Abstract
A holographic security device includes a holographic image layer which when illuminated exhibits the optically variable effect of viewing first and second overlapping patterns of elements, wherein; the first pattern of elements includes a first set of image elements and at least a second set of image elements, and the pitches and relative locations of the first and second patterns of elements are such that, upon illumination of the device; at a first viewing position of the security device the first set of image elements are exhibited by the holographic image layer and at a second, different viewing position of the security device the second set of image elements are exhibited by the holographic image layer. Also, an associated method of manufacture of the security device.
Claims
1-51. (canceled)
52. A method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the optically variable effect generated by illuminating the first and second overlapping patterns of elements, wherein; the first pattern of elements comprises a first set of image elements and at least a second set of image elements, and; the pitches and relative locations of the first and second patterns of elements are such that, upon illumination of the holographic image layer, at a first viewing position of the holographic image layer the first set of image elements are exhibited and at a second, different viewing position of the holographic image layer the second set of image elements are exhibited.
53. The method of claim 52, wherein at least one of the first and second patterns of elements, or both in combination, define indicia.
54. A method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the optically variable effect generated by illuminating the first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that, upon illumination of the holographic image layer, the holographic image layer exhibits a magnified version of at least a part of the first pattern of elements due to the moir effect, and further wherein; at least one of the first and second patterns of elements comprises a first area having a first pitch along at least one axis and a second area having a second, different pitch along said axis, whereby the moir effect causes different degrees of magnification of the first pattern of elements to occur, such that the holographic image layer exhibits areas of different depth corresponding to the first and second areas.
55. A method of manufacturing a holographic image layer for a security device, comprising: providing a holographic recording medium; providing first and second overlapping patterns of elements, and; holographically recording, in the holographic recording medium, the optically variable effect generated by illuminating the first and second overlapping patterns of elements, wherein; the pitches and/or relative rotations of the first and second patterns of elements and their relative locations are such that, upon illumination of the holographic image layer, the holographic image layer exhibits a magnified version of at least a part of the first pattern of elements due to the moir effect, and further wherein; the holographic image layer comprises a volume hologram.
56. The method of claim 54, wherein the first pattern of elements comprises an array of image elements that are compressed along at least the axis along which magnification occurs due to the moir effect.
57. The method of claim 52, wherein at least one of the first and second patterns of elements comprises a one dimensional line screen pattern or a one dimensional pattern of indicia.
58. The method of claim 52, wherein at least one of the first and second patterns of elements comprises a two dimensional line screen pattern or dot screen pattern, or a two dimensional pattern of indicia.
59. The method of claim 52, wherein the second pattern of elements comprises a one dimensional or two dimensional array of focussing elements.
60. The method of claim 59, wherein the first and second patterns of elements are spaced apart by a distance substantially equal to the focal length of the focussing elements.
61. The method of claim 52, wherein the holographic image layer comprises an embossed hologram.
62. The method of claim 52, wherein the holographic image layer comprises a volume hologram.
63. The method of claim 55, wherein the first pattern of elements comprises an array of image elements that are compressed along at least the axis along which magnification occurs due to the moir effect.
64. The method of claim 54, wherein at least one of the first and second patterns of elements comprises a one dimensional line screen pattern or a one dimensional pattern of indicia.
65. The method of claim 55, wherein at least one of the first and second patterns of elements comprises a one dimensional line screen pattern or a one dimensional pattern of indicia.
66. The method of claim 54, wherein at least one of the first and second patterns of elements comprises a two dimensional line screen pattern or dot screen pattern, or a two dimensional pattern of indicia.
67. The method of claim 55, wherein at least one of the first and second patterns of elements comprises a two dimensional line screen pattern or dot screen pattern, or a two dimensional pattern of indicia.
68. The method of claim 54, wherein the second pattern of elements comprises a one dimensional or two dimensional array of focussing elements.
69. The method of claim 55, wherein the second pattern of elements comprises a one dimensional or two dimensional array of focussing elements.
70. The method of claim 54, wherein the holographic image layer comprises an embossed hologram.
71. The method of claim 54, wherein the holographic image layer comprises a volume hologram.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0093] Examples of the present invention will now be described with reference to the attached drawings, in which:
[0094] FIG. 1 shows an exemplary security device disposed on a substrate;
[0095] FIG. 2a is a schematic illustration of a geometry for recording a H1 master hologram in a conventional H1/H2 hologram recording technique;
[0096] FIG. 2b is a schematic illustration of a geometry for using the H1 to form a surface relief H2 hologram;
[0097] FIG. 2c is a schematic illustration of a geometry for using the H1 to form a volume H2 hologram;
[0098] FIGS. 3a to 3c schematically illustrate example geometries for recording a volume hologram;
[0099] FIGS. 4a and 4b illustrate an example sampling plate;
[0100] FIGS. 5a and 5b illustrate an example artwork plate;
[0101] FIG. 6 schematically illustrates a variable image exhibited to a viewer;
[0102] FIG. 7 illustrates a further example artwork plate;
[0103] FIG. 8 schematically illustrates a further variable image exhibited to a viewer;
[0104] FIGS. 9a and 9b illustrate a further artwork plate;
[0105] FIGS. 10a and 10b illustrate a further sampling plate;
[0106] FIGS. 11a and 11b schematically illustrate a further variable image;
[0107] FIGS. 12a and 12b illustrate further examples of artwork plates;
[0108] FIGS. 13a and 13b schematically illustrate a further variable images;
[0109] FIGS. 14a and 14b illustrate a further sampling plate;
[0110] FIGS. 15a and 15b illustrate a further artwork plate;
[0111] FIG. 16 shows a frame of a further variable image;
[0112] FIG. 17 illustrates a further sampling plate;
[0113] FIGS. 18a and 18b show a further artwork plate;
[0114] FIG. 19 shows a frame of a further variable image;
[0115] FIG. 20 schematically illustrates indicia adapted to have a three dimensional appearance;
[0116] FIG. 21 is a magnified view of a further artwork plate;
[0117] FIG. 22 shows a further example of an artwork plate;
[0118] FIG. 23 shows a number of frames of a further variable image;
[0119] FIGS. 24a and 24b illustrate further examples of artwork and sampling plates;
[0120] FIG. 25 schematically illustrates a further variable image,
[0121] FIG. 26 is an example arrangement of a sampling plate comprising an array of focussing elements;
[0122] FIG. 27 is an example of an artwork plate that may be used with a sampling plate comprising an array of microlenses, and;
[0123] FIGS. 28 to 30 show varies ways in which a security device according to the invention may be incorporated into a security document.
DETAILED DESCRIPTION
[0124] For ease of reference, the description below will refer to certain directions using the notation depicted in FIG. 1. FIG. 1 shows an exemplary security document 2000 (such as a credit card) comprising a security device 1000 disposed on a substrate 1001 which sits in a substantially planar surface defined by X and Y orthogonal axes. The third orthogonal Z axis is normal to the plane of the device, and as such an observer (O.sub.1) viewing the device 1000 from any position along the Z axis has a normal viewing position. An observer O.sub.2 at an arbitrary viewing position (VP) away from the normal is shown in FIG. 1. The viewing position VP is defined by the angle (tilt angle or viewing angle) between the viewing position VP and the normal (Z axis). The change in viewing position is typically effected by an observer tilting the document (and therefore the device) about a tilt axis TA (in FIG. 1 the tilt axis being the Y axis).
[0125] For simplicity the following description will refer to tilting along either the X or Y axis in the geometry of FIG. 1, although it will be appreciated that other tilt axes within the X-Y plane are possible. The terms hologram and holographic image layer are interchangeable in the following discussion.
[0126] FIG. 2a is a schematic illustration of the geometry for recording a H1 master hologram in a conventional H1/H2 hologram recording technique. An object laser light beam (shown at 1) is incident on and directed through a light diffusing plate 3, and through overlapping first 100 and second 200 patterned plates, which are typically separated by a distance h. This distance is variable and is selected dependent on the degree of synthetic magnification and perceived depth required in the final replayed image, but is typically between 0.05 mm and 10 mm, with the typical separation for a rainbow embossed hologram being in the range on 2-10 mm. Each patterned plate comprises a pattern of elements that cooperate with each other such that a phase interference and/or moir pattern is formed on the H1 hologram plate 9, which is typically coated with a silver halide emulsion. A reference light beam (shown at 7) of collimated laser light that is coherent with the object beam is directed onto the H1 plate 9 in off-axis geometry. A H2 copy (or copies) can subsequently be produced from the master H1 as is known in the art, and example geometries for this are show in FIGS. 2b and 2c.
[0127] FIG. 2b illustrates a typical geometry used to generate a H2 resist master 9a for a typical off-axis surface relief (embossed) hologram, such as a Benton slit rainbow hologram. The H1 plate 9 is illuminated with a conjugate reference beam 1, and the resultant image beam 1a illuminates the H2 resist master 9a (the holographic image projected by the master H1 is shown at 90). A H2 reference beam 7a is also used to illuminate the H2, with both the image beam la and the reference beam 7a impinging on the H2 from the same side. The planes of interference generated by the holographic interference between the image 1a and reference 7a beams will be parallel to the bisector of the wave vector for each beam. Where these planes intercept the resist surface of the H2 will determine the grating spacing and orientation of the surface relief structure. In the case of a rainbow hologram, the image beam 1a will be directed through a Benton slit.
[0128] FIG. 2c illustrates a projection geometry that may be used to record an off-axis volume hologram 10 from the H1. The main difference between this geometry and that explained above in FIG. 2b is that the H2 reference beam 7a impinges on the H2 hologram 10 from the opposite side to the image beam 1a to form interference fringes which typically make a smaller angle with the H2 than the corresponding surface relief hologram geometry shown in FIG. 2b.
[0129] For a general literature discussion of holographic H1/H2 transfer techniques a suitable reference text is Practical Holography, Graham Saxby, published by Prentice Hall Int. (UK) Ltd. 1988.
[0130] For ease of reference in the following, the first patterned plate 100 will be referred to as the artwork plate, and the second patterned plate 200 will be referred to as the sampling plate as discussed above in the summary of the invention section.
[0131] FIG. 3a schematically illustrates an example geometry for directly recording an on-axis reflection volume hologram. A laser light beam 1 is incident on and directed through a light diffusing plate 3. The light beam 1 travels through the sampling 200 and artwork 100 plates before being reflected from a back reflector 5. The reflected beam (acting as the object beam) subsequently travels though artwork plate 100 and sampling plate 200, generating a phase interference and/or moir pattern that is recorded in master volume hologram 10. In this geometry the light beam travelling through the plates before being reflected from the back reflector acts as the reference beam.
[0132] FIG. 3b schematically illustrates an alternative geometry for directly recording an on-axis reflection volume hologram. Here, a second light diffusing plate 3 is provided in place of the back reflector 5 of FIG. 3a, and an object laser light beam 1 is directed through the artwork plate 100 and sampling plate 200 to generate a phase interference and/or moir pattern that is recorded on the master volume hologram 10. A reference beam 7 that is coherent with the object beam is provided in on-axis geometry. FIG. 3c schematically illustrates the case where the object beam 7 is provided in off-axis geometry.
[0133] In each of these cases, the resulting hologram 10 is used in the security device 1000 such as that seen in FIG. 1. Upon illumination of the security device, the hologram (or holographic image layer) diffracts incident light in order to exhibit a holographic version of the variable effect produced by the overlapping plates 100, 200 to an observer of the device.
[0134] The invention will be described with reference to a number of example effects exhibited by holographic image layers. However, these are not limiting, and the skilled person will understand that features of different examples may be combined.
First Example
[0135] FIG. 4a is an example sampling plate 200 and FIG. 5a is an example artwork plate 100 that may be used according to a first example of the invention in order to provide a phase interference effect. The sampling plate 200 illustrated in FIG. 4a (with a magnified view shown in FIG. 4b) comprises a regular array of substantially opaque rectangular elements 201 having their long axes directed along the Y axis. The rectangular elements 201 are separated along a direction perpendicular to their long axes (i.e. separated along the X axis). The rectangular elements 201 are substantially opaque, with the gaps 202 between the rectangles being substantially transparent to visible light such that light from the artwork plate can pass through. In this example each rectangular element 201 has a width of 600 m and the rectangular elements are separated by gaps 202 of 100 m.
[0136] The artwork plate 100 illustrated in FIG. 5a comprises two spaced apart star-shaped indicia 101a, 101b, with FIG. 5b illustrating a magnified view of star 101a. As can be seen in FIG. 5b, the star 101a is comprised of a plurality of sections 102a, 102b . . . 102v, with specific sub-sections (segments) of the star being viewable through the gaps 202 of the sampling plate 200 at different viewing angles. The resultant hologram exhibits the effect seen in FIG. 6, where for different viewing angles (.sub.1-.sub.7) when the device (and therefore the hologram) is tilted about the Y axis, different size stars 101a, 101b are replayed. The size ratio between the opaque and transparent areas of the sampling plate 200 control the number of frames that are replayed upon tilting the devicein this instance there are seven different frames that are exhibited.
[0137] Take for example the viewing angle .sub.1, where the left star 101a is exhibited at its maximum size, and the right star 101b is exhibited in its minimum size. Referring back to FIG. 5b, each section of the star comprises up to seven segments. For example section 102h comprises segments 103a, 103b, 103c, 103d, 103e, 103f, 103g, each having different vertical heights. Segment 103a corresponds to the frame seen at viewing angle .sub.1 where the star 101a is exhibited at its maximum size, and segment 103g corresponds to the frame seen at viewing angle .sub.7 where the star 101a is exhibited at its minimum size. In other words, referring to only section 102h for ease of reference, at viewing angle .sub.1, segment 103a is viewable through the gaps 202 in the sampling plate 200; at viewing angle .sub.2, segment 103b (which is smaller than 103a) is viewable through the gaps 202 in the sampling plate 200, and so on until at viewing angle .sub.7, segment 103g is viewable through the gaps 202 in the sampling plate. The parallax allows the sampling plate to sequentially reveal each one at a time when the hologram is tilted about the Y axis.
[0138] Right star 101b is composed of sections and segments in a corresponding manner such that the final security device, when tilted about the Y axis, displays the seven frames illustrated in FIG. 6, with one star increasing in size whilst the other star correspondingly decreases in size in order to provide a striking visual effect. Under diffuse light, this effect is minimised and the general shape of the stars at their maximum size are visible (i.e. all the segments of both stars are visible), as seen in FIG. 5a. However, under spot light, each of the seven frames is individually visible upon tilting the device. This change in appearance of the device under different lighting conditions advantageously increases the security level of the device.
Second Example
[0139] FIG. 7 illustrates an alternative artwork plate 110 that may be used with the sampling plate 200 described above. Again, the exhibited effect will be a phase interference effect. Artwork plate 110 comprises two arrays 111, 112 of overlapping circles arranged in a curved manner. Array 111 comprises circles 111a, 111b . . . 111g and array 112 comprises circles 112a, 112b . . . 112g. Each circle is comprised of an array of vertical segments (directed along the Y axis), with the arrays of each circle being offset from each other such that at a particular viewing angle of the resulting hologram, only one circle of each array 111, 112 is visible. This exhibits an animation effect with the circles appearing to change in position and size upon tilting of the hologram about the Y axis, as schematically illustrated in FIG. 8. As before, this arrangement of artwork and sampling plates generates seven frames seen at viewing angles .sub.1 to .sub.7. Under diffuse light, this effect is minimised and the general shape of the circle arrays is visible, as is FIG. 7.
Third Example
[0140] FIG. 9a illustrates an artwork plate 120 that may be used with sampling plate 210 (illustrated in FIG. 10a) in order to produce a striking contrast switch phase interference effect that is illustrated in FIGS. 11a and 11b. As seen in FIG. 11a, at a first viewing angle .sub.1 of the device 100, a first pattern of indicia is exhibited. More specifically, a shaded 5 symbol 121 is displayed against a light background, a shaded region 123 outlines a light symbol 122, and two star shapes 124, 125 are exhibited. At a second viewing angle of the device, .sub.2 (e.g. tilting the device about the Y axis), the same symbols are exhibited but the light and shade are reversed.
[0141] In contrast to the first and second embodiments, only two frames are visible here, as the sampling plate 210 comprises an array of substantially opaque rectangular elements 211 (see FIG. 10b) that are spaced apart by a distance equal to the width of each rectangular element. In other words, the substantially transparent gaps 212 between the rectangular elements and the rectangular elements themselves are substantially the same width.
[0142] The artwork plate 120 comprises two arrays 121, 122 of substantially rectangular elements as illustrated in the magnified view of artwork plate in FIG. 9b. The two arrays of the artwork plate are offset from each other such that at the first viewing angle .sub.1 only the first array is visible through the substantially transparent gap regions 212 of the sampling plate 210, and at the second viewing angle .sub.2 only the second array is visible through the gaps of the sampling plate 210.
[0143] Again, under diffuse light, the contrast switch effect will be minimised, with the general shape of the artwork plate being visible (FIG. 9a).
Fourth Example
[0144] The above examples have been directed to examples of phase interference effects that may be utilised in the present invention. Alternatively or in addition, the overlapping artwork and sampling plates can also be used to create moir magnification effects when viewing the final security device, as will be explained in the following.
[0145] The degree of magnification achieved is defined by the expressions derived in The Moire Magnifier, M. Hutley, R Hunt, R Stevens & P Savander, Pure Appl. Opt. 3 (1994) pp. 133-142. To summarise the pertinent parts of this expression, suppose the pitch of the elements of the artwork plate is A and the pitch of the elements of the sampling plate is B, then the magnification of the artwork plate elements, M is given by:
M=A/SQRT[(B cos(Theta)A).sup.2(B sin(Theta)).sup.2],(Eq. 1)
[0146] where Theta equals the angle of rotation between the elements of the artwork and sampling plates.
[0147] For small Theta such that cos(Theta)1 and sin(Theta)0 and for the case where BA, we have,
M=A/(BA).(Eq. 2)
[0148] As we can see from Eq. 2 therefore, if the artwork plate comprises an array of indicia that are compressed along an axis that is perpendicular to the long axis of the sampling plate elements, then the indicia will appear magnified along that axis when viewed through the sampling plate.
[0149] This effect is illustrated in FIGS. 12 and 13. FIG. 12a illustrates an artwork plate 130 that comprises two arrays of overlapping circles arranged in a curved manner as in the plate 110 seen in FIG. 7. Additionally, the artwork plate 130 comprises a 5 symbol in outline, within which are a plurality of arrays 132a, 132b, 132c, 132d, 132e of symbols. The individual symbols are compressed in a direction along the X axis and are regularly spaced.
[0150] In this specific example, each symbol has a width (i.e. a dimension along the X axis) of 547 m, and the spacing of the symbols is a constant 70 m. The sampling plate is the plate 200 illustrated in FIG. 4a, having a regular array of 600 m wide rectangular elements separated by 100 m wide substantially transparent regions. The rectangular elements 201 of the sampling plate and the symbols are aligned along the same (Y) axis (i.e. no rotational offset) and so we may use Eq. 2 to calculate the magnification of the artwork plate indicia. Accordingly, when viewing through the sampling plate 200, the magnification of the symbols is 617/(700617)=7.4, giving an exhibited width of 4.1 mm in the replayed hologram image.
[0151] Therefore, when viewing the final hologram image exhibited by the device, the viewer perceives the animation effect of the circles as in the second embodiment, together with 4.1 mm wide symbols appearing to have fast movement along the X axis upon tilting the hologram about the Y axis. The apparent movement of the indicia is due to the fact that changing the viewing angle causes the sampling plate to sample different parts of the artwork plate. The magnification of the symbols also provides perceived depth of the final image, providing a striking effect to the viewer. FIG. 13a illustrates the centre-view combined effect of the artwork 130 and sampling 200 plates, where the magnified symbols in the arrays 132a, 132b, 132c, 132d, 132e are easily seen.
[0152] However, as also visible in FIG. 13a, under spotlight conditions, the vertical rectangular elements 201 of the sampling plate 200 are visible, which is generally an undesirable artefact in the final image exhibited by the hologram. In order to minimise this undesirable artefact and yet still maintain the moir effects, an alternative artwork plate 135 may be used as illustrated in FIG. 12b. In this artwork plate, the area of the plate surrounding the active areas (i.e. the 5 and the two arrays of circles) is masked, ensuring that the elements of the sampling plate 200 are not visible in this area in the final hologram, as seen the FIG. 13b, which is the centre-view combined effect of the alternative artwork plate 135 and sampling 200 plates. The area around the active elements remains clear and allows the application of any other holographic backgrounds (depth, greyscale etc. . . . ) or other holographic elements without being affected or masked by the sampling plate. The sampling plate 200 in this example comprised substantially opaque 600 m wide rectangular elements separated by 100 m gaps. However, the appearance of the sampling plate elements in the final image may be mitigated by using thinner elements in the sampling plate, for example rectangular elements having a width of 210 m. In order to maintain the seven frame condition, such elements would have to be spaced apart by a constant 35 m.
[0153] When viewing the hologram under very diffuse light, there is an inherent mixing of all of the replayed holographic frames, and all of the frames are visible, with the general shape of the array of circles and the 5 being visible as a darker part of the colour background. Under spot light however, only certain frames (and ultimately single frames) replay at any given viewing angle, making the moir effects appear far more clearly. This difference in exhibited optical effect under different lighting conditions advantageously provides a secondary security feature (level two security feature) in addition to the difficulty in reoriginating the hologram.
Fifth Example
[0154] In the embodiments described above, the sampling plate 200 comprised a plurality of equally-spaced rectangular elements. By varying the spacing of the elements of the sampling plate, we can achieve further optical effects in the final hologram image, such as varying the magnification power, the rate of movement of indicia defined in the artwork plate upon tilting the hologram, and also the apparent depth of the indicia of the artwork plate. The sampling plate can be non-constant and exhibit a variation of the width of the opaque or transparent areas (or both). Taking the previously discussed sampling plate 200 as an example, the opaque rectangular elements 201 and/or the gaps 202 may vary in width (dimension along the X axis). Such variation may be linear, sinusoidal or any other mathematical function, and when combined with an artwork plate having indicia of constant width and spacing, will exhibit variable magnification in the final holographic image.
[0155] FIGS. 14a and 14b illustrate such a non-constant sampling plate 220. The sampling plate is comprised of a plurality of substantially opaque rectangular elements having their long axes along the Y axis. The elements are spaced apart along the X axis in a linearly variable manner. Each element has a width of 210 m with the gaps between the elements varying linearly from 58 m at x=0 to 30 m at x=X (see FIG. 14b).
[0156] FIG. 15a illustrates an example artwork plate 140 comprising an array of 200 m wide indicia 142, each of which have been compressed along the X axis (see FIG. 14b), and varying in height along the Y axis. The separation between each symbol along the X axis is constant 70 m. A frame of the hologram image exhibited by the combination of the artwork plate 140 with the sampling plate 220 is illustrated in FIG. 16. Here the left-most symbol 143 exhibits the strongest absolute magnification and appears forward with respect to the plane of the hologram, whereas the right-most symbol 145 exhibits the smallest absolute magnification and appears closer to the plane of the hologram (although still forward).
[0157] This depth effect can be explained by using Eq. 2 above, where the absolute magnification of the left-most symbol 143 in a given frame of the hologram image is given by M=270/(268270)=135. The absolute magnification of the right-most symbol 147 is given by M=270/(240270)=9. Note that both of these absolute magnification values are negative, hence the inversion of the indicia in the artwork plate 140 such that they appear correctly orientated in the final hologram image.
[0158] The apparent depth of the indicia elements in the final image relative to the surface plane (i.e. the plane of the hologram) derives from the familiar lens equation relating magnification of an image located a distance V from the plane of a lens of focal length f, this being,
M=V/f1.(Eq. 3)
[0159] In this instance, the distance between the artwork plate and the sampling plate (which is a constant) substitutes for the focal length in Eq. 3. Therefore, from Eq. 3, we can see that the apparent depth (V) of the left-most indicia symbol 143 is more forward (i.e. more negative) than that of the right-most indicia symbol 145.
[0160] FIG. 17 illustrates a sampling plate 230 comprising a plurality of substantially opaque rectangular elements having their long axes arranged along the Y axis. The elements are spaced apart along the X axis in a linearly variable manner. Each element has a width of 210 m with the gaps between the elements varying linearly from 50 m at x=0 to 30 m at x=X/2 and back to 50 m at x=X. FIG. 18a illustrates an artwork plate 150 comprising a plurality of arrays 151a, 151b, . . . 151f of indicia 152. These are more clearly seen in FIG. 18b which is a magnified view of the arrays 151a and 151b. Each indicia element has a width (in the X direction) of 200 m and are separated by a constant 70 m. As can be seen, the height of the indicia elements continuously varies from a maximum at x=0 to a minimum at x=X/2 and back to a maximum at x=X.
[0161] An image frame exhibited by the hologram generated by the overlapping artwork plate 150 with the sampling plate 230 is illustrated in FIG. 19, where the outermost indicia elements of the frame appear forward with respect to the plane of the hologram, and the central indicia elements appear closer to the plane of the hologram. Similarly to above, this effect can be explained through the use of Eq. 2 and Eq. 3, with the outermost indicia replaying with the largest absolute magnification (note that this is negative hence the inverted indicia in the artwork plate 150), and replaying with an apparent depth that is more forward of the hologram plane.
[0162] When the security device is tilted about the Y axis, the indicia appear to move along the X axis due to the sampling effect of the sampling plate 230. This provides a particularly striking effect to a viewer. In general, the rate of motion is proportional to the perceived image depth. Therefore, generally, the greater the absolute magnification of the indicia, the faster the apparent movement of the indicia upon tilting of the device.
[0163] The above examples use an artwork plate having array(s) of indicia of constant spacing together with a sampling plate comprising regions of different spacing in order to provide the differing depth effects in the final hologram image. However, it will be appreciated that the equivalent effects may be provided using indicia in the artwork plate having varying spacing and a sampling plate having constant spacing (see for example the sixth embodiment below). Furthermore, in some embodiments both the sampling and artwork plates may comprise elements having varying spacing.
Sixth Example
[0164] Different apparent depths of indicia exhibited in the hologram image can be utilised in order to display objects which appear three dimensional. Consider an indicia element 161 in the shape of a star (see FIG. 20). If we split the star 161 into a plurality of separate elements (here three concentric star elements 162, 163, 164), and replay each element such that it appears at a different depth in the hologram image, then the combined star indicia element 161 will appear three dimensional in the hologram image.
[0165] FIG. 21 illustrates a magnified section of a suitable artwork plate 160 that may be combined with a constant spacing sampling plate in order to replay star indicia 161 having apparent three dimensional properties. The artwork plate comprises an array of inner star elements 162, an array of intermediate star elements 163 and an array of outer star elements 164, with the elements of each array having the same dimension along the X axis and being constantly spaced, but the spacing one of array differing from the spacing of another. In this example, each indicia element has a width of 200 m, with the array of outer star elements having the smallest spacing at 50 m, the array of intermediate star elements having a spacing of 55 m and the array of outer star elements having the largest spacing at 65 m. The sampling plate used comprises an array of 210 m wide substantially opaque rectangular elements separated by a constant gap size of 35 m.
[0166] The absolute magnification of the outer star elements 164, intermediate star elements 163 and inner star elements 162 is therefore 50, 25.5 and 13.25 respectively (using Eq.2). Using Eq.3 we can therefore also see that the outer star elements 164 have the strongest absolute magnification and appear very forward with respect to the plane of the hologram, with the inner star elements 162 appearing forward of the plane of the hologram, but less so than the outer star elements. This therefore creates a striking three dimensional effect to a viewer of the hologram image, with parallax upon tilting the security device. It will be appreciated that with suitable gap dimensions between the individual elements of the artwork plate arrays, the hologram image may replay the star indicia appearing in the depth behind the plane of the hologram.
Seventh Example
[0167] A particularly striking effect is provided to a viewer of the hologram when the holographic image replays a combination of the phase interference and moir magnification effects that have been described above. FIG. 22 illustrates an example artwork plate 170 suitable for providing such an effect. The artwork plate 170 is divided into four quadrants 170a, 170b, 170c, 170d, and is designed to be used in conjunction with a sampling plate adapted to replay seven independent frames (for example the sampling plate used in the sixth embodiment above). The seven frames of the hologram image animation are illustrated in FIGS. 23a to 23f.
[0168] For ease of discussion we will now focus on the top left and bottom left quadrants and how they will replay in the final hologram image. The top left quadrant of the final hologram replays as switching between a symbol and a 5 symbol upon tilting about the Y axis (for example see the top left quadrant in the frames of FIGS. 23(d) and 23(e). In a similar manner to the artwork plate of FIGS. 5a and 7, the quadrant 170a of the artwork plate comprises an array of elements that cooperate with the sampling plate such that certain segments are sequentially replayed upon tilting the hologram. Furthermore, each array of segments that is designed to be replayed at a particular viewing angle comprises an array of compressed elements that are magnified due to Moir magnification by the sampling plate.
[0169] For example, at viewing angle .sub.d, the frame shown at FIG. 23(d) is replayed, where the top left quadrant of the image exhibits a symbol. This symbol is exhibited due to moir magnification of an array of symbols on the artwork plate that are revealed through the sampling plate at the viewing angle .sub.d. Similarly, at viewing angle .sub.e, a 5 symbol is replayed in the top left quadrant of the image frame, due to the moir magnification of an array of 5 symbols on the artwork plate that are revealed through the sampling plate at the viewing angle .sub.e. In this specific example, the arrays of the artwork plate comprise individual elements ( or 5 symbols) having a width of 200 m and a constant gap size of 40 m, giving rise to a magnification of 48 and appear at a depth behind the plane of the hologram. The magnification also provides dynamic movement upon tilting the hologram about the Y axis.
[0170] The bottom left quadrant replays a rotating ball and stick 172 upon tilting the hologram. As described above with respect to the and 5 symbols in the top left quadrant, each frame of the rotating ball and stick is due to an array of ball and stick elements of the artwork plate designed to be revealed through the sampling plate at that particular viewing angle. However, in the case of the ball and stick, the arrays at different viewing angles have different spacings such that the elements appear at different depths within the image at different viewing angles. In this specific example, the ball and stick appears at different forward planes throughout the animation, providing a further striking visual effect on top of the already memorable animation effects.
[0171] All of the above examples have used a sampling plate comprising a one dimensional line array, which provide parallax effects about one axis of tilt (e.g. tilting about the Y axis in the view of FIG. 1). However, the skilled person will appreciate that more complex effects may be generated by using more complex sampling plates (and corresponding artwork plates). For example, the sampling plate may comprises a dot pattern rather than a line pattern. Furthermore, the sampling plate may comprise a two dimensional line or dot pattern such that an observer views an image that is variable when the device is tilted about more than one axis. For example, a sampling plate may be provided having a series of substantially opaque elements arranged along both the Y axis (as in the examples above) and the X axis. When used with a corresponding artwork plate having sets of image elements arranged along the X and Y axes, then an observer will perceive variation in the image when tilting the device about the X axis and the Y axis. Such a two dimensional device is particularly suited to Lippmann (volume) holograms rather than H1/H2 holograms recorded using a Benton slit.
Eighth Example
[0172] The use of complex designs for the artwork and sampling plates further increases the level of security associated with the device, as not only will would-be counterfeiters have to calculate the patterning through which the hologram was made, but also have access to tooling capable of generating the patterning. FIG. 24a illustrates an example sampling plate 240 having a complex two dimensional line pattern, in this case in the form of a tiger's head. FIG. 24b illustrates an example artwork plate 180 that cooperates with the sampling plate 240 in order to generate a complex moir effect to be recorded in the holographic image layer. FIG. 25 illustrates a frame of the variable image exhibited by the device. The combination of the two plates generates the effects described above, for example moir magnification of the pupils (shown at 242) and apparent dynamic movement upon tilting of the device (illustrated at 244). The overall effect exhibited to an observer of the device is one of dynamic texture and volume, providing a striking effect that is difficult to counterfeit.
Ninth Example
[0173] In all of the examples described above, the sampling plate comprises a line pattern or a dot pattern. The sampling plate patterning is typically visible in the variable hologram image exhibited by the security device which may be used to create a striking visual impact (such as the tiger example of the eighth example above), but may in fact create an unwanted artefact that detracts from the primary effect.
[0174] The inventors have found that the effects set out above can also be generated using a sampling plate that comprises an array of focussing elements such as microlenses or micromirrors. FIG. 26 illustrates a schematic arrangement of a setup using an array of focussing elements 251 as the sampling plate 250, positioned in front of an artwork plate 190 in a corresponding manner to the arrangement seen in FIGS. 2 and 3a to 3c. The artwork plate 190 and sampling plate 250 are separated by a distance d substantially equal to the focal length of the focussing elements (here microlenses).
[0175] For example, the phase interference effects described in the first, second and third examples above can be generated using the array of microlenses 251. The same artwork plates as described above may be used, and only selected image segments will be directed, by the microlenses, towards the viewer at a given viewing angle, thereby generating the dynamic effects upon tilting the device.
[0176] An array of focussing elements may also be used to generate moir magnification effects. Here, the artwork plate will comprise an array of microimages that is mismatched with the array of focussing elements. Each microimage element is a complete, miniature version of the image which is ultimately observed on viewing the device, and the array of focussing elements acts to select and magnify a small portion of each underlying microimage element, which portions are combined by the human eye such that the whole, magnified image is visualised when viewing the device. The magnified array appears to move relative to the device upon tilting and can be configured to appear above or below the surface of the device.
[0177] FIG. 27 is an example of an artwork plate 190 that may be used with a sampling plate comprising an array of microlenses 250. Here the artwork plate comprises an array of microimage elements 191, with each microimage element 191 taking the form of a 20, and with each microimage element being typically tens or hundreds of times smaller in dimension that the 20s that will be replayed in the final magnified image.
[0178] At the left-hand side of the plate 190, i.e x=0, the pitch A(x=0) between adjacent microimage elements 191 (in the x-direction) is selected to replay at a first image depth. At the right-most side of the plate, i.e. x=X, the pitch A(x=X) between adjacent microimage elements is selected to return a greater image depth. Between x=0 and x=X, the pitch A continuously varies. Preferably, the pitch changes between each adjacent pair of elements 191for instance, the spacing between elements 191a and 191b is slightly less than that between elements 191b and 191c. In this way, the gradual change in image plane depth when viewing the device will be perceived as a smooth surface to the human eye. However, in some cases the same result can be achieved if two or more adjacent pairs of elements share the same spacing. Equations 1, 2 and 3 described above can be used to determine the pitch of the microimage elements and the pitch of the lens array required to obtain the desired depth effects (here the pitch variation is seen in the artwork plate but it will be appreciated that the pitch of the lens array may vary instead or in addition).
[0179] In this example, the pitch variation is only applied along the X axis but in other embodiments the pitch of the microimage element array could instead vary along the Y axis, which would result in a plane appearing to tilt towards the top or bottom edge of the device rather than the left/right edges. In still further embodiments, the pitch could vary along both the X and Y axes, in which case the image plane would appear to tilt in both directions.
[0180] It will be noted that, in FIG. 27, the size of the individual microimage elements 191 also changes from the left to the right of the array plate 190. This is not essential. If all of the microimage elements are formed at the same size, there will be distortion of the magnified image. In some implementations this can be made use of as a visual effect in itself. However, in the present example, it is desired to remove size distortion so that the magnified elements appear to have substantially the same size as each other.
[0181] The use of an array of focussing elements as the sampling plate also allows integral imaging effects to be recorded in the holographic image layer of the device. Here, the artwork plate 190 comprises an array of microimages, with each microimage being a miniature version of the final image to be exhibited. However, unlike with moir magnification, there is no mismatch between the focussing elements and the microimages, and instead the visual effect is created by arranging for each microimage to be a view of the same object but from a different viewpoint. When the device is tilted, different ones of the images are magnified by the lenses such that the impression of a three dimensional image is given to a viewer.
[0182] The security device of the present invention may be designed to be viewed in reflection or transmission. FIGS. 28, 29 and 30 depict examples of security documents in which security devices of the sorts described above have been incorporated. FIG. 28 shows a first exemplary security document, here a banknote 2000, in (a) plan view and (b) cross-section along line Q-Q. Here, the banknote 2000 is a polymer banknote, comprising an internal transparent polymer substrate 1020 which is coated on each side with opacifying layers 1030a and 1030b in a conventional manner. In some cases, the opacifying layers may be provided on one side of the substrate 1020 only. The opacifying layers 1030a and 1030b are omitted in a region of the document so as to define a window 1010, here having a square shape. Within the window region 1010 is located a security device 1000 in accordance with any of the examples discussed above. The security device 1000 may be formed by cast-curing a suitable carrier material onto the substrate 1020, in which the desired relief structure is formed. Alternatively, the security device 1000 may have been formed separately on a security article such as a transfer patch or label. In this case, the security device 1000 may be affixed to the transparent substrate 1020 inside the window region 1010 by means of a suitable adhesive. Application may be achieved by a hot or cold transfer method e.g. hot stamping.
[0183] It should be noted that a similar construction could be achieved using a paper/plastic composite banknote in which the opacifying layers 1030a and 1030b are replaced by paper layers laminated (with or without adhesive) to an internal transparent polymer layer 1020. The paper layers may be omitted from the window region from the outset, or the paper could be removed locally after lamination. In other constructions, the order of the layers may be reversed with a (windowed) paper layer on the inside and transparent polymer layers on the outside.
[0184] FIG. 28 shows the use of a full window where the regions where the opacifying layers are omitted are in register. It will be appreciated that the device 1000 may be applied in a half window, for example in a case where opacifying layer 1030b was present across window region 1010.
[0185] In FIG. 29, the banknote 2000 is of conventional construction having a substrate 1020 formed for example of paper or other relatively opaque or translucent material. The window region 1010 is formed as an aperture through the substrate 1020. The security device 1000 is applied as a patch overlapping the edges of window 1010 utilising an adhesive to join the security article to the document substrate 1020. Again, the application of the security device and document could be achieved using various methods including hot stamping. FIG. 29(b) shows a variant in which the window 1010 is omitted and the device 1000 is simply applied to a section of the substrate 1020 using any convenient application technique such as hot stamping. In such arrangements the device 1000 will of course only be viewable from one side of the security document 2000.
[0186] FIG. 30 depicts a third example of a security document, again a banknote 2000, to which a security article 1050 in the form of a security thread or security strip has been applied. Three security devices 1000 each carried on the strip 1050 are revealed through windows 1010, arranged in a line on the document 2000. Two alternative constructions of the document are shown in cross-section in FIGS. 30(b) and 30(c). FIG. 30(b) depicts the security thread or strip 1050 incorporated within the security document 2000, between two portions of the document substrate 1020a, 1020b. For example, the security thread or strip 1050 may be incorporated within the substrate's structure during the paper making process using well known techniques. To form the windows 1010, the paper may be removed locally after completion of the paper making process, e.g. by abrasion. Alternatively, the paper making process could be designed so as to omit paper in the desired window regions. FIG. 30(c) shows an alternative arrangement in which the security thread or strip 1050 carrying the security device 1000 is applied to one side of document substrate 1020, e.g. using adhesive. The windows 1010 are formed by the provision of apertures in the substrate 1020, which may exist prior to the application of strip 1050 or be formed afterwards, again for example by abrasion.
[0187] Many alternative techniques for incorporating security devices of the sorts discussed above are known and could be used. For example, the above described device structures could be formed on other types of security document including identification cards, driving licenses, bankcards and other laminate structures, in which case the security device may be incorporated directly within the multilayer structure of the document.
