Optically variable device
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.
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.