Safety element and method for manufacturing a safety element

Abstract

The invention relates to a security element for the production of valuable documents, such as banknotes, checks, or the like. The security element (4) has a front (2) and a back (78). The security element (4) has a substrate body (37) which has a lattice structure (38, 49) extending over a certain area, with a reflector layer (52) located thereon, and the lattice structure exhibits at least one color in plan view and / or transmission. The lattice structure (38, 49) has at least two sub-areas (10-14 and 22-32) that are not visible in the visible spectral range, in each of which the lattice structure (38, 49) is uniformly formed, wherein the lattice structures (38, 49) differ between the sub-areas (10-14 and 22-32) in at least one lattice structure parameter (h, d) such that the sub-areas (10-14 and 22-32) differ with respect to their absorption in the near-infrared range.

Description

The invention relates to a security element for producing value documents, such as banknotes, checks or the like, which has a front side and a rear side. The security element has a substrate body which has a grating structure extending over a region with a reflector layer located thereon, and the grating structure shows at least one color in the visible spectral range in plan view and / or transmission.Security elements for producing value documents with periodic line gratings are known, for example from WO 2013 / 053435 A1. Two-dimensionally periodic color-filtering gratings are also known, for example, from DE 102011101635 A1.It is likewise known to secure value documents such as banknotes by printing with a special printing ink which, when illuminated with radiation from the near infrared range (NIR range), that is to say in accordance with DIN 5031 in the version applicable on May 1, 2023, in the wavelength range from 780 nm to 3000 nm, exhibits a clearly different appearance compared with the illumination with light from the visible wavelength range. These known value documents have certain partial regions in the printed image which absorb radiation with a wavelength from the NIR region, while other partial regions do not absorb radiation with a wavelength from the NIR region and appear bright in reflection when illuminated with radiation of this wavelength. Such colors have so-called reflectance properties; they absorb part of the illumination spectrum and thus appear opaque to these wavelengths, and transmit or reflect another part of the illumination spectrum-this is therefore also referred to as diffuse reflection in the case of reflectance. In the known colors, different remission properties of partial areas are present in the NIR range; they are therefore not obvious in the visible spectral range. This property is therefore frequently used for machine authenticity checking in bank note processing, since a counterfeiter is usually not aware of this different appearance in the NIR range and in the visible-a surprising forgery-proof effect is produced.Conventional colors with infrared-absorbing properties generally exhibit an absorption in the NIR range from 780 nm to 3000 nm, regardless of the wavelength of the incident infrared radiation. However, colors of some manufacturers are known, such as colors from SicPa or Glidesmann, for example, which have a spectral-dependent absorption behavior in the NIR range, i.e. they absorb radiation of different NIR wavelengths to different extents. In these colors, for example, the absorption in the short-wave NIR range is lower than in the longer-wave NIR range. This property is difficult to falsify and can be used for extended authenticity checking with the aid of suitable sensors which evaluate the remission, for example, in two different IR spectral ranges.It is known that these properties of the special printing inks can also be used in the case of value documents in transmission for machine authenticity checking. The printing inks having spectrally different absorption characteristics are known both for paper and for polymer substrates.It is also known to overprint metallically embossed microstructures or nanostructures with the above-mentioned infrared-absorbing colors. The security elements produced in this way can then likewise be checked with suitable sensors for authenticity in the NIR range.In order to equip a security element with remission properties in the NIR range, printing with the described printing inks has hitherto been required. However, a printing process always also means an additional further process in the production of the security element or the value document, so that the production effort is increased. In addition, it is difficult with known printing processes to ensure a high lateral position accuracy of the print (so-called passer), as is necessary, however, in the production of security elements for value documents.The object of the invention is to create a security element which can be applied to a value document with high lateral positional accuracy and has a forgery-proof feature with NIR properties, the production complexity of which is low.The invention is defined in claims 1, 11 and 12. The dependent claims relate to preferred embodiments.A security element for producing value documents such as bank notes, checks or the like is provided. The security element has a front side and a rear side and has a substrate body which has a grating structure extending over a region with a reflector layer situated thereon. The reflector layer ensures that the grating structure exhibits at least one color in the visible spectral range-usually the wavelength range from 380 nm to 780 nm-in plan view and / or in transmission.The substrate body is a dielectric; generally a plastic film, in particular a PET film, or the substrate body itself is formed from a UV embossing lacquer. The refractive index of the materials of the substrate body is preferably approximately n=1.5.The grid structure is metallized, preferably with aluminum, which is applied to the grid structure by vapor deposition in a vacuum. Plasmonic effects are generated by this metallization, which result in resonant light absorption in the metal. This frequency-selective absorption has the consequence that the transmission or reflection for the respective wavelength is reduced. The selection of the grating period is determining for the position of plasmonic resonances in the NIR range. Plasmonic resonances are particularly pronounced in grating structures for wavelengths which have approximately the value d / n, wherein d corresponds to the period and n corresponds to the refractive index. The exact position of the resonance in the wavelength spectrum depends, however, on the exact geometry of the grating structure in the profile and the optical constants of the substrate body. The structure depth also has an influence on the transmission or reflection spectrum of the grating structure, in that cavity resonances can be excited and these shift into the long-wave range of the spectrum with increasing structure depth. Therefore, 1-dim or 2-dim periodic gratings with periods of 100 nm to 1000 nm, preferably of 200 nm to 600 nm, are suitable for the formation of plasmonic resonances or cavity resonances in the NIR range. Furthermore, the depth of the grating structure, i.e. the modulation depth of two metallic gratings which are arranged at a distance from one another, is relevant for the formation of cavity resonances. The modulation depth can be selected in the range of very flat structures with about 20 nm up to several micrometers. However, the production of such structures in an embossing process limits the choice to the range of about 40 nm to 500 nm. The plasmonic effects described arise also in the case of other metals such as Ag, Cu, Au, Cr, etc. and alloys thereof, with the result that these metals are likewise used for the metallization. In contrast, the cavity resonances depend primarily on the geometry of the grating structure and lead to a redistribution of the light intensity between the reflection and transmission portions. In practice, however, there is coupling between both effects which can cause near infrared light resonances.In the region in which the grating structure extends, at least two partial regions not recognizable in the visible spectral range are provided in the grating structure, in each of which partial regions the grating structure is formed uniformly. However, the grating structures differ between the sub-regions in each case in at least one grating structure parameter, for example a modulation height, an area fill factor or a period of the grating structure. The azimuth angle of different sub-regions can also be changed by arranging a sub-region in a tilted manner. This can be achieved, for example, by arranging the grating structure on a micromirror arrangement. The partial regions are each NIR-absorbing, but different in their absorption behavior in the NIR region, in particular spectrally differently absorbing.This achieves a surprising forgery-proof effect which becomes visible only when illuminated with radiation from the NIR range, but is camouflaged when illuminated with light, i.e. from the visible wavelength spectrum. A visual impression does not change between the sub-areas due to the IR-effective structure.The subareas preferably realize hidden codes in the NIR spectrum. These codes are formed by the fact that some partial regions are absorbent when irradiated with wavelengths from this partial spectrum of the NIR region and thus appear in transmission, while others do not. The sub-regions can also change at different wavelengths in the NIR range, because each sub-region preferably absorbs the radiation of a different wavelength or of a different wavelength range. A partial region which appears at a wavelength of 800 nm, for example, can disappear again at a wavelength of 1000 nm, for example. A further subregion which is not visible on irradiation with a wavelength of 800 nm, however, can become visible at 1000 nm, for example. In addition, the effects in reflection when illuminated with radiation of a specific wavelength from the NIR range can differ in the subareas, but can be the same in transmission, or vice versa.Preferably, the security element shows, in plan view, a representation recognizable with the naked eye in the partial areas, e.g. a pattern, image, text or the like, superimposed. It is recognizable in the visible wavelength range when illuminated and is not changed under these conditions by the grating structure, which is basically only NIR-effective.The grating structure is preferably absorbent in at least one sub-region for radiation in the NIR sub-spectrum of 780 nm to 1,100 nm. Preferably, a plurality of subareas each absorb in the NIR sub-spectrum of 780 nm to 1.100 nm, but the subareas differ in terms of their spectral absorption characteristic in this spectral range. The variation of the at least one grating structure parameter in the sub-regions ensures that the representation recognizable with the naked eye generates no effect over all sub-regions when illuminated with light from the visible wavelength range. If no further printing, etc., is provided, the partial regions appear, for example, as a unit. When illuminated with radiation from the NIR range, the partial regions absorb differently in the NIR spectral range. The subareas can implement binary transparency coding in the NIR, for example in the sense of an s / w see-through image or the like, without impairing the impression in the visible.In particular, the subareas can absorb NIR radiation in a spectrally different manner. The subareas can then realize different codings at different wavelengths in the NIR range, for example in the sense of a multicolored see-through image.The grating structure can be a one-dimensional periodic subwavelength structure with grating ridges and grating columns, such as a double line grating, as well as a two-dimensional subwavelength structure, in particular a two-dimensional periodic subwavelength structure with periodically arranged elevations or depressions. Both variants for the grating structure show a contrast difference in the case of a variation of the grating parameters, in particular of the modulation height. The period in the case of one-dimensionally periodic subwavelength structures also has an influence on the reflection behavior in the NIR range. It can therefore likewise be selected as grating structure parameter, as is explained in more detail in the exemplary embodiments on the basis of graphs.Both a one-dimensional periodic subwavelength structure with grating ridges and grating columns and a two-dimensional periodic subwavelength structure with periodically arranged elevations or depressions are suitable for producing a good contrast between the subareas in the NIR range, wherein at the same time the color phenomena in the visible are unchanged from area to area. Each sub-region may be set to absorb radiation of a different wavelength in the NIR region.Particularly preferably, the grating structure parameter, in which the grating structures differ in the sub-regions, comprises the modulation height h of the grating structure. It was established and is also shown in the exemplary embodiments on the basis of graphs that the reflection of the subareas in the NIR range decreases significantly with increasing modulation height h. Thus, in a two-dimensional periodic subwavelength structure, for example, the modulation height h=300 nm exhibits a large contrast in the NIR range, while a modulation depth of h=250 nm, for example, hardly exhibits a difference in contrast.The grating structure parameters can also be set in the subareas such that they absorb radiation in a spectrally selective manner in the NIR range. The described grating structures thus represent a further development to known IR-absorbing printing inks which do not permit different reflectance properties in subareas when irradiated with different wavelengths. Compared to printing inks that act selectively in spectral terms, the grid structures also have the advantage that they can be applied to the security element with a significantly higher lateral position accuracy than a printing process enables.A printing process for applying the IR-absorbing properties to the security element can also be completely omitted, so that the manufacturing effort is also significantly reduced due to the saving of an entire process step.The described grating structures can be combined with known optical embossed structures such as holograms, micromirrors or known grating structures which produce a color effect when illuminated with light from the visible wavelength spectrum. They can be applied to a value document as a transfer element, as well as be directly integrated into the printed image of value documents.Metallic wire or slot grids and metallic nanohole arrays or nanodot arrays can also be used as grid structures. Such grating structures are known from the literature and likewise exhibit optical resonance effects in the NIR range. See literature:◯ H. Holebiehler, "Surface polarization on metallic wire gratings studied via power loss." Physical Review B 53.15 (1996): 10289.◯ J.A. Porto et al., "Transmission responses on metallic gratings with very narrow splits." Physical review letters 83.14 (1999): 2845.◯ T.W. Ebbesen et al., "Ancillary optical transmission through subwavelength hole arrays." nature 391.6668 (1998): 667-669.◯ H. Holebihler "Security element with colour-filtering grating", AZ 10 2015 008 655.3.There is also disclosed a method for producing a security element for value documents such as banknotes, checks or the like. The security element has a front side and a rear side. A substrate body is provided in the method. A grating structure extending over a region is introduced into the substrate body, and a reflector layer is applied to the grating structure. In the visible spectral range, the grating structure shows at least one color in plan view and / or transmission, and partial regions are introduced into the grating structure, in each of which partial regions the grating structure is formed uniformly. The grating structures differ between the sub-regions in at least one grating structure parameter in such a way that the sub-regions do not differ in the visible spectral range with regard to the at least one color, but are different in their absorption and / or reflection in the near infrared range.The security element is preferably produced using photolithographically processes, in particular in an e-beam installation. Other known methods for production are interference lithography or mechanical scribing. A known manufacturer of such installations is, for example, the Nanoscribe GmbH & Co. KG, Hermann-von-Helmholtz Place 6, 76344 Eggenstein-Leopoldshafen, Germany.In the production process, first exposed originals are produced. After development of a photoresist, these exposed originals are subsequently copied over galvanically or using photopolymers (e.g. Ormocer). Such an original can also be assembled with additional original structures in a registered manner by nanoimprint methods, for example. This method is suitable in particular for combination with further known structures such as relief holograms, micromirror arrangements, moth-eye structures or known embossed nanostructures for producing structural colors. An original produced in this way is now copied over galvanically or by a nanoimprint method. Furthermore, multiple placement of the original pattern by hot embossing or nanoimprinting on a die is required to produce an embossing cylinder for later replication. Such embossing cylinders finally permit continuous replication of the original structure in UV lacquer on films in a roll-to-roll process. Hot embossing or nano casting in UV lacquer are possible here. Finally, the structured film is metallically vapor-deposited. The common vapor deposition methods are electron beam vapor deposition, thermal evaporation or sputtering. Suitable metallizations are, in particular, Al, Ag, Au, Ni or Cr and alloys of these metals. The thicknesses of the metallization are in the range of about 10 nm to about 150 nm. Alternatively, the grid structures can also be metallically printed, e.g. coated with super silver. The metallized surface of the grid structure is then preferably coated with a transparent protective layer or laminated with a cover film in order to protect it from environmental influences later. The security element produced in this way can be applied to a value document in different ways.The security element can be designed as a transfer element, that is to say as a foil element which additionally has an adhesive layer on the rear side and is therefore applied to a value document. Such transfer elements are, for example, security threads, LEAD strips or patches and serve primarily as a feature which can be perceived by the human eye. Known optically variable effects of these features are formed by embossed holograms, micromirror arrangements, microlens arrangements and color shift coatings. The transfer elements can be checked for authenticity only with difficulty by machine when illuminated with light in the visible wavelength spectrum. This shortcoming is exploited in the practice of counterfeiters, so in this invention the known optical transfer elements are supplemented by the grating structures explained above as a security feature recognizable in the NIR range, in order to enable machine authenticity checking and to change the optical appearance of the transfer element when illuminated with radiation of different wavelengths from the NIR range.The security element can also have a substrate body which is part of the value document, such that the security element is introduced directly into the substrate of the value document. This is realized, for example, in a transfer method within the printing range of a banknote. In this case, the security element can also be overprinted with printing ink.Fully nanostructured value documents are also known, for example from the aforementioned DE 10 2014 018 551 A1, in which their optical appearance, i.e. the printed image, is formed exclusively by nanostructures. These value documents can advantageously be equipped with the grid structures explained above as security features in the NIR range in order to ensure machine authenticity checking of the film elements and thus to increase forgery security. In the film composite, the front side of the security element points inward, and the rear side of the security element points outward.The invention is explained in more detail below on the basis of exemplary embodiments with reference to the appended drawings, which likewise disclose features essential to the invention. These exemplary embodiments are intended merely for illustrative purposes and should not be interpreted as restrictive. For example, a description of an embodiment having a plurality of elements or components should not be construed as requiring all of these elements or components for implementation. Rather, other embodiments may also include alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of various embodiments may be combined with each other unless otherwise indicated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary embodiments. To avoid repetitions, identical or mutually corresponding elements are denoted by identical reference symbols in different figures and are not explained multiple times. The figures show: FIG. 1 shows a value document during the illumination with light from the visible spectral range; FIG. 2 shows the value document according to FIG. 1 during the illumination with radiation from the near infrared range; FIG. 3 shows a value document in a further embodiment during the illumination with light from the visible spectral range; FIG. 4 shows the value document according to FIG. 3 during the illumination with radiation from the near infrared range; FIG. 5 shows a value document in a further embodiment during the illumination with light from the visible spectral range; FIG. 6 shows the value document according to FIG. 3 during the illumination with radiation from the near infrared range; FIG. 7 shows a one-dimensional periodic grating structure in a sectional illustration; FIG. 8A shows a two-dimensional periodic grating structure with three different sub-regions in plan view; FIG. 8B shows a two-dimensional periodic grating structure with three different sub-regions in an oblique view; FIG. 9A shows a measurement geometry of an optical sensor in remission; FIG. 9B shows a measurement geometry of an optical sensor in transmission; FIG. 10A shows an emission spectrum of an IR LED, a transmission curve of an IR filter, and a quantum efficiency of a silicon detector; FIG. 10B shows an emission spectrum of two different IR LEDs and a transmission curve of two IR filters; FIG. 11A shows a reflection of a two-dimensional periodic grating structure as a function of the wavelength at different modulation levels; FIG. 11B shows a reflection of two different two-dimensional periodic grating structures as a function of the modulation height; FIG. 12 shows a color perception in reflection of a two-dimensional periodic grating structure in the CIE color space; FIGS. 13A to 13C show a color perception in reflection of a two-dimensional periodic grating structure in the LCH color space; FIG. 14 shows a reflection as a function of the wavelength of a one-dimensional periodic grating structure at different periods; FIG. 15 shows an emission spectrum of two different IR LEDs and a transmission curve of two IR filters; FIG. 16 shows a reflection of two different one-dimensional periodic grating structures as a function of the period; FIG. 17 shows a color perception in reflection of a one-dimensional periodic grating structure in the CIE color space; FIGS. 18A to 18C show a color perception in reflection of a one-dimensional periodic grating structure in the LCH color space; FIG. 19 shows a reflection of a one-dimensional periodic grating structure as a function of the modulation depth; FIG. 20 shows a transmission of a one-dimensional periodic grating structure as a function of the modulation depth; FIGS. 21A to 21C show a color perception of a one-dimensional periodic grating structure in the LCH color space in reflection; FIGS. 22A to 22C show a color perception of a one-dimensional periodic grating structure in the LCH color space in transmission; FIG. 23 shows a value document with a security element, configured as a transfer element; FIG. 24 is a diagram of value documents with an embedded security element;FIG. 1 shows a value document 1 on the front side 2 of which a security element 4 is applied. A first pressure element 6 and a second pressure element 8 are also applied to the front side 2. FIG. 1 shows the value document 1 when illuminated with light from the visible wavelength spectrum when viewed in reflection and / or in transmission. The region in which the security element 4 is applied appears to an external observer in a uniform color.FIG. 2 shows the same value document 1 when illuminated with radiation of a wavelength from the near infrared range (NIR range) The security element 4 is applied in a region to the front side 2 of the value document 1. This region has a first partial region 10, a second partial region 12 and a third partial region 14. The subareas 10 to 14 can only be identified when illuminated with radiation of a wavelength from the NIR range reflection. Likewise, the partial regions 10 to 14 can also appear in transmission when illuminated with radiation of a wavelength from the NIR region. When illuminated with light from the visible wavelength spectrum, an external observer sees the partial regions 10 to 14, as is just shown in FIG. 1-they generate a uniform color impression and therefore cannot be recognized-neither in transmission nor in reflection.Each of the sub-regions 10 to 14 differs from the other sub-regions 10 to 14 in terms of its absorption behavior in the NIR region. For example, the first sub-region 10 absorbs radiation of a first wavelength in the NIR range, the sub-region 12 absorbs radiation of a second wavelength in the NIR range and the sub-region 14 absorbs radiation of a further different wavelength in the NIR range. The security element thereby shows different patterns in reflection or in transmission when illuminated with different wavelengths in the NIR range and conceals them again when illuminated with other wavelengths in the NIR range. When illuminated with light of a wavelength from the visible wavelength spectrum, the partial regions 10 to 14 are not visible, but rather the security element 4 appears in a uniform color.FIG. 3 shows a further embodiment of the value document 1, in which the security element 4 is divided into a first region 16, a second region 18 and a third region 20. The appearance according to FIG. 3 is shown to the observer when illuminated with light from the visible wavelength spectrum in a plan view of the front side 2 and / or in a see-through view. The first region 16 appears to the external observer in a first color, the second region 18 in a second color and the third region 20 in a third color, so that the security element 4 represents a color profile, a pattern or the like when illuminated with light from the visible spectrum.FIG. 4 shows the value document 1 in the embodiment of FIG. 3 during the illumination with radiation of a wavelength in the NIR range. The first area 16 is divided into the first sub-area 10, the second sub-area 12 and the third sub-area 14, as was already the case with FIG. 2. In addition, however, the second region 18 is also divided into a fourth subregion 22, a fifth subregion 24 and a sixth subregion 26, and analogously the third region 20 is divided into a seventh subregion 28, an eighth subregion 30 and a ninth subregion 32.Each of the sub-regions 10 to 14 and 22 to 32 can differ from the other sub-regions 10 to 14 and 22 to 32 in their absorption behavior in the NIR range, but one or more of the sub-regions 10 to 14 and 22 to 32 can also be the same. All the sub-regions 10 to 14 and 22 to 32 can absorb radiation of a different wavelength in the NIR range; some sub-regions 10 to 14 and 22 to 32 can also absorb light of the same wavelength. Therefore, different patterns are shown when illuminated with different wavelengths in the NIR range, and they disappear again when illuminated with other wavelengths in the NIR range. Preferably, a plurality of partial regions 10 to 14 and 22 to 32 can exhibit the same reflectance when illuminated with light of the same wavelength from the NIR region. When illuminated with a wavelength from the wavelength spectrum of the visible light, the partial regions 10 to 14 and 22 to 32 are not visible, but rather the security element 4 represents the color profile or the pattern according to FIG. 3.In FIGS. 5 and 6, analogously to FIGS. 1 and 2 or 3 and FIG. 4, a further embodiment of the value document 1 is shown, in which an image element 34 is applied as a security element 4 to the front side 2 of the value document 1 (FIG. 5 ). This picture element 34 disappears from the NIR region when illuminated with radiation and a QR code 36 appears, as is shown in FIG. 6. It is also possible that the QR code 36 only appears at a specific wavelength in the NIR range, but at a different wavelength a different pattern appears in reflection and / or transmission. Patterns / codes / images in transmission and reflection can also differ when illuminated with radiation of specific wavelengths in the NIR range and can be the same again when illuminated with radiation of a different wavelength in the NIR range.The described different properties of the security element 4 when illuminated with light from the wavelength range of the visible spectrum and from the NIR range enable the machine authenticity check of the value document 1.The optical appearance during illumination with visible light and the properties in the NIR region produce a metallized grating structure, as is shown by way of example in FIGS. 7, 8 aand 8 b. They can be linear periodic subwavelength structures as grating structures as shown in FIG. 7, but also two-dimensional periodic gratings as shown in FIGS. 8 aand 8 b.FIG. 7 shows a safety element 4 in a sectional view. The security element 4 has a double-line grating 38 embedded in a substrate body 37 as an example of a one-dimensional periodic grating structure. The first line grating structure of the double line grating 38 consists of first grating webs 40 with the width a, which extend along a longitudinal direction lying perpendicular to the plane of the drawing. Between the first grid webs 40 there are first grid columns 42 which have a width b. The thickness of the first grid webs 40 is indicated by t. At a modulation height h above the first grid webs 40, there is a second line grid structure with second grid webs 44; they have the width b. The second line grid structure is phase-shifted with respect to the first line grid structure in such a way that the second grid webs 44 come to lie above the first grid webs 40. At the same time, second grating gaps 46 which exist between the second grating ridges 44 are located over the first grating ridges 40. the thickness t is smaller than the modulation height h, so that no continuous film is formed from the grating ridges 40 and 44. It is essential that the modulation height h, i.e. the height difference between the first line grating structure and the second line grating structure, is greater than the sum of the thicknesses of the grating webs 40 and 44, since otherwise there would be no separation between the two line grating structures. The security element 4 of FIG. 7 reflects incident radiation as reflected radiation R. Furthermore, a radiation component is transmitted as transmitted radiation T. The reflection and transmission properties depend on the angle of incidence θ; for further explanations, reference is made to WO 2013 / 053435 A1.FIGS. 8A and 8B show a security element 4 having a two-dimensional periodic lattice structure with elevations 50 raised with respect to a flat region 48.FIG. 8A shows the safety element 4 in plan view. A grid structure 49 can be seen, which is divided into the sub-regions 10 to 14. Grating structure parameters vary from subregion to subregion, such as modulation height, period, surface fill factor, etc.FIG. 8B illustrates, by way of example, a variation of the modulation height h in an isometric illustration. In the partial region 10 of the security element 4, the elevations 50 have a first modulation height h 1, in the second partial region 12 a second modulation height h 2 and in the third partial region 14 a third modulation height h 3. The modulation height decreases from the first subregion 10 to the third subregion 14, i.e. h 3 > h 2 > h 1. Preferably, such a variation produces a uniform color impression of the subareas 10 to 14 when illuminated with light from the visible wavelength spectrum in reflection and / or in transmission, but the optical impression when illuminated with radiation in the NIR range is different in the subareas 10 to 14.FIGS. 9A and 9B show a measurement geometry of an optical sensor. FIG. 9A shows an optical sensor for measuring reflectance and FIG. 9B for measuring transmittance. The optical sensor consists of a beam source 54, an optical filter 56 and a detector 58. In such a measurement geometry, the absorbing properties of lattice structures in the NIR region are examined. In the embodiment of FIGS. 9A and 9B, therefore, these are IR LEDs as beam source 54, IR filters as optical filters 56 and a silicon line detector as detector 58.In order to be able to estimate a measured signal of such an IR sensor, knowledge of the emission spectrum of the beam source 54, the spectral emission of the filter 56 and the spectral sensitivity of the detector 58 is necessary. The graphs in FIGS. 10A and 10B plot an intensity of the radiation of the wavelength in μm on the x-axis on the y-axis. FIG. 10A shows the curves of an optical sensor with a filter which has a homogeneous spectral emission. An emission spectrum of a first beam source 60 (here an IR LED), a transmission curve of a first filter 62 (here an IR filter) and a quantum efficiency of a detector 64 (here a silicon detector) are shown. Suitable beam sources are IR LEDs which have a center-of-gravity wavelength in the NIR range. FIG. 10B shows curves for two spectrally different beam sources 54 with associated optical filters 56 (edge filters). In addition to the emission spectrum of a first beam source 60 and the transmission curve of a first filter 62, an emission spectrum of a second beam source 66 (here a second IR LED) and a transmission curve of a second filter 68 (here a second IR filter) are shown. The latter exemplary embodiment offers the possibility of evaluating the IR contrast of the feature compared to the first variant. The measurement signal I of the sensor in the remission arrangement can be estimated as follows: or for the transmission geometry: wherein E(λ) is the spectral emission of the IR LED, Q(λ) is the quantum efficiency of the detector, T F( λ) is the transmission of the edge filter and R G( λ) or T G( λ) is the reflection or transmission of the grating structure. In the case of FIG. 10B, the sensor arrangement consists of two beam sources with different emission spectra E 1( λ) and E 2( λ). The detector consists of two detector rows, the first row being equipped with the edge filter of the transmission characteristic T F1( λ) and the second row with the edge filter of the transmission characteristic T F2( λ). This sensor arrangement supplies two different signals in the respective detector rows, which are given, for example, by signals for the remission arrangement. An improvement in the measurement accuracy of the sensor arrangement is achieved by evaluating the ratio I 1 / I 2.These values determined from this are used for the further estimation of an IR signal.First, the spectral reflection of a two-dimensional periodic grating as shown in Figs. 8A and 8B is examined.FIG. 11A shows a spectral reflection of the two-dimensional periodic grating with constant period d=240 nm for different modulation heights h of 240 nm to 300 nm at light incidence of 8° in the NIR range as a function of the wavelength (the reflection in % is depicted on the y-axis and the wavelength in μm is depicted on the x-axis). The incident radiation is unpolarized. The two-dimensional periodic grating has the parameters b=120 nm, n=1.52 and is coated with aluminum with a thickness t=440 nm. The modulation levels h 1 to h 7 are drawn in, wherein the modulation level h decreases from the first modulation level h 1 to the seventh modulation level h 7 in 10 nm steps, wherein h is 1=300 nm and h is 7=240 nm.It can be seen in FIG. 11A that the reflection in the NIR range decreases significantly with increasing modulation depth h. A peak of the resonant light absorption of red shifts to the infrared region. The signal of an optical sensor according to FIGS. 9A and 9B can be calculated with the aid of the spectral curves of FIGS. 10A and 10B, respectively, by being folded with the reflection of FIG. 11A according to the above-mentioned formula.The result of the calculation explained above is shown in FIG. 11B for the two different LEDs and filter curves of FIG. 10B. FIG. 11B compares the modulation height in μm on the x-axis with the reflection signal on the y-axis measured by the sensor arrangement. The first curve k1 arises for the emission spectrum of a first beam source 66 and the transmission curve of a first filter 68, the second curve k2 arises for the emission spectrum of a second beam source 60 and the transmission curve of a second filter 62, wherein the curve k1 relates to the spectral characteristic of the curves 66, 68 of FIG. 10B and the curve k2 relates to the spectral characteristic of the curves 60, 62 of FIG. 10B. It is found that the modulation depth h=300 nm with respect to the beam source used exhibits a high contrast in the NIR range, while the modulation height h=250 nm hardly exhibits any difference.It has already been explained that as a further requirement of the security element 4 the property is required that it is not perceptible-but at most only hardly perceptible-when viewed in the visible spectral range. Thus, Figs. 12 and 13A to 13C show the color perception of a two-dimensional periodic grating structure with a variation in the modulation height between 250 nm and 300 nm in the CIE and LCh color space, respectively. For this variation of the modulation level, the contrast in the NIR range when using different beam sources and filters has also been investigated beforehand.Fig. 12 shows the colour coordinates x and y in the CIE colour space and Figs. 13A to 13C show individual colour values in the LCH colour space as a function of the modulation height h (x-axis). FIG. 13A compares the brightness in % on the y-axis with the modulation height h on the x-axis, FIG. 13B the color in % on the y-axis, and FIG. 13C the hue as the hue angle on the y-axis. It is shown that the analyzed two-dimensional periodic structure appears green in reflection. The variation of the modulation amount hardly changes the color appearance. Thus, despite structuring in the NIR range, the same color is retained in the visible spectrum, namely green.This is only an example of a grating structure that has different IR properties with a constant appearance in the visible when a grating structure parameter is changed. A large number of further variants of two-dimensionally periodic grating structures are conceivable here in order to achieve similar results. Grating structure parameters other than modulation height h, e.g., period d or an area fill factor, may also be changed.FIG. 14 now shows the reflection in % (y axis) as a function of the wavelength in μm (x axis) of a one-dimensional periodic aluminum-coated grating structure in the NIR region with constant modulation height h=300 nm for the periods d=270-370 nm, wherein the width b=d / 2, t=330 nm, n=1.52 and the angle of incidence is 8°. Curves for a first period d 1= 270 nm, a second period d 2= 290 nm, a third period d 3= 310 nm, a fourth period d 4= 330 nm, a fifth period d 5= 350 nm and a sixth period d 6= 370 nm are depicted. The corresponding one-dimensional periodic grating structure is shown in FIG. 7.It can be seen that an intensity minimum shifts towards the long-wave wavelength range with increasing period d. The minimum intensity coincides with a maximum in light absorption, which is attributable to the excitation of surface plasmons.FIG. 15 shows the optical characteristic of a sensor arrangement with two detector rows and two radiation sources of different spectral characteristics. This illustration is analogous to FIG. 10B, only with changed spectral characteristics of the IR LEDs or the edge filters and is used for the following signal calculation.FIG. 16 illustrates, analogously to FIG. 11B, the result of the calculation already explained for the two different LEDs and filter curves from FIG. 15. FIG. 16 compares the period in μm on the x-axis with the % reflection on the y-axis. FIG. 16 shows a measured IR intensity of a sensor in reflection from a one-dimensional periodic grating in the NIR range with the parameters of FIG. 14 as a function of the period d=270-370 nm. The third curve k 3 arises for the emission spectrum of a third beam source 70 and the transmission curve of a third filter 72, the fourth curve k 4 arises for the emission spectrum of a fourth beam source 74 and the transmission curve of a fourth filter 76. On the other hand, both detector lines supply somewhat the same signal for the period d=330 nm.Figure 17 shows a colour perception in reflection of a one-dimensional periodic grating in the visible wavelength range for the parameters of Figure 14 at variable period d = 270 - 370 nm in the CIE colour space. Figures 18A to 18C show the color perception in the LCH color space, analogous to Figures 13A to 13C, except that the period d is used instead of the modulation height h on the x-axis. The perceived colors extend from a neutral hue, through green, yellow, to a reddish color. On the other hand, the color brightness remains approximately constant (Fig. 18A).Furthermore, in FIG. 19, the IR signal of a one-dimensional periodic grating structure is examined in reflection in % (y-axis) as a function of the modulation height h of 100 nm to 450 nm (x-axis) at constant period d=250 nm. The remaining parameters correspond to the parameters of the grating of Fig. 14. Fig. 20 shows the same measured in transmission in % (y-axis).It is evident that the IR signal can change significantly by the corresponding selection of the modulation level h. At h≈250 nm, the detector row exhibits a maximum in reflection, wherein the reflection of the second detector row exhibits a minimum. In contrast, there is good contrast between the two detector rows in transmission at h≈400 nm. This can be used to generate contrast effects in the NIR range.Figs. 21A to 21C show a color perception of a one-dimensional periodic grating with constant period d = 250 nm and variable modulation height h = 100 to 450 nm in the LCH color space in reflection. Figures 22A-22C show the same in transmission. The scale on the y-axis corresponds to those of Figs. 13A to 13C.A relatively small change in brightness in the visible is shown both in reflection and in transmission. The hue is in the green, yellow region for reflection and blue region for transmission. This shows that, for example, two grating parameters, in particular the modulation height, can be selected such that a good contrast is present in the NIR range and the color appearance in the visible is approximately unchanged.Finally, FIGS. 23 and 24 show sectional representations of a value document 1 with a security element 4. in FIG. 23, the security element 4 has been applied as a transfer element to the front side 2 of a substrate body 77 of the value document 1. A first printing layer 80 is applied to a rear side 78 of the substrate 77. A second pressure coat 82 is applied to the front side 2. The security element 4 is also overprinted with the second printing layer 82.FIG. 24 shows the security element 4 which is incorporated directly into the substrate body 77 of the value document 1. An embodiment for this is a full-surface structured value document whose visual appearance is produced without the use of printing inks. Further embodiments are value documents on which the structure described here has been applied directly to the substrate, for example in the transfer process, and has subsequently been overprinted with pigment inks.List of reference characters1 Document of value 2 Front side 4 Security element 6 First printing element 8 Second printing element 10 First subregion 12 Second subregion 14 Third subregion 16 First region 18 Second region 20 Third region 22 Fourth subregion 24 Fifth subregion 26 Sixth subregion 28 Seventh subregion 30 Eighth subregion 32 Ninth subregion 34 Image element 36 QR code 37 Substrate body 38 Double-line grating 40 First grating webs 42 First grating column 44 Second grating webs 46 Second grating column 48 Flat region 50 Elevations 52 Metallization 54 Beam source 56 Optical filter 58 Detector 60 Emission spectrum of a first beam source 62 Transmission curve of a first filter 64 Quantum efficiency of a detector 66 Emission spectrum of a second beam source 68 Transmission curve of a second filter 70 Emission spectrum of a third beam source 72 Transmission curve of a third filter 74 Emission spectrum of a fourth Beam source 76 Transmission curve of a fourth filter 77 Substrate body 78 Rear side 80 First print layer 82 Second print layer d Period d 1 First Period d 2 Second Period d 3 Third Period d 4 Fourth Period d 5 Fifth Period d 6 Sixth Period h Modulation level h 1 First Modulation level h 2 Second Modulation level h 3 Third Modulation level h 4 Fourth Modulation level h5funft Modulation level h6secht Modulation level h 7 seventh modulation height k 1 first curve k 2 second curve k 3 third curve k 4 fourth curve L longitudinal direction R reflected radiation T transmitted radiation Θ angle of incidenceReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedWO 2013 / 053435 A1 [0002, 0042]DE 102011101635 A1

[0002] DE 10 2014 018 551 A1

[0031] Cited Non-Patent LiteratureH. Holebiehler, "Surface polarization on metallic wire gratings studied via power loss." Physical Review B 53.15 (1996): 10289

[0025] ◯ J.A. Porto et al., "Transmission responses on metallic gratings with very narrow splits." Physical review letters 83.14 (1999): 2845

[0025] T. W. Ebbesen et al., "Extraordinary optical transmission through subwavelength hole arrays." nature 391.6668 (1998): 667-669

[0025]

Claims

Security element for producing value documents, such as banknotes, checks or the like, which - has a front side (2) and a rear side (78), - has a substrate body (37) which has a grid structure (38, 49) extending over a region and having a reflector layer (52) located thereon, and - the grid structure (38, 49) shows at least one colour in the visible spectral range in plan view and / or transmission, characterized in that - the grid structure has partial regions (10-14, 22-32) in which the grid structure (38, 49) is in each case of uniform design, wherein the grid structures (38, 49) differ between the partial regions (10-14, 22-32) in at least one grid structure parameter (h, d) in such a way that the partial regions (10-14, 22-32) do not differ in the visible spectral range with respect to the at least one colour, However, they differ in their absorption and / or reflection in the near infrared range.The security element according to claim 1, which shows a representation recognizable with the naked eye from the front side (2).Security element according to one of the above claims, characterized in that at least one partial region (10-14, 22-32) absorbs radiation in the wavelength range from 780 nm to 1100 nm.The security element according to any one of the preceding claims, characterized in that the grating structure parameter, in which the grating structures (38, 49) differ in the sub-regions (10-14, 22-32), comprises a modulation height (h) of the grating structure (38, 49).The security element according to any one of the preceding claims, characterized in that the at least two partial regions (10-14, 22-32) each absorb radiation in the wavelength range between 780 nm and 1100 nm, but differ in terms of their spectral absorption characteristic in this wavelength range.Security element according to one of the above claims, characterized in that the grating structure in at least one of the sub-regions (10-14, 22-32) has a one-dimensional periodic sub-wavelength structure (38) with grating webs (40, 44) and grating columns (42, 46).The security element according to any one of the preceding claims, characterized in that the grating structure in at least one of the partial regions (10-14, 22-32) has a two-dimensional periodic subwavelength structure (49) with elevations (50) and a flat region (48).Security element according to one of the above claims, characterized in that it additionally has an adhesive layer on the rear side (78) in order to be applied as a transfer element to a value document (1).The security element according to any one of the above claims, characterized in that the substrate body (37) is part of a value document (1), and the security element (4) is introduced directly into the substrate (77) of the value document (1).The security element according to any one of the above claims, characterized in that the substrate body (37) is part of a film composite, wherein the front side (2) of the security element (4) points inward in the film composite, and the rear side (78) of the security element (4) points outward.Method for producing a security element for value documents such as banknotes, checks or the like, wherein - the security element (4) has a front side (2) and a rear side (78), - a substrate body (37) is provided, - a grid structure (38, 49) extending over a region is introduced into the substrate body (37), - a reflector layer (52) is applied to the grid structure (38, 49), and - the grid structure (38, 49) shows at least one colour in the visible spectral range in plan view and / or transmission, characterized in that - partial regions (10-14 and 22-32) are introduced into the grid structure (38, 49), in each of which the grid structure (38, 49) is formed uniformly, wherein the grid structures (38, 49) differ between the partial regions (10-14 and 22-32) in at least one grid structure parameter (h, d), that the partial regions (10-14 and 22-32) do not differ in the visible spectral range with regard to the at least one color, but are different in their absorption and / or reflection in the near infrared range.A value document having a security element (4) according to one of claims 1 to 10.

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