Security element with differently coloured sections, valuable document, and production method

Abstract

The invention relates to a security element (12) for protecting valuable objects, comprising a feature region (14) having at least two sections (16, 18) which are visible to the naked eye and which exhibit different colours when viewed from above. According to the invention, it is provided here that the feature region (14) is coated in said sections (16, 18) with a common, shape-following and colour-producing multilayer structure (30). The surface of the feature region (14) is of nanoscale smooth design in a first of said sections (16) and is provided in a second of said sections (18) with a stochastic, flat nanotexture (28) which has a multiplicity of nanostructure elements (54) with an average spacing of between 50 nm and 500 nm. As a result of the effect of the colour-producing multilayer structure (30), the first section (16) exhibits a first colour when viewed from above. In the second section (18), the colour effect of the colour-producing multilayer structure (30) is modified by the stochastic nanotexture (28) such that the second section (18) exhibits a second, different colour when viewed from above due to the combined effect of the colour-producing multilayer structure (30) and the stochastic nanotexture (28).

Description

[0001] Security element with different colored sections, value document and manufacturing process

[0002] The invention relates to a security element for protecting valuables, comprising a feature region with at least two partial regions visible to the naked eye, which display different colors when viewed from above. The invention also relates to a valuable document comprising such a security element and a method for producing such a security element.

[0003] Data storage media, such as valuables or identification documents, but also other valuable items, such as branded goods, are often provided with security elements for security purposes. These elements allow the authenticity of the data storage media to be verified and at the same time serve as protection against unauthorized reproduction.

[0004] Since embossed holograms, which have been used since the late 1980s, are now widespread and no longer offer a high level of counterfeit protection, micro-optical systems, for example based on small micromirrors, are increasingly being used for authentication. Security elements with micro-optical systems are clearly visible even under unfavorable lighting conditions and also allow for the realization of attractive optical effects.

[0005] Due to the achromatic reflection of the mirror elements themselves, micromirror effects usually appear color-neutral or metallically lustrous and are therefore usually combined with color-generating coatings to achieve a color effect. However, if a security element is intended to display various micromirror effects with different color effects, registration fluctuations during the application of the color coatings often result in transition zones in which one or more micromirror effects appear with an undesirable color effect. To avoid using such security elements with a suboptimal visual appearance, they must be laboriously sorted out during quality control.

[0006] Based on this, the invention is based on the object of providing a generic security element with an appealing visual appearance, which, in particular, features precise color matching of different colored areas. The invention also aims to provide a valuable document and a method for producing such a security element.

[0007] This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0008] According to the invention, in a security element of the type mentioned at the outset, it is provided that the feature area in the said partial areas is coated with a common, shape-following and color-producing multi-layer structure.

[0009] The surface of the feature region is nanoscale smooth in a first of the aforementioned subregions. In a second of the aforementioned subregions, the surface of the feature region is provided with a stochastic, flat nanotexturing comprising a plurality of nanostructure elements with an average spacing of between 50 nm and 500 nm. Due to the effect of the color-generating multilayer structure, the first subregion exhibits a first color in plan view, while in the second subregion, the color effect of the color-generating multilayer structure is modified by the stochastic nanotexturing, so that the second subregion exhibits a second, different color in plan view due to the combined effect of the color-generating multilayer structure and the stochastic nanotexturing.

[0010] Shallow nanotexturing is nanotexturing in which the average depth of the nanostructure elements is smaller than the average spacing between the nanostructure elements. Preferably, the average depth of the nanostructure elements is smaller than the average spacing between the nanostructure elements by more than a factor of 2, more than a factor of 3, or even more than a factor of 4.

[0011] The term stochastic nanotexturing refers to the irregular arrangement of nanostructure elements, as opposed to an ordered, regular arrangement, for example, in a lattice. Stochastic nanotexturing can be created, for example, by an etching process, but especially also by electron beam lithography. In the latter case, the stochastic nature of the texturing can be ensured, for example, by using pseudorandom numbers when creating the exposure data set.

[0012] Pseudorandom numbers are sequences of numbers that appear random but are calculated using a deterministic algorithm and are therefore not truly random numbers in the strict sense. Nevertheless, pseudorandom numbers are widely used because the statistical properties of a pseudorandom number distribution, such as the equal probability of individual numbers or the statistical independence of consecutive numbers, are sufficiently irregular for practical purposes, such as the generation of stochastic nanotexturing, and because pseudorandom numbers, unlike true random numbers, are easy to generate with computers.

[0013] In an advantageous embodiment, the color-generating multilayer structure in the second partial region has a nanoscopic variation in the optical density and / or the complex refractive index.

[0014] In a preferred embodiment, the color-generating multilayer structure is formed by a color-shifting thin-film element, in particular by a three-layer color-shifting thin-film element. Color-shifting thin-film elements with an absorber layer / spacer layer / reflector layer structure have proven particularly effective. The absorber layer is advantageously formed by a chromium layer, for example, with a layer thickness of 3 nm to 6 nm. The spacer layer is advantageously formed by a dielectric, in particular a SiCh layer with a thickness between 100 nm and 450 nm, and the reflector layer is advantageously formed by an opaque aluminum layer, for example, with a thickness of approximately 60 nm.

[0015] Color-shifting thin-film elements with a structure consisting of a semi-transparent metal layer / spacer layer / semi-transparent metal layer have also proven successful, exhibiting a color-shift effect even in transmission. Thin aluminum layers with a thickness of approximately 5 nm can advantageously be used as semi-transparent metal layers, but other semi-transparent metal layers made of Al, Cr, Ag, Cu, Fe, Ni, Au, or alloys of these elements with a layer thickness between 3 nm and 20 nm can also be considered. The spacer layer can advantageously be formed by a SiCh layer approximately 240 nm thick, but other dielectrics, such as ZnS, SiCh, MgF, or Al2O3, with a layer thickness between 50 nm and 600 nm, can also be used.

[0016] The multilayer structure can also be formed by a combination of metal layers, high-index layers, dielectrics, and true colors, preferably with 2 to 10 layers. Although the use of a multilayer structure is currently preferred, the feature area in the aforementioned subregions can in principle also be coated with a shape-following, color-generating coating in the form of a metal layer, a high-index layer, a dielectric, or a true color.

[0017] The nanostructure elements of the stochastic nanotexturing preferably exhibit a stochastic distribution in one or more of the following characteristics: spacing, depth, base area, and shape. The nanostructure elements can, in particular, exhibit a stochastic distribution in all of the aforementioned characteristics. The distribution can, for example, be a normal distribution or a Poisson distribution.

[0018] The nanostructure elements can be formed in the form of cylinders, lenses, cuboids, pyramids, stars, any top view shapes, deformed objects, or a combination of these shapes. The nanostructure elements advantageously taper from their base surface to a tip or end face of any shape, thus having a conically tapered shape. The average base surface of the nanostructure elements is preferably between 50 nm and 400 nm, particularly preferably between 100 nm and 350 nm. The base surfaces of the nanostructure elements advantageously cover between 30% and 70% of the area of ​​the second subregion.

[0019] The average spacing of the nanostructure elements is preferably between 100 nm and 350 nm, particularly preferably between 200 nm and 250 nm. The average depth of the nanostructure elements is preferably between 5 nm and 300 nm, particularly preferably between 10 nm and 175 nm. It is understood that the average depth is always smaller than the average spacing of the nanostructure elements in order to obtain a flat nanotexturing.

[0020] The nanostructure elements of the stochastic nanotexturing are advantageously formed in an embossing varnish, in particular a UV embossing varnish.

[0021] In an advantageous embodiment, the first and / or second partial region is formed as a flat region or as a microstructure region, the latter preferably with a plurality of microlenses, micromirrors, a Fresnel structure, and / or a freeform surface. A flat region or vellum region is a region without microstructures, in particular without microlenses, micromirrors, a Fresnel structure, or a freeform surface.

[0022] In this case, one of the two subregions can be designed as a flat region and the other of the two subregions as a microstructured region. In another, equally advantageous variant, both subregions are designed as microstructured regions, in particular with different coatings of microlenses, micromirrors, a Fresnel structure, and / or a freeform surface. The micromirrors in the two subregions are preferably designed to generate different micromirror effects, for example, a motion effect in one subregion and a 3D effect in the other subregion.

[0023] In advantageous embodiments, the micromirrors are designed as flat micromirrors or as micromirrors that have a curved profile in one or two directions, wherein the micromirrors in particular form a periodic or aperiodic sawtooth grating.

[0024] In particular, micromirrors can be used, as described, for example, in the pending German patent applications DE 102024 128848.5 and DE 10 2025 107278.7, the disclosure content of which is incorporated into the present application in this respect. The micromirrors can be designed with a constant inclination in a first direction and a (continuous) curvature in a second direction. Alternatively, the micromirrors can be present with a (continuous) curvature in two directions that are perpendicular to each other. The curved profile can be designed as a convex profile, a concave profile, or a partially convex and partially concave profile. Instead of a curved profile, the micromirrors can also have a varying, partially constant inclination.

[0025] It is understood that the feature region may also contain more than two subregions visible to the naked eye, which exhibit different colors when viewed from above. For this purpose, the surface of different subregions can be provided with different stochastic, flat nanotextures, which lead to different modifications of the color effect of the color-generating multilayer structure. For example, nanotextures with different texture depths can be used, leading to color shifts of varying intensity. If at least one subregion contains microlenses or micromirrors, the lateral dimensions of the micromirrors or microlenses are expediently below 50 μm, advantageously below 20 μm, and particularly preferably approximately 10 μm, i.e., between 7 μm and 13 μm.On the other hand, to avoid color splitting, the lateral dimensions of the micromirrors are advantageously above 2 gm, in particular above 3 gm or even above 5 gm. The pitch of the micromirrors is preferably less than 10 gm, more preferably less than 5 gm.

[0026] In an advantageous embodiment, the different colors of the two sub-regions visible in plan view each have at least one reflection maximum and / or at least one reflection minimum, which is shifted from one another in the different colors of the sub-regions by more than 10 nm, in particular by more than 25 nm. Specifically, for example, the reflection spectrum in the first sub-region can have reflection maxima at 380 nm and 575 nm (first color), while the reflection spectrum in the second sub-region can have reflection maxima at 390 nm and 605 nm (second color). The shift of the first, short-wave reflection maximum is then 10 nm, while the second, long-wave reflection maximum has a shift of 30 nm.

[0027] In an advantageous development of the invention, it is provided that the two aforementioned partial regions also display different colors when viewed through. The different colors of the two partial regions, visible when viewed through, advantageously each have at least one transmission maximum and / or at least one transmission minimum, which are shifted relative to one another in the different colors of the partial regions by more than 10 nm, in particular by more than 25 nm. In an advantageous embodiment, the total reflection of the color-generating multi-layer structure in the second partial region is reduced compared to the first partial region.

[0028] Furthermore, the second sub-range can have a reduced reflectivity compared to the first sub-range, particularly in the blue spectral range below 450 nm. The second sub-range can therefore have a reflectivity that is more significantly reduced in the blue spectral range compared to the first sub-range than in the red spectral range. Specifically, if the reflection spectra of the two colors each have a first, short-wave reflection maximum in the blue spectral range and a second, longer-wave reflection maximum, the short-wave reflection maximum in the second sub-range can be more strongly attenuated compared to the first sub-range than the second, longer-wave reflection maximum.

[0029] The stochastic nanotexturing advantageously does not produce an additional, static reflection peak in the second sub-area when the viewing angle changes. While the stochastic nanotexturing modifies the color effect of the applied multilayer structure, it exhibits no inherent color, or at most a very pale one. A color-shift effect of the multilayer structure thus remains fully functional.

[0030] The invention also includes a value document, in particular a banknote, with a security element of the type described.

[0031] The invention further includes a method for producing a security element, in particular of the type described, in which a feature region is created in the security element, which has at least two partial regions visible to the naked eye, which exhibit different colors in plan view. It is provided that the feature region is coated in said partial regions with a common, shape-following, and color-generating multilayer structure, the surface of the feature region in a first of said partial regions is formed to be smooth on a nanoscale, the surface of the feature region in a second of said partial regions is provided with a stochastic, flat nanotexturing, which has a plurality of upwardly conically tapering nanostructure elements with an average spacing of between 50 nm and 500 nm, the first partial region exhibits a first color in plan view due to the effect of the color-generating multilayer structure,and in the second sub-area, the color effect of the color-generating multilayer structure is modified by the stochastic nanotexturing, so that the second sub-area shows a second, different color in plan view due to the combined effect of the color-generating multilayer structure and the stochastic nanotexturing.

[0032] In a preferred process, the color-generating multilayer structure is applied simultaneously to the aforementioned subregions of the feature area, in particular by printing or vapor deposition. Particularly advantageously, the feature area with the nanoscale smooth surface and the stochastic, flat nanotexturing is created by embossing with an embossing master. The embossing can be carried out, in particular, into an embossing lacquer layer, preferably a UV lacquer. The embossing master is advantageously provided with the stochastic, flat nanotexturing by an etching step or, particularly preferably, by electron beam lithography.

[0033] If at least a partial area contains microlenses or micromirrors, these microstructures are advantageously created together with the stochastic, flat nanotexturing in the same embossing step. The embossing master is advantageously provided with both a microstructuring for creating the microstructures and the stochastic, flat nanotexturing.

[0034] The preferred embodiments and their advantages presented with reference to the respective method according to the invention apply accordingly to the security element according to the invention. The physical components of the security element according to the invention are each designed to carry out the respective steps of the method.

[0035] Further features of the invention emerge from the claims, the figures and the description of the figures.

[0036] Embodiments of the invention are explained in more detail below with reference to schematic drawings. In the figures, identical or functionally equivalent elements are provided with the same reference numerals.

[0037] In the drawings: Fig. 1 shows schematically a banknote with a security element according to the invention,

[0038] Fig. 2 shows schematically a cross-section of the security element of Fig. 1 along the line II-II,

[0039] Fig. 3 in (a) a top view of the nanotexturing of the second sub-area of ​​Fig. 2 measured by atomic force microscopy, in (b) a section of the measured height profile of the nanotexturing along a spatial coordinate and in (c) the power spectral density function in the frequency domain calculated from the measured height profile,

[0040] Fig. 4 the influence of nanotexturing of increasing average texture depth on the reflection spectrum of a color-shifting thin-film element, and

[0041] Fig. 5 in (a) and (b) schematic drawings explaining the nanoscopic variation of the optical density and the complex refractive index of a thin film element due to the influence of stochastic nanotexturing.

[0042] The invention will now be explained using the example of security elements for banknotes. Fig. 1 shows a schematic representation of a banknote 10 with an optically variable security element 12 according to the invention in the form of an adhesive-applied transfer element, and Fig. 2 shows a schematic cross-section of the security element 12 along the line II-II of Fig. 1. It is understood, however, that the invention is not limited to transfer strips and banknotes, but can be used for all types of security elements, for example labels on goods and packaging or for securing documents, ID cards, passports, credit cards, health cards and the like. For banknotes and similar documents, in addition to transfer elements (such as strips or patches with or without their own carrier film), security threads or security strips partially or completely embedded in the document substrate can also be considered.

[0043] The security element 12 contains a feature area 14 with two large, visually visible sub-areas 16, 18, each of which exhibits both a micromirror effect and a color-shift effect when viewed from above. The micromirror effects and the color-shift effects of the two sub-areas 16, 18 differ from one another, but are perfectly aligned with one another in the plane of the feature area 14.

[0044] More precisely, the first partial region 16 shows a first micro-mirror effect, for example a bulge effect, which is combined with a first color-shift effect, for example a color-shift effect from gold in a vertical view 40 to green in an oblique view 42. The second partial region 18 shows a second micro-mirror effect, for example a reflection effect, which is combined with a second color-shift effect, for example a color-shift effect from magenta in a vertical view 40 to yellow-green in an oblique view 42. The change from the first to the second micro-mirror effect takes place at exactly the same location in the feature region 14 as the change from the first to the second color-shift effect, namely at the contact line 15 of the partial regions 16, 18. There are therefore no transition zones in which the first micro-mirror effect would appear with the second color-shift effect or the second micro-mirror effect would appear together with the first color-shift effect.

[0045] The creation of the perfect registration of the micromirror effects and the color shift effects is explained in more detail below with reference to Figures 2 to 5.

[0046] With reference initially to the cross-section in Fig. 2, the security element 12 contains a transparent film carrier 20 which is provided with an embossed micromirror arrangement 22 on one side, namely the underside of the film carrier opposite the viewing direction 40, 42. The inclinations and orientations of the individual micromirrors are adjusted in a manner known per se such that they produce the desired micromirror effects when viewed, i.e. the aforementioned bulging effect in the first partial region 16 and the aforementioned rolling effect in the second partial region 18. In the exemplary embodiment, the micromirrors of the micromirror arrangement 22 are embossed into a transparent embossing lacquer layer and have dimensions of 10 μm x 10 μm and a maximum pitch of 3.5 μm.

[0047] For coloring, the micro-mirror arrangement 22 of the two subregions 16, 18 is coated over its entire surface with a common, shape-following, color-generating multilayer structure 30. In the exemplary embodiment, the multilayer structure 30 is formed by a three-layer, color-shifting thin-film element that, starting from the micro-mirror arrangement, has a thin absorber layer, a spacer layer, and an opaque reflection layer. The absorber layer of the thin-film element is formed, for example, by a 5 nm thick chromium layer, the reflector layer by an approximately 60 nm thick aluminum layer, and the spacer layer by a SiCh layer with a thickness between 100 nm and 450 nm.The thickness of the SiCh layer essentially determines the color effect of the thin-film element through constructive and destructive interference of specific wavelengths of the incident light, so that various color-shift effects can be achieved by appropriately selecting the thickness of the SiCh layer. The carrier film with the coated micromirror arrangement is applied, for example, with an adhesive layer 32 to the banknote paper 34 of the banknote 10.

[0048] In order to be able to produce different color shift effects despite the coating 30 being applied identically in the two partial areas 16, 18, the surface of the two partial areas 16, 18 is designed differently on a nanoscale.

[0049] In the first partial area 16, the surface of the micromirror arrangement 22 has a nanoscale smooth texture 26 - there, the color effect of the applied thin-film element 30 is not changed by the surface texture, so that the first partial area 16 appears with the nominal color shift effect of the thin-film element, in the exemplary embodiment, for example, with a gold-green color shift effect.

[0050] In the second subregion 18, the surface of the micromirror array 22 is provided with a stochastic, flat nanotexturing 28, which, as described in more detail below, changes the color effect of the applied thin-film element 30 and creates a magenta-yellow-green color shift effect shifted toward red. When viewed from a substantially perpendicular perspective, the first subregion 16 therefore appears gold-colored, and the second subregion 18 appears magenta with a clear contrast. Even when viewed at an angle, the two subregions appear different colors, namely the first subregion 16 green and the second subregion 18 yellow-green.

[0051] Since the color-shifting nanotexturing 28 is present only on the micromirrors of the second sub-region 18, while the micromirrors of the first sub-region 16 are nanoscale smooth, the two color-shift effects and the two micromirror effects are perfectly aligned. In the exemplary embodiment, the nanotexturing for this purpose was created by electron beam lithography on an embossing master for the micromirror arrangement 22 and was precisely introduced only into the surfaces of the micromirrors of the second sub-region, while the micromirrors of the first sub-region remained nanoscale smooth. During embossing of the micromirror arrangement 22, nanotextured micromirrors are embossed in the second sub-region 18, while nanoscale smooth micromirrors are embossed in the first sub-region 16.

[0052] For more precise characterization, Fig. 3 in (a) shows a top view 50 of the nanotexturing 28 of the second partial area 18 measured by means of atomic force microscopy. Figure 3(b) shows a section of the measured height profile 52 of the nanotexturing along a spatial coordinate and Fig. 3(c) shows the power spectral density function 56 in the frequency domain calculated from the measured height profile.

[0053] As can be seen from the illustrations in Fig. 3, the nanotexturing 28 is formed from a plurality of nanostructure elements 54 whose base area, spacing, depth, and shape exhibit a stochastic distribution. As best seen in the height profile 52 in Fig. 3(b), the nanostructure elements 54 have a maximum height of h ~ 30 nm, and the root mean square (rms) roughness of the height profile was determined to be r q = 5.4 nm calculated.

[0054] The nanostructure elements 54 have a conical shape tapering upwards (i.e., away from the micromirror surface), with the degree of tapering varying and the tapered elements ending either in a tip or in an arbitrarily shaped end face.

[0055] The average spacing of the nanostructure elements 54 is given by the center of gravity 58 of the spacing distribution, which can be taken, for example, from the power spectral density function 56 in the frequency domain, as shown in Fig. 3(c). In the exemplary embodiment, the average spacing of the nanostructure elements determined in this way is d m = 210 nm.

[0056] Nanotexturing 28 therefore represents a high-frequency and at the same time flat texturing, in which the maximum height h and the square roughness r qof the nanostructure elements are significantly smaller than the average distance d of the nanostructure elements 54. This is also visible in Fig. 3(b), since the height coordinate there is approximately 100 times higher than the position coordinate.

[0057] These flat nanostructures according to the invention differ significantly from known, deep nanostructures, such as those described in the document DE 102004 016 596 A1, in which the depth of the relief structures is greater, preferably even significantly greater, than the width and spacing of the relief structure elements. In contrast to the flat nanotextures now proposed, deep nanostructures do not lead to a color change, but rather to a complete suppression of the color shift effect of a deposited thin-film element.

[0058] In addition to the first observation of a color change due to shallow nanotexturing, it was further discovered that the extent of the color shift can be adjusted over a wide range by varying the height or depth of the nanostructural elements 54. In general, the nanotexturing 28 produces a color shift toward longer wavelengths, i.e., toward the red, and the magnitude of the color shift increases with the average depth of the nanostructural elements 54.

[0059] For further explanation, Fig. 4 illustrates the influence of nanotexturing with increasing average texture depth on the reflection spectrum of a color-shifting thin-film element. Reflection spectrum 60 shows the starting point, namely the reflection spectrum of the thin-film element on a nanoscale smooth surface in a vertical view. In particular, the spectrum contains a narrower, short-wave reflection peak 62 and a broader, long-wave reflection peak 64.

[0060] Spectra 70, 72, and 74 are reflection spectra of the same thin-film element on stochastically nanotextured surfaces with increasingly greater texture depth. As can be seen, both the short-wave reflection peak 62 and the long-wave reflection peak 64 shift to longer wavelengths with increasing texture depth, thereby changing the color effect of the thin-film element. Color shifts of reflection peaks 62 and 64 of more than 10 nm, more than 25 nm, or even more than 50 nm are achieved. In extreme cases, color shifts of up to 350 nm could be generated with flat, stochastic nanotexturing. Although Figure 4 only shows the spectra from a vertical view, completely analogous shifts of the reflection peaks also occur when the thin-film element is viewed at an angle, so that the color effect is modified accordingly by the nanotexturing.

[0061] According to current understanding, the modification of the color effect of the color-generating multilayer structure by the nanotexturing is based on a nanoscopic variation of the optical density or the complex refractive index of the shape-following multilayer structure.

[0062] For illustration, Fig. 5 (a) shows a three-layer thin-film element 80 with a chromium absorber layer 82, a SiCh dielectric layer 84, and an aluminum reflection layer 86, which is applied to a nanoscale smooth embossing lacquer layer 88. The curve 90 of the refractive index n shows a sharp, abrupt change 92 at the boundaries between the various sublayers.

[0063] If the layer sequence 82, 84, 86 of such a thin-film element 80 is applied to an embossing lacquer layer 94 with a nanotextured surface, the situation shown schematically in Fig. 5(b) results. The layers 82, 84, 86 follow the shape predetermined by the nanotexturing and are thus vertically offset from one another in a nanoscale aperiodic manner by the height or depth of the nanostructure elements 54. Furthermore, the nanotexturing can also alter the layer growth of the chromium absorber layer 82 on the embossing lacquer layer 94; for example, the nanoscale dislocations can act as nucleation nuclei for island growth of the chromium layer. As a result, the nanotexturing changes the profile 96 of the refractive index n of the thin-film element 80, namely, it is averaged in the vertical direction on the scale of the average spacing of the nanostructure elements 54 and thus, in a sense, "smeared" at high frequency.As a result, sharp jumps in the refractive index no longer occur at the layer boundaries, but rather soft, gradual transition regions 98 result. These soft transition regions lead to a change in the effective refractive index of the chromium absorber layer 82 and the effective optical path length n*d in the SiCh layer 84. Since the color effect of the thin-film element is based on the constructive or destructive interference of certain wavelengths of the incident light, the interference conditions changed by the nanotexturing lead to a shift in the maxima and minima of the reflection spectrum of the thin-film elements.

[0064] In addition to the color shift, a decrease in the short-wavelength reflection peak 62 can usually be observed with increasing depth of the nanotexturing (see Fig. 4), which is presumably due to Rayleigh scattering at the interfaces of neighboring nanostructure elements 54. Rayleigh scattering exhibits a strongly increasing scattering intensity towards shorter wavelengths, resulting in blue light being scattered more strongly than red light and thus being removed from the specular reflection spectrum.

[0065] Finally, it should be emphasized that while the shallow nanotexturing of the surface modifies the color effect of an applied multilayer structure, for example, a thin-film element, the nanotexturing itself has no inherent color, or at most, a very pale color. This means, in particular, that the nanotexturing does not generate any additional static reflection peaks in the reflection spectrum—that is, reflection peaks whose spectral position remains unchanged when the viewing angle changes. A color-shift effect of the multilayer structure therefore remains fully functional even in the presence of nanotexturing, i.e., just like the original color-shift effect, it changes color when the viewing angle changes.

Claims

P a t e n t a n s p r ü c h e 1. A security element for protecting valuables, comprising a feature region with at least two partial regions visible to the naked eye, which display different colors when viewed from above, characterized in that the feature region is coated in said partial regions with a common, shape-following, and color-generating multilayer structure, the surface of the feature region in a first of said partial regions is nanoscale smooth, the surface of the feature region in a second of said partial regions is provided with a stochastic, flat nanotexturing comprising a plurality of nanostructure elements with an average spacing of between 50 nm and 500 nm, the first partial region displays a first color when viewed from above due to the effect of the color-generating multilayer structure, and in the second partial region, the color effect of the color-generating multilayer structure is modified by the stochastic nanotexturing,so that the second sub-area shows a second, different color in plan view due to the combined effect of the color-generating multilayer structure and the stochastic nanotexturing.

2. Security element according to claim 1, characterized in that the color-generating multilayer structure in the second partial region has a nanoscopic variation in the optical density and / or the complex refractive index.

3. Security element according to claim 1 or 2, characterized in that the color-generating multi-layer structure is formed by a color-shifting thin-film element, preferably a three-layer color-shifting thin-film element, in particular with an absorber layer / dielectric layer / reflector layer structure or a semi-transparent metal layer / dielectric layer / semi-transparent metal layer structure.

4. Security element according to at least one of claims 1 to 3, characterized in that the nano structural elements of the stochastic nanotexturing have a stochastic distribution in one or more of the characteristics of distance, depth, base area and shape.

5. Security element according to at least one of claims 1 to 4, characterized in that the nano structural elements have an average distance between 100 nm and 350 nm, advantageously between 200 nm and 250 nm.

6. Security element according to at least one of claims 1 to 5, characterized in that the nanostructure elements have an average depth between 5 nm and 300 nm, advantageously between 10 nm and 175 nm.

7. Security element according to at least one of claims 1 to 6, characterized in that the first and / or second partial area is designed as a flat area or as a microstructure area, the latter preferably with a plurality of microlenses, micromirrors, a Fresnel structure and / or a freeform surface, and wherein preferably one of the two partial areas is designed as a flat area and the other of the two partial areas is designed as a microstructure area, or wherein both partial areas are designed as a microstructure area with different coverage with microlenses, micromirrors, a Fresnel structure and / or a freeform surface.

8. Security element according to claim 7, characterized in that the micromirrors are designed as flat micromirrors or as micromirrors which have a curved profile in one or two directions, wherein the micromirrors in particular form a periodic or aperiodic sawtooth grating.

9. Security element according to at least one of claims 1 to 8, characterized in that the different colors of the two partial areas visible in plan view each have at least one reflection maximum and / or at least one reflection minimum which is shifted from one another in the different colors of the partial areas by more than 10 nm, in particular by more than 25 nm.

10. Security element according to at least one of claims 1 to 9, characterized in that the two said partial areas also show different colors when viewed through, preferably in that the different colors of the two partial areas visible when viewed through each have at least one transmission maximum and / or at least one have a transmission minimum that is shifted by more than 10 nm, in particular by more than 25 nm, in the different colors of the sub-areas.

11. Security element according to at least one of claims 1 to 10, characterized in that the second partial region has a reduced reflectivity in the blue spectral range below 450 nm compared to the first partial region.

12. Security element according to at least one of claims 1 to 11, characterized in that the stochastic nanotexturing in the second partial region does not produce an additional static reflection peak when the viewing angle changes.

13. A method for producing a security element, in particular according to one of claims 1 to 12, in which a feature region with at least two partial regions visible to the naked eye, which show different colors in plan view, is produced in the security element, wherein the feature region is coated in said partial regions with a common, shape-following and color-generating multi-layer structure, the surface of the feature region in a first of said partial regions is formed to be nanoscale smooth, the surface of the feature region in a second of said partial regions is provided with a stochastic, flat nanotexturing which has a plurality of upwardly tapering Nanostructure elements with an average distance between 50 nm and 500 nm, the first partial area shows a first color in plan view due to the effect of the color-generating multilayer structure, and in the second partial area the color effect of the color-generating multilayer structure is modified by the stochastic nanotexturing, so that the second partial area shows a second, different color in plan view due to the combined effect of the color-generating multilayer structure and the stochastic nanotexturing.

14. Method according to claim 13, characterized in that the color-generating multi-layer structure is applied simultaneously to the said partial areas of the feature area, in particular by printing or vapor deposition.

15. The method according to claim 13 or 14, characterized in that the feature region with the nanoscale smooth surface and the stochastic, flat nanotexturing is produced by embossing with an embossing master.

16. The method according to claim 15, characterized in that the embossing master is provided with the stochastic, flat nanotexturing by an etching step or preferably by electron beam lithography.

Images