SECURITY DEVICE AND METHOD OF MANUFACTURE THEREOF

Abstract

A security device includes a luminescent material arranged in a first region, wherein the material luminesces in response to irradiation one excitation wavelength wherein the first region exhibits a luminescent visible colour; an array of viewing elements; and a masking layer positioned between the decorative layer and the array of viewing elements, the masking layer partially overlaps with first region, including first pattern elements defined by the masking material, and second pattern elements defined by the absence of the masking material wherein the decorative layer is visible through the second pattern elements; the masking material is non-luminescing in response to one excitation wavelength, and when the security device illuminates by at least one wavelength, where the array of pattern elements overlaps with the first region, the second pattern elements exhibit the luminescent visible colour, and the first pattern elements exhibit a visual appearance that's different to the luminescent visible colour.

Claims

1. A security device comprising: a decorative layer comprising a luminescent material arranged in a first region, wherein the luminescent material luminesces in response to irradiation at at least one excitation wavelength such that the first region exhibits a luminescent visible colour when illuminated by said at least one excitation wavelength; an array of viewing elements; and, a masking layer comprising masking material positioned between the decorative layer and the array of viewing elements, the masking layer arranged as an array of pattern elements at least partially overlapping with said first region, and comprising first pattern elements defined by the presence of the masking material, and second pattern elements defined by the absence of the masking material such that the decorative layer is visible through the second pattern elements, wherein; the masking material is non-luminescing in response to the at least one excitation wavelength and is such that, when the security device is illuminated in the absence of the at least one excitation wavelength, at least where the array of pattern elements overlaps with the first region the first and second pattern elements have substantially the same visual appearance, and when the security device is illuminated by said at least one excitation wavelength, where the array of pattern elements overlaps with the first region, the second pattern elements exhibit the luminescent visible colour, and the first pattern elements exhibit a visual appearance that is different to the luminescent visible colour; and wherein, the array of pattern elements cooperates with the array of viewing elements such that when the security device is illuminated with the at least one excitation wavelength, the security device exhibits an optically variable effect.

2. The security device of claim 1, wherein, when illuminated in the absence of the at least one excitation wavelength, the first region exhibits a non-luminescent visible colour.

3. The security device of claim 2, wherein the non-luminescent visible colour exhibited by the first region when illuminated in the absence of the at least one excitation wavelength is different to the luminescent visible colour exhibited by the first region when illuminated by said at least one excitation wavelength.

4. The security device of claim 1, wherein, when illuminated in the absence of the at least one excitation wavelength, the first region is substantially colourless.

5. The security device of claim 1 wherein, when the masking material is illuminated in the absence of the at least one excitation wavelength, the masking material is substantially transparent and substantially colourless such that the decorative layer is visible through the first pattern elements.

6. The security device of any of claim 2, wherein each first pattern element is arranged laterally within the first region and, when illuminated in the absence of the at least one excitation wavelength, the masking material exhibits a non-luminescent visible colour that is substantially the same as the non-luminescent visible colour exhibited by the first region in the absence of the at least one excitation wavelength.

7. A security device comprising: a decorative layer comprising a luminescent material arranged in a first region, wherein the luminescent material luminesces in response to irradiation at at least one excitation wavelength such that the first region exhibits a luminescent visible colour when illuminated by said at least one excitation wavelength; an array of viewing elements; and, a masking layer comprising masking material positioned between the decorative layer and the array of viewing elements, the masking layer arranged as an array of pattern elements at least partially overlapping with said first region, and comprising first pattern elements defined by the presence of the masking material, and second pattern elements defined by the absence of the masking material such that the decorative layer is visible through the second pattern elements, wherein; the masking material exhibits a non-luminescent visible colour when illuminated in the absence of said at least one excitation wavelength, and is non-luminescing in response to the at least one excitation wavelength, and wherein, when the security device is illuminated in the absence of the at least one excitation wavelength, in at least one part of the security device, the first pattern elements exhibit a visual appearance that is different to a visual appearance exhibited by the second pattern elements, whereby the array of pattern elements cooperates with the array of viewing elements such that the security device exhibits a first optically variable effect, and, when the security device is illuminated by said at least one excitation wavelength, where the array of pattern elements overlaps with the first region, the second pattern elements exhibit the luminescent visible colour, and the first pattern elements exhibit a visual appearance that is different to the luminescent visible colour, whereby the array of pattern elements cooperates with the array of viewing elements such that the security device exhibits a second optically variable effect; and wherein, the arrangement of the first region and the array of pattern elements is such that the first optically variable effect conveys first information, and the second optically variable effect conveys second information that is different to the first information.

8. The security device of claim 7, wherein the array of pattern elements defines a second region, wherein the pattern elements within the second region cooperate with the array of viewing elements to exhibit an optically variable effect, and wherein either: (i) the second region is laterally completely contained within the first region, and further wherein the first region exhibits a non-luminescent visible colour that is different to the non-luminescent visible colour exhibited by the masking material; or (ii) the second region laterally extends beyond the first region in at least a part of the device.

9.-13. (canceled)

14. The security device of claim 7, wherein the masking layer comprises at least first and second masking materials such that some first pattern elements comprise the first masking material and some first pattern elements comprise the second masking material, the first and second masking materials having different optical properties when illuminated in the absence of the at least one excitation wavelength.

15. The security device of claim 1, wherein the decorative layer comprises an ink arranged in said first region, the ink comprising said luminescent material.

16. The security device of claim 15, wherein the ink further comprises an optically detectable material that exhibits a non-luminescent visible colour when illuminated in the absence of the at least one excitation wavelength.

17. The security device of claim 8, wherein the first region is in the form of indicia or an indicium; wherein the second region is in the form of indicia or an indicium, and wherein the first and second regions complement each other.

18.-24. (canceled)

25. The security device of claim 1, wherein the masking material substantially absorbs radiation of the at least one excitation wavelength.

26. (canceled)

27. The security device of claim 1, wherein the masking material is an ink.

28. The security device of claim 1, wherein the at least one excitation wavelength is at least one UV wavelength in the range of 200 nm to 400 nm, or wherein the at least one excitation wavelength is substantially any UV wavelength in the range of 200 nm to 400 nm.

29.-40. (canceled)

41. The security device of claim 1, wherein, when the security device is illuminated in the absence of the at least one excitation wavelength, the security device conveys a first piece of information, and wherein the optically variable effect exhibited by the device when illuminated by the said at least one excitation wavelength conveys a second piece of information, the first piece of information being different to the second piece of information.

42. The security device of claim 1, wherein the array of viewing elements comprises an array of focussing elements.

43.-48. (canceled)

49. A method of manufacturing a security device, comprising: (i) providing a substantially transparent substrate; and, (ii) applying a masking layer, decorative layer and an array of viewing elements to the substrate such that the masking layer is positioned between the decorative layer and the array of viewing elements, and the array of viewing elements overlaps with the masking layer, and wherein; the decorative layer comprises a luminescent material arranged in a first region, wherein the luminescent material luminesces in response to irradiation at at least one excitation wavelength such that the first region exhibits a luminescent visible colour when illuminated by said at least one excitation wavelength; the masking layer comprises masking material and is arranged as an array of pattern elements at least partially overlapping with said first region, and comprises first pattern elements defined by the presence of the masking material, and second pattern elements defined by the absence of the masking material such that the decorative layer is visible through the second pattern elements, wherein; the masking material is non-luminescing in response to the at least one excitation wavelength and is such that, when the security device is illuminated in the absence of the at least one excitation wavelength, the first and second pattern elements have substantially the same visual appearance, and when the security device is illuminated by said at least one excitation wavelength, where the array of pattern elements overlaps with the first region, the second pattern elements exhibit the luminescent visible colour, and the first pattern elements exhibit a visual appearance that is different to the luminescent visible colour; and wherein, the array of pattern elements cooperates with the array of viewing elements such that when the security device is illuminated with the at least one excitation wavelength, the security device exhibits an optically variable effect.

50. (canceled)

51. A method of manufacturing a security device, comprising: (i) providing a substantially transparent substrate; and, (ii) applying a masking layer, decorative layer and an array of viewing elements to the substrate such that the masking layer is positioned between the decorative layer and the array of viewing elements, and the array of viewing elements overlaps with the masking layer, and wherein; the decorative layer comprises a luminescent material arranged in a first region, wherein the luminescent material luminesces in response to irradiation at at least one excitation wavelength such that the first region exhibits a luminescent visible colour when illuminated by said at least one excitation wavelength; the masking layer comprises masking material and is arranged as an array of pattern elements at least partially overlapping with said first region, and comprises first pattern elements defined by the presence of the masking material, and second pattern elements defined by the absence of the masking material such that the decorative layer is visible through the second pattern elements, wherein; the masking material exhibits a non-luminescent visible colour when illuminated in the absence of said at least one excitation wavelength and is non-luminescing in response to the at least one excitation wavelength, and wherein, when the security device is illuminated in the absence of the at least one excitation wavelength, in at least one part of the security device, the first pattern elements exhibit a visual appearance that is different to a visual appearance exhibited by the second pattern elements, whereby the array of pattern elements cooperates with the array of viewing elements such that the security device exhibits a first optically variable effect, and, when the security device is illuminated by said at least one excitation wavelength, where the array of pattern elements overlaps with the first region, the second pattern elements exhibit the luminescent visible colour, and the first pattern elements exhibit a visual appearance that is different to the luminescent visible colour, whereby the array of pattern elements cooperates with the array of viewing elements such that the security device exhibits a second optically variable effect; and wherein, the arrangement of the first region and the array of pattern elements is such that the first optically variable effect conveys first information, and the second optically variable effect conveys second information that is different to the first information.

52.-62. (canceled)

63. A security document or security article comprising a security device according to claim 1.

64. A security arrangement comprising: a security document that comprises a masking layer and an array of viewing elements; and a decorative layer; wherein the decorative layer may be arranged with the masking layer and array of viewing elements to form a security device according to claim 1.

65.-66. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0095] Preferred examples of the present invention will now be described with reference to the attached drawings, in which:—

[0096] FIG. 1(a) is a cross-sectional view of a security device according to an example embodiment of the invention, and FIGS. 1(b) to 1(d) illustrate the appearance of the device under different illumination conditions;

[0097] FIG. 2(a) is a cross-sectional view of a security device according to a further example embodiment of the invention, and FIGS. 2(b) and 2(c) illustrate the appearance of the device under different illumination conditions;

[0098] FIG. 3(a) is a cross-sectional view of a security device according to a further example embodiment of the invention, and FIGS. 3(b) and 3(c) illustrate the appearance of the device under different illumination conditions;

[0099] FIGS. 4(a) and 4(b) illustrate the appearance of a security device according to a further embodiment of the invention under different illumination conditions;

[0100] FIG. 5(a) schematically depicts a security device in accordance with a further embodiment of the invention, FIG. 5(b) showing a cross-section through the device, and FIGS. 5(c) and 5(d) illustrating the optically variable appearance of the device;

[0101] FIG. 6(a) is a cross-sectional view of a security device according to a further example embodiment of the invention, and FIGS. 6(b) and 6(c) illustrate the appearance of the device under different illumination conditions;

[0102] FIG. 7 illustrates the appearance of a security device according to a further embodiment of the invention;

[0103] FIG. 8(a) is a cross-sectional view of a security device according to a further example embodiment of the invention, and FIGS. 8(b) and 8(c) illustrate the appearance of the device under different illumination conditions;

[0104] FIGS. 9(a) and 9(b) illustrate the appearance of a security device according to a further embodiment of the invention under different illumination conditions;

[0105] FIGS. 10(a) to 10(c) illustrate the configuration of the masking layer and decorative layer in accordance with an embodiment of the invention;

[0106] FIG. 11(a) is a cross-sectional view of a the security device of FIG. 10, and FIGS. 11(b) and 11(c) illustrate the appearance of the device under different illumination conditions;

[0107] FIGS. 12(a) to 12(c) illustrate the configuration of the masking layer and decorative layer in accordance with an embodiment of the invention;

[0108] FIGS. 13(a) and 13(b) illustrate the appearance of a security device of FIG. 12 in different illumination conditions;

[0109] FIG. 14(a) illustrates the arrangement of the masking layer of a security device according to an embodiment of the invention, and FIG. 14(b) depicts the visual appearance of the security device;

[0110] FIG. 15(a) illustrates the arrangement of the masking layer of a security device according to a further embodiment of the invention, and FIG. 15(b) depicts the visual appearance of the security device;

[0111] FIG. 16(a) is a cross-sectional view of a security device according to a further embodiment of the invention, and illustrates the visual appearance of the device at different viewing angles, and FIG. 16(b) illustrates an example viewing element that may be used in the invention;

[0112] FIG. 17 is a flow diagram outlining the steps of an exemplary method for forming a security device;

[0113] FIGS. 18, 19 and 20 show three exemplary security documents carrying security devices in accordance with embodiments of the invention (a) in plan view and (b) in cross-section;

[0114] FIG. 21 illustrates a further exemplary security document carrying a security device in accordance with embodiments of the present invention (a) in front view, (b) in back view and (c) in cross-section; and

[0115] FIG. 22 illustrates a further example of a security document carrying a security device in accordance with the invention.

DETAILED DESCRIPTION

[0116] The following description sets out a number of embodiments of different security devices, and methods of manufacture thereof, according to the invention. However, it will be appreciated that the visual appearances of the devices in the examples are exemplary only, and different configurations of the viewing elements and pattern elements are envisaged so as to present different visual appearances, for example different indicia. The attached figures use different shading patterns to indicate different colours.

[0117] For ease of description, in the following examples the scenario where the security device is illuminated in the absence of the at least one excitation wavelength will be referred to as illumination under visible light, and the scenario where the security device is illuminated by the at least one excitation wavelength will be referred to as illumination under UV light. As discussed above, in this specification UV wavelengths refer to wavelengths between 200 nm and 400 nm, preferably between 235 nm and 380 nm. It should also be understood that when viewing under UV light there will also be visible light present so that colours visible under visible light illumination conditions will also contribute to the overall appearance in UV light.

[0118] The security device of the present invention is intended to be viewed primarily in reflected light from the side of the viewing elements. However, the device may be viewed from the side of the lenses in transmitted light if light is able to pass through the decorative layer.

[0119] FIG. 1(a) illustrates a cross-sectional view of a security device 100 according to a first example embodiment of the invention. In this example, the security device is a lenticular device. A substantially transparent substrate 10 is provided on one surface with an array 21 of focusing elements 20, here in the form of cylindrical lenses, and on the other, opposing, surface with a masking layer in the form of an array 15 of pattern elements, and a decorative layer 1. The masking layer is positioned between the decorative layer and the focussing elements. The pattern element array 15 comprises an arrangement of first pattern elements and second pattern elements. In this embodiment and the following example embodiments the first pattern elements are formed of a UV absorbing ink, and the second pattern elements are defined by the absence of said UV absorbing ink. The UV absorbing ink absorbs all wavelengths within the UV part of the electromagnetic spectrum such that when the device is illuminated under UV light, the first pattern elements appear black.

[0120] In this example the pattern of first and second pattern elements defines sections of two images interleaved with each other along a first direction, which in this case is along the x-axis. The image sections I.sub.1, I.sub.2 and their interleaved nature is illustrated in FIG. 1(a). The direction of interleaving is substantially perpendicular to the axial direction of the cylindrical lenses 20, which here is along the y-axis.

[0121] The lenses 20 have a focal length f that is substantially equal to the optical spacing between the lenses and the decorative layer 1, such that the focal plane of the array of lenses substantially corresponds to the plane of the pattern element array 15. In other words, the combined thickness of the lenses themselves and the transparent substrate 10 (the thickness of which is illustrated by t) is substantially equal to the focal length f of the lenses 20. Focussing elements that may be used in the present invention typically have a pitch in the range of 5-100 microns, preferably 20-60 microns; a height of 5-40 microns, preferably 5-20 microns and a focal length of 5-100 microns, preferably 5-75 microns. Typical thicknesses, t, of the transparent substrate are in the range of 9-150 microns, preferably 12-100 microns.

[0122] As best shown in the cross-section of FIG. 1(a), the pattern element array 15 and the focusing element array 21 have substantially the same periodicity as one another in the x-axis direction, such that one first image section I.sub.1 and one second image section I.sub.2 lies under each lens 20. In this case, as is preferred, the width, w, of each image section I.sub.1, I.sub.2 is approximately half that of the lens pitch. In this example, the image array is registered to the lens array 20 in the x-axis direction (i.e. in the arrays' direction of periodicity) such that a first image section I.sub.1 lies under the left half of each lens and a second image section I.sub.2 lies under the right half. However, registration between the lens array 20 and the image array in the periodic dimension is not essential.

[0123] The decorative layer 1 comprises a luminescent material. In the following example embodiments the decorative layer comprises an ink arranged in first region R, the ink comprising the luminescent material. For the purposes of this specification an ink is a composition comprising a binder carrying appropriate luminescent materials together with any dyes and/or pigments as necessary to exhibit the desired colour effects, of which examples will be given below. The ink used to form the decorative layer may be referred to herein as a “luminescent ink”.

[0124] The first region R may extend across the whole pattern element array 15, and even across the whole device, or may extend over a part (or “zone”) of the pattern element array, in which case within that zone the second pattern elements will possess the optical characteristics of the first region whereas outside that zone the second pattern elements may be transparent or may take on the colour of some underlying backing layer or substrate. In general the first region covers at least some first pattern elements and some second pattern elements such that under UV illumination the device exhibits an optically variable effect due to the perceived contrast between the first and second pattern elements where the first region R overlaps with the array. In this example the first region R overlaps with each first pattern element.

[0125] In this example the first region R is in the form of a circle when the device is viewed in plan view. Here, the luminescent ink exhibits a non-luminescent blue colour (i.e. appears blue) when illuminated by visible light, and exhibits a luminescent yellow colour (i.e. appears yellow) when illuminated under UV light.

[0126] The image sections of the pattern element array overlap with the region, R defined by the luminescent ink of the decorative layer 1. In contrast to the dimensions of the pattern elements and image sections, the dimensions of the region R are significantly larger than the pitch of the lenses 20 of the lens array such that the region R substantially does not interact with the lenses to provide an optically variable effect.

[0127] As we have seen above, the UV absorbing ink appears black when the device is illuminated by UV light. The UV absorbing ink is also substantially transparent and substantially colourless when illuminated by visible light. The consequences of these properties of the UV absorbing ink, and the configuration of the pattern element array and the luminescent ink will now be explained.

[0128] When the device 100 is viewed by an observer O from a first viewing angle θ.sub.1 within the x-z plane, each lens 20 will direct light from its underlying first image section I.sub.1 to the observer. When the device is tilted so that it is viewed by a second observer O.sub.2 at a second viewing angle θ.sub.2 within that viewing plane, now each lens 20 directs light from the second image sections I.sub.2 to the observer. In other words, at viewing angle θ.sub.1, the observer sees the first image sections in combination, and viewing angle θ.sub.2 the observer sees the second image sections in combination.

[0129] When the device 100 is illuminated with visible light and viewed in reflection from the side of the lenses, at viewing angle θ.sub.1 the observer perceives a blue circle against a white background (see FIG. 1(c)). This is because under visible light, the UV absorbing ink is substantially transparent and substantially colourless, and therefore the luminescent ink is visible through the first pattern elements. The part of the device outside of the region R is substantially transparent, represented by the white background in FIG. 1(c), At viewing angle θ.sub.2, the observer perceives the same image (i.e. a blue circle) as the luminescent ink is visible through the second pattern elements. Consequently, when the security device 100 is viewed in reflection under visible light and from the side of the lenses, as the device is tilted, the observer will perceive substantially no change in the exhibited image. In other words, no optically variable effect is exhibited.

[0130] However, when the device 100 is illuminated by UV light and again viewed in reflection from the side of the lenses 20, at viewing angle θ.sub.1 the image perceived by the observer comprises a black letter “A” against a circular yellow background, whereas at viewing angle θ.sub.2 the image perceived by the observer comprises a yellow circle in isolation with no “A” present, as illustrated in FIG. 1(b). This is because the image sections I.sub.1 together comprise an arrangement of first pattern elements and second pattern elements that together define the letter “A”. In this case, the background to the “A” is defined by second pattern elements with the luminescent ink of region R viewable therethrough.

[0131] In contrast, the second image sections I.sub.2 are formed entirely of second pattern elements (i.e. they do not comprise any UV absorbing ink) such that at viewing angle θ.sub.2, the device 100 exhibits the appearance of the luminescent ink, i.e. a yellow circle as seen in FIG. 1(b). Hence, as the security device is tilted back and forth between viewing angles θ.sub.1 and θ.sub.2 under UV illumination, the appearance of the device switches between image A and image B as shown in FIG. 1(b).

[0132] Under UV light illumination, the part of the device outside of the first region R of the decorative layer 1 appears brown/black as this part of the device does not luminesce in response to UV irradiation.

[0133] Therefore, on a change of illumination conditions from visible light to UV light, not only does the appearance of the security device 100 change, but so does the information content conveyed to the observer, as the appearance of the device alters by more than just a change in colour. Here, under visible light, there is substantially no optically variable effect whereas under UV illumination, there is an optically variable effect exhibited upon tilting the device, as illustrated in FIG. 1(b).

[0134] This change in information content (on top of different colours exhibited) under visible and UV lighting conditions provides a memorable and easily authenticatable visual effect which is difficult to counterfeit.

[0135] It is to be noted that when the device 100 is viewed from the side of the decorative layer 1 under visible light, the viewer would see the images as shown in FIG. 1(c) due to the coloured nature of the luminescent ink under visible light. Under UV light, the viewer would perceive a yellow circle corresponding to the region, R, of the luminescent ink at both viewing angles θ.sub.1 and θ.sub.2, as illustrated in FIG. 1(d). In contrast to the situation when the device is viewed from the side of the lenses, although there is a change in colour when moving from one lighting condition to the other, there is no change in information content in the manner as discussed above as there is no introduction of an optically variable effect.

[0136] FIG. 2 illustrates a further example embodiment similar to the one discussed above in relation to FIG. 1. In this example, the luminescent ink of the decorative layer 1 extends substantially completely across the (square) device. Again, the UV absorbing ink is substantially transparent and clear in visible light such that the images exhibited at viewing angles θ.sub.1 and θ.sub.2 in visible light are optically invariable blue squares as represented in FIG. 2(b).

[0137] In a similar manner to the embodiment of FIG. 1, the first pattern elements and second pattern elements are arranged in accordance with interleaved image sections I.sub.1, I.sub.2. The image sections I.sub.2 are made up of second pattern elements (i.e. do not comprise any UV absorbing ink) and therefore when the device is viewed under UV illumination, the image exhibited at viewing angle θ.sub.2 is a uniform yellow square as the luminescent ink is visible through the second pattern elements of the second image sections. The first image sections comprise both first pattern elements and second pattern elements, with the first pattern elements defining a background of a letter “A”. Therefore, under UV light and when viewed in reflection from the side of the lenses 20, at viewing angle θ.sub.1 the device exhibits a yellow letter “A” against a black background. The two images of the optically variable effect exhibited under UV light are illustrated in FIG. 2(c).

[0138] The devices of FIGS. 1 and 2 exhibiting such an “image reveal” optically variable effect under UV illumination proves a memorable and easily authenticatable optical effect, which is difficult to counterfeit.

[0139] In the example embodiments of FIGS. 1 and 2, the optically variable effect exhibited by the device under UV light was made up of a first image exhibiting a text character “A” against a background, and a second image of a single uniform colour. FIG. 3 shows a further embodiment where the optically variable effect in UV light exhibits different indicia at different viewing angles θ.sub.1 and θ.sub.2. This is achieved because the second image sections I.sub.2 do not correspond exclusively to second pattern elements. Therefore, both image sections I.sub.1 and I.sub.2 comprise first pattern elements and second pattern elements, as schematically shown in the cross-sectional view of the device in FIG. 3(a), with the first image sections labelled as “A” and the second image sections I.sub.2 labelled as “B”. In the example of FIG. 3, in the first image sections the first pattern elements define a background of a letter “A”, and in the second image sections the first pattern elements define the background of a letter “B”. Therefore, as above, when the device 100 is viewed under visible light an observer perceives an optically invariable (“static”) optical effect with the device exhibiting a blue colour at substantially all viewing angles (FIG. 3(b)). However, under illumination by UV light, at viewing angle θ.sub.1 the device exhibits a yellow “A” against a black background, and at viewing angle θ.sub.2 the device exhibits a yellow “B” against a black background, as represented in FIG. 3(c).

[0140] It will be understood that more than two images can be interleaved in this way in order to achieve a wide range of animation, morphing, zooming effects etc.

[0141] FIG. 4 shows a further example embodiment of the invention in which not only is the UV absorbing ink of the masking layer substantially transparent and substantially colourless under visible light, but in addition the luminescent ink of the decorative layer is also substantially colourless under visible light as well. Therefore, when viewed under visible light, the device 100 appears substantially colourless at substantially all viewing angles, as illustrated in FIG. 4(a). This advantageously means that the optically variable effect exhibited under UV illumination may be particularly unexpected after general viewing in visible light, providing a high level of security.

[0142] In the example of FIG. 4, the first and second pattern elements define sections of first and second images interleaved with each other periodically along the x axis. The set of first image sections comprises both first pattern elements and second pattern elements, with the second pattern elements defining a letter “A” against a background of first pattern elements. Similarly, the set of second image sections comprises both first pattern elements and second pattern elements, this time with the first pattern elements defining a letter “A” against a background of second pattern elements. This provides a striking “phase shift” optically variable effect when the device is viewed under UV light, with a yellow letter “A” visible against a black background at first viewing angle θ.sub.1 when the first image sections are visible, and a black letter “A” against a yellow background at second viewing angle θ.sub.2 when the second image sections are visible.

[0143] We will now discuss examples where the array of pattern elements is arranged in the form of a “parallax barrier” (or “venetian blind”), which provides an optically variable effect when the device is illuminated under UV light. This concept is now discussed with reference to FIG. 5. Here, the array of pattern elements 15 comprises first pattern elements P.sub.1, and second pattern elements P.sub.2. The first pattern elements P.sub.1 are formed of UV absorbing ink, and the second pattern elements are defined by an absence of the UV absorbing ink. In other words, the second pattern elements may be seen as the “gaps” between adjacent first pattern elements.

[0144] The size and shape of each of the first pattern elements P.sub.1 is substantially identical. The pattern elements in this example are elongate strips and so the overall pattern of the elements is a line pattern, with the elongate direction of the lines lying substantially parallel to the axial direction of the focusing elements 20, which here is along the Y axis.

[0145] The array of pattern elements is registered to the array of lenses 20 in the x-axis direction (i.e. in the arrays' direction of periodicity) such that a first pattern element P.sub.1 lies under the left half of each lens 20, and a second pattern element P.sub.2 lies under the right half.

[0146] As is clearly shown in FIG. 5(b), when an observer views the device 100 at a viewing angle θ.sub.1, each lens 20 will direct light from its underlying first pattern element P.sub.1 to the observer. Conversely, when the device is tilted so that it is viewed by an observer at a second viewing angle θ.sub.2, now each lens 20 directs light from the second pattern elements P.sub.2 to the observer. As the second pattern elements P.sub.2 are defined by the absence of UV absorbing ink, at viewing angle θ.sub.2, the decorative layer 1 is visible within the second pattern elements P.sub.2.

[0147] Thus, under UV illumination, at viewing angle θ.sub.1, the device exhibits an image that is a uniform black colour, as illustrated in FIG. 5(d). Under UV illumination and viewing at viewing angle θ.sub.2, the device exhibits an image defined by the appearance of the decorative layer 1, and more specifically the region, R, of the luminescent ink, as seen in FIG. 5(c). Hence, as the security device is tilted back and forth between viewing angles θ.sub.1 and θ.sub.2, the appearance of the device switched between image A and image B.

[0148] An example of this is seen in FIG. 6, where the array 15 of pattern elements is arranged as seen in FIG. 5(a), (schematically illustrated in the cross-section of FIG. 6(a)), and the luminescent ink is arranged in a first region corresponding to a serial number, here “A1111 1111”. In this example, the luminescent ink exhibits a non-luminescent black colour under visible light illumination, and luminesces to exhibit a blue colour under UV light.

[0149] Under illumination with visible light, due to the UV absorbing material being transparent and colourless under these illumination conditions, the appearance of the device at both viewing angles θ.sub.1 and θ.sub.2 is substantially the same, with the serial number being visible at substantially all viewing angles as illustrated in FIG. 6(b), However, under UV illumination, at viewing angle θ.sub.1, light from the first pattern elements P.sub.1 is directed to the observer and as such the serial number formed by the luminescent ink is not visible. However, at viewing angle θ.sub.2, the luminescent material is visible within the second pattern elements P.sub.2, and therefore the serial number is exhibited.

[0150] The luminescent ink may comprise a phosphorescent material that continues to luminesce after illumination with the UV radiation has stopped (in contrast to fluorescent materials where the luminescent effect stops substantially immediately on removal of the at least one excitation wavelength). Thus, with the UV light removed and the device 100 viewed in visible light, the serial number is visible in a green colour at viewing angle θ.sub.2, as illustrated in FIG. 7.

[0151] FIG. 8 illustrates an example embodiment where the luminescent ink of the decorative layer exhibits a complex multi-coloured photograph under both visible and UV light. As will be appreciated by the skilled person, in order to form a multi-coloured photograph the decorative layer therefore typically comprises a plurality of luminescent inks, e.g. four inks that exhibit CMYK colour mixing in visible light and corresponding colour mixing in UV light. Other colour mixing models such as RGB may be used. As with the previous examples, the UV absorbing ink of the first pattern elements is substantially transparent and substantially colourless under visible light illumination, and is in the form of a “parallax barrier” line pattern as represented by the cross-section in FIG. 8(a).

[0152] Therefore, when the device 100 is illuminated by visible light and viewed in reflection from the side of the lenses 20, an observer perceives the same colour photograph at both viewing angles θ.sub.1 and θ.sub.2, as represented by FIG. 8(b). However, upon illumination with UV radiation and viewing at viewing angle θ.sub.1, the pattern elements P.sub.1 mask the luminescent ink, thus obscuring the image. Conversely, at viewing angle θ.sub.2 under UV illumination, the colour photograph is visible within the second pattern elements P.sub.2. This optically variable effect under UV light is illustrated in FIG. 8(c).

[0153] It is to be noted that although the decorative layer and/or array of pattern elements may define an “image”, the overall appearance of the security device under a stated illumination condition is also referred to as an “image”.

[0154] FIG. 9 illustrates a variation on the embodiment described above with reference to FIG. 8. The arrangement of pattern elements is the same as that outlined above in the FIG. 8 embodiment; however, the decorative layer comprises inks that combine to exhibit a multi-coloured colour photograph under UV illumination, with each ink containing a pigment that exhibits a uniform red colour under visible lighting conditions. In the same way as described in the above embodiments, under visible light conditions the device exhibits a “static” optical effect that is substantially independent of viewing angle (here a uniform red square as shown in FIG. 9(a)). In contrast, under UV illumination, the device 100 exhibits substantially the same optically variable effect as in FIG. 9, and is represented in FIG. 9(b). As discussed above, this change in information content conveyed to the observer when switching from visible to UV illumination of the device provides a memorable effect to the observer which is easy to authenticate and yet highly difficult to counterfeit.

[0155] In the examples provided so far, the UV absorbing ink of the first pattern elements has been substantially transparent and substantially colourless under visible light. This has meant that when illuminated under visible light, the first pattern elements and second pattern elements have had substantially the same visual appearance such that there is no optically variable effect exhibited by the device. We now move on to look at examples where the device exhibits an optically variable effect under both visible light and UV illumination.

[0156] FIG. 10 illustrates an example embodiment of the invention where, under visible fight illumination, the UV absorbing material exhibits a non-luminescent visible colour (here, blue). The luminescent ink of the decorative layer 1 defines a first region R.sub.1, here in the shape of an 8-pointed star, as shown in FIG. 10(a). The luminescent ink of this exemplary embodiment comprises pigments that exhibit a blue non-luminescent colour in visible light and a luminescent material that exhibits a yellow luminescent colour in response to illumination by UV radiation.

[0157] FIG. 10(b) illustrates the arrangement of the pattern element array 15 comprising first pattern elements P.sub.1 and second pattern elements P.sub.2. In the same manner as described above in the embodiments of FIGS. 1 to 5, the pattern of first and second pattern elements define sections I.sub.1, I.sub.2 of two images interleaved with each other along a first direction, which in this case is along the x-axis. The first image is that of an 8-pointed star, having a periphery that defines a second region R.sub.2. The second image is of uniform appearance (i.e. the second image sections are made up exclusively of second pattern elements).

[0158] Here, parts (labelled Z) of the second region R.sub.2 defined by the pattern element array 15 extend beyond the periphery of the first region R.sub.1 defined by the luminescent ink of the decorative layer, as seen in FIG. 10(c). The resulting device is schematically shown in cross section in FIG. 11(a).

[0159] Under illumination by visible light, in the areas Z, the first pattern elements P.sub.1 and the second pattern elements P.sub.2 have a different visual appearance. As such, the device exhibits an optically variable effect in visible light, which is schematically shown in FIG. 11(b). More specifically, at viewing angle θ.sub.1, the first pattern elements P.sub.1 are visible such that the image perceived by the observer is a 16-pointed star that is a superposition of the regions R.sub.1 and R.sub.2. Thus the regions R.sub.1 and R.sub.2 may be said to complement each other, with the region R.sub.2 defined by the array of pattern elements in isolation being at least partially discernible at viewing angle θ.sub.1.

[0160] In contrast, upon tilting the device and viewing at viewing angle θ.sub.2 under visible light, only the second pattern elements P.sub.2 of the areas Z are visible. As there is no luminescent ink visible through the second pattern elements in the areas Z, at viewing angle θ.sub.2 the device exhibits the star shape defined of region R.sub.1 defined by the luminescent ink.

[0161] Under illumination by UV light, at viewing angle θ.sub.1, the device exhibits an image comprising the outer points of the star shape defined by region R.sub.1 in yellow, as these portions of the luminescent material are not covered by UV absorber material. At viewing angle θ.sub.2, the star shape defined by the luminescent ink is visible in a yellow colour. This is schematically shown in FIG. 11(c).

[0162] Thus, the embodiment described above with reference to FIGS. 10 and 11 exhibits an optically variable effect in both visible and UV light conditions. The optically variable effect under visible light differs from the optically variable effect under UV light not only in the colour of the luminescent ink but also in the change of the image shapes exhibited on varying the viewing angle. As such, the information conveyed to the observer under visible light is different to the information conveyed under UV light. This combination of an optically variable effect being exhibited under two different lighting conditions, together with a change in conveyed information content, provides a device that is highly secure against counterfeiting.

[0163] FIGS. 12 and 13 illustrate a further variation on the previous embodiment, where in this example the UV absorbing ink exhibits a different visible colour under visible illumination to that of the luminescent ink of the decorative layer, and also comprises a transparent region, giving rise to further complex effects.

[0164] The first and second regions R.sub.1, R.sub.2 defined by the luminescent ink and the pattern element array respectively are the same 8-pointed stars as seen in the previous embodiment, and are shown in FIGS. 12(a) and 12(b). However, here, the second region R.sub.2 comprises a sub-region (the points of the star) that exhibits a red colour in visible light, and a further sub-region (the interior of the star) that is substantially transparent and colourless in visible light. Hence, some first pattern elements P.sub.1 comprise a portion formed of a first UV absorbing ink that exhibits a red colour in visible fight and a portion formed of a second UV absorbing ink that is substantially transparent and colourless in visible light. The transparent sub-region of the pattern element array is illustrated at T in FIG. 12(b).

[0165] The overlaid appearance of the pattern element array 15 on the luminescent ink of the decorative layer is illustrated in FIG. 12(c).

[0166] Under visible light, at viewing angle θ, a multi-coloured 16-pointed star is visible, corresponding to the outer peripheries of regions R.sub.1 and R.sub.2, and at viewing angle θ.sub.2, the star shape defined by the luminescent material (i.e. region R.sub.1) is visible, as illustrated in FIG. 12(a). Under UV light, the same optically variable effect as discussed above in the previous embodiment is exhibited, which is illustrated in FIG. 13(b).

[0167] In a further variation on the embodiment of FIGS. 12 and 13, the pattern element array 15 may comprise first pattern elements formed of a UV absorbing ink that exhibits a non-luminescent colour in visible light that differs from the non-luminescent colour of the luminescent ink, with the region R.sub.2 laterally completely contained within region R.sub.1. This would still provide an optically variable effect in visible light due to the visual (i.e. colour) difference between the first and second pattern elements.

[0168] The exemplary embodiments described thus far have been directed to lenticular devices. An embodiment of the invention will now be described with reference to FIG. 14, where the security device is a moiré magnifier, comprising a pattern element array 15 that defines an array of microimages 17 and an overlapping focussing element array (here, spherical lenses) with a pitch or rotational mismatch necessary in order to achieve the moiré effect. FIG. 14(a) depicts the pattern element array 15 as it would appear without the overlapping lens array, i.e. the non-magnified microimage array, but shown at increased scale for clarity.

[0169] In this example, the second pattern elements P.sub.2 form a regular array of microimages which here each convey the digit “5”. In this case, all of the microimages are of identical shape and size, although in other embodiments the microimages may vary in shape and/or size. The first pattern elements P.sub.1 comprising the UV absorbing ink form a contiguous, uniform background surrounding the microimages 17. The luminescent ink of the decorative layer 1 extends laterally across the whole of the pattern element array, and is visible through the second pattern elements of the pattern element array.

[0170] The luminescent ink of the decorative layer 1 exhibits a blue non-luminescent colour in visible light and a yellow luminescent colour under UV illumination. Under visible light the UV absorbing material is substantially transparent and colourless. Thus, when the device is viewed under visible light, the device appears a substantially uniform blue colour with no optically variable effect exhibited as both the first and second pattern elements have the same visual appearance. Conversely, under UV illumination, the viewer perceives an optically variable effect with the moiré effect acting to magnify the microimages 17 defined by the second pattern elements P.sub.2. More specifically, the viewer perceives an array of magnified yellow microimages 17′, each in the form of the digit “5”. Upon tilting of the device (i.e. changing the viewing angle), the magnified microimages 17′ appear to move relative to the device.

[0171] FIG. 14(b) represents the appearance of the device when viewed under UV illumination, at a first viewing angle approximately normal to the plane of the device. It should be noted that the scale of FIG. 14(b) is the same as that of FIG. 14(a), with the apparent enlargement effect due to the focussing element array and the moiré effect. In this example, just two of the enlarged microimages 17′ are shown. In practice, the size of the enlarged microimages and their orientation relative to the device will depend on the degree of mismatch between the focussing element array and the array of microimages 17.

[0172] In the example security device of FIG. 14 above, the microimages 17 are all identical to one another, such that the device can be considered a “pure” moiré magnifier. However, the same principles can be applied to “hybrid” moiré 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. 15, where FIG. 15(a) shows the unmagnified pattern element array 15, without the effect of focusing elements 20, and FIG. 15(b) shows the appearance of the finished security device, i.e. the magnified image. As shown in FIG. 15(a), the microimages 17 show an object, here a cube, from different angles. The microimages 17 are formed as first pattern elements P.sub.1 corresponding to the black lines of the cubes in the Figure, with the second pattern elements forming a contiguous uniform background, although the reverse configuration could be used. The region defined by the luminescent ink extends laterally across the whole of the pattern element array so as to cover each of the microimages 17, and is visible through the second pattern elements, so that under UV light the magnified microimages 17′ appear as black lines against the luminescent colour background of the luminescent ink. In the magnified image (FIG. 15(b)), the moiré effect generates magnified, 3D versions of the cube labelled 17′.

[0173] Further interesting effects may be generated if the region R defined by the luminescent ink covers only a part of the pattern element array 15 (i.e. covers only some of the microimages), with the microimages overlapping with the first region being more strongly visible under UV illumination. In the optically variable effect visible under UV fight, as the magnified microimages appear to move relative to the device as the device is tilted, the microimages appear to move in and out of the first region, providing an optical effect with high visual impact.

[0174] In all of the above examples of security devices, the viewing element employed to cooperate with the pattern element array to generate the optically variable effect(s) is a focussing element array, FIG. 16(a) shows an example security device which exhibits an optically variable effect under at least UV light illumination wherein the viewing elements are not required to be focussing elements such as lenses. In this example, each viewing element 20 comprises a substantially opaque (to at least UV light, and preferably also to visible light) portion 20a, and a gap region 20b. A plan view of such a viewing element 20 is shown in FIG. 16(b). The array of such viewing elements may be referred to as a “masking grid”, and could be composed of an array of metal lines 20a spaced by gaps 20b, for example.

[0175] The first and second pattern elements of the pattern element array 15 are arranged as a sequence of interlaced image sections A, B, C, etc., with the direction of interleaving here being along the x-axis. Each of the complete images A, B, C, etc. from which the image sections are taken is shown under the cross-section of the device, and it will be seen that these comprise a sequence of animation steps depicting a star symbol changing in size.

[0176] The luminescent ink of the decorative layer 1 substantially completely overlaps with the pattern element array, such that the luminescent ink is visible within the second pattern elements of the pattern element array 15. The luminescent ink exhibits a blue colour in visible light and a yellow luminescent colour when irradiated with UV light. The UV absorbing material of the first pattern elements is substantially transparent and colourless in visible light.

[0177] When viewing the device under visible light, no optically variable effect will be perceptible due to the transparent and colourless nature of the UV absorbing material making up the first pattern elements. This means that, under visible light, the first and second pattern elements exhibit substantially the same visual appearance. However, when illuminated with UV light, the device exhibits an optically variable effect, as described below.

[0178] Under UV light, when the device 100 is viewed from the side of the viewing elements 20, at any one instant, the image sections from only one of the images A to E are visible. For example, in the configuration shown in FIG. 16(a), when the device is viewed straight-on, only the image sections forming the image E will be visible, and thus the device as a whole will appear to exhibit a complete reproduction of image E. (It is to be noted that dependent on the arrangement of the first and second pattern elements, the images may be exhibited as black lines against a yellow background, or vice-versa.) Provided the dimensions of the device are correctly selected, when the device is observed from different angles, the different images will become visible. For example, when the device is viewed from position A under UV light, only the image sections forming image A will be viewable through the array of viewing elements, or “masking grid”. Similarly, when viewed from position C, image C will be exhibited. As such, as the device is tilted and the observer views it at different viewing angles, different stages of the animation will be seen and, provided that the images of the pattern element array are printed in the correct order, an animation will be perceived—in the present example a star increasing or decreasing in size as the device is tilted. Thus, in this case the animation is perceived as zooming in or out but in other cases the images could be arranged to depict, for example, perceived motion (e.g. a horse galloping), morphing (e.g. a sun changing into a moon) or perceived 3D depth (by providing multiple images of the same object, but from slightly different angles). Of course, fewer images (e.g. two) could be interleaved, resulting in an a “switch” or “reveal” effect at certain angles, as described above in FIGS. 1 to 4 for example, rather than an animation effect.

[0179] In order to achieve this effect, the width of each image section, X, must be smaller than the thickness, t, of the transparent substrate 10, preferably several times smaller, such that there is a high aspect ratio of the thickness t to image section width X. This is necessary in order that a sufficient portion of the pattern element array 15 can be revealed through tilting of the device. If the aspect ratio were too low, it would be necessary to tilt the device to very high angles before any change in image will be perceived. In a preferred example, each image section has a width X of the order of 5 to 10 microns, and the thickness t of the substrate 10 is approximately 25 to 35 microns.

[0180] The dimensions of the viewing elements 20 are generally larger than those of the pattern elements, requiring opaque stripes 20a of width ((n−1)X) where n is the number of images to be revealed (here, five), spaced by transparent regions 20b of approximately the same width as that of the image sections (X). Thus, in this example the opaque regions 20a of the viewing elements have a width of around 20 to 40 microns and may be produced using conventional techniques such as printing.

[0181] Viewing elements in the form of such a “masking grid” may also be used to form devices that exhibit moiré interference patterns. In such devices each of the array of viewing elements and the pattern element array typically consist of an array of line elements. To exhibit moiré effects, a mismatch between line element arrays required in order to form moiré interference fringes. This mismatch may be provided by a rotation of one of the arrays relative to the other, or by a pitch mismatch, and/or isolated distortions in either of the patterns.

[0182] In order to achieve significant perceived motion at relatively small viewing angles, a high aspect ratio of the spacing between the two patterns (typically the thickness of the transparent substrate 10) relative to the spacing of the line elements is required. For example, where the line elements of the pattern element array and the viewing element array have a width and a spacing of 5 microns, a thickness of around 25 microns is suitable. No registration between the pattern element array and viewing element array is required.

[0183] In the above embodiments, the non-luminescent colour of the luminescent ink is blue, with the ink appearing yellow under UV illumination. Examples of suitable ink formulae for use in these embodiments is set out below, although some adjustments may be necessary as will be readily understood by a person skilled in the art to achieve desired colours. It will be noted that in these cases the ink composition includes one or more visible (non-luminescent) pigments or dyes in addition to the luminescent material, which will typically be necessary unless the luminescent materials have the desired visible body colour. In these examples, each pigment or dye is supplied in the form of a base ink which also includes a binder (ink vehicle) of conventional composition, although this could be added separately. Also included in this case are additives such as driers, to improve the performance of the ink, which are optional.

[0184] Red Ink Luminescing Green

[0185] 9C3002B Bluish Red Base ink (ex SICPA) 16.8%

[0186] 9H0011B Transparent White Base ink (ex SICPA) 32.8%

[0187] 9C5033B Yellowish Green Fluorescent Base ink (ex SICPA) 49.7% Cobalt Driers 0.7%

[0188] Red Ink Luminescing Orange

[0189] 9C3002B Bluish Red Base ink (ex SICPA) 16.8%

[0190] 9H0011B Transparent White Base ink (ex SICPA) 32.8%

[0191] 9C1979B Yellow Fluorescent Base ink (ex SICPA) 49.7%

[0192] Cobalt Driers 0.7%

[0193] The two inks described above are responsive to substantially all UV wavelengths in the range 235 to 380 nm and so both inks will display the desired colour change when illuminated with any one UV wavelength in that range (plus visible light). However this is not essential and in other cases the luminescent ink need only be responsive to one or more UV wavelengths, provided that the UV absorbing ink absorbs that one or more UV wavelengths.

[0194] Green Ink Luminescing Red

[0195] 9C1033B Reddish Yellow Base ink (ex SICPA) 7.0%

[0196] 9C5000B Green Base ink (ex SICPA) 2.6%

[0197] 9H0011B Transparent White Base ink (ex SICPA) 39.8%

[0198] 9C3901B Red Fluorescent Base ink (ex SICPA) 50.0%

[0199] Cobalt Driers 0.6%

[0200] Green Ink Luminescing Yellow

[0201] 9C1033B Reddish Yellow Base ink (ex SICPA) 7.0%

[0202] 9C5000B Green Base ink (ex SICPA) 2.7%

[0203] 9H0011B Transparent White Base ink (ex SICPA) 69.7%

[0204] 9C1979B Yellowish Fluorescent Base ink (ex SICPA) 20.0%

[0205] Cobalt Driers 0.6%

[0206] FIG. 17 is a flow diagram illustrating the steps of an example method for manufacturing a security device according to the invention. At step S101 a transparent substrate 10 is provided, typically comprising at least one transparent polymeric material, such as BOPP, PET or polycarbonate, and may be monolithic or multi-layered. The substrate may be of a type suitable for forming the basis of a security article such as a security thread, strip, patch, transfer foil or a data page for a passport, or of a type suitable for forming the basis of a security document itself, such as a polymer banknote. The substrate is typically provided in the form of a substrate web such that a plurality of security devices may be manufactured in a web-based process, although sheet-based processes are also envisaged where the substrate is provided as a plurality of sheets.

[0207] At step S102, an array of viewing elements is applied to a first surface of the substrate web (or sheet). Typically the viewing elements are focussing elements such as lenses, which may be formed by, for example, lamination or cast curing. Cast curing involves applying a transparent curable material to a 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. The support layer may be the substrate web, or could be a separate support layer which is later applied to the substrate web (e.g. a transfer foil that may be applied to the substrate web by a foiling machine).

[0208] The curable material is cured by exposing it to appropriate curing energy, typically UV radiation from a source. This preferably takes place while the curable material is in contact with the surface relief although if the material is already sufficiently viscous this could be performed after separation of the casting tool from the support layer. The curable material may be irradiated through the support layer (typically the case when the lenses are formed on a transfer foil), although the source could alternatively be positioned above the support layer, e.g. inside the casting tool if it is formed from a suitable transparent material such as quartz.

[0209] In one embodiment the curable material is partially cured while in contact with the surface relief, with a subsequent cure performed after the curable material is released from the surface relief to fully cure the curable material. The radiation applied to cure the material after it is released from the surface relief may be directed through the support layer, or from above the support layer.

[0210] If the viewing elements formed in step S102 are in the form of a masking grid (as described in relation to FIG. 16) rather than focussing elements, the array of focussing elements may be formed using the techniques described below for forming the pattern element array in step S103.

[0211] At step S103 the masking layer is applied to the opposing surface of the web (or sheet) to which the array of viewing elements was applied in step S102. As has been explained above, the masking layer is arranged as an array of pattern elements comprising first pattern elements defined by the presence of masking material (typically UV absorbing ink), and second pattern elements defined by an absence of masking material. The pattern element array may be provided using conventional printing techniques such as lithographic printing, fleoxographic printing, intaglio printing, screen printing or gravure, with line widths typically between 5-50 microns.

[0212] At step S104 a decorative layer 1 is applied over the pattern element array formed in step S103. The decorative layer comprises a luminescent material that exhibits a luminescent visible colour in response to irradiation by at least one excitation wavelength (typically within the UV spectrum). The decorative layer typically further comprises a dye and/or pigment that exhibits a non-luminescent colour under visible light illumination. Substances such as these are preferably dispersed in a binder to form an ink, for example, suitable for application by printing or coating, or could be applied by other means such as vapour deposition. Most preferably, the decorative layer is applied by a printing technique such as: laser printing, inkjet printing, lithographic printing, gravure printing, flexographic printing, intaglio printing, screen printing, letterpress or dye diffusion thermal transfer printing.

[0213] In the present invention, the high resolution detail of the optically variable effect(s) is provided by the pattern element array, and as such the decorative layer does not need to be applied using a high resolution process. Indeed, the decorative layer preferably has a dimension much larger than that of the viewing elements such that it substantially does not interact with the viewing elements. If desired, the decorative layer may be applied in more than one working (e.g. in order to provide a decorative layer in the form of a complex colour photograph). The decorative layer may also be formed on another substrate and laminated to or transferred to the pattern element array.

[0214] As the decorative layer 1 does not need to be applied at high resolution, it can be made relatively thick and may possess sufficiently high optical density to produce a good quality image by itself. However, in some cases, it is desirable to increase the optical density by applying a substantially opaque backing later over the decorative layer.

[0215] Although the viewing element array is preferably applied to the substrate web prior to the formation of the masking layer, it may alternatively be applied at any stage during the above manufacturing process, particularly if the viewing elements are in the form of a masking grid which does not require UV curing.

[0216] In the embodiments described above, the viewing element array is formed on the opposing surface of the transparent substrate to the masking layer and the decorative layer. However, it is envisaged that in some variations, the decorative layer, masking layer and viewing element layer may be formed on the same surface (i.e. all on the same side of) of the transparent substrate. Such variations would typically require an optical spacer layer between the masking layer and array of viewing element array in order to provide a suitable optical spacing.

[0217] Security devices of the sorts described above can be incorporated into or applied to any product for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into documents of value such as banknotes, passports, driving licences, cheques, identification cards etc. The various components of the security device and/or the complete security device can either be formed directly on the security document (e.g. on a polymer substrate forming the basis of the security document) or may be supplied as part of a security article, such as a security thread or patch, which can then be applied to or incorporated into such a document.

[0218] Such security articles can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 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.

[0219] The security article may be incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate at at least one window of the document. Methods of incorporating security elements in such a manner are described in EP-A-1141480 and WO-A-03054297. In the method described in EP-A-1141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

[0220] 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.

[0221] The security device may also be applied to one side of a paper substrate, optionally so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.

[0222] Examples of such documents of value and techniques for incorporating a security device will now be described with reference to FIGS. 18 to 21.

[0223] FIG. 18 depicts an exemplary document of value 50, here in the form of a banknote. FIG. 18(a) shows the banknote in plan view whilst FIG. 18(b) shows a cross-section of the same banknote along the lines Q-Q′. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 51. Two opacifying layers 53 and 54 are applied to either side of the transparent substrate 51, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 51.

[0224] The opacifying layers 53 and 54 are omitted across a selected region 52 forming a window within which a security device is located. In FIG. 18(b), the security device is disposed within window 52, with a focusing element array 21 arranged on one surface of the transparent substrate 51, and masking layer 15 and decorative layer 1 on the other, thus forming device 100.

[0225] It will be appreciated that, if desired, the window 52 could instead be a “half-window”, in which opacifying layer 54 is continued over all or part of the masking layer and decorative layer. Depending on the opacity of the opacifying layers, the half-window region will tend to appear translucent relative to surrounding areas in which opacifying layers 53 and 54 are provided on both sides.

[0226] In FIG. 19 the banknote 50 is a conventional paper-based banknote provided with a security article 55 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 56 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 55 in window regions 57 of the banknote. Alternatively the window regions 57 may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. Here, the window regions 57 are “half thickness” windows, meaning that the thread 55 is only exposed on one surface of the banknote. It should be noted that the window regions 57 may be “full thickness” windows where the thread 55 is exposed on both surfaces if preferred. The security device 100 is formed on the thread 55, which comprises a transparent substrate, a focusing array 21 provided on one side and the masking layer and decorative layer provided on the other. Windows 57 reveal parts of the device, which may be formed continuously along the thread, as illustrated in FIG. 19(b). Alternatively several security devices could be spaced from each other along the thread, with different or identical images displayed by each.

[0227] In FIG. 20, the banknote 50 is again a conventional paper-based banknote, provided with a strip element or insert 58. The strip 58 is based on a transparent substrate and is inserted between two plies of paper 56a and 56b. The security device 100 is formed by a lens array 21 on one side of the strip substrate, and masking layer 15 and decorative layer 1 on the other. The paper plies 56a and 56b are apertured across region 59 to reveal the security device, which in this case may be present across the whole of the strip 58 or could be localised within the aperture region 59, as in FIG. 19(b). It should be noted that the ply 56a need not be apertured and could be continuous across the security device.

[0228] A further embodiment is shown in FIG. 21 where FIGS. 21(a) and (b) show the front and rear sides of the document 50 respectively, and FIG. 21(c) is a cross section along line Q-Q′. Security article 58 is a strip or band comprising a security device according to any of the embodiments described above. The security article 58 is formed into a security document 50 comprising a fibrous substrate 56, 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. 21(a)) and exposed in one or more windows 59 on the opposite side of the document (FIG. 21(b)). Again, the security device is formed on the strip 58, which comprises a transparent substrate with a lens array 21 formed on one surface and a co-operating masking layer 15 and decorative layer 1 as previously described on the other

[0229] Alternatively a similar construction can be achieved by providing paper 56 with an aperture 59 and adhering the strip element 58 onto one side of the paper 56 across the aperture 59. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting.

[0230] In still further embodiments, a complete security device could be formed entirely on one surface of a security document which could be transparent, translucent or opaque.

[0231] FIG. 22 illustrates a further embodiment of the present invention. Here, the decorative layer 1 is located separately to the masking layer 15 and array of focussing elements 21. In this particular embodiment, the decorative layer 1 is printed on a different part of a banknote 50 to the masking layer and focussing element array. When the banknote 50 is folded over, the complete security device 100 is formed, with the decorative layer 1, masking layer 15 and focussing element array 21 cooperating to exhibit the visual effects described above. In the example of FIG. 22, the banknote is a polymer banknote having opacifying layers 53, 54 defining a window region in which the complete security device may be viewed. In other alternative embodiments, the decorative layer 1 may be provided on a separate security document.

[0232] The security device of the current invention can be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.