# US 12,001,011 B2 · Optical element and a method of visually authenticating an object

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- Number: US 12,001,011 B2
- Document ID: US12001011B2
- Status: Granted
- Kind: B2
- Application number: 17/917,845
- Filed: 2021-04-07
- Published: 2023-09-14
- Granted: 2024-06-04
- Earliest claimed: 2020-04-07
- Assignees: Sicpa Holding SA
- Applicants: Sicpa Holding SA
- Inventors: Yuliy Schwartzburg; Romain Testuz; Andrea Callegari
- Web page: https://patentgrep.com/patent/US12001011B2

## CPC

- B42D 25/324: Reliefs
- G02B 26/0875: {by means of one or more refracting elements}
- B42D 25/30: Identification or security features, e.g. for preventing forgery
- B42D 25/405: Marking
- B42D 25/445: using chemical means, e.g. etching
- G02B 1/10: Optical coatings produced by application to, or surface treatment of, optical elements (G02B1/08 takes precedence)
- G02B 27/18: for optical projection, e.g. combination of mirror and condenser and objective {(photographic, cine and overhead projectors G03B21/00; photographic projection printing G03B27/32; photolithographic projectors G03F7/20; projection television H04N5/74; colour projection television H04N9/31)}
- G03F 1/68: Preparation processes not covered by groups G03F1/20 - G03F1/50
- B42D: BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G03F: PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR; (phototypographic composing devices B41B; photosensitive materials or processes for photographic purposes G03C; electrophotography, sensitive layers or processes therefor G03G)

## Abstract

The invention relates to an anti-copy optical element comprising a caustic layer and a mask layer configured to simultaneously display a visible image reproducing a reference image and form a projected image containing a visible caustic pattern reproducing a reference pattern, upon illumination of the optical element with a light source, the projected image being distinct from the reference image. The invention also relates to a method for designing a relief pattern of a light-redirecting surface of said caustic layer consistently with the transmission properties of the mask layer.

## Claims

### Claim 1

An optical element comprising a caustic layer made of a piece of reflective, or refractive transparent or partially transparent, first optical material, and having a light-redirecting surface with a relief pattern,
wherein
the optical element includes a mask layer disposed, respectively, on an optical surface of the optical element or within the optical element, the mask layer comprising a mask pattern and having a variable light transmission coefficient, the mask layer being adapted to at least partially transmit incoming light upon illumination of the optical element with a point-like light source; and
the relief pattern of the light-redirecting surface of the caustic layer is configured to redirect incident light received by the optical element from the point-like light source and form a projected image containing a visible caustic pattern reproducing a reference pattern, wherein
the point-like light source is a source of light whose angular size is sufficiently small that light can be considered to arise from a single point.

### Claim 2 (depends on 1)

The optical element according to claim 1, wherein, upon illumination of the optical element with the point-like light source, the mask layer is configured to show a visible image reproducing a reference image, the visible image being distinct from the projected image.

### Claim 3 (depends on 1)

The optical element of any one of claim 1, wherein a profile of a depth of the relief pattern has abrupt variations formed by machining a surface of the piece of first optical material according to a calculated relief pattern profile having discontinuities, said machined abrupt variations corresponding to the discontinuities.

### Claim 4 (depends on 3)

The optical element according to claim 3, wherein the profile of the relief pattern has a maximum depth less than or equal to 250 μm.

### Claim 5 (depends on 1)

The optical element according to claim 1, wherein the relief pattern of the light-redirecting surface is adapted to redirect incident light received from the light source, at a distance ds from the light-redirecting surface, and form the projected image containing the caustic pattern on a wall surface at a distance di 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.

### Claim 6 (depends on 1)

The optical element according to claim 1, further comprising a lens element adjacent to the caustic layer and made of a refractive transparent or partially transparent second optical material, the lens element being configured to redirect incident light received by the optical element from the light source to form the projected image containing the visible caustic pattern reproducing the reference pattern, and wherein
the light-redirecting surface has a focal length fC; and
the lens element has a focal length fL configured to form the projected image containing the visible caustic pattern directly on a retina of an observer looking at the light source through the optical element.

### Claim 7 (depends on 6)

The optical element according to claim 6, comprising one of the following:
a) the caustic layer has a positive focal length (fC>0) and the lens element has a negative focal length (fL<0), or
b) the caustic layer has a negative focal length (fC<0) and the lens element has a positive focal length (fL>0).

### Claim 8 (depends on 7)

The optical element according to claim 7, wherein a relationship between the focal length fL of the lens element and the focal length fC of the caustic layer satisfies following equation:
R
-
(
1
f
L
+
1
f
C
-
1
d
s
)
-
1
≥
d
R
,
where:
R is distance between the caustic layer and an eye of the observer;
ds is a distance between the light source and the optical element; and
dR is a reading distance from the eye, which is at least 25 cm.

### Claim 9 (depends on 1)

The optical element according to claim 1, marking an object selected from the group comprising: consumer products, value documents, tax stamps, and banknotes.

### Claim 10 (depends on 1)

A method of visually authenticating an object, marked with the optical element with a mask layer according to claim 1, by an observer, comprising steps of:
illuminating the optical element with a point-like light source;
visually observing the projected image containing the visible caustic pattern reproducing the reference pattern; and
deciding that the object is genuine upon evaluation by the observer that the caustic pattern is visually similar to the reference pattern, wherein
the point-like light source is a source of light whose angular size is sufficiently small that light can be considered to arise from a single point.

### Claim 11 (depends on 10)

The method according to claim 10, wherein, upon illumination of the optical element with the point-like light source, the mask layer is configured to show a visible image reproducing a reference image, the method comprising a further step of visually observing the visible image reproducing the reference image, and wherein the step of deciding that the object is genuine comprises a further verification by the observer that the visible image is visually distinct from the caustic pattern.

### Claim 12

A method of designing a relief pattern of a light-redirecting surface of a caustic layer made of a piece of refractive transparent or partially transparent, or reflective, first optical material, the caustic layer including a mask layer disposed, respectively, on an optical surface of the optical element or within the optical element, the mask layer comprising a mask pattern and having a variable light transmission coefficient, the mask layer being adapted to at least partially transmit incoming light upon illumination of the optical element with a point-like light source, the caustic layer being adapted to redirect incident light received from the point-like light source and form a projected image containing a caustic pattern, the method comprising the computer implemented steps of:
providing a discrete representation of an input target image of a reference pattern comprising a set P of N image pixels pi of coordinates {(xi,yi)} in the image plane with associated nonzero target light intensities {Ii}, i=1, . . . , N, distributed within a given area of the target image and corresponding to a target caustic pattern of the target image;
computing a piecewise representation of the light-redirecting surface z=F(x,y) of the caustic layer, with height z above the (x,y) coordinates plane, based on a representation of the light-redirecting surface by means of intersecting pieces of surfaces z=fi(x,y), i=1, . . . , N, respectively obtained from the stationarity of an optical path length of rays refracted, or reflected, by the caustic layer and focused on points P(i) of the image plane of coordinates (xi,yi), i=1, . . . , N, wherein each piece of surface z=fi(x,y) is a surface of revolution around an axis passing through the point P(i) and having a vertex at point (xi,yi,zi), with height zi=fi(xi,yi), i=1, . . . , N, the piecewise representation of the light-redirecting surface associated with respective values of the heights of the N vertices being formed by an envelope of the intersections of the corresponding N pieces of surfaces z=fi(x,y), i=1, . . . , N;
for a given set of respective values of heights z1, . . . , zN of the vertices of the N pieces of surfaces, calculating a corresponding set of values of light intensities I(1), . . . , I(N) which are respectively focused on the points P(1), . . . , P(N) by the caustic layer redirecting incident light via the associated piecewise light-redirecting surface according to the variable light transmission coefficient of the mask pattern; and
calculating the respective values of the N heights z1, . . . , zN of the N vertices of the corresponding N pieces of surfaces which minimize the differences between the respective values of calculated light intensities I(1), . . . , I(N) focused on the points P(1), . . . , P(N) via the associated light-redirecting surface and the respective corresponding values of the target light intensities I1, . . . , IN,
thereby obtaining the light-redirecting surface having a relief pattern adapted to redirect incident light received from the light source by the optical element comprising the mask layer and form a projected image containing the target caustic pattern reproducing the reference pattern; wherein
the point-like light source is a source of light whose angular size is sufficiently small that light can be considered to arise from a single point.

### Claim 13 (depends on 3)

The optical element according to claim 3, wherein the profile of the relief pattern has a maximum depth less than or equal to 30 μm.

### Claim 14 (depends on 12)

The method according to claim 12, wherein each piece of surface z=fi(x,y), i=1, . . . , N, is approximated by taking, within a paraxial approximation, a Taylor expansion of order k greater or equal than two of an expression of the piece of surface obtained from the stationarity of the optical path length.

### Claim 15 (depends on 12)

The method according to claim 12, wherein the obtained light-redirecting surface is used to generate a machine-compatible representation for controlling a machining tool to machine the light-redirecting surface of the caustic layer.