Security devices

Abstract

A security device is disclosed, comprising an array of focussing elements with regular periodicity in at least a first direction, each focusing element having an optical footprint of which different portions will be directed to the viewer in dependence on the viewing angle; and an array of image elements with regular periodicity in at least the first direction overlapping the array of focusing structures, the image elements representing portions of at least two respective images, and at least one image element from each respective image being located in the optical footprint of each focusing structure. The security device includes a first region and a second region which is laterally offset from the first, the image elements in the first region being laterally shifted in at least the first direction relative to the image elements in the second region such that, at a first viewing angle, in the first region of the device the focussing structures direct image elements corresponding to a first image to the viewer such that the first image is displayed across the first region of the device, and simultaneously, in the second region of the device, the focussing structures direct image elements corresponding to a second image to the viewer such that the second image is displayed across the second region of the device, and at a second viewing angle the second image is displayed across the first region of the device and simultaneously the first image is displayed across the second region of the device. The security device further comprises a colour filter located in use between the image elements and the viewer, the colour filter overlapping at least part of the array of focussing elements and the array of image elements, and having a first colour in the first region of the device and a different colour in the second region of the device such that the colour appearance of the first and second images is different in the respective first and second regions of the device.

Claims

1. A security device, comprising: an array of focussing elements with periodicity in at least a first direction, each focusing element having an optical footprint of which different portions will be directed to a viewer in dependence on a viewing angle; and an array of image elements with periodicity in at least the first direction overlapping the array of focusing structures, the image elements representing portions of at least two respective images, and at least one image element from each respective image being located in the optical footprint of each focusing structure; wherein the security device includes a first region and a second region which is laterally offset from the first, the image elements in the first region being laterally shifted in at least the first direction relative to the image elements in the second region such that, at a first viewing angle, in the first region of the device the focussing structures direct image elements corresponding to a first image to the viewer such that the first image is displayed across the first region of the device, and simultaneously, in the second region of the device, the focussing structures direct image elements corresponding to a second image to the viewer such that the second image is displayed across the second region of the device, and at a second viewing angle the second image is displayed across the first region of the device and simultaneously the first image is displayed across the second region of the device, and the security device further comprises a colour filter located in use between the image elements and the viewer, the colour filter overlapping at least part of the array of focussing elements and the array of image elements, and having a first colour in the first region of the device and a different colour in the second region of the device such that a colour appearance of the first and second images is different in the respective first and second regions of the device, and wherein the colour filter is formed at least in part by the focussing elements of the focussing element array having different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by a pedestal layer provided between the focussing element array and a surface of a substrate on which the focussing element array is located, the pedestal layer comprising at least first and second transparent materials of different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by an image base layer provided between the image array and a surface of a substrate on which the image array is formed, the image base layer comprising at least first and second transparent materials of different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by an intermediate layer spaced from both the focussing element array and from the image array.

2. A security device according to claim 1, wherein the image array comprises a set of monochromatic image elements corresponding to the first image in the first and second regions of the device.

3. A security device according to claim 2, wherein the monochromatic image elements are either opaque; or semi-transparent or translucent.

4. A security device according to claim 3, wherein the monochromatic image elements are semi-transparent or translucent and a colour of the monochromatic image elements either is different from the colours of the colour filter in both the first and second regions of the device; or matches the colour of the colour filter in one of the first and second regions of the device.

5. A security device according to claim 1, wherein the image base layer is a tic-coat formed of curable materials for affixing the image array to the substrate.

6. A security device according to claim 1, wherein the colour filter comprises at least two colour filter layers provided at different spacings from the focussing element array and/or from the image array which are laterally offset.

7. A security device according to claim 1, wherein the security device is formed as a security thread, strip, foil, insert, label or patch.

8. An article provided with a security device according to claim 1.

9. A security device, comprising: an array of focussing elements with periodicity in at least a first direction, each focusing element having an optical footprint of which different portions will be directed to a viewer in dependence on a viewing angle; and a corresponding first image array overlapping the array of focussing elements and configured to co-operate with the array of focussing elements so as to generate a first optically variable effect which varies with viewing angle, the first image array comprising a periodic arrangement of image elements or microimages formed in a first colour across the security device, wherein the security device further comprises a colour filter located in use between the first image array and the viewer, the colour filter overlapping at least part of the array of focussing elements and the first image array, and having different colours in respective first and second regions of the device which are laterally offset from one another, the colour of the colour filter layer in the first region of the device matching the first colour of the image elements or microimages, and wherein the colour filter is formed at least in part by the focussing elements of the focussing element array having different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by a pedestal layer provided between the focussing element array and a surface of a substrate on which the focussing element array is located, the pedestal layer comprising at least first and second transparent materials of different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by an image base layer provided between the image array and a surface of a substrate on which the image array is formed, the image base layer comprising at least first and second transparent materials of different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by an intermediate layer spaced from both the focussing element array and from the image array.

10. A security device according to claim 9, wherein in the first region of the device the image elements or microimages are concealed from view by the matching colours of the image elements or microimages and the colour filter, such that the first optically variable effect is exhibited in the second region and not in the first region.

11. A security device according to claim 9, further comprising a second image array overlapping the array of focussing elements and configured to co-operate with the array of focussing elements so as to generate a second optically variable effect which varies with viewing angle, the second image array comprising a periodic arrangement of image elements or microimages formed in a second colour across the security device.

12. A security device according to claim 11, wherein the colour of the colour filter layer in the second region of the device matches the second colour of the image elements or microimages.

13. A security device according to claim 12, wherein in the second region of the device the image elements or microimages are concealed from view by the matching colours of the image elements or microimages and the colour filter, such that the second optically variable effect is exhibited in the first region and not in the second region.

14. A security device, comprising: an array of focussing elements with periodicity in at least a first direction, each focusing element having an optical footprint of which different portions will be directed to a viewer in dependence on a viewing angle; and a corresponding first image array overlapping the array of focussing elements and configured to co-operate with the array of focussing elements so as to generate a first optically variable effect which varies with viewing angle, the first image array comprising a periodic arrangement of image elements or microimages formed in a first colour across the security device, wherein the security device further comprises a colour filter located in use between the first image array and the viewer, the colour filter overlapping at least part of the array of focussing elements and the first image array, and having different colours in respective first and second regions of the device which are laterally offset from one another, the colour of the colour filter layer in the first region of the device being complementary to the first colour of the image elements or microimages, and wherein the colour filter is formed at least in part by the focussing elements of the focussing element array having different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by a pedestal layer provided between the focussing element array and a surface of a substrate on which the focussing element array is located, the pedestal layer comprising at least first and second transparent materials of different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by an image base layer provided between the image array and a surface of a substrate on which the image array is formed, the image base layer comprising at least first and second transparent materials of different colours from one another in the respective first and second regions of the device, or the colour filter is formed at least in part by an intermediate layer spaced from both the focussing element array and from the image array.

15. A security device, comprising: an array of focussing elements with periodicity in at least a first direction, each focusing element having an optical footprint of which different portions will be directed to a viewer in dependence on a viewing angle; and a corresponding first image array overlapping the array of focussing elements and configured to co-operate with the array of focussing elements so as to generate a first optically variable effect which varies with viewing angle, the first image array comprising a periodic arrangement of image elements or microimages formed in a first colour across the security device; wherein the security device further comprises: a colour filler located in use between the first image array and the viewer, the colour filter overlapping at least part of the array of focussing elements and the first image array, and having different colours in respective first and second regions of the device which are laterally offset from one another; and a backing layer located behind the first image array such that the first image array is between the colour filter and the backing layer, the backing layer comprising at least two laterally offset areas of different colour, the backing layer being visible at least between the image elements or microimages in the first image array.

16. A security device according to claim 15 wherein either two of the differently coloured areas of the backing layer correspond to the first and second regions of the device respectively; or at least two of the differently coloured areas of the backing layer are located in each of the first and second regions of the device.

17. A method of forming an image array for a security device, the image array comprising a pattern of at least one first curable material, the method comprising: (i) providing a die form, the die form having a surface comprising an arrangement of raised areas and recessed areas defining the pattern; (ii) applying the at least one first curable material to the surface of the die form such that said at least one first curable material fills the recessed areas; (iii) bringing a pattern support layer in contact with the surface of the die form such that it covers the recessed areas; (iv) separating the pattern support layer from the surface of the die form such that the first curable material in the recessed areas is removed from said recessed areas and retained on the pattern support layer in accordance with the pattern; and (v) during and/or after step (ii), at least partly curing the first curable material in one or more curing steps; wherein the method further comprises either: (ii) after step (ii) and before step (iii), covering the surface of the die form and the recessed areas filled with the at least one first curable material with a tie coat comprising at least two tie coat curable materials arranged in respective laterally offset areas; or (ii) before step (iii), applying to the pattern support layer the tie coat comprising the at least two tie coat curable materials arranged in the respective laterally offset areas; step (v) further comprises at least partly curing the at least two tie coat curable materials such that in step (iv) the tie coat and the at least one first curable material are retained on the pattern support layer; and the at least two tie coat curable materials have different optical detection characteristics from one another, whereby the image array comprises a background to the pattern of the at least one first curable material, formed by the tie coat, the background having different appearances in the respective laterally offset areas.

Description

(1) FIG. 1 schematically depicts an embodiment of a security device, in cross-section;

(2) FIG. 2 shows, in plan view, (a) an exemplary image array, (b) an exemplary colour filter, and (c) their appearance when overlapped;

(3) FIG. 3 schematically depicts a comparative example of a conventional security device: FIG. 3(a) showing a schematic perspective view of the security device; FIG. 3(b) showing a cross-section through the security device; and FIGS. 3(c) and (d) showing two exemplary images which may be displayed by the device at different viewing angles;

(4) FIGS. 4 to 7 schematically depict four security devices in accordance with embodiments of the invention, (a) in cross-section, (b) in plan view from a first viewing angle and (c) in plan view from a second viewing angle;

(5) FIG. 8(a) illustrates in plan view an exemplary image array in accordance with an embodiment of the present invention, FIG. 8(b) showing in plan view the appearance of a security device in accordance with an embodiment of the present invention incorporating the image element array of FIG. 18(a), at one viewing angle;

(6) FIG. 9(a) illustrates an exemplary image array in accordance with an embodiment of the invention, and FIG. 9(b) shows the appearance of a security device incorporating the image pattern of FIG. 9(a);

(7) FIGS. 10 and 11 schematically depict two security devices in accordance with embodiments of the invention, (a) in cross-section and (b) in plan view;

(8) FIGS. 12 to 17 schematically depict six further security devices in accordance with embodiments of the invention, (a) in cross-section, (b) in plan view from a first viewing angle and (c) in plan view from a second viewing angle;

(9) FIGS. 18 (a) and (b) illustrate an exemplary apparatus for forming a focussing element array, in accordance with embodiments of the present invention, FIG. 18(a) illustrating the apparatus from a side view and FIG. 18(b) showing a perspective view of the focussing element support layer;

(10) FIGS. 19 and 20 illustrate two variants of the apparatus shown in FIG. 18(a);

(11) FIG. 21(a) shows an exemplary focussing element array formed as a transfer elements, suitable for use in embodiments of the invention, in cross-section, and

(12) FIG. 21(b) shows a security device in accordance with an embodiment of the present invention, comprising the focussing element array of FIG. 21(a);

(13) FIG. 22a schematically depicts a security device in accordance with another embodiment of the present invention, in cross-section;

(14) FIG. 22b shows a further embodiment of exemplary apparatus suitable for forming a focussing element array such as that in the FIG. 22a embodiment;

(15) FIG. 23 schematically depicts a security device in accordance with another embodiment of the present invention, in cross-section;

(16) FIGS. 24(a) and (b) and 25 (a) and (b) show four exemplary embodiments of apparatus suitable for forming an image array such as that in the FIG. 23 embodiment;

(17) FIGS. 26 and 27 schematically depict two further security devices in accordance with embodiments of the present invention, in cross-section;

(18) FIGS. 28, 29 and 30 show three exemplary articles carrying security devices in accordance with embodiments of the present invention (a) in plan view, and (b) in cross-section; and

(19) FIG. 31 illustrates a further embodiment of an article carrying a security device in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section.

(20) Security devices in accordance with aspects of the present disclosure make use of a colour filter to modify the apparent colour of an image array. The colour filter can be incorporated into the security device in various different ways each of which will produce substantially the same end result. Some preferred arrangements of the colour filter will be summarised with reference to FIG. 1 and discussed in more detail in connection with particular embodiments below. However it should be appreciated that all of the embodiments disclosed herein can be implemented with colour filters incorporated in any of the manners now described, or a combination thereof. In all cases however the colour filter should be located so that it lies between the image array and the viewer (observer) in use.

(21) Hence, FIG. 1 schematically depicts an embodiment of a security device 1, in cross-section. The security device could be for example a moir magnifier, an integral imaging device, a lenticular device or any other security device in which an optically variable effect is generated by the co-operation between a focussing element array and an image array. The security device comprises a transparent substrate 2, which is typically polymeric, and may be monolithic or formed of multiple layers such as layers 2a, 2b in this example. Suitable polymeric substrates include polypropylene (preferably BOPP), polyethylene, polyvinylchloride and the like. The thickness of the substrate will be selected based on the desired end use. For instance if the security device is to be formed as a thread, strip, foil or other article for application to a security document, typically the substrate thickness will be 50 microns or less, more preferably 35 microns or less. In other cases, the substrate 2 could be a portion of a document substrate such as that on which a polymer banknote is based in which case the thickness will be greater, e.g. in the region of 70 to 200 microns.

(22) A focussing element array 20 is provided on one surface of the substrate 2 and comprises a regular array of focussing elements 21, such as lenses or mirrors. The particular arrangement of focussing elements 21 will depend on the nature of the optically variable effect to be generated. The array 20 may be periodic in one dimension or two dimensionsFIG. 1 depicts the array 20 as periodic in the x-axis direction but it may additionally be periodic in the orthogonal y-axis direction. The individual focussing elements could comprise elongate elements such as cylindrical focussing elements, or could be spherical or aspherical, for example. The focussing elements preferably take the form of lenses or mirrors. In the FIG. 1 embodiment, and in all the examples depicted below, the focussing element array is exemplified as lenses but in all cases could be replaced by a mirror array, in which case the observer O.sub.1 would view the effect from the opposite side of the device. The colour filter would need to be repositioned within the structure accordingly.

(23) The colour filter (generally denoted 10 in the Figures) may be integrated into another component of the security device 1 or may be provided separately. For instance, FIG. 1 shows four exemplary locations for the colour filter 10, labelled 10, 10, 10 and 10.sup.IV. In a first preferred option, the colour filter 10 is incorporated into the focussing element array 20 by forming the focussing elements 21 of differently coloured transparent materials in different regions of the device. Hence the focussing elements themselves perform dual functions of co-operating with the image array 30 to generate the optically variable effect and modifying the colour thereof. Alternatively, the colour filter 10 could be located between the focussing element array 20 and the surface of the substrate 2 on which the focussing element array 20 is located, in the form of a pedestal layer (not shown separately in FIG. 1). The pedestal layer will comprise transparent materials having different colours in different regions of the device 1.

(24) If the substrate 2 is multi-layered, the colour filter 10 could alternatively be provided at some intermediate location within the substrate 2 at an internal interface between adjacent substrate layers such as that illustrated between layers 2a and 2b. In this case, the colour filter 10 could be a printed layer of coloured inks, for example. In a fourth example, the colour filter could be located between the image array 30 and the surface of the substrate on which the image array is carried. Here the colour filter 10.sup.IV could take the form of a printed layer on top of which the image array is then placed, or more preferable could be formed as a multi-coloured tie coat of coloured curable materials, as will be described further below.

(25) For ease of manufacturing, colour filter locations 10 (pedestal layer) or 10.sup.IV in the form of a printed layer are especially preferred. However, forming the colour filter integrally with another component (e.g. in the focussing element array or as a tie coat) offers other advantages such as improved registration.

(26) However the colour filter 10 is incorporated into the device, it comprises at least two transparent materials with different visibly coloured tints (one of which may be colourless), arranged in respective regions of the device. The colour filter 10 modifies the observed colour of the underlying image array by transmitting only selected wavelengths of the visible spectrum therethrough, which are different in the different regions. To consider the effect of the colour filter 10 on the appearance of the device, the following model is adopted:

(27) The visible spectrum can be represented by red, green and blue wavebands of roughly equal width and therefore the terms R, G and B in the following equations are just label indices.

(28) We represent the reflected colour of the image elements or microimages 31 making up the image array as P(p)=(p.sub.r R, p.sub.g G, p.sub.b B) or simply (p.sub.r, p.sub.g, p.sub.b). Meanwhile, the reflected colour of the background 32 surrounding the image elements of microimages is B(b)=(b.sub.r R, b.sub.g G, b.sub.b B) or simply (b.sub.r, b.sub.g, b.sub.b). For example, for Magenta: b.sub.r, b.sub.b, p.sub.r, p.sub.b=1 and b.sub.g, p.sub.g=0 Cyan: b.sub.b, b.sub.g, p.sub.b, p.sub.g=1 and b.sub.r, p.sub.r=0 Yellow: b.sub.r, b.sub.g, p.sub.r, p.sub.g, 1 and b.sub.b, p.sub.b=0 Black: b.sub.r, b.sub.g, b.sub.b, p.sub.r, p.sub.g p.sub.b=0

(29) For the colour filter 10, the colour transmission is defined by T(t)=(t.sub.r R, t.sub.g G, t.sub.b B). For instance, a red filter as defined as that which passes only the red wave band and therefore t.sub.r=1 and t.sub.g, t.sub.b=0 etc.

(30) Given the previous representation and notation, the observed background colour exhibited by the image array 30 and colour filter 10 in combination can be denoted OB=.sub.i (b.sub.i.Math.t.sub.i) i, whilst the observed colour of the image elements or microimages 31 is OP=.sub.i (p.sub.i.Math.t.sub.i) i.

(31) To illustrate, suppose the background 32 is a pure cyan with the colour matrix B(b)=(0,1,1) and the image elements 31 are magenta with the colour matrix P(p)=(1,0,1). Suppose the colour filter 10 transmits 90% red, 5% green and 5% blue, then T=(0.9, 0.05, 0.05). Hence the observed background colour OB will be (0, 0.05, 0.05) i.e. very dark cyan, whilst the image element colour OP will be defined by (0.9, 0, 0.05) which will result in a bright red image element 31. Thus the effect of the colour filter 10 here will be transform a magenta on cyan image array 30 to a red on dark magenta observed image array. We therefore have a convenient way of qualitatively determining the observed colour for more complex colour compositions present in background 32, image elements 31 and/or the colour filter 10.

(32) FIG. 2 shows these principles at work in an illustrative example. Here, FIG. 2(a) depicts an exemplary image array 30 which here comprises a regular array of microimages, each having in this example the form of the digit 5, which are formed in magenta, on a cyan background 32. FIG. 2(b) shows an exemplary colour filter 10 having three laterally offset and non-overlapping regions R.sub.1, R.sub.2 and R.sub.3. In the first region R.sub.1, the colour filter is formed of a first material 10a having a red tint, in the second region R.sub.2, a second material 10b is provided which in this case is colourless (i.e. no tint), and in the third region R.sub.3 a third material 10c is provided which here has a green tint. FIG. 2(c) shows the colour filter 10 and image array 30 arranged to partially overlap one another. Now, the image elements 31 which appear in the first region R.sub.1 are observed as bright red against a dark cyan background, those appearing in the second region R.sub.2 are unmodified and hence are observed as magenta image elements 31 against a cyan background, and in the third region R.sub.3 the image elements 31 appear dark magenta on a green background. As a further example, if the colour filter 10 was formed of a yellow tinted material 10b in second region R.sub.2, here the image elements 31 would appear red on a green background.

(33) The above principles can be utilised to create various new and distinctive effects in optically variable security devices, of which preferred examples will now be described.

(34) First, a comparative example of a lenticular device 10 is shown in FIG. 3 in order to illustrate certain principles of operation. FIG. 3(a) shows the device 1 in a perspective view and it will be seen that an array 20 of focussing element structures, here in the form of cylindrical lenses 21, is arranged on a transparent substrate 2. An image array 30 is provided on the opposite side of substrate 2 underlying (and overlapping with) the cylindrical lens array 20. Alternatively the image element array 30 could be located on the same surface of the substrate 2 as the lenses, directly under the lenses. Each cylindrical lens 21 has a corresponding optical footprint which is the area of the image element array 30 which can be viewed via the corresponding lens 21. In this example, the image array 30 is an interlaced image array comprising a series of image slices, of which two slices 31, 32 are provided in (and fill) each optical footprint.

(35) The image slices 31 each correspond to strips taken from a first image I.sub.A whilst the image slices 32 each correspond to strips of a second image I.sub.B. Thus, the size and shape of each first image slice 31 is substantially identical (being elongate and of width equal to half the optical footprint), but their information content will likely differ from one first image slice 31 to the next (unless the first image I.sub.A is a uniform, solid colour block). The same applies to the second image slices 32. The overall pattern of image slices is a line pattern, the elongate direction of the lines lying substantially parallel to the axial direction of the focussing elements 21, which here is along the y-axis. The lenses 21 and the image slices 31, 32 are periodic in the orthogonal direction (x-axis) which may be referred to below as the first direction of the device.

(36) As shown best in the cross-section of FIG. 3(b), the image element array 30 and the focussing element array have substantially the same periodicity as one another in the x-axis direction, such that one first image slice 31 and one second image slice 32 lies under each lens 21. The pitch S of the lens array 20 and of the image element array 30 is substantially equal and is constant across the whole device. In this example, the image array 30 is registered to the lens array 20 in the x-axis direction (i.e. in the arrays' direction of periodicity) such that a first pattern element 31 lies under the left half of each lens and a second pattern element 32 lies under the right half. However, registration between the lens array 20 and the image array 30 in the periodic dimension is not essential.

(37) When the device is viewed by a first observer O.sub.1 from a first viewing angle, as shown in FIG. 3(b) each lens 21 will direct light from the underlying first image slice 31 to the observer, with the result that the device as a whole appears to display the appearance of the first image I.sub.A, which in this case is a uniform block colour as shown in in FIG. 1(c). The full image I.sub.A is reconstructed by the observer O.sub.1 from the first image slices 31 directed to him by the lens array 20. When the device is tilted so that it is viewed by second observer O.sub.2 from a second viewing angle, now each lens 21 directs light from the second image slices 32 to the observer. As such the whole device will now appear to display a second image I.sub.B, which in this example is blank, as shown in FIG. 1(c), although it could comprise any alternative image. Hence, as the security device is tilted back and forth between the positions of observer O.sub.1 and observer O.sub.2, the appearance of the whole device switches between image I.sub.A and image I.sub.B.

(38) In this example the first image elements 31 are provided by material forming the image array 30 whilst the second image elements 32 are provided by gaps therebetween. However in other cases as illustrated below the second image elements 32 could also be coloured, e.g. by providing a coloured background such as that described with reference to FIG. 2 above. It is also possible to interleave three of more images by extending the above principles accordingly.

(39) As also noted in passing, the images need not be uniform blocks of colour (or lack thereof) but could each carry any desirable graphic, such as indicia or the like, by arranging each image slice 31 to be provided only in accordance with the desired graphic rather than in a continuous form along its length, as shown.

(40) FIG. 4 illustrates an embodiment of a security device 1 in accordance with an aspect of the present invention which here is a lenticular device operating on the same principles described with respect to FIG. 3. Components of the device 1 are labelled using like reference numerals as before and so those already introduced will not be described again. The security device 1 comprises two laterally offset regions R.sub.1 and R.sub.2 which, as shown best in the plan views of FIGS. 4(b) and (c) are arranged as a circular area R.sub.2 on a rectangular surroundings R.sub.1. The device incorporates a colour filter 10 of the type described above which here is incorporated into the focussing element array 20, but could take any of the other implementations already mentioned. However in this example, the focussing elements 21a in the first region R.sub.1 are formed of a transparent material in a first colour (e.g. blue) whilst the focussing elements 21b in the second region R.sub.2 have a second colour (e.g. yellow). The image array 30 once again comprises first image slices 31 spaced by background slices 32 arranged so as to generate the above-described lenticular switching effect in combination with the focussing elements 21, upon tilting of the device. However, in the second region R.sub.2, the image array is laterally shifted in the x-axis direction relative to its translational position in the first region R.sub.1 (i.e. phase-shifted), which can be achieved through design of the image array 30. Thus, in the first region R.sub.1 the first image slices 31 sit under the left half of each focussing element 21 whilst the second image slices 32 occupy the right half, and in the second region R the arrangement is reversed. In this example, the first image slices 31 are achromatic (e.g. black) whilst the second image slices 32 are colourless.

(41) FIGS. 4(b) and (c) show the appearance of the device from two different viewing angles for respective observers O.sub.1 and O.sub.2. Observer O.sub.1 sees outer region R.sub.1 appearing dark blue/black due to the combination of the blue lenses 21a with the black image elements 31. However in the central region R.sub.2 the focussing elements 21 will direct light from the second image elements 32 to the same observer O.sub.1, due to the phase-shifted image array 30 and this in combination with the yellow lenses 21b will cause the region R.sub.2 to appear bright yellow. When the device is tilted and viewed by observer O.sub.2, now in the outer region R.sub.1, the blue lenses 21a will direct light from the second image slices 32 to the viewer causing that region to appear bright blue whilst the central region R.sub.2 will now appear dark yellow or gold due to the combination of the yellow lenses 21b and dark image elements 31. Hence, overall two different colours, each at two different darkness levels, are displayed by the device over the full range of viewing angles. In addition it will be noted that the contrast between the two regions has reversed during tilting: observer O.sub.1 sees the outer region R.sub.1 as dark compared with the centre region R.sub.2 whereas the reverse is true for observer O.sub.2. This provides a particularly strong and distinctive visual effect.

(42) By requiring both the colour filter 10 and the image array 30 to possess different characteristics in respective regions of the device 1, the device presents a significant challenge to would-be counterfeiters, since any mis-registration between the colour filter 10 and the image array 30 will be noticeable since additional colour effects will appear at the boundaries between regions.

(43) FIG. 5 illustrates another embodiment of a security device 1 which is a variant of that depicted in FIG. 4 and operates on the same principles. Here, the two regions R.sub.1, R.sub.2 are laterally offset rectangular areas of the device 1 and once again the colours of the colour filter 10 and the translational position of the image array 30 are varied between regions. In this example, however, the image elements 31 are not achromatic but themselves carry a hue which when combined with colours of the colour filter creates additional effects. To illustrate this, here the colour filter 10 (incorporated again into the focussing element array 20) is colourless in the first region R.sub.1 but carries a yellow tint in the second region R.sub.2. The image elements 31 are blue and the gaps 32 between them are colourless. Now, observer O.sub.1 sees the first region R.sub.1 as light blue and the second region R.sub.2 as yellow. Upon tilting, observer O.sub.2 sees the first region R.sub.1 as colourless and the second region R.sub.2 as green (due to the combination of blue and yellow). Hence four different colours are visible across the whole range of viewing angles, despite only three having been used in its production (counting colourless).

(44) In the FIG. 6 embodiment, new colour effects are achieved by matching at least one of the colours in the colour filter 10 to at least one colour of the image array 30. Again the construction is similar to that in the two preceding embodiments and so only the differences will be highlighted here. The image array 30 in this example is formed of yellow first image slices 31 and intervening blue second image slices 32. Unlike in the preceding embodiments, there is no phase-shift in the image array 30 between regions and the arrangement of image slices continues uniformly across the device 1. The colour filter 10, meanwhile, is blue in the first region R.sub.1 and yellow in the second region R.sub.2 such that in this example the two colours in the filter 10 match each of the two colours in the image array 30 (although this is not essential, only one matching colours is required).

(45) The described arrangement results in the generation of a new third colour which appears to move between regions upon tilting of the device. As shown in FIGS. 6(b) and (c), the first observer O.sub.1 sees the first region R.sub.1 as green due to the combination of the blue lenses 21a and the yellow first image slices 31, and the second region R.sub.2 as yellow. Upon tilting, observer O.sub.2 now sees the first region R.sub.1 as blue whilst the new green colour has moved to the second region R.sub.2.

(46) Another effect can be achieved by adding a phase-shift to the image array 30 between regions, as illustrated in the embodiment of FIG. 7. Here the construction of the device and choice of colours is the same as in the FIG. 6 embodiment, the only difference being that the image slices have been laterally shifted in region R.sub.2 relative to region R.sub.1. Now, observer O.sub.1 will perceive both regions R.sub.1 and R.sub.2 as green, whereas observer O.sub.2 will see only the two original colours: blue in region R.sub.1 and yellow in region R.sub.2. Hence the third colour, green, appears and disappears as the device is tilted.

(47) The above examples of security devices have all operated on lenticular principles but colour filters of the types just mentioned in which at least one of the colour filter regions matches a colour in the image array also have particular benefit in security devices such as moir magnifiers and integral imaging devices.

(48) To illustrate the principles of operation, comparative examples of moir magnifier and integral imaging devices will first be described with reference to FIGS. 8 and 9 respectively.

(49) FIG. 8 depicts an exemplary moir magnifier device, comprising an image element array 30 defining an array of microimages 31 and an overlapping focussing element array 20 with a pitch or rotational mismatch as necessary to achieve the moir effect. FIG. 8(a) depicts part of the image element array 30 as it would appear without the overlapping focusing element array, i.e. the non-magnified microimage array (but shown at a greatly increased scale for clarity).

(50) In contrast, FIG. 8(b) depicts the appearance of the same portion of the completed security device, i.e. the magnified microimages 34, seen when viewed with the overlapping focussing element array, at one viewing angle. It will be seen from FIG. 8(a) that the image array 30 here forms a regular array of microimages 31 which here each convey the digit 5. In this case all of the microimages 31 are of identical shape and size. The microimages 31 may be coloured or achromatic, formed of ink for example. Surrounding the microimages 31 is a contiguous, uniform background 32 which is preferably colourless but could be of a second contrasting colour. Alternatively, the arrangement could be reversed with the microimages 31 formed as negative, colourless gaps in a coloured background 32.

(51) FIG. 8(b) shows the completed security device 1, i.e. the image element array 30 shown in FIG. 8(a) plus an overlapping focusing element array 20, from a first viewing angle which here is approximately normal to the plane of the device 30. It should be noted that the security device is depicted at the same scale as used in FIG. 8(a): the apparent enlargement is the effect of the focusing element array 20 now included. The moir effect acts to magnify the microimage array such that magnified versions 34 of the microimages 31 are displayed. In this example just two of the magnified microimages are shown. In practice, the size of the enlarged images and their orientation relative to the device will depend on the degree of mismatch between the focussing element array. This will be fixed once the focusing element array is joined to the image element array. The magnified microimages will appear to move laterally relative to the device upon tilting and depending on the magnification level may be visualised above or below the surface plane of the device 1.

(52) In the above example security device, the microimages are all identical to one another, such that the devices can be considered pure moir magnifiers. However, the same principles can be applied to hybrid moir magnifier/integral imaging devices, in which the microimages depict an object or scene from different viewpoints. Such microimages are considered substantially identical to one another for the purposes of the present invention. An example of such a device is shown schematically in FIG. 9, where FIG. 9(a) shows the unmagnified microimage array, without the effect of focusing elements 21, and FIG. 9(b) shows the appearance of the finished device, i.e. the magnified image. As shown in FIG. 9(a), the microimages 31 show an object, here a cube, from different angles. It should be noted that the microimages are formed as lines of one colour corresponding to the black lines of the cubes in the Figure, the remainder of the image array 30 providing a background thereto which may be coloured or contrasting. Again this arrangement could be reversed with the lines formed as colourless gaps in a coloured background layer. In the magnified image (FIG. 9(b)), the moir effect generates magnified, 3D versions of the cube labelled 34. As the device is tilted the magnified cubes 34 will appear to move across the device, amounting to an effect with significant visual impact.

(53) FIG. 10 shows another embodiment of a security device 1 in accordance with an aspect of the invention, which here is a moir magnifier or integral imaging device. Thus whilst the physical structure of the device 1 is much the same as that described in the preceding embodiments, here the image array 30 comprises a regular array of microimages 31 rather than image slices. The microimages are arranged with a pitch mismatch and/or a rotational mismatch relative to the focussing element array 20 such that the device as a whole exhibits magnified versions of the microimages 31 as described with reference to FIGS. 8 and 9 above. The focussing element array could possess one dimensional or two dimensional periodicity, e.g. being formed of cylindrical, spherical or aspherical lenses. In a first region R.sub.1 of the device, the focussing elements 21a carry a coloured tint, e.g. yellow, whereas in a second region of the device R.sub.2, the focussing elements 21b are colourless (although could possess any other colour different to that in region R.sub.1). The image array 30 comprises microimages 31 which substantially match the colour of the filter 10 in region R.sub.1 and hence are yellow in this example, against a colourless background (e.g. white, silver-reflective or clear).

(54) As seen in the plan view of FIG. 10(b), the central second region R.sub.2 here has the shape of a star whilst the first region R.sub.1 provides a background thereto filling the remainder of the rectangular device area.

(55) In the central second region R.sub.2, the focussing elements 21b will cooperate with the microimages 31 in a standard manner to exhibit the desired optically variable effect. In the surrounding first region R.sub.1, however, the matching colours of the filter 10 and the microimages 31, together with the colourless background 32, reduce or preferably prevent the visualisation of the microimages such that the appearance of the optically variable effect is substantially diminished and preferably eliminated. As a result, the device 1 appears optically variable only across star-shaped region R.sub.2 and not elsewhere. This approach enables the shape, size and position of the optically variable area to be controlled solely through design of the colour filter 10 whilst the image array 30 can be provided in a continuous manner without modification. As such, more complex device designs can be achieved.

(56) The embodiment shown in FIG. 11 advances the same principles a step further by making use of two image arrays 30a and 30b. The construction of the device 1 is otherwise the same as in the FIG. 10 example and so will not be described again here. The two image arrays 30a and 30b are formed in different colours from one another: hence, in an example the image array 30a is yellow (as per image array 30 in the preceding embodiment) whilst image array 30b is blue. Both image arrays have colourless backgrounds 32. Both of the image arrays may be provided across the whole area of the device, overlapping one another, e.g. formed in two sequential printed workings. The image arrays 30a and 30b might each define an array of microimages 31a, 31b which co-operates with the focussing element array 20 to exhibit a moir magnification or integral imaging effect, or they could each be designed to generate different effects in combination with the focussing elements such as a moir magnification effect from image array 30a and a lenticular effect from image array 30b. In this example, each image array 30a, 30b is adapted to generate a moir magnification effect in combination with the focussing elements 20.

(57) In the first region R.sub.1, forming the outer surroundings of the device 1, as in the previous example, the optically variable effect from image array 30a is inhibited due to the colour matching between the colour filter 10 and the microimages 31a. However, the microimages 31b of the second image array 30b will not be inhibited since here the colours do not match. Thus, the optically variable effect arising from the second image array 30b (only) will be exhibited in the first region R.sub.1. In the second region R.sub.2 which again has here the shape of a star, since the colour filter 10 is colourless neither of the image arrays 30a, 30b will be inhibited and hence both optically variable effects will be displayed, superimposed on one another. The two image arrays can be designed to make best use of this superposition, e.g. through selection of the microimage contentfor instance the microimages 31a could each be signs and the microimages 31b each the digit 10 so that in combination information concerning the denomination 10 is conveyedand/or by configuring each set of magnified images to be visualised at different apparent heights or depthsfor instance one set could appear to float above the device and the other appear sunken below it.

(58) Alternatively, the colours could be selected so that each region of the colour filter matches one of the colours of the image arrays 30. This can be used to select single ones of the image arrays 30a, 30b etc to be active in each region. For instance if the FIG. 11 embodiment where modified such that the colour filer 10 is blue in region R.sub.2, now only the optically variable effect generated by the first image array 30a will be exhibited in that region, whilst that generated by the second image array 30b will be inhibited due to the matching colours.

(59) Generally, the colour of the colour filter 10 can therefore be used, by applying the principles above, to select which of a plurality of image arrays 30 is visualised in each region of the device. Any number of differently coloured image arrays 30 and regions could be combined in this way across the device, resulting in a highly complex appearance which is very difficult to replicate.

(60) In some of the above embodiments, the distinctive visual effects are achieved by matching a colour of the colour filter 10 to a colour of the image array 30. However, other strong effects can be achieved by arranging a colour of the colour filter 10 to be complementary to a colour of the image array 30. A complementary colour is one which if mixed with its corresponding colour would provide substantially all wavelengths of the visible spectrum and so appear either black or white depending on whether the colour mixing mechanism is additive or subtractive. By utilising complementary colours in this way, the contrast between the various colours exhibited by the device (either between a microimage and its surroundings viewed simultaneously, or between different images in a lenticular device) can be enhanced and hence the effect made more visually distinct.

(61) FIGS. 12 and 13 show two examples of security devices 1 utilising this principle which otherwise largely correspond in structure to the embodiments shown in FIGS. 6 and 7 respectively. Hence, only the modifications to those previous embodiments will now be described. In both examples, the security devices 1 are lenticular devices.

(62) In the FIG. 12 embodiment, the colour filter 10 is arranged to be cyan in region R.sub.1 and yellow in region R.sub.2, whilst image elements 31 of array 30 are red, spaced by colourless gaps 32. Red and cyan are complementary colours according to the RGB additive colour model and the CMY subtractive colour model.

(63) When viewed from a first viewing angle, observer O.sub.1 perceives the first region R.sub.1 to be very dark blue/indigo due to the combination of the cyan lenses 21a with the red image elements 31. The second region R.sub.2 appears orange. Upon tilting to another viewing angle, the second observer O.sub.2 sees the first region R.sub.1 as light blue and the second region R.sub.2 as yellow. Hence four different colours are visualised.

(64) The FIG. 13 embodiment is substantially the same as the FIG. 12 embodiment except that here an additional effect is introduced by phase-shifting the image array 30 between regions as in the earlier embodiments described herein. Thus, observer O.sub.1 again perceives the first region R.sub.1 to be very dark blue/indigo but now the second region R.sub.2 appears yellow. Upon tilting to another viewing angle, the second observer O.sub.2 sees the first region R.sub.1 as light blue and the second region R.sub.2 as orange. Hence the position of the darker contrast region appears to move upon tilting.

(65) In the preceding embodiments, the colour effects are achieved by the combination of the image array 30 and the overlying colour filter 10. However, still more complex effects can be achieved by additionally providing a multi-coloured backing layer which sits on the opposite side of the image array 30 and provides colour to any gaps therein between the image elements 31. FIGS. 14 to 17 provide four examples of embodiments of security devices making use of such a backing layer 40.

(66) For ease of comparison, the embodiments of FIGS. 14 and 15 correspond in all respects other than the provision of the backing layer 40 to the embodiments just described with reference to FIGS. 12 and 13, respectively. However it should be appreciated that here it is not essential for the image array 30 to be provided in a colour which is complementary to either of the colours of the colour filter 10, although this is preferred in order to provide enhanced contrast as mentioned above.

(67) Hence, the backing layer 40 can be provided as a printed layer or the like which covers at least part of the image array 30 on the side opposite from that on which the viewer is located in use. The backing layer 40 comprises at least two differently coloured materials 41a, 41b arranged in respective areas of the layer. It should be noted that these areas need not correspond to the aforementioned regions of the device, but this is preferred and in this example the first area of the backing layer containing material 41a corresponds to the first region R.sub.1 whilst the second area of the backing layer containing material 41b corresponds to the second region R.sub.2. The colours of the backing layer could be different to those of the colour filter but in this example they are the same. Hence, in region R.sub.1 the focussing elements 21a are cyan as is the backing material 41a, and in region R.sub.2, the focussing elements 21b and the backing material 41b are both yellow. The image elements 31 are red, spaced by colourless gaps 32.

(68) When viewed from a first viewing angle, observer O.sub.1 perceives the first region R.sub.1 to be very dark blue/indigo due to the combination of the cyan lenses 21a with the red image elements 31. The second region R.sub.2 appears orange. These colours are the same as in the FIG. 12 embodiment since the backing layer does not contribute here due to masking by the image elements 31. Upon tilting to another viewing angle, the second observer O.sub.2 sees now the first region R.sub.1 as bright blue and the second region R.sub.2 as bright yellow, each with increased colour intensity due to the contributions from the colour filter 10 and backing layer 40.

(69) Similarly, the FIG. 15 embodiment is identical to the FIG. 14 embodiment, save for phase-shifting of the image array 30 between the two regions. Thus, observer O.sub.1 again perceives the first region R.sub.1 to be very dark blue/indigo but now the second region R.sub.2 appears bright yellow. Upon tilting to another viewing angle, the second observer O.sub.2 sees the first region R.sub.1 as bright blue and the second region R.sub.2 as orange. Hence the position of the darker contrast region appears to move upon tilting.

(70) The complexity of the appearance can be further increased by arranging the areas of the backing layer 40 to differ from the regions R.sub.1, R.sub.2. For instance, multiple areas of the backing layer 40 could be located within any one of the regions. This is the case in the embodiments of FIGS. 16 and 17 which are otherwise structurally the same as the embodiments of FIGS. 14 and 15 respectively. Here, the backing layer 40 comprises four regions with materials 41a and 41b occupying the two halves of first region R.sub.1 and materials 41c and 41d occupying the two halves of second region R.sub.2. Materials 41a and 41c are cyan whilst materials 41b and 41d are yellow. The colour filter 10 is once again cyan in region R.sub.1 and yellow in region R.sub.2, whilst the image elements 31 are red.

(71) When the device is viewed at a first angle by observer O.sub.1, the whole of region R.sub.1 appears dark blue/indigo since once again the backing layer 40 does not contribute, and similarly the second region R.sub.2 appears orange. However upon tilting the arrangement of differently colours portions changes: now only half of region R.sub.1 appears light blue (corresponding to area 41a of the backing layer) whilst the other half and the neighbouring half of the second region R.sub.2 (corresponding to areas 41b and c) appear green, and the last half of region r.sub.2 appears yellow (area 41d). Hence five different colours are exhibited across the range of viewing angles, and the pattern of differently coloured device portions also changes.

(72) The FIG. 17 embodiment is identical to the FIG. 16 embodiment, save for phase-shifting of the image array 30 between the two regions. Thus, observer O.sub.1 again perceives the first region R.sub.1 to be very dark blue/indigo but now the second region R.sub.2 appears in two halves: green in area 41c and yellow in area 41d. Upon tilting, observer O.sub.2 now sees the first region R.sub.1 split into two halves 41a, 41b which are bright blue and green respectively while the whole of region R.sub.2 is orange.

(73) In all of the embodiments described so far the colour filter 10 has been formed integrally with the focussing elements array 20, e.g. in the form of coloured lenses. Preferred methods for forming multi-coloured focussing element arrays suitable for this purpose will now be described with reference to FIGS. 18, 19 and 20, and are disclosed in more detail in our existing International patent application no. PCT/GB2016/052082.

(74) In embodiments of the present invention, the focussing element array 20 is formed by cast-curing. This involves applying one or more transparent curable material either to the support layer or to a casting tool carrying a surface relief defining the desired focussing element array, forming the material using the casting tool and curing the material to fix the relief structure into the surface of the material.

(75) Referring to FIG. 18, a first transparent curable material 205a is applied to a support layer 201 (such as the substrate 2 shown in previous embodiments) using an application module 210a which here comprises a patterned print cylinder 211a which is supplied with the curable material from a doctor chamber 213a via an intermediate roller 212a. For example, the components shown could form part of a gravure printing system. Other printing techniques such as lithographic, flexographic, screen printing or offset printing could also be used. Print processes such as these are preferred since the curable material 205a can then be laid down on the support 201 only in first regions 202a thereof, the size, shape and location of which can be selected by control of the print process, e.g. through appropriate configuration of the pattern on cylinder 211a. The curable material 205a is applied to the support 201 in an uncured (or at least not fully cured) state and therefore may be fluid or a formable solid.

(76) A second application module 201b is then used to apply a second transparent curable material 205b to other second regions 202b of the support layer 201. The second application module is typically of the same construction as the first. The second transparent material 205b will have a different optical detection characteristic, particularly its visible colour, from the first material 205a.

(77) The support 201 is then conveyed to a casting module 220 which here comprises a casting tool 221 in the form of a cylinder carrying a surface relief 225 defining the shape of the focussing elements which are to be cast into the curable materials 205a,b. As each patch 202 (comprising regions 202a and 202b) of curable material 205 (comprising materials 205a and 205b) comes into contact with the cylinder 221, the curable material 205 fills a corresponding region of the relief structure, forming the surface of the curable material into the shape defined by the relief. The cylinder 221 could be configured such that the relief structure 225 is only provided at regions corresponding to shape and position of the patches 202 of curable material 205. However this gives rise to the need for accurate registration between the application module 210 and the casting module 220 in order that the focussing elements are accurately placed in each first region 202 of the curable material. Therefore in a particularly preferred example, the cylinder 221 carries the relief structure corresponding to the focussing elements over an area larger than that of the patch 202, preferably around its complete circumference and most preferably over substantially its whole surface (although axial regions which will not come into the vicinity of the curable material may be excluded). In this way, each entire patch 202 of curable material 205 is guaranteed to come into contact with the surface relief structure 225 such that the focussing element array is formed over the full extent of the material. As a result, the shape, size and location of the focussing element array 20 is determined solely by the application of the curable material by the application modules.

(78) Having been formed into the correct surface relief structure, the curable material 205 is cured by exposing it to appropriate curing energy such as radiation R from a source 222. This preferably takes place while the curable material is in contact with the surface relief 225 although if the material is already sufficiently viscous this could be performed after separation. In the example shown, the material is irradiated through the support layer 201 although the source 222 could alternatively be positioned above the support layer 201, e.g. inside cylinder 221 if the cylinder is formed from a suitable transparent material such as quartz.

(79) The surface relief 225 may be carried by cylinder 221 in the form of a sheet embossed or otherwise provided with the required relief, which is wrapped around the cylinder 221 and clamped in place. This may result in a noticeable join 225a where the two ends of the sheet meet, at which there is a discrepancy in the relief pattern. If replicated into one of the focussing element arrays this would cause a reduction in quality. It is therefore preferred that the casting module is at least coarsely registered to the application module so that the location of join 225a where it contacts support 201 does not coincide with any of the first regions 202 but rather is located between them, as shown by the example location labelled 225b. In cases where the curable material is applied (and retained) all over the support, or at least along a continuous strip in the machine direction MD, this join 225a is still preferably positioned outside the first region which is to be used to form the security device, advantageously in a location which will subsequently be coated with one of the opacifying layers 3. To achieve this consistently it is desirable for the process for forming the focussing element array to be registered with the opacifying layer application process, e.g. performed in the same in-line process.

(80) It will be noted that in the present example the two regions 202a, 202b (which correspond to regions R.sub.1, R.sub.2 in the preceding embodiments abut one another, as is preferred. Either the perimeter of the first region 202 as a whole, and/or the two regions 202a,b (in combination or independently of one another) preferably define indicia. The two application modules 210a,b are preferably registered to one another, e.g. performed in the same in-line process. The two curable materials 205a,b are then brought into contact with the casting cylinder 221 so as to form the surface relief into both materials, and cured as previously described. The result is a focussing element array formed of at least two materials laterally offset from one another (i.e. side by side), giving rise to an optically detectable pattern or indicia.

(81) FIGS. 19 and 20 show two alternative apparatus arrangements which may be used to form focussing element arrays of at least two materials. In these examples, the two curable materials 205a,b are applied to the casting cylinder 221 rather than to the support layer 201. Thus, in the FIG. 19 embodiment, application module 210a selectively applies a first curable material 205a to first regions 202a of the surface relief 225 on cylinder 221 and then application module 210b selectively applies a second curable material 205b to second regions 202b. In each application module 210, either or both of the rollers 211, 212 in the inking chain may be patterned. For example, rollers 212a,b may be pattered gravure rollers configured to take up resin on selected portions of their surfaces only, with respective removal means 213a, 213b such as doctor blades optionally being provided to remove any excess. Rollers 211a,b may then be uniform transfer rollers. The patterning required to form patches 202 and regions 202a, 202b could be achieved solely by the two application modules 210a,b in which case the focussing element relief structure 225 may be provided continuously across the whole surface of casting cylinder 221. Alternatively, as shown in FIG. 19, the relief structure 225 may be provided only in discrete patches on the surface of cylinder 221 and an optional removal means 213a, 213b such as respective doctor blades can be provided after each application station to remove any excess material. The precise location and extent of the patches 202 (and the regions thereof) which are ultimately formed on the support layer 201 may be determined by the manner in which the curable materials 205a,b and/or by the arrangement of the surface relief structure 225 on the cylinder 221.

(82) In a variant, shown in FIG. 20, rather than apply the two curable materials 205a,b onto the support layer 201 sequentially, the two application modules could be configured to apply the respective curable materials in the desired pattern onto some intermediate component, such as a blanket or an offset roller. The pattern of different curable materials can then be transferred onto the support layer 201 in a single application step. This has been found to improve the achievable registration. Thus, the apparatus shown in FIG. 20 corresponds largely to that of FIG. 19 except for the provision of collect roller 214 which is inserted between the application modules 210a, 210b and the casting cylinder 221. Thus, each application module 210a,b deposits its curable material 205a,b in a pattern onto the surface of collect roller 214, from which both materials 205a,b are then transferred together onto the casting cylinder 221. This approach has been found to achieve particularly accurate registration between the two curable materials 205a,b.

(83) As mentioned at the outset, the colour layer could alternatively be provided at various different locations within the security device structure, and this applies to all embodiments described above. For example, the security device could be constructed utilising a lens array (or other focussing element array) formed as a transfer element which is then affixed to a substrate carrying the image array on its opposite side. The substrate could be that of a polymer banknote, for example. FIG. 21a shows a cross-section through an exemplary lens transfer structure 50 formed using methods disclosed in our British patent application no. 1607480.9. The lens transfer structure 50 comprises a layer of carrier material 51, an upper surface of which has preferably been corona treated. On the upper surface of the carrier material is a layer of first material 52, which is substantially transparent, and has a first refractive index, of 1.35 for example. The upper surface of the first material 52 is shaped into a lens relief structure, which in this embodiment is a regular two-dimensional array of concavities suitable for functioning as a two-dimensional array of spherical lenses. Over the lens relief structure, i.e. over and in contact with the upper surface of the first material 52, is located a layer of second material 53, a lower surface of which conforms to the lens relief structure, and an upper surface of which is spaced from the lens relief structure and is substantially flat. The second material is also substantially transparent, and has a second, different refractive index, of 1.55 for example. The refractive index of the second material 26 is higher than that of the first material 21 such that the second material filling the concavities in the surface of the first material acts as an array of spherical, convex lenses, in this example. The transfer structure 50 is also provided with an adhesive layer 54 for affixing the lens structure to a substrate, although this adhesive layer could alternatively be provided on the substrate itself. The adhesive 54 is preferably heat-activated so that portions of the transfer element can be transferred to the substrate by hot stamping for example. In this embodiment, the adhesive layer 54 provides the colour filter 10. Hence, different portions of the adhesive layer 54 are differently coloured. In this example, two differently coloured regions 54a and 54b are shown to illustrate this.

(84) FIG. 21b shows the lens structure 50 described above having been transferred onto a security article, in this case a security document, however it could equally be transferred to any substrate, for example a security element such as a security thread. Transfer of the lens structure may be achieved by placing the upper surface of the lens transfer structure, i.e. the substantially flat surface of the second material 53 or the adhesive layer 54 if this is present, in contact with a surface of a substrate 2 of a security document (if the adhesive layer 54 is not provided as part of the transfer element 50 this will be pre-applied to the surface of substrate 2). The carrier layer 51 is then peeled away from the lens structure, leaving the lens structure formed by layers of the first and second material 52 and 53, and adhesive layer 54, on the surface of the substrate. The carrier layer is removed substantially without distorting the lens array provided within the lens structure since the peel strength of the bond between the carrier layer 51 and the first material 52 is relatively low, and in particular, is lower than the peel strength of the bond between the first and second materials 52 and 53.

(85) FIG. 21b shows the lens structure on a transparent substrate 2, which may for example be a polymer, such as biaxially oriented polypropylene (BOPP) as used in polymer banknotes. On a surface of the substrate 2 opposite to the lens structure is located image array 30. As previously described, the colour filter layer 54 will act to modify the appearance of the image array 30 and hence of the optically variable effect ultimately generated by the device.

(86) In a variant of this embodiment, rather than colour the adhesive layer 54, a colour filter layer may be applied between layers 53 and 54 or printed onto the surface of substrate 2 before the lens transfer structure 50 is applied.

(87) FIG. 22a schematically shows a further embodiment in which the colour filter 10 is provided in the form of a multi-coloured pedestal layer 25 located under the focussing element array on the surface of substrate 2, which is colourless (or of a single uniform colour) across the device. In this example the device has three concentric regions R.sub.1, R.sub.2 and R.sub.3 in each of which the pedestal layer is formed by a differently coloured transparent material 25a, 25b, 25c. Pedestal layers are described in more detail in our existing International Patent Application No. PCT/GB2016/052085 and typically have a preferred height in the region of at least 1 micron, more preferably at least 3 microns and most preferably at least 5 microns. The pedestal materials 10a,b,c are preferably flexible elastomers which helps improve the resilience of the device 1.

(88) A preferred method for forming a focussing element array with a pedestal layer will now be discussed with reference to FIG. 22b. Again, this involves cast-curing and any of the method variants described above could be employed for application of the curable material(s) 205 and forming thereof. However, an additional layer, referred to as a pedestal layer 25, is formed between the curable material(s) in which the focussing element array is defined and the support layer 201.

(89) Thus FIG. 22b shows an exemplary process in which, prior to application of the curable material 205 to the support layer 201, a pedestal layer 25 is formed by applying at least two transparent materials 207a,b to the support layer 201, using pedestal application modules 240a,b. Again this could involve printing or coating the transparent materials 207a,b onto the support layer using any of the same methods as previous mentioned for the application of curable material 205, such as gravure printing. In this example the material 207a is applied to a patterned gravure roller 241a from a reservoir 243a and a removal means 243a such as a doctor blade is provided for removing any excess material. If necessary, the support layer is then conveyed through a drying and/or curing section 245a to fix the material 207a. Whether the section 245a involves drying and/or curing will depend on the nature of the material 208a. Next the support layer 201 is conveyed through a second pedestal application module 240b at which a section transparent material 207b, of different colour, is applied selectively to a second region 208b of the support layer 201, in register with the first material 207a. The apparatus of the second pedestal application module 240b corresponds to that of the first pedestal application module 240a in this example. Again, a drying and/or curing section 245b may be provided for fixing the second material 207b. The pedestal materials 207a,b do need to cover at least the patches 202 in which the focussing element arrays are to be formed. In the example depicted, the areas 208a,b to which the pedestal materials 207a,b are applied are coincident with the patches 202, but this is not essential, and indeed may not be desirable since this gives rise to greater registration requirements. In more preferred examples, the each pair of regions 208a,b to which the pedestal materials 207a,b are applied are collectively larger than the extent of the respective focussing element array to be formed thereon, at least in the machine direction MD. This reduces the accuracy with which the lens application and formation stage must be registered to the pedestal application stage.

(90) The transparent materials 207a,b forming the pedestal layer may or may not also be a curable material. If not, the transparent material is preferably dried or otherwise solidified sufficiently before proceeding. If the material is curable, it may be cured during application from cylinder 241a,b or after, possibly at the same time as curing the curable material 205. However, preferably at least partial curing of material 207a,b takes place before curable material 205 is applied, which takes place at application station 210. In this example, this comprises a patterned gravure cylinder 211 onto which the curable material 205 is applied from a reservoir 213, a doctor blade 213 or similar being provided to remove excess. The curable material 205 can be applied in the same way as previously described but now is applied onto the pedestal layer 25 rather than onto the support layer 201. The curable material is then brought into contact with the casting tool 221 at casting station 220 in the same manner as previously described, and the focussing element array formed and cured into material 205.

(91) FIG. 23 illustrates an embodiment of a security device in which the colour filter 10 is again provided in a different location within the security element, as a base layer 35 located between the image array 30 and the substrate 2. Thus, the focussing element array 20 itself and any pedestal layer 25 provided can be colourless or of uniform colour across the device. The base layer 35 comprises at least two transparent materials of different colour arranged in respective concentric regions of the device. Hence in first region R.sub.1, the base layer 35 is formed of a first material 35a with a first colour, in second region R.sub.2, the base layer 35 is formed of a second material 35b with a second colour and in third region 35c the base layer 35 is formed of a third material 35c with a third colour.

(92) Such a base layer 35 could be formed in various different ways. In some preferred examples, the base layer could be printed or otherwise applied to the surface of substrate 2 using any convenient application technique, such as gravure printing or the like, the various different materials being applied in register with one another. The image array 30 would then be formed and affixed over the top of the existing base layer.

(93) However in other preferred embodiments, the coloured base layer 35 can take the form of a tie coat which is created integrally with the image array 30 and some preferred techniques for achieving this will now be described with reference to FIGS. 24 and 25.

(94) FIG. 24(a) shows a first preferred embodiment of a method for forming the image array 30, which is based on the principles disclosed in WO 2014/070079 A1, where more details can be found. The image array is formed on an image array support layer 301, which is preferably transparent, and such as the polymer substrate 2 mentioned above. The image array support layer 301 is preferably pre-primed, e.g. by applying a primer layer such as a thin, optically clear UV adhesive layer (not shown) or by raising its surface energy e.g. by corona treatment. The desired pattern of image elements which are to form the image array 30 (e.g. microimages, or slices of interleaved images) is defined by recessed areas in the surface 303 of a die form 302. Each recessed area preferably has a depth of the order of 1 to 10 microns, more typically 1 to 5 microns, and a width in the range 0.5 to 5 microns. The recessed areas are separated by raised areas of that surface 303. The die form preferably takes the form of a cylinder, but this is not essential.

(95) The recessed areas of the die form are filled with a curable material 305, which is preferably visibly coloured (including white, grey or black). The material 305 may or may not be transparent. An exemplary application module for applying the material 305 into the recessed areas is shown at 310a. This includes a slot die 312a configured to supply the curable material 305 to a transfer roller 311a from which it is applied to the die form surface 303. The shore hardness of the transfer roller 311a is preferably sufficiently low that some compression/compliance is achieved to improve the transfer of material to the die form 302, which is typically relatively rigid such as a metal print cylinder. The applied ink layer should match or exceed the depth of the recessed areas. The viscosity of the curable material may be configured so that the material 305 transfers substantially only into the recessed areas of the die form and not onto the raised surfaces but in case any of the material 305 remains on the raised surfaces it is preferred to provide a removal means such as doctor blade 315a to remove any such excess material 305 from outside the recessed areas. The material 305 in the recessed areas is preferably then at least partially cured by exposing the material 305 to appropriate curing energy, e.g. radiation, from a source 320a, although this curing could be performed at a later stage of the process.

(96) Any suitable curable material 305 could be used, such as a thermally-curable resin or lacquer. However, preferably, the curable material is a radiation curable material, preferably a UV curable material, and the curing energy source is a radiation source, preferably a UV source. UV curable polymers employing free radical or cationic UV polymerisation are suitable for use as the UV curable material. Examples of free radical systems include photo-crosslinkable acrylate-methacrylate or aromatic vinyl oligomeric resins. Examples of cationic systems include cycloaliphatic epoxides. Hybrid polymer systems can also be employed combining both free radical and cationic UV polymerization. Electron beam curable materials would also be appropriate for use in the presently disclosed methods. Electron beam formulations are similar to UV free radical systems but do not require the presence of free radicals to initiate the curing process. Instead the curing process is initiated by high energy electrons.

(97) The finished pattern should be visible (optionally after magnification) to the human eye and so the curable material comprises at least one colourant which is visible under illumination within the visible spectrum. For instance, the material may carry a coloured tint or may be opaque. The colour will be provided by one or more pigments or dyes as is known in the art. Additionally or alternatively, the curable material may comprise at least one substance which is not visible under illumination within the visible spectrum and emits in the visible spectrum under non-visible illumination, preferably UV or IR. In preferred examples, the curable material comprises any of: luminescent, phosphorescent, fluorescent, magnetic, thermochromic, photochromic, iridescent, metallic, optically variable or pearlescent pigments.

(98) If the first application module 310a achieves substantially complete filling of the recessed areas with material 305 then no further application of curable material 305 may be required. However it has been found that the recessed areas may not be fully filled by a single application process and so, in particularly preferred embodiments, a second application module (not shown) may be provided downstream of the first (and preferably of curing source 320a) for applying more of the same material 305 to the die form.

(99) Next, a tie coat 35 formed of at least two second curable materials 35a, 35b is applied over substantially the whole surface of the die form 303, i.e. coating both the filled recessed areas and the raised areas of the surface 303. The second curable materials may be of the same composition as the first curable material but are of a different appearance so as to provide a visual contrast with the first material in the finished array, as well as with each other. In particularly preferred embodiments, the tie coat composition may be selected so as to improve the adhesion between the first curable material and the support layer 301. The tie coat materials 35a, 35b are applied by respective tie coat application modules 330a,b which here each comprise a slot die 332 and a patterned transfer roller 331 which defines the different regions R.sub.1, R.sub.2 etc of the finished device.

(100) Preferably the two tie coat application modules 330a,b are registered to one another. In this way, each of the second materials 35a, 35b is applied to different respective parts of the cylinder 302 resulting in the desired differently coloured regions of the tie coat 35.

(101) The multi-coloured tie coat 35 may be partially cured at this point by a further radiation source (not shown). The die form surface carrying the filled recesses and tie coat is then brought into contact with the support layer 301, either at a nip point or, more preferably, along a partial wrap contact region between two rollers 309a, 309b as shown. The combination is then exposed to curing energy, e.g. from radiation source 335, preferably while the support layer 301 is in contact with the die form surface. The support layer 301 is then separated from the die form at roller 309b, carrying with it the tie coat 35 and the elements of material 305 removed from the recessed areas of the die form surface 303 by the tie coat 307. The material 305 is therefore present on the support layer 301 in accordance with the desired pattern, forming image array 30.

(102) The tie coat 35 is preferably at least partially cured before the die form 302 leaves contact with the support layer 301 at roller 309b, hence the preferred use of a partial wrap contact via lay on and peel off rollers 309a, b as shown which tension the web around the die form cylinder. If the material is not fully cured in this step, an additional curing station may be provided downstream (not shown) to complete the cure.

(103) In a variant, after the tie coat 35 has been applied, a removal means such as a further doctor blade could be provided to remove the tie coat 307 from the raised portions of the die form surface 303 such that the regions of the tie coat 307 are confined to the print images. These tie coat regions will most likely not be proud of the die form surface. As such the support layer 301 in this embodiment is preferably primed with a compliant adhesive layer which may be partly cured prior to contacting the die form but should still be compliant before entering the curing wrap.

(104) FIG. 24(b) shows a second preferred embodiment which corresponds in substantially all respects to that described above with reference to FIG. 24(a), the only difference being that here the two tie-coat materials 35a, 35b are each applied by the patterned rollers 331a, 331b to an intermediate collection roller 335 from which the two materials are then transferred simultaneously onto the cylinder 302 to form the tie coat 35. This approach has been found to achieve improved register between the tie coat materials.

(105) It will be appreciated that whilst in the above examples only two tie coat materials 35a, b are utilised, in practice any number of such materials could be used to form the tie coat 35 so that any number of differently coloured regions can be formed.

(106) Another embodiment of a method for forming an image array 30 is shown in FIG. 25. In many respects this is the same as described above with reference to FIG. 24 and so like items are labelled with the same reference numbers and will not be described again. The main difference is that here, the tie coat 35 is not applied to the die form surface 303 but rather to the surface of support layer 301, upstream of the point at which it is brought into contact with the die form. Thus the tie coat application module 330 is positioned upstream and is configured to apply the materials 35a,b to the surface of support layer 301. As before, each tie coat material 35a,b can be applied in a patterned manner to the support layer 301 by a respective tie coat application module 330a,b comprising for instance a slot die 332a,b feeding a patterned roller 331a,b, with an impression roller 333 being provided on the opposite side of the substrate. The tie coat application modules 330a,b are preferably registered to one another as before and result in the desired arrangement of differently coloured regions forming the tie coat 35.

(107) The support layer 201 carrying the tie coat 35 is then brought into contact with the die form surface so as to cover the filled recessed areas and adjacent raised areas with the tie coat 35. Preferably the tie coat 35 is pressed into the recessed areas so as to achieve good joining therebetween before the curing process begins. A second impression roller 334 may be provided for this purpose, located after the lay on roller 309a but before curing module 335.

(108) FIG. 25(b) shows a variant of the FIG. 25(a) method in which the two tie coat materials 35a,b, are each applied to an intermediate transfer roller 335 and then applied simultaneously to the support layer 301. Again this has been found to result in improved register between the materials.

(109) In the above embodiments, the colour filter 10 has been provided at a single location within the security device structure, i.e. either integrally with the focussing element array, as a pedestal layer, as an intermediate layer between substrates or as a base layer of the image array. However it is also possible to provide the colour filter using a combination of these approaches either in different respective regions of the device or together in the same region(s). For instance, in one region of the device the colour filter could be integrally provided in the focussing element array 20 whilst in another region it could take the form of a pedestal layer 25 and in yet another region it could be provided by a base coat 35 to the image array 30. Whilst the various parts of the colour filter will then be located at different heights within the device, this will not be apparent to the observer. Alternatively or in addition the different parts of the colour filter could overlap one another, either across the whole device or in portions thereof. In this case the effective colour of each region of the colour filter will be that created by the overlapping portions in combination with one another.

(110) FIGS. 26 and 27 illustrate two further embodiments of security devices in which the colour filter is provided at multiple locations across the device 1, although in this case all of the portions of the filter 10 are formed as a base layer to respective image arrays 30, e.g. in the form of tie-coats. In the FIG. 26 embodiment, the device substrate 2 is formed of two transparent substrate layers 2a, 2b which are laminated together. The focussing element array 20 is colourless and is formed on a first surface of layer 2a. On the second surface of layer 2a are formed two areas of a first image array 30a and in each cases these are located on a base layer 35a which is arranged in two regions of different coloured materials 10a, 10b. For instance, the base layer 35a could be formed as a tie coat using any of the methods described above in relations to FIGS. 24 and 25. The first surface of second substrate layer 2b is affixed over the first image array 30a and on its second surface a second image array 30b is provided, which also sits on a base coat 35b which here is of a single colour. Thus, the colour filter 10 as a whole is made up of three parts: 10(i) and 10(iii) which are laterally spaced portions of base coat 35a, sitting at the interface between substrate layers 2a and 2b, and 10(ii) which is formed by base coat 35b located on the outer surface of substrate layer 2b. The result is an arrangement of three concentric regions across the device 1: region R.sub.1 in which the colour filter 10 has the colour of material 10a (e.g. blue), region R.sub.2 in which the colour filter 10 has the colour of material 10b (e.g. green) and region R.sub.3 in which the colour filter has the colour of material 10c (e.g. yellow). The arrangement of colours in the filter 10 can be selected and combined with colours of the image arrays 30a,b to offer any of the enhanced security effects already described above.

(111) Since the image arrays 30a, 30b are located on different substrate surfaces, it may be desirable to vary the focal position of the focussing elements between regions, in order that the image elements remain in focus across the device. This could be achieved for instance by forming the focussing elements 21 in region R.sub.3 with a different shape for those in regions R.sub.1 and R.sub.2 such that they have a longer focal distance. Alternatively, the focussing elements in regions R.sub.1 and R.sub.2 could be placed on pedestal layers to raise them away from the surface of substrate 2a so that their focal position is raised accordingly relative to that in region R.sub.3.

(112) The construction of the exemplary security device shown in FIG. 27 is substantially the same as that in 27 except that here each of the individual base layers 35a,b making up the colour filter 10 is of a single colour and they partially overlap to create additional colours. Hence, base layer 10a extends across regions R.sub.1 and R.sub.2 and is blue in both, whilst base layer 35b extends across regions R.sub.2 and R.sub.3 and is yellow in both. As a result the device will have substantially the same appearance as in FIG. 26 with region R.sub.1 appearing blue, R.sub.2 appearing green (due to the overlapping yellow and blue filters) and R.sub.3 appearing yellow. It will be noted that the arrangement of image elements in arrays 30a,b has been modified to ensure that all portions of the colour filter are located between the arrays and the viewer.

(113) In order to achieve an acceptably low thickness of the security device (e.g. around 70 microns or less where the device is to be formed on a transparent document substrate, such as a polymer banknote, or around 40 microns or less where the device is to be formed on a thread, foil or patch), the pitch of the lenses must also be around the same order of magnitude (e.g. 70 microns or 40 microns). Therefore the width of the image slices or microimages 31 is preferably no more than half such dimensions, e.g. 35 microns or less.

(114) As mentioned above, the thickness of the device 1 is directly related to the size of the focusing elements and so the optical geometry must be taken into account when selecting the thickness of the transparent layer 2. In preferred examples the device thickness is in the range 5 to 200 microns. Thick devices at the upper end of this range are suitable for incorporation into documents such as identification cards and drivers licences, as well as into labels and similar. For documents such as banknotes, thinner devices are desired as mentioned above. At the lower end of the range, the limit is set by diffraction effects that arise as the focusing element diameter reduces: e.g. lenses of less than 10 micron base width (hence focal length approximately 10 microns) and more especially less than 5 microns (focal length approximately 5 microns) will tend to suffer from such effects. Therefore the limiting thickness of such structures is believed to lie between about 5 and 10 microns.

(115) Whilst in the above embodiments, the focusing elements have taken the form of lenses, in all cases these could be substituted by an array of focusing mirror elements. Suitable mirrors could be formed for example by applying a reflective layer such as a suitable metal to the cast-cured or embossed lens relief structure. In embodiments making use of mirrors, the image array should be semi-transparent, e.g. having a sufficiently low fill factor to allow light to reach the mirrors and then reflect back through the gaps between the image elements. For example, the fill factor would need to be less than 1/2 in order that that at least 50% of the incident light is reflected back to the observer on two passes through the image element array.

(116) In all of the embodiments described above, the security level can be increased further by incorporating a magnetic material into the device. This can be achieved in various ways. For example an additional layer may be provided (e.g. under the image array 30) which may be formed of, or comprise, magnetic material. The whole layer could be magnetic or the magnetic material could be confined to certain areas, e.g. arranged in the form of a pattern or code, such as a barcode. The presence of the magnetic layer could be concealed from one or both sides, e.g. by providing one or more masking layer(s), which may be metal. If the focussing elements are provided by mirrors, a magnetic layer may be located under the mirrors rather than under the image array.

(117) Security devices of the sort described above can be incorporated into or applied to any article 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.

(118) The security device or article 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 6 mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.

(119) The security device or article may be subsequently incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate. 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.

(120) 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.

(121) The security device may also be applied to one side of a paper substrate 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.

(122) Examples of such documents of value and techniques for incorporating a security device will now be described with reference to FIGS. 28 to 31,

(123) FIG. 28 depicts an exemplary document of value 100, here in the form of a banknote. FIG. 28a shows the banknote in plan view whilst FIG. 28b shows the same banknote in cross-section along the line Q-Q. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 102. Two pacifying layers 103a and 103b are applied to either side of the transparent substrate 102, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 102.

(124) The opacifying layers 103a and 103b are omitted across an area 101 which forms a window within which the security device is located. As shown best in the cross-section of FIG. 28b, an array of focusing elements 20 is provided on one side of the transparent substrate 102, and a corresponding image element array 30 is provided on the opposite surface of the substrate (the colour filter 10 is not shown but will be present). The focusing element array 20 and image element array 30 are each as described above with respect to any of the disclosed embodiments, such that the device 1 displays an optically variable effect in window 101 upon tilting the device (an image of the letter A is depicted here as an example). It should be noted that in modifications of this embodiment the window 101 could be a half-window with the opacifying layer 103b continuing across all or part of the window over the image element array 30. In this case, the window will not be transparent but may (or may not) still appear relatively translucent compared to its surroundings. The banknote may also comprise a series of windows or half-windows. In this case the different regions displayed by the security device could appear in different ones of the windows, at least at some viewing angles, and could move from one window to another upon tilting.

(125) FIG. 29 shows such an example, although here the banknote 100 is a conventional paper-based banknote provided with a security article 105 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 104 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 in is incorporated between layers of the paper. The security thread 105 is exposed in window regions 101 of the banknote. Alternatively the window regions 101 which may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. The security device is formed on the thread 105, which comprises a transparent substrate with lens array 20 provided on one side and image element array 30 provided on the other. In the illustration, the lens array 20 is depicted as being discontinuous between each exposed region of the thread, although in practice typically this will not be the case and the security device will be formed continuously along the thread.

(126) If desired, several different security devices 1 could be arranged along the thread, with different or effects displayed by each. In one example, a first window could contain a first device, and a second window could contain a second device, each having their focusing elements arranged along different (preferably orthogonal) directions, so that the two windows display different effects upon tilting in any one direction. For instance, the central window may be configured to exhibit a motion effect when the document 100 is tilted about the x axis whilst the devices in the top and bottom windows remain static, and vice versa when the document is tilted about the y axis.

(127) In FIG. 30, the banknote 100 is again a conventional paper-based banknote, provided with a strip element or insert 108. The strip 108 is based on a transparent substrate and is inserted between two plies of paper 109a and 109b. The security device is formed by a lens array 18 on one side of the strip substrate, and an image element array 70 on the other. The paper plies 109a and 109b are apertured across region 101 to reveal the security device, which in this case may be present across the whole of the strip 108 or could be localised within the aperture region 101. The focusing elements 20 are arranged with their long direction along the X axis which here is parallel to the long edge of the note. Hence the lenticular effect will appear to activate upon tilting the note about the X-axis.

(128) A further embodiment is shown in FIG. 31 where FIGS. 31(a) and (b) show the front and rear sides of the document 100 respectively, and FIG. 31(c) is a cross section along line Z-Z. Security article 110 is a strip or band comprising a security device according to any of the embodiments described above. The security article 110 is formed into a security document 100 comprising a fibrous substrate 102, 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 (FIG. 31(a)) and exposed in one or more windows 101 on the opposite side of the document (FIG. 31(b)), Again, the security device is formed on the strip 110, which comprises a transparent substrate with a lens array 20 formed on one surface and image element array 30 formed on the other.

(129) In FIG. 31, the document of value 100 is again a conventional paper-based banknote and again includes a strip element 110. In this case there is a single ply of paper. Alternatively a similar construction can be achieved by providing paper 102 with an aperture 101 and adhering the strip element 110 on to one side of the paper 102 across the aperture 101. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting. Again, the security device is formed on the strip 110, which comprises a transparent substrate with a lens array 20 formed on one surface and image element array 30 formed on the other.

(130) In general, when applying a security article such as a strip or patch carrying the security device to a document, it is preferable to have the side of the device carrying the image element array bonded to the document substrate and not the lens side, since contact between lenses and an adhesive can render the lenses inoperative. However, the adhesive could be applied to the lens array as a pattern that the leaves an intended windowed zone of the lens array 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 It is also worth noting that since the device only exhibits the optical effect when viewed from one side, it is not especially advantageous to apply over a window region and indeed it could be applied over a non-windowed substrate. Similarly, in the context of a polymer substrate, the device is well-suited to arranging in half-window locations.