Retroreflective element with a safety element

Description

[0001] Due to the ever-increasing miniaturization of retroreflectors, particularly in displays and films, and the resulting increased difficulty in manufacturing miniaturized structures, especially full-cube structures, there is a desire to protect these structures from counterfeiting. At the same time, there is a desire to ensure that as little reflective performance as possible is lost through anti-counterfeiting marking.

[0002] The term "retroreflector" refers to optical elements that largely redirect incident light back to the light source within a specific angular range, regardless of the exact angle of incidence. Certain deviations of the reflected beam from the direction of incidence are possible and sometimes even desirable, for example, when the light source and the intended recipient of the reflected radiation are not exactly aligned when viewed from the reflector.

[0003] Very common applications involve the retroreflector consisting of an array of multiple triples, each with essentially perpendicular reflective side surfaces, each of which reverses a component of the directional vector. In many applications, the triples consist of tetrahedral structures (e.g., US3810804 A, Applicant: Rowland dev. Corp), i.e., half cubes truncated along a diagonal. Such pyramid structures have the disadvantage of having a maximum efficiency of only approximately 66% due to dead areas, i.e., surfaces whose reflected rays cannot be reflected further.

[0004] Arrangements in which the three side surfaces of a triple are essentially square and of the same size are also very common. They then form a corner (i.e. three surfaces that meet at one point) of a cube. When projected onto a light entry surface, this structure results in a regular hexagon. Such a special triple is referred to here as a full cube. In the following, full-cube structures are used as a basis. However, without limiting the generality, the invention can also be applied to retroreflectors with pyramid structures or to retroreflectors in which the projection onto the light entry surface results in a rectangle, as described, for example, in DE 4 236 779 A1 (applicant: Hans-Erich Gubela sen.), as well as to all other triple structures with three almost vertical side surfaces.

[0005] A retroreflector marked with a hologram in one corner has long been available on the market from IMOS Gubela GmbH. However, the hologram is immediately visible when the retroreflector is illuminated at a small angle. The structures disclosed in DE 10 2018 112 043 A1 (applicant: Imos Gubela GmbH) also exhibit the problem of being immediately visible when exposed at a small angle.

[0006] DE 103 12 708 B4 (owner: OVD Kinegram AG, filing date: March 21, 2003) discloses a retroreflector made of a plastic layer composite with a relief-like matt structure, which changes the polarization direction of the reflected light but does not show any diffraction effects for visible light.

[0007] In the scientific literature ("Color-selective holographic retroreflector array for sensing applications" Authors: Rajib Ahmed, Ali K Yetisen, Seok Hyun Yun, and Haider Butt published in Light: Science & Applications (2017) 6 preview online 5 September 2016), a holographic image of a cube-corner reflector is discussed. This image exhibits the same phase properties as a normal cube-corner reflector, but lacks retroreflective properties itself. Documents DE 10 2012 105 571 A1 (applicant: OVD Kinegram AG) and DE 10 2009 012 300 A1 (applicant: Giesecke und Devrient GmbH) describe how the modulation of flat security elements affects the diffraction pattern. However, the grating structures disclosed therein, which are based on polarization effects, cannot be applied to totally reflecting retroreflectors because totally reflecting cube corner retroreflectors are not polarization maintaining.

[0008] DE 10 2005 061 749 A1 (applicant: Giesecke & Devnent GmbH) discloses security elements with an achromatically reflective microstructure in the form of a mosaic. The mosaic elements can also be retroreflectors and have a lateral dimension that is smaller than the resolving power of the human eye. They can be provided with additional security elements. EP 3 598 186 A1 (applicant: Hans Erich Gubela) shows retroreflectors in which a selected number of triples of the first type are provided with triples of the second type.WO 2020 / 074746 A1 (applicant: Bundesdruckerei GmbH) discloses a security element with a reflection-diffracting diffraction element comprising a diffraction structure for which a diffraction geometry exists in which a desired exposure direction of the light intended for diffraction forms an acute angle with a desired detection direction under which the reflectively diffracted light can be detected at the point of incidence of the diffraction element.

[0009] Important definitions can be adopted from the field of retroreflectors for road traffic: The observation angle α is the angle between the straight lines connecting the centers of an illuminating light source and a receiver with the center of a sample.

[0010] The angle of incidence β is the angle between the straight line connecting the light source with the center of the illuminated surface and the perpendicular bisector of the illuminated surface.

[0011] The angle of rotation ε is the angle by which the sample is rotated about its perpendicular bisector from an arbitrarily defined position in the direction of illumination clockwise (+ ε) or counterclockwise (- ε).

[0012] The definitions of the terms angle of incidence, angle of observation, and angle of rotation are taken from DIN 67520 Part 1 09.82. Therefore, this standard is incorporated into this application.

[0013] The present invention is based on the object of protecting retroreflectors, in particular microstructures, from unauthorized imitation and molding and of providing them with a security feature visible at an illumination angle > 30°, and at the same time losing as little reflection performance as possible in the retroreflective structures at illumination angles < 10°.

[0014] This object is achieved by a retroreflective element according to claim 1. Claim 7 describes a device for detecting a signal encoded in the security element of the retroreflective element. A mold for producing a retroreflective element according to the invention is described in claim 8.

[0015] The retroreflective element according to the invention can be, for example, a retroreflector or a retroreflective film. It comprises a regular arrangement of several reflective triplet elements, each with three side surfaces that are approximately perpendicular to one another. This means that they form an angle between 88° and 92°, preferably between 89° 50' and 90° 10'. At least one selected triplet element in the arrangement of the retroreflective element according to the invention has a security element on at least one first side surface. Of course, the security element can also be applied to other side surfaces of the selected triplet element or to other first side surfaces of other triplet elements. The security element can also be applied in a regular pattern. Selected triplet elements with the security element can themselves form lettering.

[0016] A security element is an element used for authenticity verification. Authenticity verification can be performed mechanically or through a visual or tactile impression. The security element serves to make unauthorized impressions detectable. The security element according to the invention has diffractive structures such as a simple diffraction grating or a hologram. Diffractive structures are structures based on the principle of diffraction.

[0017] The modulation depth of the security element according to the invention is selected such that the security element is not perceptible when the retroreflector is illuminated at an angle of incidence of < 10°. The modulation depth is understood to mean the amplitude of the grating modulation. The modulation depth is calculated from the maximum height difference between peak and valley within a grating line of the optical grating, or the maximum height difference between peak and valley of a hologram. The smaller the modulation depth, the smaller the path difference or phase difference caused by the path of the light in depth, i.e. along a normal vector of the first side surface, and the less intense the diffraction pattern of the security element is at small angles of incidence. At large angles of incidence, the effect of the grating spacing perpendicular to the normal vector of the first side surface predominates, and the grating structure becomes visible.According to the invention, the modulation depth of the optical structures is chosen to be as low as possible. With a grating line spacing of 1.5 µm, a modulation depth of approximately 20–30 nm has proven suitable. Such a structure makes the security feature visible when illuminated at large angles. During normal operation, e.g., in sensors, light barriers, or displays, the security feature is invisible.

[0018] To ensure that the security feature remains invisible during normal operation, it is particularly advantageous if the diffractive structures have a diffraction efficiency of less than 7%. Diffraction efficiency is defined as the ratio of the energy flowing into a first-order diffraction maximum to the zero-order diffraction maximum.

[0019] In order for the security feature to be visible to the naked eye due to its color reflections at large illumination angles, line spacings of the diffractive structures on the order of a maximum of 3 times the desired wavelength of visible light are advantageous, according to claim 1 line spacings between 500 nm and 2.0 µm. Line spacings between 1.4 µm and 1.6 µm are particularly advantageous. If several triples have security elements, it is advantageous to vary the line spacings of the security elements of adjacent triples slightly so that an optically uniform overall impression is created despite different illumination relative to the individual triple. In other words, the line spacing of a second security element is between 1.01 and 2 times greater than the line spacing of the first security element. The second security element is preferably further away from a light source ora transmitter in the preferred illumination direction than the first security element. The exact factor of the different line spacings depends on the angle difference to the light source and must be calculated individually. For machine recognition, other grating spacings can also be selected, which then, for example, appropriately diffract wavelengths outside the visible spectrum. According to claim 1, the modulation depth should be between 0.1% and 10% of the line spacing, preferably between 1% and 2% of the line spacing.

[0020] For small structures, i.e., structures with key widths of the triples between 50 µm and 500 µm according to claim 1, which are difficult to manufacture, marking with a security element is particularly interesting. The key width is defined as the distance between two parallel edges of the hexagon as projected onto the light entry surface. Application areas for such structures include, for example, displays where a viewer should not be able to detect the security feature during normal use, or flexible retroreflective films, where small key widths lead to greater film flexibility.

[0021] Depending on the desired visual effect, the proportion of the area covered by the security element on the first side of the triplet equipped with the security element can be freely selected. If as little light as possible is to be diffracted even at large angles of incidence, the covered area can be between 5% and 50%. If the security element should be found as quickly as possible, it is advisable to cover between 50% and 98% of the first side with the security element.

[0022] According to an advantageous embodiment of the invention, the selected triplet (3) has a security element (11) on each of the three side surfaces (5, 7, 9, 35). The modulation depth of the security element (11) on the first side surface (5) differs from the modulation depth of the security element on the second side surface (7). Both modulation depths, in turn, differ from the modulation depth of the security element on the third side surface (9). This allows a different brightness impression to be created from different viewing directions, thereby facilitating orientation of the retroreflective element according to the invention.

[0023] The retroreflector according to the invention can be easily aligned. Especially with round retroreflectors, the problem has so far been that the light output depends on the orientation. This requires either a visible marking on a light entry surface to indicate a preferred direction, or the structures must be arranged in different areas, as proposed in utility model DE 20 2019 001 143 U1 (owner: Imos Gubela GmbH, filing date March 11, 2019), which also leads to losses due to the cutting of individual triplet elements. With a retroreflector according to the invention, easy alignment of the reflector is possible.

[0024] Alignment is performed in the following steps: First, a retroreflective element according to the invention is provided. The retroreflective element is illuminated at a predetermined angle of incidence, for example, at an angle of incidence between 30° and 45°, and analyzed at a predetermined observation angle. The positions of a transmitter and a receiver are thus predefined. The receiver can, for example, be located in a housing with the transmitter, arranged adjacent to it. This corresponds to a small observation angle of approximately 0° to 2°. The receiver can also be arranged in an axis relative to the perpendicular bisector of the retroreflective element. In this case, the observation angle is equal to the illumination angle.

[0025] In a further step, the retroreflective element according to the invention is rotated step by step. This means that the angle of rotation ε is changed, for example, in 5° increments. The receiver, for example, a CCD camera, then records a color value. The color value is the absolute value of the vector to a color location. An evaluation unit, for example, a computer with appropriate graphics software, compares the received color value with a target value. Subsequently, a fine adjustment can be performed with a smaller increment, for example, a increment of 0.5°.

[0026] If a signal is encoded in the security element, a device for reading the encoded signal is required. The device for detecting the signal encoded in the security element of the retroreflective element comprises at least one transmitter, one receiver, a receptacle for the retroreflective element, and an evaluation unit. The transmitter emits electromagnetic waves of at least one predetermined wavelength. The wavelength is preferably in the visible light range. A single wavelength is suitable if the only requirement is to determine whether a signal is detected at all at a predetermined position of the transmitter and receiver. If a predetermined color image is to be detected, white light, e.g., with a color temperature of 2600 K, is a suitable light source for the transmitter. The receptacle determines the position of the retroreflective element. The retroreflective element is therefore stationary.The position of the retroreflective element in the mount allows the perpendicular bisector of the entrance surface to be determined. The device is characterized by the fact that the transmitter is positioned at an angle greater than 30° and less than 90° to the perpendicular bisector. The angle is preferably between 30° and 60°. Particularly preferred angles are the slope angles of the edges, i.e., the diagonal angle in a cube of 54.74°, and the complementary angle to this, i.e., 35.26°, because then a light path exists in each case where a light beam impinges perpendicularly on the grating. For reasons of symmetry and space, an angle of 45° is also preferred.

[0027] The receiver can advantageously be mounted at the position of a mirror image of the transmitter reflected at the perpendicular bisector, directly at the perpendicular bisector or in a housing with the transmitter.

[0028] According to an advantageous embodiment, the evaluation unit is configured to compare a predetermined light value with a light value measured at the receiver, log deviations, and make them visible to a human operator. If deviations are too large, the evaluation unit advantageously generates a warning or alarm.

[0029] The device for detecting the coded signal can be provided in addition to a regular light barrier in a sensor arrangement that usually operates at small illumination angles (<5°).

[0030] The device according to the invention with the retroreflective element according to the invention can also be used to detect stray rays from external light sources, for example, in sensors, since a color component can also be detected. If the color component is not detected by a receiver and an evaluation unit, but a signal is received from the retroreflector, the evaluation unit can output an error message.

[0031] The retroreflective element according to the invention is preferably manufactured by molding a master. The master, as a molding tool, is a negative of the retroreflective element according to the invention. The molding tool therefore also has a regular arrangement of triples, each with three side surfaces that form an angle between 88° and 92°, preferably between 89° 50' and 90° 10', and are particularly preferably perpendicular to one another. Low points in the molding tool become high points in the retroreflective element. The same applies to the security element and its diffractive structures. Due to inaccuracies in molding and shrinkage processes of the plastic used, the modulation depth should be approximately 1.01 times the desired modulation depth in the retroreflective element.

[0032] Injection molding has proven successful for large-scale production. For prototypes, other molding processes such as hot stamping can also be used if greater molding accuracy is required. Imprinting processes such as roll-to-roll imprinting with UV-curing polymers can be used to produce retroreflective foils. With all of these molding processes, the security element must first be applied to the master. This can be done using conventional lithographic methods such as photolithography with masks followed by etching. The modulation depth can be influenced by the etching time. However, methods using laser interference ablation are cheaper. In this process, beams from a UV ultrashort pulse laser (for example, with a wavelength of 197 nm) are caused to interfere with a grating interferometer in such a way that a standing wave is formed.Material is removed from the master at the points of constructive interference. The modulation depth of the structure increases with the intensity of the radiation used.

[0033] If the retroreflective element or the molding tool according to the invention is re-molded, for example, using electroplating processes, the security element remains in a weaker form. By comparing the intensities at illumination angles > 30°, an unauthorized counterfeit made using a retroreflective element or molding tool in circulation can be detected.

[0034] The following figures are intended to illustrate the invention in more detail without limiting the generality of the disclosure. They show: Figure 1 a retroreflective element according to the invention viewed from a light entry side Figure 2 the retroreflective element made of Figure 1 from a broader perspective Figure 3 symbolically a single triple with a security element according to the invention Figure 4 another example of a diffractive grating Figure 5 an example of a retroreflector according to the invention in comparison with various inscriptions according to the prior art.

[0035] In Fig. 1 A retroreflective element 1 according to the invention is shown, viewed from a light entry side. The retroreflective element 1 consists of a regular arrangement of triplet elements 3. Each triplet element 3 has the shape of a hexagon in this view. Each triplet element 3 has a first side surface 5, a second side surface 7, and a third side surface 9, each of which is perpendicular to one another. A security element 11 is arranged on the first side surface 5 of the regularly spaced triplet elements 3. The width across flats 17 is the distance between parallel edges of the hexagon.

[0036] Fig. 2shows the retroreflective element 1 from the perspective of the top view of the first side surfaces 5 with the regular arrangement of safety elements 11. In this perspective, the key width 17 corresponds to the diagonal of a single first side surface 5.

[0037] Fig. 3symbolically shows a single triple 3 of a retroreflector according to the invention. This figure serves to illustrate the complex light path through a retroreflective element according to the invention. The grating lines of the diffraction grating as a diffractive structure 13 with a line spacing of 15 (symbolically shown here) run parallel to a central diagonal of surface 5. For example, it is known from EP 1 894 043 B1 (owner: Imos Gubela GmbH) that light can take six different paths through a single triple 3, since with three side surfaces 5, 7, 9, six different permutations of the sequence are possible. In the hexagonal projection onto a light entry surface 19, each of the possible paths corresponds to a partial surface 21, 22, 23, 24, 25, 26.For light diffracted to the zeroth order by the security element, the order in which it passes through the side surfaces 5, 7, and 9 is irrelevant, since the zeroth order behaves like a mirror. For higher diffraction orders, the total deflection of the reflected beam to the zeroth order follows the diffraction law: . d = g ⋅ sin φ − sin φ i = n* λ with d as path difference, g as grating pitch, ϕ as diffraction angle perpendicular to the surface normal of the diffraction grating, ϕi as incidence direction perpendicular to the surface normal of the diffraction grating, n as diffraction order, and λ as the wavelength of the electromagnetic radiation.

[0038] The angle of incidence ϕ i on the diffractive structure 13 depends on both the angle of incidence and the order of the illuminated partial surfaces. For light that first falls on the side surface 5 with the diffractive structure 13, the angle of incidence is equal to the angle of inclination of the side surface 5 to the light entry surface 19 plus (calculated vectorially) the angle of incidence modified according to the law of refraction at the light entry surface. For the further path through the retroreflector via the side surfaces 7 and 9, the diffraction angle ϕ acts like a very large angular error of the side surface 5, so that the approximations of small angular errors commonly found in the literature no longer apply, provided that the side surfaces 7 and 9 are still hit at all and the beam is not lost as a stray ray. On the other hand, with this beam path, the diffractive structure 13 is effective even at such large angles of incidence that total internal reflection of the side surface 5 would normally be lost.Then higher order diffraction rays can still hit the side surfaces 7 and 9 and be reflected by them.

[0039] Even a beam that hits the first side surface 5 as a second reflection surface after reflection at the partial surface 25 of the third side surface 9 or the partial surface 23 of the second side surface 7 has a large angular error after diffraction and thus makes tracking more difficult.

[0040] Only light that first strikes the partial surfaces 24 and 26 and is reflected by them then last strikes the side surface 5 with the diffractive structure 13. Here, the exit angle of the retroreflected beam is approximately the angle of incidence, vectorially added to the diffraction angle caused by the diffractive grating 13.

[0041] The discussion of the light path demonstrates why it is desirable to suppress higher orders of diffraction as much as possible at small angles of incidence. However, the larger angles of incidence can be used specifically for coding, given a given alignment of a transmitter and receiver and a given angle of rotation (ε).

[0042] Since the light paths are very complex, as shown, the target value of a color distribution must be determined using ray tracing software or read from an experimentally determined table for the adjustment procedure described above and for the device for detecting the signal encoded in the security element of the retroreflective element.

[0043] Fig. 4shows a further example of a security element 41 with a diffractive grating 43 on a first side surface 35 of a retroreflector according to the invention in another embodiment. Here, too, the line spacing 45 is only indicated symbolically in an enlarged manner and is not to be understood as a dimensional specification. The individual grating lines of the diffraction grating run parallel to a side edge of the surface. In comparison to the grating according to Figure 3 The grating lines are thus rotated by 45°. The course of the grating lines gives side surface 35 a preferred direction, so that light incident perpendicular to it is more likely to be diffracted than light incident parallel to it.

[0044] Fig. 5 shows an example of a retroreflector according to the invention in comparison with various inscriptions according to the prior art. Figure 5 a shows a microscope image of the retroreflector in reflected light, ie, at an illumination angle of 0°. Figure 5 bshows the same section of the retroreflector illuminated at an angle of approximately 45° from the top left. Conventionally ablated letters 91 absorb incident light and thus reduce the reflective power of the reflector. The security feature 93 according to the invention is invisible when illuminated under incident light and due to the predominant diffraction into the zeroth order, the security feature 93 behaves optically like the surrounding reflective surface 97. As soon as the light source is shifted and illuminated at illumination angles greater than 30°, the security feature appears in a color spectrum that depends on the illumination angle. A conventional diffractive structure 95 is also visible under incident light, i.e. reflection power is converted into a diffraction maximum of at least first order and the grating structure is immediately noticeable even when used for sensor applications. List of reference symbols:

[0045] 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 spacing 17 Wrench size 19 Light entry surface 21, 22, 23, 24, 25, 26 Subarea 91 ablated letters 93 Security feature 95 visible diffractive structure

Claims

1. Retroreflective element (1), for example a retroreflector or a retroreflective film, comprising a regular arrangement of several reflective triples (3), each with three side surfaces (5, 7, 9, 35) which form an angle between 88° and 92°, preferably between 89° 50' and 90° 10', and particularly preferably are perpendicular to one another, wherein at least one selected triple (3) in the arrangement has a security element (11, 41) with diffractive structures (13, 43) on at least one first side surface (5, 35), characterized in that a width between flats (17) of the retroreflective element (1) is between 50 µm and 500 µm and a modulation depth of the security element (11, 41) is selected such that the security element is imperceptible when the retroreflective element (1) is illuminated at an illumination angle < 10°, the diffractive structures (13, 43) of the security element (11, 41) have a line spacing (15, 45) between 500 nm and 2.0 µm and the modulation depth is between 0.1% and 10% of the line spacing (15, 45).

2. Retroreflective element (1) according to claim 1, characterized in that a diffraction efficiency of the diffractive structures (13, 43) is at most 7%.

3. Retroreflective element (1) according to one of the preceding claims, characterized in that the width between flats (17) of the retroreflective element (1) is between 100 µm and 300 µm.

4. Retroreflective element (1) according to one of the preceding claims, characterized in that at least a second selected triple has a second security element with diffractive structures wherein a line spacing of the second security element is between 1.01 and 2 times the line spacing of the first security element.

5. Retroreflective element (1) according to one of the preceding claims, characterized in that a signal is encoded in the security element.

6. Retroreflective element (1) according to one of the preceding claims, characterized in that the selected triple (3) has security elements on each of the three side surfaces (5,7,9,35), 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 those two side surfaces differ from the modulation depth of the first security element of the third side surface.

7. Use of a retrorefractive element (1) according to claim 6 in a device for detecting a signal encoded in the security the device comprising at least one emitter, one receiver, a receptacle for the retroreflective element and an evaluation unit, wherein the emitter emits electromagnetic waves of at least one predetermined wavelength or an entire spectrum, preferably in the visible range, wherein the receptacle defines a position and a perpendicular bisector of the retroreflective element, characterized in that the transmitter is mounted at an angle > 30°, preferably between 30° and 60° with respect to the perpendicular bisector.

8. Molding tool for producing a retroreflective element according to one of claims 1 to 6, comprising a regular arrangement of several triples (3), each with three side surfaces (5, 7, 9, 35) which form an angle between 88° and 92°, preferably between 89° 50' and 90° 10', and particularly preferably are perpendicular to one another, wherein at least one selected triple (3) in the arrangement has a security element (11, 41) with diffractive structures (13, 43) on at least one first side surface (5, 35), the modulation depth of which is < 200 nm.