SECURITY DOCUMENTS AND METHODS OF MANUFACTURE THEREOF

Abstract

A security document and a method of forming a security document are disclosed. The method comprises: (a) providing a security document substrate having a security article integrated within or attached thereto, the security article being exposed within an aperture region in the security document substrate, the security article comprising a first optical effect layer that is visible within the aperture region, and; (b) applying, in said aperture region within which the security article is exposed, an array of substantially transparent refractive structures on the exposed security article, wherein the array of refractive structures cooperates with the first optical effect layer to exhibit a first optically variable effect.

Claims

1-61. (canceled)

62. A method of forming a security document, the method comprising: (a) providing a security document substrate having a security article integrated within or attached thereto, the security article being exposed within an aperture region in the security document substrate, the security article comprising a first optical effect layer that is visible within the aperture region, and; (b) applying, in said aperture region within which the security article is exposed, an array of substantially transparent refractive structures on the exposed security article, wherein the array of refractive structures cooperates with the first optical effect layer to exhibit a first optically variable effect

63. The method of claim 62, wherein the security article comprises a security article substrate, and a first adhesive layer forming a first outer layer of the security article, wherein at least a part of said first adhesive layer is in contact with the security document substrate.

64. The method of claim 63, wherein at least a part of the first adhesive layer is exposed within said aperture region, and wherein step (b) comprises applying said array of substantially transparent refractive structures on the exposed part of the first adhesive layer of the security article.

65. The method of claim 63, wherein the first adhesive layer is present substantially only in region(s) of the security article which are not exposed within the aperture region.

66. The method of claim 62, wherein the aperture region comprises an aperture in the security document substrate, and wherein the security article is exposed through said aperture, and step (b) comprises applying said array of refractive structures on the exposed security article through said aperture.

67. The method of claim 62, wherein the array of refractive structures extends outside the aperture region.

68. The method of any of claim 62, wherein step (b) comprises: (i) applying a transparent curable material on the exposed security article or to a casting tool carrying a surface relief corresponding to the refractive structures, at least over an area corresponding to the exposed security article; (ii) forming the transparent curable material with the casting tool, and; (iii) curing the transparent curable material so as to retain the surface relief.

69. The method of claim 68, wherein in step (b)(i) the transparent curable material is applied on the exposed security article and on a region of security document substrate outside the aperture region.

70. The method of claim 62, wherein step (b) comprises: (i) applying a transparent curable material to a refractive structure support layer or to a casting tool carrying a surface relief corresponding to the refractive structures, at least over an area corresponding to the exposed security article; (ii) forming the transparent curable material with the casting tool, (iii) curing the transparent curable material so as to retain the surface relief, and; either applying the refractive structure support layer to the exposed security article, or applying the retained surface relief to the exposed security article and removing the refractive structure support layer.

71. The method of claim 62, wherein the array of substantially transparent refractive structures is applied in direct contact with the exposed security article within the aperture region.

72. The method of any claim 62, further comprising the step of applying, on the exposed security article substrate in said aperture region, a substantially transparent pedestal layer, and wherein the array of substantially transparent refractive structures is applied on said pedestal layer.

73. A security document comprising; a security document substrate having a security article integrated within or attached thereto, the security article being visible within an aperture region in the security document substrate, wherein the security article comprises; an optical effect layer that is visible within the aperture region, and a first adhesive layer forming a first outer layer of the security article, wherein the first adhesive layer is in contact with the security document substrate such that the security article is adhered to the security document substrate, and a part of the first adhesive layer laterally extends across the aperture region, wherein the security document further comprises; an array of substantially transparent refractive structures on the part of the first adhesive layer that laterally extends across the aperture region, wherein said array of refractive structures cooperates with the optical effect layer to exhibit an optically variable effect.

74. The security document of claim 73, wherein the array of refractive structures is in direct contact with the first adhesive layer.

75. The security document of claim 73, further comprising a support layer positioned between the array of refractive structures and the first adhesive layer.

76. The security document of claim 73, wherein the security article comprises a security article substrate, and the first adhesive layer extends substantially continuously across a first surface of the security article substrate.

77. The security document of claim 73, wherein the array of refractive structures extends outside the aperture region such that a region of the array is on the security document substrate.

78. The security document of any claim 77, wherein a region of the security document substrate outside the aperture region that is covered by the array of refractive structures comprises a second optical effect layer that cooperates with the corresponding region of the array to exhibit a second optically variable effect.

79. The method of claim 62, wherein the first optical effect layer comprises a colour shifting layer or pattern of elements.

80. The security document of claim 73, wherein the first optical effect layer comprises a colour shifting layer or a pattern of elements.

81. The method of claim 62, wherein the array of refractive structures comprises an array of focusing elements, or wherein the array of refractive structures comprises an array of microprisms.

82. The security document of claim 73, wherein the array of refractive structures comprises an array of focusing elements, or wherein the array of refractive structures comprises an array of microprisms.

83. The method of claim 62, wherein the security document substrate comprises a fibrous substrate.

84. The method of claim 73, wherein the security document substrate comprises a fibrous substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Preferred examples of the invention will now be described with reference to the attached drawings, in which:

[0079] FIG. 1 is a flow diagram outlining the steps of a preferred method of manufacturing a security document, according to the present invention;

[0080] FIG. 2 schematically illustrates a first example embodiment of manufacturing a security document according to the invention;

[0081] FIG. 3 schematically illustrates a second example embodiment of manufacturing a security document according to the invention;

[0082] FIGS. 4 and 5 schematically depict two preferred cast-curing techniques which may be used in the method of the present invention;

[0083] FIGS. 6 to 13 schematically illustrate further example embodiments of manufacturing a security document according to the invention, and;

[0084] FIGS. 14 and 15 illustrate comparative examples.

DETAILED DESCRIPTION

[0085] FIG. 1 is a flow diagram outlining the steps of a method of manufacturing a security document according to the present invention, and will be described in more detail with reference to FIGS. 2 to 13. In the following, any of the features of the described embodiments may be used in combination with the other embodiments.

[0086] In step S100, a security document substrate is provided having a security article exposed within an aperture region of the security document substrate. The security article is integrated within or attached to the security document substrate using standard techniques, as have been discussed in the summary of the invention section above. In particular, such security articles may be applied to or incorporated into documents of value such as banknotes, passports, driving licences, cheques, identification cards etc.

[0087] The method of FIG. 1 is preferably a sheet-based method. In other words, at step 100, a plurality of security document substrates, each having a security article integrated within or attached thereto, are provided as a sheet of material and arranged in an m×n array. This may involve sheeting an initial web of such security document substrates into a plurality of sheets. The subsequent steps of the method are performed using sheet-fed machinery. However, the following examples will be described with reference to individual security documents for ease of description, without limitation. Although a sheet-based implementation is preferred, in other applications of the present invention, the method may be a web-based method, at least for steps S100 and S200.

[0088] FIG. 2 schematically illustrates a first example of manufacturing a security document according to the invention. FIGS. 2(a) and 2(b) illustrate a security document substrate 100, here in the form of a paper substrate for a banknote 1000. FIG. 2(a) shows the banknote 1000 in plan view and FIG. 2b as a cross-section along line Q-Q′. Typical thicknesses, t, of the banknote are between 50 and 200 microns, preferably between 70 and 150 microns.

[0089] In this example the security article 200 is 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 100 lie on either side of the thread. The security thread is exposed within a plurality of aperture regions 50. This can be done using the techniques described in EP0059056 where paper is not formed in the aperture regions 50 during the paper making process, thus exposing the security thread within the aperture regions 50 through apertures 60 in the paper substrate. Alternatively the aperture regions 50 may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. The security thread 200 is exposed through the apertures 60 on one surface of the banknote, and hence the apertures are “half-thickness” apertures. The aperture regions 50 are defined by the lateral shape of the apertures 60

[0090] The thread 200 comprises a transparent polymer substrate 10 having opposing first and second surfaces 10a and 10b, and the thread extends continuously along the full height of the banknote. The thread 200 further comprises an optical effect layer 30 on the second surface 10b of the thread substrate, i.e. distal to the surface of the thread that is exposed through the apertures 60. The thread is adhered into the paper substrate using transparent adhesive layers 20a, 20b on both sides of the thread 200.

[0091] In this example the optical effect layer 30 comprises an array of microimage elements schematically illustrated at 32, which are formed on surface 10b of the thread substrate 10. The optical effect layer is visible within each aperture region 50 through the apertures 60 due to the transparent properties of the thread substrate 10 and the adhesive layer 20a. In this example, the array of microimage elements extends continuously along the surface 10b of the thread substrate, but in alternative embodiments the optical effect layer may be provided only on regions of surface 10b that are in register with the apertures 60.

[0092] The array of microimage elements 30 may be provided using conventional printing techniques such as lithographic printing, fleoxographic printing or gravure, with line widths typically between 5-50 microns. One method which may be used as an alternative to the printing techniques mentioned above is used in the so-called Unison Motion™ product by Nanoventions Holdings LLC, as mentioned for example in WO-A-2005052650. This involves creating pattern elements (“icon elements”) as recesses in a substrate surface before spreading ink over the surface and then scraping off excess ink with a doctor blade. The resulting inked recesses can be produced with line widths of the order of 2 microns to 3 microns.

[0093] A different method of producing high-resolution image elements is disclosed in WO-A-2015/044671 and is based on flexographic printing techniques. A curable material is placed on raised portions of a die form only, and brought into contact with a support layer preferably over an extended distance. The material is cured either whilst the die form and support layer remain in contact and/or after separation. This process has been found to be capable of achieving high resolution and is therefore advantageous for use in forming the microimage array 30 in the present application.

[0094] Some more particularly preferred methods for generating patterns or micropatterns (i.e. a microimage array 30) on a substrate are known from US 2009/0297805 A1 and WO 2011/102800 A1. These disclose methods of forming micropatterns in which a die form or matrix is provided whose surface comprises a plurality of recesses. The recesses are filled with a curable material, a treated substrate layer is made to cover the recesses of the matrix, the material is cured to fix it to the treated surface of the substrate layer, and the material is removed from the recesses by separating the substrate layer from the matrix.

[0095] Another method of forming a micropattern is disclosed in WO 2014/070079 A1. Here it is taught that a matrix is provided whose surface comprises a plurality of recesses, the recesses are filled with a curable material, and a curable pickup layer is made to cover the recesses of the matrix. The curable pickup layer and the curable material are cured, fixing them together, and the pickup later is separated from the matrix, removing the material from the recesses. The pickup layer is, at some point during or after this process, transferred onto a substrate layer so that the pattern is provided on the substrate layer.

[0096] Referring back to FIG. 2, the first adhesive layer 20a extends continuously across the first surface 10a of the thread substrate such that it not only adheres the thread 200 to the paper “bridges” 100a between the apertures, but is also exposed within aperture regions 50 through the apertures 60 themselves. Thus, in this example the surface of the thread 200 that is exposed through the apertures 60 comprises the first adhesive layer 20a.

[0097] In an alternative embodiment (schematically illustrated in FIG. 3), the adhesive of the first adhesive layer 20a is provided in register with the paper bridges 100a such that no adhesive is exposed within the aperture regions 50 through the apertures 60. The adhesive layer 20a may there be seen as a partial layer comprising gaps registered with the aperture regions such that the adhesive is provided in register with the paper bridges 100a. In this example, the surface of the thread 200 that is exposed through the apertures 60 is the first surface 10a of the thread substrate 10.

[0098] At step S200 of the method, an array 70 of focusing elements in the form of microlenses 71 is applied to the exposed security in each aperture region, as schematically shown in FIG. 2(c). The lenses are formed by cast curing a curable material as will be explained in more detail below. As will be appreciated, in the example of FIG. 2(c), the focusing elements are formed directly onto the first adhesive layer 20a which is exposed through the apertures 60. In the embodiment shown in FIG. 3, the focusing elements will be formed directly onto the first surface 10a of the transparent thread substrate 10. The microlenses 71 cooperate with the microimage elements 32 to exhibit an optically variable effect to an observer of the banknote 1000 using any of the mechanisms that have been discussed in the summary of the invention (e.g. moiré magnification, lenticular effects etc.).

[0099] The resulting security document thus comprises security devices 1 defined by the optical effect layer and corresponding arrays of refractive structures, as illustrated in FIG. 2(c).

[0100] The focusing elements have a focal length f that is substantially equal to the optical spacing between the lenses and the microimage array 30, such that the focal plane of the arrays 70 substantially corresponds to the plane of the microimage array (i.e. the second surface 10b of the security thread). In other words, the combined thickness of the curable material, adhesive layer 20a and transparent substrate 10 is substantially equal to the focal length of the focusing elements. The thickness, h, of the curable material in which the lenses are formed is controlled in the casting process such that the correct optical spacing is achieved.

[0101] The most preferred method of forming the focusing element arrays 70 is by cast-curing. This involves applying a transparent curable material to the exposed security thread or to a casting tool carrying a surface relief defining the desired focusing element array, forming the material using the casting tool and curing the material to fix the relief structure into the surface of the material. FIGS. 4 and 5 schematically depict two preferred cast-curing techniques which may be used. Components common to both methods are labelled with the same reference numbers. In both cases the process is shown as applied to a support layer 201 which may be the aforementioned exposed security article 200, or could be a separate support layer which is later applied to the exposed security article 200 (e.g. a transfer foil that may be applied to the exposed security article by a foiling machine). In each case, Figure (a) depicts the apparatus from a side view, and Figure (b) shows the support layer in a perspective view, the manufacturing apparatus itself being removed for clarity.

[0102] In the FIG. 4 embodiment, a transparent curable material 205 is first applied to the support layer 201 using an application module 210 which here comprises a patterned print cylinder 211 which is supplied with the curable material from a doctor chamber 213 via an intermediate roller 212. For example, the components shown could form part of a gravure printing system. Other printing techniques such as lithographic, flexographic, screen printing or offset printing could also be used. Print processes such as these are preferred since the curable material 205 can then be laid down on the support 201 only in first regions 202 thereof, the size, shape and location of which can be selected to conform to the apertures by control of the print process, e.g. through appropriate configuration of the pattern on cylinder 211. However, in other cases, an all over coating method could be used, e.g. if the focusing element array is to be formed all over the support 201. The curable material 205 is applied to the support 201 in an uncured (or at least not fully cured) state and therefore may be fluid or a formable solid.

[0103] The support 201 is then conveyed along the machine direction or sheet path MD to a casting module 220 which here comprises a casting tool 221 in the form of a cylinder carrying a surface relief 225 defining the shape of the focusing elements which are to be cast into the curable material 205. The surface relief 225 may be formed in the cylinder surface itself, or on a plate mounted to the cylinder. As each region 202 of curable material 205 comes into contact with the cylinder 221, the curable material 205 fills a corresponding region of the relief structure, forming the surface of the curable material into the shape defined by the relief. The cylinder 221 could be configured such that the relief structure 225 is only provided at regions corresponding to shape and position of the first regions 202 of curable material 205. However this gives rise to the need for accurate registration between the application module 210 and the casting module 220 in order that the focusing elements are accurately placed in each first region 202 of the curable material. Therefore in a particularly preferred embodiment, the cylinder 221 carries the relief structure corresponding to the focusing elements over an area larger than that of the first region 202, preferably around its complete circumference and most preferably over substantially its whole surface (although axial regions which will not come into the vicinity of the curable material may be excluded). In this way, each entire first region 202 of curable material 205 is guaranteed to come into contact with the surface relief structure 225 such that the focusing element array is formed over the full extent of the material. As a result, the shape, size and location of the focusing element array 20 is determined solely by the application of the curable material by the application module.

[0104] Having been formed into the correct surface relief structure, the curable material 205 is cured by exposing it to appropriate curing energy such as radiation R (typically UV radiation) from a source 222. This preferably takes place while the curable material is in contact with the surface relief 225 although if the material is already sufficiently viscous this could be performed after separation. In the example shown, the material is irradiated through the support layer 201 (typically the case when the lenses are formed on a transfer foil) although the source 222 could alternatively be positioned above the support layer 201, e.g. inside cylinder 221 if the cylinder is formed from a suitable transparent material such as quartz.

[0105] In one embodiment the curable material 205 is partially cured while in contact with the surface relief 225, 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 201, or from above the support layer.

[0106] In a variation to the process shown in FIG. 4, the print cylinder 211 may be a screen print unit 211 carrying a screen corresponding to the first regions, where the curable material 205 is pushed from the inside of the roller 211 out of the screen by a squeegee directly on to the support 201.

[0107] FIG. 5 shows a further variant of the above process in which, rather than apply the curable material 205 to the support layer 201, it is applied instead to the surface of the casting cylinder 225. Again this is preferably done in a patterned manner, using a print cylinder 211 to transfer the curable material 205 only onto the first regions 202 on the casting cylinder 221. Upon contact with the support layer 201, the regions 202 of curable material 205 affix to the support layer 205 and curing preferably takes place at this stage to ensure strong bonding. The so-formed focusing element arrays 70 again have a shape, size and location determined solely by the application module 210.

[0108] In both the processes illustrated in FIGS. 4 and 5, a counter impression cylinder (not shown) may be used on the opposing side of the support layer 201 to the casting cylinder 221.

[0109] In FIGS. 4 and 5, the surface relief of the casting cylinder 225 defines the arrays of focusing elements seen in FIG. 2(c). However, it will be appreciated that the surface relief may take substantially any form so as to form a variety of refractive structures in the curable material 205, and thus the cast methods described above in relation to FIGS. 4 and 5 may be used in any of the herein-described embodiments. Examples of different refractive structures will be described herein.

[0110] The transparent curable material 205 in which the lenses are formed can be of various different compositions as discussed in the summary of the invention section.

[0111] Other examples of security document substrates and the application of refractive structures will now be described.

[0112] FIG. 6(a) is a cross sectional view of a security document substrate 1000 for a banknote similar to FIG. 2(b). As with FIG. 2(b), the security thread 200 is partially embedded within paper substrate 100 and exposed within aperture regions 50 through apertures 60 in the security document substrate 100 such that the optical effect layer 30 is visible through the apertures. In this example, the optical effect layer comprises a colour shifting layer that exhibits different colours at different tilt angles. In this example the colour shifting layer is a partially transparent liquid crystal film that is formed on the second surface 10b of the thread substrate 10. The liquid crystal film is partially transparent and thus an absorbing layer 35 is provided on a side of the liquid crystal film distal to the exposed surface of the thread. Here the light absorbing layer acts to absorb light in the visible part of the electromagnetic spectrum (approximately 400 to 750 nm), and is preferably substantially transparent to UV radiation such that the curing process described above in FIGS. 4 and 5 may be easily implemented. The thread is adhered into the paper substrate 100 using adhesive layers 20a and 20b.

[0113] At step S200, an array 80 of linear microprisms 81 is applied to the exposed security article in each aperture region, as illustrated at FIG. 6(b). The linear microprisms cooperate with the colour shifting layer to change the colour response upon tilting as compared to if no prisms were present.

[0114] In the example shown in FIG. 6(b), the linear microprisms are cast such that their long axes are aligned with the width of the banknote (i.e. along the x-axis in FIG. 2(a)). This provides the strongest optically variable effect when the banknote 1000 is tilted about a tilt axis parallel with the x-axis (i.e. about an axis parallel with the long axes of the microprisms). It will be appreciated that the microprisms may be applied in other orientations, for example with their long axes aligned with the height of the banknote (along the y-axis). Furthermore, each array of microprisms may comprise regions (or “sub-arrays”) of microprisms having different orientations and/or geometry. This provides a particularly striking visual effect as each aperture region of the banknote may exhibit different colours (corresponding to the different orientations of microprisms) at a particular viewing angle (i.e. tilt angle).

[0115] The arrays of linear microprisms may be applied on the exposed security thread so as to only partially cover the exposed security thread within each aperture region 50. For example, each array may be in the form of indicia, as illustrated in FIG. 6(c) where the arrays are applied in the form of a star shape. When viewing the security document, at a particular viewing angle, the star shape and the surrounding region 85 where the liquid crystal layer is visible in isolation (i.e. not through the micoprisms) will appear in different colours, providing a striking effect to the user. In some embodiments, the arrays of refractive structures may be applied having different forms in each aperture region 50 so as to exhibit different indicia.

[0116] FIG. 7 illustrates a further example of manufacturing a security document according to the invention, where the array of refractive structures 75 that is applied in step S200 also extends across regions of the security document substrate outside the aperture regions 50. In step S100, a security document substrate and security article are provided as described above in relation to FIGS. 2(a) and 2(b). In step S200, the transparent curable material is applied to both the exposed security article within each aperture region 50, and also to the paper substrate between each aperture region.

[0117] The curable material 205 is initially applied such that it “fills” the apertures 60 of the apertures regions and extends over the paper substrate outside the aperture regions. This ensures that the refractive structures (in this case microlenses) are located on substantially the same plane within the finished security document after embossing. Applying the curable material in this manner so as to extend outside the aperture regions reduces the registration tolerances required when embossing the refractive structures. The curable material within the aperture regions 50 (shown at 75a) has a height, h1, that is greater than the height h2 of the curable material on the paper substrate outside of the apertures (shown at 75b). Accordingly, the dimensions of the cast microlenses at least in the aperture regions will be such that their focal length is substantially equal to the combined thickness of the thread substrate 10, adhesive layer 20a and curable material 205.

[0118] The resulting security document exhibits a particularly striking effect in that an observer will perceive the above-described optically variable effect within the aperture regions 50 due to the refractive properties of the microlenses, as well as a bright “flash” at certain viewing angles due to specular reflection off the lenses that are positioned on the paper bridge regions. This specular reflection effect is more pronounced in embodiments where the refractive structures that are cast in step S200 have planar facets (e.g. linear microprisms).

[0119] In the embodiment illustrated in FIGS. 7(a) and 7(b), the curable material 205 is applied substantially continuously over each bridge region 100a, 100b, 100c and 100d, and within the apertures of each aperture region 50a, 50b, 50c (see FIG. 7(b)) such that the resulting microlens array 75 extends substantially continuously between each aperture region over the full height of the banknote. Here, the microlens array 75 has substantially the same lateral width as the aperture regions 50, but may have a lateral width that is different to that of the aperture regions. However, in other embodiments, the curable material 205 (and consequently the microlens array 75) may be applied so as to extend only partially over the bridge regions, or only between certain aperture regions. Furthermore, the curable material 205 may be applied so as to surround the complete circumference of an aperture region, as schematically illustrated at 110 in FIG. 7(b).

[0120] In general, in such an embodiment the region(s) of the security document substrate outside the aperture regions on which the curable material is applied is substantially laterally contiguous with an aperture region.

[0121] FIG. 8 schematically illustrates an example in which the paper bridges 100a, 100b, 100c, 100d have thereon a second optical effect layer 35 in the form of an array of microimage elements. Here, the microlenses of the array that are formed over the bridge regions (schematically represented at 75b) cooperate with the array of microimage elements 35 in order to exhibit an optically variable effect, which may be for example a lenticular- or moiré-based effect in the same manner as described for the aperture regions.

[0122] Here, the thickness h2 of the curable material outside the aperture regions is substantially equal to the focal length of the microlenses outside the aperture regions. Consequently, the microlenses that are formed laterally outside the aperture regions have different dimensions (typically height) to those formed laterally within the aperture regions. The surface relief of the casting tool therefore comprises regions corresponding to the regions of the array 75a that are applied within the aperture regions, and to regions of the array 75b laterally outside of the aperture regions.

[0123] In other embodiments, the second optical effect layer 35 may comprise a colour shifting layer, with the refractive structures that are formed outside of the aperture regions and over the colour shifting layer comprising microprisms. In such embodiments preferably the whole array 75 comprises microprisms and the first optical effect layer 30 comprises a colour shifting layer.

[0124] FIG. 9 illustrates a yet further example of a method of manufacturing a security document according to the present invention. Here, the structure provided in step S100 is illustrated in FIGS. 9(a) and 9(b), and is again a paper banknote 1000, where FIG. 9(a) is a view of the front side of the banknote, and FIG. 9(b) is a cross section along line Q-Q′. Security article 200 is a strip or band comprising a transparent substrate 10 and an optical effect layer 30, which in this example comprises an array of microimage elements.

[0125] The security article 200 is formed into a security document 1000 comprising a fibrous substrate 100, using a method described in EP-A-1141480. The paper substrate 100 comprises a full thickness aperture 60 defining aperture region 50. The aperture may be formed during papermaking or after papermaking, for example by die-cutting or laser cutting. The strip 200 is adhered on to one side of the paper 100 across the aperture 60 using adhesive layer 20 such that it extends across the aperture 60 and is exposed through the aperture 60 within aperture region 50.

[0126] As can be seen in FIG. 9(b), the strip 200 is fully exposed on one side of the document and exposed through the aperture 60 on the opposite side of the document (FIG. 9(a)). There is no adhesive on the surface of the strip that is distal to the paper substrate 100.

[0127] In step S200, and as shown in FIG. 9(c), an array 70 of microlenses 71 is formed on the adhesive layer 20 that is exposed within aperture region 50 through the aperture 60, using any of the techniques discussed above, wherein the array of microlenses cooperates with the array of microimage elements 30 to exhibit an optically variable effect. In an alternative arrangement, similarly to FIG. 3, the adhesive layer 20 may be omitted across aperture region 50 such that the array 70 of focusing elements as applied directly to the security article substrate 10.

[0128] FIG. 9(d) illustrates a further embodiment where the array 70 of microlenses is applied on the side of the security article 200 that is fully exposed, i.e, the side of the security article that is distal to the aperture 60 itself. In this embodiment, the array 70 is applied laterally within the aperture region 50, as shown in FIG. 9(d). The thickness, h, of the curable material 205 that is used to form the array of microlenses is controlled such that its thickness is substantially equal to the focal length of the microlenses. In an alternative embodiment, as shown in FIG. 9(e), a transparent pedestal layer 90 may be applied between the security article 200 and the curable material in which the array of microlenses is defined. The combined thickness, H, of the pedestal layer 90 and the curable material in which the array of microlenses is defined is substantially equal to the focal length of the lenses since here the microimage array 30 is on the same side of the security article as the array of microlenses. In variations to FIGS. 9(d) and 9(e), the optical effect layer may be provided on the opposing side of the security article substrate (i.e. proximal to the aperture 60) such that the optical spacing between the lenses and microimage array includes the transparent security article substrate 10.

[0129] Such a pedestal layer 90 may be applied prior to the application of the array of microlenses by applying a transparent material to the security article or to a separate support layer that is subsequently affixed to the security article. This could involve printing or coating the pedestal material onto the security article or separate support layer using any of the methods described above for the application of the curable material 205, for example gravure printing. The pedestal material is preferably applied in a selective manner to at least the desired region within which the array of microlenses is to be formed. In the example illustrated in FIG. 9(e), the regions in which the pedestal layer 90 and array 70 of microlenses are applied are substantially the same, but this is not essential, and indeed may not be desirable as this gives rise to greater registration requirements. It will be appreciated that such a pedestal layer 90 may be used in combination with other refractive structures, such as microprisms.

[0130] In both FIGS. 9(d) and 9(e), the array 70 of microlenses is applied on the exposed security article laterally within the aperture region 50. However, in further embodiments (not shown), the array may extend outside the aperture region.

[0131] In the embodiments that have been described so far, the optical effect layer 30 has been positioned on a side of the security article substrate that is distal from aperture(s) in the security document substrate 100. Hence, the security article substrate is substantially transparent such that the optical effect layer is visible within the aperture regions(s) through the aperture(s) 60. However, in any of the embodiments described herein, the optical effect layer 30 may be positioned on a side of the security article substrate that is proximal to aperture(s) in the security document substrate, as schematically illustrated in FIG. 10. The optical effect layer may be applied to the first surface 10a of the security article substrate, for example.

[0132] Here, the optical effect layer is in the form of an array of microimage elements, with the refractive structures formed as microlenses. The thickness, h, of the curable material used to form the microlenses is controlled appropriately such that the optical spacing between the lenses and the microimage elements is approximately equal to the focal length of the lenses. A pedestal layer (not shown) may also be used to control the optical spacing.

[0133] Furthermore, in such embodiments where the optical effect layer is provided on a side of the security article proximal to the cast refractive structures, the security article substrate need not be transparent, and may be substantially opaque to visible light. This may be particularly advantageous where the optical effect layer comprises a substantially transparent colour shifting layer, as the security article substrate may act as a light absorbing layer. Such a substrate would preferably be transparent to UV radiation for ease of implementation of the cast curing process.

[0134] FIGS. 11(a) and 11(b) illustrate a further example structure that may be provided in step S100, again in the form of a banknote 1000. In FIG. 11(a) the banknote 1000 is a conventional paper-based banknote, provided with a strip element or insert as the security article 200. The strip 200 is exposed through full thickness aperture 60 formed in the paper substrate 100, with said aperture 60 defining aperture region 50. In this example, an array of refractive structures may be applied to the exposed strip on the front surface of the banknote (here, on the adhesive layer 20a within aperture region 50) that cooperates with optical effect layer 30 in order to provide an optically variable effect when viewed from that side of the document. It is also envisaged that a second array of refractive structures may be applied to the opposing exposed surface of the strip (i.e. on adhesive layer 20b) such that an optically variable effect is exhibited when the banknote 1000 is viewed from both sides.

[0135] FIGS. 12(a) and 12(b) schematically illustrate a yet further example of a structure that may be provided in S100, again in the form of a paper banknote 1000. In this example, the security article 200 is in the form of a strip that is fully exposed within aperture region 50 through partial thickness aperture 60 that laterally extends across the full height of the banknote. As shown in FIG. 12(b) which is a cross section along line X-X′, the strip comprises a transparent substrate 10 and optical effect layer 30, and is adhered into the paper substrate using adhesive layers 20a and 20b. The adhesive layer 20a that is on a side of the strip 200 that is exposed through the aperture 60 is in the form of elongate strips, or “tramlines” that extend continuously along the lateral edges of the strip. The exposed side of the strip 200 is therefore adhered into the paper substrate only at its lateral edge regions, such that the surface of the strip that is exposed within the aperture region 50 through the aperture 60 comprises the substrate 10. The opposing side of the strip is adhered to the document substrate using adhesive layer 20b that extends substantially continuously over the opposing surface of the substrate 10.

[0136] FIG. 13 illustrates a further example similar to the one set out above with reference to FIG. 9. In FIG. 13, the security document substrate is a substantially transparent polymer substrate such as PET or BOPP comprising an aperture region 50 having a full thickness aperture 60. The strip 200 is adhered on to one side of the polymer 100 across the aperture using adhesive layer 20 such that it is exposed within the aperture region 50 through the aperture 60. The array of refractive structures is then applied on the exposed adhesive layer of the strip in the same manner as has been described.

[0137] In such examples where the security document substrate comprises a transparent polymer, one or more opacifying layers 120 are typically applied to at least one surface of the polymer substrate so as to clearly define the aperture through which the security article is exposed. The opacifying layers are substantially opaque to light in the visible part of the electromagnetic spectrum.

[0138] FIGS. 14 and 15 illustrate comparative examples. FIG. 14(a) illustrates a cross section of a security document 1000, again in the form of a banknote, which may be provided before application of refractive structures. Here, the security document substrate 100 comprises a transparent BOPP substrate. A window region 500 is defined by application of opacifying layers 120a and 120b on front 100a and reverse 100b sides of the banknote substrate 100 respectively. Here the window region is a “full thickness” window region in that the security article 200 is visible from both sides of the banknote.

[0139] The security article 200 is in the form of a laminate foil and is adhered to the reverse surface 100b of banknote substrate 100 within window region 500. The security article 200 comprises a substrate 10, optical effect layer 30 and adhesive layer 20 used to adhere the article 200 to the banknote substrate 100. The optical effect layer 30 in these comparative examples is in the form of a microimage array, and is visible within the window region 500 from both sides of the banknote.

[0140] FIG. 14(b) schematically illustrates the application of an array of refractive structures 70 on part of the security document substrate 100 that is exposed within window region 500 on the front side of the banknote. Here, the array of refractive structures is in the form of an array of microlenses that cooperate with the microimage array to exhibit a first optical effect. The array 70 may be applied using any of the techniques that have been discussed above.

[0141] FIGS. 15(a) and 15(b) illustrate a variation in which the opacifying layers 120a and 120b are arranged such that the security article 200 is visible within window region 500 on only one side of the banknote, as opacifying layer 120a is provided to as to laterally extend continuously across window region 500. In this example, as shown in FIG. 15(b), the array of microlenses 70 is applied to the security article itself that is exposed within the window region.

[0142] In the comparative examples of FIGS. 14 and 15, the security article 200 is in the form of a laminate foil. In other comparative examples, the security article may be in the form of a transfer or release foil comprising an adhesive layer and optical effect layer.

[0143] Referring back to FIG. 1, after the refractive structures have been applied in step S200, the method may optionally continue to steps S300, S400 and S500 in order to form the finished security document.

[0144] At step S300, a graphics layer is applied, typically by way of security printing techniques. For example, the graphics layer may be printed by any conventional printing technique, or combination of techniques, such as intaglio printing, lithographic printing, offset printing, flexographic printing, gravure printing and the like. The graphics layer typically comprises high resolution patterns such as fine line patterns and guilloches, portraits, and other indicia. In the examples where the security document substrate is a paper substrate, one or more graphics layers may be printed directly onto the paper substrate. Where the security document substrate comprises a transparent polymer substrate, such a graphics layer is applied to one or more opacifying layers 120 that are provided to at least one of the surfaces of the polymer substrate.

[0145] In step S400, which is also optional, any additional security devices or articles such as threads, strips, patches etc., are applied to the substrate. Any conventional techniques for applying such components can be utilised, including bonding by adhesives, lamination, hot stamping, transfer methods and the like. The security devices could be of any known type, such as holograms, kinegrams and other diffractive elements, iridescent or colour-shift material, etc. Steps S300 and S400 could take place in either order and/or as a series of sub-steps which could be intermingled with one another. Finally, the processed sheet material is cut into individual security documents in step S500.

[0146] In the examples that have been described above, the security document has been in the form of a banknote. However, as will be appreciated by the skilled person, the security document may take other forms such as cheques, passports, identity cards, certificates of authenticity, fiscal stamps, visas or other documents for securing value or personal identity.