DE102008046128B4 - Optically variable security element with matt area - Google Patents

Abstract

Optically variable security element for securing valuables, with an achromatic, matt shiny matt area, characterized in that the matt area contains a reflective matt structure made of a large number of microelements, which are determined by the parameters lateral dimension, height, outline shape, relief shape, spatial position and spatial orientation are characterized, wherein - the lateral dimension of the microelements is below the resolution limit of the human eye and - at least one of the characterizing parameters of the microelements varies randomly within the matt structure, in that at least one of the randomly varying characterizing parameters varies in a predetermined fluctuation range around a predetermined mean value, or at least one of the randomly varying characterizing parameters is concentrated on a predetermined fluctuation range around a predetermined mean value, wherein in the case of random variation the varied parameter obeys a specific probability function or density function which has a single maximum.

Description

The invention relates to an optically variable security element for securing valuables with an achromatic, matt, shiny matte area, a method for producing such a security element and a data carrier equipped with such a security element.

Data carriers, such as valuables or identification documents or other valuables, such as branded items, are often provided with security elements for security purposes, which allow the authenticity of the data carrier to be checked and which also serve as protection against unauthorized reproduction. The security elements can be designed, for example, in the form of a security thread embedded in a banknote, a tear thread for product packaging, an applied security strip or a self-supporting transfer element, such as a patch or a label that is applied to a document of value after its production. The applied security elements can also be located above a recess in the document of value.

In order to prevent the security elements from being recreated even with high-quality color copiers, these often have optically variable elements that give the viewer a different image impression from different viewing angles and, for example, show a different color impression or different graphic motifs. The optical effectiveness of holograms and hologram-like diffraction structures is based not least on the color splitting when polychromatic light falls on the diffraction structure. However, the resulting play of colors has become so commonplace in recent years that its effect as an attractive security feature is already diminishing significantly. In addition, the image or color impressions created by optical diffraction can often only be seen completely and clearly from certain viewing directions and in good lighting conditions. In particular, the visibility of holographic motifs is often severely limited in poor lighting conditions, such as diffuse lighting.

Optically variable, dull-appearing security elements have therefore been proposed which consist of modified diffraction gratings, the grating lines of which are subject to a stochastic fluctuation in their spatial orientation. The distribution of the emitted light is determined by the grating period, the average direction of the grating lines and the fluctuation range of the grating line alignment. Such modified diffraction gratings have a strong anisotropy of the outgoing light. Since the diffraction grating structures are designed for minimal light absorption, it is practically impossible to adjust the total light intensity emitted. The production of such diffraction gratings is usually carried out using electron beam lithography and therefore requires a lot of equipment.

Matting and darkening of a surface can also be achieved by using so-called moth-eye structures. These are regular or stochastic knob structures made up of knobs with a size of 50 to a few hundred nanometers. To produce such subwavelength structures, for example, a PMMA layer can be etched with an ion beam or a diamond surface can be processed with a laser beam. Moth eye structures absorb a high proportion of incident light and therefore always appear dark. Surfaces that appear dull and bright at the same time cannot be achieved with moth-eye structures. In addition, the production of subwavelength structures is very complex in terms of equipment.

From the DE 10 2005 061 749 A1 an optically variable security element and an associated manufacturing process are known. The WO 2007/115648 A1 relates to a security element and a method for its production. The EP 0 360 969 B1 concerns a diffraction element.

Proceeding from this, the invention is based on the object of improving a security element of the type mentioned at the outset and, in particular, of creating an optically variable security element with a matt area, the spatial radiation characteristics and the overall brightness of which can be adjusted as desired over a wide range.

This task is solved by the security element with the features of the main claim. A method for producing such a security element and a data carrier equipped with such a security element are specified in the independent claims. Further developments of the invention are the subject of the subclaims.

• - die laterale Abmessung der Mikroelemente unterhalb der Auflösungsgrenze des menschlichen Auges liegt und
• - zumindest einer der charakterisierenden Parameter der Mikroelemente innerhalb der Mattstruktur zufällig variiert.

According to the invention, in a security element of the type mentioned at the outset, it is provided that the matt area contains a reflective matt structure made up of a large number of microelements, which are characterized by the parameters lateral dimension, height, outline shape, relief shape, spatial position and spatial orientation, whereby

• - the lateral dimension of the microelements is below the resolution limit of the human eye and
• - At least one of the characterizing parameters of the microelements varies randomly within the matt structure.

For the purposes of the invention, a random variation of a parameter means that the parameter obeys a specific probability function (for discrete parameters) or density function (for continuous parameters), which indicates the probability of occurrence or the probability density of this value for each value of the parameter. If the frequency of the parameter values of the entirety of the microelements within the matt structure is considered, the associated probability function or density function of this parameter is reconstructed when plotted. However, the parameter value for a specific, arbitrarily selected microelement cannot be predicted in an obvious way.

The probability functions or density functions of different parameters are fundamentally independent of one another. In some embodiments, however, it may be advisable to link the probability functions or density functions of two or more parameters with one another, for example in order to set a high aspect ratio of the microelements to produce darkened areas.

According to the invention, the varied parameter obeys a certain probability function or density function in the random variation, which has a single maximum. The probability functions or density functions in question can extend over a certain range, can be discrete (probability functions), i.e. the microelements only have certain, discrete values with respect to the parameter under consideration, can be continuous or quasi-continuous (density functions), i.e. they occur all values in a certain continuous range, or they can also be continuous or quasi-continuous in some areas and discrete in some areas.

The microelements can in particular have a relief shape with a preferred direction. Microelements with a preferred direction reflect the incident light to different degrees in different solid angle ranges, so that their shape makes them suitable for generating an anisotropic brightness distribution. The spatial position is determined, for example, by a specific density distribution of the individual microelements.

The lateral dimension of the microelements is advantageously below 50 µm, in particular below 30 µm. On the one hand, this ensures that the microelements cannot be perceived as individual elements with the naked eye and that the microelements can be produced using established film technology. On the other hand, the small element size enables a variety of spatial arrangements, variations and combinations of different microelements, thus allowing the creation of desired visual appearances.

On the other hand, the lateral dimension of the microelements is preferably above 0.6 μm, preferably above 2 μm or even above 5 μm, in order to avoid wavelength-dependent light diffraction effects and to ensure that the incident light is reflected achromatically by the microelements without disturbing color effects becomes.

The microelements expediently have a square, rectangular, round, oval, honeycomb or polygonal outline shape. The lateral dimension of the microelements is advantageously not more than five times the dimension in any of the other directions in any direction.

In an advantageous variant of the invention, the microelements have a simple relief shape with exactly one reflection surface that is inclined towards the surface of the security element. The angle of inclination of the reflection surface of the microelements is expediently between 0° and approximately 35°. The reflection surface of the microelements can be flat, concave or convex in order to reflect the incident light into a larger solid angle range. To produce a matting effect, in particular the spatial orientation of the microelements within the matt structure, specified by an azimuth angle φ, is varied randomly and/or the inclination angle θ of the inclined reflection surface of the microelements within the matt structure is varied randomly and/or the outline shape of the microelements within the matt structure is random varies.

In another, also advantageous variant of the invention, the microelements have a relief shape with two or more reflection surfaces that are inclined in different directions towards the surface of the security element. In particular, the microelements can have a roof structure or a multi-sided pyramidal structure. In this variant too, the angle of inclination of the reflection surfaces of the microelements is expediently between 0° and approximately 35° and the reflection surfaces of the microelements can be flat, concave or convexly curved. To produce a matting effect, in particular that specified by an azimuth angle φ spatial orientation of the microelements within the matt structure varies randomly and/or the inclination angles θ _i of the inclined reflection surfaces of the microelements within the matt structure are varied randomly and/or the outline shape of the microelements within the matt structure is varied randomly.

According to a further advantageous variant of the invention, the microelements are formed by depressions in the surface of the matt area. In order to produce a matting effect, in this variant, in particular the outline shape and/or the sharpness of the edge on the outline line, and/or the depth and/or the relief shape and/or the spatial position of the depressions within the matt structure are varied randomly.

In a further variant of the invention, the microelements are formed by elevations in the surface of the matt area. In order to produce a matting effect, in this variant, in particular the outline shape and/or spatial orientation and/or the height and/or the relief shape and/or the spatial position of the elevations within the matt structure are varied randomly. It goes without saying that depressions and elevations can be combined with one another in a matt area, just as the different configurations of the microelements can generally be combined with one another.

According to the invention, it is provided that at least one of the randomly varying characterizing parameters varies in a predetermined fluctuation range around a predetermined mean value, or is concentrated in a predetermined fluctuation range around a predetermined mean value. Such a concentration can be achieved, for example, by using a density function in the form of a Gaussian bell curve, i.e. by a density proportional to exp(-(xm) ² /2s ² ), where m represents the desired mean value and s indicates the range of fluctuation. In order to take into account a limited value range of a parameter, the density function can be truncated accordingly at the value range limits.

In a further development of the invention, the parameters lateral dimension, height, outline shape and relief shape are coordinated with one another in order to create darkened areas in the reflective matt structure due to a high aspect ratio of the microelements. This tuning of the parameters can be achieved in particular by tuning the probability or density functions of the parameters involved.

In all variants of the invention, the reflective matt structure can have an opaque metal layer, a partially transparent metal layer, a transparent, high-refractive index layer, a thin-film element with a color-shifting effect or a liquid-crystalline layer as a reflection layer. Thin-film elements with a color-shift effect typically have a reflection layer, an absorber layer and a dielectric spacer layer arranged between the reflection layer and the absorber layer. The color shift effect in such thin-film elements is based on viewing angle-dependent interference effects caused by multiple reflections in the various sub-layers of the element. The embossed relief representing the matt structure can also have a coating with locally varying properties. The layer thickness of a metallization can vary between zero and a predetermined maximum value in a predetermined manner. For example, in a homogeneous matt structure, a raster image can be metallized or demetallized. The layers or at least one layer of a thin-film element can also vary locally.

In advantageous embodiments, the matt area of the security element forms a homogeneous, matt shiny surface with a predetermined spatial radiation characteristic and a predetermined overall brightness.

In other, equally advantageous designs, the matt area is part of a predetermined motif made up of motif areas with different spatial radiation characteristics and/or a different overall brightness. The predetermined motif can in particular be a black and white image or a grayscale image.

The different contrasts of the black-and-white image or the different gray levels of the gray-scale image are preferably caused by a different spatial radiation characteristic and/or a different overall brightness of the matt area and a surrounding area or by a different spatial radiation characteristic and/or a different overall brightness of Partial areas of the matt area with different characterizing parameters.

The overall brightness can be controlled, for example, via the density distribution of the microelements. The closer the microelements are arranged, the lower the brightness.

It is understood that the matt area has holographic or hologram-like diffraction structures, subwavelength gratings, moth-eye structures or other security features, such as incorporated magnetic substances, in particular with magnetic coding Targeted electrical conductivity, in particular through a targeted thickness of a metallic reflection layer, can be combined with color shift effects, with colored embossing varnish, with luminescent substances or the like.

The invention also includes a method for producing a security element of the type described, in which the surface profile of the matt structure is embossed into a UV-curable or thermoplastic lacquer layer. The embossed lacquer layer is advantageously coated with a reflection layer, in particular an opaque metal layer, a transparent, highly refractive layer, a thin-film element with a color-shifting effect or a liquid crystal layer. Alternatively, the lacquer layer can also be applied to a reflection layer, in particular an opaque metal layer, a transparent, highly refractive layer, a thin-film element with a color-shifting effect or a liquid crystal layer and only embossed after application.

In preferred embodiments, the matt structure, optionally with an applied reflection layer, is leveled by a transparent cover layer in order to prevent the matt structure from being molded.

The invention further contains a data carrier, in particular a document of value, such as a banknote, ID card or the like, with a security element of the type described.

Further exemplary embodiments and advantages of the invention are explained below with reference to the figures, which are not reproduced to scale and proportions in order to increase clarity.

• 1 eine schematische Darstellung einer Banknote mit einem erfindungsgemäßen Sicherheitselement,
• 2 bis 4 einige der für die Erfindung infrage kommenden Mikroelemente, jeweils einzeln dargestellt,
• 5 in (a) und (b) jeweils einen Ausschnitt einer Aufsicht auf die Oberfläche einer erfindungsgemäßen Mattstruktur, bei der die Mikroelemente jeweils durch die Erhebung der 4(b) gebildet sind,
• 6 eine Mattstruktur mit einem Aspektverhältnis t/b,
• 7 einen typischen Aufbau einer Folie mit einer erfindungsgemäßen Mattstruktur, und
• 8 eine Folie nach einer weiteren Erfindungsvariante, bei der die Lichtbrechung an der strukturierten Oberfläche eines transparenten Materials ausgenutzt wird.

Show it:

• 1 a schematic representation of a banknote with a security element according to the invention,
• 2 to 4 some of the microelements in question for the invention, each shown individually,
• 5 in (a) and (b) each shows a section of a top view of the surface of a matt structure according to the invention, in which the microelements are each formed by the elevation of the 4(b) are formed,
• 6 a matt structure with an aspect ratio t/b,
• 7 a typical structure of a film with a matt structure according to the invention, and
• 8th a film according to a further variant of the invention, in which the refraction of light on the structured surface of a transparent material is exploited.

The invention will now be explained using the example of a banknote. 1 shows a schematic representation of a banknote 10 with a security element 12 according to the invention in the form of a glued-on transfer element, which has a silvery, matt shiny area 16 in addition to a metallic reflecting area 14. Other metal-colored, matt shiny areas 16, for example copper or gold-colored, are also conceivable. Of course, it is also possible to combine several different metal-colored matt shiny areas.

As explained in more detail below, the matt area 16 contains a large number of small microelements, which, however, do not cause any color splitting, so that the matt area 16 appears achromatic. The matting effect of the matt structure is based on reflection, absorption, scattering and/or refraction effects by the microelements involved and typically leads to a silvery matt appearance that stands out visually clearly from the highly shiny metallic area 14 outside the matt area 16.

The arrangement and design of the microelements according to the invention allows both the solid angle areas into which the incident light is emitted from the matte area 16, i.e. the spatial radiation characteristics of the matte area, as well as the proportion of the total emitted light, i.e. the overall brightness of the matte area 16, to be determined Wide ranges can be adjusted as desired. This gives the designer a lot of scope to design visually appealing security elements.

It is understood that the invention is not limited to transfer elements and banknotes, but can be used in all types of security elements, for example in labels on goods and packaging or in securing documents, ID cards, passports, credit cards, health cards and the like.

In order to create the silvery matt impression of the matt area 16, it contains a reflective matt structure made up of a large number of microelements, which are characterized by the parameters lateral dimension, height, outline shape, relief shape, spatial position and spatial orientation and at least one of these characterizing parameters within the area of the matt structure varies randomly. A random variation means that the corresponding parameter of a certain probability function or Density function obeys, as explained in more detail above. All microelements are so small that they cannot be perceived as individual elements with the naked eye. In particular, their lateral dimension is below 50 µm, preferably even below 30 µm.

Some of the microelements in question for the invention are individually in the 2 to 4 shown.

2(a) shows as the first microelement an achromatic reflecting micromirror 22, which has a reflection surface 24 with a certain reflectivity that is inclined towards the surface 20 of the security element. In the exemplary embodiment shown, the micromirror has a square base area A, therefore a square outline shape, and a lateral dimension of 15 x 15 μm ² . The relief shape of the micromirror is determined by the single flat reflection surface 24, which includes an angle of inclination θ with the surface 20 of the security element. At an inclination angle of θ = 30 °, the micromirror 22 shown has, for example, a structure height h of h = 15 μm * tan (30 °) = 8.7 μm.

The spatial orientation of the micromirrors 22 is specified by an azimuth angle φ, which is defined as the angle between a reference direction R and the vector that is created by projecting the normal vector of the reflection surface 24 onto the base surface. The spatial position of each micromirror 22 can be specified by an x and a y coordinate relative to the surface 20 of the security element.

Each of the multitude of micromirrors 22 reflects incident light 26 in accordance with the specifications by its characterizing parameters into a specific solid angle range 28. A matting effect arises if, in a matt structure made up of a multitude of small micromirrors 22, at least one of the characterizing parameters of the micromirrors 22 has a random variation , so that the micromirrors 22 do not all radiate into the same solid angle range 28. The greater the variation of the characterizing parameter(s), the larger the solid angle range into which the incident light is directed by the entirety of the micromirrors 22 is generally.

For example, the azimuth angle φ and the inclination angle θ of the micromirrors 22 of a matte structure may have a random variation. If these parameters are chosen completely randomly and equally distributed within the possible range of values, here between 0° and 360° for the azimuth angle φ and between 0° and 90° for the inclination angle θ, the matt structure appears equally matt from all viewing directions. If, on the other hand, the values that occur for the azimuth angle φ and/or the inclination angle θ are limited to certain angular ranges or concentrated in certain angular ranges, the matt structure appears matt on all sides, but shines brighter in some solid angle ranges than in others.

For example, the azimuth angle φ can vary randomly between 0° and 360° and the inclination angle θ can be limited or concentrated in the range between 20° and 25° in order to achieve a rotationally symmetrical matt appearance with a particularly high luminosity when viewed obliquely at approximately 45° and vertically To obtain light incidence on the surface 20. If the azimuth angle φ is not uniformly distributed, the brightness varies depending on the direction. By determining the proportion of reflection surfaces oriented in certain directions, practically any desired spatial radiation characteristic of the matt area can be obtained.

In generalization of the square form of the 2(a) The shape of the micromirrors can be chosen arbitrarily and can also vary randomly from element to element in order to achieve or reinforce a matting effect.

In further exemplary embodiments, the reflection surfaces 24 do not have to be flat, but can also be convex or concave or, as in the cross section of the 2 B) shown, be convexly curved. In the latter case, the solid angle range 30 from which the reflected radiation 28 can be seen is enlarged compared to the case of flat reflection surfaces 24.

The transition between individual micromirrors can be abrupt or continuous. Abrupt transitions in the form of edges lead to the scattering of the incident light and therefore enlarge the areas of space into which light is emitted. In the case of continuous transitions, depending on the type of transition, light is reflected through the transition regions into additional solid angle regions, which can lie, for example, between the solid angle regions of the interconnected microelements.

Instead of a simple relief profile, the micromirrors can also have a more complex relief shape with two or more reflection surfaces inclined in different directions towards the surface 20 of the security element, such as those in 2(c) shown roof shape 32 or in 2(d) shown multi-sided pyramidal relief shape 34. It goes without saying that the reflection surfaces are also at These more complex profile shapes can be convex or concave.

Another type of microelements according to the invention is described in 3 shown depressions 42, 44, 46 in the surface 40 of a matt area. The depressions 42, 44, 46 form scattering centers for incident light and thus produce a matting effect when at least one of their characterizing parameters varies randomly. To illustrate, show the 3(a) to (c) depressions with different outline shapes (circular outline for depressions 42 and 44, hexagonal outline for depression 46), with different relief shapes (dome-shaped relief for depression 42, cylindrical relief for depressions 44 and 46) and with different depths t ₁ to t ₃ . A varying relief shape can also be achieved by varying the sharpness of the edges 48 on the outline lines.

In the 4 The elevations 52, 54 shown in the surface 50 of a matt area represent a further type of microelements according to the invention. The elevations 52, 54 form scattering and reflection centers for the incident light and thus produce a matting effect with random variation of at least one of their characterizing parameters. Shows for illustration 4(a) a dome-shaped elevation 52 and 4(b) an elevation 54 in the form of a straight pyramid on a symmetrical hexagonal base.

In the matt structure, the rotationally symmetrical elevations 52 reflect the incident light independently of the azimuthal direction of incidence and thus lead to a substantially isotropic light distribution of the emitted light. In contrast, microelements reflect with a preferred direction, such as the elevations 54 of the 4(b) , the incident light has different intensity in different angular ranges, so that with such elements an anisotropic brightness distribution can be achieved due to their shape.

5 shows in (a) a section 56 of a top view of the surface 50 of a matt structure according to the invention, in which the microelements pass through the elevations 54 of the 4(b) are formed. The elevations 54 are all aligned with the same azimuth angle φ, so that due to the hexagonal shape of the pyramids 54, an anisotropic azimuthal brightness distribution results in the reflected light. In addition, the spatial position of the elevations 54 has a random variation and thus creates a matt appearance of the matt structure shown.

$$\text{y}=\text{j}\ast \text{dy}+\text{dy/2}\ast \text{rand}\left(-\mathrm{1,1}\right)$$

gegeben sein, wobei dx und dy einen Gitterabstand in x- bzw. y-Richtung darstellen und rand(-1,1) eine Zufallszahl oder geeignete erzeugte Pseudozufallszahl aus dem Intervall (-1,1) darstellt. Die Lage einer Erhebung 54 weicht dann von dem Gitterpunkt (x_i,y_j) = (i*dx, j*dy) um einen zufälligen Versatz ab, der in x-Richtung zwischen -dx/2 und +dx/2 und in y-Richtung zwischen -dy / 2 und + dy / 2 liegt.The position of the surveys 54 can be determined, for example, by the relationships $$\text{x}=\text{i}\ast \text{dx}+\text{dx/2}\ast \text{edge}\left(-1.1\right)$$

$$\text{y}=\text{j}\ast \text{dy}+\text{dy/2}\ast \text{edge}\left(-1.1\right)$$

be given, where dx and dy represent a grid spacing in the x and y directions, respectively, and rand(-1,1) represents a random number or a suitable generated pseudo-random number from the interval (-1,1). The position of an elevation 54 then deviates from the grid point (x _i , y _j ) = (i*dx, j*dy) by a random offset, which in the x direction is between -dx/2 and +dx/2 and in y direction is between -dy / 2 and + dy / 2.

An anisotropic brightness distribution can also be achieved in that the individual microelements (recesses, elevations or both) are arranged with different densities along two spatial directions of the plane of the security element. 5(b) shows a section 58 of an exemplary embodiment in which the microelements again pass through the elevations 54 of the 4(b) are formed. The average distance dx of the elevations 54 in the x direction is chosen to be smaller than the average distance dy of the elevations 54 in the y direction, so that depending on the different densities, a different intensity of reflection occurs in the x and y directions. In the exemplary embodiment shown, the anisotropy created thereby is added to the anisotropy due to the hexagonal shape of the pyramids 54 and creates a complex azimuthal brightness distribution of the matte area.

In all designs, increased light absorption of the matt structure can be generated through a high aspect ratio, i.e. through a high ratio of structure depth to structure width. Particularly in metallized structures with high surface density and high aspect ratio, the absorption of light plays an important role alongside reflection or scattering. With surface structures with a high aspect ratio, metallic surfaces can be darkened in a targeted manner, which can be used to create image motifs, for example halftone images, as explained below. Regarding 6 the aspect ratio of a matt structure 60 is given by the ratio of structure depth t to structure width b. Significant light absorption occurs for aspect ratios t/b above 1, in particular above 3 or especially above 5.

With the microelements described, matt areas can be formed in the form of a homogeneous, matt shiny surface with a desired spatial radiation characteristic and a desired overall brightness.

In addition, as already in 1 shown by combining one or more matts Areas with metallic reflective surrounding areas generate “black and white” images, i.e. binary representations with two different contrast values. The necessary image contrast between “black” and “white” results from the invention due to the different radiation characteristics of the matt and metallic reflecting areas.

For example, the white image areas in a black-and-white image to be displayed can be filled with matt structures, while the black image areas are metallically reflective. Of course, it is also possible to fill the black image areas with another matt structure instead of an unstructured metallic area, which has a sufficient contrast to the matt structure of the white image areas in at least one spatial direction.

In more complex embodiments, grayscale images can also be generated in this way, with the grayscales being generated by screening “black”, i.e., for example, metallic reflective pixel elements, and “white”, i.e. matt pixel elements, as explained above.

In addition to these binary designs, grayscale images can also be displayed with real grayscales, i.e. not generated by black and white screening. For this purpose, several different matt structures with different brightnesses or different radiation characteristics are used depending on the desired number of gray levels.

For example, the degree of darkening of a partial area while maintaining the same lateral extent can be determined by the aspect ratio t/b of the matt structure used. A true grayscale image can therefore be created by making the depth of the matt structure larger in dark pixels and smaller in bright pixels. The depth of the matt structure is selected according to the specifications of the grayscale image to be displayed as a function of the location.
In addition to the aspect ratio, other characterizing parameters of the microelements can of course also be used to influence the radiation characteristics and overall brightness and thus to produce a black-and-white or grayscale image.

A spatial anisotropy of the radiation characteristic can also be used to display different images in different spatial directions. For example, the matt structures of a black and white image can be selected so that the microelements that reflect strongly in a first spatial direction reflect only slightly in a second spatial direction, while the microelements that reflect strongly in the second spatial direction reflect in the first spatial direction reflect only slightly. When the viewing direction changes from the first to the second spatial direction, the contrast of the displayed image is reversed.

In the general case, the different microelements can also be coordinated with one another in such a way that they jointly produce a first motif in a first spatial direction and jointly produce a second motif in a second spatial direction. When the viewing direction changes from the first to the second spatial direction, the first motif merges into the second motif for the viewer.

The production of the described homogeneous matt structures or those designed as image motifs can be done in a variety of ways. A particularly suitable method for producing an embossing master is based on exposing photoresist with a focused laser beam.

For this purpose, a substrate, for example made of quartz or soda lime, is coated with photoresist of a suitable thickness, for example by spin coating. Suitable paints include the AZ® series paints, which belong to the diazonaphtoquinone group and can cover a layer thickness range from 300 nm to more than 15 µm.

After a soft bake step to remove the solvent, the paint is exposed using a focused laser beam. Different areas of the structure can be exposed to different dose values, so that different structural depths result after the development step. Areas that are more exposed have a greater structural depth than areas that are less exposed. In this way, very complex surface structures can be created with a resolution of less than 1 µm. The depth of the structures can significantly exceed 10 µm and aspect ratios of up to around 10 can be achieved.

The original created in this way can be reproduced after galvanic molding and further processed into an embossing tool. Using the embossing tool, the structure can be embossed on a film web in a UV-curable lacquer or a thermoplastic. After the embossing step, the film webs can be vapor-coated with a reflective metal layer. In some embodiments it is also possible to vapor deposition color-shifting interference layer systems, which work in conjunction with the matt structures Create powerful optical effects, especially color effects.

A typical structure of a film 70 with a matt structure 72 according to the invention is shown in 7 shown. The film 70 contains, in layer sequence from bottom to top, a carrier film 74, a UV embossing lacquer layer 76 with the embossed matt structures 72, a metallic layer 78, for example a 50 nm thick aluminum layer, and a protective layer 80 made of transparent material, which levels the matt structure 72 and thus protects against unwanted impressions.

The metallization 78 can cover the matt structures 72 completely or only partially. Instead of the metallization 78, a thin-film element with a color-shift effect can also be provided, which typically has a reflection layer, an absorber layer and a dielectric spacer layer arranged between the reflection layer and the absorber layer. The individual layers are usually vapor deposited or applied via sputtering; this can be done perpendicularly or at an oblique angle to the surface to be vaporized.

In a further inventive variant of a film structure, the refraction of light on the structured surface of a transparent material is exploited. Regarding 8th In the exemplary embodiment 82 shown, a thin-film element 84 with a color-shifting effect is applied over a carrier film 74. In other embodiments, the thin-film element 84 can be replaced by a metallization layer or can be omitted without replacement.

A UV embossing lacquer layer 76 is arranged on the thin-film element 84, into which the desired matt structures 72 are embossed. In order to obtain a sufficiently high difference in refractive index on the surface of the security element 82, the matt structures 72 are exposed directly on the surface. Alternatively, the matt structures 72 can also be leveled with a material with a sufficiently high refractive index, provided there is a sufficiently large difference in refractive index between the embossing lacquer and the leveling layer (preferably Δn > 0.3).

Additional matting can be achieved by providing the embossing varnish 76 with light-scattering particles. These light-scattering particles are generally particles that have a different refractive index than the surrounding embossing lacquer 76, such as metal particles. The scattering properties of the embossing varnish/particle system depend on the nature of the particles and their density in the varnish. Additionally or alternatively, the light-scattering particles can also be contained in the protective varnish that levels the embossed matt structures. If the particles are only provided in the protective varnish, this has the advantage that the particles cannot have any disruptive influence on the UV curing during the embossing process.

It is understood that the matt structures described in this application can be combined with other security features to further increase protection against counterfeiting. In particular, a matt area with holographic or hologram-like diffraction structures, with sawtooth structures, with subwavelength gratings, moth-eye structures, with the above-mentioned thin-film elements with color-shifting effect or other color effects, with incorporated magnetic substances, in particular with magnetic coding, with specifically adjusted electrical conductivity, in particular by a targeted set thickness of a metallic reflection layer, with colored embossing varnish or with luminescent materials of any kind.

Claims (25)

Optically variable security element for securing valuables, with an achromatic, matt shiny matt area, characterized in that the matt area contains a reflective matt structure made of a large number of microelements, which are determined by the parameters lateral dimension, height, outline shape, relief shape, spatial position and spatial orientation are characterized, wherein - the lateral dimension of the microelements is below the resolution limit of the human eye and - at least one of the characterizing parameters of the microelements varies randomly within the matt structure, in that at least one of the randomly varying characterizing parameters varies in a predetermined fluctuation range around a predetermined mean value, or at least one of the randomly varying characterizing parameters is concentrated on a predetermined fluctuation range around a predetermined mean value, wherein in the case of random variation the varied parameter obeys a specific probability function or density function which has a single maximum. security element Claim 1 , characterized in that the microelements have a relief shape with a preferred direction. security element Claim 1 or 2 , characterized in that the microelements have a lateral dimension of less than 50 μm, preferably less than 30 μm. Security element according to at least one of the Claims 1 until 3 , characterized in that the microelements have a relief shape with exactly one reflection surface that is inclined towards the surface of the security element. security element Claim 4 , characterized in that the spatial orientation of the microelements within the matt structure, specified by an azimuth angle φ, varies randomly and/or the inclination angle θ of the inclined reflection surface of the microelements varies randomly within the matt structure and/or the outline shape of the microelements varies randomly within the matt structure. security element Claim 4 or 5 , characterized in that the reflection surface of the microelements is flat, concave or convexly curved. Security element according to at least one of the Claims 1 until 3 , characterized in that the microelements have a relief shape with two or more reflection surfaces inclined in different directions towards the surface of the security element, the microelements in particular having a roof structure or a multi-sided pyramidal structure. security element Claim 7 , characterized in that the spatial orientation of the microelements within the matt structure, indicated by an azimuth angle φ, varies randomly and/or the inclination angles θ _i of the inclined reflection surfaces of the microelements vary randomly within the matt structure and/or the outline shape of the microelements varies randomly within the matt structure. security element Claim 7 or 8th , characterized in that the reflection surfaces of the microelements are flat, concave or convexly curved. Security element according to at least one of the Claims 1 until 3 , characterized in that the microelements are formed by depressions in the surface of the matt area. security element Claim 10 , characterized in that the outline shape and/or the sharpness of the edge on the outline line and/or the depth and/or the relief shape and/or the spatial position of the depressions within the matt structure varies randomly. Security element according to at least one of the Claims 1 until 3 , characterized in that the microelements are formed by elevations in the surface of the matt area. security element Claim 12 , characterized in that the outline shape and/or spatial orientation and/or the height and/or the relief shape and/or the spatial position of the elevations within the matt structure varies randomly. Security element according to at least one of the Claims 1 until 13 , characterized in that the parameters lateral dimension, height, outline shape and relief shape are coordinated with one another in order to create darkened areas in the reflective matt structure due to a high aspect ratio of the microelements. Security element according to at least one of the Claims 1 until 14 , characterized in that the reflective matt structure has, as a reflection layer, an opaque metal layer, a transparent, highly refractive layer, a thin-film element with a color-shifting effect or a liquid-crystalline layer. Security element according to at least one of the Claims 1 until 15 , characterized in that the matt area forms a homogeneous, matt shiny surface with a predetermined spatial radiation characteristic and a predetermined overall brightness. Security element according to at least one of the Claims 1 until 15 , characterized in that the matt area is part of a predetermined motif made up of motif areas with different spatial radiation characteristics and/or a different overall brightness. security element Claim 17 , characterized in that the predetermined motif is a black and white or a grayscale image. security element Claim 18 , characterized in that the different contrasts of the black-and-white image or the different gray levels of the gray-scale image are caused by a different spatial radiation characteristic and/or a different overall brightness of the matt area and a surrounding area or by a different spatial radiation characteristic and/or a different overall brightness of partial areas of the matt area with different characterizing parameters. Security element according to at least one of the Claims 1 until 19 , characterized in that the matt area is equipped with holographic or hologram-like diffraction structures, subwavelength gratings, moth-eye structures or with further security features, such as incorporated magnetic substances, in particular with magnetic coding, with specifically adjusted electrical conductivity, in particular through a specifically adjusted thickness of a metallic reflection layer, with color-shifting effects, with colored embossing varnish, with luminescent materials or the like. Method for producing a security element according to at least one of Claims 1 until 20 , in which the surface profile of the matt structure is embossed into a UV-curable or thermoplastic lacquer layer. Procedure according to Claim 21 , characterized in that the embossed lacquer layer is coated with a reflection layer, in particular an opaque metal layer, a transparent, high-refractive index layer, a thin-film element with a color-shifting effect or a liquid crystal layer. Procedure according to Claim 21 , characterized in that the lacquer layer is applied to a reflection layer, in particular an opaque metal layer, a transparent, high-refractive index layer, a thin-film element with a color-shifting effect or a liquid crystal layer and is embossed after application. Method according to at least one of the Claims 21 until 23 , characterized in that the matt structure, optionally with an applied reflection layer, is leveled by a transparent cover layer. Data carrier, in particular a document of value, such as a banknote, ID card or the like, with a security element according to one of the Claims 1 until 20 .

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