Optically variable security element comprising a colour-shifting interference layer element, item of value, and manufacturing method

Abstract

The invention relates to an optically variable security element (12) for securing security papers, value documents, and other items of value, said security element comprising at least one colour-shifting, multilayer interference layer element (22) having a reflector layer (28), at least one absorber layer (24), and at least one dielectric spacer layer (26) that is positioned between the reflector layer (28) and the at least one absorber layer (24), wherein the multilayer interference layer element (22), owing to its layered structure, produces an intrinsic colour-shifting effect with a first colour when viewed from a first viewing direction and a different second colour when viewed from a second viewing direction. According to the invention, a colour-selective scattering layer (30, 32) is arranged above and / or below the at least one multilayer interference layer element (22), said scattering layer having nanoscale variations in refractive index produced by cracks (34) and / or air inclusions, and, owing to its colour-selective scattering, modifying the intrinsic colour-shifting effect of the at least one multilayer interference layer element (22) such that the security element exhibits a colour-shifting effect between a third colour when viewed from the first viewing direction (40) and a different fourth colour when viewed from the second viewing direction (42), wherein the third colour differs from the first colour in hue and / or colour purity, and / or the fourth colour differs from the second colour in hue and / or colour purity.

Description

[0001] Optically variable security element with color-shifting

[0002] Interference layer element, valuable object and manufacturing process

[0003] The invention relates to an optically variable security element for protecting security papers, valuable documents, and other valuables, comprising at least one color-shifting, multilayer interference layer element. The invention also relates to a method for producing such a security element and a correspondingly equipped valuable object.

[0004] Valuable items, such as security paper, valuables, or identification documents, are often provided with security elements for protection. These allow the authenticity of the valuable item to be verified and also serve as protection against unauthorized reproduction. Security elements with viewing-angle-dependent effects play a special role in authenticity assurance, as these cannot be reproduced even with the most modern copying machines. These security elements are equipped with optically variable elements that convey a different image impression to the viewer at different viewing angles, for example, displaying a different color or brightness impression and / or a different graphic motif depending on the viewing angle.

[0005] Security elements with multilayer thin-film elements are often used, whose color impression changes for the viewer with the viewing angle (hereinafter referred to as the color shift effect). The layer primarily responsible for the color effect is a thin dielectric layer, typically arranged between an absorber layer and a reflective layer. These thin-film elements represent interference layer elements, in which the colors generated in reflection are produced by interference of the light rays reflected from various sublayers of the element. Therefore, such interference layer elements can only produce specific color changes compatible with the physical interference conditions.In some cases, the colors produced are also created by mixing spectral colors, for example red and blue, and then often have a dark and unattractive appearance due to the lack of color purity.

[0006] Based on this, the invention is based on the object of specifying a generic security element which avoids the disadvantages of the prior art and which in particular enables the generation of color shift effects with novel color changes or with color changes between colors of high color purity.

[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] The invention contains an optically variable security element for protecting security papers, valuable documents and other valuables.

[0009] The security element contains at least one color-shifting, multilayer interference layer element with a reflector layer, at least one absorber layer, and at least one dielectric spacer layer arranged between the reflector layer and the at least one absorber layer. The multilayer interference layer element, with its layer structure, generates an intrinsic color-shift effect with a first color in a first viewing direction and a different second color in a second viewing direction.

[0010] A color-selective scattering layer is arranged above and / or below the at least one multilayer interference layer element. This layer has nanoscale refractive index variations generated by cracks and / or air inclusions and, through its color-selective scattering, modifies the intrinsic color-shift effect of the at least one multilayer interference layer element. The security element therefore exhibits a color-shift effect between a third color in the first viewing direction and a different fourth color in the second viewing direction, wherein the third color differs from the first color in hue and / or color purity and / or the fourth color differs from the second color in hue and / or color purity.

[0011] The color-shift effect of the security element as a whole is also referred to as an extrinsic color-shift effect in this description to distinguish it from the intrinsic color-shift effect of the interference layer element. The extrinsic color-shift effect arises as a combination effect through the influencing and modification of the intrinsic color-shift effect by the color-selective scattering layer. This allows, in particular, a targeted change of the first and / or second color towards colors of greater color purity or to colors that cannot be produced by interference effects alone. In the context of this application, nanoscale refractive index variations are refractive index variations on a length scale below the wavelength of visible light, i.e., below 380 nm, in particular below 300 nm.

[0012] The security element may contain one or more color-selective scattering layers. Statements made in this description for a color-selective scattering layer apply to at least one of the color-selective scattering layers in designs with multiple color-selective scattering layers, or advantageously even to all color-selective scattering layers.

[0013] In an advantageous embodiment, the security element has precisely one color-selective scattering layer, which is arranged above or below the at least one multilayer interference layer element. In a currently particularly preferred embodiment, the security element has two color-selective scattering layers, which are present on opposite sides of a carrier film and are both arranged above or below the at least one multilayer interference layer element. The two color-selective scattering layers are preferably formed with identical properties, in particular with regard to material, layer thickness, and density of the cracks or air inclusions.

[0014] According to a preferred design, the color-selective scattering layer is traversed by a dense, interconnected network of crack edges. The crack edges have a width below the wavelength of visible light, i.e., below 380 nm, in particular below 300 nm. A color-selective scattering layer with cracks is often referred to below as a crack layer. Instead of or in addition to cracks, the color-selective scattering layer can also have a plurality of air inclusions, which in particular have dimensions below the wavelength of visible light, i.e., below 380 nm, in particular below 300 nm.

[0015] The color-selective scattering layer is advantageously formed from a crack-forming coating, preferably in the form of a dispersion, especially a colloidal dispersion. Dispersions of SiCh nanoparticles or acrylic resin nanoparticles, for example, are particularly suitable. Furthermore, the crack-forming coating can be based on a polymer in solution. Such a polymer solution is applied, especially printed, to a carrier film, creating a thin polymer film that forms cracks during drying.

[0016] Crack formation depends on the choice of raw materials, the choice of carrier film material, the layer thickness of the crack-inducing coating, and the drying parameters. The layer thickness of the crack-inducing coating is preferably between 10 nm and 5 μm. The width of the cracks is advantageously in the range of 150 nm to 300 nm. The dimensions of the slabs separated by cracks can be larger, but are particularly advantageously also predominantly in the range of 150 nm to 300 nm.

[0017] In a particularly advantageous embodiment, the color-selective scattering layer is formed by a cracked acrylate coating.

[0018] Further advantageous designs of the color-selective scattering layer can be found in the publications WO 2016 / 19258 A1 and EP 4 013 188 A1, the disclosure content of which is incorporated into the present application. The at least one dielectric spacer layer of the interference layer element is advantageously formed from SiCh, TiCh, ZnS, MgF, or Al2O3 and has a layer thickness between 100 nm and 500 nm. If two dielectric spacer layers are provided in the multilayer interference layer element, they are advantageously formed from the same material and with the same layer thickness.

[0019] The reflector layer is preferably made of aluminum, an aluminum-iron alloy, an iron alloy, or silver. The at least one absorber layer is advantageously made of chromium, an iron alloy, a chromium-iron alloy, or titanium. If two absorber layers are provided in the multilayer interference layer element, they are advantageously made of the same material. The security element can also contain more than one reflection layer, as explained in more detail below.

[0020] In an advantageous embodiment, with the intrinsic color-shift effect of the multilayer interference layer element, the first color is blue and the second color is a dark red of low color purity. With the extrinsic color-shift effect of the security element, the third color is a lighter blue compared to the first color, and the fourth color is a lighter and more color-pure red compared to the second color. In the context of this description, the term "lighter blue" does not refer to a blue of higher radiation intensity, but rather to a brighter, lighter, or more pastel blue appearance of the color.

[0021] A blue-to-red color shift effect is desirable in numerous applications. However, conventional designs based on the intrinsic color shift effect of the interference layer elements suffer, particularly from a dark red with low color purity when viewed at an angle, which is often difficult to detect and visually unattractive.

[0022] In another, equally advantageous embodiment, the first color of the intrinsic color-shift effect of the multilayer interference layer element is gold and the second color is green, while the third color of the extrinsic color-shift effect of the security element is orange and the fourth color is green or yellow. A color-shift effect with such a color change cannot be easily realized with conventional designs.

[0023] In a further, equally advantageous embodiment, the first color of the intrinsic color-shift effect of the multilayer interference layer element is magenta and the second color is green, while the third color of the extrinsic color-shift effect of the security element is red and the fourth color is green or yellow. A color-shift effect with such a color change is not easily realized with conventional designs.

[0024] The first viewing direction is preferably a vertical viewing direction and the second viewing direction is an oblique viewing direction of 45°.

[0025] In an advantageous variant of the invention, the security element comprises a carrier foil and a thin-film element applied over a large area to the carrier foil, which is formed by a single one of the aforementioned interference layer elements. The security element can then, in particular, represent a foil security element, such as a security thread, a security band, or a security patch. The thin-film element is advantageously formed by a three-layer interference layer element with a reflector layer, an absorber layer, and a dielectric spacer layer arranged between the reflector layer and the absorber layer.

[0026] In this variant of the invention, the color-selective scattering layer(s) are advantageously applied directly to the carrier film. Particularly advantageously, two color-selective scattering layers are provided on opposite sides of the carrier film.

[0027] The thin-film element is advantageously applied to a main surface of the carrier film opposite the color-selective scattering layer, or is applied to the or to one of the color-selective scattering layers, so that the color-selective scattering layer lies between the carrier film and the thin-film element.

[0028] In an advantageous development of the invention, the security element additionally contains an embossed lacquer layer with an embossed relief structure, which is coated with the aforementioned thin-film element. The color-shift effect of the thin-film element is then combined with an image motif created by the relief structure.

[0029] Particularly advantageous structures contain the following layer sequence: first color-selective scattering layer - carrier foil - second color-selective scattering layer - thin-film element or the following layer sequence: first color-selective scattering layer - carrier foil - second color-selective scattering layer - embossed lacquer layer with relief structure - thin-film element. The color-selective scattering layers advantageously represent cracked acrylate coatings. In an advantageous further development, the security element contains a translucent color layer with a red body color, which is preferably applied to the carrier foil, the large-area thin-film element, or a color-selective scattering layer.

[0030] In another, equally advantageous variant of the invention, the security element contains a printed layer containing, in a binder, a plurality of color-shifting, platelet-shaped pigments, each of which has a so-called interference layer element. In this variant, the security element can, in particular, be a printed element applied to a valuable object, but also a foil security element, such as a security thread, a security band, or a security patch.

[0031] The platelet-shaped pigments advantageously each comprise an interference layer element with at least five layers, comprising an outer first absorber layer, a first dielectric spacer layer, a central reflector layer, a second dielectric spacer layer, and an outer second absorber layer. The platelet-shaped pigments are advantageously constructed symmetrically, so that the two absorber layers and the two dielectric spacer layers are each formed from the same material and have the same layer thicknesses.

[0032] For magnetic alignment and / or the mechanical detection of their magnetic properties, the platelet-shaped pigments can in particular also contain at least one magnetic layer. Thus, an additional magnetic layer can be incorporated into the structure and / or the absorber and / or reflector layer can be formed from magnetic metal or a magnetic alloy. The magnetic layer can also form a middle layer in the layer structure of the platelet-shaped pigments, which, if the magnetic layer is not itself highly reflective, is expediently sandwiched between two reflective layers. The sandwich of reflective layer, magnetic layer, and second reflective layer can then be provided on each side with a dielectric spacer layer and an absorber layer, as described above, to form a preferably symmetrical interference layer element.

[0033] In this variant of the invention, the color-selective scattering layer is advantageously formed by an overcoating of the said printing layer.

[0034] In an advantageous further development, the security element contains a translucent ink layer with a red body colour, which is preferably applied to the printing layer with the colour-shifting, platelet-shaped pigments or a colour-selective scattering layer.

[0035] The invention also includes a valuable object, in particular a security paper, a value document or an identity document with a security element of the type described.

[0036] Finally, the invention also includes a method for producing a security element of the type described, in which at least one color-shifting, multi-layer interference layer element is provided, which has a reflector layer, at least one absorber layer, and at least one dielectric spacer layer arranged between the reflector layer and the at least one absorber layer, wherein the multi-layer interference layer element, with its layer structure, generates an intrinsic color-shift effect with a first color in a first viewing direction and a different second color in a second viewing direction, a color-selective scattering layer is arranged above and / or below the at least one multi-layer interference layer element, which has nanoscale refractive index variations generated by cracks and / or air inclusions,and which, through its color-selective scattering, modifies the intrinsic color-shift effect of the at least one multilayer interference layer element, so that the security element exhibits a color-shift effect between a third color in the first viewing direction and a different fourth color in the second viewing direction, wherein the third color differs from the first color in hue and / or color purity and / or the fourth color differs from the second color in hue and / or color purity.

[0037] In an advantageous variant of the invention, a carrier film is provided and a crack-forming coating is applied, in particular printed, to one or both sides of the carrier film. A thin-film element formed by a single one of the aforementioned interference layer elements is then applied over a large area to the coated carrier film. The partial layers of the thin-film element can be vapor-deposited for this purpose, for example, using a PVD process. The crack-forming coating cracks upon drying or upon application of the thin-film element and forms a crack layer with a dense, interconnected network of crack edges. In another advantageous variant of the invention, a security printing ink is used to produce the security element. This ink contains, in a binder, a plurality of color-shifting, platelet-shaped pigments, each having an aforementioned interference layer element.With such a security printing ink, a print layer is applied directly to a target valuable item or to an intermediate carrier. A color-selective scattering layer is then applied, specifically printed, to the print layer. For example, a crack-forming coating can be applied that cracks during drying, thereby forming a dense, interconnected network of crack edges.

[0038] In all variants of the invention, gravure printing, flexographic printing or an inkjet process is advantageously used when printing the color-selective scattering layer

[0039] Further embodiments and advantages of the invention are explained below with reference to the figures, in which a true-to-scale and true-to-proportion reproduction has been omitted in order to increase clarity.

[0040] They show:

[0041] Fig. 1 schematically shows a banknote with two optically variable security elements according to embodiments of the invention,

[0042] Fig. 2 shows schematically the layer structure of the first security element of Fig. 1 in cross section, Fig. 3 is a diagram illustrating the occurrence of the intrinsic color shift effect of the interference layer element,

[0043] Fig. 4 shows a combination element illustrating the influence of a color-selective scattering layer on the intrinsic color shift effect of the interference layer element,

[0044] Fig. 5 is a plan view of the crack layer of the combination element of Fig. 4,

[0045] Fig. 6 shows schematically the layer structure of the second security element of Fig. 1 in cross section,

[0046] Fig. 7 one of the platelet-shaped pigments of Fig. 6 separately, and

[0047] Fig. 8 schematically shows the layer structure of a further development of the security element of Fig. 2.

[0048] The invention will now be explained using banknotes as an example. Figure 1 shows a schematic representation of a banknote 10 provided with two optically variable security elements 12 and 14 according to embodiments of the invention. The first security element represents an adhesively bonded transfer element 12, the layer structure of which is shown schematically in cross-section in Figure 2. The second security element is formed by a printing element 14 printed onto the paper substrate 16 of the banknote 10, which is described in more detail below in connection with Figures 6 and 7. Referring initially to Figure 2, the optically variable security element 12 shows an image motif with a blue-to-red color shift effect, wherein the color impression of the image motif for a viewer alternates between a strong blue when viewed vertically 40 and a strong red of high color purity when viewed obliquely 42.

[0049] The security element 12 contains a transparent carrier film 20, each of which is provided on its opposite sides with a crack layer 30 and 32, respectively. In the exemplary embodiment, the crack layers 30, 32 are formed by acrylate coatings interspersed with a dense, interconnected network of narrow crack edges 34.

[0050] On the underside of the carrier foil 20, an embossed lacquer layer 25 is applied to the lower crack layer, which is coated with a color-shifting three-layer interference layer element 22 with an absorber layer 24 made of chromium, a dielectric spacer layer 26 made of SiCh and an opaque reflector layer 28 made of aluminum.

[0051] The relief structure embossed into the embossing lacquer layer 25, together with the interference layer element 22, creates the desired image motif in reflection, combined with a color-shift effect. For example, the thickness of the absorber layer for generating a blue-to-red color-shift effect can be between 2 nm and 8 nm, the thickness of the dielectric spacer layer between 220 nm and 330 nm, and the thickness of the aluminum layer between 20 nm and 50 nm.

[0052] The color shift effect generated by the interference layer element 22 alone is referred to in this description as the intrinsic color shift effect of the interference layer element 22. It is essentially determined by the material and layer thickness of the dielectric spacer layer 26. With the specified layer structure, the interference layer element 22 in the exemplary embodiment generates an intrinsic blue-to-red color shift effect, which, however, alternates relatively unattractively between a strong blue in the vertical viewing direction 40 and a dark red of low color purity in the oblique viewing direction 42.

[0053] To create the visually attractive color-shift effect exhibited by the security element 12, the carrier film is provided on both sides with cracked acrylate coatings 30 and 32, respectively, which act as color-selective scattering layers. As explained in detail below, the cracked layers 30, 32, through their color-selective scattering, significantly modify the intrinsic color-shift effect of the at least one multilayer interference layer element 22, such that, when viewing the security element 12 as a whole, the visually significantly more appealing color-shift effect results, ranging between a lighter blue and a pure red.

[0054] On the underside of the reflector layer 28, the security element 12 is also provided with an adhesive layer 38 with which it is glued to the paper substrate 16 of the banknote 10.

[0055] Figure 3 first illustrates the origin of the intrinsic color shift effect of the interference layer element 22. Diagram 50 shows the reflection R of the interference layer element 22 as a function of the wavelength X at a nearly vertical viewing angle of 8° (reflection curve 52) and at an oblique viewing angle of 45° (reflection curve 54). As can be seen from the figure, the reflection spectrum of the interference layer element 22 has two peaks in the visible and near infrared ranges: a narrow, short-wavelength peak 52-B or 54-B in the wavelength range of 400-500 nm, and a broad, long-wavelength peak 52-R or 54-R in the wavelength range of 700-900 nm.

[0056] When viewed essentially perpendicularly (reflection curve 52), the maximum of the long-wavelength peak 52-R lies in the near infrared, while the maximum of the short-wavelength peak 52-B lies in the blue region at approximately 465 nm. When viewed perpendicularly, the color impression of the interference layer element 22 is therefore determined by the short-wavelength peak 52-B in the blue region. The increase in reflection at approximately 800 nm hardly affects the color purity of the blue color impression, since the spectral sensitivity of the human eye is already very low above 700 nm.

[0057] When viewed at an angle (reflection curve 54), the maximum of the long-wavelength peak 54-R has migrated into the visible red range, while the short-wavelength peak 54-B has not completely disappeared into the UV range, which is invisible to the human eye, but still exhibits a high intensity in the visible blue spectral range. Therefore, when viewed at an angle, the color impression of the interference layer element 22 is determined by an overlay of the two peaks 54-B and 54-R, which creates a mixed color perceived as dark red. Due to the low color purity, the dark red of the shift color, in particular, appears visually unattractive.

[0058] A visually attractive blue-red color-shift effect is created by combining the interference layer element 22 with the cracked acrylate coatings 30, 32 applied to both sides of the carrier film. Figures 4 and 5 schematically illustrate the influence of a color-selectively scattering crack layer 30 on the intrinsic color-shift effect of the interference layer element 22 when viewed obliquely from a combination element 60 consisting of an interference layer element and a crack layer. The principle of the invention is explained using a simple exemplary embodiment of the invention in which only one color-selective scattering layer 30 is provided on the carrier film 20 and in which an embossed relief structure is omitted. However, the explanation applies equally to more complex variants of the invention. Figure 4 shows the aforementioned simple combination element 60 with carrier film 20, interference layer element 22, and crack layer 30 in cross-section, Fig.5 shows a plan view of a section of the crack layer 30 alone.

[0059] During the production of the combination element 60, a carrier film 20 is provided and provided with a crack-forming layer, for example, the acrylate coating mentioned in connection with Fig. 2. The crack-forming layer is applied in particular by printing technology, for example by gravure printing, flexographic printing, or an inkjet process.

[0060] During the drying of the crack-forming layer and / or during the subsequent vapor deposition of layers 24, 26, 28 of the interference layer element 22, the crack-forming layer tears open and forms a crack layer with a multitude of small slabs 36 separated from one another by a dense, interconnected network of narrow crack edges 34 (Fig. 5). The inventors have surprisingly found that the resulting crack edges 34 often have a very small width below the wavelength of light, in particular less than 300 nm. Without wishing to be bound to a specific explanation, according to current understanding, the crack edges 34, due to their small width, generate nanoscale refractive index variations in the crack layer 30, at which incident visible light is scattered in a highly color-selective manner. In particular, as with Rayleigh scattering, blue light is scattered significantly more strongly at the crack edges than red light.

[0061] In Rayleigh scattering, incident visible light is scattered elastically, largely isotropically, and, in particular, with strong color selection. The cross section or scattering intensity I of a particle is strongly dependent on the wavelength, I oc X 4 The scattering intensity increases with the fourth power as the wavelength gets shorter. This means that, for example, blue light with a wavelength of 420 nm is scattered about 10 times more strongly than red light with a wavelength of 750 nm.

[0062] Analogously, a high blue component in the scattered light is observed at the crack edges 34 of the crack layer 30 when viewed under a microscope, so that it is currently assumed that Rayleigh scattering dominates at the crack edges 34.

[0063] Returning to the cross-sectional view of Fig. 4, white light 62 incident on the combination element 60 contains, in addition to its other color components, in particular red light 62-R and blue light 62-B. At an oblique angle of incidence of approximately 45°, these two light components are reflected by the interference layer element 22 with comparable intensity, as shown in Fig. 3. The color-selective scattering layer 30, with its crack network 34, now scatters the blue light component of both the incident light and the light reflected by the interference layer element very effectively in all directions 64-B, while the red light component is only slightly influenced by the crack network 34. In the reflected light 66, the light component 64-B scattered away by the color-selective scattering layer 30 is no longer available, so that the blue light component 66-B is significantly reduced compared to the red light component 66-R.The combination element 60 therefore creates a red color impression of high color purity for the viewer 44 in the oblique viewing direction by suppressing the disturbing blue light components.

[0064] When viewed vertically, where the reflected light 66 is determined by the short-wavelength peak 52-B in the blue range, the cracked layer 30 also reduces the intensity of the blue light component, but only slightly brightens the blue color impression, since the long-wavelength peak 52-R, due to its maximum in the near infrared, contributes little to the overall color impression. When viewed vertically, the combination element 60 therefore displays a blue whose hue appears somewhat lighter and more pastel-like than the intrinsic blue, but which is still perceived as a strong, pure blue.

[0065] Returning to the design of Fig. 2, there a crack layer is provided as a color-selective scattering layer on both the top side and the bottom side of the carrier film 20, resulting in strong color-selective scattering. The security element 12 exhibits an attractive color-shift effect between a strong blue in plan view 40 to a red of high color purity in oblique view 42 due to the combination of the interference layer element 22 with the two color-selective scattering layers 30. While in the embodiment of Fig. 2 a crack layer is provided as a color-selective scattering layer on both the top side and the bottom side of the carrier film 20, a color-selective scattering layer can also be provided only on the top side (as in Fig. 4, for example) or only on the bottom side of the carrier film 20.The extent of modification of the intrinsic color shift effect of the interference layer element 22 can be adjusted within a wide range by the number of color-selective scattering layers, the thickness of the color-selective scattering layers and the density of the crack network within the scattering layers.

[0066] Figure 8 shows a further development of the embodiment of Figure 2, in which the security element 12 is additionally provided with a translucent color layer 39 with a red body color. The translucent red body color layer 39 can, as shown in the figure, be arranged between the interference layer element 22 and the carrier foil 20, but it can also be provided on the side of the carrier foil 20 facing the viewer. It has been found that such a translucent red body color layer 39 further improves the color effect of the blue-to-red color shift effect.

[0067] With reference again to the illustration in Fig. 1, the banknote 10 contains, as a second security element, an optically variable printing element 14, which is shown schematically in cross-section in Fig. 6. The printing element 14, like the transfer element 12, exhibits a blue-to-red color shift effect, in which the color impression for a viewer changes between a strong blue when viewed vertically 40 and a red of high color purity when viewed obliquely 42. The printing element 14 is formed by a security printing ink 70 printed on the paper substrate 16 of the banknote 10 and a color-selectively scattering overcoat 90.

[0068] The security printing ink 70 contains a transparent binder 72 with a plurality of color-shifting platelet-shaped pigments 74, one of the platelet-shaped pigments 74 being shown separately in more detail in Fig. 7. Each of the platelet-shaped pigments 74 represents a color-shifting five-layer interference layer element with a symmetrical structure and consists of an outer first chromium absorber layer 80, a first SiO spacer layer 82, a middle, opaque aluminum reflector layer 84, a second SiO spacer layer 86, and an outer second chromium absorber layer 88. The layer thicknesses of the dielectric spacer layers, the reflector layer, and the absorber layers are advantageously within the ranges mentioned above. For a symmetrical structure, the two SiO spacer layers 82, 86 and the two absorber layers 80, 88 are each substantially the same thickness.

[0069] The platelet-shaped pigments 74 distributed in the binder 72 of the security printing ink 70 are aligned essentially parallel to the surface of the paper substrate 16 during printing by the forces acting during printing. Due to the symmetrical structure of the pigments 74, it is irrelevant for the observable color effect which of the two chromium absorber layers 80 or 88 faces the paper substrate or the viewer.

[0070] The overcoat 90 is formed by a color-selective scattering layer of the type described above, for example by an acrylate coating which, after drying, has a dense, coherent network of crack edges 34.

[0071] The intrinsic color shift effect of the platelet-shaped pigments 74 corresponds to the color shift effect of the interference layer element 22 of Fig. 2 due to the analogous layer structure, thus showing a relatively unattractive change between a strong blue in the vertical viewing direction 40 and a dark red of low color purity in the oblique viewing direction 42.

[0072] To create the visually attractive color-shift effect exhibited overall by the printing element 14, the intrinsic color-shift effect of the platelet-shaped pigments 74 is modified by the color-selective scattering overcoat 90 such that the viewer sees a visually significantly more appealing color-shift effect between a lighter blue and a pure red. The mode of action is analogous to the explanation given above: The nanoscale crack network 34 of the overcoat 90 scatters incident blue light much more strongly than red light, so that the blue light component in the reflected light is significantly reduced compared to the red light component.

[0073] In the embodiment of Fig. 6, the color effect of the blue-to-red color shift effect can also be further enhanced by a translucent red body color layer. Such a translucent red body color layer can be applied, in particular printed, to the security printing ink 70 or the overcoat 90 of Fig. 6.

Claims

P a t e n t a n s p r ü c h e 1. An optically variable security element for protecting security papers, valuable documents, and other valuables, comprising at least one color-shifting, multilayer interference layer element with a reflector layer, at least one absorber layer, and at least one dielectric spacer layer arranged between the reflector layer and the at least one absorber layer, wherein the multilayer interference layer element, with its layer structure, generates an intrinsic color-shift effect with a first color in a first viewing direction and a different second color in a second viewing direction, characterized in that a color-selective scattering layer is arranged above and / or below the at least one multilayer interference layer element, which has nanoscale refractive index variations generated by cracks and / or air inclusions,and which, through its color-selective scattering, modifies the intrinsic color-shift effect of the at least one multi-layer interference layer element, so that the security element displays a color-shift effect between a third color in the first viewing direction and a different fourth color in the second viewing direction, wherein the third color differs from the first color in hue and / or, the color purity and / or the fourth color differs from the second color in hue and / or color purity.

2. Security element according to claim 1, characterized in that the color-selective scattering layer is traversed by a dense, interconnected network of crack edges.

3. Security element according to claim 1 or 2, characterized in that the color-selective scattering layer is formed from a crack-forming coating in the form of a dispersion, in particular a colloidal dispersion.

4. Security element according to at least one of claims 1 to 3, characterized in that the color-selective scattering layer is formed by a cracked acrylate coating.

5. Security element according to at least one of claims 1 to 4, characterized in that the at least one dielectric spacer layer is formed from SiCh, TiCh, ZnS, MgF or Al2O3 and has a layer thickness between 100 nm and 500 nm, and / or that the reflector layer is formed from aluminum, an aluminum-iron alloy, or silver, and / or that the at least one absorber layer is formed from chromium, an iron alloy, a chromium-iron alloy or titanium.

6. Security element according to at least one of claims 1 to 5, characterized in that the first colour is blue and the second colour is a dark red of low colour purity and the third colour is a lighter blue than the first colour and the fourth colour is a lighter and more colour-pure red than the second colour, or that the first colour is gold and the second colour is green, and the third colour is orange and the fourth colour is green or yellow, or that the first colour is magenta and the second colour is green, and the third colour is red and the fourth colour is green or yellow.

7. Security element according to at least one of claims 1 to 6, characterized in that the first viewing direction is a vertical viewing direction and the second viewing direction is an oblique viewing direction of 45°.

8. Security element according to at least one of claims 1 to 7, characterized in that the security element contains a carrier film and a thin-film element applied over a large area to the carrier film or to a relief layer present on the carrier film, which thin-film element is formed by a single one of the said interference layer elements.

9. Security element according to claim 8, characterized in that the thin-film element is formed by a three-layer interference layer element with a reflector layer, an absorber layer and a dielectric spacer layer arranged between the reflector layer and the absorber layer.

10. Security element according to claim 8 or 9, characterized in that the color-selective scattering layer(s) are applied directly to the carrier film.

11. Security element according to at least one of claims 8 to 10, characterized in that the security element contains a translucent color layer with a red body color, which is preferably applied to the carrier film, the large-area thin-film element or a color-selective scattering layer.

12. Security element according to at least one of claims 1 to 7, characterized in that the security element contains a printing layer which contains, in a binder, a plurality of color-shifting, platelet-shaped pigments, each having a said interference layer element.

13. Security element according to claim 12, characterized in that the platelet-shaped pigments each have an at least five-layer interference layer element with an outer first absorber layer, a first dielectric spacer layer, a central reflector layer, a second dielectric spacer layer and an outer second absorber layer, and are preferably constructed symmetrically.

14. Security element according to claim 12 or 13, characterized in that the platelet-shaped pigments each contain at least one magnetic layer and can thus be aligned by means of external magnetic fields.

15. Security element according to at least one of claims 12 to 14, characterized in that the color-selective scattering layer is formed in an overcoat of the printing layer.

16. Security element according to at least one of claims 12 to 15, characterized in that the security element contains a translucent ink layer with a red body color, which is preferably applied to the printing layer with the color-shifting, platelet-shaped pigments or a color-selective scattering layer.

17. Valuable object, in particular security paper, value document or identity document with a security element according to one of claims 1 to 16.

18. A method for producing a security element according to one of claims 1 to 16, in which at least one color-shifting, multi-layer interference layer element is provided, which has a reflector layer, at least one absorber layer and at least one dielectric spacer layer arranged between the reflector layer and the at least one absorber layer, wherein the multi-layer interference layer element with its layer structure generates an intrinsic color-shift effect with a first color in a first viewing direction and a different second color in a second viewing direction, characterized in that a color-selective scattering layer is arranged above and / or below the at least one multi-layer interference layer element, which has nanoscale refractive index variations generated by cracks and / or air inclusions, and which by its color-selective Scattering modifies the intrinsic color shift effect of the at least one multi-layer interference layer element, so that the security element shows a color shift effect between a third color in the first viewing direction and a different fourth color in the second viewing direction, wherein the third color differs from the first color in hue and / or color purity and / or the fourth color differs from the second color in hue and / or color purity.

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