EP1392520B1 - Diffractive safety element - Google Patents

Abstract

A security element ( 2 ) comprising a reflective, optically variable surface pattern ( 3 ) which is embedded in a layer composite of plastic material and which can be visually recognized from predetermined observation directions is formed from a mosaic of optically active surface elements ( 13 ). In the mosaic of the surface pattern ( 3 ) at least two of the mosaic surfaces ( 11; 12 ) of the surface pattern ( 3 ) are arranged substantially adjacent and have microscopically fine light-diffractive relief structures ( 4 ). The spatial frequencies of the relief structures in the mosaic surfaces ( 11; 12 ) are of values from predetermined spatial frequency ranges in such a way that, in the case of illumination beams which are incident obliquely relative to a normal onto the plane of the layer composite, the relief structures of the mosaic surfaces ( 11; 12 ) deflect visible monochromatic light parallel to the normal ( 32 ). The relief structures of the mosaic surfaces ( 11; 12 ) differ only in respect of spatial frequency, the difference in the spatial frequencies in the adjacent mosaic surfaces ( 11; 12 ) being at most 40 lines/mm.

Description

The invention relates to a diffractive security element according to the Preamble of claim 1.

Such diffractive security elements are used for authentication of authenticity of a document and are characterized by an optically variable pattern by turning or tilting for the observer in a conspicuous way and Way intended changes.

Such diffractive security elements are known from many sources, Representative here are EP 0 105 099 B1, EP 0 330 738 B1, EP 0 375 833 B1 called. They are characterized by the brilliance of the patterns and the Moving effect in the pattern out, are in a thin plastic laminate are embedded in the form of a mark on documents such as banknotes, Securities, identity cards, passports, visa, identity cards etc. glued on. Materials which can be used to produce the security elements are described in EP 0 201 323 B1.

Modern color copiers and scanner facilities are capable of such Seemingly to duplicate the document in true color. The diffractive Security elements are copied, although the brilliance and the Motion effect is lost, so that the original under a single predetermined viewing angle visible patterns with the inks of the Color copying is displayed. Such copies of documents may be included poor lighting conditions or inattention with the original be confused.

The human eye recognizes colored, arranged next to each other Squares a color contrast when the wavelengths of the spectral colors in distinguish the faces by less than ten nanometers (nm). In particular, in the area 470 nm to 640 nm an observer still fall Differences of 1 nm to 2 nm (W.D. Wright & F.H.G. Pitt "Hue discrimination in normal clour vision ", Proc. Physical Society (London) Vol. 46, p. 459 (1934)).

Known is the on the differences in the spectral sensitivity of human eye and the color copier based idea, documents with one to equip colored background and on the background the information in print a different color, the information against the background of a Contrast perceivable by the human eye, however, of the Color copiers can not be reproduced.

From EP 0 281 350 B1 such a colored security paper is known which as a background a repetitive, e.g. Plaid, pattern of two colors A and B , wherein the information with a further color S on the Background pattern is printed. The spectral reflectivities of the colors A, B and S are chosen so that the color copier, although in areas with the color A a contrast between A and S, in areas of color B but not between B and S can recognize and reproduce. On the copy are therefore of the Information only visible in the areas A parts visible.

The US 5,338,066 points to two methods, a manufactured by printing to distinguish colored original from its color copy. That has a method chemical constituents of the printing ink after and the other method is supported on the different dynamic areas in image processing of the Color copier compared with the image processing of the human eye. in the Original ones show the colors of the background and the information in theirs spectral reflectivity to a modulation of ± 5%, with the background color the maximum in the green spectral range of visible light and the color for the information each have a maximum in the blue and red spectral range. The spectral reflectivities of both complementary colors have averaged over the visible spectral range the same value and together form the Color white. While the eye is the purple information before the green Background, the color copier registers only a white one to slightly gray area.

The reference to additional chemical detection for the detection of Authenticity of a suspected original, which in practice difficult to dominant technique of color mixing on.

The invention is based on the object, a cost-effective diffractive Security element to create that of a color copier not has reproducible information.

The object mentioned is achieved by the im Claim 1 specified features solved. Advantageous embodiments of Invention will be apparent from the dependent claims.

Embodiments of the invention are shown in the drawing and will be described in more detail below.

Figur 1

einen Querschnitt durch eine Reliefstruktur,

Figur 2

ein Flächenmuster,

Figur 3

einen Querschnitt durch eine Abtastvorrichtung eines Farbkopierers,

Figur 4

ein Farbdiagramm,

Figur 5

eine Beugungsebene und

Figur 6

Beugungsgitter.

Show it:

FIG. 1

a cross section through a relief structure,

FIG. 2

a surface pattern,

FIG. 3

a cross section through a scanning device of a color copier,

FIG. 4

a color chart,

FIG. 5

a diffraction plane and

FIG. 6

Diffraction grating.

In FIG. 1, a cross-section is glued onto a document 1 Security element 2 shown with a surface pattern 3. With the documents 1 are mainly ID cards, banknotes, visas, securities, entrance cards, etc. meant, which serve as a substrate for the security element 2 and their authenticity is authenticated by the glued security element 2. The microscopic fine, mechanically or holographically generated, optically active structures 4 of the surface pattern 3 are embedded in a laminate of plastic. For example, the layer composite consists of a crystal-clear, transparent Covering layer 5, through which the surface pattern 3 of predetermined Observation directions is visually recognizable. Under the cover layer 5 is a Lacquer layer 6 is arranged, in which the microscopically fine structure 4 is molded. The structure 4 is only symbolically drawn as a simple rectangular structure and stands for a mosaic of the surface pattern 3 of the optically active structures 4 of surface elements. The structure 4 is such with a protective lacquer layer 7 covers up that furrows of the structures 4 are filled by the protective lacquer layer 7 and the structure 4 between the lacquer layer 6 and the protective lacquer layer 7 is embedded. Between the document 1 and the protective lacquer layer 7 is a Adhesive layer 8 is arranged to secure the security element 2 to the document 1 to connect. The layers 5 and 6 or 7 and 8 can in others Be made of the same material, leaving an interface between the layers 5 and 6 or 7 and 8 deleted. The structure 4 determines a Interface 9 between layers 6 and 7. The optical effectiveness of Interface 9 increases with the difference of the refractive indices of the materials in the both adjacent layers, the lacquer layer 6 and the protective lacquer layer 7. To enhance the optical efficiency of the interface 9, the structure 4 is present the application of the protective lacquer layer 7 with a compared to the depths of Furrows coated thin, metallic or dielectric reflective coating. Other versions of the security element 2 and the transparent ones or non-transparent security elements 2 usable materials are in described in EP 0 201 323 B1 mentioned above. The one shown in FIG Structure 4 is only symbolically drawn as a simple rectangular structure and stands for general optically active structures 4, such as light-diffracting relief structures, light-scattering relief structures or mirror surfaces. Well-known light-diffractive Relief structures are linear or circular diffraction gratings and holograms. The light-scattering relief structures also include matt structures.

In FIG. 2, the security element 2 attached to the document 1 is shown in FIG a tablet 10 shown. The tablet 10 is part of a diffractive Surface pattern 3. The writing plate 10 has a simple design at least two abutting mosaic surfaces 11, 12. In others Designs form the mosaic surfaces 11, 12 a pattern with a Background area 12 and patches 11. The areas of the background area 11 and the partial surfaces 12 are covered with diffraction structures. Further mosaic-like arranged, having a diffractive, diffusive or reflective property Surface elements 13 complete the surface pattern 3. Single band-shaped Surface elements 13 may also extend over the tablet 10.

For example, the writing tablet 10 has an inscription "TEXT". The inscription consists of the sub-areas 11, which within at least one background area 12 are arranged. Incident white light is emitted at the diffraction structures bent so that an observer under predetermined Viewing conditions both the background surface 12 and the Part surfaces 11 appear in predetermined colors and the partial surfaces 11th stand out by a color contrast to the background surface 12.

FIG. 3 shows a schematic cross section through a scanning device a color copier. A glass plate 14 serves as a support for the copy to be copied Document 1 with the glued security element 2 (Fig. 1). The document 1 contacted with its surface to be copied, the glass plate 14. A White light source 15 illuminates a narrow strip 16 on the glass plate 14, wherein in this representation, the strip 16 in the direction y perpendicular to Drawing plane of Figure 3 extends and therefore in the drawing only as a point is visible. A direction x is parallel to the plane of the drawing of Figure 3 and Surface of the glass plate 14 directed. An illumination beam 17 from the White light source 15 is refracted through the glass plate 14 toward the solder so that the illumination beam 17 at an angle of about 30 ° to the surface of the document 1 and the diffraction structure and a to the strip 16 parallel area 18 of the document 1 illuminates. In the area 18 becomes the light reflected by the diffraction structure and reflected on the surface of the Document 1 scattered or reflected. Part of the bowed, reflected or scattered light again penetrates the glass plate 14 and is in the half-space sent back over the glass plate 14. Only the glass plate 14 vertical penetrating light rays 19 pass directly or via a deflection mirror as deflected light rays 21 into a light receiver 20. All other light, e.g. Second rays 22, 23, which does not penetrate vertically through the glass plate 14, falls on screens not shown here and does not reach the light receiver 20. Die White light source 15 and the light receiver 20 are on a carriage 24th arranged on rails 25 for optically scanning the document 1 in the Direction x is displaceable. The white light source 15, the deflection mirror, the Light receiver 20 and the carriage 24 extend parallel to the strip 16 linear in the y direction. Not shown in the drawing is the appearance of the Light receiver 20 for guiding the light beams 19, 21 between the area 18th and the light receiver 20. During scanning, the area 18 moves in steps so about the document 1 that the color copier a strip-shaped image of the Document 1 or security element 2 after the other. The entire substrate 10 is scanned optically in steps.

FIG. 4 shows a color diagram. (from "Optical Document Security", van Renesse, Editor, ISDN number 0-89006-982-4, page 135). Places of spectral pure colors form the outer tongue-shaped boundary 26. The three-digit Numbers along the boundary 26 indicate the wavelength of the light Nanometers (nm). The straight connecting line 27 between 380 nm and 770 nm is the place of purple. All mixed colors are in the color range 28 within the border 26 and the connecting line 27. The in the color range 28 lying polygon 29 includes the range of color reproduction of high-quality color printing machines and the inner, polygonal, hatched Color range 30 with the white point 31 is from color copiers played. Are the colors of document 1 with the color press printed, lie the color values within the polygon 29. The color copier with the color value range 30 restricts that in the copy of the document 1 as they are all assigned to color value range 30 are. An outside of the color value range 30 e.g. on a line 34 within the Color range 28 lying color value is in color copy as color point 35 played. The color point 35 lies on the line 34, which is the location of the color value and the white point 31 connects.

Return to FIG. 2: The illustrated writing tablet 10 generates with its microscopic fine, mechanically or holographically produced, optically effective Structures 4 (Fig. 1) when illuminated with white light (daylight) for the Observers almost spectrally pure colors lying on the boundary 26 (Fig.4). When copying the tablet 10 is under the ruling in the color copier Lighting conditions and according to the parameters of the lit. Diffraction structures in the background area 12 and / or in the partial areas 11 Registered light with almost spectrally pure color in the light receiver 20 (Fig. 3). Of the Color copier is in the copy of the tablet 10 with him available standing color values from the color value range 30 again. The surface pattern 3 (FIG. 2) is under the scanning and lighting conditions in the color copier in FIG the registered for the light receiver 20 pattern with the colors of the Color value range 30 is reproduced in the color copy. The playback of the Surface pattern 3 in the color copy therefore also depends on the scanning direction from.

According to the invention, for the background area 12 and for the partial areas 11, the spatial frequencies f _H and f _T are selected such that the eye of the observer determines a color contrast between the background area 12 and the partial areas 11 and can distinguish the partial areas 11 from the background area 12. Therefore, the observer recognizes in the original of the security element 2, the information shown with the sub-areas 11. If the spatial frequencies f _H and f _{T are} closely adjacent, the color copier reproduces both the background area 12 and the areas 11 in the same color value. The background area 12 and the partial areas 11 are thus indistinguishable in the copy. The information represented by the faces 11 are alphanumeric characters as shown in FIG. 2 and / or graphic patterns or characters.

FIG. 5 shows the illumination and observation conditions in FIG Color copier. The security element 2 with the optically active structure 4 is located in the x-y plane of the document 1. Is the structure 4 a plane in the x-y plane Mirror surface, which is inclined at an angle -α Illuminating beam 17 at the angle + α to the normal 32 as a second beam 22nd reflected. The flat mirror surface registers the color copier therefore as black Area. The illumination beam 17, second beam 22 and the normal 32 set a Diffraction plane 33 fixed.

For example, if the optically active structure 4 is a linear diffraction grating 40 (FIG. 6), the light beam 19 (FIG. 3) diffracted into the negative k th diffraction order falls only into the light receiver 20 (FIG. 3) when the light beam 19 is diffracted parallel to the normal 32. The spatial frequencies f and f _H and f _T are therefore according to the equation sin (δ = 0 °) - sin (α) = ± k • λ • f predetermined, where α is the angle of incidence and δ = 0 °, the diffraction angle between the normal 32 and the diffracted light beam 19. The diffracted light beam 19 has the wavelength λ in the k-th diffraction order. The beam diffracted under the positive k-th diffraction order, the secondary beam 23, encloses the angle 2α to the normal 32. For an angle of incidence α of 25 ° to 30 ° and at k = 1, the usable range of the spatial frequencies f is from 725 lines / mm to 1025 lines / mm; at k = 2, the usable spatial frequencies f are between 350 lines / mm and 550 lines / mm, so that the diffracted light reaches the light receiver 20. The range limits are determined by the optics, geometry and color sensitivity of the light receiver 20. To compensate for any unevenness of the surface pattern 3, it is advantageous to modulate the spatial frequency f, wherein the spatial frequency f over at least part of a period or over several periods of 0.5 mm to 10.0 mm with a stroke of 3 lines / mm to 20 lines / mm changes. This modulation is visible to the naked eye in daylight, but is not reproducible by the color copier. In one embodiment, two mosaic surfaces 11 and 12 abut one another with the spatial frequencies f _H and f _T , wherein along the common boundary of the two mosaic surfaces 11 and 12 the modulation of the spatial frequencies f _H and f _T is shifted by one phase angle, for example in the region of 90 ° to 180 °.

This consideration holds only as long as the grating vector of the diffraction grating in the Diffraction plane 33 and thus parallel to the scanning direction. The grid vectors the diffraction grating in the background area 12 (FIG. 2) and in the partial areas 11 (Fig. 2) are therefore arranged substantially parallel, so that the diffraction gratings in the background area 12 and in the partial areas 11 in contrast to the rest Surface pattern 3 (Fig. 2) optically substantially under the same conditions be scanned.

In any scan direction, the grating vector is an azimuth  Diffraction plane 33 on. The effective spatial frequency f decreases at one linear diffraction grating with increasing azimuth , so that the of Diffraction grating in the direction of the normal 32 produced spectral color both in the Background area 12 and in the sub-areas 11 changed, with the Difference in the wavelengths of the diffracted light beams 19 from the Background surface 12 and hardly changed from the partial surfaces 11 and the Color copier does not reflect the small color differences.

Once no visible diffracted light rays 19 more in the light receiver 20 reach the light receiver 20 receives only stray light; the Irrespective of its spatial frequency f, the diffraction grating acts like a dark one Matt structure and is reproduced by the color copier in a gray tone. The Reproducing the document 1 (FIG. 2) with the security element 2 with the Diffraction optical surface pattern 3 depends on the orientation of the Surface pattern 3 on the glass plate 18 (Fig. 3) from, wherein the tablet 10 is always is reproduced in one color, so that the contained in the sub-areas 11 Information is not recognizable.

Advantageously, the spatial frequencies for the diffraction gratings of background area 12 and subareas 11 are selected such that upon scanning of area pattern 3 with white illumination beam 17 (Figure 3), light at wavelength λ in the range of 615 nm to 700 nm enters the light receiver 25 (Fig. 3) passes. The spatial frequencies f _H and f _T are therefore to be selected from the spatial frequency range of 770 lines / mm to 820 lines / mm, the spatial frequencies f _H , f _{T being} a spatial frequency difference Δf = ± (f _H -f _T ) of 5 lines / mm to 40 lines / mm for k = 1 or 20 lines / mm for k = 2, where k is the order of diffraction. In one embodiment of the surface pattern 3, the diffraction grating of the background surface 12 has the spatial frequency f _H = 810 lines / mm or 860 lines / mm and the diffraction gratings in the sub-areas 11 the spatial frequency f _T = 800 lines / mm and 890 lines / mm respectively. Of course, the values for f _H and f _{T are} interchangeable. The background area 12 with f _H = 810 lines / mm shines for the observer in daylight in a red with the wavelength 617 nm, while the sub-areas 11 with f _T = 800 lines / mm appear in a darker red of 625 nm wavelength. In this range, the human eye distinguishes a wavelength difference of at least 2 nm. For the observer, the difference of 8 nm produces a clear color contrast, so that the information is easily recognizable. The color copier is unable to reproduce this color difference in the copy.

In order to reduce the dependency of the reproduction of the writing tablet 10 (FIG. 2) in the color copy from the scanning direction, the diffraction gratings of the background surface 12 (FIG. 2) and the partial surfaces 11 (FIG. 2) advantageously have the embodiments shown in FIG which differ only in the spatial frequencies f _H and f _T. The diffraction gratings satisfy square or hexagonal patches 36 having a largest dimension h of less than 0.3 mm. The surface pieces 36 are of the same shape and the same size and fill the background surface 11 and the partial surfaces 12 completely. In one embodiment, the mosaic surfaces 11, 12 are composed of the patches 36 having the character of PixeIn. The shapes of the diffraction gratings satisfying the patches 36 are circular diffraction gratings 37, linear cross gratings 38, hexagonal diffraction gratings 39. In the circular diffraction grating 37, circular grooves are concentrically arranged in the sheet 36 at the spatial frequency f. The linear cross grating 38 has two or more crossed linear diffraction gratings, preferably with the same spatial frequency f. In the hexagonal diffraction grating 39, the grooves have a hexagonal shape and are arranged concentrically in the surface piece 36, advantageously of hexagonal shape, with the spatial frequency f. Instead of the hexagonal diffraction grating 39, groupings of linear diffraction gratings 40 with grid vectors distributed regularly in the azimuth can be used; this grouping of the linear diffraction gratings 40 is a generalization of the hexagonal diffraction grating 39. Also, the linear diffraction gratings 40 are advantageous, which were considered in the description of Figures 1 to 5.

Claims (9)

1. Security element (2) having a reflective, optically variable surface pattern (3) which is embedded in a layer composite made of plastic, can be detected visually from predetermined observation directions and is formed from a mosaic of optically active surface elements (13), characterized in that, in the mosaic of the surface pattern (3), at least two of the mosaic areas (11; 12) of the surface pattern (3) are arranged substantially adjacently and are occupied by microscopically fine light-diffracting relief structures (4) having substantially parallel grating vectors, in that the spatial frequencies (f) of the relief structures (4) have values from predetermined spatial frequency ranges such that, with illumination beams (19) falling on the plane of the layer composite at an angle to a normal (32), the relief structures (4) of the mosaic areas (11; 12) deflect visible monochromatic light parallel to the normal (32), in that at least one of the adjacent mosaic areas (11; 12), as a background area (12), encloses one or a plurality of the other mosaic areas (subareas 11) such that the subareas (11) are arranged on the background area (12) in such a way that the subareas (11) form an item of information that can be detected with the naked eye, and in that the relief structures (4) of the mosaic areas (11; 12) forming the background areas and subareas differ in spatial frequency f such that for an observer the background area(s) (12) and subareas (11) differ by means of a colour contrast, while in a colour copy the adjacent background area(s) (12) and subareas (11) are reproduced in the same colour or grey tone.
2. Security element (2) according to Claim 1, characterized in that the difference in the spatial frequencies (f) of the relief structures (4) in the adjacent mosaic areas (11; 12) is at most 40 lines/mm.
3. Security element (2) according to Claim 1, characterized in that the spatial frequency (f) of the relief structures (4) of at least one of the mosaic areas (11; 12) is modulated with a period of 0.5 mm to 10 mm and a range of 3 lines to 20 lines.
4. Security element (2) according to Claim 3, characterized in that the modulation of the spatial frequency (f_H) of the relief structure (4) in one mosaic area (11) is displaced by a phase angle from the range 60° to 180° with respect to the modulation of the spatial frequency (f_T) of the relief structure (4) in the other mosaic area (12).
5. Security element (2) according to one of Claims 1 to 4, characterized in that the spatial frequencies (f) of the relief structures (4) in the adjacent mosaic areas (11; 12) lie in the range from 350 lines/mm to 550 lines/mm and/or from 725 lines/mm to 1025 lines/mm.
6. Security element (2) according to one of Claims 1 to 4, characterized in that the spatial frequencies (f) of the relief structures (4) in the adjacent mosaic areas (11; 12) lie in the range from 800 lines/mm to 820 lines/mm and/or 860 lines/mm to 890 lines/mm.
7. Security element (2) according to one of Claims 1 to 6, characterized in that, in the adjacent mosaic areas (11; 12), subareas (11) have the form of words or alphanumeric characters or a graphic design.
8. Security element (2) according to one of Claims 1 to 7, characterized in that the adjacent mosaic areas (11; 12) are divided up into part areas (36) having a largest dimension (h) of 0.3 mm, and in that the part areas (36) of the adjacent mosaic areas (11; 12) have circular diffraction gratings (37) or crossed gratings (38) or hexagonal diffraction gratings (39) as relief structures (4).
9. Security element (2) according to one of Claims 1 to 7, characterized in that the adjacent mosaic areas (11; 12) have linear diffraction gratings (40) as relief structures (4).

Images