阅读说明：本技术 反射型防伪元件 (Reflective security element ) 是由 W.霍夫米勒 C.亨格 K.H.谢雷尔 M.索博尔 于 2018-10-22 设计创作，主要内容包括：本发明涉及一种用于用偏振光(18)检测原真性的反射型防伪元件(40),包括逆反射层(42)和以结构化方式布置在所述逆反射层(42)上的双折射层(44)。(The invention relates to a reflective security element (40) for detecting authenticity using polarized light (18), comprising a retroreflective layer (42) and a birefringent layer (44) arranged in a structured manner on the retroreflective layer (42).)

1. A reflective security element for checking authenticity by polarized light has a retroreflective layer and a birefringent layer arranged in a structured manner on the retroreflective layer.

2. A reflective security element according to claim 1, wherein said birefringent layer is configured to have a profile in the form of a pattern, character or code.

3. Reflective security element according to claim 1 or 2, wherein the birefringent layer comprises two or more regions with different optical effects, said regions being configured in the form of a pattern, character or code.

4. Reflective security element according to at least one of claims 1 to 3, wherein the retroreflective layer comprises a multi-reflective micro-prism layer.

5. Reflective security element according to claim 4, wherein the micro-prism layer comprises embossed structures having a depth of between 10 μm and 1 mm and/or a period length of between 10 μm and 1 mm.

6. Reflective security element according to at least one of claims 1 to 5, characterized in that the retroreflective layer comprises a focused single reflective structure, in particular a spherical gradient index lens with a mirror coating on the back side.

7. A reflective security element according to claim 6, wherein the spherical gradient index lens has a diameter between 20 microns and 200 microns.

8. Reflective security element according to at least one of claims 1 to 7, characterized in that the birefringent layer comprises a liquid crystal layer, in particular a nematic liquid crystal layer.

9. Reflective security element according to claim 8, wherein the liquid crystal layer is arranged directly on an alignment layer, preferably formed by a linear photopolymer, a fine structure layer, or a layer that is aligned under shear forces.

10. Reflective security element according to at least one of claims 1 to 9, characterized in that the birefringent layer forms a λ/4 layer.

11. Reflective security element according to at least one of claims 1 to 10, characterized in that the security element appears colorless and/or unstructured under unpolarized light at least in the region of the structurally arranged birefringent layer.

12. Reflective security element according to at least one of claims 1 to 11, characterized in that the security element has a hologram or a diffractive structure similar to a hologram in a local area.

13. Reflective security element according to claim 12, wherein the hologram or the diffractive structure similar to a hologram is formed by an embossed layer which simultaneously represents an alignment layer for the alignment of the liquid crystal layer.

14. Reflective security element according to claim 12 or 13, wherein the hologram or the diffractive structure resembling a hologram is provided with a metallised layer or a transparent high refractive layer.

15. A data carrier, in particular a license plate, having a security element as claimed in any one of claims 1 to 14.

16. A method for checking the authenticity of a security element having a polarizing feature, wherein the security element is exposed to polarized light from any arbitrary exposure direction, the light reflected by the security element is visually or machine-captured through a polarizer substantially from the exposure direction, and the polarizing feature becomes visible or a predetermined change in its appearance in the polarized light is considered an authenticity indicator of the security element.

Technical Field

The invention relates to a reflective security element, a data carrier provided with such a security element, and a method for checking the authenticity of a reflective security element.

Background

Data carriers such as banknotes, stocks, bonds, deeds, vouchers, checks, high-value entrance tickets and other documents that face the risk of counterfeiting, such as passports or other identity documents, are often provided for protection purposes with security elements that allow the authenticity of the value document to be checked while also serving to prevent unauthorized copying.

In many cases, special properties of liquid-crystal materials are used for this purpose, and particular attention is paid to the viewing-angle-dependent color impression and to the special light-polarization properties of the liquid-crystal layer.

The reflective security elements with liquid crystals can be divided into at least two groups. The first group of security elements comprises a reflector which reflects circularly polarized light only in an oriented manner and which is formed on the basis of cholesteric liquid crystals. The security elements of the first group can also typically be sensed from a relatively large distance, since the source of the light reflected by the security elements is irrelevant.

The second group of security elements comprises reflectors which do not only reflect circularly polarized light. For example, on the basis of nematic liquid crystals, an optically anisotropic layer is arranged above a metal reflector without depolarizing effect. In order to check authenticity, a polarizing filter is used, which generally has to be arranged directly on the security element, since the light falling on the security element must already be present in polarized form to ensure that the reflected light is also polarized and can be sensed with the polarizing filter, which thus acts as an analyzer. It is particularly desirable to arrange the polarizing filters directly to minimize unpolarized spurious light.

Cholesteric reflectors included in security elements of the first group are undesirable in many applications, since such reflectors reflect only in a limited wavelength region and at most only half of the incident light in this wavelength region. Furthermore, false light must be prevented by an absorptive background.

In principle, the second security element group is not suitable for checking authenticity from a large viewing distance, since in all cases both the incident light and the reflected light have to pass through the polarizer/analyzer. Due to the reflection condition of "angle of incidence equal to angle of reflection", authenticity checking is only possible if the security element is almost completely perpendicular to the illumination direction and the viewing direction or if the illumination unit and the evaluation unit are arranged symmetrically at exactly the same angle relative to the security element. In many applications, this requirement is not fulfilled or is not fulfilled at all, so that the use of such security elements of the second group can only be implemented to a limited extent.

Disclosure of Invention

On these premises, the object of the invention is to provide a reflective security element which avoids the disadvantages of the prior art and, in particular, can also be easily checked for authenticity from a large viewing distance.

This object is achieved by the features defined in the independent claims. Improvements of the invention are the subject of the dependent claims.

The invention provides a security element for checking authenticity by polarized light, comprising a retroreflective layer and a birefringent layer arranged on the retroreflective layer in a structured manner.

The combination of a structured birefringent layer with a retroreflective layer provides a decisive advantage in that the optical anisotropy of the birefringent layer can easily be interrogated from large distances of several meters or even tens of meters. The use of a retroreflective layer enables incident light to be reflected onto the light source itself and onto small angular areas around the light source. Disturbing extraneous light, such as sunlight, room lighting, or license plate lighting in the case of license plates, is at the same time strongly suppressed, since only a very small portion of the extraneous light is reflected in the direction of the illumination lamp/viewer.

In an advantageous configuration of the security element, the birefringent layer is configured with a contour in the form of a pattern, a character or a code. Alternatively or additionally, the birefringent layer may comprise two or more regions having different optical effects, the regions being arranged in the form of a pattern, character or code. In both cases, the required image contrast is produced during the plausibility check of polarized light. In the first case, contrast is produced between the areas with birefringent layer and the areas without birefringent layer, and in the second case, the areas of different effect appear with different brightness and/or color during the plausibility check, depending on the type and position of the polarizing filter of the analyzer.

The retroreflective layer preferably comprises a multi-reflective micro-prism layer that includes, inter alia, embossed structures having a depth of between 10 microns and 1 mm and/or embossed structures having a period length of between 10 microns and 1 mm.

Alternatively or additionally, the retroreflective layer may also comprise a focused single reflective structure, particularly a spherical gradient index lens with a mirror coating on the back, also known as a Lenberg lens. The prism lens consists of spheres of dielectric material which are as loss-free as possible and which have a position-dependent dielectric constant. Due to the mirror coating on the back side, the lens can accurately reflect incident light back in the direction of its light source, thereby acting as a retro-reflector. The refractive index within the sphere is chosen so that parallel incident rays are focused as far as possible at a point opposite the point of contact of the wavefront. For this reason, this value decreases with the distance r from the center and substantially follows the following relationship:

n(r)＝Sqrt[2-(r/R)²]，

where Sqrt [ ] represents a square root function, R represents the radius of the sphere, and R represents the distance from the center of the sphere.

The spherical gradient index lens mirror-coated on the rear side has in particular a diameter of between 20 and 200 micrometers.

The birefringent layer of the security element preferably comprises a liquid crystal layer, in particular a nematic liquid crystal layer. The birefringent layer may also be formed from a single liquid crystal layer, in particular a nematic liquid crystal layer. Basically, the birefringent layer may also be formed by an optically anisotropic expanded foil (e.g. a PET foil or a PP foil), by a birefringent polycarbonate foil, by mica, or by a layer with birefringent pigments.

The liquid crystal layer is preferably disposed directly above an alignment layer, which is preferably formed from a linear photopolymer, a fine structure layer, or a layer that is aligned by the application of shear forces.

The birefringent layer is preferably formed as a lambda/4 layer. Other contrast mechanisms may be based on the use of dichroic dyes or foils which absorb light to varying degrees depending on the polarization in the aligned form. The illuminated unpolarized light is selectively linearly polarized by the dye or foil, while the other polarized parts are absorbed. With a corresponding analyzer, unpolarized light (background of the security element) can be distinguished from linearly polarized light (in the region with the dichroic dye or foil).

In an advantageous configuration, the security element appears colorless and/or unstructured in unpolarized light at least in the region of the birefringent layer arranged in a structured manner, so that the structured birefringent layer together with the retroreflective layer forms a hidden security feature which can only be read with the aid of an auxiliary tool. In particular in unpolarized light, the security element may look completely colorless and/or unstructured, so that the presence of the security element cannot be recognized at all without the aid of auxiliary tools.

In some configurations, the security element may also have a hologram or a diffractive structure similar to a hologram in a localized area. The hologram or a diffraction structure similar to a hologram is preferably formed here by an embossing layer which at the same time also represents an alignment layer for the alignment of the liquid crystal layer. Advantageously, the hologram or the diffractive structure similar to the hologram is provided with a metallised layer or a transparent high refractive layer.

The invention also relates to a data carrier having a security element of the type described. The data carrier may be, inter alia, a license plate or other number plate, a value document (e.g. a banknote, stock certificate, bond, certificate, voucher, check, premium admission ticket), or an identification card (e.g. a credit card, bank card, cash card, authorization card, national identity card, passport or personalized page).

The invention also includes a method for checking the authenticity of a security element having a polarizing feature, wherein the security element is exposed to polarized light from any arbitrary exposure direction, light reflected by the security element is visually or machine-captured through a polarizer substantially from the exposure direction, and the polarizing feature becomes visible or a predetermined change in its appearance in the polarized light is considered an authenticity indicator of the security element. In the case of planar security elements, the exposure direction is in particular not perpendicular to the face of the security element.

In an advantageous variant of the invention, the polarization features are not even recognizable in unpolarized light and become visible in polarized light only when viewed through a polarizer.

Capturing the reflected light is preferably done visually, but may also be done by machine, e.g. by a sensor. The same polarizer can be used for polarization of the exposure radiation and analysis of the reflected radiation. However, since the reflected light is reflected as a small retro-reflected cone of light, it is also possible to use a second analyzing polarizer, which is arranged at a small angular distance from the first polarizer. At this point, the second polarizer may be designed to be a different type of polarization (or circular rather than linear) and/or a different polarization direction than the first polarizer in order to obtain an image of the structured birefringent layer with as high a contrast as possible.

In the following, some further details and preferred configurations of the proposed solution will be discussed:

if nematic liquid crystals are used for the birefringent layer and arranged above the counter reflector, the polarization characteristics can be detected by observation through a polarizing filter after the security element has been illuminated with polarized light in the vicinity of the light source. In a simple example, security elements with anisotropic λ/4 layers (45 ° orientation) present in certain regions are illuminated with linearly polarized light (polarizing filter position 0 °). The incident light is converted into circularly polarized light by the lambda/4 layer. This light is reflected at the interface of the prism lens (e.g. at the metal layer). When the reflected circularly polarized light passes through the optically anisotropic layer again, it becomes linearly polarized light whose polarization plane is rotated by 90 °. If a user views the security element through the analyzer (in this example a linear polarizing filter) at the 0 position, the element appears dark. However, the adjacent area without the optically anisotropic layer appeared bright. If one or both polarization filters are rotated, the contrast ratio may change, or even reverse.

The explained principles can be used, for example, to illuminate a security tag with a polarized flashlight, or to illuminate a license plate through one or more polarized front headlights of a police car, and to make corresponding observations through a polarized filter. Another simple detection technique is to use a camera with a polarizer in front of the flash and in front of the objective. Here, separate polarizers having selectable positions relative to each other may be used. In this way, the polarization characteristic can be detected even in the case of lighting which is very disadvantageous in practice (large amounts of false light), since the polarized flash only has to exceed the false light in a very short exposure time.

As an anisotropic layer, nematic liquid crystals have a phase shift effect in both the visible and adjacent wavelength regions (uv, ir). This also enables detection by using invisible light irradiation. For example, in the speed monitoring process, the authenticity of the license plate can be checked by means of an infrared flash light, with the aid of (where applicable) a suitable analyzer with a wavelength filter. The sensors of a common digital camera are already sensitive enough to infrared light to perform this analysis.

Basically, the polarization properties of nematic liquid crystals can be produced in different ways. In order to generate anisotropy, the liquid crystal needs to be aligned. In order to create different optical conditions at different locations on the security element, the optically anisotropic layer can be applied as a pattern, the optically anisotropic layer can be applied in a locally resolved manner with different alignment, or the layer material can be fixed in a locally resolved manner in different states. For example, the fixation may be achieved by irradiation with ultraviolet light.

The alignment may be achieved, for example, by printing the liquid crystal (or a solution containing a latent liquid crystal substance) onto a substrate that supports alignment. The substrate may be a PET foil with good surface quality. If the alignment does not appear to be sufficiently uniform, the uniformity can be improved by mechanical pre-treatment, such as rubbing with velvet or a softer felt or a suitable cloth in the preferred direction as desired. By using additional alignment layers almost any desired substrate can be made suitable for alignment. Suitable alignment layers are, for example, polyimides, but also polyvinyl alcohols or gum arabic. In general, the solubility of the polymer forming the alignment layer in the liquid crystal substance is very low. The chemical substances are preferably mechanically pre-structured, although one disadvantage of mechanical pre-structuring is that it is sometimes difficult to achieve regions of different alignment with local resolution. For example, locally resolved alignment may be achieved by photoalignment. For this purpose, a substance is applied as alignment layer, for example by exposing the substance to polarized light (uv light) to obtain a structure supporting alignment in a defined direction of polarization of the uv light. High resolution patterns can be produced by mask exposure and subsequent exposure with different polarizations.

Another technique for locally resolved alignment is to use an imprinted structure. The orientation of the imprinted structure results in a corresponding orientation of the liquid crystal applied thereon. Since in principle any orientation is possible, a gray-scale image can be produced in the security element downstream. However, when only two orientations are selected, an optimal contrast, i.e. a black and white contrast, is obtained.

The above measures can be carried out on the target substrate or on a temporary carrier. Adhesion of the intermediate layer on the target substrate is a great challenge when directly realizing the construction. On the temporary carrier, removability must be ensured. This can be achieved by using a conventional release layer or, for example, using a uv-curable lacquer layer which adheres less strongly to the temporary carrier itself.

The liquid crystal material may be applied by dissolving the liquid crystal in a suitable solvent, such as butyl acetate, butyl propionate, cyclopentanone, THF, MEK, toluene, and mixtures thereof. Such solutions have a low viscosity and can be applied using conventional printing/coating methods, such as flexographic printing, gravure printing, inkjet, nozzle spray, and the like. After physical drying, alignment and crosslinking can be achieved, for example, using ultraviolet light or ESH. Line widths of a minimum of about 80 microns can be printed without problems using conventional printing methods.

Alternatively, it can be carried out without a solvent. Here, the liquid crystal mixture is melted and printing is performed in a melted state. The viscosity can be adapted to the desired printing process by temperature control. Screen printing and flexographic printing are particularly advantageous. In case the chosen printing process does not support an acceptable local resolution, a structured alignment layer has to be used, which enables a different orientation of the liquid crystals in the painted area. If the printing method supports sufficient local resolution, the desired pattern can be printed directly, where a uniform alignment in the printed area is acceptable and appropriate.

Other layers in the finished product are also important for the optical effect. The cast foil is generally optically isotropic and does not interfere with the polarization effect. In case an optically anisotropic layer (e.g. an intumescent foil) is present or scattering (e.g. by pigments or fillers) occurs in the whole viewing area (the surface of the product that is transmitted through the reflective layer/area), this is not detrimental to the overall effect, for example, when the dispersion of the additional optically anisotropic layer in the region of the light wavelengths being observed is not too strong.

The retroreflective layer must be designed to reflect polarized light back when illuminated with polarized light. The polarization of the light may be changed but strong depolarization does not occur and the polarization change that may occur should be substantially uniform over the entire area of the security element. Lenticular and microprismatic structures have proven to be particularly suitable retroreflective layers. High retroreflection and as little polarization disturbance as possible are advantageous.

The security element may extend over the entire area of the data carrier, for example the entire license plate, but may also be used as a strip or patch over a part of this area. When the security element is used as a transfer patch from which the carrier foil is removed in a subsequent step, a plurality of concentric circumferential lines may be punched out to prevent uncontrolled fluttering. Furthermore, further lines and patterns can be punched in the patch, which makes it more difficult to remove them from the subsequent data carrier, but does not hamper the manufacturing process.

Although in the field of banknotes, polarization features based on nematic liquid crystals usually have a polarizing filter placed directly on the security feature and therefore the polarizer and the analyzer are constructed identically (i.e. inevitably identical) and are also present in the same place, the requirements and possibilities for labels and license plates having retroreflective properties are different. For example, when a light source for authentication is provided with a linear polarization filter, it is likely that different polarized light (for example, elliptically polarized light) is returned due to a birefringent layer in the path of the light, and analysis must be performed with an optimum contrast. Thus, in one configuration, the polarizer or analyzer may be provided with an additional birefringent layer to achieve optimal contrast overall.

Another embodiment is to introduce an additional full-area birefringent layer for the security feature to compensate for the birefringent layer already present for technical reasons. This layer may be a fully domain liquid crystal layer of suitable orientation or may for example be a birefringent foil, such as an intumescent foil. This effect can also be exploited for polarization features with less fine structure.

If the birefringent foil is laminated or used as an intermediate layer from which certain characters, patterns, symbols or the like are punched, the pattern can be detected in the same way as a positive pattern, symbol or code.

Another possibility is to destroy the birefringence of the foil present after manufacture by appropriate post-treatment. This can be achieved by brief intense heating (for example with a laser) or by dissolving the foil material (if this can be done by local application of a solvent) and (where applicable) drying/resolidifying, whereby in some areas there is a birefringent expanded foil and in other areas there is a foil similar to a cast foil.

Drawings

Further exemplary embodiments and advantages of the present invention will be described below with reference to the accompanying drawings, which are not drawn to scale for the sake of clarity.

In the drawings:

fig. 1(a) schematically shows the basic principle of checking the authenticity of a counter-reflective security element present on a data carrier, e.g. a license plate, fig. 1(b) shows the colorless and unstructured appearance of the security element under normal illumination conditions, fig. 1(c) shows the appearance of the security element with the text "OK" under polarized light in an analyzer;

fig. 2(a) and 2(b) show two variants of the basic structure of the security element according to the invention;

fig. 3 gives a more detailed explanation of the way in which the security element of fig. 2a works in an exploded view;

FIG. 4 is a cross-sectional view of a first polarization feature;

FIG. 5 is a schematic illustration of another polarization feature shown in FIG. 4;

fig. 6(a) to 6(c) show three configurations of the second polarization feature;

FIG. 7 illustrates further processing of the polarization features of FIG. 6 into stamped structured patches;

FIG. 8 shows the security element of FIG. 7 with a retroreflective layer and a patch; and

fig. 9(a) to 9(c) show a license plate with the security element of fig. 8, with a patch in the form of a shield logo in fig. 9(a), a full-area security foil with a plurality of patches in the form of a shield logo in fig. 9(b), and a full-area security foil with a groove in the form of a shield logo in fig. 9 (c).

Detailed Description

The invention will now be explained in more detail by way of an example of a security element for license plates. However, it is to be understood that the described security element can also be used as a security label for value documents or for marking products.

Fig. 1(a) schematically illustrates the basic principle of checking the authenticity of a retroreflective security element 30 present on a data carrier such as license plate 10, according to an exemplary embodiment of the present invention. For purposes of illustration, the security element 30 is shaded in fig. 1(a), and under normal lighting conditions the security element 30 appears virtually colorless and unstructured, as shown in fig. 1(b), and therefore its presence is not readily discernible.

To check authenticity, the retroreflective security elements 30 of license plate 10 are exposed to polarized light and the light retroreflected by the security elements 30 is observed through an analyzer, as shown in figure 1 (a). For example, user 12 polarizes unpolarized light 14 by means of linear polarizer 16 and exposes license plate 10 to polarized light 18. Due to the retroreflective nature of security element 30, reflected light 20 returns to user 12 within a small retroreflective cone of light and thereby passes again through linear polarizer 16. As explained in more detail below, light 22 after passing through linear polarizer 16 is no longer unstructured, but rather exhibits a desired appearance 32 as proof of authenticity due to previous effects on the polarization state of light in security element 30. For example, under polarized light, the security element 30 may appear with the text "OK" in the analyzer, as shown in fig. 1 (c).

In particular, such authenticity checks may be performed by user 12 from virtually any location, as the retroreflective properties of security element 30 ensure that incident light 18 is always reflected back to user 12.

Fig. 2 shows the basic structure of the security element according to the invention. The mode of operation of the security element is explained in more detail in the exploded view of fig. 3 by way of example of the arrangement of fig. 2 (a).

Referring first to fig. 2(a), a security element 40 of the present invention includes a retroreflective layer 42 and a birefringent layer 44 applied in certain areas in the form of the text "OK". The retroreflective layer 42 is formed, for example, on the basis of a microprismatic structure, while the birefringent layer is, for example, a nematic liquid crystal layer which, due to its layer thickness, acts as a λ/4 layer.

In the variant of fig. 2(B), the birefringent layer 46 is present over the entire area and comprises different areas 48A, 48B with different optical effects, which are configured in the form of the word "OK". For example, region 48A represents the letter of the word "OK" and region 48B represents a complementary background region.

Referring to fig. 2(a) and 3, in this configuration the retroreflective layer 42 of security element 40 is applied only in certain areas (i.e., in the form of the text "OK"), so that there are areas 50 without nematic liquid crystal layer in addition to areas 52 with nematic λ/4 liquid crystal layer 44.

Now, corresponding to the explanation of FIG. 1(a), if unpolarized light 54 emitted by user 12 from a light source is polarized by linear polarizer 16, polarized light 56 in regions 50 where nematic layer 44 is absent impinges on retroreflective layer 42 and is reflected back in the direction of incidence without substantially changing the polarization state of the incident light. Thus, reflected light 58 has the same polarization state as incident light 56 and can pass through linear polarizer 16 unimpeded (reference numeral 60). The area 50 thus appears bright to the user 12 in polarized light.

In the region 52 with the lambda/4 nematic layer 44, the linearly polarized light 56 is converted by the nematic layer into circularly polarized light 62. Circularly polarized light 62 impinges on retroreflective layer 42 and is reflected back by the retroreflective layer in the direction of incidence. The reflected circularly polarized light 64 passes through the lambda/4 nematic layer 44 again and is thereby converted into linearly polarized light 66, but the polarization vector of the linearly polarized light 66 is now perpendicular to the initial polarization. Thus, linearly polarized light 66 cannot pass through linear polarizer 16 (reference numeral 68), and thus region 52 is dark to viewer 12.

Since the aperture cone retro-reflecting is small but in practice limited, the polarizer used to polarize the incident light and the analyzer used to observe the light reflected from the security element can also be slightly separated from each other. For example, the polarizer may be disposed on the front car light of a police car, while the analyzer is located in glasses worn by a police officer sitting in the police car.

If the polarizer and analyzer are spatially separated, they may also be configured differently. For example, the polarizer may be a linear polarizer and the analyzer may be a circular polarizer or a linear polarizer with different polarization vectors.

Examples of specific configurations of the security element of the invention will now be described in more detail with reference to fig. 4 to 9.

First, fig. 4 shows a cross-section of the first polarization feature 70. To produce the first polarizing feature 70, a PET foil 72 of 23 μm thickness is provided, provided with a uv lacquer as a separating layer 73 and a further uv embossing lacquer layer 74. In the embossing lacquer layer 74, the desired hidden pattern with the alignable structures 76 is embossed. Furthermore, the hologram embossing can also be performed in the same processing step. A nematic liquid crystal solution is printed onto the alignable structure 76. After physical drying, the layer thickness of the nematic layer 78 is between 0.8 and 3 microns, preferably about 1.2 microns. During and after physical drying, the liquid crystals are aligned by the alignment structure 76. Subsequently, the liquid crystal is crosslinked (for example by uv exposure), preferably under conditions of reduced oxygen concentration (nitrogen inertization). The arrangement in which the PET foil 72 remains in the finished security element is constructed without the separating layer 73.

Subsequently, a structured or unstructured metal layer, for example aluminum or chromium, can be applied. Structuring can be achieved, for example, by covering part of the area with a washable ink, metallizing it and then removing the washable ink on which the metallization layer is applied. Of course, other structuring methods, such as etching, can also be used.

For further processing, a primer and heat seal lacquer or other adhesive is applied over the polarizing feature 70 and the polarizing feature is applied to the desired target substrate. The manufacturing may also include cutting and/or stamping operations to transfer polarizing features 70 having a desired shape. The application is such that only partial areas of the formed polarization features are transferred, while other partial areas remain on the carrier foil 72. In other configurations, partial areas of the polarizing features may be removed from the carrier foil 72 prior to transfer, and the remaining partial areas may then be completely transferred.

The polarizing feature 80 of fig. 5 is substantially similar in construction to the polarizing feature 70, wherein the ultraviolet imprint lacquer layer 74 in the embodiment of fig. 5 is provided with an imprint layer 82, which imprint layer 82 represents an alignment imprint for aligning the liquid crystals of the nematic layer 78 as well as a holographic imprint. In the local region 84, a metallisation layer 86 is applied to the embossing layer 82, rather than a liquid crystal layer, and the reflection hologram becomes visible when viewed.

Fig. 6(a) shows an exemplary embodiment of the second polarization feature 90. To produce the second polarizing feature 90, a smooth PET foil 92 with good surface quality and 23 micron thickness is provided and printed directly with a liquid crystal solution to form the desired covert pattern, for example in gravure printing. The liquid crystal solution is subsequently dried and crosslinked. More precisely, the printing solution itself is not yet in the liquid crystalline state, but instead the substance contained only goes into the nematic liquid crystalline state during and after physical drying and forms a structured nematic liquid crystal layer 94. For transferring the nematic layer, a transfer auxiliary layer 96 in the form of a uv-curable lacquer layer is provided. The surface thereof can be adjusted so that both the PET foil layer 92 and the liquid crystal 94 can be coated without any problem. If this is not desired in some structures, mechanically forced wetting of the liquid crystals can also be carried out during the crosslinking process or immediately after this process.

The variation of fig. 6(b) is based on the configuration of fig. 6. Further, in the polarizing feature 100 of fig. 6(b), an ultraviolet embossing lacquer 102 is applied over the ultraviolet cured lacquer layer 96, a holographic embossing pattern 104 is embossed, and in some areas a metallization layer 106 is applied. In the polarizing feature 110 of fig. 6(c), an ultraviolet imprint lacquer 102 is also applied over the ultraviolet cured lacquer layer 96, imprinting the holographic imprint pattern 104, and covering the high refractive ultraviolet cured lacquer 112. In this exemplary embodiment, the holographically embossed holographic pattern becomes visible by the difference in refractive index of the lacquer layers 102, 112.

Further processing of the polarization characteristics of fig. 6 can be implemented as with the polarization characteristics of fig. 4 and 5. Fig. 7 shows the structured patch further processed into a stamp. The starting point here is a polarization feature 120 with a carrier foil 122, for example one of the embodiments shown in fig. 4, 5 or 6(a), 6(b) or 6 (c).

A PET foil 124 of approximately 12 microns thickness is laminated with a laminating adhesive 126 over the painted surface of the polarizing feature 120 of fig. 7. On the other side, a support foil 128, also 12 microns thick, is laminated using a laminating adhesive 126. Other layers, such as a primer layer 130 and a suitable heat seal layer 132, are then applied to the foil of the previous finish. The layered composite thus obtained is then punched out from the lacquer side (reference numeral 134) in order to punch out the polarization features 120 with the incorporated liquid crystal layer 78 or 94 and, where applicable, also with the incorporated transfer assist layer 96. Ideally, the stamping ends at the carrier foil 122, but the edge stamping of the carrier foil 122 is not critical, as the support foil 128 prevents further tearing.

The intermediate regions between the patches 136 created in this manner may be peeled off. It is preferred to print the control indicia that may be desired on the other side or to retain such control indicia during the stripping process. Finally, the foil with the layered composite is suitably cut. The geometry of the stamp for this application is not critical, as the adhesive is only present in the area of the patch 132. Only the required units are transferred separately. Removal from the carrier foil 122 may be supported by appropriate adjustment of the peel angle, for example by means of a dispensing wedge.

Fig. 8 shows a security element 140 of the invention having a retroreflective layer 42, the retroreflective layer 42 having in certain areas patches 136 applied thereto via a suitable intermediate layer 142 as shown in fig. 7. The layer sequence 121 of the polarization features 120 is here configured, for example, in a manner similar to fig. 4, i.e. it comprises a nematic layer 78 of approximately 1.2 μm thickness and an ultraviolet embossing lacquer layer 74 for alignment of the liquid crystals. The patch 136 is applied, for example, in the outline of the desired symbol (e.g., a shield logo) or in the outline of the desired word (e.g., the word "OK" shown in fig. 1). After application, a suitable coating 144, such as a protective layer, is applied over the patch.

Under normal lighting conditions, the patch 136 is colorless and unstructured, and only appears when illuminated with polarized light and when reflected light is viewed through a polarizing filter.

To illustrate, fig. 9(a) shows a license plate 150, and in a partial region, a security element 140 as shown in fig. 8 is laminated on the license plate 150, the security element 140 having a patch 136 in the form of a shield logo. Under normal lighting conditions, the shield logo 136 is not visible, but only when the license plate 150 is illuminated with polarized light and when the reflected light is viewed through the polarizing filter. In the exemplary embodiment of fig. 9(a), a conventional holographic patch 152 is also shown, which is also visible under normal lighting conditions.

In the exemplary embodiment of fig. 9(b), a security foil 154 has been laminated over the entire area of license plate 150, which security foil 154 is substantially configured like security element 140 of fig. 8 and carries a plurality of regularly spaced patches 136 in the shape of a shield logo. Fig. 9(c) shows a reverse arrangement in which a security foil 156 of the type described in fig. 8 is laminated over the entire area of the license plate 150, from which security foil 156 shield-shaped symbols 158 have been previously punched.

In both configurations, the male shield logo of fig. 9(b) and the female shield logo-shaped groove of fig. 9(c) are not visible under normal lighting conditions, only when the license plate 150 is illuminated with polarized light and when the reflected light is viewed through the polarizing filter.

List of reference numerals

10 license plate

12 users

14 unpolarized light

16 linear polarizer

18 polarized light

20 reflect light

22 light passing through

30 security element

32 appearance

40 Security element

42 retroreflective layer

44 birefringent layer, nematic layer

46 birefringent layer

48A, 48B region

50,52 area

54 emitted unpolarized light

56 polarized light

58 reflect light

60 light passing through

62 circular polarized light

64 reflected circularly polarized light

66 linearly polarized light

68 blocked light

70 polarization characteristic

72 PET foil

73 separating layer

74 ultraviolet ray impression lacquer layer

76 alignable structure

78 nematic layer

80 polarization characteristic

82 embossing

84 local area

86 metallization

90 polarization characteristic

92 PET foil

94 liquid crystal

96 transfer auxiliary layer

100 polarization characteristic

102 ultraviolet imprint varnish

104 holographic embossing

106 metallization

110 polarization characteristic

112 high-refraction ultraviolet curing paint

120 polarization characteristic

Layer sequence of 121 polarization features

122 carrier foil

124 PET foil

126 laminating adhesive

128 PET foil

130 primer layer

132 Heat seal layer

134 stamping

136 paster

140 security element

142 middle layer

144 coating layer

150 license plate

152 holographic patch

154 antifake foil

156 security foil

158 stamping shield-shaped symbol