Optical security element

Patent metadata

Field	Value
Publication number	US-11292283-B2
Application number	US-201816652321-A
Country	US
Kind code	B2
Filing date	Sep 28, 2018
Priority date	Sep 29, 2017
Publication date	Apr 5, 2022
Grant date	Apr 5, 2022

Abstract

The invention relates to a thin optical security element comprising a reflective or refractive light-redirecting surface having a relief pattern operable to redirect incident light from a light source and form a projected image on a projection surface, the optical parameters of this optical security element fulfilling a specific projection criterion such that the projected image comprises a caustic pattern reproducing a reference pattern that is easily visually recognizable by a person. The invention also relates to a method for designing a relief pattern of an optical security element.

First claim

The invention claimed is:

1. An optical security element comprising a reflective light-redirecting surface, or a refractive transparent or partially transparent light-redirecting surface of refractive index n, having a relief pattern of depth δ adapted to redirect incident light received from a point-light source, at a distance d s from the light-redirecting surface, and form a projected image containing a caustic pattern on a projection surface disposed at a distance d i from the light-redirecting surface, said caustic pattern reproducing a reference pattern,
wherein upon illumination by the light source of an area of value A of the relief pattern and delivering an illuminance value E A by the optical security element to the projection surface, an average illuminance value E α1 over a circular area of value α 1 selected within an area of the projected image on the projection surface fulfills the following projection criterion E α1 ≤E A (½+α 0 /α 1 +√(¼+α 0 /α 1 )), with scaling area parameter α 0 =4π d i δ for the reflective light-redirecting surface, or α 0 =2π(n−1) d i δ for the refractive light-redirecting surface, and α 1 is smaller than the area value A.

2. The optical security element according to claim 1 , wherein a value of d i is less than or equal to 30 cm and a value of the ratio d s /d i is greater than or equal to at least 5.

3. The optical security element according to claim 1 , wherein a value of depth δ of the relief pattern is less than or equal to 30 μm.

4. The optical security element according to claim 1 , wherein a value of depth δ of the relief pattern is less than or equal to 250 μm.

5. The optical security element according to claim 1 , wherein the relief pattern is disposed over a flat base, an overall thickness of the optical security element being less than or equal to 100 μm.

6. A method for designing a relief pattern of depth less than or equal to a value δ of a reflective light-redirecting surface, or a transparent or partially transparent light-redirecting surface of refractive index n, adapted to redirect incident light received from a point-light source, at a distance d s from the light-redirecting surface, and form a projected image containing a caustic pattern on a flat projection surface disposed at a distance d i from the light-redirecting surface, so that upon illumination by the light source of an area of value A of the relief pattern and delivering of an illuminance value E A by the optical security element to the projection surface, an average illuminance value E α1 over a circular area of value α 1 selected within an area of the projected image on the projection surface fulfills the following projection criterion E α1 ≤E A (½+α 0 /α 1 +√(¼+α 0 /α 1 )), with scaling area parameter α 0 =4πd i δ for the reflective light-redirecting surface, or α 0 =2π(n−1) d i δ for the refractive light-redirecting surface, and α 1 is smaller than the area value A, said method comprising:
a) selecting a digital image of a reference pattern to be reproduced by the caustic pattern on the projection surface, the digital image comprising a total number of pixels N A and a sum of all pixel values over the digital image being I A , by checking that for each circular area of N pixels within the digital image, with N integer and 1≤N≤N A , a value I(N) of a sum of each pixel value of the N pixels in the circular area is less than a value I max (N)=N(I A /N A )(½+N 0 /N+√(¼+N 0 /N)), wherein N 0 is a number of pixels given by N A (α 0 /A) within the digital image;
b) calculating a relief pattern of depth less than or equal to 6 corresponding to the reference pattern on the digital image selected at a); and
c) machining a surface of an optical material substrate to form a light-redirecting surface reproducing the relief pattern calculated at b), thereby obtaining an optical security element comprising said machined light-redirecting surface.

7. The method according to claim 6 , wherein a), selecting a digital image of a reference pattern comprises modifying the digital image of the reference pattern of which a part does not fulfill a selection criterion that I(N) is less than I max (N), by adapting the pixel values within said part of the digital image, by making said part of the digital image with adapted pixel values to comply with the selection criterion for any N, with 1≤N≤N A , thereby providing a modified digital image to be selected.

8. The method according to claim 7 , wherein the pixel values of the digital image are adapted by filtering with a filter the image to reduce image contrast.

9. The method according to claim 6 , wherein the machining of the surface of the optical material substrate comprises any one of ultra-precision machining, laser ablation, and lithography.

10. The method according to claim 6 , further comprising that the machined light-redirecting surface is a master light-redirecting surface to be used to build a replica.

11. The method according to claim 10 , further comprising replicating the machined light-redirecting surface on a substrate.

12. The method according to claim 10 , wherein replication comprises one of UV casting and embossing.

13. A method of visually authenticating an object, marked with an optical security element, by a user, the optical security element comprising a reflective light-redirecting surface, or a refractive transparent or partially transparent light-redirecting surface of refractive index n, having a relief pattern of depth δ adapted to redirect incident light received from a point-light source, at a distance d s from the light-redirecting surface, and form a projected image containing a caustic pattern on a projection surface disposed at a distance d i from the light-redirecting surface, said caustic pattern reproducing a reference pattern,
wherein upon illumination by the light source of an area of value A of the relief pattern and delivering an illuminance value E A by the optical security element to the projection surface, an average illuminance value E α1 over a circular area of value α 1 selected within an area of the projected image on the projection surface fulfills the following projection criterion E α1 ≤E A (½+α 0 /α 1 +√(¼+α 0 /α 1 )), with scaling area parameter α 0 =4πd i δ for the reflective light-redirecting surface, or α 0 =2π(n−1) d i δ for the refractive light-redirecting surface, and α 1 is smaller than the area value A,
the method comprising:
illuminating the light-redirecting surface of the optical security element with a point light source at the distance d s from the light-redirecting surface;
visually observing on the caustic pattern as projected on the projection surface at distance d i form the optical security element; and

deciding that the object is genuine upon evaluation by the user that the projected caustic pattern is visually similar to the reference pattern.

Assignees

• Sicpa Holding Sa

Inventors

• Callegari Andrea
• Degott Pierre
• Dinoev Todor
• Garnier Christophe
• Mayer Alain
• Schwartzburg Yuliy
• Testuz Romain
• Pauly Mark

Classifications

• G02B5/1861

  Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials (G02B5/1809, G02B5/1828, G02B5/1833, G02B5/1838 and G02B5/1847 take precedence) · CPC title

• B42D25/351

  Translucent or partly translucent parts, e.g. windows · CPC title

• B42D25/435

  using electromagnetic radiation, e.g. laser · CPC title

• G02B5/1842

  Gratings for image generation (G02B5/1847 takes precedence) · CPC title

• B42D25/324 Primary

  Reliefs · CPC title