Abstract
A reflective security element for checking the authenticity with polarized light, has a retroreflective layer and a birefringent layer arranged in a structured manner on the retroreflective layer.
Claims
1.-16. (canceled)
17. A reflective security element for checking the authenticity with polarized light, having a retroreflective layer and a birefringent layer arranged in a structured manner on the retroreflective layer.
18. The reflective security element according to claim 17, wherein the birefringent layer is configured having an outline in the form of patterns, characters or a coding.
19. The reflective security element according to claim 17, wherein the birefringent layer includes two or more regions with different optical effects, which regions are configured in the form of patterns, characters or a coding.
20. The reflective security element according to claim 17, wherein the retroreflective layer comprises a multi-reflective microprismatic layer.
21. The reflective security element according to claim 20, wherein the microprismatic layer comprises embossed structures having a depth between 10 m and 1 mm and/or a period length between 10 m and 1 mm.
22. The reflective security element according to claim 17, wherein the retroreflective layer comprises focusing, single-reflective structures, in particular spherical gradient-index lenses mirror-coated on the backside.
23. The reflective security element according to claim 22, wherein the spherical gradient-index lenses have a diameter between 20 m and 200 m.
24. The reflective security element according to claim 17, wherein the birefringent layer comprises a liquid-crystal layer, in particular a nematic liquid-crystal layer.
25. The reflective security element according to claim 24, wherein the liquid-crystal layer is arranged directly above an alignment layer, which is preferably formed from a linear photopolymer, a finely structured layer or a layer aligned by the action of shear forces.
26. The reflective security element according to claim 17, wherein the birefringent layer forms a /4 layer.
27. The reflective security element according to claim 17, wherein the security element appears colorless and/or structureless in unpolarized light at least in the region of the birefringent layer arranged in a structured manner.
28. The reflective security element according to claim 17, wherein the security element in a partial region has a hologram or a hologram-like diffraction structure.
29. The reflective security element according to claim 28, wherein the hologram or the hologram-like diffraction structure is formed by an embossing which at the same time represents an alignment layer for the alignment of the liquid-crystal layer.
30. The reflective security element according to claim 28, wherein the hologram or the hologram-like diffraction structure is furnished with a metallization or a transparent highly refractive layer.
31. A data carrier, in particular license plate, having a security element according to claim 17.
32. A method for checking the authenticity of a security element having a polarization feature, in which the security element is exposed to polarized light from any arbitrary direction of exposure, the light reflected by the security element is captured visually or by machine through a polarizer substantially from the direction of exposure, and the polarization feature becoming visible or the predetermined change in its appearance in polarized light is considered to be a sign of authenticity of the security element.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] Further embodiment examples as well as advantages of the invention will be explained hereinafter with reference to the figures, in whose representation a rendition that is true to scale and to proportion has been dispensed with in order to increase the clearness.
[0042] There are shown:
[0043] FIGS. 1(a) to 1(c) in 1(a) schematically the basic principle of checking the authenticity of a retroreflective security element present on a data carrier, for example a license plate, in 1(b) the colourless and structureless appearance of the security element under normal illumination conditions, and in 1(c) the appearance of the security element in polarized light in the analyzer with the writing OK,
[0044] FIGS. 2(a) and (b) the basic construction of security elements according to the invention in two variants,
[0045] FIG. 3 a more detailed explanation of the mode of function of the security element of FIG. 2(a) in an exploded representation,
[0046] FIG. 4 a cross-section of a first polarization feature,
[0047] FIG. 5 a representation as in FIG. 4 for another polarization feature,
[0048] FIGS. 6(a) to 6(c) three configurations of a second polarization feature,
[0049] FIG. 7 an illustration of the further processing of the polarization features of FIGS. 6(a) to 6(c) into a punched-out structured patch,
[0050] FIG. 8 a security element with a retroreflective layer and a patch according to FIG. 7, and
[0051] FIGS. 9(a) to 9(c) license plates having security elements according to FIG. 8, in 9(a) with a patch in the form of a coat of arms, in 9(b) with a full-area security foil with a plurality of coat-of-arms-shaped patches and in 9(c) with a full-area security foil with coat-of-arms-shaped recesses.
DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS
[0052] The invention will now be explained in more detail by the example of security elements for license plates. It is to be understood, however, that the security elements described can also be used, for example, as security labels for value documents or for marking products.
[0053] FIG. 1(a) schematically illustrates the basic principle of checking the authenticity of a retroreflective security element 30 present on a data carrier, for example a license plate 10, according to an embodiment example of the invention. The security element 30 is hatched in FIG. 1(a) for illustration purposes, under normal illumination conditions the security element 30 actually appears colourless and structureless as shown in FIG. 1(b), so that its presence is not readily recognizable.
[0054] For checking the authenticity, the retroreflective security element 30 of the license plate 10 is exposed to polarized light and the light retroreflected by the security element 30 is viewed through an analyzer, as shown in FIG. 1(a). For example, a user 12 polarizes unpolarized light 14 by a linear polarizer 16 and the license plate 10 is exposed to the polarized light 18. Due to the retroreflective properties of the security element 30, the reflected light 20 travels back to the user 12 within a small retroreflection cone and in doing so passes the linear polarizer 16 again. As will be explained in more detail below, the light 22 that has passed through the linear polarizer 16 is no longer structureless due to the previous influence on the polarization state of the light in the security element 30, but rather shows a desired appearance 32 as proof of authenticity. For example, in polarized light the security element 30 may appear in the analyzer with the writing OK, as shown in FIG. 1(c).
[0055] As a specialty, this authenticity check can be carried out by the user 12 from practically any locations, since it is ensured by the retroreflective properties of the security element 30 that the incident light 18 is always reflected back to the user 12.
[0056] FIGS. 2(a) and 2(b) show the basic construction of security elements according to the invention. The mode of function of the security elements is explained in more detail in the exploded representation in FIG. 3 by the example of the configuration of FIG. 2(a).
[0057] With reference first to FIG. 2(a), a security element 40 according to the invention comprises a retroreflective layer 42 and a birefringent layer 44 applied in certain regions in the form of the writing OK. The retroreflective layer 42, for example, is formed on the basis of microprismatic structures, and the birefringent layer is, for example, a nematic liquid-crystal layer which due to its layer thickness acts as a /4 layer.
[0058] In the modification of FIG. 2(b) the birefringent layer 46 is present over the full area and includes different regions 48A, 48B with different optical effects, which are configured in the form of the writing OK. For example, the regions 48A represent the letters of the writing OK and the regions 48B the complementary background regions.
[0059] With reference to FIG. 2(a) and FIG. 3, the retroreflective layer 42 of the security element 40 in this configuration is applied only in certain regions, namely in the form of the writing OK, so that in addition to the regions 52 in which a nematic /4 liquid-crystal layer 44 is present, there are also regions 50 without a nematic liquid-crystal layer.
[0060] If now, corresponding to the explanation for FIG. 1(a), unpolarized light 54 sent out by the user 12 from a light source is polarized by a linear polarizer 16, the polarized light 56 in the regions 50 without nematic layer 44 impinges on the retroreflective layer 42 and is reflected back in the direction of incidence substantially without changing the polarization state of the incident light. The reflected light 58 therefore has the same polarization state as the incident light 56 and can pass the linear polarizer 16 unhindered (reference sign 60). For the user 12, the regions 50 therefore appear bright in polarized light.
[0061] In regions 52 with the /4 nematic layer 44, the linearly polarized light 56 is converted by the nematic layer into circularly polarized light 62. The circularly polarized light 62 impinges on the retroreflective layer 42 and is reflected back by it in the direction of incidence. The reflected circularly polarized light 64 passes through the /4 nematic layer 44 again and in doing so is converted into linearly polarized light 66 whose polarization vector, however, is now perpendicular to the initial polarization. The linearly polarized light 66 can therefore not pass the linear polarizer 16 (reference sign 68), so that the regions 52 appear dark to the observer 12.
[0062] Because of the small but in practice finite opening cone of the retroreflection, the polarizer for polarizing the incident light and the analyzer for viewing the light reflected from the security element may also be slightly apart from each other. For example, the polarizer may be arranged on the headlamp of a police car, while the analyzer is present in a pair of glasses worn by a police officer sitting in the police car.
[0063] If polarizer and analyzer are spatially separated, they can also be configured differently. For example, the polarizer can be a linear polarizer and the analyzer can be a circular polarizer or a linear polarizer with a different polarization vector.
[0064] Examples of concrete configurations of security elements of the invention are now described in more detail with reference to FIGS. 4 to 9.
[0065] First, FIG. 4 shows a cross-section of a first polarization feature 70. For manufacturing the first polarization feature 70, a PET foil 72 with a thickness of 23 m is provided and furnished with a UV lacquer as a release layer 73 and a further UV embossing lacquer layer 74. Into the embossing lacquer layer 74 the desired hidden motif is embossed with an alignable structure 76. Additionally, a hologram embossing can also be performed in the same processing step. A nematic liquid-crystalline solution is printed onto the alignable structure 76. After physical drying, the nematic layer 78 is present in a layer thickness between 0.8 m and 3 m, preferably of about 1.2 m. During and after physical drying, the liquid crystals are aligned by the alignment structure 76. Subsequently, the liquid crystals are crosslinked, for example by UV exposure, preferably at reduced oxygen concentration (nitrogen inertization). Configurations in which the PET foil 72 is to remain in the finished security element are configured without release layer 73.
[0066] Subsequently, a structured or unstructured metal layer, for example of aluminium or chrome, can be applied. The structuring can be effected, for example, by covering a partial region with a washable ink, metallization and subsequently removing the washable ink with the metallization applied thereon. Of course, other structuring methods, such as etching methods, can also be used.
[0067] For further processing, the polarization feature 70 is furnished with primer(s) and heat-seal lacquers or other adhesives and applied onto the desired target substrate. The manufacturing may also include a cutting and/or punching operation to transfer the polarization feature 70 with a desired shape. The application can be carried out in such a way that only partial regions of the formed polarization feature are transferred, while other partial regions remain on the carrier foil 72. In other configurations, partial regions of the polarization feature can be removed from the carrier foil 72 before the transfer and the remaining partial regions can then be transferred completely.
[0068] The polarization feature 80 of FIG. 5 is constructed basically like the polarization feature 70, where the UV embossing lacquer layer 74 in the embodiment example of FIG. 5 is furnished with an embossing 82 which represents an alignment embossing for aligning the liquid crystals of the nematic layer 78 as well as a hologram embossing. In partial areas 84, in which instead of the liquid-crystal layer a metallization 86 is applied on the embossing 82, a reflection hologram becomes visible upon viewing.
[0069] FIG. 6(a) shows an embodiment example of a second polarization feature 90. For manufacturing the second polarization feature 90, a smooth PET foil 92 with good surface quality and a thickness of 23 m is provided and directly printed with a liquid-crystalline solution to form the desired hidden motif, for example in gravure printing. Subsequently, the liquid-crystalline solution is dried and crosslinked. More precisely, the printed solution itself is not yet in the liquid-crystalline state, rather the substances included enter into the nematic liquid-crystalline state only during and after physical drying and form a structured nematic liquid-crystal layer 94. For the transfer of the nematic layer, a transfer auxiliary layer 96 in the form of a UV lacquer layer is provided. The surface energy thereof can be adjusted such that both PET foil 92 and liquid crystals 94 can be coated without any problems. If this is not desired in some configurations, a mechanical, forced wetting of the liquid crystals during or immediately after crosslinking can also be effected.
[0070] The variant of FIG. 6(b) is based on the configuration of FIGS. 6(a) to 6(c). In addition, in the polarization feature 100 of FIG. 6(b), a UV embossing lacquer 102 is applied on the UV lacquer layer 96, embossed with a hologram embossing 104, and furnished with a metallization 106 in some regions. In the polarization feature 110 of FIG. 6(c), likewise, a UV embossing lacquer 102 is applied on the UV lacquer layer 96, provided with a hologram embossing 104, and overlaid with a higher-refractive UV lacquer 112. The hologram motif of the hologram embossing becomes visible, in this embodiment example, through the difference in the refractive index of the lacquer layers 102, 112.
[0071] The further processing of the polarization features of FIGS. 6(a) to 6(c) can be effected as in the polarization features of FIGS. 4 and 5. FIG. 7 illustrates this further processing into a punched-out structured patch. The starting point here is a polarization feature 120 having a carrier foil 122, for example according to one of the embodiment examples of FIG. 4, 5 or 6(a), (b) or (c).
[0072] Onto the lacquer side of the polarization feature 120 of FIG. 7 an approximately 12 m thick PET foil 124 is laminated with a laminating adhesive 126. On the opposite side, a supporting foil 128, also 12 m thick, is laminated with a laminating adhesive 126. Further layers, such as primer layers 130 and suitable heat-sealing layers 132, are then applied onto the foil of the former lacquer side. The thus resulting layered composite is then punched from the lacquer side (reference number 134) to such an extent that the polarization feature 120 with the included liquid-crystal layer 78 or 94 and the, where applicable, also included transfer auxiliary layer 96 is punched. Ideally, the punching ends at the carrier foil 122, but a marginal punching of the carrier foil 122 does not matter, as the supporting foil 128 prevents a further tearing.
[0073] The intermediate regions between the patches 136 produced in this way can be stripped off. Possibly required control marks are advantageously printed onto the opposite side or are retained in the process of stripping off. Finally, the foil with the layered composite is suitably cut. As the adhesive is only present in the region of the patches 132, the geometry of a stamp employed for the application is not critical. Only the desired unit is respectively transferred. The removal from the carrier foil 122 can be supported by suitable adjustment of the peel angle, for example with dispensing wedges.
[0074] FIG. 8 shows a security element 140 according to the invention having a retroreflective layer 42, which in certain regions has patches 136 applied thereon via suitable intermediate layers 142 according to FIG. 7. The layer sequence 121 of the polarization feature 120 here is configured, for example, analogous to FIG. 4, i.e. it comprises an approximately 1.2 m thick nematic layer 78 and a UV embossing lacquer layer 74 for the alignment of the liquid crystals. The patches 136 are applied for example with the outline of a desired symbol, such as a coat of arms, or with the outline of a desired writing, such as the writing OK shown in FIGS. 1(a) to 1(c). After the application, the patches are then furnished with suitable cover layers 144, for example a protective layer.
[0075] The patches 136 are colourless and structureless under normal illumination conditions and only appear when illuminated with polarized light and when the reflected light is viewed through a polarizing filter.
[0076] For illustration, FIG. 9(a) shows a license plate 150 which in a partial region has laminated thereon a security element 140 according to FIG. 8 with a patch 136 in the form of a coat of arms. The coat of arms 136 is not visible under normal illumination conditions, but only appears when the license plate 150 is illuminated with polarized light and when the reflected light is viewed through a polarizing filter. In the embodiment example of FIG. 9(a), a conventional hologram patch 152 is additionally shown, which is also visible under normal illumination conditions.
[0077] In the embodiment example of FIG. 9(b), the license plate 150 has laminated over the full area thereon a security foil 154 which is basically configured like the security element 140 of FIG. 8 and carries a plurality of regularly spaced, coat-of-arms-shaped patches 136. FIG. 9(c) shows an inverse configuration, in which there has been laminated over the full area of the license plate 150 a security foil 156 of the type described in FIG. 8 from which coat-of-arms-shaped symbols 158 have previously been punched out.
[0078] In both configurations, the positive coats of arms of FIG. 9(b) and the negative coat-of-arms-shaped recesses of FIG. 9(c) are not visible under normal illumination conditions, but only appear when the number plate 150 is illuminated with polarized light and when the reflected light is viewed through a polarizing filter.
