RETROREFLECTIVE ELEMENT HAVING A SECURITY ELEMENT

Abstract

A retroreflective element, for example a retroreflector or a retroreflective film, includes a regular arrangement of multiple reflective triples, each having three side surfaces that form an angle between 88° and 92°, preferably between 89° 50′ and 90° 10′ relative to one another, and particularly preferably stand perpendicularly on one another. At least one selected triple in the arrangement has a security element having at least one diffractive structure on at least one first side surface. A modulation depth of the security element is selected in such a manner that the security element cannot be perceived when the retroreflective element is illuminated at an illumination angle <10°.

Claims

1. A retroreflective element comprising a regular arrangement of multiple reflective triples, each triple having first, second, and third side surfaces that form an angle between 88° and 92° relative to one another; wherein at least a first selected triple in the arrangement has a first security element having at least one diffractive structure on the first side surface of the first selected triple; and wherein the first security element has a modulation depth selected so that the first security element is imperceptible when the retroreflective element is illuminated at an illumination angle <10°.

2. The retroreflective element according to claim 1, wherein the angle formed by the first, second, and third side surfaces is between 89° 50′ and 90° 10′ relative to one another.

3. The retroreflective element according to claim 1, wherein a diffraction efficiency of the at least one diffractive structure is at most 7%.

4. The retroreflective element according to claim 1, wherein the at least one diffractive structure of the first security element has a line distance between 500 nm and 2.0 μm.

5. The retroreflective element according to claim 4, wherein the modulation depth is between 0.1% and 10% of the line distance.

6. The retroreflective element according to claim 1, wherein a width between flats of the retroreflective element is between 50 μm and 500 μm.

7. The retroreflective element according to claim 1, wherein a width between flats of the retroreflective element is between 100 μm and 300 μm.

8. The retroreflective element according to claim 1, wherein at least a second selected triple has a second security element having a second diffractive structure; and wherein a line distance of the second security element is between 1.01 and 2 times the line distance of the first security element.

9. The retroreflective element according to claim 8, wherein the line distance of the second security element is between 1.01 and 1.1 times the line distance of the first security element.

10. The retroreflective element according to claim 1, wherein a signal is coded in the first security element.

11. The retroreflective element according to claim 1, wherein the first selected triple has first security elements on the first, second, and third side surfaces, respectively; and wherein the modulation depth of the first security element of the first side surface differs from the modulation depth of the first security element of the second side surface, and the modulation depths of the first security elements of the first and second side surfaces differ from the modulation depth of the first security element of the third side surface.

12. A method for orientation of a retroreflective element, the method comprising the following steps: i. providing the retroreflective element, wherein the retroreflective element comprises a regular arrangement of multiple reflective triples, each triple having first, second, and third side surfaces that form an angle between 88° and 92° relative to one another, wherein at least a first selected triple in the arrangement has a first security element having at least one diffractive structure on the first side surface of the first selected triple, and wherein the first security element has a modulation depth selected so that the first security element is imperceptible when the retroreflective element is illuminated at an illumination angle <10°; ii. illuminating the retroreflective element at a predetermined illumination angle, and observing and evaluating a received signal at a predetermined observation angle; iii. changing a rotation angle ε in increments; iv. comparing a received color value of the first security element with a predefined color value, wherein the color value is an absolute value of a vector to a color location; and v: repeating steps iii and iv until the received color value agrees within a tolerance with the predefined color value.

13. The method according to claim 12, wherein the illumination angle is between 30° and 60°.

14. The method according to claim 12, wherein the illumination angle is 45°.

15. The method according to claim 12, wherein the observation angle is equal to the illumination angle ±1°.

16. The method according to claim 12, wherein the observation angle is <1°.

17. An apparatus for detection of a signal coded in a security element of a retroreflective element comprising: a transmitter; a receiver; a holder for the retroreflective element; and an evaluation unit; wherein the transmitter is configured to transmit electromagnetic waves having at least a predetermined wavelength or an entire spectrum; wherein the holder defines a position and a center vertical line of the retroreflective element; and wherein the transmitter is affixed at an angle >30° with reference to the center vertical line.

18. The apparatus according to claim 17, wherein the transmitter is affixed at an angle between 30° and 60° with reference to the center vertical line.

19. A forming tool for production of a retroreflective element, the forming tool comprising a regular arrangement of multiple triples, each triple having first, second, and third side surfaces that form an angle between 88° and 92° relative to one another; wherein at least one selected triple in the arrangement has a security element having at least one diffractive structure on the first side surface, the modulation depth of the at least one diffractive structure being <200 nm.

20. The forming tool according to claim 19, wherein the first, second, and third side surfaces of each triple form an angle between 89° 50′ and 90° 10′ relative to one another.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Other objects and features of the invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention.

[0040] In the drawings,

[0041] FIG. 1 shows a retroreflective element according to the invention, viewed from a light entry side;

[0042] FIG. 2 shows the retroreflective element from FIG. 1 from a further perspective;

[0043] FIG. 3 shows, symbolically, a single triple having a security element according to the invention;

[0044] FIG. 4 shows a further example of a diffractive grating; and

[0045] FIGS. 5A and 5B show an example of a retroreflector according to the invention in comparison with different lettering according to the state of the art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0046] In FIG. 1, a retroreflective element 1 according to the invention is shown, seen from a light entry side. The retroreflective element 1 consists of a regular arrangement of triples 3. Each triple 3 has the shape of a hexagon in this view. Each triple 3 has a first side surface 5, a second side surface 7, and a third side surface 9, which stand perpendicular on one another, in each instance. A security element 11 is arranged on the first side surface 5 of selected triples 3, at a regular distance from one another. The width across flats 17 is the distance between parallel edges of the hexagon.

[0047] FIG. 2 shows the retroreflective element 1 from the perspective of a top view of the first side surfaces 5 with a regular arrangement of security elements 11. In this perspective, the width across flats 17 corresponds to the diagonal of an individual first side surface 5.

[0048] FIG. 3 symbolically shows an individual triple 3 of a retroreflector according to the invention. Using this figure, the complex light path through a retroreflective element according to the invention will be explained as an example. The grating lines of the diffraction grating as a diffractive structure 13 with the line distance 15 (shown symbolically here) run parallel to a center diagonal line of the surface 5. It is known from EP 1894043 B1 (proprietor: Imos Gubela GmbH), for example, that light can take six different paths through a single triple 3, because in the case of three side surfaces 5, 7, 9, six different permutations of the sequence are possible. In the hexagonal projection onto a light entry surface 19, a sub-surface 21, 22, 23, 24, 25, 26 corresponds to each of the possible paths. For light that is diffracted to the zeroth order by the security element, the sequence in which light passes through the side surfaces 5, 7, 9 does not play any role, because the zeroth order behaves like a mirror. At higher orders of diffraction, the total deflection of the reflected beam to the zeroth order follows the diffraction law:

d=g.Math.(sin φ−sin φ.sub.i)=n*λ

with d as the optical path difference, g as the grating distance, φ as the angle of diffraction perpendicular to the surface normal line of the diffraction grating, φ as the incidence direction perpendicular to the surface normal line of the diffraction grating, n as the order of diffraction, and λ as the wavelength of the electromagnetic radiation.

[0049] The incidence angle φ.sub.i onto the diffractive structure 13 depends not only on the illumination angle but also on the sequence of the illuminated sub-surfaces. For light that first falls on the side surface 5 having the diffractive structure 13, the incidence angle is equal to the diffraction angle of the side surface 5 relative to the light entry surface 19 plus (calculated vectorially) the illumination angle modified at the light entry surface in accordance with the law of diffraction. The diffraction angle φ acts like a very large angle error of the side surface 5 for the further path through the retroreflector, by way of the side surfaces 7 and 9, so that the approximations of small angle errors that are usual in the literature no longer apply, if the side surfaces 7 and 9 are still impacted at all, and the beam is not lost as a misdirected beam. On the other hand, in the case of this beam path, the diffractive structure 13 still works at such great illumination angles when normally a total reflection of the side surface 5 would be lost. Then diffraction beams of a higher order can nevertheless impact the side surfaces 7 and 9 and be reflected by them.

[0050] A beam that hits the first side surface 5 as a second reflection surface after reflection at the sub-surface 25 of the third side surface 9 or the sub-surface 23 of the second side surface 7 also has a great angle error after diffraction and is therefore difficult to follow.

[0051] Only light that impacts first on the sub-surfaces 24 and 26 and is reflected by them then impacts last onto the side surface 5 having the diffractive structure 13. Here, the exit angle of the retroreflected beam is approximately the illumination angle, vectorially added to the diffraction angle that is caused by the diffractive grating 13.

[0052] By explaining the light path, it is shown why it is desirable to suppress higher orders of diffraction at small illumination angles if at all possible. The greater illumination angles, however, can be used in targeted manner for coding, at a predetermined orientation of a transmitter and a receiver and at a predetermined rotation angle (c).

[0053] Because the light paths as shown are very complex, the reference value of a color distribution must be determined by means of ray-tracing software or read out from a table determined by experiments, for the adjustment process described above and for the apparatus for detection of the signal coded in the security element of the retroreflective element.

[0054] FIG. 4 shows a further example of a security element 41 having a diffractive grating 43 on a first side surface 35 of a retroreflector according to the invention in a different embodiment. Here, too, the line distance 45 is indicated only symbolically enlarged, and should not be understood as being to an indication of dimension. The individual grating lines of the diffraction grating run parallel to a side edge of the surface. In comparison with the grating according to FIG. 3, the grating lines are therefore rotated by 45°. The course of the grating lines imparts a preferential direction to the side surface 35, so that light coming in perpendicular to it is more likely to be diffracted than light coming in parallel to it.

[0055] According to an advantageous embodiment of the invention, the selected triple 3 has a security element 11 on each of the three side surfaces 5, 7, 9, 35. In this regard, the modulation depth of the security element 11 of the first side surface 5 differs from the modulation depth of the security element of the second side surface 7. The two modulations depths in turn differ from the modulation depth of the security element of the third side surface 9. As a result, a different brightness impression can be produced from different viewing directions, and thereby orientation of the retroreflective element according to the invention is facilitated.

[0056] FIGS. 5A and 5B show an example of a retroreflector according to the invention in comparison with different lettering in accordance with the state of the art. FIG. 5A shows a microscope image of the retroreflector with light from above, i.e. at an illumination angle of 0°. FIG. 5B shows the same section of the retroreflector with illumination at an angle of approximately 45° at a slant from top left. Conventionally ablated letters 91 absorb incoming light and thereby reduce the reflection capacity of the reflector. The security feature 93 according to the invention is invisible under light from above, and due to the predominant diffraction to the zeroth order, the security feature 93 optically behaves like the reflection surface 97 that surrounds it. As soon as the light source is shifted and illumination takes place at illumination angles greater than 30°, the security feature appears in a color spectrum that is dependent on the illumination angle. A conventional diffractive structure 95 is also visible under light from above, i.e. reflection power is issued in a diffraction maximum of at least the first order, and the grating structure is noticeable immediately, even when used for sensor applications.

[0057] Although only a few embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.

TABLE-US-00001 Reference Symbol List: 1 retroreflective element 3 triple 5, 35 first side surface 7 second side surface 9 third side surface 11, 41 security element 13, 43 diffractive structure 15, 45 line distance 17 width across flats 19 light entry surface 21, 22, 23, 24, sub-surface 25, 26 91 ablated letters 93 security feature 95 visible diffractive structure