EP2134551A1

EP2134551A1 - Safety element

Info

Publication number: EP2134551A1
Application number: EP08734745A
Authority: EP
Inventors: Hans Lochbihler
Original assignee: Giesecke and Devrient GmbH
Current assignee: Giesecke and Devrient GmbH
Priority date: 2007-04-03
Filing date: 2008-03-25
Publication date: 2009-12-23
Anticipated expiration: 2028-03-25
Also published as: DE102007016394A1; EP2134551B1; WO2008119487A1

Abstract

The invention relates to a safety element (12) for machine-based optical authenticity testing using light that has a prescribed test wavelength, wherein the safety element according to the invention has a layer (20) made of a highly conductive material, the layer comprising a one or two-dimensional lattice structure made of a periodic arrangement of a plurality of lattice elements (22), the lateral dimensions (d) thereof and/or lateral distances (a<SUB>0</SUB>) thereof being smaller than the test wavelength.

Description

security element

The invention relates to a security element for mechanical optical authentication with light, which contains a predetermined test wavelength. The invention further relates to an associated manufacturing method for the security element, a correspondingly equipped data carrier and a method and a device for mechanical optical authentication of such a security element.

Data carriers, such as valuables or identity documents, but also other valuables, such as branded goods, are often provided with security elements for the purpose of security, which permit verification of the authenticity of the data carrier and at the same time serve as protection against unauthorized reproduction. The security elements can be embodied, for example, in the form of a security thread embedded in a banknote, a covering film for a banknote with a hole, an applied security strip or a self-supporting transfer element which is applied to a value document after its manufacture.

To increase the security against forgery, often diffractive structures, such as holograms or hologram-like diffraction structures, are used as security features. These structures serve primarily as human features and, because of their generally inhomogeneous lateral design and the presence of higher diffraction orders, are only of limited use for mechanical authenticity checking. In addition, the previously known optical security features have obvious diffraction properties, which can also be detected and imitated relatively easily by counterfeiters. Proceeding from this, the object of the invention is to avoid the disadvantages of the prior art. In particular, a security element of the aforementioned type is to be specified, which can be tested for authenticity with high security against forgery in a simple manner.

This object is achieved by the security element having the features of the main claim. A corresponding production method, a data carrier equipped with such a security element as well as a method and a device for the mechanical optical authenticity check of such a security element are specified in the coordinated claims. Further developments of the invention are the subject of the dependent claims.

According to the invention, it is provided in a generic security element that the security element has a layer of a highly conductive material. The layer of a highly conductive material is also referred to hereinafter as a "highly conductive layer" or "highly conductive layer". The highly conductive layer may on the one hand be a substantially flat layer (surface layer). Such a surface layer has a very high conductivity substantially in the entire plane defined by the security element. On the other hand, the highly conductive layer can also be a layer which does not have a very high conductivity essentially in the entire plane defined by the safety element, but only in a part of the plane, eg. Along substantially a direction of this plane. The latter is z. In the case of a highly conductive layer, for example, a one-dimensional lattice structure with a large number of highly conductive lattice elements ensures the high conductivity of the layer. essentially in only one direction of the plane defined by the security element. More detailed descriptions of various highly conductive layers can be found in the following.

In the following, a highly conductive material or a highly conductive layer is always used if the conductivity of the material or of the layer is so great that the effect of the security element according to the invention is evident. In the context of the present invention, the term "conductivity" usually refers to the electromagnetic conductivity: When an electromagnetic wave, as in the context of this invention, strikes a specific material, the electromagnetic wave penetrates a certain depth (penetration depth) into the material and The electromagnetic wave is characterized by an electromagnetic conductivity, which depends in particular on the material and the wavelength of the electromagnetic wave, and the electromagnetic conductivity can be defined by the complex refractive index m (see chapter 2, page 21 ff "Convention confusions" of the "Handbook of Optical Constants of Solids II", Editor: Edward D. Palik, Academic Press, 1998).

Where:

m = n + ik.

Where m is the complex refractive index, n is the real part of the complex refractive index and k is the imaginary part of the complex refractive index. For highly conductive materials according to the present invention, the following applies:

k ²

-> 1 n

With particular preference, highly conductive materials in the context of the present invention fulfill the condition:

ie the quotient k ² / n is significantly greater than 1.

The criterion for highly conductive materials in the context of the present invention (k ² / n> 1) is z. B. by numerous metals such as gold, copper, silver, chromium, aluminum, or by alloys such. As brass or stainless steel met.

As will be explained below, it is particularly particularly preferred for the electromagnetic wave, which acts on the highly conductive material, to use light from the visible spectral range or light from the spectral range of the near infrared. Accordingly, it is particularly preferred for the highly conductive material, if the criterion k ² / n> 1 for light from the visible spectral range or light from the spectral range of the near infrared is met.

In addition, it should also be mentioned here that the penetration depth of the electromagnetic wave in the highly conductive material generally corresponds approximately to the so-called "skin depth" and a characterization of highly conductive materials. Details of the terms "skin depth" and "surface impedance" can be found in the "Handbook of Optical Constants of Solids II", Editor D. Palik, Academic Press, 1998. The same applies to electromagnetic conductivities of various materials.

Finally, it should be mentioned at this point that a material with a high electromagnetic conductivity according to the above definition usually also has a high electrical conductivity.

The reciprocal of the electrical conductivity is referred to as a specific electrical resistance. Consequently, the higher the electrical conductivity of a material at a given temperature, the lower the specific electrical resistance at that temperature. Without wishing to be set to exact numerical values, a highly conductive layer of the present invention generally will be characterized by an electrical resistivity which is less than about 1 C • 10- ⁴ ohm-cm at 20 ^0th This criterion is z. As by numerous metals such as gold, copper, silver, chromium, aluminum, but also by alloys such. As brass or stainless steel met. Specific resistances and electrical conductivities of various materials are known to those skilled in the art. In addition, specific electrical resistances of Table 4-2, page 227 of the textbook "Physics for Engineers" Hering, Martin, Stohrer, 5th edition, VDI-Verlag, 1995 or the website "http://de.wikipedia.org / wiki / Specific_Wider stand ".

As already mentioned, however, any material can be used with which the effect of the security element according to the invention is brought about and for which the criterion k ² / n> 1 in relation to the electromagnetic conductivity is fulfilled (see above).

According to the invention, the highly conductive layer comprises a one- or two-dimensional lattice structure comprising a periodic arrangement of a multiplicity of lattice elements whose lateral dimensions and / or lateral distances are smaller than the test wavelength. A periodic arrangement of lattice elements is understood below to mean any arrangement whose periodicity satisfies the definition given in Paperback of Mathematics "Bronstein, Semendjajew, 25th edition." The lattice elements forming the lattice structure are therefore regularly arranged, ie the lattice elements have, for example, a relation Furthermore, the term "periodic arrangements" also encompasses "almost periodic arrangements." A fast-periodic arrangement is understood below to mean any arrangement which is not exact, but only approximately periodic can show the effect of the security element according to the invention if the deviation from a periodic arrangement is not too great On the other hand, they can be mechanically checked for authenticity, whose diffraction properties are difficult to recognize and imitate for potential counterfeiters.

In a preferred embodiment of the security element according to the invention, the lateral dimensions and / or the lateral distances are at least a factor of 1.5, preferably even at least a factor of 2 smaller than the test wavelength. The security element is designed in particular for a mechanical optical authenticity check with light from the visible spectral range or for light from the spectral range of the near infrared. The authenticity check can be carried out with monochromatic light or also with polychromatic light which contains the defined test wavelength or the defined test wavelengths.

Within the scope of the invention, the highly conductive layer with the lattice structure is advantageously designed so that the light incident on the authenticity test under a predetermined test angle excites surface polarities and / or cavity resonances in the layer. Without wishing to be bound by this explanation, the present understanding of the physical background of the effects exploited according to the invention will be explained in more detail below.

The lattice structure of the highly conductive layer can be formed both as a reflection grating and as a transmission grating. Depending on the intended use of the security element, a reflection or a transmission grating may prove to be more suitable. For example, when using the security element on an opaque carrier expediently a reflection grating is used, while offering a transmission grating when used as a see-through element.

In an advantageous variant of the invention, the grid elements are formed by parallel, highly conductive grid lines in order to form a one-dimensional periodic grid structure. The period length is preferably smaller than the test wavelength, and is preferably at least a factor of 1.5 or even at least a factor of 2 smaller than the Prüfwellen- length. In a development of this variant of the invention, the grid structure has a substructure within a period.

In another likewise advantageous variant of the invention, the grid elements are formed by regularly arranged perforations in an otherwise continuous highly conductive surface layer in order to form a two-dimensional periodic lattice structure. The diameter of the perforations is preferably smaller than the test wavelength, preferably at least a factor of 1.5, more preferably at least a factor of 2 and most preferably at least a factor of 4 smaller than the test wavelength.

Additionally, but not necessarily, the lateral spacing of the perforations may be smaller than the test wavelength.

Particularly good results can be achieved if the ratio of the thickness of the surface layer to the diameter of the perforations is between 0.5 and 2, in particular about 1.0. The wavelength and angle ranges with characteristic diffraction properties are then particularly sharp and clearly formed.

In all of the stated embodiments, the thickness of the surface layer can be between 50 nm and 2 μm, preferably between 100 nm and 1 μm. The highly conductive material of the highly conductive layer is preferably a metal, in particular one of the metals gold, silver, copper, aluminum or chromium.

Of course, the highly conductive material may also be a mixture of different materials, such. B. an alloy of two or more than two metals. Advantageously, the metals of the alloy are selected from the group comprising gold, silver, copper, aluminum or chromium. The prerequisite for the use of a highly conductive material in connection with the invention described in this application is the possibility of producing a highly conductive layer containing the one- or two-dimensional lattice structure of the security element according to the invention.

The described optical authenticity check can also be carried out if the grid structure is embedded in a dielectric. Among others, glass or plastics are considered as dielectrics, in which latter case the security element expediently has a plastic film.

Suitable materials for the film are in particular PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PEN (polyethylene naphthalate), PP (polypropylene), PA (polyamide) and PE (polyethylene). The film may also be monoaxially or biaxially stretched. The stretching of the film, inter alia, leads to it receiving polarizing properties that can be used as another security feature. The tools required for exploiting these properties, such as polarization filters, are known to the person skilled in the art.

As a dielectric and paper, especially cotton vellum paper, conceivable. Of course, it is also possible to use paper which contains a proportion of polymeric material in the range of 0 <x <100% by weight.

It may also be expedient for the dielectric to be a multilayer composite comprising at least one layer of paper or a paper-like layer. gene material has. Further dielectrics can be found on the website "http://wikipedia.org/ wiki / Specific_Wid erstand".

To further increase the security against forgery or for design reasons, the security element can be combined with a visually verifiable human feature. In particular, the machine-testable highly conductive layer can be part of the visually verifiable human feature and can not easily be recognized as a machine-testable security feature by the normal user.

In advantageous embodiments, the security element is a security thread, a label, a transfer element or a see-through security element.

The invention also includes a method for producing a security element of the type described, in which a layer of a highly conductive material is provided with a one- or two-dimensional lattice structure of a periodic arrangement of a plurality of lattice elements, their lateral dimensions and / or lateral distances are smaller than the test wavelength.

Furthermore, the invention encompasses a data carrier, in particular a brand article or a value document, such as a banknote, a passport, a document, a banderole, an identification card or the like, which is provided with a security element of the type described. The security element can be arranged in particular in a window area of the data carrier. The window area can advantageously be punched out of the data carrier or produced by the action of laser radiation be. Of course, it is also conceivable in principle to form the window area before the application of the security element, for example in the sense of WO 03/054297 A2 during the production of the data carrier substrate, for. B. the papermaking.

The invention further relates to a method for mechanical optical authentication of a security element of the type described, in which

a) at least one test wavelength and at least one illumination geometry for the authenticity test is determined;

b) the highly conductive layer of the security element is exposed to light of the at least one defined test wavelength under the at least one defined illumination geometry,

c) the light reflected or transmitted by the highly conductive layer is detected at at least one predetermined test wavelength, and

d) the authenticity of the security element is assessed on the basis of the intensity and / or the polarization of the detected light.

In an advantageous variant of the method, the light reflected in the zeroth diffraction order is detected in step c). In another likewise advantageous variant, the light transmitted in the zeroth diffraction order is detected in step c).

In a further development of the method, at least two different test wavelengths are defined in step a), the light input in step c) detected at the at least two fixed test wavelengths and compared with each other in step d), wherein the authenticity of the security element is concluded when the light intensity is reduced at least one of the test wavelengths beyond a set level.

The surface layer can be acted upon in step b) simultaneously or sequentially with light of at least two different test wavelengths.

According to another development of the method according to the invention, at least two different illumination angles are determined in step a), the light intensities are detected upon application of the at least two fixed illumination angles in step c) and compared with one another in step d), wherein the authenticity of the security element is determined. conclude when the light intensity is reduced beyond a fixed level at at least one of the illumination angles.

The light intensities are detected in step c) expediently with a spatially resolving detector.

In a further advantageous development, the highly conductive layer in step b) is subjected to polarized light of preferably a predetermined test wavelength, and in step c) the polarization direction of the reflected or transmitted light is detected, wherein in step d) from a change in the polarization direction via a fixed level on the authenticity of the security element is closed.

The invention further comprises a device for mechanical optical authentication of a security element of the type described, with at least one light source for applying the layer of a highly conductive material of the security element to be tested with light of at least one specified test wavelength under at least one defined illumination geometry,

at least one detection device for detecting the light reflected or transmitted by the highly conductive layer, and

Means for evaluating the intensity and / or the polarization of the detected light at the at least one predetermined test length and / or at least one fixed illumination geometry, and for assessing the authenticity of the security element to be tested based on the evaluation made.

In a preferred embodiment, the light source is designed to act on the highly conductive layer of the security element with at least two different test wavelengths, and a wavelength-sensitive detection device is provided.

In another advantageous embodiment, the light source for exposing the highly conductive layer of the security element is designed with light from at least two different illumination angles, and it is a spatially resolving detection device, such as a diode array provided.

In a further advantageous embodiment for authenticity testing using the polarization conversion is in the beam path between the light source and the security element to be tested, a first polarizer, and in the beam path between the security element to be tested and the detection device is provided in the reverse direction to the first polarizer oriented second polarizer.

Of course, the security elements described can be combined with other visual and / or machine-readable security features. For example, the highly conductive layer can be provided with further functional layers, such as polarizing, phase-shifting, conductive, magnetic or luminescent layers, provided that they do not prevent the described effects according to the invention.

It is also conceivable embedding the security element according to the invention in a polarizing film or between two opposing polarizing films, wherein the polarization directions of the two films are crossed to each other.

Further exemplary embodiments and advantages of the invention are explained below with reference to the figures, in the representation of which a representation in terms of scale and proportions has been dispensed with in order to increase the clarity.

Show it:

1 is a schematic representation of a banknote with a security element according to the invention,

2 is a more detailed plan view of a detail of the security element of FIG. 1, 3 shows schematic transmission curves which show the intensity I of the light transmitted by a security element according to the invention as a function of the wavelength λ, in (a) for vertical incidence of light θ = 0 ° and in (b) for an incidence angle θ = θo, for example θ = 0.5 °,

4 shows in (a) and (b) two variants of the mechanical optical authenticity check of a security element according to the invention in transmission,

5 shows a security element with a one-dimensional periodic lattice structure according to a further exemplary embodiment of the invention,

Fig. 6 in (a) to (c) in cross-section security elements for further

Embodiments of the invention with one-dimensional grid structures having a substructure within a period, and

7 shows in (a) a test device for measuring the polarization conversion in reflection and in (b) a test device designed for measuring the polarization conversion in transmission.

The invention will now be explained in more detail using the example of a banknote. 1 shows a schematic representation of a banknote 10, which has a security element 12 according to the invention for a mechanical optical authenticity check. As explained in detail below, the security element 12 is designed primarily for a mechanical verification of the authenticity, but can of course for design reasons and / or for further increase anti-counterfeiting security with a human feature such as a hologram.

With reference to the detailed plan view of a detail of the security element 12 shown in FIG. 2, the security element 12 comprises a layer 20 of a highly conductive / highly conductive material, in particular a metal, which contains a two-dimensional lattice structure consisting of a regular arrangement of microperforations 22. The layer 20 is formed in the embodiment shown as a surface layer in the sense of the above-mentioned definition. Ie. the surface layer has a very high conductivity essentially in the entire plane E defined by the security element 12. In Fig. 2, the plane E is defined by the two vectors ei and β2.

The microperforations 22 form in the embodiment shown

Square gratings, however, all other periodic arrangements known to those skilled in the art are also suitable. According to the invention, at least one of the two quantities is below the wavelength of the light used for the authenticity check, so that the microperforations form a sub-wavelength structure which, as explained in detail below, has characteristic properties in zeroth diffraction order shows.

Since the characteristic diffraction properties, unlike conventional diffractive optical structures, already occur in zeroth order of diffraction, the security elements according to the invention can be mechanically tested without great effort for authenticity. In addition, hitherto known optical security features have obvious diffraction properties and are therefore easier to imitate than the sub-wavelength structures according to the invention, whose diffraction properties are difficult to detect and imitate in zeroth diffraction order for potential counterfeiters.

The particular diffraction properties of the highly conductive layers with subwavelength structures can be detected in the mechanical authentication by transmission measurements, reflection measurements or by a measurement of the polarization conversion, since they, as explained in more detail below, for each wavelength a strongly modified transmission, reflection or a characteristic Have polarization conversion.

Without wishing to be bound by any particular explanation, the effects described in this application are currently interpreted as resonance effects on the highly conductive layer, which occur for certain properties of the surface layer (geometry, arrangement, material properties) for specific wavelengths and illumination angles of the incident test light. Physically, this resonance effect is currently explained by the excitation of surface polarites in the highly conductive layer, which can occur when momentum transfer of the incident photons to the surface polaritons is ensured and a component of the electric field of the incident radiation is perpendicular to the surface of the highly conductive layer. The excited in this way collective oscillations of the electrons of the highly conductive layer are generally referred to as Oberflächenpolaritonen or as Oberflächenplasmonen.

The excitation of surface polaritons by the incident radiation has effects on the reflected or transmitted light. By the Formation of electromagnetic waves at the boundary layer leads to a high field enhancement at the surface. As a result of the propagation of surface waves, increased ohmic losses occur in the highly conductive layer. This energy lacks the propagating diffraction orders, so that polaritone excitation can be detected as a sink in the reflected or transmitted light intensity.

Furthermore, the excitation of the surface polarites leads to a redistribution of the energies of the propagating diffraction orders. Surface polarization excitation may therefore also result in increased transmission at certain wavelengths on transmission grids having a one- or two-dimensional periodicity. Cavity resonances in the interstices of highly conductive grating structures can also cause resonance phenomena in the reflected or transmitted diffraction orders. The location of these resonance wavelengths is also characteristic of the geometry of the lattice structure, in particular the thickness and their optical constants.

The basic behavior of security elements according to the invention with sub-wavelength structures is explained with reference to the schematic transmission curves 30, 32 of FIG. 3, which show the intensity I of the light transmitted by a security element according to the invention for vertical incidence of light θ = 0 ° (FIG. 3 (a)), and for an incident angle, θ = θ o, for example, θ = 0.5 ° (Fig. 3 (b)) depending on the wavelength λ.

As can be clearly seen in FIG. 3, the transmission 32 has a characteristic depression 34 through the security element at the slightly oblique angle of incidence θo at a resonance wavelength λ ₂ , while at a normal angle of incidence no significantly reduced transmission in the case of FIG Wavelength λ2 is observed. In the context of the explanation given above, the depression 34 can be explained at oblique light incidence by the then possible excitation of surface polaritons, while the photons do not have a component perpendicular to the surface of the surface layer 20 under normal incidence of light and therefore have no impulse on the surface

Surface polarity transmitted. Experiments show that the position of the resonance wavelength X ₂ depends not only on the grating period of the grating elements, but also strongly on the surface geometry and the material properties of the layer 20, so that it is extremely difficult for a potential forger, the characteristic diffraction properties of a given To replicate security elements.

The mechanical optical authenticity check of such a security element can, as illustrated in FIG. 4 (a), be effected, for example, by first determining an illumination angle and two test lengths, for example the slightly oblique angle of incidence θ ₀ and those denoted by X ₁ and λ ₂ Wavelengths of Fig. 3. Then, the high-conductivity layer of the security element 40 with a light source 42 from the fixed illumination angle θo successively or simultaneously applied to light the two test wavelengths A ₁ and X ₂ and the transmission of zeroth diffraction order is detected by a wavelength-sensitive detector 44 , The authenticity of the security element can then be assessed by comparing the 40 transmitted in the test wavelengths Xi and X ₂ light intensities Ii and I, respectively. ₂ As can be seen from FIG. 3, a security element according to the invention exhibits a significantly reduced transmission at the wavelength X ₂ compared to the wavelength X ₁ , whereas an imitated security element will not exhibit this characteristic reduction of the transmission. Thus, for a ratio I2 / I1 <Ithres with a suitable threshold Wes, the authenticity of the While I2 / I1 ≥ Ithres indicates an imitated security element.

Another way of checking the authenticity is illustrated in Fig. 4 (b). Instead of defining an illumination angle and two different test wavelengths, the authenticity check can also be carried out at a fixed wavelength, for example at the wavelength designated by λ ₂ in FIG. 3, and two predetermined illumination angles θi and θ2, for example θi = 0 ° (vertical incidence). and Θ2 = θo (see Fig. 3). The zeroth diffraction order transmitted by the security element 40 at the respective angles of incidence is detected by a spatially resolving detector 46, for example a diode array.

As can be seen from FIG. 3, a security element according to the invention exhibits a markedly reduced transmission at the wavelength λ2 at the angle of incidence Θ2 compared with the angle of incidence θi, while an imitated security element will not exhibit this characteristic reduction of the transmission. Thus, for a ratio I2 / I1 <Ithres with an appropriate threshold Ithres can be inferred on the authenticity of the security element, while I2 / I1 ≥ Ithres an imitated security element. The illumination can be done in this variant by two separate illumination sources 42, or even a flat extended illumination source, such as an LED array.

It is understood that in both test variants, instead of the transmitted zeroth diffraction order, the reflected zeroth diffraction order can also be used for the authenticity check. Returning to the sub-wavelength structure of FIG. 2, the surface layer 20 is preferably formed by a metal layer of gold, silver, copper, aluminum or chromium having a thickness t between 50 nm and 2 μm, in the exemplary embodiment by a silver layer having a thickness of 200 nm. det. The microperforations 22 have a diameter d between about 50 nm and 1 .mu.m, in the embodiment of 200 nm on. The distance ao adjacent perforations is preferably between 400 nm and 2 microns, in the embodiment at 900 nm. For the sharpness of the areas with exceptional reflection or transmission, it has been found advantageous if the ratio of the layer thickness t to the diameter d of the perforations t / d is between 0.5 and 2, in particular about 1, although values outside this range are of course not excluded.

In addition to the previously described designs with two-dimensional lattice symmetry, security elements with one-dimensional periodic lattice structures are also suitable. Thus, the embodiment of FIG. 5 shows a security element 50, in which a plurality of parallel, metallic grid lines 54 is applied to a carrier foil 52. While in the design shown in Fig. 2, the grid elements are formed by the micro-perforations 22, ie by non-conductive regions in an otherwise highly conductive layer, the grid elements in the design of Fig. 5 are formed inversely by highly conductive grid lines 54, between which, ie in the intermediate spaces, non-conductive areas exist. In the embodiment of FIG. 5, the highly conductive layer is thus formed by the highly conductive grid lines 54. The plane E of this security element defined by the vectors e; i and e ₂ comprises a highly conductive layer which has a very high conductivity in the sense of the invention essentially only in the direction of the vector e2. The very high conductivity in Accordingly, in the embodiment shown in FIG. 2, direction β2 corresponds to the multiplicity of grating lines 54, which are oriented in the direction of the vector e2 and which form the highly conductive layer.

According to the invention, the width b of the grating lines 54 and / or the grating period ao are below the wavelength of the light 56 used for the authenticity check. If the grating structure of FIG. 5 is exposed to p-polarized light 56 at an angle of incidence θo, its electric field vector 58 is perpendicular is the grating lines 54, then, according to the explanation given above at optical wavelengths λ, which are greater than

X ₀ = ao * (I + I sin θo I)

are only the zeroth reflected or transmitted diffraction order propagate. The first evanescent orders excite surface polarites in the lattice structure when the condition

is satisfied, where ksp denotes the real part of the wave vector of the surface polarity, ko the wave vector of the incident light in vacuum and G = 2π / ao a reciprocal lattice vector.

One-dimensional grid structures may also have a substructure within a period, as illustrated by FIG. 6. Fig. 6 (a) first shows a simple grating structure 60 having a slot 62 of width d within the period length ao. The width b of the grid lines 64 is given by b = ao-d in this case. FIGS. 6 (b) and (c) show configurations with three or five slots 62 of the same width d within one period. denlänge, so with a sub-structure within a period length, which of course also a different number of slots or different slot widths come into consideration.

The slot widths d are selected so that they are smaller than the wavelength of the light used for the authenticity check. Depending on the material parameters, the width and the number of slots, security elements with such grating structures can have an increased transmission at certain resonant wavelengths or even narrow valleys within areas of resonantly excessive transmission. This characteristic increased or reduced transmission can then, as already explained in connection with FIG. 4, be used for checking the authenticity of the security elements.

Lattice structures with one-dimensional periodicity additionally open up a further possibility for checking the authenticity of security elements according to the invention. The excitation of surface polarites in lattices with one-dimensional periodicity causes, according to the current state of knowledge under suitable incident conditions, a rotation of the polarization plane of the diffracted light relative to the polarization vector of the incident light. This polarization conversion can be detected, for example, by test devices as shown schematically in FIG.

In the test device of FIG. 7 (a) designed for measuring the polarization conversion in reflection, the incident light 72 is p-polarized by a first polarizer 74 before the security element 70 to be tested is acted upon. The light 76, which is reflected by the security element 70, arrives at the first polarizer in the reverse direction. second polarizer 78 to a detector 75, which detects the intensity of the transmitted through the second polarizer 78 s-polarized light.

If the security element 70 to be tested contains a sub-wavelength structure according to the invention having a one-dimensional periodicity, the result is an increased detector signal due to the polarization conversion described, the maximum signal strength being achieved if the grating vector of the grating structure is at 45 ° to the plane of incidence 72, 76 of the light , In the case of an intensity signal which is increased in relation to a threshold value, it is therefore possible to conclude that surface polarites are excited and thus the authenticity of the tested security element.

Instead of in reflection, the polarization conversion can also be checked in transmission, as shown in Fig. 7 (b). In this case, the incident light 82 passes through a first polarizer 84 and the security element 80 to be tested. The transmitted light 86 arrives at the detector 85 via a second polarizer 88 oriented in the reverse direction relative to the first polarizer. Here, as well, an increased detector signal can be used for the corresponding position of the Security element 80 on the excitation of Oberflächenpolaritonen and thus on the authenticity of the security element to be tested 80 closed.

Claims

Patent claims

1. A security element for mechanical optical authentication with light containing a predetermined test wavelength, characterized in that the security element comprises a layer of a highly conductive material containing a one- or two-dimensional lattice structure of a periodic arrangement of a plurality of lattice elements, whose lateral dimensions and / or lateral distances are smaller than the test wavelength.

2. Security element according to claim 1, characterized in that the lateral dimensions and / or the lateral distances by at least a factor of 1.5, preferably by at least a factor of 2, are smaller than the test wavelength.

3. Security element according to claim 1 or 2, characterized in that the security element is designed for a mechanical optical authenticity check with light from the visible spectral range.

4. Security element according to claim 1 or 2, characterized in that the security element is designed for a mechanical optical authenticity check with light from the spectral range of the near infrared.

5. The security element according to claim 1, characterized in that the highly conductive layer with the lattice structure is designed so that the light incident on the authenticity test under a predetermined test angle excites surface polarities and / or cavity resonances in the highly conductive layer ,

6. Security element according to at least one of claims 1 to 5, characterized in that the grid structure is formed as a reflection grating.

7. Security element according to at least one of claims 1 to 5, characterized in that the grid structure is formed as a transmission grating.

8. The security element according to at least one of claims 1 to 7, characterized in that the grid elements are formed by parallel, highly conductive grid lines to form a one-dimensional periodic lattice structure.

9. Security element according to claim 8, characterized in that the period length is smaller than the test wavelength, preferably at least by a factor of 2 smaller than the test wavelength.

10. Security element according to claim 8 or 9, characterized in that the grid structure has a substructure within a period.

11. The security element according to claim 1, characterized in that the highly conductive layer is a substantially highly conductive surface layer and that the grid elements are formed by regularly arranged perforations in the surface layer in order to form a two-dimensional periodic lattice structure.

12. The security element according to claim 11, characterized in that the diameter of the perforations is smaller than the test wavelength, preferably at least a factor of 2 smaller than the test wavelength, in particular ders preferred is at least a factor of 4 smaller than the test wavelength.

13. A security element according to claim 11 or 12, marked thereby, that the lateral distance of the perforations is smaller than the test wavelength.

14. The security element according to at least one of claims 11 to 13, characterized in that the ratio of the thickness of the surface layer to the diameter of the perforations is less than 2, preferably between 0.5 and 1.5, in particular about 1.0.

15. A security element according to at least one of claims 1 to 14, characterized in that the thickness of the layer of a highly conductive material is between 50 nm and 2 μm, preferably between 100 nm and 1 μm.

16. The security element according to at least one of claims 1 to 15, characterized in that the highly conductive material of the layer is a metal, in particular one of the metals gold, silver, copper, aluminum or chromium.

17. Security element according to at least one of claims 1 to 16, characterized in that the lattice structure is embedded in a dielectric.

18. Security element according to at least one of claims 1 to 17, characterized in that the security element is combined with a visually verifiable human feature.

19. A security element according to at least one of claims 1 to 18, characterized in that the security element is a security thread, a label, a transfer element or a see-through security element.

20. A method for producing a security element according to one of claims 1 to 19, wherein a layer of a highly conductive material is provided with a one- or two-dimensional grid structure of a periodic arrangement of a plurality of grid elements whose lateral dimensions and / or lateral distances smaller as the test wavelength are.

21. Data carrier, in particular branded article or valuable document, such as banknote, passport, document, banderole, identity card or the like, with a security element according to one of claims 1 to 19.

22. A data carrier according to claim 21, characterized in that, the security element is arranged in a window region of the data carrier.

23. Use of a security element according to at least one of claims 1 to 19 or a data carrier according to claim 21 or 22 for

Counterfeiting of goods of any kind.

24. A method for mechanical optical authentication of a security element according to one of claims 1 to 19, wherein

a) at least one test wavelength and at least one illumination geometry for the authenticity test is determined, b) the highly conductive layer of the safety element is exposed to light of the at least one defined test wavelength under the at least one defined illumination geometry,

25. The method according to claim 24, characterized in that in step c) the reflected light in the zeroth diffraction order is detected.

26. The method according to claim 24, characterized in that in step c) the light transmitted in the zeroth diffraction order is detected light.

27. The method according to at least one of claims 24 to 26, characterized in that in step a) at least two different test wavelengths are determined in step c) the light intensities detected at the at least two predetermined test wavelengths and compared in step d), wherein in the case of a light intensity which is reduced beyond a defined extent, the authenticity of the security element is concluded at at least one of the test wavelengths.

28. The method according to claim 27, characterized in that the surface layer is acted upon in step b) simultaneously with light of at least two different test wavelengths.

29. The method according to claim 27, characterized in that the layer of a hochleitf ähigen material in step b) is successively acted upon by light of the at least two different test wavelengths.

30. Method according to claim 24, characterized in that in step a) at least two different illumination angles are defined, in step c) the light intensities are detected upon application of the at least two fixed illumination angles and compared with each other in step d) In the case of a light intensity which is reduced beyond a defined extent, the authenticity of the security element is concluded at at least one of the illumination angles.

31. The method according to claim 30, characterized in that the light intensities are detected in step c) with a spatially resolving detector.

32. The method according to at least one of claims 24 to 26, characterized in that the layer of a highly conductive material in

Step b) is applied with polarized light of the at least one predetermined test wavelength, and in step c) the polarization direction of the reflected or transmitted light of the at least one predetermined test wavelength is detected, wherein in step d) from a change in the polarization direction at least a predetermined test wavelength a fixed amount is added to the authenticity of the security element.

33. A device for automated optical authenticity testing of a security element according to one of claims 1 to 19, with

at least one light source for subjecting the highly conductive layer of the security element to be tested to light of at least one specified test wavelength under at least one specified illumination geometry,

Means for assessing the intensity and / or polarization of the detected light at the at least one specified test wavelength and / or at least one predetermined illumination geometry, and for assessing the authenticity of the security element to be tested based on the evaluation made.

34. Apparatus according to claim 33, characterized in that the light source for applying the highly conductive layer of the security element is designed with at least two different test wavelengths, and a wavelength-sensitive detection device is provided.

35. Apparatus according to claim 33, characterized in that the light source for applying the highly conductive layer of the security element is designed with light from at least two different illumination angles, and a spatially resolving detection device, such as a diode array, is provided.

36. The apparatus of claim 33, characterized in that in the beam path between the light source and the security element to be tested, a first polarizer and in the beam path between the security element to be tested and detection means is provided in the reverse direction to the first polarizer oriented second polarizer.