WO1998010324A1

WO1998010324A1 - Surface pattern with at least two different light-diffracting relief structures for optical security elements

Info

Publication number: WO1998010324A1
Application number: PCT/EP1997/004608
Authority: WO
Inventors: Wayne Robert Tompkin; René Staub
Original assignee: Electrowatt Technology Innovation Ag
Priority date: 1996-09-04
Filing date: 1997-08-25
Publication date: 1998-03-12

Abstract

The invention concerns a surface pattern (1) with at least two part areas (15; 16) comprising microscopically fine light-diffracting relief structures. The edging around the two part areas (15; 16) forms information ('V') which, when illuminated with unpolarized light and viewed without polarizing devices cannot be recognized or can only be recognized with difficulty, and which, when illuminated in a predetermined direction with unpolarized light and viewed with polarizing devices from a viewing direction dependent on the illumination direction can be recognized comparatively clearly. The relief structures in the two part areas (15; 16) are grids with different profile heights and/or profile shapes having an optical effect. Surface patterns of this type are suitable as optical security features for protecting identity cards or objects of all kinds.

Description

SURFACE PATTERN WITH AT LEAST TWO DIFFERENT LIGHT-BREAKING RELIEF STRUCTURES FOR OPTICAL SECURITY ELEMENTS

The invention relates to a surface pattern of the type mentioned in the preamble of claim 1.

Such surface patterns have a microscopic relief structure and are suitable as optical security elements, for example to increase counterfeit security and / or the conspicuous identification of objects of all kinds, and can be used in particular for securities, ID cards, means of payment and similar objects to be secured.

Surface patterns with microscopic, light-diffractive relief structures are known from various patents, e.g. from European patent EP 105 099.

The invention has for its object to propose further optical effects that increase the security of such surface patterns.

The invention consists in the features specified in claims 1, 7 and 9. Advantageous configurations result from the dependent claims.

The invention is based on the knowledge that the diffraction capacity of a light-diffraction grating, with a suitable choice of its profile height and / or profile shape, depends on the polarization of the incident light. A surface pattern with such a grating appears when viewed in non-polarized light, but through a polarizer held in front of the eyes, in a brightness and color that depends on the orientation (rotational position) of the polarizer.

The use of two light diffraction gratings with different polarization properties allows the surface pattern to be equipped with information that is barely visible to the unarmed eye when viewed in non-polarized light, but that is clearly recognizable when viewed through a polarizer. In a preferred embodiment, two grids are used which differ only in the profile height of their furrows, while the other grid parameters line spacing, azimuthal orientation and profile shape are identical.

The grids are preferably implemented as microscopic relief structures, which allows simple mass reproduction by embossing. In order to compensate for deviations from the desired profile shape that arise during manufacture, the embossing die is advantageously designed in such a way that the embossed relief structure achieves the desired profile shape as far as possible.

Although the polarization properties of the grids can be calculated per se, the computational effort to optimize a grid can be very large. It is therefore often necessary to experimentally optimize the profile height that is adapted to the optical effect. Representative of mathematical methods, algorithms and numerical computer programs for the calculation of the diffraction properties of metallic gratings are mentioned: The book "Electromagnetic Theory of Grätings" by R. Petit, Springer Verlag, the article "Rigorous coupled-wave analysis of metallic surface-relief gratings" by M.

CONFIRMATION COPY G. Moharam and TK Gaylord, published in the Journal of the Optical Society of America A, Vol. 3 (1 1), pp. 1780-1787, 1986, and the software package "GSOLVER V2.0".

One method for producing such relief structures with dimensions in the submicron range is, for example, electron beam lithography.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawing.

1 shows a surface pattern in cross section, FIG. 2 shows the surface pattern in plan view,

3 shows the relative spectral sensitivity of the human eye, FIG. 4 shows another cross-sectional area pattern, FIG. 5 shows the diffraction efficiency of the zeroth diffraction order for a first grating,

6 shows the diffraction efficiency of the zeroth diffraction order for a second grating, FIG. 7 multiplied the diffraction efficiency of the zeroth diffraction order for the two gratings when illuminated with unpolarized light, and FIG. 8 multiplied by the spectral sensitivity of the human eye

Diffraction efficiencies of the zeroth diffraction order for the two gratings for TE-polarized light, FIG. 9 multiplied by the spectral sensitivity of the human eye

Diffraction efficiencies of the zeroth diffraction order for the two gratings for TM polarized light,

10 diffraction efficiencies multiplied by the spectral sensitivity of the human eye of the first diffraction order for the two gratings when illuminated with unpolarized light, FIG. 11 diffraction efficiencies multiplied by the spectral sensitivity of the human eye of the first diffraction order for the two gratings

Illumination with TE-polarized light, FIG. 12 diffraction efficiencies of the first diffraction order multiplied by the spectral sensitivity of the human eye for the two gratings when illuminated with TM-poarized light, FIG. 13 shows a surface pattern with brightness adjustment,

14 shows the diffraction efficiency of the zeroth diffraction order for a first grating in

15 shows the diffraction efficiency of the zeroth diffraction order for a second grating as a function of the angle of incidence, and FIG. 16 shows the contrast of these gratings when illuminated with white light as a function of the

Angle of incidence. 1 shows in cross section a surface pattern 1 which is constructed as a layer composite 2. An intermediate layer 4, a first lacquer layer 5, a reflective reflection layer 6, a second lacquer layer 7 and an adhesive layer 8 are applied to a carrier layer 3 in the order given. The layers 3 to 8 form the layer composite 2. Between the lacquer layers 5 and 7, microscopically fine relief structures 9, 10 are embedded, which diffract light 12 incident through the lacquer layer 5 and at least partially reflect it. The profile height of the relief structures 9 and 10 is h | or h ₂ , the line spacing with d- or d ₂ . The refractive index of the lacquer layer 5 is designated n5. The layer composite 2 is glued to a substrate 11 to be protected, for example a document, the adhesive layer 8 resting on the substrate 11. After the surface pattern 1 has been stuck onto the substrate 11, the carrier layer 3 is removed. The light 12 strikes the surface pattern 1 from the side of the first lacquer layer 5 at an angle θ to the normal 13. An observer sees the light 14 diffracted and reflected on the relief structures 9 and 10.

2 shows the surface pattern 1 in a top view. The surface pattern 1 consists of two adjacent partial surfaces 15 and 16, which, as shown, preferably have common boundary lines and serve for mutual referencing. A first grid G1 is present in the partial area 15 as a microscopically fine relief structure 9 (FIG. 1), and a second grid G2 is present in the partial area 16 as a microscopic fine relief structure 10. The enlarged furrows 17 of the grids G l and G2 are parallel, i.e. they have the same azimuthal orientation. The common boundary of the two partial surfaces 15 and 16 is chosen in the example so that information "V" is formed. In addition to meaningful characters, the information can also be a graphic representation, for example a logo, a coat of arms, a pattern, etc.

Several examples are described below as to how the grids G1 and G2 are to be designed so that the information “V” formed by the shape of the sub-areas 15 and 16 is barely or comparatively weakly recognizable when viewed in unpolarized light, when viewed in polarized light or by a polarizer through it is clearly recognizable. In addition, the relative spectral sensitivity of the human eye is shown in FIG. 3, as specified in the book "Principles of Optics" by M. Born and E. Wolf.

example 1

The grids Gl and G2 have the same line spacing of 1 μm and a sinusoidal profile shape. The optically effective profile height h- * n5 (FIG. 1) of the first grating Gl is 70 nm, the optically effective profile height h ₂ * n5 of the second grating G2 is 300 nm. The reflection layer 6 consists of aluminum.

1, different optically effective profile heights h - "- ^* n5 and h ₂ * n5 are realized by the geometric profile heights h- and h _{2 being of} different sizes. In another solution shown in FIG Geometric profile heights h and h _{2 are of the} same size, instead of the single lacquer layer 5, which covers both partial areas 15 and 16, the first partial area 15 is provided to cover a lacquer layer 5a with a first refractive index n5a and the second partial area 16 with a lacquer layer 5b with a second refractive index n5b. The lacquer layers 5a and 5b can advantageously be applied with an ink jet printer. At the same time, this represents a simple way of providing the surface pattern 1 with individual information.

Fig. 5 shows the diffraction efficiency of the first grating Gl as a function of the wavelength λ for the zeroth diffraction order for TE or TM polarized light as well as for unpolarized light which is at the angle θ = 0 ° (Fig. 1), i.e. perpendicular, falls on the grid Gl. In the figures of the drawing, a dotted line 18 is used for TE-polarized light, a dashed line 19 for TM-polarized light and a solid line 20 for unpolarized light. The diffraction efficiency for unpolarized light corresponds to the arithmetic mean of the diffraction efficiencies of the TE and TM polarized light. The term TE polarization means that the electric field of the light 12 (FIG. 1) oscillates parallel to the furrows 17 (FIG. 2), the magnetic field perpendicular to the furrows 17 of the grating Gl, while TM polarization means that the magnetic field oscillates parallel to the furrows 17 and the electric field perpendicular to the furrows 17 of the grid G l. In the area that is clearly visible to the human eye, i.e. In the range of the wavelengths λ of approximately 475 to 650 nm, light with TE polarization and light with TM polarization are diffracted into the zeroth diffraction order with approximately the same efficiency.

FIG. 6 shows the diffraction efficiency of the second grating G2 as a function of the wavelength λ for TE and TM polarized light and for unpolarized light 12 that falls perpendicularly on the grating G2 for the zero order of diffraction.

FIG. 7 shows the curves of FIGS. 5 and 6 multiplied by the spectral sensitivity of the human eye for unpolarized light. It can be seen from this that the human eye perceives the two gratings G1 and G2 as almost the same and therefore cannot recognize the information “V” (FIG. 2) present in the partial areas 15 and 16.

8 and 9 show the curves of FIGS. 5 and 6 multiplied by the spectral sensitivity of the human eye in the event that the grids Gl and G2 are illuminated with TE or TM polarized light or illuminated with unpolarized light, but by a polarizer that is transparent to TE or TM polarized light can be viewed through. It follows from this that in TE-polarized light the partial area 16 with the grating G2 appears approximately twice as bright as the partial area 15 with the grating Eq. In addition, the human eye perceives partial area 16 in blue-green, partial area 15 in light brown color. In TM-polarized light, the partial area 15 is brighter than the partial area 16, but both partial areas 15 and 16 appear in the same color.

The previous considerations apply to vertical incidence. If the light 12 (FIG. 1) falls obliquely onto the gratings G1 and G2, then the incident light always contains TE and TM polarized light, except in special cases. The surface pattern 1 (FIG. 2) is therefore advantageously viewed through a polarizer that emits either the TE or the TM polarized light that is on the grating Gl or G2 has been bent and reflected, absorbed. When the angle of incidence θ (FIG. 1) changes, the spectra shown in FIGS. 5 to 9 change, which can lead to the fact that the information “V” formed by the edges of the partial areas 15 and 16 is at least weakly visible from certain observation directions . When viewing the surface pattern 1 through the polarizer, however, the brightness and the color of the partial surfaces 15 and 16 change with the rotational position of the polarizer.

The subject of the previous investigations has been the light diffracted and reflected in the zero diffraction order, i.e. the light reflected in specular reflection. Neither the profile shape nor the line spacing of the grating Gl or G2 have any influence on the direction of the zeroth diffraction order. Both the profile shape and the line spacing of the two gratings G 1 and G 2 can thus be optimized such that even when the light 12 is at an angle, it is ensured that the information “V” is only visible in at least partially polarized light or thanks to the use of a polarizer.

The diffraction efficiency of the gratings Gl and G2 also depends on its polarization for the light diffracted in the first diffraction order. 10-12 show the spectra of the first diffraction order for unpolarized light, TE-polarized or TM-polarized light that already take the spectral sensitivity of the human eye into account. In spite of the shift of the maximum of the intensity of the light diffracted at the grating G 1 compared to the intensity of the light diffracted at the grating G 2 of approximately 30 nm and the slightly different maximum diffraction efficiency of 14 and 17 percent, respectively, that is due to the two partial surfaces 15 and 16 (FIG. 2) information "V" formed with the gratings G1 and G2, respectively, when viewed in unpolarized light, is difficult or impossible to recognize.

1 1 and 12 it can be seen that the light diffracted into the first diffraction order on the grating Gl is predominantly TM-polarized or on the grating G2 is predominantly TE-polarized. When the two partial areas 15 and 16 are illuminated under normal lighting conditions and the two partial areas 15 and 16 are viewed through a polarizer, the appears

Partial area 15 lighter than partial area 16 or vice versa, depending on the orientation of the polarizer: the information "V" can be made visible by means of the polarizer.

With the selected line spacing of d- = d ₂ - = lμm, the diffraction angle, based on the normal to the surface pattern 1, is 26.7 ° with perpendicular incidence of the light at a wavelength of λ = 450 nm and 40.5 ° at a wavelength of λ = 650 nm. The first diffraction order thus shows

Dispersion effects, i.e. when tilted about an axis parallel to the furrows 17 (FIG. 2) of the grids G 1 and G 2, the colors and brightnesses of the two partial surfaces 15 and 16 change.

Example 2

The grids Gl and G2 are the same as in the first example, with the exception of the optical profile height h ₂ * n5 of the second grating G2, which is h ₂ * n5 = 312.5 nm. At this profile height h ₂ the difference is Intensity of the light diffracted into the first diffraction order between TE or TM polarization is even more pronounced.

The strong dependence of the difference in diffraction power for TE or TM polarized light for the light diffracted in the first diffraction order on the optically effective profile height results in a high level of security against counterfeiting, since maintaining an exact profile height places very high demands on the quality of the production poses.

Example 3

It may be that with profile heights h- and h selected for certain criteria for the two gratings G1 and G2, even when viewed in non-polarized light, the two partial surfaces 15 and 16 (FIG. 2) are visible in different brightness and the information "V" is thus recognizable. To reduce the brightness of the lighter of the two sub-areas 15 and 16, part of the area occupied by the lighter of the two sub-areas 15 and 16 can therefore be eliminated and covered with a relief structure that reflects as little light as possible. 13 shows such a surface pattern 1. Within the partial area 15, partial areas 21 are arranged in an island-like manner, the largest dimension of which is less than 0.3 mm in any direction, so that the partial areas 21 cannot be seen by the eye from a normal visual distance of typically 30 cm. The partial areas 21 contain e.g. a diffusely scattering matt structure or, preferably, a grating structure consisting of two superimposed gratings, the furrows 17 (FIG. 2) of which are arranged perpendicular to one another, with a number of lines which is greater than approximately 1800 lines / mm, it being assumed that the refractive index n5 the lacquer layer 5 (FIG. 1) has the value 1.5. Such a crossed lattice structure acts like a black surface for visible light, since the visible light is neither diffracted nor reflected, but is absorbed (see e.g. G.H. Derrick et al “Applied Physics, Vol. 18, pp. 39-52 (1979)). Thanks to the partial areas 21, the brightness of the partial area 15 is reduced to the brightness of the partial area 16.

With a number of lines of 1800 lines / mm, visible light becomes, depending on its wavelength, in the first diffraction order with perpendicular incidence compared to the normal at an angle of 54 °

(λ = 450 nm) or 82 ° (λ = 550 nm) diffracted and reflected, while the first diffraction order for light with λ = 650 nm no longer occurs.

With a number of lines of 2250 lines / mm, no more diffracted blue light occurs. How large the number of lines of the grating of the crossed grating structure should be chosen depends on whether it is sufficient if the diffracted and reflected light reaches a diffraction angle of, for example, 70 ° or 80 ° or more, so that it can be seen by the person on the street is usually ignored.

It is also possible for some of the partial surfaces 21 to be designed as a reflecting surface and others of the partial surfaces 21 as a lattice structure with crossed gratings. The reflectivity of a mirror is usually greater than the diffraction power of a grating. With reflecting sub-areas 21, the brightness of the sub-area 15 can thus be increased for an observer who sub-area 15 in the zeroth Diffraction order, ie in mirror reflection to the light source, considered. With partial surfaces 21, which are designed as a crossed lattice structure with a high number of lines, the brightness of the partial surface 15 can be reduced for all diffraction orders. The use of these two types of partial areas 21 thus enables the brightnesses of the partial areas 15 and 16 to be matched to one another in such a way that the two partial areas 15 and 16 appear in the same brightness both when viewed in the zero order and in the first order of diffraction in non-polarized light.

Example 4

The grids Gl and G2 of the partial areas 15 and 16 have the same profile shape and the same optically effective profile height h | * n5 * = h ₂ * n5 = 300 nm, so that they at least partially polarize the light diffracted into the zeroth diffraction order (see 8 and 9). The furrows 17 (FIG. 2) of the grid G 1 are twisted azimuthally by a predetermined angle φ, for example φ = 90 °, relative to the furrows 17 of the grid G2. When viewing the surface pattern 1 in polarized light or in non-polarized light, but through a polarizer, and in the zero order of diffraction, ie in mirror reflection, either the partial surface 15 is markedly brighter than the partial surface 16 or vice versa. Since the parameters of the two gratings G1 and G2 are identical with the exception of their azimuthal orientation, a contrast reversal takes place when the polarizer is rotated through the angle φ = 90 °. If the angle φ is less than 90 °, for example φ = 30 °, then the relative brightnesses of the partial areas 15 and 16 change when the polarizer is rotated by the angle φ = 30 °.

The polarizing properties of the surface pattern 1 can also be verified mechanically with an appropriately equipped reading device.

Example 5

The grids G 1 and G 2 of the partial surfaces 15 and 16 (FIG. 2) have a sinusoidal profile shape with an optically effective profile height h- * n5 = 170 nm and h ₂ * n5 = 355 nm. The line spacing is 0.4 μm. The reflection layer 6 (Fig. 1) consists of aluminum. 14 and 15 show the diffraction efficiency for the light diffracted to the zeroth order on the gratings G1 and G2, respectively, as a function of the angle of incidence θ of the light 12 (FIG. 1), the wavelength of which is 550 nm. Roughly speaking, three angular ranges can be distinguished. At small angles of incidence θ in the range from 0 to approximately 25 °, the light on both gratings Gl and G2 is diffracted with comparable intensity and hardly polarized, so that the surface pattern 1 (FIG. 2) appears to a viewer as a reflecting surface. At angles of incidence θ in the range from approximately 25 ° to approximately 60 °, the grating G 1 polarizes the incident light comparatively strongly, while the grating G2 polarizes the incident light only slightly. However, the diffraction efficiency for unpolarized light is approximately the same for both gratings G 1 and G2. In normal lighting with unpolarized light and viewing without polarizing aids, the surface pattern 1 still appears to a viewer as a reflecting surface: the information "V" is not recognizable. When viewed through a polarizer, however, the information “V” is visible in the surface pattern 1. At angles of incidence θ above approximately 60 ° The two gratings Gl and G2 polarize the incident light differently, but the diffraction efficiency for unpolarized light is now different for both gratings Gl and G2, so that the two gratings Gl and G2 appear differently bright at large angles of incidence θ from approximately 80 °. This makes the information "V" recognizable even in unpolarized light.

16 shows the contrast as a function of the angle of incidence θ when illuminated with white, unpolarized light. The contrast is low for angles of incidence θ that are smaller than approximately 75-80 ° and increases significantly for angles of incidence θ above approximately 80 °.

The surface pattern 1 shown in FIG. 2 has the two partial surfaces 15 and 16. The partial surfaces 15 and 16 can of course be shaped as desired and, for example, also represent a thin line.

To optimize the polarization effects, all parameters that influence the diffraction efficiency of the relief structure can be varied. These are primarily the profile shape of the grids G1 and G2, but also the materials of the reflective layer 6 (FIG. 1) and the lacquer layer 5. It is e.g. possible that the reflection layer 6 for the partial surface 15 made of a first metal, e.g. Gold, and the reflection layer 6 for the partial surface 16 made of a second metal, e.g. Tellurium exists.

Claims

PATENT CLAIMS

1. surface pattern (1) with at least a first and a second partial surface (15; 16) which contain microscopic, light-diffractive relief structures (9; 10), characterized in that the edges of the two partial surfaces (15; 16) contain information ( "V") or a pattern forms that the information when illuminated with unpolarized light and viewed without polarizing aids is not or only weakly recognizable and that the information when illuminated in a predetermined direction with unpolarized light and when viewed from a direction dependent on the direction of illumination Direction of observation with polarizing aids is comparatively clearly recognizable.

2. surface pattern (1) according to claim 1, characterized in that the relief structure (9) of the first partial surface (15) is a grating Gl with an optically effective profile height, which is chosen so that the diffraction efficiency of the grating Gl for the in a Light incident in a predetermined direction and diffracted in a predetermined diffraction order is hardly dependent on the polarization of the light and that the relief structure (10) of the second partial surface (16) is a grating G2 with an optically effective profile height that is different from the optically effective profile height of the grating G l and is selected such that the diffraction efficiency of the grating G2 for the light incident in the predetermined direction and diffracted in the predetermined diffraction order is comparatively strongly dependent on the polarization of the light.

3. surface pattern (1) according to claim 1 or 2, characterized in that when illuminated with unpolarized light, the first partial surface (15) is brighter than the second partial surface (16) when the polarizing aid has a first orientation and that the second partial surface (16) is lighter than the first partial surface (15) if the polarizing aid has a second orientation.

4. surface pattern (1) according to one of claims 1 to 3, characterized in that the first partial surface (15) and the second partial surface (16) are visible in a first and second color when the polarizing aid has a first orientation, and that the first partial surface (15) and the second partial surface (16) are visible in a third and fourth color, respectively, if the polarizing aid has a second, different orientation.

5. surface pattern (1) according to one of claims 1 to 4, characterized in that the first partial surface (15) has third partial surfaces (21) for comparing the relative brightness of the first and second partial surface (15; 16).

6. surface pattern (1) according to claim 5, characterized in that the third partial surfaces (21) have a grid structure of two superimposed grids, and that the number of lines of the grid is greater than 1800 lines / mm.

7. surface pattern (1) with at least a first and a second partial surface (1 1; 12) containing microscopic, light diffractive relief structures (9; 10), characterized in that the edges of the two partial surfaces (15; 16) provide information ("V") forms that the two partial surfaces (15; 16) change their color and / or their contrast when viewed in polarized light or through a polarizing aid if the polarization of the light or the polarizing aid changes its orientation.

8. surface pattern (1) according to claim 7, characterized in that the relief structure (9) of the first partial surface (1 1) is a grating G l with an optically effective profile height, which is chosen so that the diffraction efficiency of the grating Gl for the Light incident in a predetermined direction and diffracted in a predetermined diffraction order is dependent on the polarization of the light, and in that the relief structure (10) of the second partial surface (11) is the same grating Gl, however the grating Gl in the first partial surface (15) is rotated by a predetermined angle with respect to the grid Gl in the second partial surface (16).

9. surface pattern (1) with at least a first and a second partial surface (1 1; 12) containing microscopic, light-diffractive relief structures (9; 10), characterized in that the edges of the two partial surfaces (15; 16) provide information ("V") means that the information ("V") does not when illuminated with unpolarized light (12) which strikes the surface pattern (1) at an angle θ to the surface pattern (1) with respect to the normal (13) is recognizable if the angle θ is smaller than a predetermined angle and that the information ("V") is recognizable if the angle θ is greater than the predetermined angle.

10. surface pattern (1) according to claim 9, characterized in that the relief structures (9; 10) of the first partial surface (15) and the second partial surface (16) are grids with different optically effective profile heights.