structure providing an achromatic effect.

Further, in some cases the recesses of Figure 19a could be provided with an ink or the debossed regions or bumps in Figure 19b could be provided with an ink. The latter is shown in Figure 19h where ink layers 1910 are provided on the bumps 1900. Thus each image element could be created by forming appropriate raised regions or bumps in a resin layer provided on a transparent substrate. This could be achieved for example by cast curing or embossing. A coloured ink is then transferred onto the raised regions typically using a lithographic, flexographic or gravure process. As explained previously, in some examples, two structures (or more) each of a different colour may be provided. The image elements of one structure could be printed with one colour and other image elements of the other could be printed with a second colour. Again, magnetic and/or conductive ink(s) could be utilised.

Finally, Figure 19i illustrates the use of an Aztec structure.

Figures 20 shows a cross-sectional view of an example of a security device 2000 in accordance with a further embodiment of the invention. Like the exemplary devices described previously (for example that of Figure 9), the security device 2000 includes a substrate 107 with a first surface 107a and a second surface 107b. On the first surface is an array of focusing features 101 as described previously with reference to Figure 1. On the second surface 107b is a first structure of a first colour which comprises a first active zone 2013 and a first static zone 2011 formed of screen pattern elements 2103. Also on the second surface 107b is a second structure of a second colour which comprises a second active zone 2017 and a second static zone 2015 formed of screen pattern elements 2003. Like in the Figure 9 embodiment, the peripheries of the first active zone 2013 and second active zone 2017 are configured in accordance with different images and the arrangement of the arrangement of the first-colour image elements 2105 and second-colour image elements 2005 is such that the two images appear “on” at different viewing angles (as shown in Figures 21 (a) and 21 (b)). However, in this embodiment, the first static zone 2011 and the second static zone 2015 do not overlap the second active zone 2017 and the first active zone 2013 respectively. Outside of the two active zones 2013, 2017, the first static zone 2011 and second static zone 2015 overlap one another, and in this area of overlap the two static zones 2011 , 2015 in combination form a composite static zone that comprises the first colour and the second colour.

Figure 21 (a) shows the security device 2000 as it would appear to a viewer at a viewing angle at which the second active zone is “off” (i.e. appears at the second- colour minimum tone) and the first active zone 2013 is “on” (i.e. appears at the first-colour maximum tone). Figure 21(b) shows the device 2000 at a different viewing angle, where the second active zone 2017 is “on” (i.e. appears at the second-colour maximum tone) and the first active zone 2013 is “off” (i.e. appears at the first-colour minimum tone). Because both the first colour and the second colour are present in the composite static zone 2101 while only the second colour is present in the second active zone 2017, the tones and colours of the two active zones in their “off” states do not precisely match those of the composite static zone 2101 . However, the tones of the first colour and second colour in the composite static zone 2101 are chosen such that the appearance of the composite static zone is reasonably closely matched to that of each of the active zones in their “off” states. This can be achieved by ensuring that the tone and colour of the composite static zone (i.e. the tone and colour achieved by the combination of the first and second structures in the area across which the first-colour static zone and second- colour static zone overlap one another) is within a specified Euclidean distance AE in CIE L*a*b* colour space of those of the first-colour minimum tone and the second-colour minimum tone, for example AE less than or equal to 15 (preferably less than or equal to 10) and greater than 5. Advantageously, the tone and colour of the composite background could be equidistant from those of the first-colour minimum tone and the second-colour minimum tone. The ratio of the first colour to the second colour in the composite static zone can be varied while still achieving the required effect, but it has been found that a ratio of substantially 50:50 (i.e. equal coverage of the two colours on the second surface 107b) consistently achieves a good intermediate appearance that effectively reduces the perceptibility of both ghost images. As noted previously, arranging the each static zone so as to be non-overlapping with the active zone of the other structure allows for the implementation of two different colour workings with a background that is not excessively saturated and thus strongly contrasts with the appearance of each image in its “on” state. The combined tone of the first-colour static zone and the second-colour static zone in combination in the composite static zone could be less than or equal to 20%, for example.

Figure 22(a) shows the second working of the Figure 21 device 2000. Like in previous embodiments, second-colour image elements 2005 are arranged in the second active zone, the periphery of which defines a first image (in this case the character “A”). The screen pattern elements 2003 of the second static zone 2017 are not present in the first active zone 2013. Figure 22(b) shows the first working of the Figure 21 device 2000. The image elements 2105 are arranged within the first active zone 2013, the periphery of which defines an image in the shape of the character “B”. The screen pattern elements 2103 of the first static zone 2013 do not extend into the second active zone 2017. Figure 22(c) shows the two workings as they are arranged on the second surface 107b of the substrate 107, where they are superimposed on one another such that the first static zone 2011 and second static zone 2015 in combination form a composite static zone 2101.

Claims

1. A security device comprising: a substrate having a first side and a second side; an array of focusing features disposed on the first or second side of the substrate and arranged periodically along at least a first direction, each focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; and a first structure of a first colour disposed on or in the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone that, when viewed under the constant lighting conditions, appears to the naked eye substantially the same as the tone of the first first-colour image at one of the first-colour maximum tone and the first- colour minimum tone, whereby the tone of the first static zone substantially matches that of the first first-colour image in the first active zone when the viewing angle is such that the first first-colour image appears at said one of the first-colour maximum tone and the first-colour minimum tone.

2. The security device of claim 1 , wherein the periphery of the first active zone further defines a second first-colour image, within which second first-colour image extends a second array of first-colour image elements, wherein the first- colour image elements of the second first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the second first-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the second first-colour image contains at least a portion of a respective first-colour image element of the second first-colour image and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the second first- colour image, and wherein the pitch between the first-colour image elements of the second first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the second first-colour image appears varies between the first-colour minimum tone and the first-colour maximum tone.

3. The security device of claim 2, wherein the first-colour image elements of the first first-colour image are offset from the first-colour image elements of the second first-colour image along the first direction such that the second first-colour image appears in the first-colour maximum tone at a different viewing angle to that at which the first first-colour image appears at the first-colour maximum tone.

4. The security device of claim 3, wherein the first first-colour image and the second first-colour image at least partially overlap one another such that the optical footprint of each focusing feature in the area across which said images overlap one another contains at least a portion of a respective first-colour image element of each of said images.

5. The security device of any of claims 2 to 4, wherein in the area(s) across which the first first-colour image and the second first-colour image overlap one another the first-colour image elements of the second first-colour image are in the regions that space the first-colour image elements of the first first-colour image from one another.

6. The security device of any preceding claim, further comprising a second structure of a second colour on or in the substrate, the second structure comprising: a second active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the second active zone having a periphery which defines at least a first second-colour image, within which first second-colour image extends a first array of second-colour image elements, wherein the second-colour image elements of the first second-colour image are spaced from one another at least in the first direction by regions in which no second-colour image elements of the first second-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first second-colour image contains at least a portion of a respective second-colour image element and at least a portion of a region which spaces said second-colour image element from an adjacent second-colour image element of the first second- colour image, and wherein the pitch between the second-colour image elements of the first second-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first second-colour image appears varies between a second-colour minimum tone and a second-colour maximum tone, and a second static zone laterally adjacent the second active zone and configured as an area of a substantially uniform tone that, when viewed under the constant lighting conditions, appears to the naked eye substantially the same as the tone of the first second-colour image at one of the second-colour maximum tone and the second-colour minimum tone, whereby the tone of the second static zone substantially matches that of the first second-colour image in the second active zone when the viewing angle is such that the first second-colour image appears at said one of the second-colour maximum tone and the second-colour minimum tone.

7. The security device of claim 6, wherein the first structure and the second structure at least partially overlap one another.

8. The security device of claim 6 or claim 7, wherein the first-colour image elements of the first first-colour image are offset from the second-colour image elements of the first second-colour image along the first direction such that the first first-colour image appears in the first-colour maximum tone at a different viewing angle to that at which the first second-colour image appears at the second-colour maximum tone.

9. The security device of any of claims 6 to 8, wherein the first and second structures are arranged such that the first static zone at least partially overlaps the second active zone and/or the second static zone at least partially overlaps the first active zone.

10. The security device of any of claims 6 to 9, wherein the first and second structures are arranged such that the first active zone at least partially overlaps the second active zone.

11 . The security device of claim 10, wherein the first image elements and the second image elements are arranged alternately along the first direction.

12. The security device of any of claims 6 to 11 , wherein at least some of the second-colour image elements of the first second-colour image are in the regions that space the first-colour image elements of the first first-colour image from one another.

13. The security device of any preceding claim, wherein the periphery of the first active zone is shaped such that the first first-colour image is repeated a plurality of times across the security device.

14. The security device of claim 13, wherein the periphery of the first active zone is shaped such that the second first-colour image is repeated a plurality times across the security device.

15. The security device of claim 13 or claim 14, wherein the periphery of the second active zone is shaped such that the first second-colour image is repeated a plurality of times across the security device.

16. The security device of any of claims 13 to 15, wherein the repeats of the first first-colour image and/or the second first-colour image and/or the first second- colour image are arranged in accordance with a regular pattern, wherein preferably the regular pattern is periodic in at least the first direction.

17. The security device of any of claims 13 to 16, wherein the repeats of the first first-colour image and/or the second first-colour image and/or the first second- colour image are arranged in a tessellating or pseudo-tessellating pattern.

18. The security device of any of any preceding claim, wherein the first first- colour image is bounded by a keyline which does not match the appearance of the tone of the first static zone when viewed under the constant lighting conditions at substantially all viewing angles.

19. The security device of claim 18, wherein the first colour is absent in the keyline.

20. The security device of any preceding claim, wherein in the first and/or second structure the respective static zone is formed by an array of screen elements, the screen elements preferably being arranged in accordance with a halftone pattern.

21. The security device of any preceding claim, wherein in the first static zone, the substantially uniform tone is in the range 5% to 35% or 75% to 100%, preferably 20% to 30% or 80% to 100%, more preferably 20% to 30% or 80% to 90%, where 100% is the tone of a solid area of the first colour and/or wherein in the second static zone, the substantially uniform tone is in the range 5% to 35% or 75% to 100%, preferably 20% to 30% or 80% to 100%, more preferably 20% to 30% or 80% to 90%, where 100% is the tone of a solid area of the second colour.

22. The security device of any preceding claim, wherein the array of focusing features extends across at least part of the first static zone and/or, if provided, the second static zone.

23. The security device of any preceding claim, wherein the focusing features are elongate focusing features each extending along a direction that is nonparallel to the first direction, preferably perpendicular to the first direction.

24. The security device of any preceding claim, wherein the first image elements and/or, if provided, the second image elements, are elongate elements extending along a direction non-parallel to the first direction, preferably perpendicular to the first direction.

25. The security device of any preceding claim, wherein the first and/or second structure is a print working, preferably printed by a gravure, intaglio, microintaglio, flexographic or lithographic technique.

26. The security device of any of claims 1 to 25, wherein the first and/or second structure comprises any of: a laser marking; a relief structure configured to generate structural colour, preferably a diffractive or plasmonic relief structure; a relief structure carrying a marking material in the recesses thereof or on the elevations thereof; or a demetallised metal or metal alloy layer.

27. A security article comprising the security device of any preceding claim, wherein the security article is formed as a security thread, strip, foil, insert, label or patch.

28. A security document provided with the security device of any of claims 1 to 26, wherein preferably the security document is selected from banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other documents for securing value or personal identity.

29. A method of manufacturing a security device, the method comprising, in any order:

(a) providing a substrate having a first side and a second side;

30. The method of claim 29, further comprising, before step (b): providing a test device comprising: a test substrate having a first side and a second side; an array of test focusing features on the first or second side of the test substrate, wherein the test focusing features are arranged periodically along at least a test array direction, each test focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; and a test structure of the first colour on or in the first or second side of the test substrate, the test structure comprising: one or more test active zones overlapping the array of test focusing features and disposed substantially in the focal plane thereof, the or each test active zone having a periphery which defines at least a test image, within which test image extends an array of test image elements, wherein the test image elements of the test image are spaced from one another at least in the test array direction by regions in which no test image elements are present and arranged such that the optical footprint of each test focusing feature overlapping the test image contains at least a portion of a respective test image element and at least a portion of a region which spaces said test image element from an adjacent test image element of the test image, and wherein the pitch between the test image elements of the test image along the test array direction is substantially equal to the pitch between the test focusing features along the test array direction such that, as the viewing angle is varied while the test device is viewed under constant lighting conditions, the tone in which the test image appears varies between the first-colour minimum tone and the first-colour maximum tone, and a plurality of test static zones laterally adjacent the test active zone(s) and each configured as an area of a different respective substantially uniform tone of the first colour; identifying one of the test static zones having a tone that most closely matches the first-colour minimum tone or the first-colour maximum tone when viewed under the constant lighting conditions, the tone of the identified zone being designated the selected tone; and in step (c), producing the first-colour static zone in the selected tone.

31 . The method of claim 29 or claim 30, further comprising applying a second structure of a second colour on or in the substrate, the second structure comprising: a second active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the second active zone having a periphery which defines at least a first second-colour image, within which first second-colour image extends a first array of second-colour image elements, wherein the second-colour image elements of the first second-colour image are spaced from one another at least in the first direction by regions in which no second-colour image elements of the first second-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first second-colour image contains at least a portion of a respective second-colour image element and at least a portion of a region which spaces said second-colour image element from an adjacent second-colour image element of the first second- colour image, and wherein the pitch between the second-colour image elements of the first second-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first second-colour image appears varies between a second-colour minimum tone and a second-colour maximum tone, and a second static zone laterally adjacent the second active zone and configured as an area of a substantially uniform tone that, when viewed under the constant lighting conditions, appears to the naked eye substantially the same as the tone of the first second-colour image at one of the second-colour maximum tone and the second-colour minimum tone, whereby the tone of the second static zone substantially matches that of the first second-colour image in the second active zone when the viewing angle is such that the first second-colour image appears at said one of the second-colour maximum intensity tone and the second- colour minimum tone.

32. The method of claim 31 , wherein the first structure and the second structure are formed on or in the substrate in register with one another.

33. The method of any of claims 29 to 32, wherein the focusing features are applied in register with the image elements of the first and/or second structure.

34. The method of any of claims 29 to 33, wherein the first and/or second structure is a print working, formed by a printing technique, preferably a gravure, intaglio, micro-intaglio, flexographic or lithographic technique.

35. The method of any of claims 29 to 34, wherein the first and/or second structure is 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 onon the elevations thereof; or demetallisation of a metal or metal alloy layer.

36. The method of any of claims 29 to 35, wherein the focusing features are applied to the first side of the substrate and the first and/or second structures are 80 applied to the second side of the substrate simultaneously at the same location along the substrate.

37. The method of any of claims 29 to 36 adapted to produce the security device of any of claims 1 to 26.

38. A security device comprising: a substrate having a first side and a second side; an array of focusing features disposed on the first or second side of the substrate and arranged periodically along at least a first direction, each focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; and a first structure of a first colour disposed on or in the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and 81 a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone in the range 5% to 35% or 75% to 100%, where 100% is the tone of a solid area of the first colour.

39. The security device of claim 38, further comprising: a second structure of a second colour disposed on or in the substrate, the second structure comprising: a second active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the second active zone having a periphery which defines at least a first second-colour image, within which first second-colour image extends a first array of second-colour image elements, wherein the second-colour image elements of the first second-colour image are spaced from one another at least in the first direction by regions in which no second-colour image elements of the first second-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first second-colour image contains at least a portion of a respective second-colour image element and at least a portion of a region which spaces said second-colour image element from an adjacent second-colour image element of the first second- colour image, and wherein the pitch between the second-colour image elements of the first second-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first second-colour image appears varies between a second-colour minimum tone and a second-colour maximum tone, and a second static zone laterally adjacent the second active zone and configured as an area of a substantially uniform tone in the range 5% to 35% or 75% to 100%, where 100% is the tone of a solid area of the second colour.

40. The security device of claim 38 or claim 39, further comprising the features of any of claims 2 to 5, 7 to 20 or 22 to 26.

41. The security device of claim 39, wherein the first and second structures are arranged such that the first static zone at least partially overlaps the second 82 active zone and/or the second static zone at least partially overlaps the first active zone.

42. The security device of claim 39 or claim 40, wherein the first static zone does not overlap the second active zone and the second static zone does not overlap the first active zone.

43. The security device of claim 42, wherein the first static zone and the second static zone overlap one another, wherein the overlapping parts of the first static zone and the second static zone together form a composite static zone comprising the first colour and the second colour.

44. The security device of claim 43, wherein the tone of the composite static zone is less than or equal to 60%, preferably less than or equal to 40%, more preferably less or equal to than 30%, most preferably less than or equal to 20%.

45. The security device of claim 43 or claim 44, wherein the ratio of the first colour to the second colour in the composite static zone is in the range of 40:60 to 60:40, preferably substantially 50:50.

46. A method of manufacturing a security device, the method comprising in any order:

(a) providing a substrate having a first side and a second side;

(c) forming a first structure of a first colour on or in the first or second side of the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image 83 extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone in the range 5% to 35% or 75% to 100%, where 100% is the tone of a solid area of the first colour.

47. A security device comprising: a substrate having a first side and a second side; an array of focusing features disposed on the first or second side of the substrate and arranged periodically along at least a first direction, each focusing feature having a respective optical footprint of which different parts will be directed to the viewer in dependence on the viewing angle; a first structure of a first colour disposed on or in the substrate, the first structure comprising: a first active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the first active zone having a periphery which defines at least a first first-colour image, within which first first-colour image extends a first array of first-colour image elements, wherein the first-colour image elements of the first first-colour image are spaced from one another at least in the first direction by regions in which no first-colour image elements of the first first- colour image are present and arranged such that the optical footprint of each 84 focusing feature overlapping the first first-colour image contains at least a portion of a respective first-colour image element and at least a portion of a region which spaces said first-colour image element from an adjacent first-colour image element of the first first-colour image, and wherein the pitch between the first- colour image elements of the first first-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first first-colour image appears varies between a first-colour minimum tone and a first-colour maximum tone, and the first structure further comprising a first static zone laterally adjacent the first active zone and configured as an area of a substantially uniform tone; and a second structure of a second colour disposed on or in the substrate, the second structure comprising: a second active zone overlapping the array of focusing features and disposed substantially in the focal plane thereof, the second active zone having a periphery which defines at least a first second-colour image, within which first second-colour image extends a first array of second-colour image elements, wherein the second-colour image elements of the first second-colour image are spaced from one another at least in the first direction by regions in which no second-colour image elements of the first second-colour image are present and arranged such that the optical footprint of each focusing feature overlapping the first second-colour image contains at least a portion of a respective second-colour image element and at least a portion of a region which spaces said second-colour image element from an adjacent second-colour image element of the first second- colour image, and wherein the pitch between the second-colour image elements of the first second-colour image along the first direction is substantially equal to the pitch between the focusing features along the first direction such that, as the viewing angle is varied while the device is viewed under constant lighting conditions, the tone in which the first second-colour image appears varies between a second-colour minimum tone and a second-colour maximum tone, and 85 the second structure further comprising a second static zone laterally adjacent the second active zone and configured as an area of a substantially uniform tone; wherein the first static zone does not overlap the second active zone and the second static zone does not overlap the first active zone; and wherein the first static zone and the second static zone overlap one another, the overlapping parts of the first static zone and the second static zone together forming a composite static zone comprising the first colour and the second colour, wherein the tone and colour of the composite static zone are between those of the first-colour minimum tone and the second-colour minimum tone.

48. The security device of claim 47, wherein the tone and/or colour of the composite static zone are substantially equidistant from those of the first-colour minimum tone and those of the second-colour minimum tone.

49. The security device of claim 47 or claim 48, wherein the colour and/or tone of the composite static zone are separated from those of each of the first- colour minimum tone and the second-colour minimum tone in CIE L*a*b* colour space by a Euclidean distance AE less than or equal to 20, preferably less than or equal to 10.

50. The security device of any of claims 47 to 49, wherein the ratio of the first colour to the second colour in the composite static zone is in the range of 40:60 to 60:40, preferably substantially 50:50.

51. The security device of any of claims 47 to 50, wherein the tone of the composite static zone is less than or equal to 60%, preferably less than or equal to 40%, more preferably less than or equal to 30%, most preferably less than or equal to 20%.