OPTICAL DEVICE AND METHOD OF MANUFACTURE THEREOF

Abstract

An optical device is disclosed, comprising; a colour shifting layer that exhibits different colours dependent on the angle of incidence of incident light, and; an array of substantially transparent microstructures covering at least a part of the colour shifting layer and configured to modify the angle of light incident to, and reflected from, the colour shifting layer, said array of microstructures arranged in accordance with a plurality of pixels of a colour image to be exhibited by the optical device, each pixel exhibiting a uniform colour, wherein; the array of microstructures comprises at least first and second sub-arrays of microstructures corresponding to respective first and second colour channels, each sub-array covering an area within a pixel corresponding to the proportion of the respective colour channel within the pixel such that the pixel exhibits the uniform colour, and further wherein; the microstructures of the first sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a first manner such that, at a substantially normal viewing angle of the optical device, the first sub array exhibits a base colour and at a first viewing angle of the optical device, the first sub-array exhibits a first colour, wherein said first viewing angle corresponds to viewing the optical device along a direction that is off the normal of the optical device and; the microstructures of the second sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a second manner different to the first such that, at a substantially normal viewing angle of the optical device, the second sub-array exhibits said base colour and at said first viewing angle, the second sub-array exhibits a second colour different from the first colour; such that at a substantially normal viewing angle, the optical device exhibits the base colour, and at said first viewing angle, the optical device exhibits the colour image. Methods of manufacturing such optical devices are also disclosed.

Claims

1. An optical device comprising; a colour shifting layer that exhibits different colours dependent on the angle of incidence of incident light, and; an array of substantially transparent microstructures covering at least a part of the colour shifting layer and configured to modify the angle of light incident to, and reflected from, the colour shifting layer, said array of microstructures arranged in accordance with a plurality of pixels of a colour image to be exhibited by the optical device, each pixel exhibiting a uniform colour, wherein; the array of microstructures comprises at least first and second sub-arrays of microstructures corresponding to respective first and second colour channels, each sub-array covering an area within a pixel corresponding to the proportion of the respective colour channel within the pixel such that the pixel exhibits the uniform colour, and further wherein; the microstructures of the first sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a first manner such that, at a substantially normal viewing angle of the optical device, the first sub array exhibits a base colour and at a first viewing angle of the optical device, the first sub-array exhibits a first colour, wherein said first viewing angle corresponds to viewing the optical device along a direction that is off the normal of the optical device and; the microstructures of the second sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a second manner different to the first such that, at a substantially normal viewing angle of the optical device, the second sub-array exhibits said base colour and at said first viewing angle, the second sub-array exhibits a second colour different from the first colour; such that at a substantially normal viewing angle, the optical device exhibits the base colour, and at said first viewing angle, the optical device exhibits the colour image.

2. The optical device of any of claim 1, wherein the array of microstructures further comprises a third sub-array corresponding to a respective third colour channel, the third sub-array covering an area within a pixel corresponding to the proportion of the third colour channel within the pixel such that the pixel exhibits the uniform colour, wherein; the microstructures of the third sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a third manner different to the first and second manners such that, at a substantially normal viewing angle of the optical device, the third sub-array exhibits said base colour and at said first viewing angle, the third sub-array exhibits a third colour different from the first and second colours.

3. The optical device of claim 2, wherein the first, second and third sub-arrays correspond to red, green and blue channels respectively.

4. The optical device of claim 1, wherein the microstructures of the sub-arrays are configured to modify the angle of light incident to and reflected from the colour shifting layer in differing manners due to differences in one or more of: (a) facet angle; (b) orientation; (c) refractive index.

5. The optical device of claim 1, wherein each microstructure comprises at least one planar or curved face which makes a facet angle of more than 0° and less than or equal to 90° with the plane of the colour shifting layer.

6-13. (canceled)

14. The optical device of claim 1, wherein each microstructure has a primary axis orientated in a first direction lying in the plane of the optical device, wherein the microstructures are prisms extending along their primary axis or wherein the microstructures are substantially pyramidal.

15. (canceled)

16. The optical device of claim 1, wherein each pixel that exhibits a non-zero proportion of a colour channel comprises at least three microstructures of the respective sub-array corresponding to that colour channel.

17. The optical device of claim 1, wherein the coloured image exhibited at the first viewing angle is a part of a larger image exhibited by the optical device.

18. The optical device of claim 1, wherein the colour shifting layer is an infra-red to red colour shifting layer or an infra-red to infra-red colour shifting layer.

19. The optical device of claim 1, further comprising a second array of substantially transparent microstructures covering at least a part of the colour shifting layer and configured to modify the angle of light incident to, and reflected from, the colour shifting layer, and arranged in accordance with a plurality of pixels of a second colour image to be exhibited by the optical device, each pixel of the second colour image exhibiting a uniform colour, wherein; the second array of microstructures comprises at least first and second sub-arrays of microstructures corresponding to respective first and second colour channels of the second colour image, each sub-array covering an area within a pixel corresponding to the proportion of the respective colour channel within the pixel such that the pixel exhibits the uniform colour, and further wherein, within the second array; the microstructures of the first sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a first manner such that, at a substantially normal viewing angle of the optical device, the first sub array exhibits the base colour and at a second viewing angle of the optical device, the first sub-array exhibits a first colour, wherein said second viewing angle corresponds to viewing the optical device along a direction that is off the normal of the optical device, said second viewing angle being different to said first viewing angle and; the microstructures of the second sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a second manner different to the first such that, at a substantially normal viewing angle of the optical device, the second sub-array exhibits said base colour and at said second viewing angle, the second sub-array exhibits a second colour different from the first colour; such that at a substantially normal viewing angle, the optical device exhibits the base colour, and at said second viewing angle, the optical device exhibits the second colour image.

20. (canceled)

21. The optical device of claim 19, wherein the first viewing angle lies within a viewing plane that intersects the plane of the device along a first viewing direction, and the second viewing angle lies within a viewing plane that intersects the plane of the device along a second viewing direction, and wherein the first and second viewing directions are non-parallel.

22. The optical device of claim 21, wherein each microstructure has a primary axis orientated in a first direction lying in the plane of the optical device, wherein the first viewing direction is substantially perpendicular to the primary axes of the microstructures of the first array, and the second viewing direction is substantially perpendicular to the primary axes of the microstructures of the second array.

23. The optical device of claim 19, wherein the microstructures of the first array are orientated at an angle of between 0° and 180° to the microstructures of the second array.

24. The optical device of any of claim 19, wherein the first array is arranged as a plurality of first image segments that in combination form the first colour image, and the second array is arranged as a plurality of second image segments that in combination form the second colour image, and wherein the plurality of first image segments are interlaced with the plurality of second image segments.

25-32. (canceled)

33. The optical device of claim 1, wherein the optical device is a security device.

34. A security article comprising an optical device according to claim 33, wherein the security article is formed as a security thread, strip, foil, insert, label or patch or a substrate for a security document.

35. A security document comprising an optical device according to claim 33.

36-37. (canceled)

38. A method of manufacturing an optical device, comprising; providing a colour shifting layer that exhibits different colours dependent on the angle of incidence of incident light, and; providing an array of microstructures so as to cover at least a part of the colour shifting layer and configured to modify the angle of light incident to, and reflected from, the colour shifting layer, whereby the array of microstructures is formed in accordance with a template defining a plurality of pixels of a colour image to be exhibited by the optical device, each pixel exhibiting a uniform colour, the array of microstructures comprising at least first and second sub-arrays of microstructures corresponding to first and second colour channels of the template, each sub-array covering an area within a pixel corresponding to the proportion of the respective colour channel within the pixel such that the pixel exhibits the uniform colour, wherein; the microstructures of the first sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a first manner such that, at a substantially normal viewing angle of the optical device, the first sub array exhibits a base colour and at a first viewing angle of the optical device, the first sub-array exhibits a first colour, wherein said first viewing angle corresponds to viewing the optical device along a direction that is off the normal of the optical device and; the microstructures of the second sub-array are configured to modify the angle of light incident to, and reflected from, the colour shifting layer in a second manner different to the first such that, at a substantially normal viewing angle of the optical device, the second sub-array exhibits said base colour and at said first viewing angle, the second sub-array exhibits a second colour different from the first colour; such that at a substantially normal viewing angle, the optical device exhibits the base colour, and at said first viewing angle, the optical device exhibits the colour image.

39. The method of claim 38, wherein the template is generated by; providing a source colour image comprising a plurality of image pixels, each image pixel exhibiting a uniform colour, and; for each image pixel of the source colour image, creating a corresponding template pixel based on the colour of the respective image pixel, each template pixel comprising an arrangement of at least two colour channels and their relative proportions required to generate the uniform colour for that pixel, wherein the colour image exhibited by the device at the first viewing angle is a version of the source colour image.

40. The method of claim 39, wherein each template pixel comprises colour zones defining the relative proportions of the first and second colour channels to be exhibited by the device based on the colour of the corresponding image pixel, and wherein; the sub-arrays are provided according to the colour zones of template pixels.

41-67. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0093] Examples of optical devices and their methods of manufacture will now be described in relation to the accompanying drawings, in which:

[0094] FIG. 1 is a schematic arrangement schematically used to aid understanding of the invention;

[0095] FIGS. 2a and 2b are schematic plan views of a security document comprising an optical device 100 according to the invention;

[0096] FIGS. 3a and 3b are flow diagrams outlining the steps of manufacturing a security device according to the invention;

[0097] FIG. 4 illustrates an example source image and associated image pixels;

[0098] FIG. 5 illustrates an example template and associated template pixels that may be used to form a device according to the invention;

[0099] FIG. 6 schematically illustrates the colour channels of a template pixel;

[0100] FIGS. 7a and 7b schematically illustrate template pixels that may be used to form a device according to the invention;

[0101] FIG. 8 is a plan view of an array of microstructures of an exemplary device according to the invention;

[0102] FIG. 9 illustrates a portion of an array of microstructures in greater detail;

[0103] FIGS. 10a and 10b illustrate alternative colour channel arrangements for a template pixel;

[0104] FIG. 11 schematically illustrates a portion of a look-up table that may be used to form a device according to the invention;

[0105] FIG. 12 schematically illustrates the optical effect exhibited by an exemplary device according to the invention;

[0106] FIG. 13 schematically illustrates the optical effect exhibited by a further exemplary device according to the invention;

[0107] FIGS. 14a to 14c schematically illustrate interlacing of microstructure arrays that may be used to form a device according to the invention;

[0108] FIG. 15 illustrates exemplary microstructures that may be used according to the invention;

[0109] FIGS. 16 to 20 illustrate examples of incorporating an optical device according to the invention into a security document;

[0110] FIG. 21 schematically illustrates a pixel of further embodiment of the invention, and;

[0111] FIGS. 22 and 23 schematically illustrate further embodiments of the invention.

DETAILED DESCRIPTION

[0112] FIG. 1 is a schematic diagram of an arrangement that will be used to aid understanding of the invention. The arrangement comprises first 20a and second 20b substantially transparent microstructures positioned on (here in contact with, but generally meaning above and in optical communication with) a colour shifting layer 10. Here, the microstructures 20a, 20b are linear symmetrical triangular microprisms having their long axes extending into the plane of the paper. FIG. 1 is a light ray diagram showing the effect of the microprisms 20a, 20b on the angle of incidence of light on a colour shifting layer 10, and the subsequent effect on the optical effect exhibited to an observer O.

[0113] Microprism 20a comprises facets arranged at an angle α.sub.1 (facet angle) to the colour shifting layer, and microprism 20b makes a facet angle α.sub.2 with the colour shifting layer 10. The microprisms 20a, 20b have substantially the same height, h, which is typically in the range of 1 μm to 100 μm, preferably 5 μm to 40 μm. In this example, the colour shifting layer 10 is a partially transparent liquid crystal layer, and as such an absorbing layer 12 is positioned on a distal side of the colour shifting layer with respect to the observer O in order to absorb transmitted light. The absorbing layer is typically black, although other colours may be used. In a case where a colour shifting layer or structure comprises individual layers, (e.g. an absorber layer, dielectric layer and reflective layer), for the purposes of this description, such a structure is referred to as a “colour shifting layer”.

[0114] As is understood in the art, when light is incident upon a colour shifting layer 10, some of the light is reflected and undergoes Bragg reflection. The wavelength (and hence the colour exhibited to the observer) of the reflected light is dependent on the angle of incidence of light onto the colour shifting layer. In other words, the colour shifting layer exhibits different colours dependent on the angle of incidence.

[0115] In the schematic diagram of FIG. 1, an observer views the arrangement at a viewing angle θ.sub.va, where θ.sub.va is the angle with respect to the normal of the colour shifting layer (taken as the plane of the arrangement). Light incident on the arrangement corresponding to the viewing angle θ.sub.va (i.e. having an angle of incidence of θ.sub.va) is refracted at the facets of the microprisms 20a, 20b to varying extents dependent on the facet angle aαConsequently, the angle of incidence of light incident on the colour shifting layer (θ.sub.i) differs dependent on whether the light was initially incident upon microprism 20a or microprism 20b.

[0116] More specifically, microprism 20a has a larger facet angle a than microprism 20b, and as a result, for viewing angle θ.sub.va, light incident on microprism 20a undergoes a larger amount of refraction as compared to microprism 20b. As such, a light beam refracted at microprism 20a impinges on the colour shifting layer 10 with a larger angle of incidence θ.sub.i as compared to light refracted at microprism 20b. Consequently, when the arrangement is viewed at viewing angle θ.sub.va, different colours are exhibited by the regions of the first and second microprisms. In other words, the facet angle of the microstructures can be utilised in order to control the colours exhibited to an observer of the device at a particular viewing angle.

[0117] It is noted that in the schematic diagram of FIG. 1, the microprisms are substantially symmetrical, which is a particularly preferred arrangement of the microstructures, although this is not essential.

[0118] It will also be appreciated that when the viewing angle θ.sub.va of the device 100 changes, the colours exhibited by the device change as the angle of incidence of light on the colour shifting layer will vary. A change in viewing angle O.sub.va is typically achieved by “tilting” the device 100 with respect to the observer. Here, the optical effects exhibited by the combination of the microprisms and the colour shifting layer are most readily observed when the device is viewed in a direction (orientation) substantially perpendicular to the long axes of the microprisms (here indicated by arrow V), and therefore a change in colour may be observed by tilting the device 100 relative to the observer about an axis substantially parallel with the long axes of the microprisms.

[0119] In general, microprisms having a larger facet angle a refract light incident on the arrangement to a greater extent than microprisms having a smaller facet angle α, thereby increasing the angle of incidence θ.sub.i of the light incident on the colour shifting layer, which gives rise to shorter wavelength light reflected from the colour shifting layer.

[0120] FIG. 2a is a schematic plan view of a security document 1000 (in this case a banknote) comprising an optical device 100 according to the invention. Here, the optical device takes the form of a security device. FIG. 2a illustrates the optical effect exhibited by the device 100 when the banknote 1000 is viewed along a direction substantially parallel to the plane normal of the optical device, i.e. θ.sub.va equals 0°. Such a viewing angle will be referred to hereafter as a “normal viewing angle”, or “normal viewing”. At this viewing angle, the security device 100 exhibits a substantially uniform colour, which for the purposes of this description is referred to as the “base” colour of the colour shifting layer. Here “uniform” is used to mean that each part of the security device exhibits substantially the same colour.

[0121] It will be appreciated that the colour shifting layer 10 will exhibit the same colour for a range of incident angles. At normal viewing of the device 100, although the microprisms covering the colour shifting layer will refract the light to different extents, the subsequent angles of incidence of light on the colour shifting layer are within a range such that the device exhibits the colour that would be exhibited by the colour shifting layer in the absence of microprisms, when viewed normally. This colour is referred to as the base colour. In this particular embodiment, the colour shifting layer 10 is a partially transparent “infra-red to red” colour shifting layer, with a black absorbing layer 12 positioned beneath it (as in FIG. 1). Consequently, at substantially normal viewing angle, the colour shifting layer reflects light in the infrared portion of the electromagnetic spectrum (and therefore appears black as the black absorbing layer 12 is visible), and when tilted significantly away (˜75°) from a substantially normal viewing angle, exhibits light in the red portion of the electromagnetic spectrum. At the normal viewing angle depicted in FIG. 2a, the device 100 therefore appears black to an observer.

[0122] The device 100 comprises an array of microprisms having differing facet angles (as will be explained later) arranged on the colour shifting layer. The microprisms are linear microprisms with their primary axes extending along the x direction in the view of FIGS. 2a and 2b. Therefore, when the viewing angle of the device 100 is changed by rotating (“tilting”) the banknote 1000 relative to the observer about an axis O-O′ substantially parallel with the long axes of the microprisms, the colours exhibited by the device 100 differ in the regions of microprisms having different facet angles. At least at one viewing angle θ.sub.va=θ.sub.image the exhibited colours cooperate with each other to form a colour image as schematically shown in FIG. 2b. This effect of “revealing” a colour image on tiling the device 100 provides a surprising and memorable effect for an observer, which improves the ease of checking authenticity, whilst also increasing the difficulty of counterfeiting. This effect is most readily observed when the device 100 is viewed at an orientation substantially perpendicular to the long axes of the microprisms (as indicated by the arrow labelled V).

[0123] A more detailed explanation of the invention will now be provided with reference to FIGS. 3a to 9. FIG. 3a is a flow diagram setting out selected steps of a preferred method for manufacturing an optical device according to the invention, and FIGS. 4 to 9 illustrate stages in the method with respect to an exemplary device.

[0124] The process begins at step S101 by obtaining a source image which is to be exhibited by the optical device 100 (as shown in FIG. 2b). The source image is in the form of a pixelated image with pixels of the desired size. The source image may be provided by converting an original input image accordingly. Therefore, such an original input image could be of any file type such as a bitmap, jpeg, gif or the like, and is a colour image. The pixel size is selected so that, preferably, the individual pixels are not readily disenable to the naked human eye whilst, desirably, keeping the overall number of pixels low so as to keep down the computational demands on the system. For instance, the original input image may be at a higher resolution which is beyond that necessary to create a good visual effect in the final device 100, and so step S101 may optionally involve reducing the resolution of the input image, e.g. by combining groups of original pixels into new single pixels with greater size and applying the average colour of the original pixels to that new pixel for the source image. In preferred cases, the pixelated source image at the end of step S101 will have a pixel size between 50 μm and 500 μm, preferably between 100 μm and 300 μm, and more preferably between 50 μm and 150 μm. For instance, in a particularly preferred example, a pixel size of 264×264 microns was adopted and found to produce good results. Of course, the pixel size may be chosen dependent on the application. For example, if the security device is to be incorporated on or within a security thread for a banknote (typically having a width of ˜4 mm), smaller pixel sizes may be chosen to ensure good resolution of the exhibited image.

[0125] FIG. 4a is an example of a source image P that may be used in the present invention. As shown in FIG. 4b, which is an enlarged view of a section of the image P, the image P is made up of a plurality of image pixels 10, optionally generated via a conversion process as described above, each of which is the same size and shape as one another and exhibits a single uniform colour. Three exemplary pixels 10x, 10y and 10z are highlighted. Pixel 10x has a uniform blue colour, pixel 10z has a uniform red colour and pixel 10y is a uniform pink-brown colour.

[0126] At step S102, for each image pixel 10 of the source image P, a corresponding template pixel 11 is created, based on the colour of the corresponding image pixel 10 in the source image P. FIG. 5b illustrates a plurality of template pixels corresponding to the image pixels of FIG. 4(b), with template pixels 11x, 11y and 11z—corresponding to image pixels 10x, 10y and 10z respectively—highlighted. FIG. 5a illustrates the template pixels arranged to form a template T of the source image (step S103).

[0127] The template pixels 11 are each divided into red (15a), green (15b) and blue (15c) sectors of equal area, as seen in FIG. 6. These sectors are the colour channels for the image. In this exemplary embodiment, each sector (colour channel) is an elongate linear strip of equal width, although other geometries of colour channel arrangement are envisaged, as shown for example in FIGS. 10a and 10b.

[0128] Each template pixel 11 defines the proportion of each colour channel that should be exhibited by that pixel such that when the optical device 100 is viewed by an observer at viewing angle θ.sub.image, the pixel 11 appears in the desired uniform colour corresponding to the source image. For each colour channel, a template pixel 11 comprises a colour zone that defines the proportion of the colour channel within that pixel that should be exhibited in order that the pixel exhibits the desired uniform colour. In other words, each template pixel 11 comprises a red colour zone 16a covering a percentage of the red colour channel 15a, a green colour zone 16b covering a percentage of the green colour channel 15b, and a blue colour zone 16c covering a percentage of the blue colour channel 15c. The percentage coverage of a colour channel provided by a colour zone may range between 0% and 100% inclusive (e.g. if no blue colour is to be displayed by the pixel, the blue colour zone is not present). For example, referring to FIG. 10b, if the pixel is desired to display a uniform green colour, the green sectors 15b would have a 100% coverage by a colour zone, and the red and blue sectors 15a, 15c would have 0% coverage. In arrangements of the type seen in FIG. 10b, a template pixel 11 may have more than nine colour sectors, with the percentage coverage of each colour sector preferably being either 0% or 100%.

[0129] For example, template pixel 11z (corresponding to image pixel 10z) comprises a red colour zone 16a that substantially fills the red colour channel 15a, with the green 16b and blue 16c colour zones covering minimal areas of their respective colour channels 15b, 15c, as seen in FIG. 7a. In contrast, template pixel 11y (corresponding to the pink-brown original pixel 10y) has a more equal portion of red, green and blue such that the colour zones cover more equal percentages of their respective colour channels, as seen in FIG. 7b.

[0130] It is to be noted that in the presently described example, the colour zones are in the form of elongate strips having the same dimension of the pixel in the y direction, with the respective areas of the colour channels varying due to a variation in the width (i.e. dimension along the x axis) of each colour zone. However, other geometries of colour zone such that the required areas of the colour channels are covered are envisaged—for example a variation in the dimension of the colour zone along the y axis. In general, the colour zones may take any geometry such that the requisite areas of the colour channels are covered.

[0131] At step S103, the template pixels 11 are arranged in accordance with the relative positions of the original image pixels 10 from which each derives, to form the template T (see FIG. 5a) corresponding to the source image P. Each template pixel 11 is placed in the position of the original image pixel 10 from which it was generated, resulting in an array of red, green and blue colour zones having widths (here along the x axis) corresponding to the amounts of that colour to be exhibited within the pixels. This array of colour zones is clearly seen in FIG. 5b. The red colour zones of the plurality of template pixels form a first sub-array; the green colour zones of the template pixels form a second sub-array and the blue colour channels can form a third sub-array of the overall array of colour zones.

[0132] In this example, the colour channels are elongate in the y direction and repeat periodically in the x direction, i.e. RGBRGBRGB . . . along the x axis. It is the “width” (i.e. dimension in the x direction) of each colour zone that determines the relative proportion (intensity) of that colour to be exhibited at that part of the device.

[0133] Next, in step S104, an array 200 of microstructures is formed based on the generated template T. The array 200 of microstructures corresponding to the template portion of FIG. 5b is shown in FIG. 8, and comprises a plurality of microstructures arranged in accordance with the colour zones of FIG. 5b. Here, each microstructure is a symmetrical triangular linear microprism 20 having its primary axis arranged along the x axis. As a result, the optical effects of the array 200 are predominantly observable when the device is viewed along a direction parallel to the primary axis (i.e. along the y axis in the illustration of FIG. 7). The portion of the array 200 corresponding to a template pixel may be referred to as an “array pixel” 12.

[0134] The array 200 comprises microprisms 20a having a facet angle configured to exhibit red wavelength light when the device is viewed at a viewing angle θ.sub.image (“red” microprisms); microprisms 20b having a facet angle configured such that green light is exhibited at viewing angle θ.sub.image (“green” microprisms); and microprisms 20c having a facet angle that is configured to exhibit blue wavelength light at the same viewing angle of θ.sub.image (“blue” microprisms). For the avoidance of doubt, the “red”, “green” and “blue” microprisms are each substantially transparent and colourless, with their colour labelling used here for ease of description.

[0135] The “red” microprisms 20a are formed in accordance with the red colour zones of the template pixels to form a first sub-array 200a comprising the “red” microprisms. In a similar manner, the “green” microprisms are arranged in accordance with the green colour zones of the template pixels to form a second sub-array 200b, and the “blue” microprisms are arranged in accordance with the blue colour zones of the template pixels in order to form third sub-array 200c. Each microprism of the array is orientated along the width of the colour zones (i.e. primary axes of the microprisms are parallel with the x-axis), and the microprisms within each colour zone are arranged substantially abutting one another along the direction of the y-axis. In this manner, the area of the array 200 covered by “red” microprisms corresponds to the area of the red colour zones of the template pixels; the area of each array pixel 12 covered by “green” microprisms corresponds to the area of the green colour zones of the template pixels and the area of each array pixel 12 covered by the “blue” microprisms corresponds to the area of the blue colour zones of the template pixels.

[0136] Consequently, the proportion of “red”, “green” and “blue” microprisms in each pixel 12 of the microprism array 200 corresponds to the relative amounts of red, green and blue to be displayed by that pixel of the device 100 in order to exhibit the uniform colour for that pixel of the source image. Take, for example, pixel 12y of the microprism array (which corresponds to image pixel 10y and template pixel 11y), and is shown more detail in FIG. 9. The red 15a, green 15b and blue 15c colour channels for the pixel are schematically shown in FIG. 9, together with the “red”, “green” and “blue” microprisms 20a, 20b, 20c. As can be seen, the microprisms have differing dimensions along the x axis corresponding to the width of the corresponding colour zones of the corresponding template pixel 11y (see FIG. 7b). As can also be seen from FIG. 9, the pixel comprises more than three microprisms corresponding to each colour channel. This provides a large amount of facet area corresponding to each colour channel within that pixel such that the device exhibits a good colour representation of the source image.

[0137] In this example, the “red” microprism sub-array has a pitch (along the y-axis) of 40 μm; the “green” microprism sub-array has a pitch of 30 μm and the “blue” microprism sub-array has a pitch of 20 μm. As the microprisms of each sub-array abut one another along the y-axis, the pitch of a sub-array corresponds to the width of a microprism. Each microprism has the same height (8 μm in this example). The colour image is exhibited when the device is tilted between approximately 30° and 60° away from the normal (i.e. at viewing angle θ.sub.image of between 30° and 60°), dependent on where the incident light originates.

[0138] FIG. 3b sets out the steps of an example embodiment for determining the facet angles of the “red”, “green” and “blue” microprisms such that the device exhibits the colour image at the desired tilt angle. As will be appreciated, the facet angle of the microprisms determines the pitch of the microprisms in an array.

[0139] At step S201, the colour shifting layer to be used is determined. A number of different types of colour shifting materials and structures may be used, as have been outlined herein. Preferably, in isolation the colour shifting layer exhibits an infra-red to red, or an infra-red to infra-red wavelength shift upon tilting, such that when viewing the device substantially along its normal, the device appears a uniform black colour. In the case of a colour shifting layer that exhibits an IR-IR wavelength shift, the applied microstructures refract the light incident upon the device such that the light that is ultimately reflected from the colour shifting layer and exhibited by the device is in the visible part of the electromagnetic spectrum.

[0140] At step S202, microstructures of differing facet angles are applied to the colour shifting layer, and the exhibited colours (i.e. wavelengths) for each facet angle are measured as a function of viewing angle (tilt angle). For example, arrays of symmetrical linear microprisms having facet angles of between 25 degrees and 70 degrees at 5 degree intervals may be applied to the colour shifting layer and the exhibited colours as a function of viewing angle measured for facet angle.

[0141] At step S203, the tilt angle at which the device should exhibit the colour image is determined. Typically, this may be between 30 degrees and 60 degrees away from the normal of the device.

[0142] At step S204, based on the data obtained in step S203, the facet angles that exhibited red, green and blue colours at the desired viewing angle are chosen. The array of microstructures may then be formed based on the chosen facet angles that will provide for R,G,B colour mixing. For example, in this embodiment, linear microprisms having the required facet angles are formed in accordance with the colour zones of the template pixels.

[0143] In this example, the microprisms of the array 200 are arranged to cover the area of the respective colour zones by orientating each microprism such that its primary axis extends along the “width” of the colour zones (i.e. along the x axis). In this way, the final device 100 is intended to be viewed primarily along the direction of the y axis, i.e. substantially perpendicular to the direction of the primary axes of the microprisms. In other words, the viewing angle lies within a viewing plane that intersects the plane of the device along the y axis. However, it will be appreciated that the microprisms of the array 200 may be arranged in alternative configurations in order to fill the corresponding colour zones of the template pixels. For example, it is envisaged that the microprisms could be orientated with their primary axes extending along the length of the colour zones (i.e. along the y axis); in which case the device would be intended to be viewed primarily along the x axis.

[0144] In the current example, each colour channel 15a, 15b, 15c of the template pixels is arranged as an elongate linear strip, which advantageously makes arrangement of the colour zones and subsequent microprism array fabrication easier. However, other colour channel arrangements are envisaged with each colour channel having the same area within a pixel, for example as seen in FIGS. 10a and 10b.

[0145] In the present example, each microprism of the array has the same orientation within the plane of the device, and the different colours exhibited by the “red”, “green” and “blue” microprisms are due to the differences in facet angles causing differing amounts of refraction. FIG. 21 schematically shows a pixel 12 of an alternative embodiment, where the different amounts of refraction are provided due to differences in orientation of the “red”, “green” and “blue” microprisms in the respective sub-arrays, with each microprism having the same facet angle. In the view of FIG. 21, the device is intended to be viewed along a direction parallel with the y-axis.

[0146] At step S105 a colour shifting layer is provided and the array of microstructures is provided on the colour shifting layer in order to form the device. As has been explained earlier, a particular advantage of the present invention is the fact that the array of microstructures does not have to be registered with the colour shifting layer. Examples of such a colour shifting layer include photonic crystals, liquid crystals, interference pigments, pearlescent pigments, structured interference materials or thin film interference structures including Bragg stacks and Fabry-Perot stacks. In this example, the colour shifting layer is a partially transparent IR-red colour shifting layer used in combination with a black absorbing layer. As the device is to be viewed in reflection in other examples a substantially opaque colour shifting layer such as a printed ink comprising an optically variable pigment may be used.

[0147] Referring back now to FIG. 9, each colour channel has an area (shown generally at “A”) that is not covered by microprisms, and exhibits the optical effect displayed by the colour shifting layer in isolation. Here, the colour shifting layer is an “IR-red” colour shifting layer. The “red”, “green” and “blue” microprisms are configured to exhibit their respective colours at a viewing angle θ.sub.image where the colour shifting layer in isolation reflects light in the infra-red range of the electromagnetic spectrum and therefore appears black in combination with a black absorbing layer 12. Therefore, at θ.sub.image, the uncovered areas A of the array pixels 13 appear black and do not contribute to the overall colour exhibited by that pixel. In this example, θ.sub.image is between 30° and 60°, with the shift from infra-red to red light reflected by the colour shifting layer occurring at a greater angle of tilt of approximately 75°.

[0148] Referring back to FIG. 3a, in step S102, the arrangement of the colour zones for each pixel can be generated in various different ways. One preferred implementation is to use a look-up table which stores in memory a template pixel for a variety of colours. FIG. 11 schematically illustrates a portion of such a look-up table, which in this case provides template pixel arrangements for six exemplary colours H1 to H6 using red, green and blue colour channels arranged as elongate linear strips as have been discussed above. Each colour H1 to H6 may be defined in the memory by a range of colour values, e.g. in CIELab colour space or the like.

[0149] In this example, colour H1 is red, and so the stored template pixel arrangement includes a single colour zone 16 that covers the entirety of the red colour channel, with the green and blue colour channels uncovered by colour zones. Similarly, colour H2 is green and the stored template pixel arrangement comprises a single colour zone covering the entirety of the green colour channel. Colour H3 is blue and the stored pixel arrangement comprises a single colour zone covering the entirety of the blue colour channel.

[0150] Colour H4 is purple, and in order to achieve this colour, contributions from the red and blue colour channels are required. Consequently, the template pixel arrangement for colour H4 includes a colour zone covering the entirety of the red colour channel, and a colour zone covering the entirety of the blue colour channel such that equal amounts of red and blue light are exhibited by that pixel.

[0151] Colour H5 is turquoise and requires a 2:1 ratio of blue to green light to be exhibited by the pixel in order for the human eye to perceive the correct colour. Consequently, the template pixel arrangement for colour H5 comprises a colour zone covering the entirety of the blue colour channel, and a colour zone covering half of the green colour channel. Colour H6 is black, and as such the template pixel arrangement comprises no colour zones such that in the final device, such a “black” pixel comprises a region with no microstructures. Such a pixel appears black as the black absorbing layer is visible through the colour shifting layer at a viewing angle of θ.sub.image.

[0152] The use of such a look-up table has the benefit that it is computationally efficient, but the drawback that only a finite number of colours will be represented in the table. Whilst the colour value ranges associated with each of the colours can be arranged to encompass the full colour spectrum such that every input colour can be captured and a suitable template pixel generated, this may reduce the number of different colours in the image exhibited by the device as compared with the original image.

[0153] To avoid this, in an alternative implementation, rather than use a look-up table, step S102 may involve the use of an algorithm for generating a template for each source image pixel directly from the detected colour. For instance, the algorithm may involve determining the proportion of each of the available colour channels (e.g. RGB) that are required to recreate the detected colour, and then selecting appropriate colour zones such that the necessary relative proportions of the colour channels are exhibited. In this way there is no limitation on the number of colours, but the process is more computationally expensive.

[0154] FIGS. 12a and 12b schematically illustrate an example device 100 where the image revealed at a viewing angle of θ.sub.image is part of a larger image displayed by the device 100. FIG. 12a illustrates such a device 100, where a part of the larger “complete” image is visible, in this example in the form of a “border” 30. This part of the image is visible at all viewing angles and may be provided by printing, for example. The device 100 also comprises an IR-red colour shifting layer 10 positioned within the border and array of microprisms (not shown) in the manner as described above. In this example, the primary axes of the microprisms are orientated along the x axis.

[0155] At a substantially normal viewing angle θ.sub.va=0°, the colour shifting layer and array of microprisms appear black. The device therefore appears as illustrated in FIG. 12a, as a coloured border surrounding a black region. If the device 100 is viewed along a direction parallel to the y axis and tilted such that it is subsequently viewed at a viewing angle θ.sub.image, the region of the device that appeared black at normal incidence viewing exhibits a colour image portion such that the device exhibits the complete image. This is schematically shown in FIG. 12b, where the border 30 and region of colour shifting layer 10 are illustrated.

[0156] As has been explained above, in order to reveal the colour image, the device 100 is designed to be tilted, relative to the observer O, about an axis substantially parallel with the primary axes of the microstructures. In a further embodiment which will now be described, the device may comprise a further array of microstructures interlaced with, and orientated in a different direction to, the first array of microstructures. Therefore, when observed at different viewing orientations, the device exhibits different colour images, creating an aesthetically pleasing effect that is easy to authenticate and yet difficult to counterfeit.

[0157] This effect is schematically illustrated in FIGS. 13a-c. FIGS. 13a-c illustrate an example security document, here in the form of a banknote 1000, comprising an optical device 100, here acting as a security device. As seen in FIG. 13a, at a substantially normal angle of viewing, the security device 100 exhibits the base colour of the colour shifting layer, which in this case is black due to the use of an IR-red colour shifting layer. The device 100 comprises first and second arrays of linear microprisms, with the microprisms of the first array being orientated such that their primary axes are parallel with the x axis, and the microprisms of the second array are orientated such that their primary axes are parallel with the y axis. In other words, the primary axes of the microprisms of the first array are substantially perpendicular to the primary axes of the microprisms of the second array (although other relative angles of orientation are envisaged). The first array of microprisms are arranged in accordance with a plurality of pixels of a first colour image, and the second array of microprisms are arranged in accordance with a plurality of pixels of a second colour image, in the manner as has been described above.

[0158] Therefore, when the device 100 is viewed at a first viewing angle θ.sub.image1 (defined by a direction of viewing perpendicular to the primary axes of the microprisms of the first array (i.e. viewed along a direction parallel to the y-axis), and a particular angle of tilta about O-O′), the device 100 exhibits the first image, as illustrated in FIG. 13b.

[0159] When the device 100 is viewed at a second viewing angle θ.sub.image2 (defined by a direction of viewing perpendicular to the primary axes of the second array (i.e. viewed along a direction parallel to the x-axis), and a particular angle of tilt about P-P′), the device 100 exhibits the second image, as illustrated in FIG. 13c.

[0160] The direction of viewing (viewing orientation) may be varied by rotating the banknote about its normal axis.

[0161] In this example, both of the first and second colour images are RGB images, and the “red” microprisms of the first and second arrays have the same facet angle. Similarly, the “blue” microprisms of the first and second arrays have the same facet angle, and the “green” microprisms of the first and second arrays have the same facet angle. Consequently, the respective tilt angles about the axes O-O′ and P-P′ of viewing angles θ.sub.image1 and θ.sub.image2 are the same. However, it is envisaged that the microprisms of the first and second arrays may differ, with the first and second images in such a differing arrangement being exhibited at different tilt angles as well as at different viewing orientations (directions).

[0162] FIGS. 14a to 14c will now schematically illustrate how the first and second microprism arrays (corresponding to the first and second colour images) may be interlaced. For ease and clarity of description, a plurality of template pixels 11 are shown for the device 100, from which the arrays of microprisms may be arranged, as has been explained above. The colour zones have also been omitted from FIG. 14a for clarity purposes, with only the colour channels 15a, 15b, 15c for each template pixel shown.

[0163] Each of the first and second source images is divided into a plurality of image segments that together cooperate to form the respective image. In the example of FIG. 14a the image segments are image strips that are elongate linear strips extending along the y axis, although other geometries are envisaged. As can be seen in FIG. 14a, the image strips I.sub.1 of the first image are interlaced with the image strips I.sub.2 of the second image, with the image strips I.sub.1 and I.sub.2 alternating along the x axis (i.e. along a direction perpendicular to their orientation). Due to the interlacing of the two images, each template pixel of each image is halved in size along the direction of interlacing (i.e. here along the x axis) as compared to the case where the device exhibits a single image.

[0164] In the example of FIG. 14a, each image strip I.sub.1, I.sub.2 is one pixel in width, although the image strips may be two or more pixels wide. Preferably, each image strip has a dimension (e.g. in FIG. 14a a “width” along the x axis) smaller than is perceptible by the naked human eye such that each image strip is not discernible to the naked human eye.

[0165] Each image strip I.sub.1 of the first image comprises a plurality of template pixels 11 (corresponding to respective image pixels 10 of the original first image) having their colour channels arranged as elongate linear strips extending along the y axis. For each template pixel of the first image the respective colour zones are provided according to the desired colour of that pixel, and the microprisms of the first array are formed in accordance with the colour zones, as has been described above. In other words, the microprisms formed in accordance with the colour zones of the image strips I.sub.1 of the first image form the first array.

[0166] The microprisms of the first array are orientated with their primary axes extending along the “width” of the colour channels of the first image strips I.sub.1 (i.e. along the x axis in this example). The colour channels 15a, 15b and 15c are shown for exemplary first image template pixel 11a.

[0167] Each image strip I.sub.2 of the second image comprises a plurality of template pixels 11 (corresponding to image pixels 10 of the original second image) having their colour channels arranged as elongate linear strips extending along the x axis, i.e. perpendicular to the colour channels of the first image. For each template pixel of the second image the respective colour zones are provided according to the desired colour of that pixel, and the microprisms of the second array are orientated with their primary axes extending along the “width” of the colour channels of the second image strips I.sub.2 (i.e. along the y axis in this example). The colour channels 15a, 15b, 15c are shown for exemplary second image template pixel 11b.

[0168] Although the image strips I.sub.1, I.sub.2 in this example are elongate strips that are interlaced along the x axis, other arrangements of interlacing are envisaged. For example, the first and second images may be divided into image strips that extend along the x axis, with the direction of interlacing being along the y axis.

[0169] In the example of FIG. 14a, the image segments are in the form of elongate image strips that have been interlaced along one direction (perpendicular to the direction of elongation of the image strips). In other embodiments, the image segments may be arranged in the form of a grid pattern giving rise to two dimensional interlacing. Such arrangements are illustrated in FIGS. 14b and 14c, which schematically show arrangements of the arrays of microstructures that may be used in embodiments of the invention. In FIG. 14b, image segments defining two images (labelled “A” and “B” for simplicity) have substantially square geometry and are arranged in a grid pattern such that they are interlaced in two dimensions. FIG. 14c schematically illustrates an arrangement of four arrays of microstructures of a device that exhibits four different images (labelled “A”, “B”, “C” and “D”), where the image segments are arranged in a grid pattern and the interlacing is in two dimensions.

[0170] It is envisaged that three or more images (i.e. three or more arrays of microstructures) may be interlaced in either a one dimensional or two dimensional a manner as described above. It will be appreciated that due to the pixel size restrictions due to the interlacing, the colour saturation of the final images exhibited by the device 100 will be reduced as compared to a device exhibiting a single image. For example, a device exhibiting two interlaced images will have a colour saturation reduction of 50% as compared to a single image device.

[0171] FIG. 22 schematically illustrates a further embodiment of a device 100 according to the present invention. Here, the device 100 comprises first and second arrays of microstructures. However, rather than the first and second arrays being interlaced as described above, in this embodiment the first array and second arrays are laterally spaced apart. The first array of microstructures exhibits a first colour image and is schematically represented at I.sub.1. The second array of microstructures exhibits a second colour image and is schematically illustrated at I.sub.2, with I.sub.2 being laterally spaced from I.sub.1. The first and second arrays may be laterally spaced such that there is a gap between them, as in FIG. 22, or may substantially abut each other such that there is no gap between them. As in the previously described embodiments, the microstructures of both arrays are elongate linear microprisms, here with the direction of elongation being along the x-axis. The microprisms of the first and second arrays have the same orientation within the plane of the device. Hence, the device is intended to be viewed in a viewing plane that intersects the device along a direction parallel with the y-axis.

[0172] In this embodiment, the arrangement of the microstructures within each array is substantially the same. In other words, the microstructures of the first array are arranged in accordance with a plurality of pixels of a source colour image, and the microstructures of the second array are arranged in accordance with the same plurality of pixels of the same source colour image. However, the corresponding microstructures of the arrays have different facet angles such that they exhibit their respective colours (in combination with the colour shifting layer) at different viewing angles (here different angles of tilt about O-O′). For example, the “red”, “green” and “blue” microprisms of the first array may have facet angles such that they exhibit their respective colours at a viewing angle of ˜40°, whereas the “red”, “green” and “blue” microprisms of the second array may have facet angles such that they exhibit their respective colours at a viewing angle of ˜60°. As will be appreciated, these viewing angles are exemplary and the facet angles of the microprisms may be varied to provide the respective images at different desired viewing angles.

[0173] Thus, at a first viewing angle (here ˜40°), the device will exhibit two images in adjacent regions I.sub.1, I.sub.2 corresponding to the first and second arrays. The colour image exhibited by the first array in region I.sub.1 will be a “true colour” version of the source image, i.e. have substantially the same colours as the source image. In contrast, the image exhibited by the second array in region I.sub.2 will be a “false colour” version of the same image (e.g. each pixel colour of the original source image is replaced by another colour). In other words, the pixels of the second array will still exhibit colours due to light being reflected from the colour shifting layer; however, the facet angles of the microprisms of the second array are such that these colours do not define the R,G,B contributions as in the source image.

[0174] Similarly, at a second viewing angle (here ˜60°), the image exhibited by the second array in region 12 will be a “true colour” version of the original source image, with the image exhibited by the first array in region I1 being a “false colour” version of the original source image.

[0175] Such a device exhibiting side-by-side true colour and false colour versions of the same source image in this manner provides a straightforward means of authentication which is simultaneously extremely difficult to counterfeit. In the description of FIG. 22 above, the microprisms of the first and second arrays differ through their respective facet angles. In alternative embodiments, the microprisms of the first and second arrays may have the respective same facet angles, but differ in refractive index such that the visual effect of laterally adjacent “true colour” and “false colour” image versions is exhibited.

[0176] FIG. 23 schematically illustrates a variation on this embodiment, in which the device 100 comprises three laterally spaced arrays of microstructures, labelled at I.sub.1, I.sub.2 and I.sub.3. In a similar manner to the embodiment of FIG. 22 described above, the arrangement of the microstructures within each array is substantially the same, and the microprisms of each array have the same orientation within the plane of the device. However, the respective facet angles of the microprisms in each array differ from each other such that, on tilting the device about O-O′ (i.e. changing the viewing angle), at a first viewing angle θ1 the first array exhibits a true colour version of the source image in region I1; at a second viewing angle θ2 the second array exhibits a true colour version of the source image in region I2, and at a third viewing angle θ3 the third array exhibits a true colour version of the source image in region I3. Typically θ1<θ2<θ3 such that the true colour images appear to move sequentially across the device upon tilting so as to provide a particularly striking visual effect, especially as the perceived direction of movement of the images is perpendicular to the direction of tilting. In other embodiments, the arrays may not exhibit their true colour images in sequential order.

[0177] As with the embodiment described in FIG. 22, the respective microprisms of the different arrays may differ in refractive index rather than facet angle in order to achieve the same effect.

[0178] Of course, in further variations, the images exhibited by the arrays may be different (i.e. a different arrangement of pixels). However, the use of the same image being exhibited in different colour configurations (i.e. true colour or false colour) is particularly difficult to replicate and thus increases the security level of the device.

[0179] In both of the above described embodiments of FIGS. 22 and 23, the device exhibits the base colour (preferably black) at normal viewing.

[0180] The above figures have been described with reference to the microstructures being microprisms having a symmetrical triangular cross-section. FIG. 15a shows a perspective view of a portion of an array of such microstructures. Other microstructure geometries are envisaged however, for example as seen in FIGS. 15b to 15f. FIG. 15c illustrates a portion of an array comprising a plurality of microprisms each having a “saw-tooth” structure, in that one facet (shown here at 41) defines a more acute angle with the colour shifting layer than the opposing facet 42. Multi-faceted microprisms (i.e. having more than two facets) may be used, as in the portion of the array shown in FIG. 15d. A lenticular array having a curved surface structure may be used, as illustrated at FIG. 15b. In each case, a primary axis of the microstructures, D, has been shown, with the optical effects most strikingly exhibited when viewed at an orientation substantially perpendicular to the primary axes.

[0181] The above examples may be seen as “one dimensional” microstructures in that their refractive effects are primarily observed in one rotational viewing direction with respect to an individual microstructure (typically perpendicular to its long axis). However, arrays of “two dimensional” microstructures are also envisaged where the optical effects due to the presence of the microstructures are readily observed at two or more rotational viewing directions, due to such structures having facets along more than one axis that make a facet angle of less than 90°. Examples of such two-dimensional microstructures include square based pyramids as seen in FIG. 15e, and hexagonal based pyramids, as illustrated in FIG. 15f.

[0182] Optical devices of the sort described above, in the form of security devices, can be incorporated into or applied to any article for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into documents of value such as banknotes, passports, driving licences, cheques, identification cards etc.

[0183] The security device or article can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 6 mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.

[0184] The security device or article may be subsequently incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate. Methods of incorporating security elements in such a manner are described in EP-A-1141480 and WO-A-03054297. In the method described in EP-A-1141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

[0185] Base substrates suitable for making security substrates for security documents may be formed from any conventional materials, including paper and polymer. Techniques are known in the art for forming substantially transparent regions in each of these types of substrate. For example, WO-A-8300659 describes a polymer banknote formed from a transparent substrate comprising an opacifying coating on both sides of the substrate. The opacifying coating is omitted in localised regions on both sides of the substrate to form a transparent region. In this case the transparent substrate can be an integral part of the security device or a separate security device can be applied to the transparent substrate of the document. WO-A-0039391 describes a method of making a transparent region in a paper substrate. Other methods for forming transparent regions in paper substrates are described in EP-A-723501, EP-A-724519, WO-A-03054297 and EP-A-1398174.

[0186] The security device may also be applied to one side of a paper substrate so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.

[0187] Examples of such documents of value and techniques for incorporating a security device will now be described with reference to FIGS. 16 to 19.

[0188] FIG. 16 depicts an exemplary document of value 1000, here in the form of a banknote. FIG. 16a shows the banknote in plan view (and at viewing angle θ.sub.image) whilst FIG. 16b shows the same banknote in cross-section along the line Q-Q′. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 102. Two opacifying layers 103a and 103b are applied to either side of the transparent substrate 102, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 102.

[0189] The opacifying layers 103a and 103b are omitted across an area 101 which forms a window within which the security device 100 is located. As shown best in the cross-section of FIG. 16b, the microstructures (shown generally at 20) are provided on one side of the transparent substrate 102, and a colour shifting layer 10 is provided on the opposite surface of the substrate. The microstructures 20 and colour shifting layer 10 are each as described above with respect to any of the disclosed embodiments, such that the device 100 reveals a colour image at a first viewing angle, as schematically shown in FIG. 16a. As the device 100 is to be viewed in reflection it is desirable to use a substantially opaque colour shifting layer such as a printed ink comprising an optically variable pigment, although a partially transparent colour shifting layer may be used in conjunction with an absorbing element as described above.

[0190] It should be noted that in modifications of this embodiment the window 101 could be a half-window with the opacifying layer 103b continuing across all or part of the window over the security device 100.

[0191] FIG. 17 shows such an example, although here the banknote 1000 is a conventional paper-based banknote provided with a security article 105 in the form of a security thread, which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 104 lie on either side of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread. The security thread 105 is exposed in window regions 101 of the banknote. Alternatively, the window regions 101 may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. The security device is formed on the thread 105, which comprises the array(s) of transparent microstructures 20 provided on one side and colour shifting layer 10 provided on the other.

[0192] If desired, several different security devices 100 could be arranged along the thread, with different or identical images displayed by each. In one example, a first window could contain a first device, and a second window could contain a second device, each having different microstructure arrays, so that the two windows display different images.

[0193] In FIG. 18, the banknote 1000 is again a conventional paper-based banknote, provided with a strip element or insert 108. The strip 108 is based on a transparent substrate and is inserted between two plies of paper 109a and 109b. The security device is formed by an array(s) of microstructures 20 disposed on one side of the strip substrate, and a colour shifting layer 10 disposed on the opposing side. The paper plies 109a and 109b are apertured across region 101 to reveal the security device, which in this case may be present across the whole of the strip 108 or could be localised within the aperture region 101.

[0194] A further embodiment is shown in FIG. 19 where FIGS. 19(a) and (b) show the front and rear sides of the document 1000 respectively, and FIG. 19(c) is a cross section along line Q-Q′. Security article 110 is a strip or band comprising a security device according to any of the embodiments described above. The security article 110 is formed into a security document 1000 comprising a fibrous substrate 102, using a method described in EP-A-1141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (FIG. 19(a)) and exposed in one or more windows 101 on the opposite side of the document (FIG. 19(b)). Again, the security device is formed on the strip 110, which comprises a transparent substrate with microstructures 20 formed on one surface and colourshifting layer 10 formed on the other.

[0195] In FIG. 19, the document of value 1000 is again a conventional paper-based banknote and again includes a strip element 110. In this case there is a single ply of paper. Alternatively a similar construction can be achieved by providing paper 102 with an aperture 101 and adhering the strip element 110 on to one side of the paper 102 across the aperture 101. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting. Again, the security device is formed on the strip 110, which comprises a transparent substrate with the array(s) of microstructures 20 formed on one surface and colour shifting layer 10 formed on the other.

[0196] In the examples of FIGS. 16 to 19, the colour shifting layer and the array(s) of microstructures are described as being on opposing sides of a transparent substrate. However, in other examples they may be provided on the same side of the transparent substrate.

[0197] FIGS. 20a and 20b schematically illustrate how a security device 100 according to the invention may be incorporated into a substrate 1100 for a security document such as a plastic identity card or passport. FIG. 20a is a schematic cross-sectional diagram of an example substrate 1100 for a security document. The substrate 1100 comprises a plurality of polymer layers that are joined together, typically be lamination (see FIG. 20b). The substrate 1100 has a first outer surface 1031a and a second outer surface 1037a. The thickness of the substrate 1100, which is the distance between the first and second outer surfaces 31a, 37a, is preferably at least approximately 150 μm and more preferably at least approximately 300 μm. In particular, the substrate 1100 may be between approximately 300 μm and 1000 μm thick and, for example, may be approximately 800 μm thick. The substrate 1100 may be substantially rigid or at least semi-rigid by virtue of its thickness and polymer (typically plastic) composition.

[0198] Within the substrate 1100 is a colour shifting layer 10 as described in any of the embodiments above. In this case the colour shifting layer 10 is partially transparent and a dark absorbing layer 12 is therefore utilised as described above. As will be understood, a substantially opaque colour shifting layer may alternatively be used.

[0199] An array of microstructures (shown generally at 20) is formed in the first outer surface 31a of the substrate 1100 so that the microstructures are positioned above and in register (i.e. aligned with) with the colour shifting element 10, such that light from the colour shifting element passes through the microstructures 20 before reaching the observer O.

[0200] FIG. 20b schematically illustrates the structure of such a substrate 1100. As illustrated in FIG. 20b, a plurality of typically planar self-supporting polymer layers 1031, 1032, 1033, 1034, 1035, 1036 and 1037 are provided in a (typically fully) overlapping manner. Layers 1031 and 1037 are first and second outer layers respectively, and the outer surface 1031a of the first outer layer defines the first outer surface 1031a of the substrate 1100, and similarly the outer surface 1037a of second outer surface 1037a defines the second outer surface of the substrate 1100. The first and second outer layers are typically substantially transparent.

[0201] As can be seen in FIG. 20b, a plurality of internal layers 1032, 1033, 1034, 1035 and 1036 are provided positioned between the first and second outer layers 1031, 1037. For the purposes of this description, moving in a direction from the first (“top”) outer layer 1031 to the second (“bottom”) outer layer 1037, layer 1032 is the first internal layer, layer 1033 is the second internal layer, layer 1034 is the third internal layer, layer 1035 is the fourth internal layer and layer 1036 is the fifth internal layer.

[0202] A colour shifting layer 10 is provided on and in contact with a first surface the second internal layer 1033. Here the first surface is the uppermost surface of second internal layer 1033 and is the surface of second internal layer proximal the first outer layer 1031. The colour shifting layer may be provided on the second internal layer 1033 by a variety of methods, such as lamination, printing or sputtering via vacuum deposition which would typically be the case for the different layers of a thin film multilayer interference structure (in the case of optically variable pigments for example). Such a thin film interference structure forms a “colour shifting layer” for the purposes of this description.

[0203] For the case where the colour shifting layer is at least partially transparent, an absorbing element 12 is provided on and in contact with the second surface of the second internal layer 1033. Here the second surface is the bottommost surface of the second internal layer 1033 and is the surface of second internal layer distal the first outer layer 1031. In other embodiments the colour shifting layer and absorbing layer 12 may be provided on the same surface of internal layer 1033.

[0204] The first outer layer 1031 and the first internal layer 1032 are substantially transparent such that visible light can pass through them. This allows visible light to be incident to and reflected from the colour shifting layer 10 such that the colour shifting layer 10 is visible through the first outer layer 1031 and the first internal layer 1032. The second internal layer 1033 upon which the colour shifting layer 10 is positioned is also substantially transparent. In the case where an absorbing element is not required (for example where the colour shifting layer is substantially opaque, such as metal-dielectric multilayer thin films or a printed optically variable pigment), the second internal layer 1033 may be transparent or opaque. The third 1034, fourth 1035 and fifth 1036 internal layers are substantially opaque. In general the internal layers positioned between the colour shifting layer 10 and the first (“top”) outer layer are substantially transparent (or at least have a substantially transparent region) such that the colour shifting layer 10 is visible through the top of the finished substrate and the optical variable effects of the colour shifting element are exhibited to a viewer. Typically the internal layers positioned between the colour shifting layer 10 and the second (“bottom”) outer layer are substantially opaque. Furthermore, the substantially opaque internal layers may comprise marking additives such that they can be laser marked, as is known in the art.

[0205] Although in general the internal layers positioned between the colour shifting layer 10 and the first (“top”) outer layer are substantially transparent, the colour shifting layer 10 may be viewable through a substantially transparent window region in a layer positioned between the colour shifting layer 10 and the first outer layer 1031.

[0206] The polymer layers are typically formed from a plastic material such as polycarbonate, polyethylene terephthalate (PET) or polyethylene terephthalate glycol-modified (PETG). Polycarbonate is particularly suitable due to its high durability and ease of manufacture. Each of the layers may be between approximately 30 and 200 μm thick. Although in this example seven layers are shown, in other examples a different number of layers may be used.

[0207] The microstructure array is formed in at least the first outer layer 1031, and may be formed in the first outer layer 1031 and first internal layer 1032. This is typically performed by embossing, and may be carried out subsequent to lamination of the polymer layers, or substantially simultaneously with the lamination.

[0208] In other embodiments, the colour shifting layer may be inserted into a pre-formed polymer substrate by insertion of a “plug” containing the colour shifting layer into a corresponding aperture in the substrate.