HK1182773B - Security element comprising an optically variable surface pattern - Google Patents

Description

Security element with optically variable surface pattern

Technical Field

The invention relates to a security element for security papers, value documents and further data carriers which contain an optically variable surface pattern in the surface region. The invention also relates to a method for producing such a security element and to a data carrier with such a security element.

Background

Data carriers such as value documents or authentication documents and further value articles such as markers are often provided with security elements for security protection, which allow the authenticity of the data carrier to be verified and at the same time serve as protection against unauthorized copying.

Security elements with viewing-angle-dependent effects play a particular role in the authenticity protection, since these cannot be copied even with the most modern copying devices. The security element is designed here with an optically variable element which makes it possible for the observer to obtain different image impressions at different viewing angles and, depending on the viewing angle, for example, to display a further color or brightness impression and/or a further graphic theme.

It is often desirable to have the optically variable security element display the appearance of being as colorless as possible and therefore free of color separation. For this purpose, periodic structures (diffraction grids) and/or periodic arrangements of structures, for example pixel arrangements of micromirrors, are generally used. The known optically variable security elements can be roughly divided into: a) a reflective element that substantially complies with the laws of geometric optics, and b) a diffractive element that complies with the laws of diffractive optics.

In the elements of group b), there is a very small structural size in a large area, the periodic structure appearing distinctly colored to the observer. To create a colorless impression, it is therefore necessary to use a non-periodic structure or to combine gratings of different periods such that the color of the individual gratings complements the color white in the diffraction pattern. In the elements of group a), the structural dimensions are then always so large that the precise period does not play a significant role.

From document EP0868313B1 an optically variable surface pattern of a sawtooth grid based on diffractive optical action is known. The period is selected here such that the diffractive reflections at different wavelengths in the visible spectrum are as close together as possible, so that they can no longer be separated by the observer. Virtually all colors are diffracted in the direction given by the tilt angle of the sawtooth element and the security element appears colourless under normal diffraction conditions.

But at very small periods of the grid, the diffraction reflections of the grid are clearly visible. Thus, such an arrangement with a reduced grid period is always chromatic. Furthermore, this diffractive effect also interferes with the dynamic effect of the security element or of the image of the appearance of the elevations caused by the reflective behaviour of the surfaces of the elevations. However, in many cases as small a cycle time as possible is desirable, since thin structures are easier to manufacture and can be integrated more easily in banknotes or further data carriers.

Disclosure of Invention

Starting from this, the object of the present invention is to further develop a security element of the type mentioned at the outset and in particular to provide an optically variable security element which is simple and inexpensive to produce and has a high security against forgery and an attractive visual appearance. Ideally, the security element exhibits a colourless appearance without interfering colour separation or overlapping diffraction patterns.

This object is achieved by the features of the independent claims. The invention extends to the subject matter of the dependent claims.

According to the invention, a security element of the type mentioned at the outset comprises an optically variable surface pattern formed by an at least partially periodic arrangement of reflective elements which essentially act as beam optics, and wherein the reflective elements are non-periodically staggered with respect to one another over the surface region by their height.

In the case of a non-periodic staggering of the reflective elements, there is no simple, regular relationship between the heights of adjacent reflective elements, unlike in the case of a periodic staggering. As a result, constructive interference of the light reflected on adjacent reflective elements and the resulting occurrence of superimposed diffraction patterns are reliably prevented.

Preferably, the height of the reflective elements varies over the surface area according to a pseudo random number distribution. A pseudo-random number is a sequence of numbers that, although appearing random, is computed by a deterministic algorithm and is therefore not a true random number in the strict sense. However, pseudo-random numbers are widely used, since the statistical properties of the pseudo-random number distribution, such as the equal probability of a single number or the statistical independence of successive numbers, are generally sufficient for practical purposes and are easy to generate by computers compared to true random numbers. The pseudo-random number distribution is always non-periodic in the sense of the present invention, since in the pseudo-random number distribution there is no fixed, constant distance ("period") of successive values.

The non-periodic variation of the height values is of course not limited to a pseudo-random number distribution, but can also be achieved by a further irregular distribution of the height values.

Preferably, the reflective elements are staggered with respect to each other by a height offset between 0 and Δ H_maxIn which Δ H_maxThe value of (A) is between 50nm and 2000nm, preferably between 100nm and 500 nm. Here,. DELTA.H_maxIs suitably chosen to be larger than half the wavelength in the wavelength range of interest. If the optically variable surface pattern is embedded in the refractive index n>1, for example embedded in a transparent protective lacquer layer, the height offset can be reduced to 1/n of the value given above. For example, when embedded in a protective lacquer with n =1.5, the maximum height offset Δ H is therefore greater_n=260nm in order to cover all wavelengths in the visible spectral range, e.g. by Δ H_n= 780 nm/2)/n.

In an advantageous inventive variant, the arrangement of the reflective elements forms an at least partially periodic one-dimensional grid, or an at least partially periodic two-dimensional Bravais grid. The reflective elements can be arranged globally periodically or can be arranged only locally, the local period parameters changing only slowly as a function of the period length. For example, the local period parameter can be modulated periodically by an extension of the security element, wherein the modulation period is preferably greater than 20 times, preferably greater than at least 50 times, particularly preferably greater than at least 100 times the length of the local periodicity. Such slow changes of the local period parameters do not affect the basic local features, in particular the diffraction features of the grid. Due to the slow change of the period parameters, this arrangement can be described locally with sufficient accuracy by a Bravais grid with constant grid parameters.

Within the scope of the invention, the periodic or local periodic length is advantageously at least 1 μm to 40 μm, preferably at least 2 μm to 30 μm, and particularly preferably at least 3 μm to 10 μm. Accordingly, the dimension of the reflective element in the periodic direction is 1 μm to 40 μm, preferably 1.5 μm to 30 μm, and in particular 2 μm to 10 μm. In particular, the size of the reflective element may also be smaller than the period length.

The optically variable surface pattern particularly shows a colorless image without color separation or overlapping diffraction patterns.

The reflective element may have various forms and is preferably formed by a micro-flat mirror, a micro-concave mirror, a micro-convex mirror or a reflective pyramid mechanism. The reflective element is advantageously formed by providing the relief structure with a reflective layer, in particular by embossing the relief structure. The reflective layer can be formed here, for example, by a reflective, opaque metal layer or by a thin or rasterized, translucent metal layer. The reflective layer can also be formed by a colored or counter-colored reflective layer, preferably by a thin-layer element with a reflective layer, an absorbing layer and a dielectric spacer layer arranged between the reflective layer and the absorbing layer, or by a thin-layer element with two absorbing layers and a dielectric spacer layer arranged between the two absorbing layers. Finally, the reflective layer can also be advantageously formed by a highly refractive dielectric layer or a cholesteric liquid crystal layer.

In an advantageous development, the optically variable surface pattern comprises a plurality of subregions which show different representations. The optically variable surface pattern may also be combined with further security features, for example with a hologram. The total thickness of the security element is preferably less than 50 μm, preferably less than 30 μm, and particularly preferably less than 20 μm. Such a small overall thickness can be produced according to the invention, since, by means of the aperiodic height offset, it is also possible to use smaller grid periods with correspondingly low structure heights without disturbing color separation and overlapping diffraction patterns.

The invention also comprises a data carrier, in particular a security paper or a value document, with a security element of the described type. The data carrier can be in particular a value document, for example a banknote, in particular a paper banknote, a polymer banknote or a film composite banknote, but also a document, for example a credit card, a bank card, a cash payment card, a qualification card, an identity card or a passport personalization sheet.

Another object of the invention is an embossing die with a preferably metallic body for producing an embossed structure containing an optically variable surface pattern in the surface region, which optically variable surface pattern shows different representations in different illumination and/or viewing directions, wherein the optically variable surface pattern consists of an at least partially periodic arrangement of reflective elements which essentially act as ray optics, and wherein the reflective elements are non-periodically staggered relative to one another in the surface region over their height.

The imprint mold is typically a negative of the desired surface pattern in the imprint structure. By means of the embossing die, the target substrate, for example a thermoplastic or UV-curable lacquer layer applied to the film, is embossed and thus provided with the desired surface pattern. Corresponding methods are known to the expert and are described in the prior art.

The invention further relates to a method for security elements for the security protection of security papers, value documents and further data carriers, wherein a substrate is provided and the substrate is provided in a surface region with an optically variable surface pattern which displays different representations in different illumination and/or viewing directions, wherein the optically variable surface pattern is formed by an at least partially periodic arrangement of reflective elements which essentially act as beam optics and the reflective elements are non-periodically offset from one another over the surface region in accordance with their height.

The type of illustration of the security element according to the invention is not limited. For example, with the optically variable surface pattern according to the invention it is possible to produce an inversion diagram with two or more regions with different subjects that flash brightly from different viewing directions, as described in documents EP0868313B1 and EP1397259B1 for a grid structure that functions for diffractive optics. Dynamic effects can also be produced using the optically variable surface pattern according to the invention, as described in documents EP0868313B1 and WO 2007/079851. The disclosure of the aforementioned documents is included in this description in this connection.

Drawings

Further embodiments and advantages of the invention are explained below with reference to the drawings, in which the figures are omitted from the illustration on a true scale and to scale in order to improve the clarity.

The figures are as follows:

figure 1 shows a schematic representation of a banknote according to an embodiment of the present invention,

figure 2 shows a security element with a dynamic effect in the form of a moving bright strip,

figure 3 shows a cross-section of a sawtooth grid arranged on the surface of the security element in figure 2,

fig. 4 shows a diffraction pattern of a sawtooth grid with N =10 sawtooth elements, with a slant angle =6 ° and a periodic length a =6 μm,

FIG. 5 shows in FIG. 5 (a) a sawtooth grid in which the jth sawtooth element is staggered in its height by Δ H relative to the other sawtooth elements_jAnd in fig. 5 (b), a sawtooth grid is shown, in which all sawtooth elements are staggered in a non-periodic manner,

fig. 6 diffraction pattern calculated for a wavelength of 500nm shows in fig. 6 (a) the diffraction pattern of a sawtooth grid without height variation and in fig. 6 (b) to 6 (d) the diffraction patterns of a sawtooth grid according to the invention with N =10 sawtooth elements each and with different maximum height variations ah_max，

Fig. 7 shows in fig. 7 (a) a diffraction diagram of a sawtooth grid as shown in fig. 6 (d), but with N =100 sawtooth elements, and in fig. 7 (b) a smooth diffraction diagram in fig. 7 (a) taking into account the resolution of the human eye at an observation distance of 30cm, and

fig. 8 shows the structure according to the invention with two-dimensional grid symmetry in each case in a plan view in fig. (a) to 8 (e).

Detailed Description

The invention will now be explained by way of example of a security element for banknotes. To this end, fig. 1 shows a schematic representation of a banknote 10 provided with two security elements 11 and 12 according to the invention. The first security element is a security thread 11 present on the surface of the banknote 10 defining window regions, while the security thread 11 is embedded within the interior of the banknote 10 in the region located between the window regions. The second security element is formed by an adhesively bonded transfer element 12 of arbitrary shape.

When viewed, the security elements 11 and 12 each show a dynamic effect, wherein the bright strip 14 moves back and forth between the lower edge 16 and the upper edge 18 of the transfer element 12 or in the window region of the security thread 11 protruding on the surface of the banknote when the viewer changes his position relative to the security elements or tilts the banknote back and forth.

The security elements 11 and 12 according to the invention are distinguished in particular in that they have a small embossing depth of, for example, 3 μm or less and therefore a particularly small overall thickness on the one hand, while on the other hand they exhibit a highly bright, completely colorless appearance, i.e. without disruptive color separation or overlapping diffraction patterns at the high brightness of the moving strip 14. Furthermore, the bright strip 14 moves continuously and without disruptive jumps (flickers) of brightness fluctuations on the surface of the security element 11 or 12 when the banknote 10 is tilted.

In order to make the invention easier to understand and to be able to better judge its utility, the difficulties and disadvantages of the known constructions are first explained with reference to fig. 2 and 3.

To this end, fig. 2 shows a security element 20, which security element 20, like the security element 12 according to the invention, shows a dynamic effect in the form of a moving bright strip. For this purpose, the security element 20 has a reflective sawtooth grid 30 on its surface 22, a small part of said sawtooth grid 30 being shown in fig. 3. The sawtooth grid 30 comprises a plurality of reflective sawtooth elements 32 with a periodic length a and a tilt angle, wherein the tilt angle of the sawtooth elements 32 is not constant but rises linearly from 0 ° to 10 ° from the lower edge 24 to the upper edge 26 of the security element 20. This rise in the angle of inclination of the sawtooth element 32 is illustrated in three detail sections 28 in fig. 2.

The observer 44 always sees a bright flickering of the region of the security element 20 in which the tilting angle of the sawtooth element 32 is exactly equal to half the viewing angle θ, i.e. in which the sawtooth element 32 optically reflects 46 the rays of light 42 emitted from the light source 40 to the observer 44. If the observer moves from θ =0 ° to θ =20 °, the bright strip moves from its field of view from the lower edge 24 to the upper edge 26 of the security element. The same effect is obtained when the viewing angle of the viewer is fixed and the security element 20 is tilted in the direction of arrow 48.

If the periodic length a of the sawtooth grid 30 is large, for example a =1mm or more, no disturbing color separation or diffraction pattern is observed. Of course, a large periodic length determines a large structural height of the sawtooth element 32, since the height of the sawtooth element 32 is proportional to its width or to its periodic length a.

In practice, small periodic lengths are generally desirable because of the limited height of structures that can be achieved, for example, with imprint techniques. Even within the dimensions of structures that can be produced with embossing techniques, smaller structures are less expensive due to the thinner embossing lacquer layer and are easier to emboss than thicker structures in view of possible bubble influences. A thin embossing lacquer layer can furthermore have a better durability, since the thin embossing lacquer layer breaks more slowly when deformed than the thick embossing lacquer layer. A large periodicity length finally also reduces the resolution of the representation produced by the sawtooth grid, which is readily visible, for example, in the case of a detail-rich inversion or multiple inversion or dynamic effects.

However, if the periodic length a of the sawtooth grid 30 is reduced, diffractive refraction of the grid in the display appearing superimposed on the desired illustration increases. As the grid period a decreases, the desired representation, which is practically colorless, therefore always appears colored. If a graphical representation of the reflection behavior of a raised surface is produced with a sawtooth grid, as described, for example, in the also related application DE102009056934.0, which simulates the reflection behavior of the raised surface, an undesired abrupt change may occur when the sawtooth grid is tilted, since, unlike macroscopically raised surfaces, the sawtooth grid cannot reflect monochromatic light in any direction but only in discrete directions. When simulating a raised surface by means of a sawtooth grid, the light reflection therefore often does not move continuously over the surface during rotation, but rather abruptly from one discrete position to the next. If white light is used for viewing, disturbing unwanted colored diffraction patterns often form. A similar situation applies to dynamic effects which, based on a diffraction grating with a periodicity of small periods, also do not behave arbitrarily softly, as is further detailed below in connection with fig. 2 to 4.

The physical principle based on overlapping diffraction patterns can be explained from the illustration of fig. 3, which fig. 3 shows a simple model for the diffraction of normally incident light 50 on a sawtooth grid 30 with a period a and an oblique angle. The light rays 50-1, 50-2 and 50-3 should be reflected at an angle theta on the sawtooth grid 30. The optical path difference Δ of the light reflected on the first and second saw-tooth elements 32₁₂Here is Δ₁₂And = a sin θ. Precisely, the optical path difference between the light rays reflected on the second and third sawtooth elements is the same as the optical path difference between the light rays reflected on the third and fourth sawtooth elements, etc. In the case of a large number of sawtooth elements 32, a significant reflection therefore occurs overall in the case of constructive interference, i.e. when the path length difference of the light reflected at adjacent sawtooth elements is an integer multiple of the wavelength λ.

Belonging direction theta_mIs the direction of the corresponding grid diffractive arrangement and is given by:

mλ=asinθ_m(1)

wherein m is an integer.

More precisely, the diffraction on the sawtooth grid 30 is described in the Fraunhofer approximation in wave optics, i.e. at a large distance from the sawtooth grid.

The starting point for this wave optics consideration is firstly a diffraction grating comprising N identical slits with a width b and a period a. The complex amplitude E of the electric field in the diffraction pattern of this grid is determined by the amplitude E_jObtained by superposition (see e.hecht, Optics, 4)^thedition, AddisionWesley, section 2002, 10.2.3)

Here, the contribution E of the jth slot_jContribution E from the first gap₁Obtained by the following formula:

E_j=E₁exp(ikΔ_1j)

wherein, Delta_1j= a (j-1) sin θ is an optical path difference of light waves emitted from the first and jth slits, and k =2 π/λ represents a wave number. Thereby obtaining:

and no ray is emitted to intensity I = | E +of reflected light²Obtaining:

intensity I of the first single slit at diffraction angle θ₁Here given by:

in the formula with a constant I₁₀And β = (kb/2) sin θ in the case of a periodic grid, the summation of relation (2) forms a geometric sequence, and the intensity in the diffraction pattern for the slot grid yields:

here, I₁₀Again constant and α = (ka/2) sin θ the term in the first bracket gives the diffraction behaviour of a single slit or mirror, while the second term results from a periodic grid arrangement.

In a simple model for a sawtooth grid it can now be assumed that b and a are given by the period of the sawtooth grid and that the tilt angle of the sawtooth element 32 shifts the diffraction pattern by only double the tilt angle. Thus, the intensities in the diffraction pattern for the sawtooth grid result in:

wherein γ = (kb/2) sin (θ -2).

For explanation, fig. 4 shows a diffraction pattern 60 of a sawtooth grid 30 with N =10 sawtooth elements 32 with a slant angle =6 ° and a periodic length a =6 μm.

The inclination angle influences the first term in relation (3) in such a way that the envelope 62 of the individual sawtooth elements is shifted accordingly, while the second term in relation (3) ensures that, in monochromatic illumination, the intensity is only obtained at the position predefined by the grid period. In the case of a wavelength of 500nm, these positions are approximately 5 ° apart from one another in this example.

At a tilt angle =6 °, the maximum of the single sawtooth diffraction pattern is at 12 °, as given by the envelope 62. As is further directly visible from fig. 4, the regular grid arrangement prevents a significant intensity from occurring at =12 °. The sawtooth grid 30 in this case therefore cannot reflect precisely in the direction predetermined by the inclination angle. Instead, the intensity of the reflection is distributed according to relation (1) at the next position 64 of the grid reflection, said position being at about θ =10 ° or θ =15 °.

Thus, if the security element 20 of fig. 2 is realized, for example, with a periodic sawtooth grid 30 of fig. 3, whose periodic length a =6 μm, the diffractive effect of the grid structure results in the observer 44 seeing bright areas on the security element 20 only at discrete positions of the grid reflection, i.e. here, for example, at θ =0 °, θ =5 °, θ =10 °, θ =15 ° and θ =20 °. These positions correspond exactly to the local maxima of the diffraction pattern 60 shown in fig. 4.

If the observer 44 now moves from θ =0 ° to θ =20 °, the recognizable bright strip does not move continuously from the lower edge 24 to the upper edge 26, but jumps in the form of strong brightness fluctuations towards the position of the next grid reflection. Flicker is observed in the actual case of white illumination and the color of the bars changes according to the viewing direction, since the observer in different viewing directions is at a diffraction maximum of different wavelengths. The same effect also occurs when the viewer and light source positions remain fixed and the security element is rotated for this purpose. In the case of grids with a small period, this effect is very disturbing, and in addition the dynamic effect also relates to the representation described above in which there is also a convex behavior of the continuously changing tilt angle.

In this case, the highly aperiodic offset of the sawtooth element 32 according to the invention enables an effective correction. For the sake of explanation, fig. 5 (a) first shows a sawtooth grid 30 with a plurality of sawtooth elements 32 in a simple ray-optical diagram. The jth sawtooth element 72-j is here offset in its height by a height difference Δ H relative to the further sawtooth element 32_j. The non-staggered zigzag elements 32-j are drawn in dashed lines for illustration in the figure. By staggering the sawtooth elements 72-j, the optical path difference between the reflected ray 50-j and the further reflected ray 50 is changed by about 2 Δ H_j. If the optical path difference Δ H_jRandomly chosen in the range between zero and a minimum of half a wavelength, a random change of the optical path difference of the reflected light ray 50-j and the further reflected light ray in the range between zero and a minimum of full wavelength is obtained.

Now, if all sawtooth elements are staggered in a non-periodic, in particular irregular manner, with a height offset between zero and a minimum of half a wavelength, as shown by the sawtooth grid 70 according to fig. 5 (b), then there is no offsetAll optical path difference Δ H of the sawtooth grid 30_jkThe value between zero and minimum full wavelength is changed in an irregular manner. The light rays 50-j and 50-k reflected on the different sawtooth elements 72-j and 72-k appear in a random phase relationship such that the sawtooth grid 70 no longer functions as a diffraction grid despite the regular and periodic arrangement of the sawtooth elements 72 within the surface area 74. The optically variable surface pattern 76 of fig. 5 (b) thus comprises a periodic arrangement of substantially ray-optically active reflective elements 72-j, 72-k which are non-periodically staggered with respect to one another in terms of their height over the surface area 74 and show a colorless image without color separation or overlapping diffraction patterns.

If the transition from a live ray-optical image to wave-optical processing in the fraunhofer approximation is made, the offset of the jth sawtooth element 72-j can be considered as a replacement of the optical path difference Δ 1j from the first sawtooth element in relation (2) by the following equation:

Δ_1j=a(j-1)sinθ+2ΔH_j

this relationship, although strictly only holds true at the limit case → 0, also provides an acceptable approximation at higher tilt angles as are present in practical sawtooth grids. Instead, the last term in relation (2) no longer provides a geometric sequence and must be changed by Δ H according to the height_jThe specific selection of (a) is calculated accordingly. For the intensities in the diffraction pattern, we therefore get:

in the formula with a constant I_H0。

Fig. 6 now shows, by way of example, a diffraction diagram of a sawtooth grid 70 according to the invention with N =10 sawtooth elements 72 each, calculated for a wavelength of 500 nm. Fig. 6 (a) shows, as a reference, first again the diffraction pattern 60 with the grid reflection 64 of the sawtooth grid 30 without height variation. For the diffraction patterns of FIGS. 6 (b) to 6 (d), the height change Δ H_jAt zero and a maximum value Δ H, respectively, according to a pseudo-random distribution_maxTo select between. The diffraction patterns 62 of the individual sawtooth elements are each depicted by dashed lines.

As shown in fig. 6 (b), at the maximum value Δ H_maxA significantly higher intensity between the grid maxima than in the diffractogram 60 of fig. 6 (a) has occurred in diffractogram 82 at 70nm (≈ λ/7). At Δ H_maxAt =125nm (corresponding to a quarter wavelength), the intensity of the diffraction pattern 84 at a location between the grid reflections 64 is already higher than the intensity on the grid reflections 64, as illustrated in fig. 6 (c). Finally, as shown in FIG. 6 (d), at Δ H_maxIn contrast, =250nm, i.e. at random height variations up to half wavelength, the original diffraction maximum 64 of fig. 6 (a) is no longer visible in the diffraction diagram 86. Furthermore, diffraction maxima 88 are significantly more compact here than in the case of sawtooth grids 30 without random height variations, so that the color and flicker effects described above are significantly reduced and are no longer visible in many lighting conditions.

The more sawtooth elements 72 a sawtooth grid 70 contains, the more closely packed the diffraction maxima 88 are and may eventually no longer be separated for the viewer. This is illustrated in fig. 7, fig. 7 shows in fig. 7 (a) a diffraction pattern of a sawtooth grid 70 as in fig. 6 (d), but with N =100 sawtooth elements. The diffraction maxima 92 which occur here are no longer discernible by the observer under coherent illumination. If it is assumed that the pupil diameter is 5mm and the viewing distance is 30cm, the eye collects light from an angular range of about 1 degree.

To the viewer, the visual impression thus corresponds to the correspondingly smooth diffraction pattern 94 illustrated in fig. 7 (b). As shown by comparison with the diffraction pattern 62 (dashed line) of a single sawtooth element, the diffraction pattern 94 (solid line) of the sawtooth grid 70 with highly random variations largely corresponds to the diffraction pattern 62 of a single sawtooth. The global maximum 96 of the diffraction pattern 94 is at about 12 deg., and therefore at a position where the sawtooth grid 30, without height variations, does not actually reflect the intensity. In the sawtooth grid 70 according to the invention with N =100 sawtooth elements, unwanted color flicker or flicker is therefore no longer present.

Although the invention has been explained so far with the example of a one-dimensional grid, the above observations apply also to a two-dimensional grid. The reflective elements are in this case preferably formed by small micro-mirrors, micro-concave mirrors, micro-convex mirrors or reflective prism structures arranged at the grid positions of a two-dimensional Bravais grid.

Fig. 8 is a part illustrating some possible constructions according to the invention, respectively shown in top view.

All periodic floor plans can be associated with five Bravais grids, with the Bravais grid with the highest symmetry being selected for unambiguous correlation. For example, FIG. 8 (a) shows an optically variable surface pattern 104 comprising a plurality of square micromirrors 102 having dimensions of 10 μm by 10 μm within a surface area 100. The micromirrors 102 are arranged on grid points 106 of a square grid and have respective tilt angles with respect to the plane of the surface area 100. The tilt angles of the individual micromirrors are selected such that surface pattern 104 produces a desired optically variable representation upon illumination or viewing. The surface pattern 104 may for example show moving bright stripes or another dynamic effect, a graphic representation or a perspective view appearing convexly.

In order to avoid disturbing diffraction patterns due to the periodic grid arrangement of the micromirrors 102, the micromirrors 102 are offset from one another in their height over the surface area 100 and in the embodiment randomly in the range from zero to 400 nm. Such random height misalignments can be determined for each micromirror 102 by pseudo-random distribution, for example, with the aid of a computer. This ensures that the phase difference of the light reflected by the different micromirrors 102 lies randomly between 0 and 2 pi for all wavelength ranges of visible light, so that no constructive interference and thus no diffraction patterns occur.

If the optically variable surface pattern 104 is embedded in the refractive index n>1, for example by coating the surface pattern 104 with a protective lacquer layer, the height offset can be reduced to a value of 1/n times. If, for example, the sawtooth grid 70 of fig. 6 (d) is embedded in a protective lacquer with n =1.5, the maximum height offset Δ H is obtained_max=250nm can be reduced to Δ H with the same optical effect_n=170nm。

Fig. 8 (b) shows a variable optical surface pattern 110 comprising a plurality of rectangular reflective elements 112 of size 10 μm × 5 μm arranged on grid points of a rectangular grid in the surface area as a second example. In the embodiment of fig. 8 (c), the reflective elements are formed by diamond-shaped micro-mirrors 114 arranged on grid points of a diamond-shaped grid, i.e. a grid unit comprising a grid with equal side lengths and arbitrary intermediate angles. In the embodiment of fig. 8 (d), the reflective elements are formed by hexagonal micro-mirrors 116 arranged on grid points of a hexagonal grid. Finally, the reflective elements are in the general case also arranged on grid points of a parallelogram grid, as illustrated in fig. 8 (e), and are formed, for example, by parallelogram micro-mirrors 118.

The security element according to the invention may also contain a plurality of regions with different grid symmetries. Such different grid symmetries can be used as higher-level authenticity marks in different regions of the security element, which can only be checked with a magnifying glass or microscope. Due to the random height variations according to the invention, the surface pattern in particular does not produce overlapping diffraction patterns which can give an indication of the grid symmetry of the reflective element on which it is based.

Furthermore, for example in security elements for document authentication, the same representation is produced by an arrangement with different grid symmetries. The grid symmetry used may differ, for example, depending on the year of issuance, the region of issuance, the serial number or a special group of people, and thus form a further hidden authenticity feature.

The construction of the security element according to the invention advantageously leads to a uniform visual appearance of the security element in addition to the elimination of diffractive reflections. In conventional grid arrangements, the regions with a tilt angle of 0 ° often represent brighter, smoother and more specular regions than the regions with a greater tilt angle, since in these regions with a tilt angle of 0 ° the sawtooth grid or micromirrors merge into a large plane, wherein no more significant scattering at the mirror edges occurs. By random height offset, the regions with a tilt angle of =0 ° now also contain edges between adjacent reflective elements, so that a uniform visual impression is given for all tilt angles.

List of reference marks

10 banknote

12 Security element

14 bright strip

16 lower edge

18 upper edge

20 Security element

22 surface of

24 lower edge

26 upper edge

28 details section

30 sawtooth grid

32 saw tooth element

40 light source

42 incident light

44 observer

46 reflect light

48 direction of inclination

50 incident light

50-1, 50-2, 50-3, 50-j, 50-k rays

60 diffraction pattern

62 envelope

64 grid reflection

70 sawtooth grid

72 saw tooth element

74 surface area

76 optically variable surface pattern

82. 84, 86 diffraction patterns

88 maximum of diffraction

90 diffraction pattern

92 diffraction maximum

94 smooth diffraction pattern

96 global maximum

100 surface area

102 micro-plane mirror

104 optically variable surface pattern

106 grid points

110 optically variable surface pattern

112 rectangular reflecting element

114 diamond micro-plane mirror

116 hexagonal micro-plane mirror

118 parallelogram micro-plane mirror

Claims (31)

1. A security element for security papers and data carriers with a substrate which contains an optically variable surface pattern in a surface region, which surface pattern displays different representations in different illumination and/or viewing directions, wherein the optically variable surface pattern consists of an at least partially periodic arrangement of reflective elements which essentially act as ray optics, and wherein the reflective elements are non-periodically staggered with respect to one another by their height over the surface region and the height of the reflective elements varies over the surface region according to a pseudo-random number distribution.

2. A security element according to claim 1, wherein the height of the reflective element varies in a non-regular manner over the surface area.

3. Security element according to claim 1, characterized in that the reflective elements are offset in their height over the surface area from one another by a height offset between 0 and Δ H_maxIn which Δ H_maxIs between 50nm and 2000 nm.

4. A security element according to claim 3, characterized in that the reflective elements are offset in their height from one another over the surface area by a height offset between 0 and ah_maxIn which Δ H_maxIs between 100nm and 500 nm.

5. A security element as claimed in any one of claims 1 to 4 in which the arrangement of reflective elements forms at least in part a periodic one-dimensional grid.

6. Security element according to one of claims 1 to 4, characterized in that the arrangement of reflective elements forms at least partially a periodic two-dimensional Bravais grid.

7. Security element according to one of claims 1 to 4, characterized in that the periodic length or the local periodic length in at least one periodic direction lies between 1 μm and 40 μm.

8. Security element according to claim 7, characterized in that the periodic length or the local periodic length in at least one periodic direction is between 2 μm and 30 μm.

9. Security element according to claim 8, characterized in that the periodic length or the local periodic length in at least one periodic direction is between 3 μm and 10 μm.

10. A security element according to any one of claims 1 to 4 wherein the optically variable surface pattern shows a colourless appearance with no colour separation or overlapping diffraction patterns.

11. The security element according to any one of claims 1 to 4, characterized in that the reflective element is formed by a micro-flat mirror, a micro-concave mirror, a micro-convex mirror or by a reflective pyramidal structure.

12. Security element according to one of claims 1 to 4, characterized in that the reflective element is formed by a relief structure provided with a reflective layer.

13. A security element as claimed in claim 12 in which the reflective element is formed by an embossed relief structure provided with a reflective layer.

14. Security element according to claim 12, characterized in that the reflective layer is formed by a reflective opaque metal layer or by a thin or rasterized semi-transparent metal layer.

15. A security element according to claim 12, wherein the reflective layer is formed by a coloured or counter-coloured reflective layer.

16. Security element according to claim 15, characterized in that the reflective layer is formed by a thin-layer element with a reflective layer, an absorbing layer and a dielectric spacer layer arranged between the reflective layer and the absorbing layer, or by a thin-layer element with two absorbing layers and a dielectric spacer layer arranged between two absorbing layers.

17. A security element as claimed in claim 12 in which the reflective layer is formed by a highly refractive dielectric layer or a cholesteric liquid crystal layer.

18. A security element as claimed in any one of claims 1 to 4 in which the optically variable surface pattern comprises a plurality of sub-regions which display different illustrations.

19. A security element as claimed in any one of claims 1 to 4 in combination with at least one further security feature, wherein the further security feature is a diffractive hologram structure.

20. A security element as claimed in claim 1, characterized in that the data carrier is a value document.

21. Security element according to claim 20, characterized in that the value document is a banknote, or a certificate, passport or certification card.

22. The security element of claim 21, wherein the banknote is a paper banknote, a polymer banknote, or a film composite banknote.

23. A data carrier with a security element according to any one of claims 1 to 20.

24. A data carrier as claimed in claim 23, characterized in that the data carrier is a value document.

25. A data carrier as claimed in claim 24, characterized in that the document of value is a banknote, or a certificate, a passport or a certification card.

26. A data carrier as claimed in claim 25, characterized in that the banknote is a paper banknote, a polymer banknote or a film composite banknote.

27. An impression mold for producing an impression structure, having a body which contains, in a surface region, an optically variable surface pattern which shows different representations in different illumination and/or viewing directions, wherein the optically variable surface pattern consists of an at least partially periodic arrangement of reflective elements which essentially act as ray optics, and wherein the reflective elements are non-periodically staggered with respect to one another by their height over the surface region, and the height of the reflective elements varies over the surface region according to a pseudo-random number distribution.

28. A method for producing a security element for the security protection of security papers and data carriers, wherein a substrate is provided and the substrate is provided in a surface region with an optically variable surface pattern which displays different representations in different illumination and/or viewing directions, wherein the optically variable surface pattern is formed by an at least partially periodic arrangement of reflective elements which essentially act as beam optics, and wherein the reflective elements are non-periodically offset from one another by their height over the surface region and the height of the reflective elements varies over the surface region according to a pseudo-random number distribution.

29. A method as claimed in claim 28, characterized in that the data carrier is a value document.

30. The method according to claim 29, wherein the value document is a banknote, or a certificate, passport or identification card.

31. The method of claim 30, wherein the banknote is a paper banknote, a polymer banknote, or a film composite banknote.