EP1434695A1

EP1434695A1 - Security element

Info

Publication number: EP1434695A1
Application number: EP02776977A
Authority: EP
Inventors: René Staub; Wayne Robert Tompkin; Andreas Schilling
Original assignee: OVD Kinegram AG
Current assignee: OVD Kinegram AG
Priority date: 2001-10-12
Filing date: 2002-09-04
Publication date: 2004-07-07
Anticipated expiration: 2022-09-04
Also published as: KR20040051601A; WO2003033274A1; PL367433A1; US7145723B2; JP2005518956A; DE10150293B4; PL202807B1; ATE288364T1; EP1434695B1; KR100910098B1; DE50202189D1; CA2462924A1; CN1568264A; CA2462924C; ES2236594T3; TW542798B; PT1434695E; MY126197A; RU2004114260A; RU2255000C1

Abstract

A security element (2) in the form of a layer composite may be used for the certification of a document (1). The layer composite comprises at least one transparent protective layer, a transparent paint layer and an adhesive layer, whereby the paint layer is arranged between the protective layer and the adhesive layer. A boundary surface embodied as a reflection layer separates the adhesive layer and the paint layer. The boundary surface is divided into regions of a pattern (25) with planar partial surfaces and relief structures moulded into the paint layer. The planar partial surfaces form background surfaces (3) and are mirror surfaces for light incident in the layer composite, whilst the regions with relief structures form pattern elements (4) with a given optically effective structural depth. The relief structures of the pattern elements (4) absorb the incident light. The dark pattern elements (4) form a strong contrast in the reflected light from the mirror background surfaces (3) and the pattern is clearly visible. The contrast between the background surfaces (3) and the pattern elements (4) disappears in directions other than that of the reflected light, such that a copier device reproduces the background surfaces (3) and the pattern elements (4) as black surfaces.

Description

security element

The invention relates to an optically diffractive security element according to the preamble of claim 1.

Such security elements are used to authenticate documents, such as securities, checks, banknotes, credit cards, all kinds of ID cards, tickets, tickets, etc., the security elements being glued to the document, for example, as a thin layer composite.

Modern copiers for color copies represent a considerable risk potential for printed documents, because the visual differences between the original and the copy are so small that only an expert equipped with the appropriate tools can distinguish the original from the copy, often with different criteria, such as intaglio printing, watermarks, fluorescence, optically variable security elements with diffraction structures, etc., must be used as the appearance of the printed image.

It is known from EP 0 522 217 B1 that reflective pieces of film arranged on a document provide good protection against unauthorized copying of such documents. The difference between the original with the reflective pieces of film and a copy is clearly recognizable, since the copying machines reproduce reflective surfaces in black. However, reflective foils are readily available commercially. The black areas in such copies are therefore easy to cover with reflective film to make the copy look more real.

The DE _. 44 10 431 A1 describes further developments of the film pieces described above. The security element is a piece of film cut from a layered composite with a flat, reflective one

Reflective layer. In parts of the surface that form an individual identifier on the surface of the piece of film, the reflection layer is removed, so that one under the Reflective layer arranged black layer is visible. On the copy made by the copying machine, the black identifier disappears in the reproduction of the remaining mirror surface, since the parts of the surface in which the reflection layer has been removed and the remaining mirror surface in the copy appear uniformly black. Another security element has a hologram structure with the identifier instead of the flat mirror surfaces and, when copied, behaves like the diffraction structures dealt with in the next paragraph. The identifier can therefore be seen in the copy in the copied image of the hologram.

It is also e.g. from GB 2 129 739 B to equip valuable documents with an optically variable security element with diffraction structures (for example holograms, mosaic-like surface patterns made of diffractive surface elements, for example according to EP 0 105 099 A1, EP 0 330 738 A1, EP 0 375 833 A1, etc.) , These security elements have a pattern or image that changes depending on the viewing condition. These are for unauthorized persons

Imitating security elements only with great effort. Unfortunately, the color copy of the document reproduces one of the patterns or images of the security element, which is visible in the original under the viewing condition specified in the copier for the image. Of course there is no change in the pattern or picture in the copy when changing the

Viewing condition more recognizable, but if the recipient is inattentive, a copy can easily be mistaken for the real document.

EP 0 401 466 A1 and US 4,856,857 describe embodiments of the layer composite for the security elements and materials that can be used for this purpose. The object of the invention is to create an inexpensive, optically variable security element that cannot be reproduced by a copying machine and also not with it holographic methods can be copied.

According to the invention, this object is achieved by the features specified in the characterizing part of claim 1. Advantageous embodiments of the invention result from the dependent claims. Embodiments of the invention are shown in the drawing and are described in more detail below.

1 shows a document,

FIG. 2 shows the document tilted about an axis,

3 shows a security element in cross section, FIG. 4 shows the interface of a relief structure,

FIG. 5 shows a first observation condition,

FIG. 6 shows a second observation condition,

Figure 7a, 7b, the security element with gray levels and

Figure 8 is a relief structure. In FIG. 1, 1 means a document, 2 a security element, 3 a

Background area, 4 a sample element and 5 an imaginary tilting axis lying in the plane of document 1. Document 1 is illuminated laterally and obliquely from above in directed artificial light and viewed vertically from above. The security element 2 is fastened on the document 1. The security element 2 has a pattern 25 of the pattern elements 4, which are surrounded by the background areas 3, for identification. In order to make the drawing of FIG. 1 clear, the pattern 25 consists of a single pattern element 4 and forms a simple "V-sign. In a practical embodiment, several of the background areas 3 and the pattern elements 4 are arranged to form the pattern 25. Under the lighting mentioned - and

Viewing conditions, the pattern 25 is not visible to an observer since there is no contrast between the pattern element 4 and the background surface 3 and both surfaces, both the background surface 3 and the pattern element 4, appear dark, for example metallic matt. In contrast, in diffuse daylight or in diffuse room lighting and under certain lighting conditions mentioned below, the pattern element 4 stands out darkly from the bright background surface 3 and is therefore clearly visible to the observer.

As FIG. 2 shows, the document 1 with the security element 2 is tilted about the tilt axis 5 in such a way that the background surface 3 shines light into the eye of the observer, the observer recognizes the pattern 25, since the pattern element 4 remains dark and stands out from the background surface 3 with high contrast. Under these observation conditions, the reflection condition for the observer is fulfilled. A rotation of the security element 2 in its plane does not change the appearance of the pattern 25 in the reflection condition for the observer, ie an azimuthal alignment of the security element 2 is not necessary.

FIG. 3 shows the security element 2 (FIG. 2) in cross section, the sectional plane containing, for example, the tilt axis 5 (FIG. 2). The security element 2 consists of a layer composite 6 composed of a plurality of layers 7, 8, 9 and 11. Examples of the structure of the layer composite 6 and the materials suitable for the layer composite 6 can be found in EP 0 401 466 A1 and US Pat. No. 4,856,857 remove.

In the simplest case, the layer composite 6 comprises at least one protective layer 7, an adhesive layer 8, a lacquer layer 9 arranged between the protective layer 7 and the adhesive layer 8. The adhesive layer 8 connects the security element 2 to the document 1. An interface between the adhesive layer 8 and the Lacquer layer 9 reflects incident light 10 through the top layer 7 and the lacquer layer 9 when the refractive index at the boundary layer changes abruptly during the transition from the lacquer layer 9 into the adhesive layer 8. With the materials in Table 6 of US 4,856,857, the difference in the refractive indices is too small to obtain a strong reflection. The reflectivity is therefore increased by a reflective layer 11 located at the interface, which is a thin layer (<0.4 micron) of a metal or of a metal coated with a suitable inorganic dielectric layer, the dielectric layer being on the incident light 10 facing side of the metal is arranged.

The materials for the reflective layer 11 are contained in Tables 1 to 5 of US 4,856,857; Tables 1 through 6 are expressly included in this description. The tellurium not mentioned in Table 5 is also suitable for the reflection layer 11. The incident light 10 means daylight or visually visible polychromatic light with wavelengths between 380 nm and 780 nm. In one embodiment of the layer composite 6, the surface of the cover layer 7 of the layer composite 6 facing away from the lacquer layer 9 is connected to a carrier tape 13 by means of a separating layer 12 in order to facilitate the transfer of the fragile layer composite 6 to the document 1. The carrier tape 13 made of paper or a plastic film, for example PC or PETP, can be removed after the layer composite 6 has been stuck on, so that the pattern 25 (FIG. 2) is visible through the protective layer 7 and the lacquer layer 9. For this, reference is made to GB 2 129 739 B already mentioned at the beginning.

As can be seen from FIG. 3, a relief structure 14 with a geometric profile depth p is molded into the lacquer layer 9 in the area of the pattern elements 4. In the area of the background surfaces 3, the lacquer layer 9 is flat and smooth and is parallel to the other layers of the layer composite 6. The material of the adhesive layer 8 fills the depressions of the relief structure 14. The interface with or without an additional reflection layer 11 follows both the relief structure 14 as well as mirror planes of the background areas 3. The relief structure 14 is a cross grating composed of two basic gratings with periods d smaller than a cut-off wavelength λ at the short-wave end in the spectrum of visible light, ie λ = 380 nm to λ = 420 nm and has an optically effective structure depth h, that is the profile depth p multiplied by the refractive index of the lacquer layer 9, preferably in the range from h = 50 nm to h = 500 nm. Such relief structures 14 absorb almost all visible light 10 incident on the pattern elements 4 and scatter a small fraction of the incident light 10 back into the half space above the pattern element 4. The percentage of the absorbed light 10 depends in a non-linear manner on the structure depth h and can be controlled between 50% and approximately 99% by means of the selection of the structure depth h in the above-mentioned range, whereby the flatter the relief structure 14 the more incident light 10 is scattered back and the less light 10 is absorbed. The percentages given apply to the relief structure 14 with a reflection layer 11 made of, for example, aluminum. Colliding areas of the pattern elements 4 with different structure depths h therefore show a gray gradation.

The embodiment of the relief structure 14 shown in FIG. 4 is a cross grating formed by two sinusoidal base grids crossing at right angles. The sine function of the first that extends along the coordinate x

The basic grid has a period d _x and an amplitude h _x , while the sine function of the second basic grid, which extends along the coordinate y, has a period d _y and an amplitude h _y . Above the plane spanned by the coordinates x and y, the interface h (x, y) formed by the cross lattice follows the function in the layer composite 6 (FIG. 3), for example

h ( ^χ . y) ⁼ [hx ⁺ h _y ] «sin ² (πx d _x )« sin ² (πy / dy).

In other versions, h (x, y) = h _x «sin ² (πx / d _x ) + h _y « sin ² (πy / d _y ), rectangular or pyramid structures are used as the interface h (x, y).

In one embodiment, the two periods are d _x , d _y and the structure depths h _x ; h _y same, different in other versions. The structure depth h = [h _x + h _y ] can be chosen to be greater than the period d, but the relief structure 14 is difficult to produce with today's manufacturing methods. The interface h (x, y) is like an egg carton and is shown in FIG. 4.

The optical behavior of the security element 2 in a first observation condition is explained with reference to FIG. The incident light 10 forms an angle of approximately 40 ° with a normal 15 to the level of the security element 2. In one example, the pattern elements 4 with the above-described relief structure 14 absorb up to 95% of the incident light 10 in the visible region, the rest is scattered. The reflective background surface 3, however, absorbs only about 10% of the incident light 10 and reflects the rest. Since surface parts of the pattern elements 4 adjoin the reflective background surfaces 3, the observer obtains such a high contrast that the pattern elements 4 arranged on a predetermined background surface 3 of the security element 2 in the predetermined pattern 25 are easily recognizable as information. Pattern 25 represents a logo, text, image or other graphic character.

The drawing in FIG. 5 corresponds to the lighting conditions in the copier. Depending on the model of the copier, the directional light 10 of the copier incident on the document 1 and the security element 2 with the normal 15 forms the angle of incidence α in the range from approximately 40 ° to 50 °. Document 1 scatters the incident light 10 throughout the half space. Scattered light thereby arrives in a direction arranged in the direction of the normal 15 Light receiver 16 of the copier. In contrast, the light 17 reflected by the background surface 3 is deflected according to the law of reflection with the same angle α in a viewing direction 18 of the observer 19 and does not reach the light receiver 16. If the light 10 falls on the pattern element 4 at the same angle of incidence α, the incident light 10, however, is practically absorbed; neither the light receiver 16 nor the observer 19 register any light from the pattern element 4. The pattern element 4 is therefore dark.

The background surfaces 3 form the plane mirror surfaces of the pattern 25 for the light 10 incident in the layer composite 6, while the pattern elements 4, as absorber surfaces, largely swallow the incident light 10. Therefore, the observer 19 recognizes in the reflected light 17 the background areas 3 as intensely bright partial areas and the pattern elements 4 as dark partial areas of the pattern 25. In directions other than that of the reflected light 17, the security element 2 scatters only a small part of the incident light 10 Intensities per unit area of the on the background areas 3 and

Pattern elements 4 of scattered light are practically of the same size, so that there is no contrast between the background surfaces 3 and the pattern elements 4. When illuminated with the directionally incident light 10, the pattern 25 formed from the background surfaces 3 and the pattern elements 4, in contrast to a black-and-white image produced by printing technology, can only be recognized in the light 17 reflected by specular reflection.

In the copier, the background area 3 and the pattern element 4 throw such a small proportion of the incident light 10 into the light receiver 16 that the copier reproduces the background field 3 and the pattern element 4 as black areas without distinction. The advantage of this security element 2 is that the copier cannot reproduce the information represented by the pattern element 4, while the observer 19, who almost automatically tilts the security element 2 in the case of incident light 10 in such a way that he looks at the background surface 3 in reflection, sees the information of the pattern element 4 with great contrast in front of the background surface 3. In this way, the security element 2 can easily be distinguished by an attentive observer from reflective metal foils on good color copies of the document 1. Another advantage is the use of the relief structure 14 in the Security element 2 with the periods d _x (FIG. 4), d _y (FIG. 4), which are shorter than the wavelengths of the coherent light sources usable for holographic copying methods; a copy of the security element 2 cannot therefore be produced using the holographic methods.

FIG. 6 shows a second lighting condition for the two observers 19, 20 of the security element 2. A polychromatic radiation source 21, for example a halogen lamp, incandescent lamp, etc., is arranged above the second observer 20 and sends the incident light 10 onto the pattern element 4 at a large angle of incidence α of approximately 60 ° to 80 °. The first observer 19 recognizes the pattern 25 (Fig. 2) of the pattern elements 4 against the background 3 (Fig. 5) at the reflection angle α, as stated above. If the periods d _x (FIG. 4), d _y (FIG. 4) of the relief structure 14 are in the range of half and a whole cut-off wavelength λ; ie λ>d> λ / 2, where d = d _x or d _y , part of the incident light 10 is deflected at a large diffraction angle β into the minus first order as diffracted light 22. The second observer 20 can recognize the diffracted light 22. The diffracted light 22 comprises the short-wave part of the visually visible spectrum of the electromagnetic radiation. The diffracted light 22 is therefore dependent on the diffraction angle β and the periods d _X) d _y in a blue-green to violet color. The color of the diffracted light 22 observed at a predetermined diffraction angle β to the normal 15 also depends in intensity on the azimuth. Note: In the above consideration, the refractive influence of the protective layer 7 has been neglected.

The first observer 19, on the other hand, looks in the direction of the reflected light 17 and recognizes the background areas 3 as shiny, bright partial areas and the pattern elements 4 as dark partial areas of the pattern 25.

If the period d _x or d _{y is} less than λ / 2, the diffracted light 22 can no longer be seen by the second observer 20 in the direction of the coordinate x or y, since the relief structure 14 no longer bends visible light 22 , The first observer 19, who observes the security element 2 at the reflection angle α, sees the pattern elements 4 unchanged in a dark brown to black color under these conditions. The color of the pattern elements 3 visible under the reflection angle α depends on the nature of the reflection layer 11, since different combinations of the materials in and on the reflection layer 11 do not reflect the incident light 10 uniformly in the entire spectral range of the visible electromagnetic radiation. Deep black pattern elements 3 advantageously have a gradual transition of the refractive index from the lacquer layer 9 to the reflection layer 11; the transition is produced by means of at least one layer of an inorganic dielectric 23 between the lacquer layer 9 and a metal layer 24 of the reflection layer 11. The reflection layer 11 formed from the dielectric 23 and the metal layer 24 has no noticeable effect on the flat mirror surface of the background surfaces 3. In the case of the relief structure 14, on the other hand, this reflection layer 11 causes, due to interference, an almost complete extinction of the incident light 10, which occurs above all uniformly over the entire spectral range of the visible electromagnetic radiation. One example has a 50 nm thick layer of dielectric 23 made of ZnS and 100 nm aluminum as metal layer 24. A further advantage is the structure depth h, which is increased by the high refractive index for ZnS of n = 2.4 compared to the refractive index of the lacquer layer 9 of n = 1.5, while the profile depth p of the relief structure 14 remains the same.

In addition to the gray gradations with pattern elements 4 with different structure depths h, in one embodiment of the security element 2

Gradation of gray by means of different densities with halftone dots of. less than 0.4 mm dimension. It is irrelevant whether the raster points are arranged as a background field 3 in a pattern element 4 or as a pattern element 4 in the background field 3. FIGS. 7a and 7b show further examples for the generation of

Shades of gray shown within a security element 2 from the dark pattern element 4 to the light-shiny background field 3. In FIG. 7a, differently sized screen dots are used in a fixed grid with a maximum spacing of 0.5 mm depending on the gray level. In a slightly lightened zone 26, the grid points touch, in a lightened zone 27 they point

Screen dots have an average dimension of about 0.25 mm, while in a slightly darkened zone 28 the screen dots have about 0.15 mm. In FIG. 7b there is a line grid with a maximum of 0.5 mm instead of the dot grid Distance used. A corresponding line width causes the gray gradation in zones 26 (FIG. 7a) to 28 (FIG. 7a).

In one of the zones 26 to 28, the grid points of the pattern areas 4 have the same dimensions. A very fine gray gradation is achieved by means of the correspondingly graduated structure depths h in the relief structures 14 (FIG. 6), which is sufficient for the reproduction of a black and white photo.

8 shows two patterns 25 of the security element 2 as a simple example. In the upper half of the security element 2, the pattern 25 consists of a band 29 with a star 30. The band 29 is formed from the dark pattern element 4. The surroundings of the band 29 and the star 30 form the light background areas 3. Without restricting what has been described so far, the background areas 3 and the pattern elements 4 are interchangeable, as is shown in the lower half of the security element 2.

The security element 2 in FIG. 1 becomes even more difficult to imitate if the pattern 25 forms a background for a mosaic-like surface pattern 31 with diffraction structures whose spatial frequencies have values in the range from 300 lines per mm to 2000 lines per mm. Such mosaic-like surface patterns 31 are known from EP 0 105 099 A1, EP 0 330 738 A1, EP 0 375 833 A1 mentioned at the outset. The content of these patents is hereby included in the description.

Claims

1. Security element (2) with a pattern (25) of partial areas and in the form of a layer composite (6) for attesting a document (1) that has at least one transparent protective layer (7), a transparent lacquer layer (9) and an adhesive layer (8 ), wherein the lacquer layer (9) is arranged between the protective layer (7) and the adhesive layer (8) and the refractive index at the interface between the adhesive layer (8) and the lacquer layer (9) changes abruptly, and the partial surfaces of the Assemble patterns (25) from background areas (3) and pattern elements (4), characterized in that in the area of the background areas (3) the varnish layer (9) is smooth and flat and in the area of the pattern elements (4) relief structures (14) a predetermined optically effective structure depth (h) is molded into the lacquer layer (9), that the background surfaces (3) for light (10) incident in the layer composite (6) are flat mirror surfaces and that the reliefs structures (14) from basic grids with periods (d _x ; d _y ) are formed cross gratings and the periods (d _x ; d _y ) are shorter than a predetermined limit wavelength (λ) at the short-wave end in the spectrum of visible light (10), so that the pattern elements (4) the incident light (10) absorb and scatter, the ratio of the absorbed and the scattered light in each relief structure (14) depending on the optically effective structure depth (h) prevailing in the relief structure (14).

2. Security element (2) according to claim 1, characterized in that that the cross lattice of the relief structures (14) is composed of two basic lattices with the periods (d _x ; d _y ) arranged essentially at right angles to one another.

3. Security element (2) according to claim 1 or 2, characterized in that the base grating is sinusoidal.

4. Security element (2) according to one of claims 1 to 3, characterized in that at least one of the periods (d _x ; d _y ) is longer than half the limit wavelength (λ) but shorter than the limit wavelength (λ).

5. Security element (2) according to one of claims 1 to 4, characterized in that the cut-off wavelength (λ) is selected in the range between 380 nm and 420 nm.

6. Security element (2) according to one of claims 1 to 5, characterized in that the periods (d _x ; d _y ) of the two base grids have the same value.

7. Security element (2) according to one of claims 1 to 6, characterized in that the values for the optically effective structure depth (h) of the relief structures (14) are selected in the range from h = 50 nm to h = 500 nm.

8. Security element (2) according to one of claims 1 to 7, characterized in that the reflection layer (11) contains a metal from the group aluminum, silver, gold, chromium, copper, nickel and tellurium.

9. Security element (2) according to claim 8, characterized in that the reflection layer (11) on the side of the metal layer (24) facing the lacquer layer (9) has at least one layer of an inorganic dielectric (23).

10. Security element (2) according to claim 9, characterized in that the layer of the inorganic dielectric (23) consists of ZnS and the metal layer (24) consists of aluminum.

11. Security element (2) according to one of claims 1 to 10, characterized in that the pattern (25) has zones (26; 27; 28) with gray levels, and that the pattern elements (4) of the zones (26; 27; 28) with different shades of gray due to the optically effective structure depth (h) of the relief structures (14).

12. Security element (2) according to one of claims 1 to 10, characterized in that the pattern (25) has zones (26; 27; 28) with gray levels, that the pattern elements (4) have the same values of the optically effective structure depths (h) have, and that the zones (26; 27; 28) differ by different densities of grid points with dimensions of less than 0.4 mm.

13. Security element (2) according to one of claims 1 to 12, characterized in that the pattern (25) forms a background for a mosaic-like surface pattern (31) from diffraction structures with spatial frequencies in the range from 300 lines per mm to 2000 lines per mm.