Security element and method for producing a security element

Abstract

The invention relates to a security element for producing value documents, such as bank notes, cheques, or the like. The security element (4) has a front side (2) and a back side (78). The security element (4) has a substrate body (37) which has a grid structure (38, 49) which extends over a region and has a reflector layer (52) thereon, and the grid structure shows at least one colour when viewed from above and / or in transmission. The grid structure (38, 49) has at least two partial regions (10-14 and 22-32) which cannot be perceived in the spectral range, in each of which partial regions the grid structure (38, 49) is designed in a uniform manner, wherein the grid structures (38, 49) differ between the partial regions (10-14 and 22-32) by way of at least one grid structure parameter (h, d) such that the partial regions (10-14 and 22-32) differ in respect of their absorption in the near infrared range.

Description

[0001]Security element and method for producing a security element. The invention relates to a security element for producing value documents, such as banknotes, checks, or the like, which has a front and a back. The security element comprises a substrate body having a grating structure extending over a region with a reflector layer located thereon, and the grating structure displays 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 also known to protect valuable documents such as banknotes by printing them with a special printing ink which, when illuminated with radiation from the near infrared (NIR) range – i.e., according to DIN 5031 in the version valid from May 1, 2023, in the wavelength range from 780 nm to 3000 nm – appears significantly different than when illuminated with light from the visible wavelength range. These known valuable documents have certain areas in the print image that absorb radiation with a wavelength from the NIR range, while other areas do not absorb radiation with a wavelength from the NIR range and appear bright in reflection when illuminated with radiation of this wavelength. Such inks exhibit so-called remission properties; they absorb part of the illumination spectrum and thus appear opaque for these wavelengths, and transmit or reflect light.reflect a different part of the illumination spectrum – therefore, remission is also referred to as diffuse reflection. Known colors have different remission properties of sub-ranges in the NIR range, making them invisible in the visible spectral range. Therefore, this property is often used for automated authentication in banknote processing, as a counterfeiter is usually unaware of this difference in the NIR and visible range – creating a surprising anti-counterfeiting effect. Common colors with infrared-absorbing properties generally exhibit absorption in the NIR range from 780 nm to 3000 nm, regardless of the wavelength of the incident infrared radiation. However, some manufacturers' colors, such as SicPa or Gleitsmann, have spectrally dependent absorption behavior in the NIR range, i.e.They absorb radiation of different NIR wavelengths to varying degrees. With these inks, for example, absorption in the short-wave NIR range is lower than in the longer-wave NIR range. This property is difficult to counterfeit and can be used for enhanced authenticity testing with the help of suitable sensors that evaluate reflectance, for example, in two different IR spectral ranges. It is known that these properties of the special printing inks can also be used in transmission mode for machine authentication of valuable documents. Printing inks with spectrally different absorption behavior are known for both paper and polymer substrates. It is also known to overprint metallic embossed micro- or nanostructures with the above-mentioned infrared-absorbing inks.The security elements produced in this way can then also be tested for authenticity in the NIR range using suitable sensors. To equip a security element with reflectance properties in the NIR range, printing with the described printing inks was previously required. However, a printing process always means an additional process in the manufacture of the security element or value document, thus increasing the manufacturing effort. Furthermore, it is difficult to guarantee the high lateral positioning accuracy of the print (so-called registration) required in the manufacture of security elements for value documents using known printing processes. The object of the invention is to create a security element that can be applied to a value document with high lateral positioning accuracy and has an anti-counterfeiting feature with NIR properties, the manufacturing effort of which is low.The invention is defined in claims 1, 11, and 12. The subclaims relate to preferred embodiments. A security element for producing value documents, such as banknotes, checks, or the like, is provided. The security element has a front and a back side and comprises a substrate body having a grating structure extending over a region with a reflector layer located thereon. The reflector layer ensures that the grating structure displays at least one color in plan view and / or in transmission in the visible spectral range—usually the wavelength range from 380 nm to 780 nm. The substrate body is a dielectric; usually a plastic film, in particular a PET film, or the substrate body itself is formed from a UV embossing varnish. The refractive index of the materials of the substrate body is preferably approximately n = 1.5.A grating structure is incorporated into the substrate body, for example, into the UV embossing lacquer by embossing, lasering, or etching. The grating structure is metallized, preferably with aluminum, which is applied to the grating structure by vacuum vapor deposition. This metallization creates plasmonic effects that lead to resonant light absorption in the metal. This frequency-selective absorption results in reduced transmission or reflection for the respective wavelength. The choice of the grating period determines the position of plasmonic resonances in the NIR range. Plasmonic resonances are particularly pronounced in grating structures for wavelengths approximately equal to d / n, where d corresponds to the period and n to the refractive index. The exact position of the resonance in the wavelength spectrum, however, depends on the exact geometry of the grating structure in the profile as well as the optical constants of the substrate body.The structure depth also influences the transmission or reflection spectrum of the grating structure, as cavity resonances can be excited, and these shift into the long-wavelength 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 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 arranged at a distance from each other, is relevant for the formation of cavity resonances. The modulation depth can range from very flat structures of approximately 20 nm to several micrometers. However, the production of such structures in an embossing process limits the choice to the range of approximately 40 nm to 500 nm.The described plasmonic effects also occur in other metals such as Ag, Cu, Au, Cr, etc. and their alloys, so that these metals are also used equally for 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 components. In practice, however, there is a coupling between the two effects, which can cause light resonances in the near infrared. In the region in which the grating structure extends, the grating structure contains at least two sub-regions that are not visible in the visible spectral range, in each of which the grating structure is uniformly formed.However, the grating structures differ between the sub-areas 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-areas can also be changed by arranging a sub-area at an angle. This can be achieved, for example, by arranging the grating structure on a micromirror array. The sub-areas are each NIR-absorbing, but differ in their absorption behavior in the NIR range, in particular they absorb spectrally differently. This achieves a surprising anti-counterfeiting effect, which is only visible when illuminated with radiation from the NIR range, but is camouflaged when illuminated with light, i.e. from the visible wavelength spectrum.The visual impression does not change between the sub-regions due to the IR-effective structure. The sub-regions preferably realize hidden codes in the NIR spectrum. These codes arise because some sub-regions are absorbing when irradiated with wavelengths from this sub-spectrum of the NIR range 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 preferentially absorbs radiation of a different wavelength or wavelength range. A sub-region that appears at a wavelength of 800 nm, for example, can disappear again at a wavelength of 1000 nm. Another sub-region that is not visible when irradiated with a wavelength of 800 nm can, for example, become visible at 1000 nm.In addition, the effects in reflection when illuminated with radiation of a specific wavelength from the NIR range can differ in the sub-ranges, but be the same in transmission, or vice versa. Preferably, the security element shows a top view of a representation visible to the naked eye in the sub-ranges, e.g. a pattern, image, text, etc., superimposed. This is visible when illuminated in the visible wavelength range and under these conditions is not changed by the grating structure, which is fundamentally only effective in the NIR. Preferably, the grating structure is absorbent for radiation in the NIR sub-spectrum from 780 nm to 1,100 nm in at least one sub-range. Preferably, several sub-ranges each absorb in the NIR sub-spectrum from 780 nm to 1,100 nm, but the sub-ranges differ in terms of their spectral absorption characteristics in this spectral range.The variation of at least one lattice structure parameter in the sub-regions ensures that the image visible to the naked eye when illuminated with light from the visible wavelength range does not produce any effect across all sub-regions. As long as no further printing etc. is provided, the sub-regions appear, for example, as a single unit. When illuminated with radiation from the NIR range, the sub-regions absorb differently in the NIR spectral range. The sub-regions can implement binary transparency coding in the NIR, e.g. in the sense of a black and white transparent image or similar, without impairing the impression in the visible range. In particular, the sub-regions can absorb NIR radiation spectrally differently. The sub-regions can then implement different coding at different wavelengths in the NIR range, e.g. in the sense of a multi-colored transparent image.The grating structure can be either a one-dimensional periodic subwavelength structure with grating ridges and grating gaps, such as a double-line grating, or a two-dimensional subwavelength structure, in particular a two-dimensional periodic subwavelength structure with periodically arranged elevations or depressions. Both grating structure variants exhibit a contrast difference when varying the grating parameters, in particular the modulation height. The period in one-dimensional periodic subwavelength structures also influences the reflection behavior in the NIR range. It can therefore also be selected as a grating structure parameter, as explained in more detail in the exemplary embodiments using graphs.Both one-dimensional periodic subwavelength structures with grating ridges and slits, and a two-dimensional periodic subwavelength structure with periodically arranged elevations or depressions are suitable for generating good contrast between the sub-regions in the NIR range, while at the same time the color appearance in the visible range remains unchanged from region to region. Each sub-region can be adjusted to absorb radiation of a different wavelength in the NIR range. Particularly preferably, the grating structure parameter in which the grating structures differ in the sub-regions includes the modulation height h of the grating structure. It has been observed, and is also demonstrated in the exemplary embodiments using graphs, that the reflection of the sub-regions in the NIR range decreases significantly with increasing modulation height h.For example, in a two-dimensional periodic subwavelength structure, a modulation height of h = 300 nm results in a high contrast in the NIR range, while a modulation depth of, for example, h = 250 nm shows hardly any difference in contrast. The grating structure parameters can also be adjusted in the sub-areas so that they absorb radiation spectrally selectively in the NIR range. The described grating structures thus represent a further development of known IR-absorbing printing inks, which do not permit different remission properties in sub-areas when irradiated with different wavelengths. The grating structures also have the advantage over spectrally selective printing inks that they can be applied to the security element with significantly higher lateral positioning accuracy than is possible with a printing process.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 elimination of an entire process step. The grating structures described can be combined with known optical embossed structures such as holograms, micromirrors or known grating structures that 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 or directly integrated into the printed image of value documents. Metallic wire or slot gratings and metallic nanohole arrays or nanodot arrays can also be used as grating structures. Such grating structures are known from the literature and also exhibit optical resonance effects in the NIR range. See literature: o H.Lochbihler "Surface polaritons on metallic wire gratings studied via power losses." Physical Review B 53.15 (1996): 10289. o JA Porto et al. "Transmission resonances on metallic gratings with very narrow slits." Physical review letters 83.14 (1999): 2845. o TW Ebbesen et al. "Extraordinary optical transmission through sub-wavelength hole arrays." Nature 391.6668 (1998): 667-669. o H. Lochbihler "Security element with color-filtering grating," AZ 10 2015008655.3. Also disclosed is 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 back side. A substrate body is provided in the method. A grating structure extending over an area is introduced into the substrate body, and a reflector layer is applied to the grating structure.The grating structure displays at least one color in the visible spectral range in plan view and / or transmission, and subregions are incorporated into the grating structure, each of which has a uniform grating structure. The grating structures differ between the subregions in at least one grating structure parameter such that the subregions do not differ in the visible spectral range with regard to the at least one color, but differ in the near infrared range with regard to their absorption and / or reflection. The security element is preferably produced using photolithographic processes, in particular in an e-beam system. Alternatively, systems based on 2-photon absorption are also suitable. Other known production methods are interference lithography or mechanical scribing. One well-known manufacturer of such systems is, for example, Nanoscribe GmbH & Co.KG, Hermann-von-Helmholtz-Platz 6, 76344 Eggenstein-Leopoldshafen, Germany. In the manufacturing process, exposed originals are first created. After developing a photoresist, these exposed originals are then copied electrolytically or using photopolymers (e.g., Ormocere). Such an original can also be precisely combined with other original structures using, for example, nanoimprinting processes. This process is particularly suitable for combining with other known structures such as relief holograms, micromirror arrays, moth-eye structures, or known embossed nanostructures to create structural colors. An original produced in this way is then copied electrolytically or using a nanoimprinting process. Furthermore, a multiple arrangement of the original pattern by hot stamping or nanoimprinting on a matrix is ​​required to create an embossing cylinder for later duplication.Such embossing cylinders ultimately allow the continuous duplication of the original structure in UV varnish on foils in a roll-to-roll process. Hot stamping or nano-casting in UV varnish are possible options here. Finally, the structured foil is metallized. Common metallization processes are electron beam vapor deposition, thermal vapor deposition, or sputtering. Al, Ag, Au, Ni, or Cr, and alloys of these metals, are particularly suitable as metallizations. The metallization thicknesses range from approximately 10 nm to approximately 150 nm. Alternatively, the grid structures can also be metallically printed, e.g., coated with supersilver. The metallized surface of the grid structure is then preferably coated with a transparent protective layer or laminated with a cover film to protect it from environmental influences.The security element produced in this way can be applied to a valuable document in a variety of ways. The security element can be designed as a transfer element, i.e., a foil element that additionally has an adhesive layer on the back and is thus applied to a valuable document. Such transfer elements include, for example, security threads, LEAD strips, or patches, and primarily serve as features that can be perceived by the human eye. Known optically variable effects of these features are created by embossed holograms, micromirror arrays, microlens arrays, or color-shift coatings. The transfer elements are difficult to verify for authenticity by machine when illuminated with light in the visible wavelength spectrum.This deficiency is exploited in practice by counterfeiters; therefore, in this invention, the known optical transfer elements are supplemented with the grating structures described above as a security feature detectable in the NIR range. This enables machine authentication and changes 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 that is part of the value document, so that the security element is incorporated directly into the substrate of the value document. This is achieved, for example, in a transfer process within the printing area 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 102014018551 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-based authenticity verification of the film elements and thus increase security against forgery. In the film composite, the front side of the security element faces inwards, and the back side of the security element faces outwards. The invention is explained in more detail below using exemplary embodiments with reference to the attached drawings, which also disclose features essential to the invention. These exemplary embodiments serve merely as illustrations and are not to be interpreted as restrictive.For example, a description of an embodiment with a large number of elements or components should not be interpreted to mean that all of these elements or components are necessary for implementation. Rather, other embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the embodiments may also be applicable to other embodiments. To avoid repetition, identical or corresponding elements in different figures are designated by the same reference numerals and are not explained more than once. The figures show: Fig. 1 a value document when illuminated with light from the visible spectral range; Fig.2 shows the value document according to FIG. 1 when illuminated with radiation from the near infrared range; FIG. 3 shows a value document in a further embodiment when illuminated with light from the visible spectral range; FIG. 4 shows the value document according to FIG. 3 when illuminated with radiation from the near infrared range; FIG. 5 shows a value document in a further embodiment when illuminated with light from the visible spectral range; FIG. 6 shows the value document according to FIG. 3 when illuminated with radiation from the near infrared range; FIG. 7 shows a one-dimensional periodic grating structure in a sectional view; FIG. 8A shows a two-dimensional periodic grating structure with three different sub-regions in a 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 wavelength at different modulation levels; Fig. 11B shows a reflection of two different two-dimensional periodic grating structures as a function of modulation level; 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 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; Fig. 18A to 18C shows 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; Fig. 21A to 21C shows a color perception of a one-dimensional periodic grating structure in the LCH color space in reflection; Fig. 22A to 22C shows 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, designed as a transfer element; Fig. 24 shows a value document with an embedded security element; Fig.1 shows a value document 1 to the front side 2 of which a security element 4 is applied. A first printing element 6 and a second printing 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 area in which the security element 4 is applied appears to an external observer to be 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 one area on the front side 2 of the value document 1. This area has a first partial area 10, a second partial area 12 and a third partial area 14. The partial areas 10 to 14 are only visible when illuminated with radiation of a wavelength from the NIR range (reflection).Likewise, the sub-regions 10 to 14 can also appear in transmission when illuminated with radiation of a wavelength from the NIR range. When illuminated with light from the visible wavelength spectrum, an external observer sees the sub-regions 10 to 14 as shown in Fig. 1 - they create a uniform color impression and are therefore not recognizable - neither in transmission nor in reflection. Each of the sub-regions 10 to 14 differs from the other sub-regions 10 to 14 in its absorption behavior in the NIR range. 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.When illuminated with different wavelengths in the NIR range, the security element therefore displays different patterns in reflection or transmission, and then hides these patterns again when illuminated with other wavelengths in the NIR range. When illuminated with light of a wavelength from the visible wavelength spectrum, the sub-regions 10 to 14 are not visible, but 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 visible to the viewer when illuminated with light from the visible wavelength spectrum in a plan view of the front side 2 and / or in transmission.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, when illuminated with light from the visible spectrum, displays a color gradient, a pattern, or the like. Fig. 4 shows the valuable document 1 in the embodiment of Fig. 3 when illuminated with radiation of a wavelength in the NIR range. The first region 16 is divided into the first subregion 10, the second subregion 12, and the third subregion 14, as was already the case in Fig. 2. In addition, 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 its 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 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, when illuminated with different wavelengths in the NIR range, different patterns appear, and when illuminated with other wavelengths in the NIR range, they disappear again. Preferably, several sub-regions 10 to 14 and 22 to 32 can show the same remission when illuminated with light of the same wavelength from the NIR range.When illuminated with a wavelength from the wavelength spectrum of visible light, the sub-regions 10 to 14 and 22 to 32 are not visible, but the security element 4 represents the color gradient or pattern according to Fig. 3. In Figs. 5 and 6, analogous to Figs. 1 and 2 or 3 and 4, a further embodiment of the value document 1 is shown, in which an image element 34 is applied to the front side 2 of the value document 1 as the security element 4 (Fig. 5). This image element 34 disappears when illuminated with radiation from the NIR range and a QR code 36 appears, as shown in Fig. 6. It is also possible for the QR code 36 to appear only at a certain wavelength in the NIR range, but for a different wavelength to appear a different pattern in reflection and / or transmission.Patterns / codes / images can also differ in transmission and reflection when illuminated with radiation of certain wavelengths in the NIR range and become 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 authentication of the valuable document 1. This creates a surprising anti-counterfeiting effect which makes the reproducibility of the valuable document 1 more difficult. The optical appearance when illuminated with visible light as well as the properties in the NIR range creates a metallized grating structure, as shown by way of example in Figures 7, 8a and 8b. The grating structures can be linear periodic sub-wavelength structures, as shown in Fig.7, but also two-dimensional periodic gratings, as shown in Figs. 8a and 8b. Fig. 7 shows a security 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 grating webs 40 there are first grating gaps 42 which have a width b. The thickness of the first grating webs 40 is given by t. At a modulation height h above the first grating webs 40 there is a second line grating structure with second grating webs 44; they have the width b.The second line grating structure is phase-shifted relative to the first line grating structure such that the second grating webs 44 come to lie above the first grating webs 40. At the same time, second grating gaps 46, which exist between the second grating webs 44, lie above the first grating webs 40. The thickness t is smaller than the modulation height h, so that no continuous film is formed from the grating webs 40 and 44. It is essential that the modulation height h, i.e. the difference in height 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. Figures 8A and 8B show a security element 4 with a two-dimensional periodic grating structure with elevations 50 raised relative to a flat region 48. Both the flat region 48 and the elevations 50 are provided with a metallization 52. Figure 8A shows the security element 4 in plan view. A grating structure 49 can be seen, which is divided into the subregions 10 to 14. Grating structure parameters such as modulation height, period, area fill factor, etc. vary from subregion to subregion. Figure 8B shows an example of a variation in the modulation height h in an isometric representation.In the partial area 10 of the security element 4, the elevations 50 have a first modulation height h1, in the second partial area 12 a second modulation height h2 and in the third partial area 14 a third modulation height h3. From the first partial area 10 to the third partial area 14, the modulation height decreases, i.e. h3>h2>h1. Such a variation preferably produces a uniform color impression of the partial areas 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 partial areas 10 to 14. Figs. 9A and 9B show a measuring geometry of an optical sensor. Fig. 9A shows an optical sensor for measuring remission and Fig. 9B for measuring transmission. The optical sensor consists of a beam source 54, an optical filter 56 and a detector 58. In addition, in Fig.9A and 9B depict a valuable document 1 to be examined. In such a measurement geometry, the absorbing properties of grating structures in the NIR range are examined. In the embodiment of Figs. 9A and 9B, therefore, IR LEDs serve as the radiation source 54, IR filters serve as the optical filters 56, and a silicon line detector serves as the detector 58. In order to estimate a measured signal from such an IR sensor, knowledge of the emission spectrum of the radiation source 54, the spectral emission of the filter 56, and the spectral sensitivity of the detector 58 is required. The graphs in Figs. 10A and 10B plot the radiation intensity on the y-axis against the wavelength in μm on the x-axis. Fig. 10A shows the curves of an optical sensor with a filter having 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. IR LEDs, which have a center wavelength in the NIR range, can be used as the beam source. Figure 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 embodiment offers the possibility to evaluate 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: where E(λ) is the spectral emission of the IR LED, Q(λ) is the quantum efficiency of the detector, TF(λ) is the transmission of the edge filter, and RG(λ) and TG(λ) are the reflection and transmission of the grating structure, respectively. In the case of Fig. 10B, the sensor arrangement consists of two beam sources with different emission spectra E1(λ) and E2(λ). The detector consists of two detector rows, the first row with the edge filter of transmission characteristic T F1 (λ) and the second line with the edge filter of the transmission characteristic T F2 (λ). This sensor arrangement delivers two different signals in the respective detector rows, which can be I2= ∫ E2(λ) * Q(λ) * T F2 (λ) * R G(λ) dλ are given for the remission arrangement. An improvement in the measurement accuracy of the sensor arrangement is achieved by evaluating the ratio I1 / I2. These values ​​determined from this are used for further estimation of an IR signal. First, the spectral reflectance of a two-dimensional periodic grating, as shown in Figures 8A and 8B, is investigated. Figure 11A shows a spectral reflectance of the two-dimensional periodic grating with a constant period d = 240 nm for various modulation heights h from 240 nm to 300 nm with a light incidence of 8° in the NIR range as a function of the wavelength (the reflectance in % is on the y-axis and the wavelength in μm is 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 = 40 nm. The modulation heights h1bish7 is plotted, where the modulation height h decreases from the first modulation height h1 to the seventh modulation height h7 in 10 nm steps, where h1=300 nm and h7=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 into the infrared range. This absorption is synonymous with the excitation of surface plasmons (see above). The signal of an optical sensor according to Figs. 9A and 9B can be calculated using the spectral curves of Figs. 10A and 10B, respectively, by convolving them with the reflection of Fig. 11A according to the formula mentioned above. The result of the calculation explained above is shown in Fig. 11B for the two different LED and filter curves of Fig. 10B. Fig. 11B compares the modulation height in μm on the x-axis with the reflection signal measured by the sensor array on the y-axis. The first curve k1is created for the emission spectrum of a first beam source 66 and the transmission curve of a first filter 68, the second curve k2 is created for the emission spectrum of a second beam source 60 and the transmission curve of a second filter 62, whereby curve k1 refers to the spectral characteristics of curves 66, 68 of Fig. 10B and curve k2 refers to the spectral characteristics of curves 60, 62 of Fig. 10B. It can be seen that the modulation depth h = 300 nm, related to the beam source used, shows a large contrast in the NIR range, while the modulation height h = 250 nm shows hardly any difference. It has already been explained that a further requirement of the security element 4 is the property that it is not - or at most only barely - perceptible when viewed in the visible spectral range. Figs. 12 and 13A to 13C therefore show the color perception of a two-dimensional periodic lattice structure at aVariation of the modulation height between 250 nm and 300 nm in the CIE or LCh color space. In addition to this variation in the modulation height, the contrast in the NIR range was previously investigated when using different light sources and filters. Fig. 12 shows the color coordinates x and y in the CIE color space, and Figs. 13A to 13C show individual color values ​​in the LCH color space as a function of the modulation height h (x-axis). Fig. 13A compares the modulation height h on the x-axis with the brightness in % on the y-axis, Fig. 13B with the chroma in % on the y-axis, and Fig. 13C with the hue as a hue angle on the y-axis. It can be seen that the analyzed two-dimensional periodic structure appears green in reflection. Varying the modulation height 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 just one example of a grating structure that has different IR propertieswith a constant appearance in the visible range when a grating structure parameter is changed. A multitude of other variants of two-dimensional periodic grating structures are conceivable in order to achieve similar results. Grating structure parameters other than the modulation height h, e.g. the period d or an area fill factor, can also be changed. Fig. 14 shows the reflection in % (y-axis) as a function of the wavelength in μm (x-axis) of a one-dimensional periodic grating structure coated with aluminum in the NIR range with a constant modulation height h = 300 nm for the periods d = 270 - 370 nm, where the width b = d / 2, t = 30 nm, n = 1.52 and the angle of incidence is 8°. Curves are shown for a first period d1 = 270 nm, a second period d2 = 290 nm, a third period d3 = 310 nm, a fourth period d4 = 330 nm, a fifth period d5 = 350 nm, and a sixth period d6 = 370 nm. The correspondingdimensional periodic grating structure is shown in Fig. 7. It can be seen that an intensity minimum shifts towards the long-wavelength range with increasing period d. The intensity minimum coincides with a maximum in the light absorption, which is due to the excitation of surface plasmons. Fig. 15 shows the optical characteristics of a sensor arrangement with two detector rows and two radiation sources with different spectral characteristics. This representation is analogous to Fig. 10B, only with changed spectral characteristics of the IR LEDs and the edge filters and is used for the following signal calculation. Fig. 16 shows the result of the calculation already explained for the two different LED and filter curves from Fig. 15, analogous to Fig. 11B. Fig. 16 compares the period in μm on the x-axis with the reflection in % on the y-axis. Fig.16 shows a measured IR intensity of a sensor inReflection 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 k3 is generated for the emission spectrum of a third beam source 70 and the transmission curve of a third filter 72; the fourth curve k4 is generated for the emission spectrum of a fourth beam source 74 and the transmission curve of a fourth filter 76. A good contrast is evident in the NIR range in the two detector rows for the period d = 280 nm. In contrast, both detector rows deliver approximately the same signal for the period d = 330 nm. Fig. 17 shows color perception in reflection of a one-dimensional periodic grating in the visible wavelength range for the parameters of Fig. 14 with a variable period d = 270 – 370 nm in the CIE color space. Fig. 18A to 18C show the color perception in the LCH color space analogous to Fig. 13A to 13C, with the exception that instead of the modulation height h on the x-axis, thePeriod d is used. The perceived colors range from a neutral hue through green, yellow, and even a reddish color. In contrast, the color brightness remains approximately constant (Fig. 18A). Furthermore, Fig. 19 examines the IR signal of a one-dimensional periodic grating structure in reflection in % (y-axis) as a function of the modulation height h from 100 nm to 450 nm (x-axis) with a constant period d = 250 nm in reflection. The other parameters correspond to the parameters of the grating in Fig. 14. Fig. 20 shows the same, measured in transmission in % (y-axis). It is evident that the IR signal can change significantly by selecting the appropriate modulation height h. At h ≈ 250 nm, the detector row shows a maximum in reflection, while the reflection of the second detector row has a minimum. In contrast, there is a good contrast between the two detector lines in transmission at h ≈ 400 nm. This can be used to create contrast effectsin the NIR range. Figures 21A to 21C show the color perception of a one-dimensional periodic grating with a constant period d = 250 nm and a variable modulation height h = 100 to 450 nm in the LCH color space in reflection. Figures 22A to 22C show the same in transmission. The scales on the y-axis correspond to those in Figures 13A to 13C. There is a relatively small change in brightness in the visible range, both in reflection and in transmission. The color tone lies in the green and yellow range for reflection and in the blue range for transmission. This shows that, for example, two grating parameters, in particular the modulation height, can be selected such that good contrast is achieved in the NIR range and the color appearance in the visible range is almost unchanged. Finally, Fig.23 and 24 show sectional views of a value document 1 with a security element 4. In Fig.23, the security element 4 was applied as a transfer element to theFront side 2 of a substrate body 77 of the value document 1. A first printing layer 80 is applied to a back side 78 of the substrate body 77. A second printing layer 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. One exemplary embodiment is a fully structured value document whose visual appearance is created without the use of printing inks. Further exemplary embodiments are value documents on which the structure described here was applied directly to the substrate, e.g., using a transfer process, and then overprinted with pigment inks. List of reference symbols 1 Value document 2 Front side 4 Security element 6 First printing element 8 Second printing element 10 First partial area 12 Second partial area 14 ThirdSubregion 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 bars 42 first grating slits 44 second grating bars 46 second grating slits 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 Back 80 first print layer 82 second print layer d period d1first period d2second period d3third periodd4 fourth period d5 fifth period d6 sixth period h modulation height h1 first modulation height h2 second modulation height h3 third modulation height h4 fourth modulation height h5 fifth modulation height h6 sixth modulation height h7 seventh modulation height k1 first curve k2 second curve k3 third curve k4 fourth curve L longitudinal direction R reflected radiation T transmitted radiation Θ angle of incidence

Claims

Patent Claims 1. Security element for producing value documents, such as banknotes, checks or the like, which - has a front side (2) and a back side (78), - comprises a substrate body (37) which has a grating structure (38, 49) extending over a region with a reflector layer (52) located thereon, and - the grating structure (38, 49) displays at least one color in the visible spectral range in plan view and / or transmission, characterized in that - the grating structure has partial regions (10-14, 22-32) in each of which the grating structure (38, 49) is formed uniformly, wherein the grating structures (38, 49) differ between the partial regions (10-14, 22-32) in at least one grating structure parameter (h, d) such that the partial regions (10-14, 22-32) do not differ in the visible spectral range with regard to at least one color,in the near infrared range, however, are different with regard to their absorption and / or reflection.

2. Security element according to claim 1, which shows a representation that is visible to the naked eye from the front side (2).

3. Security element according to one of the above claims, characterized in that at least one partial area (10-14, 22-32) absorbs radiation in the wavelength range from 780 nm to 1100 nm.

4. Security element according to one of the above claims, characterized in that the lattice structure parameter in which the lattice structure, - 2 - structures (38, 49) in the partial regions (10-14, 22-32), a modulation height (h) of the grating structure (38, 49).

5. Security element according to one of the above 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 with regard to their spectral absorption characteristics in this wavelength range.

6. Security element according to one of the above claims, characterized in that the grating structure in at least one of the partial regions (10-14, 22-32) has a one-dimensional periodic subwavelength structure (38) with grating bars (40, 44) and grating gaps (42, 46).Security element according to one of the above claims, characterized in that the grating structure in at least one of the partial regions (10-14, 22-32) has a two-dimensional periodic sub-wavelength structure (49) with elevations (50) and a flat region (48).

8. Security element according to one of the above claims, characterized in that it additionally has an adhesive layer on the back (78) in order to be applied to a value document (1) as a transfer element.

9. Security element according to 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). - 3 - 10. Security element according to 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 inwards in the film composite, and the back side (78) of the security element (4) points outwards.

11. A 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 back side (78), - a substrate body (37) is provided, - a grating structure (38, 49) extending over a region is introduced into the substrate body (37), - a reflector layer (52) is applied to the grating structure (38, 49), and - the grating structure (38, 49) displays at least one color 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 grating structure (38, 49),in each of which the grating structure (38, 49) is uniformly formed, wherein the grating structures (38, 49) between the partial regions (10-14 and 22-32) differ in at least one grating structure parameter (h, d) such that the partial regions (10-14 and 22-32) do not differ in the visible spectral range with respect to the at least one color, but differ in the near infrared range with respect to their absorption and / or reflection.

12. A value document with a security element (4) according to one of claims 1 to 10.

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