Optical element and method for visually authenticating an object

Abstract

The present invention relates to a copy protection optical element comprising a caustic layer and a mask layer configured to simultaneously display a visible image that reproduces a reference image and to form a projected image including a visible caustic pattern that reproduces a reference pattern upon illumination of the optical element by a light source, the projected image being different from the reference image. The present invention also relates to a method for designing a relief pattern in the light redirecting surface of the caustic layer that matches the transmission properties of the mask layer.

Claims

1. An optical element comprising a cosine layer formed from a first optical material that is reflective or refractive, transparent or partially transparent, and having a light direction conversion surface with a relief pattern, wherein the optical element includes a mask layer disposed on the optical surface of the optical element or within the optical element, the mask layer having a mask pattern and a variable light transmittance coefficient, the variable light transmittance coefficient varying locally from 0 when the incident light is blocked by the opaque portion of the mask pattern to 1 when the incident light is completely transmitted through the unmasked portion of the cosine layer, and the mask layer being configured to at least partially transmit the incident light when the optical element is illuminated by a point light source, wherein the relief pattern of the light direction conversion surface of the cosine layer is configured to deflect the incident light received by the optical element from the point light source to form a projected image including a visible cosine pattern that reproduces a reference pattern, wherein when the optical element is illuminated by the point light source, the mask layer is configured to show a visible image that reproduces a reference image, the visible image being different from the projected image, and the projected image formed by the optical element does not include the reference image.

2. The optical element according to claim 1, wherein the depth profile of the relief pattern has a sharp change portion formed by machining the surface of the single-piece first optical material according to a calculated relief pattern profile having a discontinuity, and the machined sharp change portion corresponds to the discontinuity.

3. The optical element according to claim 1 or 2, wherein the profile of the relief pattern has a maximum depth of 30 μm or less.

4. The optical element according to claim 1 or 2, wherein the profile of the relief pattern has a maximum depth of 250 μm or less.

5. The relief pattern of the light direction conversion surface is separated from the light direction conversion surface by a distance d ｓ to redirect incident light received from the light source, and forms the projected image including the coarse stick pattern on a wall surface separated from the light direction conversion surface by a distance d ｉ configured as such, where the value of d ｉ is 30 cm or less, and the ratio d ｓ / d ｉ is 5 or more. The optical element according to any one of claims 1 to 4

6. further comprising a lens element formed from a second refractive, transparent or partially transparent optical material adjacent to the cosine layer, the lens element being configured to deflect the incident light received by the optical element from the light source to form the projected image including the visible cosine pattern that reproduces the reference pattern, The light direction conversion surface has a focal length f ｃ and The focal length f is set such that the lens element forms the projected image including the visible cosine pattern directly on the retina of an observer looking at the light source through the optical element. Ｌ having The optical element according to any one of claims 1 to 5.

7. a) the course stick layer has a positive focal length (f ｃ > 0), and the lens element has a negative focal length (f Ｌ < 0), or b) the course stick layer has a negative focal length (f ｃ < 0), and the lens element has a positive focal length (f Ｌ > 0), The optical element according to claim 6, including one of them.

8. the focal length f of the lens element Ｌ and the focal length f of the cosine layer ｃ The relationship between them satisfies the following formula 【Number 1】 Satisfying Here, R is the distance between the cositic layer and the observer's eye, d ｓ is the distance between the light source and the optical element, d Ｒ is a comfortable reading distance from the eyes and is at least 25 cm The optical element according to claim 7.

9. The optical element according to any one of claims 1 to 8, which marks an object selected from the group including consumer products, securities, tax stamps, and banknotes.

10. A method for visually authenticating an object marked with the optical element having the mask layer according to any one of claims 1 to 9 by an observer, Illuminating the optical element with a point light source; Visually observing the projected image including the visible cositic pattern that reproduces the reference pattern; Determining that the object is genuine based on the observer's evaluation that the cositic pattern is visually similar to the reference pattern; A method including

11. When illuminating the optical element with the point light source, the mask layer is configured to show a visible image that reproduces a reference image, and the method includes a further step of visually observing the visible image that reproduces the reference image, and the step of determining that the object is genuine includes a further verification by the observer that the visible image is visually different from the cositic pattern. The method according to claim 10.

12. A method for designing a relief pattern of a light direction conversion surface of a cositic layer formed from a refractive transparent or partially transparent or reflective piece of a first optical material, wherein the cositic layer is a part of an optical element including a mask layer disposed on or within an optical surface of the optical element, the mask layer includes a mask pattern and has a variable light transmittance coefficient, the variable light transmittance coefficient locally varies from 0 when incident light is blocked by an opaque portion of the mask pattern to 1 when incident light completely passes through an unmasked portion of the cositic layer, the mask layer is configured to at least partially transmit incident light when the optical element is illuminated by a point light source, the cositic layer is configured to redirect incident light received from the point light source to form a projected image including a cositic pattern, and the method includes Coordinates in the image plane {(x ｉ , y ｉ ),} of N image pixels p ｉ of a set P of a reference pattern for a discrete representation of an input target image, for the associated non-zero target light intensities {I ｉ} distributed within a given region of the target image and corresponding to the target course tic pattern of the target image, i = 1, …, N, and a computer-implemented step of providing refracted or reflected by the course stick layer to coordinates (x ｉ , y ｉ ), from the constancy of the optical path length of the light rays converging on the points P(i) of the image plane for i = 1, …, N, respectively obtained are a plurality of intersecting surface segments z = f ｉ (x, y), i = 1, …, N, based on the representation of the light direction conversion surface, a computer-implemented step of calculating a piecewise representation of the light direction conversion surface z = F(x, y) of the course stick layer having a height z above the (x, y) coordinate plane, wherein each surface segment z = f ｉ (x, y) passes through the point P(i) and has a height z ｉ = f ｉ (x ｉ , y ｉ ), i = 1, …, N, and is a rotational surface about an axis having a vertex at the point (x ｉ , y ｉ , z ｉ ), and the piecewise representation of the light direction conversion surface associated with the respective values of the heights of the N vertices is formed by the envelope of the intersection lines of the corresponding N surface segments z = f ｉ (x, y), i = 1, …, N, a computer-implemented step; The height z of the vertices of the N surface segments １ , …, z Ｎ For a given set of respective values, according to the variable light transmittance coefficient of the mask pattern, a computer-implemented step of calculating a corresponding set of values of the light intensities I(1), …, I(N) respectively focused on the points P(1), …, P(N) by the cosine layer that redirects incident light through the associated segmented light-direction-changing surface The respective values of the calculated light intensities I(1), …, I(N) focused on the points P(1), …, P(N) via the associated light direction conversion surface, and the target light intensity I １ , …, I Ｎ The N heights z １ , …, z Ｎ Of the respective values of the N vertices of the corresponding N surface segments, which minimize the difference between the respective corresponding values, and a computer-implemented step of calculating the respective values of Including A method of obtaining the light direction conversion surface having a relief pattern configured to form a projection image including a target cos tic pattern that reproduces a reference pattern while redirecting incident light received from the light source by the optical element including the mask layer.

13. Each surface section z = f ｉ The method according to claim 12, wherein (x, y), i = 1, ..., N are approximated by taking a Taylor expansion of degree k of 2 or more of the formula of the surface section obtained from the stationarity of the optical path length in the paraxial approximation.

14. The method according to claim 12 or 13, generating a machine compatibility expression for controlling a machining tool to machine the light direction conversion surface of the cos tic layer using the designed light direction conversion surface.

Images