Optically variable device
An optically variable device and method of manufacturing the device is disclosed, the device including a plurality of zero-order diffraction grating elements some of which are modulated such that a coloured at least partially polarized image is visible to a person viewing the device, observing a first optically variable effect when the device is rotated about an axis substantially perpendicular to the plane, and a second optically variable effect when the device is viewed under polarized light and rotated about an axis substantially perpendicular to or parallel with the plane. The device is particularly suitable for security documents such as bank notes. Preferably, the first optically variable effect is produced by a change in brightness and/or color of the grating elements and the second optically variable effect may likewise be produced by a change in brightness and/or color of the grating elements. The device advantageously provides two different types of optically variable effect may be provided in a single device. This is possible due to the use of zero-order grating elements as the “pixels” of the colored image. Each pixel, under illumination by unpolarized light, produces reflected light of a defined color which is also at least partially polarized.
1. An optically variable device, including:
a plurality of zero-order diffraction grating elements disposed in a plane, wherein unpolarised light and polarised light are incident on the plurality of zero-order diffraction grating elements;
a group of said zero-order diffraction grating elements in at least one region of the device being modulated within said region by varying the spatial frequency, groove curvature and/or groove depth within the grating elements such that a coloured, at least partially polarised first image is visible to a person viewing the device under the unpolarised light, wherein at least one grating parameter of the zero-order diffraction grating element in said region is further modulated within at least some of said zero-order diffraction grating elements to produce variations in brightness,
said group of zero-order diffraction grating elements in said region having at least two orientations, such that a second image, different to the first image, is visible to a person viewing the device under the polarized light,
wherein the group of said zero-order diffraction grating elements form a plurality of pixels of each of the first image and the second image, and further
wherein each of the plurality of pixels, under illumination by the unpolarised light, produces a reflected light of a defined colour, wherein the reflected light is at least partially polarised; and
wherein the person viewing the device observes a first optically variable effect when the device is viewed under unpolarised light and rotated about an axis substantially perpendicular to the plane, and a second optically variable effect when the device is viewed under polarised light and rotated about an axis substantially perpendicular to or parallel with the plane.
2. An optically variable device according to claim 1 , wherein the first and/or the second optically variable effect is produced by a change in brightness and/or colour of the grating elements.
3. An optically variable device according to claim 1 , wherein at least one grating parameter of the zero-order diffraction grating elements in said region is modulated within at least some of said zero-order diffraction grating elements.
4. An optically variable device according to claim 1 , wherein the zero-order diffraction grating elements comprise rectilinear or curvilinear grooves.
5. An optically variable device according to claim 4 , wherein the surface density of the zero-order diffraction grating elements within said region is modulated to produce variations in brightness.
6. An optically variable device according to claim 4 , wherein the spatial frequency of the zero-order diffraction grating elements is varied according to a harmonic or non-harmonic function.
7. An optically variable device according to claim 4 , wherein the grooves are electrically conductive and are separated by non-conducting regions.
8. An optically variable device according to claim 7 , wherein the grooves are formed from a conducting material which includes metallic nanoparticles.
9. An optically variable device according to claim 7 , wherein the grooves are formed of a non-conducting material to which a conducting material is applied.
10. An optically variable device according to claim 4 , wherein the grooves are formed in a first dielectric material which is in contact with a second dielectric material, the second dielectric material having a different refractive index to the first dielectric material.
11. A security document including an optically variable device according to claim 1 and, further including a polariser for producing polarised light to view the optically variable device, wherein the optically variable device and polariser are spaced apart on the security document and can be brought into register by folding the security document.
12. An optically variable device according to claim 1 , wherein the first and/or second optically variable effect is a change in shape of the image.
13. An optically variable device according to claim 1 , wherein at least one grating parameter of the zero-order diffraction grating elements in said region is modulated relative to grating parameters of neighbouring zero-order diffraction grating elements.
14. A method of manufacturing an optically variable device, including the steps of:
forming a plurality of zero-order diffraction grating elements in or on a surface, wherein unpolarised light and polarised light are incident on the plurality of zero-order diffraction grating elements and modulating a group of the zero-order diffraction grating elements in at least one region of the device within said region by varying the spatial frequency, groove curvature and/or groove depth within the grating elements such that a coloured, at least partially polarised first image is visible to a person viewing the device under the unpolarised light, wherein at least one grating parameter of the zero-order diffraction grating element in said region is further modulated within at least some of said zero-order diffraction grating elements to produce variations in brightness,
said group of zero-order diffraction grating elements in said region having at least two orientations, such that a second image, different to the first image, is visible to a person viewing the device under the polarized light, and wherein the group of said zero-order diffraction grating elements form a plurality of pixels of each of the first image and the second image, and further wherein each of the plurality of pixels, under illumination by the unpolarised light, produces a reflected light of a defined colour, wherein the reflected light is at least partially polarized,
wherein the person viewing the device observes a first optically variable effect when the device is viewed under unpolarised light and rotated about an axis substantially perpendicular to the surface, and a second optically variable effect when the device is viewed under polarised light and rotated about an axis substantially perpendicular to or parallel with the surface.
15. A method according to claim 14 , wherein the first and/or second optically variable effect is produced by a change in brightness and/or colour of the grating elements.
16. A method according to claim 14 , further including the step of applying a layer of a first dielectric material to the surface, wherein the zero-order diffraction grating elements are formed in the first dielectric material.
17. A method according to claim 16 , wherein the zero-order diffraction grating elements are embossed in the first dielectric material.
18. A method according to claim 16 , further including the step of curing the first dielectric material in at least the embossed regions of the first dielectric material.
19. A method according to claim 14 , further including the step of coating the zero-order diffraction grating elements with a second dielectric material, the second dielectric material having a different refractive index to the first dielectric material.
20. A method according to claim 14 , wherein the first and/or second optically variable effect is a change in shape of the image.