Security element with chromic nanostructures

Abstract

The present application relates to a security element comprising a structured layer (8) comprising a base surface (20) and recesses (12) or protrusions which are recessed or raised relative to the base surface, and a reflector layer (14) arranged on the structured layer (8), the recesses (12) or protrusions being designed as chromogenic nanostructures (10) which are arranged in the form of nanostructures (10) with respect to their dimensions along the base surface (20). The invention relates to a structured layer (8) of which the vertical extension (t) perpendicular to the base plane (20) and the arrangement thereof on the base plane (20) are formed as chromogenic nanostructures (10), the structured layer (8) comprising regions (16, 18; 25a-d), in which the vertical extension (t) of the recess (12) or projection (14) varies in the direction (22) according to a function having no constant value, such that the vertical extension (t) of the recess (12) or projection (14) in the plan view changes in the direction (22) in such a way that the region (16, 18; 25a-d) in each of the plurality of regions (25a-d; 25a-d).

Description

[0001] The present invention relates to a security element with color-generating nanostructures, comprising: a structured layer having a base surface and lowered recesses or raised protrusions, which are lowered or raised relative to the base surface, and a reflector layer arranged on the structured layer, wherein the protrusions or recesses are formed as color-generating nanostructures with regard to their size along the base surface, their vertical size perpendicular to the base surface and their arrangement on the base surface, and wherein the structured layer comprises a plurality of areas providing a color pattern or image, which preferably form pixels.

[0002] The invention further relates to a method for producing a security element, wherein the method comprises: creating a structured layer, the structured layer comprising a base surface and lowered recesses or raised protrusions, which are lowered or raised relative to the base surface, and arranging a reflector layer on the structured layer, wherein the protrusions or recesses are configured as color-producing nanostructures with respect to their dimensions along the base surface, their vertical dimensions perpendicular to the base surface and their arrangement on the base surface, and wherein the structured layer is formed with a plurality of areas providing a color pattern or image, which areas preferably form pixels.

[0003] Such a security element and such a production method are known from DE 10 2012 105 571 A1.

[0004] Nanostructures with dimensions in the subwavelength range are known in the prior art, which allow for a colored representation. For this purpose, projections or recesses are arranged on the surface. If the structure is mirror-coated, when the projections / recesses are formed as nanostructures, the color depends on the structural parameters of the projections / recesses, i.e., the structure whose dimensions are below the wavelength of visible light. Such structures are also known, for example, from DE 2009056934 A1, DE 102010048262 A1, DE 102010049832 A1, U.S. 2012 / 236415 A1, WO 2019 / 140572 A1 or WO 2019 / 1804 60 A1. EP 3572852 A1 also discloses a security element having projections / recesses that realize a color-forming nanostructure.

[0005] It is known to provide a chromogenic nanostructure in the region constituting a pixel or sub-pixel. In each pixel or sub-pixel, a predetermined configuration of a protrusion or recess will be provided according to the size along the base region, the vertical size perpendicular to the base surface and the arrangement on the base surface, depending on the color that the pixel or sub-pixel is configured to produce. This nanostructure produces a color that is usually affected by vertical extension. In the prior art, it is known to use uniform vertical extension in pixels or sub-pixels, i.e., use uniform height in the case of a protrusion or use uniform depth in the case of a recess. In the case of a sub-pixel, for example, the above-mentioned DE102012105571A1 (Figures 14b and 14c in this document) provides sub-pixels that are different in height / depth of the protrusion / recess, so that a color is set by combining several, for example, RGB sub-pixels. The formation of a region with a separate vertical extension that is different from region to region therefore allows the generation of a color image, for which these regions are realized in the form of sub-pixels. The sub-pixels each generate a color (e.g., red, green, or blue) and set the desired color of the pixel together.

[0006] On this basis, the present invention is based on the object of specifying a security element based on chromogenic nanostructures which has an improved color effect and in particular does not require subpixels.

[0007] The invention is defined in the independent claim. The dependent claims relate to preferred embodiments.

[0008] The security element comprises a structured layer, which has a base surface and a recessed inwardly concave notch or a raised outwardly raised protrusion that is generally a relief relative to the base surface. Any reference to the depression / recess below is intended to be used as an example only. The reflector layer is arranged on the structured layer. The projection or recess is designed as a color-producing nanostructure in view of its structural parameters, particularly their size along the base surface, their vertical extension perpendicular to the base surface and their arrangement on the base surface, as known from the prior art and particularly known from the EP 3572852 A1 already mentioned. The vertical extension is the height of the projection or the depth of the recess. The projection or recess can have a top surface parallel to the base surface or the tangent of the base surface. The term "nanostructure" refers to the vertical extension of the projection / recess, which is usually lower than 500nm. On the other hand, the size along the substrate surface and the distance between adjacent projections / recesses are usually in the range of micrometers.

[0009] The nanostructure produces a colored pattern or image and includes several areas in which the vertical extension of the protrusion or recess varies in one direction in each area according to a function that does not have a constant value. When viewed from above, this causes a color to be produced in each of these areas, which is produced as a mixed color due to the change in vertical extension. The term "mixed color" means that the color is produced by mixing the effects of the vertical extension of the protrusion or recess, and the size of the protrusion or recess in the area is variable. The term "mixed color" is to be distinguished from the mixing of colors because it is caused by several sub-pixels or pixels, where the nanostructure in each area (i.e., sub-pixel or pixel) has no mixing effect due to the non-variable extension of the protrusion or recess and just produces a single color, and only several pixels / sub-pixels mix their single colors together. Due to the expected change in the vertical extension of the protrusion or recess within the area (e.g., pixel), the color of each area is produced by the mixing effect rather than as a single color.

[0010] This simplifies the generation of images, since no structuring in the subpixels is required for the generation of colors; instead, each area, for example each pixel, can provide a mixed color. This color is set in the respective area by varying the vertical extension, in particular the area over which the change occurs and the distribution of the vertical extension in the area, and the average vertical extension, i.e. the average value of the distribution.

[0011] The colors that contribute to the mixture are set, among other factors, by the degree of variation of the vertical extension. If the vertical extension varies over a large range, more colors are combined into the mixed color than if the vertical extension varies over a smaller range. The average depth of the varied vertical extension corresponds to a color with a hue equivalent wavelength that is also located in the middle of the hue equivalent wavelengths of all colors that are combined to form the mixed color. It is therefore preferred that, for each region, the center of the hue equivalent wavelength of the (mixed) color produced by the region is set by the average depth of the varied vertical extension.

[0012] In order to produce a color image, it is particularly preferred that at least some of these areas differ in function. The difference may lie in the range of variation of the vertical extension. The difference may also lie in how the various vertical extensions are distributed in the different areas, for example, whether large vertical extensions occur more frequently than small vertical extensions. This then sets the center of distribution of the hue equivalent wavelengths of the mixed color asymmetrically within the range covered by all hue equivalent wavelengths of the mixed color. It is therefore preferred to combine different ranges which differ in the range of the vertical extension and / or in the distribution of different vertical extensions within the range of variation.

[0013] Furthermore, optionally, in particular in the latter case, the distribution of the vertical extension is asymmetrical about the center of the covered variation range of the vertical extension.

[0014] Particularly good color mixing is obtained when the vertical extension of the protrusions or recesses increases or decreases along the direction according to the gradient. In this way, hue-equivalent wavelengths distributed over a range are combined, wherein if the gradient is a linear gradient, all hue-equivalent wavelengths contribute equally to the mixed color. If the gradient is non-linear, such as quadratic, etc., the individual hue-equivalent wavelengths can contribute different influences to the mixed color.

[0015] When these nanostructures are combined with microstructures, i.e. with microstructure elements, particularly preferred embodiments are obtained, wherein the microstructure elements have a size higher than the wavelength of the visible spectral range in at least one direction, for example at least 5 μm, preferably 10 μm, 50 μm or more. Then, the region can be formed in each case as a microstructure element equipped with a height profile (i.e. height variation), such as an inclined mirror. At least some of the regions differ from each other in terms of the height profile and / or shape of the microstructure element (in a top view of the base plane). The nanostructure is formed on the microstructure element, whose vertical extension varies on the corresponding microstructure element. In this embodiment, the region then corresponds to the microstructure element.

[0016] It is particularly preferred that the vertical extension increases or decreases with the height of the microstructure element (e.g. the mirror), since this allows a particularly simple production which exploits local differences in the sensitivity of the photoresist that occur due to the microstructure. At the upper edge of the mirror there is then a projection / recess with a greater vertical extension and at the lower edge of the mirror there is a projection / recess with a smaller vertical extension, or vice versa. Between the upper and lower edge of the mirror, the reduction in vertical extension is parallel to the inclination of the mirror.

[0017] The microstructure elements (eg mirrors) can be formed in such a way that they produce a relief effect which is intensified by the color-imparting nanostructures and is colored, ie produces an improved 3D effect.

[0018] The direction in which the nanostructures vary relative to the vertical extension is preferably aligned at an angle to the gradient of the height profile, preferably following the gradient. This can be used as an additional covert security feature, as it is invisible to the naked eye, but can be detected by machine-based analysis of the security element.

[0019] Furthermore, in this case, the vertical extension can be varied depending on the inclination angle of the mirror. This can be done with a uniform or varying mirror length along the gradient of the mirror.

[0020] Nanostructures can be described by the following parameters:

[0021] The protrusions / recesses can be arranged in a regular or irregular pattern, which means a quasi-statistical distribution of the protrusions / recesses on the base surface. In the case of a regular arrangement, the lattice structure used is a relevant parameter, such as a square base lattice, a hexagonal base lattice, etc.

[0022] The distance between the structures, ie the elevations / recesses, influences the color effect. In the case of irregularly arranged elevations / recesses, this applies to the average distance and / or area coverage of the elevations / recesses on the base surface compared to the rest of the base surface.

[0023] The lateral dimensions of the elevations / recesses further characterize the printed layer. The elevations / recesses can be rotationally symmetrical in top view, but can also be formed to extend in one direction. This direction can be uniform, but it can also vary. The vertical extension of the elevations or recesses perpendicular to the base surface is another parameter that particularly influences the color produced.

[0024] The basic shape of the protrusions / recesses (i.e. the appearance in a top view) characterizes the embossed structure as much as the side shape (i.e. the profile of the protrusions / recesses in a cross section perpendicular to the base surface). In a top view, basic shapes such as squares, hexagons, circles, ellipses, rectangles, etc. can be used. In a cross-sectional view, the profile can take the form of a sine wave, a parabola, a rectangular structure with almost vertical sides, etc.

[0025] The regions may preferably be pixels of a displayed image or pattern. Preferably, these regions form pixels which are preferably arranged in a regular pattern (e.g., a square or hexagonal grid) and generate a color pattern. This applies to all further versions or options described herein.

[0026] After the reflector layer has been provided, the nanostructures can be coated with a dielectric material. The dielectric material covers the depressions / protrusions preferably formed in the embossing lacquer layer. The refractive index of the dielectric material is preferably the same as the refractive index of the embossing lacquer layer, but can also be different.

[0027] The present invention will be explained in more detail below with reference to the accompanying drawings, which also disclose the essential features of the present invention. These embodiments are for illustrative purposes only and should not be construed as restrictive. For example, the description of an embodiment including multiple elements or components does not mean that all these elements or components are necessary to implement the present invention. On the contrary, other embodiments may utilize different elements and components, fewer elements or components, or additional elements or components. Unless otherwise stated, the elements or components of the various embodiments may be combined with each other. The modifications and variations described for one embodiment in the embodiment may also be applicable to other embodiments. In order to avoid repetition, the same or corresponding elements in different drawings are represented by the same reference numerals and are not explained more than once. The accompanying drawings show:

[0028] Figure 1 A banknote with a security element is shown,

[0029] Figure 2 The first embodiment shows Figure 1 The cross section of the safety element,

[0030] Figure 3 A similar cross section of the second embodiment is shown,

[0031] Figure 4 A diagram for explaining the operating principle of the embodiment is shown.

[0032] Figure 5 shows a top view of a security element of a second embodiment,

[0033] Figure 6 A SEM image of the security element is shown.

[0034] Figure 1 A security document, in this case a banknote 2, is shown in a top view, which comprises a security element 4 intended to prevent the security document from being forged. The security element 4 displays a color image in the top view. For this purpose, it has a structured surface, which can be Figure 2 as seen in the cross-sectional view.

[0035] The security element is constructed on a substrate 6 on which there is an embossed lacquer layer 8 in which nanostructures 10 are embossed. A preferred alternative to embossing will be explained later.

[0036] The nanostructure 10 comprises a plurality of recesses 12 and is provided with a reflector layer 14. Such nanostructures are known in principle to the skilled person. When viewed from above, they produce a color.

[0037] The security element comprises at least two regions 16 , 18 which differ with regard to the nanostructure 10 , namely with regard to the vertical extension of the recess 12 relative to the base surface 20 , with respect to which the recess 12 is recessed.

[0038] In the region 16, the vertical extension, ie the depth t of the recess 12, increases along the direction 22. In the region 18, the depth t varies along the direction 22 according to another function, ie a substantially sinusoidal function.

[0039] The depth t of each recess 12 influences the color effect that appears when viewed from above. Since sixteen different depths are arranged along the direction 22, a mixed color is produced, as will be referred to below. Figure 4 Because different functions are used in region 18, different mixed colors are created.

[0040] In this embodiment, the mixed color between regions 16 and 18 is also different because the depth range is different. In region 18, recesses 12 are significantly lighter than in region 16. Thus, a mixed color 18 is created that is composed of other hue equivalent wavelengths than in region 16, where deeper recesses 12 are used.

[0041] Figure 2 The formation of a nanostructure 10 with a recessed portion lowered relative to the base surface 20 is shown purely by way of example (based on Figure 2 ). Also possible are designs in which projections are arranged. As a vertical extension, the height of these projections is of course variable.

[0042] For the variation of the vertical extension along the direction 22, a gradient is particularly preferred. Such a gradient allows a continuous mixing of colors from colors that are close to each other in the color space. The type of mixing of the individual hue-equivalent wavelengths can be set by the configuration of the gradient, which is, for example, linear or quadratic. Similarly, the distance between the recesses 12 or protrusions can also be varied. This also affects the color produced.

[0043] A particularly advantageous combination of using nanostructures 10 is obtained when the nanostructures are combined with microstructures 24. Figure 3 Schematically shown in FIG. Here, the microstructure 24 is shown as a series of tilted micromirrors 25a-25d, wherein the inclination of each micromirror 25a-25d relative to a base plane is different, and the base plane can be defined as the surface of the substrate 6 or a plane parallel thereto. The nanostructure 10 is formed on each micromirror 25a-25d representing a microstructure element, which is then covered with a reflector layer 14. Figure 6 A SEM image of the microstructure 24 is shown.

[0044] This results in a combination of the effect produced by the microstructure 24 and the effect produced by the coloring in each individual microstructure element. In this way, for example, the relief effect produced by the microstructure 24 can be additionally supplemented by a color effect beyond the uniform coloring, since the individual microstructure elements can produce an individual mixed color depending on how the function is selected along the direction 22.

[0045] The function is preferably designed to follow the gradient of the shape of the microstructure, i.e. the gradient direction 22 corresponds to the surface course of the microstructure 24, for example the tilt direction of the micromirrors 25a-25d. Figure 3 In the illustration of , direction 22 therefore corresponds to Figure 2 ,and Figure 3 The cross-sectional view shows the cross-sectional plane where the gradient of the mirror tilt lies. It is schematically indicated by 25e. Figure 3 A simplified case is shown, in which all micromirrors 25a-25d have a gradient 25e in the same direction, i.e. they are all tilted in the same direction. Of course, this is not mandatory. Each microstructure element can have its own gradient direction. Then, the direction 22 of the nanostructure 10 on the corresponding microstructure element is aligned with this gradient direction, in particular in the same direction.

[0046] The registration of the slope gradient of the microstructure elements (e.g. the gradient 25e of the micromirror 25d) can be used as a hidden security element. Steep micromirrors, i.e. microstructure elements with large height variations, include large gradients in the nanostructure, while microstructure elements with low height variations have lower variations in the height of the depressions or protrusions. The following values ​​can be used in embodiments:

[0047]

[0048] In variant A, this results in a depth variation T of 200 nm, whereas in variant B the depth variation is only 100 nm.

[0049] In order to produce a protruding effect, for example, the mirrors 25a to 25d can be arranged from a low slope, for example between 0 and 5°, for micromirror 25d, to a large slope, for example 30°, at micromirror 25a. By means of a nanostructure 10, for example, producing a mixed color in the yellow range, wherein the protrusions or recesses are formed in a hexagonal arrangement and have a grating period of 280nm, an edge length of a square cross section of the recess / protrusion of 140nm and a minimum depth of 80nm. Depending on the inclination angle of the micromirror, starting from the minimum depth of 80nm, the variation of the vertical extension of the recess increases to different degrees. Thus, for example, the steeper the mirror, the darker the yellow shade, while the flatter the mirror provides the brighter the yellow shade. This enhances the plasticity of the three-dimensional impression of the protruding effect.

[0050] This is Figure 5Here, the steep mirror is located at the edge marked 26, while the flat mirror is located in the center of the corresponding number marked 28. Similarly, the mixed color that is darker than in the 28 area is located in the area 26. This leads to the following values, for example:

[0051]

[0052] Of course, these conditions can also be reversed and a large gradient of depth variation can be used on a flat mirror and a small gradient on a steep mirror. This, depending on the design, defines the mixed color to be created. Of course, it is also possible to choose the depth range differently on the individual mirrors so that the mixed colors also differ more, for example in terms of the combined hue-equivalent wavelength.

[0053] Figure 3 and Figure 6 An example microstructure 24 using micromirrors 25a to 25d is shown. Of course, other microstructures can also be used, for example with concave mirrors, Fresnel structures, lens structures, buffer structures, etc., whereby in each case the local gradient of the microstructure elements, i.e. the height profile, can also specify the gradient direction of the depth variation of the nanostructure.

[0054] Another advantage of arranging the nanostructures 10 on the microstructures 24 is that production fluctuations can be compensated. Errors in imprinting the nanostructures 10 may lead to undesired local variations in their depth. When varying the depth and producing mixed colors, such variations are less noticeable because the mixed colors are not strongly affected by errors in the imprinting of the individual recesses / protrusions.

[0055] The effect of depth variation is as follows Figure 4 shown. Figure 4 The curve 30 in FIG. 1 depicts the dependence of the wavelength λ, which represents the color spectrum or hue, on the depth t of the depression 12. A similar dependence exists for the height of the protrusions. If the depth is varied over a depth variation range T, a wavelength mixture L is automatically generated. For the sake of clarity, Figure 4 A linear relationship is suggested. However, because the relationship is non-linear, no axis scale is shown. Shallow structures do not automatically result in shorter wavelengths. Experimental studies have shown that different colors or "hues" are overlaid as the depth of the structure increases.

[0056] In example A, the color changes from flat structure to deep structure, i.e., from light yellow to dark yellow to gold as the depth of the structure increases.

[0057] In example B, the color changes from flat structure to deep structure, i.e., from orange to magenta to purple and from blue to green as the depth of the structure increases.

[0058] As mentioned above, the security element 4 may be manufactured by embossing. Alternatively, it may be produced by photolithography which exploits the local differences in the sensitivity of the photoresist which occur due to the structured microstructure.

[0059] In order to achieve different vertical extensions of the recesses and protrusions of the nanostructure, the fact that the microstructure has been exposed is utilized. This automatically prevents all recesses of the nanostructure 10 from being formed with the same vertical extension. When the photoresist is exposed to the microstructure, a light dose gradient (corresponding to the height profile of the microstructure elements) has been deposited in the photoresist. A relatively small dose is placed at the upper edge of each mirror, while more dose is introduced into the resist at the lowest point of the mirror to achieve the mirror tilt. Therefore, the photoresist is increasingly bleached from the upper edge of the mirror; the bleaching increases towards the lowest point of the height profile. When the nanostructure is projected, the photoresist, which has been bleached to different degrees, therefore reacts differently to the same exposure dose, depending on how much dose was previously placed to produce the microstructure. Therefore, it is possible to work with a uniform exposure dose for the recesses 12 of the nanostructure 10 and still obtain recesses 12 of different depths. The depth is proportional to the dose that has been previously placed to produce the microstructure 24, and the depth is therefore automatically aligned to the height of the microstructure.

[0060] In order to vary the depth of the nanostructures in different ways, a dose wedge is used for patterning the nanostructures 10 to specifically vary their depth. The dose wedge must take into account the dose of the first exposure (i.e., the exposure made for the microstructures 24), as well as the shape and direction of the gradient to be achieved for the vertically extending gradient.

[0061] As an alternative to photolithographic implementation, Figure 2 and Figure 3 The mentioned embossing can be carried out in an embossed lacquer 8 .

[0062] After the reflector layer 14 is applied, the nanostructures 10 are preferably overcoated with a dielectric material (not shown). The dielectric material covers the protrusions / recesses, so that the nanostructures 10 are completely or partially flat. This increases the durability and resistance to contamination of the security element. In addition, the color effect can be further adjusted, because the refractive index of the dielectric material affects the color. The refractive index of the dielectric material is preferably the same as the refractive index of the layer in which the nanostructures 10 are formed (e.g. the embossing lacquer layer 8), but it can also be different. In the case where the nanostructures 10 are located (e.g. Figure 2 In the case where the nanostructure 10 is formed on a flat base structure (as shown in FIG. 1 ), the overcoating of the dielectric material can be performed by applying and scraping off the excess material. Figure 4 As shown), a deposition process can be used to apply the dielectric material.

[0063] Reference numerals list

[0064] 2 Banknotes

[0065] 4Security Elements

[0066] 6 base

[0067] 8 embossed varnish layer

[0068] 10 Nanostructure

[0069] 12 recesses

[0070] 14Reflection layer

[0071] Area 16,18

[0072] 20 base surfaces

[0073] 22 Directions

[0074] 24 Microstructure

[0075] 25a-d Microscope

[0076] 25e Gradient

[0077] Area 26,28

[0078] 30 Curve

[0079] tDepth

[0080] z represents the wavelength of the color hue

[0081] T Depth Change

[0082] L wavelength mixing

Claims

1. A security element, comprising: a structured layer (8) comprising a base surface (20) and recesses (12) that are recessed relative to the base surface or protrusions that are elevated relative to the base surface, and a reflector layer (14) arranged on the structured layer (8), wherein the recesses (12) or protrusions are formed as color-forming nanostructures (10) with respect to their dimensions along the base surface (20), their vertical extension (t) perpendicular to the base surface (20) and their arrangement on the base surface (20), - wherein the structured layer (8) comprises a plurality of regions (16, 18; 25a-d) providing a colored pattern or image, the regions (16, 18; 25a-d) preferably forming pixels, - wherein, in each of the plurality of regions (16, 18; 25a-d), the vertical extension (t) of the recess (12) or protrusion varies along the direction (22) according to a function that does not have a constant value, so that in each of the plurality of regions (16, 18; 25a-d) in a top view, the color resulting from the variation in the vertical extension is a mixed color.

2. The security element according to claim 1, wherein: At least some of the regions (16, 18; 25a-d) differ with respect to the function.

3. The security element according to claim 1, wherein: The vertical extension of the recess (12) or protrusion increases or decreases along the direction (22) according to a gradient (25e).

4. The security element according to claim 1, wherein: The recesses (12) or raised areas have the same dimensions along the base area (20) and / or have the same arrangement on the base area (20).

5. The security element according to one of the preceding claims, wherein The regions (25a-d) are each formed as an element of a microstructure (24), in particular as an inclined mirror surface, in which the base surface (20) is inclined relative to a base plane, and at least some of the regions (25a-d) differ from each other in terms of the inclination and / or shape of the mirror surface.

6. The security element according to claim 5, wherein: The direction (22) in each region (25a-d) follows a gradient (25e), in particular a mirrored gradient (25e), of an element of the microstructure (24).

7. The security element according to claim 5 or 6, wherein: The vertical extension (t) increases or decreases with the height of the mirror surface above the base plane.

8. The security element according to any one of claims 5 to 7, wherein: The microstructure (24) produces a swelling effect.

9. A method for manufacturing a security element, the method comprising - creating a structured layer (8) comprising a base surface (10) and recesses (12) that are recessed relative to the base surface or elevations that are elevated relative to the base surface, and - arranging a reflector layer (14) on the structured layer (8), - wherein the recesses (12) or protrusions are formed as color-producing nanostructures (10) with respect to their dimensions along the base surface (20), their vertical extension (t) perpendicular to the base surface (20) and their arrangement on the base surface (20), and - wherein the structured layer (8) is formed with a plurality of areas (16, 18; 25a-d) providing a colored pattern or image, the areas (16, 18; 25a-d) preferably forming pixels, -in, In each of the plurality of regions (16, 18; 25a-d), the vertical extension (t) of the recess (12) or the protrusion varies along a direction (22) according to a function that does not have a constant value, so that in each of the plurality of regions (16, 18; 25a-d) in a top view, the color produced by the variation in the vertical extension is a mixed color.

10. The method according to claim 9, wherein: Production of a security element according to any one of claims 1 to 8.

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