Optically variable device

Abstract

A security element, security device and method of forming a security device wherein the security element includes focusing elements, a first group of image elements, and a second group of image elements, each image element being located in an object plane to be viewable through a focusing element, and being located a distance from the focusing element such that the focal point width of the focusing element in the object plane is substantially equal to the size of the image element or differs from the size of the image element by a predetermined amount.

Claims

1. A security element, including: a plurality of focusing elements, a first group of image elements, and a second group of image elements, each image element being located in an object plane to be viewable through a focusing element, and being located a distance from the focusing element such that the focal point width of the focusing element in the object plane differs from the size of the image element by a predetermined amount, so that the focusing elements are out of focus in the object plane, wherein the predetermined amount by which the focal point width varies from the size of the image elements is not more than 20% of the size of the image elements, wherein image elements of the first group are visible in a first range of viewing angles and image elements of the second group are visible in a second range of viewing angles, and wherein a second image formed in the second range of viewing angles is a contrast-inverted version of a first image formed in the first range of viewing angles, and wherein cross-talk between the first and second images is reduced by the focusing elements being out of focus in the object plane.

2. The security element of claim 1, wherein the image elements are a colour other than black.

3. The security element of claim 1, wherein the image elements have a size distribution or a spatial distribution corresponding to the grey levels or brightness levels of an input monochromatic image.

4. The security element of claim 1, wherein the image elements are printed image elements.

5. The security element of claim 1, wherein the image elements are embossed image elements.

6. The security element of claim 5, wherein the image elements are diffractive elements or sub-wavelength grating elements.

7. The security element of claim 1, wherein the first group of image elements is a different colour to the second group of image elements.

8. The security element of claim 1, wherein the focusing elements are on one side of a transparent or translucent substrate and the image elements are on the opposite side of the transparent or translucent substrate.

9. The security element of claim 1, wherein the image elements are line elements.

10. The security element of claim 1, wherein the focusing elements are selected from the group comprising: refractive or diffractive part-cylindrical lenses; refractive or diffractive part-spherical or polygonal-base microlenses; or zone plates.

11. The security element according to claim 1, wherein in either the first or second range of viewing angles, cross-talk from one of the second or first image elements forms the background for the image resulting from the other image elements.

12. A method of forming a security device, including the steps providing a transparent or translucent substrate; applying a plurality of focusing elements to a first surface of the substrate; and applying a first group of image elements and a second group of image elements to an image surface of the substrate, each image element being located in an object plane to be viewable through a focusing element, and being located a distance from the focusing element such that the focal point width of the focusing element in the object plane differs from the size of the image element by a predetermined amount, so that the focusing elements are out of focus in the object plane, wherein the predetermined amount by which the focal point width varies from the size of the image elements is not more than 20% of the size of the image elements, whereby image elements of the first group are visible in a first range of viewing angles and image elements of the second group are visible in a second range of viewing angles, and whereby a second image formed in the second range of viewing angles is a contrast-inverted version of a first image formed in the first range of viewing angles, and wherein cross-talk between the first and second images is reduced by the focusing elements being out of focus in the object plane.

13. The method of claim 12, wherein the image elements are a colour other than black.

14. The method of claim 12, wherein the image elements have a size distribution or a spatial distribution corresponding to the grey levels or brightness levels of an input monochromatic image.

15. The method of claim 12, wherein the focusing elements are applied by embossing.

16. The method of claim 15, wherein the focusing elements are embossed in a layer of radiation-curable ink applied to the first surface of the substrate.

17. The method of claim 12, wherein the image elements are applied by a printing method.

18. The method of claim 12, wherein the image elements are applied by embossing.

19. The method according to claim 12, wherein in either the first or second range of viewing angles, cross-talk from one of the second or first image elements forms the background for the image resulting from the other image elements.

20. A security document, including a security element according to claim 1.

21. The security document of claim 20, wherein the security element or security device is located within a window or half-window region of the security document.

22. A security element, including: a transparent or translucent substrate, a plurality of focusing elements on one side of said transparent or translucent substrate, a first group of image elements, and a second group of image elements, the image elements being on the opposite side of said transparent or translucent substrate from said one side of said transparent or translucent substrate each image element being located in an object plane to be viewable through a focusing element, and being located a distance from the focusing element such that the focal point width of the focusing element in the object plane is substantially equal to the size of the image element or differs from the size of the image element by a predetermined amount, so that the focusing elements are out of focus in the object plane, wherein the predetermined amount by which the focal point width varies from the size of the image elements is not more than 20% of the size of the image elements, wherein image elements of the first group are visible in a first range of viewing angles and image elements of the second group are visible in a second range of viewing angles, and wherein a second image formed in the second range of viewing angles is a contrast-inverted version of a first image formed in the first range of viewing angles, and wherein cross-talk between the first and second images is reduced by the focusing elements being out of focus in the object plane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Disclosed embodiments will now be described, by way on non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1(a) to 1(d) show a lenticular device of known type;

(3) FIGS. 2(a) to 2(d) show a modified version of the lenticular device of FIGS. 1(a) to 1(d);

(4) FIG. 3 shows a flipping image effect produced by at least one disclosed embodiment;

(5) FIG. 4 is a perspective view of part of a security element producing the effect of FIG. 3;

(6) FIG. 5 is a cross-section through the security element of FIG. 4;

(7) FIGS. 6 and 7 schematically depict a method of producing artwork for another disclosed embodiment; and

(8) FIG. 8 shows the effect generated by the disclosed embodiment of FIGS. 6 and 7.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

(9) In at least one disclosed embodiment, the focusing elements are on one side of a transparent or translucent substrate. The image elements may be on the opposite side of the transparent or translucent substrate.

(10) The image elements may be line elements, but may be any other suitable shape, for example dots or geometrical shapes.

(11) In at least one disclosed embodiment, the focusing elements are refractive or diffractive part-cylindrical lenses, or zone plates. Alternatively, the focusing elements may be refractive or diffractive part-spherical or polygonal-base micro lenses.

(12) Another disclosed embodiment provides a method of forming a security device, including the steps of providing a transparent or translucent substrate, applying a plurality of focusing elements to a first surface of the substrate, and applying a first group of image elements and a second group of image elements to an image surface of the substrate, each image element being located in an object plane to be viewable through a focusing element, and being located a distance from the focusing element such that the focal point width of the focusing element in the object plane is substantially equal to the size of the image element or differs from the size of the image element by a predetermined amount, whereby image elements of the first group are visible in a first range of viewing angles and image elements of the second group are visible in a second range of viewing angles, and whereby a second image formed in the second range of viewing angles is a contrast-inverted version of a first image formed in the first range of viewing angles.

(13) The focusing elements may be applied by embossing, optionally by embossing in a layer of embossable radiation-curable ink applied to the first surface of the substrate.

(14) Optionally, the image elements are applied by a printing method. Optional methods are gravure-printing, offset-printing, screen-printing or flexographic-printing. The image elements may also be applied by embossing.

(15) Another disclosed embodiment provides a security document, including a security element according to any one of the disclosed embodiments, a security device according to at least one disclosed embodiment, or a security device manufactured according to any of the methods disclosed above. In at least one disclosed embodiment, the security element or security device is located within a window or half-window region of the security document.

(16) Referring initially to FIG. 1, there is shown part of a lenticular device 10 of known design, having a plurality of focusing elements in the form of part-cylindrical lenses 14. The device 10 includes a substrate 11 having an upper surface 12 and a lower surface 13. The focusing elements 14 are applied to the upper surface 12, and the lower surface 13 is an object plane carrying a first group of image elements 16 and a second group of image elements 17. Image elements 17 are shown slightly offset in the cross-sectional view of FIG. 1(a) for purposes of clarity.

(17) The left-hand edges of neighbouring image elements 16 of the first group are aligned with the left-hand edges of associated focusing elements 14 through which the image elements 16 are to be viewed. The left-hand edges of image elements 17 of the second group are aligned with the optical axes associated of focusing elements 14. Image elements 16 and 17 are in interleaved relationship in the object plane 13 (FIG. 1(d)) to form first and second channels of a flipping image.

(18) Object plane 13 is placed substantially at the focal length of the focusing elements 14. This results in a very narrow region 15 in the object plane over which incoming rays are focused, much narrower than the width of image elements 16, 17.

(19) In FIG. 1(a), the security element substrate 11 has a thickness of approximately 75 microns. In order to keep the total security element thickness to less than 90 microns, the sag height of the lenses 14 is less than 15 microns, and the lens diameter is of the order of 45 to 50 microns. If image elements 16, 17 are applied by gravure printing, their width will be larger than half the lens diameter. This results in a cross-talk region of viewing angles in which both image elements 16 and image elements 17 are visible. In the cross-talk region, for example at viewing angles which view positions 20 of the device (FIG. 1(c)), the image elements 16 and 17 are each spanned by the entire width of focal region 15, resulting in identical apparent brightnesses 26 and 27 (FIG. 1(b)) to the viewer. The ability to distinguish between images 36 (character 5) and 37 (character A) is thus completely lost due to the cross-talk between the two groups of image elements 16, 17.

(20) The cross-talk of FIG. 1 can be reduced by using off-focus lens designs such as the one shown in FIG. 2. In FIG. 2(a), a lenticular device 100 includes focusing elements 114 applied to a first surface 112. The focusing elements 114 have a focusing region 115 in the object plane 113 which is almost as wide as the image elements 116 of a first group and image elements 117 of a second group. The focusing region may have a width which is up to 20% smaller or 20% larger than the width of the image elements 116 or 117.

(21) The use of a non-focusing design reduces cross-talk because in a cross-talk region, a reduced portion of the focal region overlaps with an image element which is not intended to be seen by the viewer. For example, in region 120 shown in FIG. 2(c), image elements 116 of the first group should be visible to display the first channel of the flipping image, while image elements 117 of the second group should not be seen. The entire width of the focal region 115 overlaps with image elements 116 at the viewing angle shown in FIG. 2(c), to produce an apparent intensity 126 (FIG. 2(b)). On the other hand, only part of image element 117 overlaps with the focal region 115, so that a reduced intensity 127 is seen by the viewer.

(22) The net impression to the viewer emerging from the viewing angle shown in FIG. 2(d) is a first image 136 comprising a foreground region 126a in the form of the character 5, produced by the first group of image elements 116. Due to the presence of cross-talk 128 from the second group of image elements 117, a shadow 127a of character A is seen in the background. As the device is tilted, character A becomes more prominent, due to a greater proportion of the width of focusing region 115 viewing the image elements 117, and character 5 becomes gradually more muted, until the two characters 5 and A become undistinguishable. On further tilting, the character A dominates and forms the foreground 126b of an image 137, with the cross-talk 128 from image elements 116 of character 5 forming the background 127b.

(23) While the device 100 of FIG. 2 gives an improved result compared to device 10 of FIG. 1, the amount of cross-talk between the two channels 126a, 126b of the flipping image may be unacceptably large for security document applications. It has therefore been found greatly advantageous to select a design in which the two images of the flipping image are contrast-inverted versions of each other, as shown in the disclosed embodiment of FIGS. 3 to 5.

(24) Referring to FIGS. 3, 4 and 5, there is shown a perspective view of part of a security element 200 having a substrate 211 with upper surface 212 and lower surface (object plane) 213. A first group of image elements 216 and a second group of image elements 217, in the form of gravure-printed lines, are applied to lower surface 213. The image elements 216, 217 are viewable through associated focusing elements (part-cylindrical lenses) 214 applied to the upper surface 212 of the substrate 211.

(25) In the cross-sectional view of FIG. 5, image elements 217 are slightly offset from lower surface 213 for reasons of clarity. Image elements 217 of the first group are viewable in a first range of viewing angles from direction 230 to direction 231, while image elements 216 of the second group are viewable in a second range of viewing angles from direction 231 to direction 232. There is also a range of viewing angles from direction 231a to 231b, in which the whole of cross-talk region 220 is seen by the viewer.

(26) When the device 200 is viewed from angle 232, a first image 236 is visible, in which the image elements 216 of the first group are brightest and produce the impression of a character 5. Similarly, when the device 200 is viewed from angle 230, a second image 237 is visible, in which the image elements 217 of the first group are brightest and produce the impression of a contrast-inverted character 5.

(27) In first image 236, first image elements 216 thus form the foreground region 226a while second image elements 217 form a uniform background region 227a. Conversely, in second image 237, second image elements 217 form the foreground region 226b while first image elements form a uniform background region 227b. In each case, the cross-talk 228 becomes a uniform background to the image 236, 237 which is desired to be projected.

(28) Referring now to FIGS. 6 to 8, a method of producing a more complex security element is depicted schematically.

(29) In FIG. 6, a monochromatic input image in the form of a portrait 300 is shown. Portrait 300 is a greyscale bitmap having 256 grey levels. This is then converted to a binary bitmap 302, for example by applying a frequency-modulated dithering, error diffusion, or random or stochastic screening. The result is a two-level (binary) bitmap which appears as a tonal portrait due to the spatial distribution of the black pixels. A region with a higher spatial density of black pixels will tend to appear darker, while a sparser distribution will appear lighter.

(30) FIG. 7 shows a close-up of one region 304 of the bitmap 302, a subregion 306 of which is shown in further close-up. Region 306 includes black pixelated regions 316a, 316b, 316c, 316d, and white pixelated regions 317a, 317b, 317c.

(31) To produce a flipping image with contrast inversion, the black regions are first mapped to a first group of image elements 321a, 321b, 321c and 321d respectively, which are applied to security element 400 as a series of gravure-printed lines with their left-hand edges substantially aligned with left-hand edges of lenses 314. The gravure-printed lines 321a-321d each have a length corresponding to the length of the corresponding black pixelated region 316a-316d.

(32) The white pixelated regions are mapped to a second group of image elements 322a, 322b and 322c respectively, which are applied to security element 400 as a second series of gravure-printed lines with their right-hand edges substantially aligned with right-hand edges of associated lenses 314. Gravure-printed lines 322a-322c each have a length corresponding to the length of the corresponding white pixelated region 317a-317c.

(33) In a first range of viewing angles, a first image 336, substantially reproducing the portrait 300, is seen by a person viewing the device 400 (FIG. 8). This includes foreground region 326a from image elements 321a-321d, and uniform background region 327a due to cross-talk from image elements 322a-322c. As the device 400 is tilted through the first range of viewing angles, the amount of reflected light from background region 327a decreases, until the viewer reaches a second range of viewing angles in which background 327a begins to dominate. In the second range of viewing angles, a second, contrast-inverted image 337 is seen, in which image elements 322a-322c form the foreground 327b, while cross-talk from image elements 321a-321d forms the uniform background 326b.

(34) In a representative example of a method for manufacturing security elements substantially as described above, a layer of embossable radiation curable ink, for example UV-curable ink, is applied to one side of a 75 micron thick biaxially oriented polypropylene (BOPP) film. The UV-curable ink is then embossed with lens structures 214 or 314 and cured to produce a lenticular substrate with a total thickness of approximately 85 to 90 microns.

(35) The surface opposite the lens structures is gravure printed with image elements 216, 217 (FIG. 4 to 6) or 321a-321d, 322a-322c (FIG. 7) of a single color. A color for the image elements is one which will produce sufficient contrast yet is difficult to imitate. Blue, magenta, violet or scarlet are optional colors.

(36) In a representative gravure printing process, a gravure cylinder engraved with the resolution of 10,160 dpi (smallest incremental change in image element position of 2.5 microns) is used. The corresponding gravure engraving file is a binary digital image of the image elements, compensated for the anticipated growth in size of the digital image elements after they are printed.

(37) In order to design lenses of appropriate characteristics for the particular substrate thickness being used, the lenses should have a focal point width which is substantially equal to the image element size, or differs from the image element size by a predetermined amount, optionally no more than 20%. A suitable method is described in PCT application PCT/AU2010/000243, and includes a measurement of the width of the image elements.

(38) Measurement of the characteristics of the gravure-printed lines can be accomplished using a variety of known methods. For example, the average line width can be determined by printing a press calibration template consisting of swatches of lines of a given size and having various densities, where each swatch typically represents a density value from one percent to ninety nine percent. The template is subsequently imaged to film or plate, and printed onto the smooth side of an optical effect substrate. The printed result is then scanned using a densitometer, or similar tool, to determine the printed line width.

(39) Alternatively, the average line width can be measured directly, for example using a microscope fitted with a reticle displaying increments of measurement. In the direct method, a sample of lines can be measured in each tonal value range, recorded, and their sizes averaged.

(40) In order to obtain lens parameters suitable for image elements of a given width and a substrate of given thickness, the following relation between gauge thickness t and lens parameters s (sag height), w (width), R (radius of curvature), P (the conic constant of the lens) and n (refractive index) is optimized:

(41) $\begin{array}{cc}t=s+\frac{h-w}{A},& \left(1\right)\end{array}$
with h being the measured half-width of a printed line, and A being given by

(42) $\begin{array}{cc}A=-\mathrm{Tan}\left[?\left(s\right)-\mathrm{Arc}\mathrm{Sin}\left(\frac{\mathrm{Sin}\left(?\left(s\right)\right)}{n}\right)\right],\mathrm{where}& \left(2\right)\\ ?\left(s\right)=\mathrm{Arc}\mathrm{Tan}\left(\frac{w}{\sqrt{R^2-P*w^2}}\right).& \left(3\right)\end{array}$

(43) The thickness t can be optimised with respect to one or more of the lens parameters R, n, P, w and s in the usual way, i.e., by taking the partial derivatives of the expression in Eq (2) with respect to one or more of those parameters and setting the partial derivatives equal to zero. The resulting system of equations can be solved analytically or numerically in order to find the set of lens parameters which gives the optimal lens thickness.

(44) The optimisation may be a constrained optimisation. For example, for banknote substrates, it is desirable to limit t to a range of values between about 85 microns and 100 microns. Constrained optimisation methods are known in the art.

(45) We have found that as long as the focal point size does not exceed the average width of a printed halftone dot by more than 20%, the quality of the image is not compromised. We have also found that simply producing an arbitrary non-focussing design severely degrades the image quality, resulting in an objectionably blurred image. The focal point size may also be slightly smaller than the average width, optionally no more than 20% smaller.

(46) Many variations of the disclosed embodiments are possible without departing from the spirit and scope of the present invention. For example, the security elements described above may be manufactured separately, and then applied to a security document, or may be applied to a security document in situ, for example within a window or half-window region.