SECURITY SUBSTRATES, SECURITY DEVICES AND METHODS OF MANUFACTURE THEREOF

Abstract

A security substrate has: a polymer substrate having first and second surfaces; array of focussing elements; an optical adjustment layer and masking layer. The array is formed of a surface relief. The optical adjustment layer is on the transparent base layer across a second region including the first. The optical adjustment layer has a first surface contacting the surface relief and an opposing second surface having a profile not operative to focus visible light. The optical adjustment layer has a first transparent material extending across a first sub-region of the array, the first transparent material having a refractive index different from that of the transparent base layer. The security substrate has a masking layer over the optical adjustment layer across a third region of the polymer substrate, the third region defining a gap in the masking layer(s), such that functional focusing elements of the array are revealed through the gap.

Claims

1-72. (canceled)

73. A security substrate, comprising: a polymer substrate having first and second surfaces; an array of focussing elements in the form of a surface relief across a first region of the polymer substrate, the surface relief being defined in the surface of a transparent base layer, wherein the transparent base layer comprises either the polymer substrate or a layer disposed thereon; an optical adjustment layer disposed on the transparent base layer across a second region of the polymer substrate, which second region includes at least the first region, the optical adjustment layer having a first surface in contact with the surface relief of the transparent base layer and an opposing second surface having a profile which is not operative to focus visible light, the optical adjustment layer comprising a first transparent material extending across a first sub-region of the array of focussing elements, the first sub-region comprising all or only part of the first region, the first transparent material having a refractive index different from that of the transparent base layer, whereby the focussing element(s) in the first sub-region of the array are functional focussing element(s); and at least one first masking layer, comprising a reflective and/or non-transparent material, disposed over the optical adjustment layer across a third region of the polymer substrate, the third region defining at least one gap in the first masking layer(s) which gap includes at least part of the first sub-region, such that functional focusing elements of the array are revealed through the at least one gap.

74. A security substrate according to claim 73, wherein the at least one gap in the first masking layer(s) reveals only a subset of the functional focusing elements of the array, other functional focusing elements of the array being substantially concealed by the first masking layer(s), at least when the security substrate is viewed in reflected light from the side of the security substrate on which the first masking layer(s) are disposed.

75. A security substrate according to claim 73, wherein the optical adjustment layer further comprises a second transparent material extending across a second sub-region of the array of focussing elements, the second transparent material having a refractive index different from that of the transparent base material and from the first transparent material such that the focussing element(s) in the second sub-region of the array are functional focussing element(s) with a focal length different from that of the focussing elements in the first sub-region of the array.

76. A security substrate according to claim 73, wherein the optical adjustment layer further comprises a third transparent material extending across a third sub-region of the array of focussing elements, the third transparent material having a refractive index substantially the same as that of the transparent base material, such that the focussing element(s) in the third sub-region are non-functional focussing element(s).

77. A security substrate according to claim 73, wherein the second region covered by the optical adjustment layer extends across substantially the whole area of the polymer substrate.

78. A security substrate according to claim 73, wherein the profile of the second surface of the optical adjustment layer is substantially planar.

79. A security substrate according to claim 73, wherein the array of focussing elements comprises a convex surface relief structure defined in the surface of the transparent base material, the transparent base material having a higher refractive index than that of the first transparent material and, if provided, the second transparent material; or wherein the array of focussing elements comprises a concave surface relief structure defined in the surface of the transparent base material, the transparent base material having a lower refractive index than that of the first transparent material and, if provided, the second transparent material.

80. A security substrate according to claim 73, further comprising an image array in at least part of the first sub-region of the polymer substrate, the image array located in a plane spaced from that of the focussing elements by a distance substantially corresponding to a focal length of the functional focusing elements in the at least part of the first sub-region, such that the functional focusing elements in the first sub-region exhibit a substantially focussed image of the image array.

81. A security substrate according to claim 80, wherein the image array is located on the second surface of the polymer substrate; or wherein the transparent base layer comprises a layer disposed on the first surface of the polymer substrate and the image array is located on the first surface of the polymer substrate.

82. A security substrate according to claim 73, wherein the security substrate is a security document, being any of a banknote, an identity document, a passport, a license, a certificate, a cheque, a visa or a stamp, and the at least one first masking layer is at least one opacifying layer comprising a non-transparent material.

83. A security substrate according to claim 73, wherein the security substrate is a security article, being a security thread, strip, patch or foil.

84. A method of manufacturing a security substrate, comprising: providing a polymer substrate having first and second surfaces; forming an array of focussing elements across a first region of the polymer substrate as a surface relief in the surface of a transparent base layer, wherein the transparent base layer comprises either the polymer substrate or a layer disposed thereon; applying an optical adjustment layer onto the transparent base layer across a second region of the polymer substrate, which second region includes at least the first region, the optical adjustment layer having a first surface in contact with the surface relief of the transparent base layer and an opposing second surface having a profile which is not operative to focus visible light, the optical adjustment layer comprising a first transparent material extending across a first sub-region of the array of focussing elements, the first sub-region comprising all or only part of the first region, the first transparent material having a refractive index different from that of the transparent base layer, whereby the focussing element(s) in the first sub-region of the array are functional focussing element(s); and applying at least one first masking layer, comprising a reflective and/or non-transparent material, over the optical adjustment layer across a third region of the polymer substrate, the third region defining at least one gap in the first masking layer(s) which gap includes at least part of the first sub-region, such that functional focusing elements of the array are revealed through the at least one gap.

85. A method according to claim 84, wherein the optical adjustment layer is formed by applying at least the first transparent material across the first sub-region region by printing or coating.

86. A security device, comprising: a polymer substrate having first and second surfaces; an array of focussing elements in the form of a surface relief across a first region of the polymer substrate, the surface relief being defined in the surface of a transparent base layer, wherein the transparent base layer comprises either the polymer substrate or a layer disposed thereon, the array of focussing elements being configured to have a uniform base focal length across the first region when the surface relief is in contact with air; an optical adjustment layer disposed on the transparent base layer across a second region of the polymer substrate, which second region includes at least a first sub-region of the array of focussing elements, the first sub-region not including the whole of the first region, the optical adjustment layer having a first surface in contact with the surface relief and an opposing second surface having a profile which is not operative to focus visible light, the optical adjustment layer comprising a first transparent material extending only across the first sub-region of the array of focussing elements, the first transparent material having a refractive index different from that of the transparent base layer, whereby the focussing element(s) in the first sub-region of the array have a first focal length which is different from the base focal length; and a first image array in at least part of the first sub-region of the substrate, the first image array being located in a plane spaced from that of the focussing elements by a distance substantially corresponding to the first focal length, such that the focusing elements in the first sub-region exhibit a substantially focussed image of the first image array and the focusing elements outside the first sub-region do not.

87. A security device according to claim 86, wherein the first image array is located on the second surface of the polymer substrate.

88. A security device according to claim 86, wherein the optical adjustment layer further comprises a second transparent material extending across a second sub-region of the array of focusing elements, the second transparent material having a refractive index different from that of the transparent base layer and different from that of the first transparent material, whereby the focussing element(s) in the second sub-region of the array have a second focal length which is different from the first and base focal lengths.

89. A security device according to claim 88, further comprising a second image array in at least part of the second sub-region of the substrate, the second image array being located in a plane spaced from that of the focussing elements by a distance substantially corresponding to the second focal length, such that the focusing elements in the second sub-region exhibit a substantially focussed image of the second image array and the focusing elements outside the second sub-region do not.

90. A security device according to claim 86, wherein the optical adjustment layer further comprises a third transparent material extending across a third sub-region of the array of focusing elements, the third transparent material having a refractive index substantially equal to that of the transparent base layer, whereby the focussing element(s) in the third sub-region of the array are non-operative.

91. A security device according to claim 90, wherein the third transparent material extends over the first transparent material and/or, if provided, the second transparent material, contacting the surface relief only in the third sub-region.

92. A security device according to claim 86, wherein the optical adjustment layer is absent across a fourth sub-region of the array of focusing elements, whereby in the fourth sub-region the focussing elements are exposed to air and have the base focal length.

93. A security device according to claim 92, further comprising a further image array in at least part of the fourth sub-region of the substrate, the further image array being located in a plane spaced from that of the focussing elements by a distance substantially corresponding to the base focal length, such that the focusing elements in the fourth sub-region exhibit a substantially focussed image of the further image array and the focusing elements outside the fourth sub-region do not.

94. A security device according to claim 86, wherein the second region covered by the optical adjustment layer extends across substantially the whole area of the polymer substrate.

95. A security device according to claim 86, wherein the array of focussing elements comprises a convex surface relief structure defined in the surface of the transparent base material, the transparent base material having a higher refractive index than that of the first transparent material and, if provided, the second transparent material; or wherein the array of focussing elements comprises a concave surface relief structure defined in the surface of the transparent base material, the transparent base material having a lower refractive index than that of the first transparent material and, if provided, the second transparent material.

96. A security device according to claim 86, wherein the profile of the second surface of the optical adjustment layer is substantially planar.

97. A security article comprising a security device according to claim 86, wherein the security article is a security thread, strip, foil, insert or patch.

98. A security document comprising a security device according to claim 86 or a security article being a security thread, strip, foil, insert or patch, wherein the security document is a banknote, an identity document, a passport, a license, a certificate, a cheque, a visa or a stamp.

99. A security document according to claim 98 wherein the polymer substrate constitutes the document substrate and the security document further comprises at least one first opacifying layer, comprising a non-transparent material, over the optical adjustment layer across a third region of the polymer substrate, the third region defining at least one gap in the first opacifying layer(s) which gap includes at least part of the first sub-region, such that a focussed image of the first image layer is exhibited through the gap.

100. A method of manufacturing a security device, comprising: providing a polymer substrate having first and second surfaces; forming an array of focussing elements as a surface relief across a first region of the polymer substrate, the surface relief being defined in the surface of a transparent base layer, wherein the transparent base layer comprises either the polymer substrate or a layer disposed thereon, the array of focussing elements being configured to have a uniform base focal length across the first region when the surface relief is in contact with air; applying an optical adjustment layer onto the transparent base layer across a second region of the polymer substrate, which second region includes at least a first sub-region of the array of focussing elements, the first sub-region not including the whole of the first region, the optical adjustment layer having a first surface in contact with the surface relief and an opposing second surface having a profile which is not operative to focus visible light, the optical adjustment layer comprising a first transparent material extending only across the first sub-region of the array of focussing elements, the first transparent material having a refractive index different from that of the transparent base layer, whereby the focussing element(s) in the first sub-region of the array have a first focal length which is different from the base focal length; and forming a first image array in at least part of the first sub-region of the substrate, the first image array being located in a plane spaced from that of the focussing elements by a distance substantially corresponding to the first focal length, such that the focusing elements in the first sub-region exhibit a substantially focussed image of the first image array and the focusing elements outside the first sub-region do not.

101. A method according to claim 100, wherein the optical adjustment layer is formed by applying at least the first transparent material across the first sub-region region by printing or coating.

Description

[0092] Examples of security devices, security substrates and methods of manufacture thereof will now be described with reference to the accompanying drawings, in which:

[0093] FIG. 1 shows an exemplary security document in accordance with embodiments of the first and/or second aspects of the invention, in plan view;

[0094] FIG. 2 shows a portion of an exemplary security document in accordance with an embodiment of the first aspect of the invention, (a) in plan view and (b) in cross-section;

[0095] FIG. 3 shows a portion of an exemplary security document in accordance with an embodiment of the second aspect of the invention, (a) in plan view and (b) in cross-section;

[0096] FIG. 4 shows a portion of an exemplary security document in accordance with an embodiment of the second aspect of the invention, (a) in plan view and (b) in cross-section;

[0097] FIG. 5 shows a portion of an exemplary security document in accordance with an embodiment of the first and second aspects of the invention, (a) in plan view and (b) in cross-section;

[0098] FIG. 6 shows a portion of an exemplary security document in accordance with an embodiment of the first and second aspects of the invention, (a) in plan view and (b) in cross-section;

[0099] FIG. 7 shows a portion of an exemplary security document in accordance with an embodiment of the second aspect of the invention, (a) in plan view and (b) in cross-section;

[0100] FIG. 8 shows a portion of an exemplary security document in accordance with an embodiment of the second aspect of the invention, (a) in plan view and (b) in cross-section;

[0101] FIG. 9 shows a portion of an exemplary security document in accordance with an embodiment of the first aspect of the invention, (a) in plan view and (b) in cross-section;

[0102] FIG. 10 shows a portion of an exemplary security document in accordance with an embodiment of the second aspect of the invention, (a) in plan view and (b) in cross-section;

[0103] FIG. 11 shows a portion of an exemplary security document in accordance with an embodiment of the first and second aspects of the invention, (a) in plan view and (b) in cross-section;

[0104] FIG. 12 schematically depicts selected components of a security device suitable for use in embodiments of the first and/or second aspects of the invention, (a) in perspective view and (b) in cross-section, FIGS. 12(c)(i) and (ii) showing two exemplary images exhibited by the security device at different viewing angles;

[0105] FIG. 13 schematically depicts selected components of a security device suitable for use in embodiments of the first and/or second aspects of the invention in plan view; and

[0106] FIGS. 14 A to J depict different examples of image elements as may be employed in embodiments of the first and/or second aspects of the invention.

[0107] The description below will focus in the main part on deployment of the disclosed security devices in security documents based on polymer document substrates, such as polymer banknotes, since the devices are particularly well suited to this application. Security documents are examples of security substrates and as discussed below it will be appreciated that the disclosed structures could instead be provided on polymer substrates forming security articles such as threads, strips or patches which can then be applied to any form of security document, including polymer-based documents but also conventional documents such as paper-based documents.

[0108] Consequentially, in the examples described below, the masking layer referred to above in connection with the first aspect of the invention is implemented as an opacifying layer. However, in other cases this may be replaced by a masking layer of another composition, such as a metal layer, a semi-transparent metal layer, metallic ink layer, iridescent ink layer or any of the other materials mentioned previously. In general, the masking layer preferably takes the form of a non-fibrous material, and may comprise multiple layers of such materials (which may or may not be the same in each layer). The masking layer(s) are preferably printed or coated layer(s) and are typically not self-supporting (that is, they are supported by the underlying substrate).

[0109] FIG. 1 shows an exemplary security document 1, such as a bank note, in plan view. The majority of the surface of the document 1 is covered by one or more opacifying layers 2 which are typically printed, e.g. with security patterns, graphics, currency and denomination information and the like (not shown). The opacifying layer(s) are non-transparent and preferably light in colour, e.g. comprising a white, off-white or grey pigment such as titanium oxide dispersed in a binder. The opacifying layers are omitted across at least one gap 5 to form a window as discussed further below. The window 5 preferably takes the form of indicia, i.e. an item of information, which here is a circle but could be an alphanumeric character, a symbol, or a graphic etc. A security device 10 is arranged in the region of the window 5. The security device 10 comprises an array of focussing element 11 of which the perimeter is shown in dashed lines. In preferred embodiments, the opacifying layer 2 overlaps at least some of the array of focussing elements 11 although this is not essential in all cases. Inside the window 5, an optically variable effect will be exhibited by the security device due to the focussing effect of the focussing elements, at least across a sub-region 15a, the perimeter of which may or may not be visible in the window 5 as explained below. In the example shown, the perimeter of the sub-region 15a has the form of an indicia, here a star, but any other indicia could be chosen. The manner in which such indicia are formed will be explained below.

[0110] The focussing elements could be lenses or mirrors, e.g. cylindrical, spherical, aspherical, elliptical or Fresnel structures. In the examples described below the focussing elements are lenses and hence the focussing element array may be referred to as a lens array for convenience. However it will be appreciated that the lenses could be substituted by other types of focussing elements.

[0111] FIG. 2 shows an embodiment of the invention (a) in plan view and (b) in cross section along the line X-X. Only a portion of the security document 1 is depicted. It should be noted that the cross-section is not to scale (this applies to all Figures). Reference numbers already introduced in relation to FIG. 1 are used for the same components here and throughout the description below. In this example, the whole area of the security device 10 revealed by window 5 in the opacifying layer 2 exhibits a first focussed image, represented by an array of crosses. This focussed image is preferably optically variable, e.g. being a lenticular effect, a moir magnification effect or an integral imaging effect as discussed below.

[0112] As shown in FIG. 2(b), the lens array 11 is formed as a surface relief in a transparent base material 12, which in this case is a layer, e.g. of curable resin, disposed on the first surface 3 of a polymer substrate 3 which here is transparent. The polymer substrate 3 can be monolithic or multi-layered and in preferred examples is formed of a thermoplastic such as polypropylene (PP) (most preferably bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof. For banknote applications, the polymer substrate typically has a thickness in the region of 70 microns but should the device be formed on a polymer substrate for use as a thread, strip or other article this will be thinner (e.g. 30 microns). Where the security document is a card such as a bank card or ID card, the polymer substrate could be thicker, e.g. 150 microns or more. The layer 12 may be applied directly onto the first surface 3 of the polymer substrate 3 or one or more intermediate layers such as a primer may exist therebetween (not shown). The surface relief 11 can be formed in the surface of the transparent base material 12 by cast-curing, for example. In this case, the material 12 is applied to the polymer substrate 3 over the required area in the form of a curable fluid resin which is then brought into contact with an appropriately-shaped die and cured by exposure to heat and/or radiation such as UV. The surface relief 11 is shaped such that the lenses function as focusing elements when the surface relief is exposed to air, i.e. they focus light transmitted therethrough to a point with a focal length (the base focal length, f.sub.B) less than infinity. The lenses are preferably uniform in shape, size (and hence focal length, in the absence of any additional measures) across the array 11. In this example, the lenses are concave lenses (i.e. formed as depressions into the surface of layer 12), although convex lenses can alternatively be used in all embodiments as discussed below in relation to FIG. 10.

[0113] The lens array 11 is covered by an optical adjustment layer which here comprises a first transparent material 15a disposed across the whole of the lens array 11 and beyond. In some implementations it is preferred that the optical adjustment layer covers at least the whole lens array in order to present an outer surface 15 which is smooth relative to the lens array, ideally planar, such that first opacifying layer(s) 2a can be applied thereto more readily. Most preferably the optical adjustment layer covers a region extending beyond the lens array 11, as shown, and advantageously across substantially the whole polymer substrate 3 in order to level the outer surface for application of opacifying layer(s) 2a.

[0114] The first transparent material 15a has a refractive index n which is different from that of the transparent base material 12, preferably by at least 0.1, more preferably at least 0.15, most preferably at least 0.2. In this case, since the lenses 11 are concave lenses, in order to achieve focussing in the correct direction, the first transparent material 15a will have a refractive index higher than that of the transparent base material 12. Preferably, the transparent base material 12 is a low refractive index (LRI) material having a refractive index of 1.45 or less. Examples of suitable materials will be given below. The first transparent material 15a is preferably a high refractive index (HRI) material having a refractive index of 1.55 or more. The difference in refractive index n between the transparent base material 12 and the first transparent material 15a preserves the focussing nature of the interface between the two materials at which the surface relief defining the lenses 11 is located. Hence in this case, all of the lenses 11 remain functional after application of the first transparent layer 15a.

[0115] The first surface 15 of the optical adjustment layer contacts the surface relief where it is present, and contacts the polymer substrate 3 elsewhere. The second surface 15 of the optical adjustment layer is preferably substantially planar (i.e. flat) but if any profile is present it will be non-focussing. Thus, the second surface 15 of the optical adjustment layer does not contribute to the optical effect as a result of its shaping. In preferred embodiments, the second surface of the optical adjustment layer may be smoothed, e.g. by polishing or calendaring, after application of the layer by printing, coating or another suitable method.

[0116] The focal length f.sub.1 of the lenses 11 where the first transparent material 15a makes contact with the surface relief (all over, in this case) will be different from their focal length in air (the base focal length, f.sub.B). The focal length depends on the difference in refractive index between the first transparent material 15a and the transparent base material 12, according to the expression:

[00003]$f\frac{r}{\left(n_B-n_1\right)}=\frac{r}{.Math..Math.n_1}$

where r is the radius of the focussing elements, n.sub.B is the refractive index of the transparent base material 12, n.sub.1 is the refractive index of the first transparent material 15a, and n.sub.1 is the refractive index difference between them. In air, the refractive index difference will generally be around 0.5, whereas the refractive index difference between the transparent base material 12 and a first transparent material 15a will typically be less, e.g. around 0.15, so the focal length f.sub.1 will usually be longer than the base focal length f.sub.B.

[0117] In this example, therefore, the whole of the lens array 11 is functional and can display a focussed image if an image is placed at the appropriate focal length. However, the lateral extent over which the focussed image is visible is in this case determined by the arrangement of the opacifying layer(s) 2a applied over the optical adjustment layer. In the present case, the gap 5 in the opacifying layer(s) 2a is shaped and sized so as to reveal only some and not all of the functional lenses 11. That is, some of the functional lenses are concealed under the opacifying layer 2a (at least in reflected light). As such, the lateral perimeter of the optical effect displayed by the lenses is determined solely by the extent of the opacifying layer 2a. This can be adapted such that the gap 5 has the form of any indicia, here a circle, which allows the complexity of the visual effect to be increased and also permits incorporation of the device into the rest of the banknote design more readily. Since the opacifying layers 2(a) can be applied onto a substantially flat surface 15 this enables the use of many application techniques, such as gravure printing, without encountering difficulties which would otherwise be caused by the uneven surface presented by lens array 11.

[0118] The second surface 3 of the polymer substrate is preferably provided with further opacifying layers 2b, which may correspond in extent to opacifying layers 2a (in which case the window 5 is a full window), or may continue across all or part of the gap(s) defined in opacifying layer 2a, resulting in a half window as discussed below.

[0119] The lenses 11 by themselves provide a security effect that cannot be replicated e.g. by photocopying the device. For instance, the lenses 11 could be used to view any scene therethrough or held against a suitable surface to view an item thereon. However, in most preferred embodiments, the security device 10 further includes an image array 19a located in a plane spaced from the lenses 11 by a distance substantially corresponding to the focal length f.sub.1 of the lenses (e.g. to within +/5%) so that the lenses 11 display a focused image of the image array 19a to an observer viewing the image array 19a through the lenses 11. The image array may preferably be configured to co-operate with the lenses 11 to form an optically variable effect, e.g. such that the resulting device is a moir magnifier, an integral imaging device or a lenticular device (or some combination thereof as discussed below). Examples of such devices will be described with reference to FIGS. 12 and 13.

[0120] In this example, the focal length f.sub.1 is substantially equal to the distance between the lenses 11 and the second surface 3 of the polymer substrate, as is preferred. This can be achieved through appropriate choice of the lens dimensions, the refractive indices of the transparent base material 12 and the first transparent material 15a, and of the thicknesses of the transparent base material 12 (if provided as an additional layer) and the polymer substrate 3. If necessary an additional optical spacing layer can be provided on the second surface 3 of the polymer substrate to carry the image array as discussed below. The image array 19a can be formed by various methods, including printing techniques such as gravure, lithographic or flexographic printing. However, in order to achieve good results, the elements forming the image array 19a typically need to be formed at high resolution, e.g. with dimensions of 20 microns or less, preferably 10 microns or less, most preferably 5 microns or less. Some specialist methods adapted to achieve such high resolutions are disclosed in WO-A-2014/070079, US-A-2009/0297805, WO-A-2011/102800 WO-A-2005/052650, WO-A-2015/044671, or our British patent application no. 1510073.8, and are preferred for use here. The image array can alternatively be formed as an array of recesses as will be discussed with reference to FIG. 14 below. The image array can be formed directly on the second surface 3 of polymer substrate 3 or could be formed on a separate carrier and then applied (e.g. laminated) to the polymer substrate 3.

[0121] FIG. 3 shows another embodiment of a security document 1 equipped with a security device 10. Components already discussed above are labelled with like reference numbers and will not be described in detail again, except where differences arise. This applies equally to all subsequent embodiments.

[0122] In contrast to the previous example, in the FIG. 3 embodiment, the optical adjustment layer does not extend over the full area of the lens array 11 but instead the first transparent material 15a is provided over only a first sub-region of the lens array 11, which excludes some of the lenses. Thus, whilst the upper surface 15 of the optical adjustment layer is still preferably planar (and does not contribute to any focussing effect) since this only covers a portion of the surface relief, the same benefits discussed in relation to FIG. 2 of achieving a level surface on which to apply the opacifying layers 2a are not achieved in this case. As such, it may be preferable to apply the opacifying layers 2a only outside the lens array 11, although they could be disposed over part of the lens array 11 if a suitable technique is adopted (e.g. spraying of the opacifying layers 2a).

[0123] The first transparent material 15a again has a refractive index different from that of the transparent base material 12 such that where it is applied (the first sub-region), the lenses remain operative and have a focal length f.sub.1. Outside the first sub-region, in the sub-regions labelled 15b, the optical adjustment layer is absent and the surface relief remains exposed to air. Hence, in the sub-regions 15b, the lenses have the base focal length f.sub.B which as discussed above is typically shorter than the first focal length f.sub.1 (in FIG. 3, the base focal length is indicated as being equal to the distance between the lenses and the first surface of the polymer substrate, but this is not essential here). As a result, the optical effect exhibited in the sub-regions 15b will be different from that in the first sub-region 15a. In particular, the sub-regions 15b will not exhibit a focussed image of image array 19a. In the example shown, image array is provided only in the first sub-region 15a. However, even if the image array 19a extends partially into or fully across the sub-regions 15b, it still will not be imaged by the lenses 11 in those sub-regions since they are not focussed on it. This allows the first image array 19a to be applied without the need for high registration between it and the sub-region defined by the presence of first transparent material 15a, or indeed all over the polymer substrate 3. The resulting security device will display the focussed image of image array 19a only in sub-region 15a, which preferably defines indicia by its lateral extend. In this case this takes the form of a star but any other indicia such as alphanumeric characters, symbols, currency identifiers, logos etc could be formed instead though the selective application of first transparent material 15a. Preferably a selective application technique such as printing, e.g. gravure printing, lithographic printing, flexographic printing, offset printing or screen printing is used to apply the first transparent material. As before its second surface may be calendared or polished to improve its flatness.

[0124] Outside the first sub-region 15a, in the sub-regions 15b the device 10 may exhibit no focussed image. For example, in the FIG. 3 embodiment, the base focal length f.sub.B coincides with the first surface of the polymer substrate 3 which does not carry any image and hence no focussed image will be generated by the lenses.

[0125] FIG. 4 shows a variant of the FIG. 3 embodiment in which the optical effect of the device is enhanced by providing a second image array 19b on the first surface of the polymer substrate 3, i.e. spaced from the lenses 11 by the base focal length f.sub.B. As such, the lenses 11 in the sub-regions 15b now exhibit a focussed image of the second image array 19b. The optical effects in each of the sub-regions 15a and 15b are different, as indicated by the array of crosses shown in sub-region 15a and by the array of circles in sub-region 15b of FIG. 4(a). This can be achieved in a variety of ways. In some cases the first and second image arrays 19a, 19b are different from one another. For example, one could generate (in combination with the lens array 11) a moir magnification effect whilst the other a lenticular effect. Alternatively both could define effects operating on the same mechanism as one another but with different images, e.g. the first array 19a could carry an array of microimages exhibiting the digit 10 whilst the second array carries an array of microimages exhibiting the symbol . The two arrays 19a, 19b could also be formed using different techniques and/or in different colours from one another. If both of the image arrays 19a, 19b generate moir magnification or integral imaging devices with the lens array 11, the apparent image planes in which the respective magnified images will appear to sit will be on different levels from one another, because the depth (or height) of such an image plane is proportional to (fm), where f is the focal length and m is the magnification factor. Therefore, the image arrays 19a, 19b could be the same as one another since this will still result in different optical effects in the two sub-regions 15a, 15b. For example, in one sub-region the magnified image may appear to float in front on the device and in the other it may appear to be sunken behind the device.

[0126] The image array 19b could be formed on the first surface 3 of the polymer substrate 3 prior to application of the transparent base layer 12 and formation of the lenses 11, e.g. by cast-curing as discussed above. Alternatively the lenses 11 could be formed on a base transparent material in the form of a carrier layer on a separate component such as a patch or stripe, and then laminated onto the first surface of the polymer substrate 3. In this case it may be convenient to form the image array 19b onto the underneath surface of the separate component carrying the lenses before it is applied to the polymer substrate 3.

[0127] In this example, the second image array 19b is depicted as extending across the first sub-region 15a in addition to the sub-regions 15b, although it could be provided in the sub-regions 15b only. Provided the second image array 19b is of sufficiently low optical density, this will not significantly obstruct the view of image array 19a in the first sub-region 15a since the lenses here do not focus on the second image array 19b. Image arrays of the sort used to generate moir magnification effects or integral imaging effects are preferred for use as the second image array in this case, since they typically have a low fill factor (and hence low optical density) compared to lenticular device image arrays. Thus, in a preferred example, the second image array 19b may define a moir magnification or integral imaging effect whilst the first image array 19a may define a lenticular effect (since it is the image array furthest from the lenses 11, there is no further image array for it to obstruct).

[0128] The constructions shown in FIGS. 3 and 4 does have the disadvantage that some of the lenses 11 are left exposed during handling, which leaves them susceptible to soiling and damage. In addition, since the transparent base layer 12 will be formed of a material with a lower refractive index than the first transparent material 15a (since the lenses are concave, as discussed above), this may be of relatively low density and hence less robust.

[0129] FIG. 5 shows an alternative embodiment which does not result in such problems. In this case, the optical adjustment layer comprises two transparent materials 15a and 15c, contacting different laterally offset sub-regions of the lens array 11. Thus, the optical adjustment layer as a whole covers the whole of the lens array and preferably extends beyond it thereby presenting a level surface for the application of opacifying layers 2(a) in the manner described with reference to FIG. 2. However, in this example the extent of the focussed optical effect is not determined solely by the arrangement of the opacifying layers 2a, but rather by the application of the first transparent material 15a as described in relation to FIGS. 3 and 4. In variants, the final area over which the optical effect is visible could be defined in part by the extent of the first transparent material 15a and in part by the arrangement of the opacifying layer 2a, e.g. if the opacifying layer 2a extends over part of first transparent material 15a but some of its perimeter remains visible.

[0130] Outside the first sub-region 15a, the optical adjustment layer comprises a transparent material 15c with a refractive index substantially equal to that of the transparent base layer 12, e.g. with a difference of 0.1 or less. For instance, the transparent material 15c could be of the same composition as the transparent base layer 12 although this is not essential. Thus the lenses 11 in the sub-regions 15c are indexed-out, with the interface between the transparent base material and the transparent material 15c no longer operating as a focussing surface, or one with a very long focal length (tending to infinity as the difference in refractive index tends to zero). As a result, no focussed image is exhibited in the sub-region 15c.

[0131] In a variant of the FIG. 5 embodiment, the transparent material 15c could be applied over the first transparent material 15a, still contacting the surface relief 11 only in the sub-regions 15c. The transparent material 15c could also be applied after the opacifying layer 2a, e.g. extending across all or part of the opacifying layers and across the window region 5, potentially across the whole area of the security document as an over-varnish. This removes the benefit of the optical adjustment layer providing a level surface for application of the opacifying layers, but the finished optical appearance will be the same as described.

[0132] FIG. 6 shows a further embodiment in which the complexity of the device can be increased still further by providing the optical adjustment layer with another transparent material 15d. In this example, the optical adjustment layer includes all three materials 15a, 15c and 15d but in other cases it could include only materials 15a and 15d, depending on the desired effects. The transparent material 15d, like the first transparent material 15a, has a refractive index different from that of the transparent base layer 12 so the focusing function of the lenses in the sub-region 15d is preserved. However, the transparent material 15d has a different refractive index from that of the first transparent material 15a so the focal length f.sub.2 of the lenses in the sub-region 15d is different from the focal length f.sub.1 of the lenses in the sub-region 15a. Preferably, an additional image array 19d is provided at the focal length f.sub.2 so that a focussed image of the image array 19d is exhibited in the sub-region 15d. The image array 19d could be confined to sub-region 15d or may extend outside it, e.g. across the whole lens array as shown. The same considerations as discussed with respect to the image arrays 19a, 19b in FIG. 4 apply equally to the image arrays 19a, 19d in FIG. 6 and again they are desirably configured to give rise to different optical effects in each respective sub-region 15a, 15d. Both image arrays may be provided on the second surface of the substrate 3, e.g. by locating one image array directly on that second surface (here, image array 19d), and the other on an optical spacing layer 4 applied to that second surface, in order to locate the two image arrays at the different respective focal lengths f.sub.1, f.sub.2.

[0133] In this example, another sub-region 15c of the lens array 11 is coated with index-matching transparent material 15c so the lenses here are non-functional as described in relation to FIG. 5. Hence in this sub-region no focussed image of either image array will be exhibited. The result is a complex arrangement of different optical effects as illustrated in FIG. 6(a). The transparent material 15c can be applied only to the sub-regions in which it is required to contact the surface relief, or can be applied all over, as shown. As mentioned in relation to FIG. 5, it could be applied over the top of the opacifying layers 2a.

[0134] Any number of further transparent materials with different refractive indices (which are also different from that of the transparent base layer) could be arranged in respective sub-regions to provide additional different focal lengths if desired. A corresponding number of image arrays in the appropriate focal planes may also be formed.

[0135] FIG. 7 shows a further embodiment in which the optical adjustment layer does not extend over the whole of the lens array 11, but leaves a sub-region 15b of the lenses exposed to air, as in the FIG. 4 embodiment. The other sub-regions of the array are coated with first transparent material 15a and index-matching transparent material 15c, respectively. Thus, in the first sub-region 15a, the lenses are operative and have focal length f.sub.1, generating a focussed image of image array 19a. In the sub-regions 15b, the lenses are operative with the base focal length f.sub.B and generate a focussed image of image array 19b. In the sub-region 15c, the lenses are non-functional and do not display any focussed image.

[0136] The same considerations as to the nature of image arrays 19a, 19b apply as discussed above in relation to FIG. 4.

[0137] In all of the above embodiments, the different optical effects generated by the lenses in each distinct sub-region of the device are arranged in laterally offset areas which are distinguishable from one another by the naked eye, by virtue of their size. This enables the various different optical effects to define indicia through the shape, size and location of their peripheries. However, the principles described above can also be utilised to achieve the superposition of different optical effects, as illustrated in FIG. 8. Here, the optical adjustment layer comprises an index-matching transparent material 15c (which is optional) located outside a star-shaped sub-region labelled 15a,b inside which elemental areas of the first transparent material 15a are arrayed with elemental areas 15b in which the optical adjustment layer is absent (i.e. the lenses are exposed to the air). The elemental areas 15a, 15b are arranged periodically across the area 15a,b, e.g. on a regular one or two dimensional grid, preferably as line or dot patterns. The elemental areas 15a, 15b have such small dimensions that they are not individually resolvable to the naked eye. For example, each elemental area 15a may have dimensions (e.g. line width) of 300 microns or less. It should be noted that whilst the Figure illustrates each elemental area 15a, 15b as corresponding to one lens, this will generally not be the case in practice (usually a plurality of lenses will fall inside each elemental area) and indeed no registration between the lens surface relief and the elemental areas 15a, 15b is required (although could be desirable in some cases).

[0138] In the elemental areas where the first transparent material 15a is present, the lenses have the first focal length f.sub.1 and generate a focussed image of image array 19a. In the elemental areas 15b where the lenses are exposed to the air, the lenses have the base focal length f.sub.B and generate a focussed image of image array 19b. Since the different elemental areas, and hence the focussed images exhibited by each, cannot be distinguished by the naked eye, the two optical effects appear superimposed on one another across the star-shaped region 15a,b. Outside that region, the lenses are indexed-out by material 15c and no focussed image is visible.

[0139] The same considerations as to the nature of image arrays 19a, 19b apply as discussed above in relation to FIG. 4. For example, image array 19b may comprise a microimage array suitable for generating a moir magnification effect. Image array 19a could comprise a lenticular array of interleaved images, e.g. of two different solid colours. Hence the star-shaped region may display moir-magnified versions of the microimages against a uniform background, the colour of which will depend on the viewing angle and will switch as the device is tilted.

[0140] In a variant, the regions 15b could be filled-in with a second transparent material 15d with a refractive index different from that of the transparent base layer 12 and from the first transparent material 15a, as described with respect to FIG. 6. The locations of the first and second image arrays 19a, b may be repositioned accordingly. This will help to protect the lens structure and reduce soiling.

[0141] In some cases it may be desirable to register the application of the elemental areas 15a, 15b (or 15d) to the image arrays 19a, 19b. For example, if it is desired to superimpose two effects requiring image arrays of high optical density it may be necessary to confine each image array to the elemental areas in which it will be focussed. Since the transparent material(s) defining the optical adjustment layer and the image arrays can both be applied by printing, high register between them is achievable, e.g. by applying both simultaneously or at least as part of the same in-line process.

[0142] In all of the above embodiments, the focussing elements 11 have been formed in a layer 12 applied to the first surface of the polymer substrate 3, e.g. by cast-curing or lamination. However the focussing elements 11 can alternatively be formed in the first surface of the polymer substrate 3 itself, in which case this constitutes the transparent base material 12. An example is shown in FIG. 9. The surface relief defining the lenses 11 can be formed for instance by embossing into polymer substrate 3. This has the advantage that the overall thickness of the structure can be reduced and it is also more flexible due to the nature of the thermoplastic substrate 3. In the embodiment depicted, the optical adjustment layer is formed of a single first transparent material 15a all over the polymer substrate, as described with reference to FIG. 1, but this is not essential and embossed lenses such as those shown in FIG. 9 could be used in any of the other embodiments described above.

[0143] FIG. 9 also shows an alternative implementation of the opacifying layers 2a, 2b which could be employed in any embodiment. In this example, the first opacifying layers 2a define a gap 5 in the shape of a cloud indicia whilst the second opacifying layers 2b extend across the whole of gap 5. As a result the window formed is not transparent, but is nonetheless of lower optical density than surrounding portions of the document 1. This is referred to as a half-window. In still further variants, the second opacifying layer could define a gap which only partially overlaps that in opacifying layer 2a. This results in some half-window areas and some full-window areas. In general, all of the embodiments could be configured as full-windows, half-windows or some combination of the two. The security devices could also be applied to non-windowed regions of a substrate, e.g. if formed on a polymer substrate such as a thread or patch substrate which is then applied over a non-window region of a document.

[0144] In the embodiments so far, the focusing elements have been implemented as concave lenses which have a number of advantages in the disclosed overcoated system. However, the lenses could alternatively be convex lenses, i.e. formed as protrusions extending away from the body of the transparent base layer 12. This applies to all of the above examples and a further embodiment is shown in FIG. 10. This embodiment corresponds in all other respects to that of FIG. 4 above, except here the first region 15a has the form of a digit 5. Since the exposed lenses in regions 15b are convex, they are less susceptible to soiling than the concave lenses used previously. Also, the transparent base layer will need to be of a higher refractive index than the first transparent material 15a meaning that it will be more robust than in the FIG. 4 embodiment and hence the exposed lenses less prone to damage. However, the first transparent material will now need to be of lower refractive index and hence may now be less robust and more easily damaged. To mitigate this problem, it is preferable that the transparent base layer 12 is formed of a very high refractive index material (e.g. at least 1.6) and the first transparent layer 15a is formed of a moderate refractive index material (e.g. 1.5 or less) which is still relatively robust.

[0145] The appearance and complexity of any of the embodiments can be yet further enhanced by arranging the or each material 15a, 15c, 15d (or some of them) forming part of the optical adjustment layer to have a optically detectable characteristic such as a visibly coloured tint, or a property such as fluorescence, phosphorescence, luminescence or the like which may be exhibited in response to certain stimuli (e.g. non-visible wavelength illumination such as UV) and emit a response which may be visible or non-visible. Thus the characteristic may be detectable by machine only, e.g. IR absorption. The various materials 15a, 15c, 15d may carry different optically detectable characteristics, e.g. colour, to further emphasise the different sub-regions already described above. Alternatively or in addition, any one of the materials 15a, 15c, 15d could be made up of more than one material with the same refractive index but different optically detectable characteristics e.g. colour. In this way, the different colours or other characteristics of the materials can be used to introduce yet further indicia or patterns to the device which are not constrained to the arrangement of the different optical effects already described.

[0146] FIG. 11 shows an example of this which corresponds broadly to the embodiment shown in FIG. 5 above. Thus, the star-shaped region exhibits a focussed image of image array 19a and the outer areas do not, since here the lenses are indexed-out. However, in this case both sub-regions are each formed of two different transparent materials. Hence, the left half of the star-shaped sub-region is formed of material 15a whilst the right half comprises material 15a which has the same refractive index as material 15a but a different optical characteristic, e.g. colour. For instance, the left half of the star may carry a blue tint whilst the right half is colourless. Similarly, the left half of the sub-regions which are indexed-out carry a material 15c of substantially equal refractive index to that of the transparent base layer 12, and the right half carries a different material 15c of the same refractive index but different optical characteristic e.g. colour. In this case, the material 15c carries the same optical characteristic as the material 15a so that the whole left hand side of the circular area defined by gap 5 appears blue. In other cases the material 15c could be another colour again, e.g. red.

[0147] The transparent base layer 12 could also carry a detectable optical characteristic e.g. coloured tint or fluorescence, phosphorescence or luminescence if desired.

[0148] Examples of suitable materials for forming the transparent base layer 12 or the first or second transparent materials 15a, 15d will now be provided. In each case the transparent material 15a, 15d could be formed of just one of the material components indicated below, but more usually will comprise a mixture (co-polymer or blend) of two or more of the components listed, in order to achieve not only the required optical properties but also desirable mechanical properties. The high refractive index materials listed below have a refractive index of about 1.55 or more, and the low refractive index materials about 1.45 or less. Where available, the approximately refractive index (RI) of each component is indicated below. One or both of the materials 15a, 15d could optionally also comprise a curable component and examples of these are provided below. It will be appreciated that whether the high RI or low RI material is deployed as the transparent base layer 12 or the first or second transparent materials 15a, 15d will depend on whether the lenses are convex or concave.

[0149] Examples of High Refractive Index Components

[0150] Metal containing acrylates: [0151] zirconium acrylate (Sigma Aldrich Cat. No. 686239) [0152] hafnium acrylate (Sigma Aldrich Cat. No. R686212) [0153] zirconium carboxyethyl acrylate (Sigma Aldrich Cat. No. 686247) [0154] hafnium carboxyethyl acrylate (Sigma Aldrich Cat. No. 686220)

[0155] Fluorene acrylates based monomers [0156] (Miramer is a trade name of Miwon Chemicals, Korea) [0157] Miramer HR6040 [0158] Miramer HR6042 [0159] Miramer HR6060 [0160] Miramer HR6100

[0161] High RI Nano particulate dispersions [0162] Unidic EPC-1027 (DIC Corporation, Japan) [0163] SHR 1075 (Miwon Chemicals, Korea)

[0164] Sulfur containing acrylate [0165] Phenylthioethyl acrylate, (Dichem Korea)RI 1.560 [0166] 1-naphthylthio ethyl acrylate (Dichem Korea)RI 1.61

[0167] Standard acrylates [0168] Miramer M240 (Bisphenol A ethoxylated acrylate)RI 1.537 [0169] Miramer M2100 (Phenoxy Benzyl Acrylate)RI 1.565 [0170] Miramer M1142 (1-Ethoxylated-o-phenylphenol acrylate)RI 1.577 [0171] Miramer HR2582 (Urethane Acrylate)RI 1.595 [0172] Miramer HR2200 (Epoxy acrylate)RI 1.559 [0173] Miramer HR3000 (Urethane acrylate)RI 1.571 [0174] Miramer HR3200 (Urethane acrylate)RI 1.565 [0175] Miramer HR3700 (Urethane acrylate)RI 1.585 [0176] Miramer HR3800 (Urethane acrylate)RI 1.573 [0177] HR4000 (Urethane acrylate, RI 1.582)

[0178] Examples of Low Refractive Index Components: [0179] Fluoro-acrylate monomers from the following [0180] PDFApentadecafluorooctyl acrylateRI 1.3390 [0181] TFA=2,2,2-trifluoroethyl acrylate [0182] HFBAheptafluorobutyl acrylateRI 1.3670 [0183] HDFA=1H,1H,2H,2H-heptadecafluorodecyl acrylate, [0184] HFIPA=hexafluoroisopropy acrylate, [0185] TDFA=1H, 1H,2H,2H-tridecafluorooctyl acrylate [0186] Tetrafluoro-3-(heptafluoropropoxy)propyl acrylateRI 1.3460 [0187] Tetrafluoro-3-(pentafluoroethoxy)propyl acrylateRI 1.3480 [0188] TetrafluoroethyleneRI 1.3500 [0189] Undecafluorohexyl acrylateRI 1.3560 [0190] Nonafluoropentyl acrylateRI 1.3600 [0191] Tetrafluoro-3-(trifluoromethoxy)propyl acrylateRI 1.3600 [0192] Pentafluorovinyl propionateRI 1.3640 [0193] Trifluorovinyl acetateRI 1.3750 [0194] Octafluoropentyl acrylateRI 1.3800 [0195] Methyl 3,3,3-trifluoropropyl siloxaneRI 1.3830 [0196] Pentafluoropropyl acrylateRI 1.3850 [0197] 1H, 1H-Heptafluorobutyl(meth)acrylate, [0198] 1H, 1H,5H-octafluoropentyl(meth)acrylate, [0199] 2,2,3,4,4,4-Hexafluorobutyl(meth)acrylate, [0200] perfluorooctylethyl(meth)acrylate, [0201] trifluoroethyl(meth)acrylate, [0202] trifluoroethyl(meth)acrylate, and [0203] perfluorooctylethyl(meth)acrylate

[0204] Preferred commercially available examples include: [0205] Defensa OP-188 (from DIC Japan) [0206] Defensa OP-3801 [0207] Defensa OP-4002 [0208] Defensa OP-4003, [0209] Defensa OP-4004, [0210] Sartomer CN 4002 (from Sartomer) [0211] Viscoat 8F (from Kowa Europe GmbH) [0212] Viscoat 3F [0213] Fluorolink MD 700 (from Solvay Solexis Inc.), [0214] Fluorolink MD 500, and [0215] Fomblin MD 40

[0216] The high refractive index formulation and the low refractive index formulation may each optionally further include one or more components with higher functionality (meaning in this case a higher number of acrylic groups in the material), to increase the degree of cross-linking, which leads to reduced tackiness and improved mechanical properties. Examples of suitable higher functional acrylate components include: [0217] trimethylolpropane triacrylate, [0218] pentaerythritol triacrylate, [0219] ethoxylated (3) trimethylolpropane triacrylate, [0220] ethoxylated (3) trimethylolpropane triacrylate, [0221] propoxylated (3) trimethylolpropane triacrylate, [0222] ethoxylated (6) trimethylolpropane triacrylate, [0223] tris(2-hydroxy ethyl) isocyanurate triacrylate, [0224] dipropylene glycol diacrylate, [0225] propoxylated (3) glyceryl triacrylate, [0226] propoxylated (3) glyceryl triacrylate, [0227] pentaerythritol tetraacrylate.

[0228] A curing agent may also be included in one or both of the formulations. A range of suitable photo- and thermo-initiators are commercially available. Photo-polymerisation is preferred for the current application due to faster cure, although thermo initiation can also be used. Some examples of suitable free radical type photo-initiators are given below: [0229] 1-phenyl-2-hydroxy-2-methyl-1-propanone, [0230] 2 hydroxy 2-methyl 1-phenyl propan-1-one, [0231] 2,2-dimethoxy-1,2-di(phenyl)ethanone [0232] 1-hydroxycyclohexyl phenyl ketone, [0233] benzophenones, [0234] bis-acyl phosphine oxide (BAPO), [0235] aminoketones, [0236] thioxanthones, [0237] (2,4,6-trimethylbenzoylphenyl phosphinate), [0238] 2-Benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone

[0239] A preferred example formulation of a high refractive index transparent material is: [0240] 40 wt % DIC Unidic EPC 1027, [0241] 30 wt % Miramer HR 6042, [0242] 25 wt % Miramer HR3700. [0243] 5 wt % Photo-initiators and common surface active additives

[0244] A preferred example formulation of a low refractive index transparent material is: [0245] 50 wt % Defensa OP-188 [0246] 30 wt % Viscoat 8F [0247] 15 wt % Ethoxylated (3) trimethylolpropane triacrylate, [0248] 5 wt % Photo-initiators and additives

[0249] Examples of optically variable effects which can be deployed in the above security devices will now be described.

[0250] An example of a lenticular device is shown in FIG. 12 in order to illustrate certain principles of operation. FIG. 12(a) shows the device in perspective and it will be seen that an array of cylindrical lenses 11 is arranged on the transparent polymer substrate 3. An array 19a of image elements or strips is provided on the opposite side of substrate 3 underlying (and overlapping with) the cylindrical lenses 11 and, as shown best in the cross-section of FIG. 12(b), each of the image strips corresponds to a portion of one of several images labelled A to G (only 2 images A and B are labelled in FIG. 12(a)). Under each lens of the lenticular array 11, one image slice from each of regions A to G is provided, forming a set of image elements. Under the first lens, the strips in the set will each correspond to a first segment of the respective image A to G and under the next lens, the strips in the next set will each correspond to a second segment of respective images A to G and so forth. Each lens is arranged to focus substantially in the plane of the image array 19a using the principles described above such that, ideally, only one strip can be viewed from one viewing position through each lens 11. As such, at any viewing angle, only the strips corresponding to one of the images (A, B, C etc.) will be seen through the lenses. For example, as depicted in FIG. 12(b), when the device is viewed straight-on (i.e. parallel to the Z axis), each strip of image D will be viewed such that a composite image of image D is displayed (observer O.sub.1). When the device is tilted about the Y axis in a first direction, only the image strips from image C will be viewed (observer O.sub.2), whereas when tilted in the opposite direction, only the image strips from image E will be viewed (observer O.sub.3).

[0251] The strips are arranged as slices of an image, i.e. the strips A are all slices from one image, similarly for strips B and C etc. As a result, as the device is tilted a series of different images will be seen. The images could be related or unrelated. The simplest device would have two images that would flip between each other as the device is tilted. An example of the images which might be seen from two different viewing positions is shown in FIG. 12(c), where (i) shows a first image A visible at one set of viewing angles and (ii) shows a different image B exhibited at a different set of viewing angles. Alternatively, the images could be a series of images that have been shifted laterally strip to strip, generating a lenticular animation effect so that the image appears to move. Similarly, the change from image to image could give rise to more complex animations (e.g. parts of the image changing in a quasi-continuous fashion), morphing (one image transforms in small steps to another image) or zooming (an image gets larger or smaller in steps).

[0252] FIG. 13 illustrates the principles of operation of a moir magnification device, in plan view. For illustration, both the microimages 21 forming the image array 19a and the magnified versions 22 thereof are shown in the Figure. However, in reality when the device is viewed through the lenses 11 only the magnified images 22 are visible. In this example, the image array 19 comprises an array of identical microimage 21 each displaying a banner symbol. The microimages have a similar periodicity to the array of lenses 11, which here are spherical or aspherical lenses arranged in a 2D grid, but the pitches of the two arrays are slightly mismatched. Alternatively they can be the same but have rotationally mismatched orientations. The lenses sample different points of each microimage across the array, resulting in the display of synthetically magnified versions 22 of the microimages which appear to sit on an image plane the height of which depends on the focal length and magnification factor. As the device is tilted the position of the magnified image appears to move relative to the reference frame of the device.

[0253] An integral imaging device would have a similar construction to that shown in FIG. 13 but comprises microimages which vary from one to the next across the array, all of the same object but different angles, resulting in a 3D magnified image of the object.

[0254] As mentioned above, focal length of the lenses is directly related to their size (radius) and for an image array 19a on the second surface of the polymer substrate to be in focus, the optical geometry must be taken into account when selecting the thickness of the transparent layer 12. In preferred examples the 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 diameter and more especially less than 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.

[0255] The periodicity and therefore maximum base diameter of the focusing elements is preferably in the range 5 to 200 m, more preferably 10 to 60 m and even more preferably 20 to 40 m. The f number for the lenticular focusing elements is preferably in the range 0.1 to 16 and more preferably 0.5 to 4. The lenses could be cylindrical, spherical, aspherical or any other type as required by the application.

[0256] In all of the above examples, the image array(s) 19a, 19b etc could be formed in various different ways. For example, the image elements could be formed of ink, for example printed onto the substrate 3 or onto another layer which is then positioned adjacent to the substrate 3 or applied onto it. However, in other examples the image elements can be formed by a relief structure and a variety of different relief structure suitable for this are shown in FIG. 14. Thus, FIG. 14a illustrates image regions of the image elements (IM), in the form of embossed or recessed regions while the non-embossed portions correspond to the non-imaged regions of the elements (NI). FIG. 14b illustrates image regions of the elements in the form of debossed lines or bumps.

[0257] In another approach, the relief structures can be in the form of diffraction gratings (FIG. 14c) or moth eye/fine pitch gratings (FIG. 14d). Where the image elements are formed by diffraction gratings, then different image portions of an image (within one image element or in different elements) can be formed by gratings with different characteristics. The difference may be in the pitch of the grating or rotation. This can be used to achieve a multi-colour diffractive image which will also exhibit a lenticular optical effect such as an animation through the mechanism described above. For example, if the image elements had been created by writing different diffraction tracks for each element, then as the device is tilted, lenticular transition from one image to another will occur as described above, during which the colour of the images will progressively change due to the different diffraction gratings. A preferred method for writing such a grating would be to use electron beam writing techniques or dot matrix techniques.

[0258] Such diffraction gratings for moth eye/fine pitch gratings can also be located on recesses or bumps such as those of FIGS. 14a and b, as shown in FIGS. 14e and f respectively.

[0259] FIG. 14g illustrates the use of a simple scattering structure providing an achromatic effect.

[0260] Further, in some cases the recesses of FIG. 14a could be provided with an ink or the debossed regions or bumps in FIG. 14b could be provided with an ink. The latter is shown in FIG. 14h where ink layers 200 are provided on bumps 210. Thus the image areas of each image element could be created by forming appropriate raised regions or bumps in a resin layer provided on a transparent substrate. This could be achieved for example by cast curing or embossing. A coloured ink is then transferred onto the raised regions typically using a lithographic, flexographic or gravure process. In some examples, some image elements could be printed with one colour and other image elements could be printed with a second colour. In this manner when the device is tilted to create the lenticular animation effect described above, the images will also be seen to change colour as the observer moves from one view to another. In another example all of the image elements in one region of the device could be provided in one colour and then all in a different colour in another region of the device.

[0261] Finally, FIG. 14i illustrates the use of an Aztec structure.

[0262] Additionally, image and non-image areas could be defined by combination of different element types, e.g. the image areas could be formed from moth eye structures whilst the non-image areas could be formed from gratings.

[0263] Alternatively, the image and non-image areas could even be formed by gratings of different pitch or orientation.

[0264] Where the image elements are formed solely of grating or moth-eye type structures, the relief depth will typically be in the range 0.05 microns to 0.5 microns. For structures such as those shown in FIGS. 14 a, b, e, f, h and i, the height or depth of the bumps/recesses is preferably in the range 0.5 to 10 m and more preferably in the range of 1 to 2 m. The typical width of the bumps or recesses will be defined by the nature of the artwork but will typically be less than 100 m, more preferably less than 50 m and even more preferably less than 25 m. The size of the image elements and therefore the size of the bumps or recesses will be dependent on factors including the type of optical effect required, the size of the focusing elements and the desired device thickness. For example if the diameter of the focusing elements is 30 m then each image element may be around 15 m wide or less. Alternatively for a smooth lenticular animation effect it is preferable to have as many views as possible, typically at least five but ideally as many as thirty. In this case the size of the elements (and associated bumps or recesses) should be in the range 0.1 to 6 m. In theory, there is no limit as to the number of image elements which can be included but in practice as the number increases, the resolution of the displayed images will decrease, since an ever decreasing proportion of the devices surface area is available for the display of each image.

[0265] The security substrates and devices of the current invention can optionally be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.