Optically variable security element

Abstract

The invention relates to an optically variable security element (12) for securing security documents, valuables, and other data carriers, with a motif area (20) that displays different image information (14-A, 14-B, 14-C) from different viewing directions. According to the invention, the motif area (20) consists of two or more groups of congruent pixel elements (22-A, 22-B, 22-C), at least two of which (22-A, 22-B, 22-C) generate image information (14-A, 14-B, 14-C) from different viewing directions. The pixel elements (22-A, 22-B, 22-C) form the tiles of a tessellation of the plane and completely and without overlapping cover the motif area (20). The pixel elements (22-A, 22-B, 22-C) have a complex outline shape that is neither triangular, square, hexagonal nor circular, and the pixel elements (22-A, 22-B, 22-C) have a maximum dimension of 500 µm or less.

Description

[0001] The invention relates to an optically variable security element for securing security documents, valuable documents and other data carriers, with a motif area that shows different image information from different viewing directions.

[0002] Data carriers, such as valuables or identification documents, but also other valuables, such as branded goods, are often equipped with security elements for protection, which allow verification of the authenticity of the data carrier and also serve as protection against unauthorized reproduction.

[0003] Examples of conventional security elements are disclosed in WO 2014 / 086531 A1 and DE 10 2006 029850 A1.

[0004] Security features with viewing-angle-dependent effects play a particularly important role in ensuring authenticity, as these cannot be reproduced even with the most modern copying equipment. These security features are equipped with optically variable elements that convey a different visual impression to the viewer from different viewing angles, for example, displaying a different color or brightness impression and / or a different graphic motif depending on the viewing angle.

[0005] To ensure that an optically variable security element displays different image information from different viewing directions, the image area of ​​the security element is conventionally divided into a multitude of square or circular pixels or a multitude of rectangular strips. The pixels or strips are further subdivided into several groups, with each group of pixels or strips generating the image information for a specific viewing direction. Based on this, the invention aims to provide an optically variable security element of the type mentioned above with particularly high counterfeit protection.

[0006] This problem is solved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0007] The invention comprises four aspects, claims 1 to 4. In all four aspects, a generic optically variable safety element is provided that The motif area of ​​the security element consists of two or more groups of congruent pixel elements, at least two of which generate image information in different viewing directions; the pixel elements form the tiles of a tessellation of the plane and cover the motif area completely and without overlap; the pixel elements have a complex outline shape that is neither triangular, square, hexagonal nor circular; and the pixel elements have a maximum dimension of 500 µm or less.

[0008] Tessellation, in this context, refers to a continuous and non-overlapping covering of the plane by uniform sub-areas (hereinafter referred to as tiles). While the actual tessellation of the plane, by definition, extends to infinity, for the purposes of this invention only a finite sub-area of ​​the plane is relevant, namely the sub-area formed by the motif area. Within this sub-area, the pixel elements of the motif area correspond to the tiles of the tessellation. Just as the tiles cover the plane continuously and without overlap, the pixel elements also cover the motif area continuously and without overlap. Since the pixel elements within the motif area represent the tiles of the tessellation, the terms pixel element and tile are often used synonymously in this description.

[0009] As explained in more detail below, all tiles of the tessellation can have the same shape, or there can be a finite set of different tile templates (hereinafter referred to as proto-tiles) that determines the shape of the tiles appearing in the tessellation.

[0010] Each pixel element (tile) has a maximum dimension that is below 500 µm and preferably between 5 µm and 500 µm.

[0011] Advantageously, each tile of the tiling is congruent to an element of a set of distinct proto-tiles, wherein the cardinality of the proto-tile set is advantageously less than or equal to 6, and in particular is 1, 2, 3, or 4. In particularly advantageous embodiments, the cardinality of the proto-tile set is equal to 1, so that each tile of the tiling is congruent to exactly one proto-tile. Such a tiling is called a monohedral tiling.

[0012] Except in the second aspect of the invention (claim 2), in an advantageous embodiment the tiling is periodic, that is, the symmetry group of the tiling contains two linearly independent translations. As is customary, a mapping that maps the tiling onto itself is called the symmetry of the tiling. The set of all such symmetries forms a group called the symmetry group of the tiling.

[0013] In the first aspect of the invention (claim 1), the symmetry group of the tessellation contains at least one rotational symmetry of order 3 or 6. In the other aspects of the invention, in advantageous embodiments, the symmetry group of the tessellation contains at least one rotational symmetry of order 2, 3, 4 or 6.

[0014] In the second aspect of the invention (claim 2), the tiling is aperiodic and, in particular, represents a Penrose tiling. Aperiodic tilings can be composed of one or more proto-tiles.

[0015] The boundary points of the tiles where at least three tiles touch are called nodes, as in graph theory, and the boundary regions between the nodes are called edges. Advantageously, the tiles of the tessellation, especially those of a monohedral tessellation, do not have straight edges. This means that none of the edges of the single proto-tile (monohedral tessellation) or of all proto-tiles are straight. In this way, the complete covering of the plane can be combined with complex outline shapes of the individual tiles, thus providing the designer with great design freedom. In the third aspect of the invention (claim 3), the edges are each composed of two or more straight line segments, each forming an angle. In the other aspects of the invention, the edges can also be curved and described, for example, by second-degree Bézier curves, B-splines, or NURBS (Non-Uniform Rational B-splines).

[0016] In the fourth aspect of the invention (claim 4), the edge of at least one proto-tile represents a natural motif, namely a plant, an animal, a human, or a mythical creature. In the other aspects of the invention, the edge of at least one proto-tile advantageously represents a natural motif, in particular a plant, an animal, a human, or a mythical creature. Such designs have the advantage that they can usually be recognized and identified at first glance. In the field of art, such tessellations with recognizable tile motifs have been used in numerous works, particularly by the Dutch artist and graphic designer M.C. Escher.

[0017] In the fourth aspect of the invention (claim 4), the motif area includes a picture motif visible to the naked eye, the outline of which corresponds to the outline of a proto-tile. The picture motif visible to the naked eye advantageously has a dimension of more than 5 mm, in particular more than 10 mm. The size ratio of the picture motif and the corresponding proto-tile is preferably 10 or more, in particular 20 or more, or even 50 or more.

[0018] The pixel elements of the groups generating image information are suitably covered with diffractive structures, holographic structures, subwavelength gratings, printed structures, color-tilting thin-film elements, color-giving nanoparticles, polarizing elements, refractive structures such as micromirrors, microlenses or microfresnel lenses, or other functional layers.

[0019] The structures can, for example, have Gaussian or sinusoidal profiles or be formed by rectangular grids, cones, triangular grids or subwavelength grids.

[0020] At least some of the pixel elements can be additionally covered with further functional layers, in particular with electrical or magnetic functional layers, which can generate an additional, machine-verifiable coding in the security element.

[0021] The pixel elements can also be assigned multiple structures simultaneously, for example, to generate mixed colors. It is also not necessary for all pixel elements to be assigned the same type of structure. For example, one group of pixel elements can be assigned holographic structures, while another group is assigned micromirrors or printed structures.

[0022] It can also be intended that a group of pixel elements does not generate any image information, so that these pixel elements act like cutouts in the subject area.

[0023] Further embodiments and advantages of the invention are explained below with reference to the figures, in the representation of which a scale and proportion-accurate reproduction has been omitted in order to increase clarity.

[0024] They show: Fig. 1 a schematic representation of a banknote with a security element according to the invention, Fig. 2 a schematic top view of the motif area of ​​the security element of the Fig. 1 , Fig. 3 shows a tile from the motif area to illustrate the tile properties. Fig. 2 and additionally the adjacent tiles shown in dashed lines, Fig. 4 schematically a top view of the motif area of ​​a security element according to another embodiment of the invention, Fig. 5 schematically a top view of the motif area of ​​a security element according to a further embodiment of the invention, and Fig. 6 in (a) two proto-tiles of a Penrose tiling and in (b) schematically a top view of the motif area of ​​a security element according to the invention with a Penrose tiling using the proto-tiles of (a).

[0025] The invention will now be explained using the example of security elements for banknotes and other valuable documents. Fig. 1 Figure 1 shows a schematic representation of a banknote 10, which is provided with a security element according to the invention in the form of an affixed transfer element 12. In the exemplary embodiment, the transfer element 12 represents a changing image that presents different image information 14A, 14B and 14C to the viewer from three different viewing directions.

[0026] In the illustrated embodiment, the image information is formed by holographic letters "A", "B", and "C", whereby the viewer perceives the letter "A" when viewing obliquely from the left, the letter "B" when viewing vertically from above, and the letter "C" when viewing obliquely from the right. By tilting the banknote, the appearance of the security feature alternates between the three image information sets 14A, 14B, and 14C. It is understood that in practice, more complex motifs, such as geometric patterns, portraits, architectural, technical, or natural motifs, are typically used instead of the letters shown here for illustration. Furthermore, a changing image can contain only two or more than three image sets, or the security feature can display a moving image, a pump image, a morphing image, or a stereoscopic image instead of a changing image.

[0027] In such images with different image information for different viewing directions, the subject area typically consists of several groups of pixel elements, each generating the image information for one of the different viewing directions. The pixel elements are conventionally designed in the form of squares, circles, or parallel stripes, as explained in more detail above.

[0028] The special feature of the present invention lies in a particular design and arrangement of the pixel elements, the realization of which places the highest technological demands on the security and therefore significantly increases the counterfeit protection of the security element.

[0029] As an example, show Fig. 2 A schematic top view of the motif area 20 of the security element 12. The motif area consists of three groups of congruent pixel elements 22-A, 22-B, and 22-C, which are distinguished in the figure by different hatching. The pixel elements 22-A, 22-B, and 22-C form the tiles of a tessellation and, despite their complex shape and congruity, completely and without overlapping cover the motif area 20.

[0030] As from Fig. 2 As can be seen further, the pixel elements (or tiles) all have the same shape, namely the shape of a lizard, which is represented as proto-tile 24 next to the section of the motif area 20. The tiles 22 of the tiling are all congruent to proto-tile 24 and result from it by translation and rotation.

[0031] In this example, the tiles measure 100 µm (the maximum dimension of the lizard shape, here from the left front to the right rear foot), but can generally range between 5 µm and 500 µm. In the lower part of the specified range, the lizard shape of the tiles is not discernible to the human eye without aids, while at the upper end of the range it is visually perceptible as an irregular lizard shape.

[0032] In the exemplary embodiment, the pixel elements or tiles 22 are each covered with holographic structures, namely the pixel elements or tiles 22-A with first holographic structures (wide hatching) which, when illuminated perpendicularly, reconstruct the letter "A" diagonally to the left in the first viewing direction, the pixel elements or tiles 22-B with second holographic structures (medium hatching) which, when illuminated perpendicularly, reconstruct the letter "B" vertically upwards in the second viewing direction, and the pixel elements or tiles 22-C with third holographic structures (narrow hatching) which, when illuminated perpendicularly, reconstruct the letter "C" diagonally to the right in the third viewing direction.

[0033] When viewing banknote 10 with the naked eye, the special tiling of motif area 20 is not visible. Rather, the elements of the group of pixel elements 22-A, 22-B, and 22-C work together to reconstruct the holographic image of the letters "A," "B," or "C" in the respective viewing direction. Only when viewed with a magnifying glass or microscope does the reptilian tiling of motif area 20 become visible and thus serves as a higher-level security feature detectable only with such aids.

[0034] At the in Fig. 2 The tessellation shown is a periodic tessellation, i.e., a tessellation whose symmetry group contains two linearly independent translations. The translation vectors 26 of the two linearly independent translations are in Fig. 2 shown for illustration. Furthermore, the symmetry group of the tessellation contains rotational symmetries of order 3, as shown in particular in the representation of the Fig. 3 to recognize, which shows an arbitrarily selected tile 22-A and, additionally, the adjacent tiles 22-B, 22-C with dashed lines.

[0035] The boundary points of tile 22-A (and any other tile) where at least three tiles touch represent the nodes 60, 62 of the tile outline; the boundary regions 64 between nodes 60, 62 represent the edges of the tile outline. Every second node 60 (full points in Fig. 3 ) is the point of intersection of the rotation axis of a rotational symmetry of order 3. As in Fig. 3 As shown, the nodes 60 and 62 of the lizard tiles lie on the vertices of a regular hexagon 66. However, to create the lizard shape, the edges of the hexagon 66 were replaced by odd edges 64 composed of several straight segments. Due to the symmetry of the tiling, three of these odd edges result from the other three edges by rotating them about one of the nodes 60. The use of tiles 22 with exclusively odd edges leads to complex outline shapes of the tiles, which are difficult to reproduce and offer the designer a high degree of creative freedom.

[0036] A further characteristic of the tessellations according to the invention is that the symmetry group of the tessellations does not contain any mirror symmetry.

[0037] Instead of holographic structures, the pixel elements can also be assigned other structures that generate image information, as described above. As in Fig. 4 As shown, within a motif area 30, a group of pixel elements 32-A can also be unoccupied and generate no image information. While pixel elements 32-B and 32-C each generate image information, for example, of a rotating image, the unoccupied areas 32-A within the motif area 30 appear as cutouts with a complex outline shape.

[0038] With reference to Fig. 5 The shape of the tiles 22 can also accommodate a macroscopically existing outline of a view 44 of the motif area 42. For example, the motif area 42 of a security element 40 can show a holographic view 44 of a lizard, which is designed, for example, as a three-dimensional, true-color hologram. The individual color pixels of the true-color hologram are formed by the tiles 46-A, 46-B, and 46-C of the tiling of the motif area. In the exemplary embodiment, the tiling already described in [reference missing] was used. Fig. 2 The described tessellation uses three groups of pixel elements 46-A, 46-B, 46-C, all of which are lizard-shaped and congruent to the proto-tile 24 and the outline of the holographic view 44.

[0039] The maximum dimension of view 44 is, for example, 5 mm, while the corresponding maximum dimension of the tiles 46 is only 100 µm, so that the size ratio of view 44 and the corresponding proto-tile 24 in the exemplary embodiment is 50. The correspondence between the microscopic information formed by the outline of the tiles 46 and the macroscopic information formed by the outline of view 44 can be used as an additional authentication feature.

[0040] While in the previously described embodiments there exists exactly one proto-tile to which every tile of the tiling is congruent, there can also be several different proto-tiles. An embodiment with two different proto-tiles 50-A, 50-B is described with reference to Fig. 6 illustrated. The in Fig. 6(a) The depicted proto-tile 50-A is a rhombus with interior angles of 72° and 108°, and the proto-tile 50-B is a rhombus with interior angles of 36° and 144°. These proto-tiles can be used to create a so-called Penrose tiling 54 of a motif area 52, which, as shown in Fig. 6(b) The diagram shows that the tiling consists of pixel elements or tiles 56-A and 56-B. Tiles 56-A are all congruent to proto-tile 50-A, and tiles 56-B are all congruent to proto-tile 50-B. The special feature of these Penrose tilings is that they allow a gapless and overlap-free, yet aperiodic, covering of the plane.

[0041] In the exemplary embodiment, the pixel elements or tiles 56-A, 56-B are each covered with holographic structures for different viewing directions, resulting in a reversible image with two views. Aperiodic tessellations and Penrose tessellations can also be formed from more than two proto-tiles, which can be used accordingly for reversible images with more than two views.

[0042] Since the proto-tiles of a Penrose tiling, such as proto-tiles 50-A and 50-B, allow the generation of an infinite number of different Penrose tilings, the Penrose tiling implemented in a security feature can be used as an additional authentication mark. For example, a suitable algorithm can generate a unique Penrose tiling from the serial number of a banknote or security document and use it to create the motif area of ​​the associated security feature. During authentication, the serial number can be read, the corresponding Penrose tiling generated, and checked for consistency with the tiling implemented on the security feature.

[0043] Although the invention has been primarily described with reference to variable images, it is in no way limited to variable images or the aforementioned variants. Rather, the invention can be applied to all optically variable security elements where a motif area displays different image information from different viewing angles. The invention is also not limited to the transfer elements used for illustration in banknotes, but can, for example, also be used in security threads, wide security strips, or cover films arranged over a window area or a continuous opening in a document. Bezugszeichenliste

[0044] 10 Banknote 12 Transfer element 14-A, 14-B, 14-C Image information 20 Motif area 22, 22-A, 22-B, 22-C Pixel elements, tiles 24 Proto-tile 26 Displacement vectors 30 Motif area 32-A, 32-B, 32-C Pixel elements, tiles 40 Security element 42 Motif area 44 Holographic view of a lizard 46, 46-A, 46-B, 46-C Pixel elements, tiles 50-A, 50-B Proto-tiles 52 Motif area 54 Penrose tiling 56-A, 56-B Pixel elements, tiles 60, 62 Nodes 64 Edges 66 Regular hexagon

Claims

1. An optically variable security element (12) for securing security papers, value documents and other data carriers, having a motif region (20) that, from different viewing directions, shows different pieces of image information (14-A, 14-B, 14-C): - the motif region (20) consisting of two or more groups of, in each case, congruent pixel elements (22-A, 22-B, 22-C), of which at least two groups (22-A, 22-B, 22-C) produce a piece of image information (14-A, 14-B, 14-C) in different viewing directions, - the pixel elements (22-A, 22-B, 22-C) forming the tiles of a tiling of the plane and covering the motif region (20) with no gaps or overlaps, - the pixel elements (22-A, 22-B, 22-C) comprising a complex contour shape that is neither triangular, quadratic, hexagonal nor circular, - the pixel elements (22-A, 22-B, 22-C) having a maximum dimension of 500 µm or less, and - the symmetry group of the tiling includes at least one rotational symmetry of the order 3 or 6.

2. An optically variable security element (12) for securing security papers, value documents and other data carriers, having a motif region (20) that, from different viewing directions, shows different pieces of image information (14-A, 14-B, 14-C): - the motif region (20) consisting of two or more groups of, in each case, congruent pixel elements (22-A, 22-B, 22-C), of which at least two groups (22-A, 22-B, 22-C) produce a piece of image information (14-A, 14-B, 14-C) in different viewing directions, - the pixel elements (22-A, 22-B, 22-C) forming the tiles of a tiling of the plane and covering the motif region (20) with no gaps or overlaps, - the pixel elements (22-A, 22-B, 22-C) comprising a complex contour shape that is neither triangular, quadratic, hexagonal nor circular, - the pixel elements (22-A, 22-B, 22-C) having a maximum dimension of 500 µm or less, and - the tiling is aperiodic, especially constitutes a Penrose tiling.

3. An optically variable security element (12) for securing security papers, value documents and other data carriers, having a motif region (20) that, from different viewing directions, shows different pieces of image information (14-A, 14-B, 14-C): - the motif region (20) consisting of two or more groups of, in each case, congruent pixel elements (22-A, 22-B, 22-C), of which at least two groups (22-A, 22-B, 22-C) produce a piece of image information (14-A, 14-B, 14-C) in different viewing directions, - the pixel elements (22-A, 22-B, 22-C) forming the tiles of a tiling of the plane and covering the motif region (20) with no gaps or overlaps, - the pixel elements (22-A, 22-B, 22-C) comprising a complex contour shape that is neither triangular, quadratic, hexagonal nor circular, - the pixel elements (22-A, 22-B, 22-C) having a maximum dimension of 500 µm or less, and - the tiles (22-A, 22-B, 22-C) of the tiling comprise no straight edges, but rather the edges are each composed of two or more straight-line pieces that each form an angle, wherein those boundary points of the tiles where at least three tiles touch are called nodes, and the boundary areas between the nodes are called edges.

4. An optically variable security element (12) for securing security papers, value documents and other data carriers, having a motif region (20) that, from different viewing directions, shows different pieces of image information (14-A, 14-B, 14-C): - the motif region (20) consisting of two or more groups of, in each case, congruent pixel elements (22-A, 22-B, 22-C), of which at least two groups (22-A, 22-B, 22-C) produce a piece of image information (14-A, 14-B, 14-C) in different viewing directions, - the pixel elements (22-A, 22-B, 22-C) forming the tiles of a tiling of the plane and covering the motif region (20) with no gaps or overlaps, - the pixel elements (22-A, 22-B, 22-C) comprising a complex contour shape that is neither triangular, quadratic, hexagonal nor circular, - the pixel elements (22-A, 22-B, 22-C) having a maximum dimension of 500 µm or less, - each tile (56-A, 56-B) in the tiling is congruent to an element of a set of different prototiles (50-A, 50-B), and - the motif region (20) includes an image motif that is visible with the naked eye and whose contour shape corresponds to the contour shape of a prototile, the size ratio of the image motif and the corresponding prototile preferably being 10 or more, especially 20 or more or even 50 or more, and wherein the border of at least one prototile constitutes a natural motif, namely a plant, an animal, a person or a mythical creature.

5. The security element according to at least one of claims 1, 3 or 4, characterized in that the tiling is periodic, such that the symmetry group of the tiling includes two linearly independent displacements.

6. The security element according to at least one of claims 1, 3, 4 or 5, characterized in that the symmetry group of the tiling includes no mirror symmetry.

7. The security element according to at least one of claims 1 to 3, 5 or 6, characterized in that the border of at least one prototile constitutes a natural motif, especially a plant, an animal, a person or a mythical creature.

8. The security element according to at least one of claims 1 to 7, characterized in that the pixel elements (22-A, 22-B, 22-C) of the groups that produce a piece of image information are covered with diffractive patterns, holographic patterns, subwavelength gratings, refractive patterns such as mi-cromirrors, microlenses or micro-Fresnel lenses, printed patterns, color-shifting thin-film elements, chromophoric nanoparticles or polarizing elements.

9. The security element according to at least one of claims 1 to 8, characterized in that a group of pixel elements produces no piece of image information.

10. A data carrier having a security element according to at least one of claims 1 to 9.

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