The invention relates to an optically variable see-through security element for securing value objects, with a flat, optically variable area pattern which in transmission shows a colored appearance with a viewing-angle-dependent, polychrome color change.
Data carriers, such as value documents or identification documents, but also other value objects, such as branded articles, are frequently provided for securing purposes with security elements which permit a check of the authenticity of the data carrier and which at the same time serve as protection against unauthorized reproduction. Here, see-through security features, such as see-through windows in banknotes, are becoming increasingly attractive.
Conventional transparent or semitransparent security elements with a viewing-angle-dependent, polychrome color change in transmitted light have various disadvantages, however. Thus, it is known for example to produce diffraction colors in transmitted light with transparently or semitransparently coated hologram gratings or transmission gratings, wherein it can be achieved by suitable choice of the grating periods and the azimuth angles of the gratings that different representations with changing colors emerge at different viewing angles. The appearance of such grating images, however, strongly depends on the lighting conditions. When illuminated with a point light source, individual subregions can flash very brightly and disappear quickly again at certain angles, while in diffuse ambient light only a very weak or possibly even no diffraction effect may be visible. Also, the perceived color does not only depend on the viewing angle to the security element, but also on the direction to the light source, wherein in addition a corresponding security element must not be held directly in front of a light source for viewing the diffraction colors of the first order, but the security element must be held somewhat out of the direct connecting line. Further, upon tilting the security element all rainbow colors are run through, so that the color changes occurring are largely undefined and the observed color effects are frequently perceived as simply colorful by the untrained viewer. Finally, holographic techniques have become common also outside the security sector and therefore now offer only a limited protection against imitation.
In a different solution, colors are produced with thin film systems through interference in incident light and in transmitted light, which colors change in dependence on the viewing angle. Different colors are therein usually realized by a variation of the layer thicknesses, for example the thickness of a dielectric spacer layer in a three-layer structure of absorber/dielectric/absorber. The adjustment of a desired color by adjusting the layer thicknesses is technologically very elaborate, however. One possibility is the regional printing of one or a plurality of dielectric layers, however very high demands are placed on the uniformity of the printed layers and the lateral resolution is limited to the resolution attainable by the corresponding printing methods. Moreover, motif changes upon tilting can practically not be implemented with such thin film systems.
A further solution is to produce colors in incident light and in transmitted light with transparently or semitransparently coated subwavelength structures, which colors change upon tilting the structures. Such subwavelength structures are very challenging to produce and difficult to manufacture on the required industrial scale, however.
Proceeding therefrom, it is the object of the present invention to specify a see-through security element of the type mentioned at the outset that avoids the disadvantages of the state of the art. In particular, the see-through security element is to combine an appealing visual appearance with high falsification security, and ideally be manufacturable on the industrial scale required in the security sector.
According to the invention, in a generic optically variable see-through security element it is provided that
Since the inclination angle and azimuth angle in the above-mentioned subregions of the optically variable area pattern are equal in each case for all facets, the subregions each represent exactly the regions of identically oriented facets.
In an advantageous embodiment, the facets of a subregion do not only have the same orientation, but also the same shape and size. The area occupied by each subregion on the optically variable area pattern in advantageous embodiments is at least 50 times, preferably at least 100 times, particularly preferably at least 1000 times greater than the area occupied on average by one individual facet of said areal region. The subregions thus usually include a very large number of individual facets.
In an advantageous embodiment, the facets of the at least two subregions differ from each other with respect to the inclination angle relative to the plane by 5° or more, preferably by 10° or more, particularly preferably by 20° or more. Alternatively or additionally, the facets of the at least two subregions differ from each other with respect to the azimuth angle in the plane by 45° or more, preferably by 90° or more, in particular by 180°.
The facets of the area pattern are preferably formed by flat area pieces that are respectively distinguished by their shape, size and orientation. The orientation of a facet is specified by the inclination a relative to the plane of the area pattern and by an azimuth angle θ in the plane of the area pattern. The azimuth angle θ therein is the angle between the projection of the normal vector of the facet to the plane of the area pattern and a reference direction in the plane. Since the azimuth angle θ depends on the choice of the reference direction its absolute value is not important, but the difference of the azimuth angles of different subregions all the more, since it describes the different relative orientation of the facets in the associated subregions. In principle it is also possible, although presently not preferred, to provide curved facets. Also in the case of these curved facets, the orientation can be specified by a normal vector averaged over their area and thus by an averaged inclination angle α and an averaged azimuth angle θ.
The dimension of the facets is preferably so large that little or no diffraction effects occur, so that the facets act in a substantially ray-optical manner only. In particular, the facets advantageously have a smallest dimension of more than 2 μm, preferably of more than 5 μm, in particular of more than 10 μm. In particular for application in banknotes and other value documents, the facets preferably have a height below 100 μm, preferably below 50 μm, in particular of less than 10 μm. The facets can be arranged regularly, for example in the form of a one- or two-dimensional periodical grid, such as a sawtooth grating, or also aperiodically.
A further possibility to suppress unwanted diffraction effects is to mutually offset the facets aperiodically in their height above the area region. When the facets are offset aperiodically, there is no simple, regular connection between the heights of adjacent facets, so that a constructive interference of light reflected at adjacent facets and thus the emergence of a superimposed diffraction pattern can be prevented reliably. Details of such an aperiodic offsetting can be gathered from the publication WO 2012/055506 A1, the disclosure of which is incorporated in the present application in this respect.
As interference layer in principle all coatings come into question which show a viewing-angle-dependent color change in transmitted light. A first example of an advantageous interference layer is a thin film element with semitransparent metal layers and a dielectric spacer layer, in particular with a structure of absorber/dielectric/absorber, wherein for example metals such as Ag, Au, Cr or Al can be used as absorber layers and SiO2, MgF2, or polymers can be used as dielectric layer. Also dielectric layer systems, in particular multilayer systems, can be considered as interference layer, in particular layer structures with at least one highly refractive layer, such as TiO2 or ZnS, preferably combined with at least one lowly refractive layer, such as SiO2 or MgF2. The thin film element can also include semiconductive layers, such as Si, for example a thin film structure of the layer sequence Si/SiO2/Si can be employed. As dielectric spacer layers, also polymers can be used here for example instead of oxides. Finally, also liquid-crystalline layers, especially with color-changing cholesteric liquid crystals, can be used as interference layer.
The entire optically variable area pattern is advantageously supplied with the same interference layer which is applied simultaneously to all facets. The interference layer can be structured after application by subsequent process steps to produce interference-layer-free regions. The interference layer can also have a locally different thickness depending on the inclination of the facets, as explained in more detail below.
In one advantageous embodiment, the interference layer has a layer thickness which is not substantially dependent on the inclination angle of the coated facets. Such a substantially constant layer thickness can be achieved for example by undirected coating methods or results from a coating with cholesteric liquid crystals in the form of a constant spacing of the planes with the same refractive index.
In a further, particularly advantageous embodiment, the facets are supplied with an interference layer the layer thickness of which varies with the inclination angle α of the facets, in particular decreases with an increasing inclination angle α. The present inventors have surprisingly found that such an interference layer makes it possible to produce particularly strong color differences between facets of different inclination. Thereby, on the one hand a particularly wide range of colors for the color appearances is available, which even allows the production of true-color images, on the other hand strongly pronounced color changes upon tilting the area patterns can be realized in this fashion. Such a varying layer thickness of the interference layer can be achieved for example by directed coating processes, such as vacuum vapor deposition. In such methods, the inclination angle of the facets leads to an enlargement of the effective surface, so that on inclined facets less material is deposited per area unit and the resulting layer thickness is thus strongly dependent on the inclination angle of the facets.
The facets are advantageously embossed into an embossing lacquer layer having a first refractive index. Above the interference layer a lacquer layer with a second refractive index is applied, which differs from the first refractive index of the embossing lacquer layer by less than 0.3, particularly less than 0.1. Through this substantially equal refractive index of the two lacquer layers, incident light passes through the security element independently of the local inclination angle α of the facets substantially without direction deflection, and thus ensures a uniform brightness distribution in the plane of the area pattern.
In an advantageous embodiment, the at least two subregions are arranged in the form of a motif, wherein the optically variable area pattern shows the motif formed by the subregions in transmission with two or more different colors, at least in certain tilted positions of the security element. For this purpose the inclination angles α and the azimuth angles θ of the facets and the interference layer in the two subregions are advantageously mutually coordinated such that the subregions show the same colors in one certain tilted position and different colors in different tilted positions. Overall, the security element then shows a motif which, upon tilting, emerges from an area of homogeneous apparition or disappears into an area of homogeneous apparition.
Since the full color effect of the coated facets depends not only on their orientation, but also on the properties of the specifically chosen interference layer, both the inclination angles α of the facets, the azimuth angles θ of the facets and the interference layer must be mutually coordinated in the subregions such that the desired color effect is achieved.
In an advantageous further development, the optically variable area pattern includes at least three subregions which are arranged in the form of a background region and of two foreground regions and in which the inclination angles α and the azimuth angles θ of the facets and the interference layer are mutually coordinated such that the optically variable area pattern in transmission
Advantageously, the optically variable area pattern in a further development includes at least four subregions which are arranged in the form of a background region, of two foreground regions and one overlap region, and in which the inclination angles α and the azimuth angles θ of the facets and the interference layer are mutually coordinated such that the optically variable area pattern in transmission
In all configurations, the optically variable area pattern advantageously includes at least two subregions in which the facets have the same inclination angle α, but azimuth angles θ which differ from each other by 180°. The inclination angles α are advantageously larger than 5°, particularly preferably larger than 10°, and for example amount to 15°, 20° or 25°. As explained in more detail below, in this fashion a tilt image can be realized with a motif tilting out from a homogeneous area or tilting into a homogeneous area.
When the optically variable area pattern includes at least four subregions, it is advantageously provided that the optically variable area pattern includes a first and second subregion in which the facets have the same inclination angle α0, but azimuth angles θ differing from each other by 180°, and further includes a third and fourth subregion in which the facets have different inclination angles α1 and α2 and in which the azimuth angle θ differs from the azimuth angle of the first and second subregion by 90° or 270°. The inclination angles α0 are advantageously larger than 5°, particularly preferably larger than 10°, and for example amount to 15°, 20° or 25°. As explained in more detail below, in this fashion a tilt image with two different motifs can be realized in a particularly easy way.
In principle, tilt images can be realized with two different, also overlapping motifs already with an optically variable area pattern with only three subregions. However, in the case of at least partially overlapping motifs this usually requires a nesting of the subregions assigned to the motifs in which, as described in more detail below, the area pattern is divided into narrow strips or small pixels.
In an advantageous further development, the optically variable area pattern includes at least three subregions in which the inclination angles α and the azimuth angles θ of the facets and the interference layer are mutually coordinated such that the subregions appear in a tilted position in transmission in red, green, or blue. Preferably, these colors are produced in the non-tilted security element, thus when viewed perpendicularly in transmission. In an advantageous further development, the optically variable area pattern can additionally have in the subregions a black mask placed in register with the inclined facets, said black mask serving to adjust the brightness in transmission of the facets in the respective subregions. The three subregions can, optionally together with the black mask placed in register, each represent the color separations of a true-color image advantageously. In this fashion, true-color images can be represented which appear realistic in transmission in the chosen tilted position.
The invention also includes a data carrier with a see-through security element of the type described, wherein the see-through security element is preferably arranged in or above a window region or a through opening of the data carrier. The data carrier can in particular be a value document, such as a banknote, in particular a paper banknote, a polymer banknote or a foil composite banknote, but also an identification card, such as a credit card, a bank card, a cash card, an authorization card, a national identity card or a passport personalization sheet.
The invention further includes a method for manufacturing an optically variable see-through security element in which a substrate is made available and the substrate is supplied with a flat, optically variable area pattern which in transmission shows a colored appearance with a viewing-angle-dependent, polychrome color change. According to the invention, the optically variable area pattern is produced with a multiplicity of facets which act in a substantially ray-optical manner, the orientation of which is distinguished in each case by an inclination angle α relative to the plane of the area pattern, which lies between 0° and 45°, and by an azimuth angle θ in the plane of the area pattern, the facets are supplied with an interference layer with a viewing-angle-dependent color change in transmitted light, and the optically variable area pattern is produced with at least two subregions, respectively having a multiplicity of identically oriented facets, wherein the facets of the at least two subregions differ from each other with respect to the inclination angle relative to the plane and/or with respect to the azimuth angle in the plane.
In an advantageous process variant, the facets are coated with the interference layer in a directed coating method, particularly in a vacuum vapor deposit method.
Further exemplary embodiments as well as advantages of the invention will be explained hereinafter with reference to the figures, in the representation of which a rendition that is true to scale and to proportion has been dispensed with in order to increase the clearness.
There are shown:
The invention will now be explained by the example of security elements for banknotes.
In the embodiment example of
The security element 12 has a flat, optically variable area pattern which includes a multiplicity of facets 32 which act in a substantially ray-optical manner. The facets 32 are formed by flat area pieces and are respectively distinguished by their shape, size and orientation. As already explained generally above, the orientation of a facet 32 is specified by the inclination α relative to the plane 30 of the area region and by an azimuth angle θ in the plane 30, wherein the azimuth angle θ is the angle between the projection of the normal vector 46, 48 of a facet 32 to the plane 30 and a reference direction Ref.
As shown in
The facets 32 of the area pattern are embossed into a preferably transparent embossing lacquer 34 and have a square outline with a dimension of 20 μm×20 μm in the embodiment example. The facets 32 are further supplied with a nearly transparent or at least semitransparent interference coating 36, which produces a viewing-angle-dependent color impression in transmission.
The interference coating 36 can for example be formed of a three-layer thin film structure with two metallic semitransparent layers, for example of aluminum, silver, chromium, gold or copper, and an interposed dielectric spacer layer, for example of SiO2, MgF2 or a polymer. In the embodiments examples first described the thickness of the interference coating 36 is independent of the inclination angle α of the facets 32.
Above the interference coating 36 a further lacquer layer 38 is applied, which has substantially the same refractive index as the lacquer layer 34, which ensures that incident light passes through the layer sequence of the security element 12 independently of the local inclination angle α of the facets 32 substantially without direction deflection, thus producing a uniform brightness distribution in the plane of the area pattern.
The interference coating 36 of the facets produces a color impression in transmitted light which depends both on the direction of incidence of the light relative to the plane normal of the optically variable area pattern and the individual inclination angle of the facets 32, since both factors influence the angle of incidence of the light with reference to the normal of the interference coating 36.
When in the embodiment of
In
In the subregion 16, the angle between the incident light 40 and the interference layer normal is 46 is reduced by the tilting to the right by ß=20°, so that the light 40 now incides perpendicularly on the interference layer 36 there (ϕ=0°). As can be gathered from
Upon tilting by 20° to the left, the conditions are reversed correspondingly, so that then the light 40 incides perpendicularly on the interference layer 36 in the subregion 18, producing a red color in transmission there, while it incides at an angle of ϕ=40° on the interference layer 36 in the subregion 16, producing a green color in transmission.
The monochrome homogeneous color impression at perpendicular light incidence in
A security element 60 according to the invention can also show a tilt image in which different motifs are visible in different tilted positions, as explained now with reference to
The interference layer 36 in this embodiment example is chosen so that it produces an orange color in transmission at perpendicular light incidence (ϕ=0°), a yellow color in transmission at light incidence at ϕ=10°, a green color in transmission at light incidence at ϕ=20° and a blue color in transmission at light incidence at ϕ=30°.
In the non-tilted position of
In the position of
Conversely, in the position of
In the embodiment examples of
In the embodiment example of
However, the nesting of overlapping representations with three subregions having different facet orientations usually leads to the chromaticity and/or the contrast of the colors in transmission not reaching the maximally possible values, since partly only mixed colors can be produced due to the nesting, and mixed colors usually have a lower chromaticity than the original colors.
Very high-contrast and colorful images can be realized, however, by employing four subregions realize with different facet orientations, as shown in
In the security element 80, the optically variable area pattern is divided into four subregions 82, 84, 86, 88, which are arranged in the form of a background region 82, a first foreground region 84 (square without circular segment 88), a second foreground region 86 (circular disk without circular segment 88) and an overlap region 88 (circular segment). The first foreground region 84 together with the circle segment 88 forms the complete square as the first motif to be represented, the second foreground region 86 together with the circular segment 88 forms the complete circular disk as the second motif to be represented. Although the two motifs to be represented overlap in the overlap region 88, their color in transmission is not to arise from color mixing.
The inclinations and azimuth angles of the facets in the four subregions for this purpose are chosen so that the security element 80 in a first tilted position in transmitted light shows the complete square (first foreground region 84 and circular segment 88 together) as the first motif to be represented with a uniform motif color, and shows the remaining area pattern (second foreground region 86 and background region 82) in a background color different from the motif color. In a second tilted position, the security element 80 in transmitted light shows the complete circle (second foreground region 86 and circular segment 88 together) as the second motif to be represented with the uniform motif color, whereas the remaining area pattern (first foreground region 84 and background region 82) appears with the background color.
To achieve this, the inclination and the azimuth angle of the facets in the background region 82 are thus chosen such that they produce the background color in each case, in both the first and in the second tilted position. The inclination and the azimuth angle of the facets in the first foreground region 84 are chosen so that they produce the motif color in the first tilted position and the background color in the second tilted position, while the facets in the second foreground region 86 are chosen so that they produce the background color in the first tilted position and the motif color in the second tilted position. In the overlap region 88 finally the inclination and azimuth angle of the facets are chosen so that they produce the motif color in each case, in both the first and the second tilted position. Altogether, four subregions with different orientations of the facets are thus required.
The required inclinations and azimuth angles in the various subregions can be ascertained for example by the following procedure, wherein it is presumed specifically that the first tilted position is caused by a tilting 90-O of the security element 80 by a certain angle from the horizontal upwards, while the second tilted position 80 is caused by a downward tilting 90-U of the security element by the same angle.
First, for the facets of the first and second foreground region 84, 86, the azimuth angle in the tilting direction 90-O, 90-U is determined, thus at θ=270° or θ=90° with reference to the reference direction Ref shown in the figure. As inclination angle α that angle is determined for both foreground regions which produces the desired motif color in the first and second tilted position upon an upward or downward inclination of the mirrors. This corresponds substantially to the procedure already described in connection with
Similar to
Further, it was ascertained in a series of experiments at which inclination angles the facets coated with the chosen interference coating show the motif color or the background color in the first tilted position at an azimuth angle of 0° or 180°. These inclination angles generally depend on the type of interference coating, the dependence of the interference layer thickness on the inclination angle of the facets and the refractive indices of the embedded lacquer layers, but can be readily ascertained by a simple series of experiments. For example, the result is that the facets show the motif color in the first tilted position show at an azimuth angle of 0° and an inclination angle αM and the background color at an inclination angle αH. Due to the symmetry of the arrangement it is then ensured that the facets show these colors also in the second tilted position, since said position is reached by tilting the security element by the same the same angular amount as the first tilted position.
The facets in the overlap region 88 are then formed with an inclination angle α=αM and an azimuth angle of θ=0° or θ=180°, while the facets in the background region 82 are formed with a inclination angle α=αH and an azimuth angle of θ=0° or θ=180°. The associated projected normal vectors 98 and 92 are drawn for θ=0° in
In the embodiments described so far, the thickness of the interference coating was independent of the inclination angle of the facets. Particularly strong color differences can be produced, however, when a coating method is chosen for applying the interference coating in which the achieved layer thickness depends on the inclination of the facets. This can be achieved by subjecting the facets to directed vacuum vapor deposition, for example, wherein there results a layer thickness by vertical vapor deposition that is substantially proportional to the cosine of the inclination angle α, i.e.
d=d0 cos α
with the nominal film thickness do which is obtained in non-inclined facets. As the inventors have surprisingly found, the color differences between differently inclined facets shown in
As shown by a comparison of
The embodiment examples described above can be realized not only with an interference coating of constant thickness, but advantageously also with an interference coating of inclination-dependent thickness, whereby it is possible to produce tilt images with particularly strong color contrasts, for example.
It is particularly noteworthy and surprising in this context that there are certain layer thicknesses in some interference layer systems in which the primary colors red, green and blue can be produced as colors in transmission with one and the same interference coating depending on the inclination angle of the facets. In the layer system shown in
In this fashion, true-color images can be produced in transmission by suitably arranging small red, green and blue color regions, since any desired color can be represented as an additive color mixture of these three primary colors. For this purpose the subregions are formed for example in the form of small pixels or strips like in a conventional RGB display.
To be able to produce realistic true-color images, it has to be possible to adjust the brightness of the color regions in the individual pixels in targeted fashion. For this purpose, the color regions of individual pixels can be printed over in black or covered with an opaque metallization, wherein the technological challenge consists in the arrangement of the overprint or the coating in exact register.
Specifically, an optically variable area pattern for representing a true-color image can be manufactured with a black mask in exact register in the fashion described with reference to
With reference to
Subsequently, the embossed lacquer layer 34, as shown in
Then, as shown in
Now, a blackened photoresist 118 is applied to the opposite side of the area pattern, as shown in
In another method variant, in the step of
In principle, the black mask can also be produced by other methods, however, for example by metal transfer methods, etching methods or also directly or indirectly by laser ablation controlled by embossed structures.
Number | Date | Country | Kind |
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10 2014 019 088 | Dec 2014 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2015/002414 | 12/1/2015 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2016/096094 | 6/23/2016 | WO | A |
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Number | Date | Country | |
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20180037049 A1 | Feb 2018 | US |