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1. (WO2019063778) THIN OPTICAL SECURITY ELEMENT AND METHOD OF DESIGNING IT
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CLAIMS

1. Optical security element comprising a reflective light-redirecting surface, or a refractive transparent or partially transparent light-redirecting surface, having a relief pattern of depth 5f adapted to redirect incident light received from a point-like source and form a projected image containing a caustic pattern on a projection surface, said caustic pattern reproducing a reference pattern and being visually recognizable, characterized in that:

a profile of the relief pattern has abrupt variations formed by machining a surface of an optical material piece according to a calculated relief pattern profile having discontinuities, said machined abrupt variations corresponding to the discontinuities.

2. The optical security element according to claim 1, wherein the calculated relief pattern profile having discontinuities is obtained by slicing an initial relief pattern profile of a model light-redirecting surface into smaller contiguous profile portions, said initial relief pattern profile having a depth δι greater than 5f and being operable to reproduce by optical path simulation said caustic pattern on the projection surface under illumination by the point-like source, the slicing generating a boundary surface between any two contiguous profile portions which extends parallel to an optical axis of said model light-redirecting surface, and by collapsing along the optical axis each profile portion comprised between two consecutive boundary surfaces, thereby forming the calculated relief profile having a discontinuity along each boundary surface.

3. The optical security element according to claim 2, wherein the operation of collapsing a profile portion of the initial relief pattern profile, of which height is measured with respect to the optical axis of said model light-redirecting surface and which extends above a base plane perpendicular to said optical axis, is obtained by translating, parallel to the optical axis and toward the base plane, the profile portion by a distance value corresponding to a minimal height at which its boundary surfaces intersect said profile portion, thereby obtaining the calculated relief profile having a relief pattern of reduced depth less than

4. The optical security element of any one of claims 1 to 3, wherein the profile of the relief pattern has a low depth 5f less than or equal to 30 μνα.

5. The optical security element of any one of claims 1 to 3, wherein the profile of the relief pattern has a low depth 5f less than or equal to 250 μτ .

6. The optical security element according to any one of claims 1 to 4, wherein the reflective light-redirecting surface, or a refractive transparent or partially transparent light-redirecting surface, is disposed over a flat base substrate, an overall thickness of the optical security element being less than or equal to 100 μιτι.

7. The optical security element according to any one of claims 1 to 6, wherein its relief pattern is adapted to redirect incident light received from the point-like source, at a distance ds from the light -redirecting surface, and form the projected image containing the caustic pattern on the projection surface at a distance d± from the light-redirecting surface, with a value of di less than or equal to 30 cm and a value of the ratio ds/di greater than or equal to 5.

8. The optical security element according to any one of claims 1 to 7, marking an object selected from the group comprising: consumer products, tax stamps, ID cards, passports, credit cards and banknotes.

9. Method of designing a reflective light -redirecting surface, or a refractive transparent or partially transparent light-redirecting surface, having a relief pattern of depth 5f, of an optical security element adapted to redirect incident light received from a point-like source and form a projected image containing a caustic pattern on a projection surface, said caustic pattern reproducing a reference pattern and being visually recognizable, characterized by comprising the steps of :

a) calculating a relief pattern profile having discontinuities; and

b) machining a surface of an optical material piece according to the relief pattern profile having discontinuities calculated at step a) , thereby having a machined profile of the relief pattern with abrupt variations corresponding to the discontinuities of the relief pattern profile calculated at step a) .

10. The method according to claim 9, wherein, at step a), calculating the relief pattern profile having discontinuities is performed by the following further steps of:

slicing an initial relief pattern profile of a model light-redirecting surface into smaller contiguous profile portions, said initial relief pattern profile having a depth δι greater than 5f and being operable to reproduce by optical path simulation said caustic pattern on the projection surface under illumination by the point-like source, the slicing generating a boundary surface between any two contiguous profile portions which extends parallel to an optical axis of said model light-redirecting surface; and

collapsing along the optical axis each profile portion comprised between two consecutive boundary surfaces, thereby forming the calculated relief profile having a discontinuity along each boundary surface .

11. The method according to claim 10, wherein:

at step a) , the further step of collapsing a profile portion of the initial relief pattern profile, of which height is measured with respect to the optical axis of said model light-redirecting surface and which extends above a base plane perpendicular to said optical axis, is performed by translating, parallel to the optical axis and toward the base plane, the profile portion by a distance value corresponding to a minimal height at which its boundary surfaces intersect said profile portion, thereby obtaining the calculated relief profile having a relief pattern of reduced depth less than δι; and

at step b) , the surface of the optical material piece is machined according to the calculated relief pattern profile of reduced depth less than δι,

thereby obtaining the light-redirecting surface of the optical security element with the relief pattern of reduced depth 5f less than δ .

12. The method according to any one of claims 9 to 11, wherein the machining of the surface of the optical material piece comprises any one of ultra-precision machining, laser ablation and lithography.

13. The method according to any one of claims 9 to 12, further comprising that the machined light-redirecting surface is a master light-redirecting surface to be used to build a replica of the light-redirecting surface.

14. The method according to claim 13, further comprising replicating the machined light-redirecting surface on a substrate .

15. The method according to any one of claims 13 and 14, wherein replication comprises one of UV casting and embossing.