Security element and method for producing security element

Abstract

The invention relates to a security element for producing value documents, such as banknotes, checks or the like. The security element (4) has a front side (2) and a rear side (78). The security element (4) has a main body (37) which comprises a lattice structure (38, 49) extending over a region, on which a reflective layer (52) is arranged, and which shows at least one color in plan view and / or transmissive. The lattice structure (38, 49) has at least two sub-regions (10-14 and 22-32) which are not recognizable in the visible spectral range, in which sub-regions the lattice structure (38, 49) are each uniformed, and wherein the lattice structure (38, 49) differs between the sub-regions (10-14 and 22-32) with respect to at least one lattice structure parameter (h, d), the sub-regions (10-14 and 22-32) are arranged such that the sub-regions (10-14 and 22-32) differ with respect to their absorption in the near-infrared range.

Description

[0001] The present invention relates to a security element for the production of value documents, such as banknotes, checks or the like, said security element having a front side and a back side. The security element has a substrate which has a grid structure extending over an area, on which grid structure a reflective layer is present, and which grid structure displays at least one color in the visible spectral range in plan view and / or in transmission.

[0002] Security elements for the production of value documents having a periodic line grid are known, for example, from WO 2013 / 053435 A1. Two-dimensionally periodic color filter grids are likewise known, for example, from DE 102011101635 A1.

[0003] It is also known to protect value documents, such as banknotes, by printing with special printing inks which exhibit a distinctly different appearance when irradiated with radiation in the near-infrared range (NIR range), i.e. in the wavelength range from 780 nm to 3000 nm according to the standard DIN 5031 in the version valid from May 1, 2023, than when irradiated with light in the visible wavelength range. These known value documents have specific sub-areas in the printed image which absorb radiation of the NIR range wavelengths, while other sub-areas do not absorb radiation of the NIR range wavelengths and appear bright in reflection when irradiated with radiation of this wavelength. Such inks have so-called diffuse properties; they absorb a portion of the incident spectrum and thus appear opaque to these wavelengths, and transmit or reflect another portion of the incident spectrum - hence the term Remission also being referred to as diffuser Reflexion. For the known inks, the sub-areas have different diffuse properties in the NIR range, so they are not noticeable in the range of the visible spectrum. This property is therefore often used in machine authenticity testing in banknote processing, since counterfeiters are generally not aware of this difference in appearance in the NIR range and in the visible light, an unexpected security effect is created.

[0004] Common inks with infrared absorption properties generally exhibit absorption in the NIR range from 780 nm to 3000 nm independently of the wavelength of the incident infrared radiation. However, inks from some manufacturers, such as SicPa or Gleitsmann, are known to have a spectral-dependent absorption behavior in the NIR range, i.e. they absorb different amounts of radiation of different NIR wavelengths. For example, these inks absorb less in the short-wave NIR range than in the long-wave NIR range. This property is difficult to counterfeit and enables an authenticity test which can be extended by means of suitable sensors, for example, which analyze the remission in two different IR spectral regions.

[0005] These properties of special printing inks in value documents are known to be used in transmission also for machine authenticity testing. Printing inks with different absorption behavior in the spectral range are known both for paper and polymer substrates.

[0006] Furthermore, it is known to overprint micro- and nanostructures of metal embossings with the above-mentioned infrared-absorbing inks. The security elements thus produced can then likewise be subjected to authenticity testing in the NIR range by means of suitable sensors.

[0007] In order to impart diffuse properties in the NIR range to the security element, hitherto it has been necessary to print with the printing inks described. The printing process always means, however, an additional process in the production of the security element and / or value document, resulting in an increased production outlay. Furthermore, it is difficult to guarantee a high lateral position accuracy (so-called register) of the printing using the known printing processes, which is necessary in the production of security elements for value documents.

[0008] The technical problem addressed by the present application is to realize a security element which can be applied to a value document with high lateral position accuracy and which has an anti- counterfeit feature with NIR properties, the production outlay of which is low.

[0009] The application is defined in claims 1, 11 and 12. The dependent claims relate to preferred design variants.

[0010] A security element for the production of value documents, such as banknotes, checks or the like, is specified. The security element has a front side and a back side and has a substrate which has a grid structure extending over a region, on which a reflective layer is present. The reflective layer ensures that the grid structure displays at least one color in the case of planar and / or transmission in the range of the visible spectral range, usually the wavelength range of 380 nm to 780 nm.

[0011] The substrate is a dielectric; usually a plastic film, in particular a PET film, or the substrate itself is formed by a UV embossing lacquer. The refractive index of the substrate material is preferably about n = 1.5. The grid structure is introduced into the substrate, i.e. by embossing, for example, into a UV embossing lacquer, laser treatment or etching.

[0012] The grid structure is metallized, preferably with aluminum, which is applied to the grid structure by vacuum evaporation. By this metallization, a plasmonic effect is generated, which leads to a resonant light absorption in the metal. This frequency-selective absorption leads to a reduced transmission or reflection for the respective wavelength. The position of the plasmonic resonance in the NIR range is determined by the choice of the grid period. The plasmonic resonance is particularly pronounced for grid structures with a wavelength of approximately the value d / n, wherein d is the period and n is the refractive index. However, the exact position of the resonance in the wavelength spectrum depends on the exact geometry of the grid structure profile and on the optical constants of the substrate. The structure depth also has an influence on the transmission or reflection spectrum of the grid structure, since cavity resonances can be excited, and these cavity resonances move towards the long-wave region of the spectrum with increasing structure depth. Thus, a 1 -dimensional or 2-dimensional periodic grid with a period of 100 nm to 1000 nm, preferably 200 nm to 600 nm, is suitable for forming a plasmonic resonance or a cavity resonance in the NIR range. Furthermore, the depth of the grid structure, i.e. the modulation depth of the two spaced-apart metal grids, is relevant for the formation of cavity resonances. The modulation depth can be chosen in a range from approximately 20 nm to several micrometers for very flat structures. However, the choice is limited to a range of approximately 40 nm to 500 nm for manufacturing such structures in an imprint process. The described plasmonic effect also occurs in other metals such as Ag, Cu, Au, Cr, etc. and their alloys, so that these metals can likewise be used for the metallization. In contrast, the cavity resonance is mainly dependent on the geometry of the grid structure and leads to a redistribution of the light intensity between the reflection and transmission shares. In practice, however, there is a coupling between the two effects, which can lead to optical resonances in the near infrared.

[0013] Within the area in which the grid structure extends, at least two sub-areas are provided in the grid structure, which are not recognizable in the visible spectral range, in which the grid structure is respectively uniformly configured. Between the sub-areas, however, the grid structure respectively differs in at least one grid structure parameter, such as the modulation height, the area fill factor or the period of the grid structure. It is also possible to change the azimuth angle of the different sub-areas by arranging the sub-areas obliquely. This can be achieved, for example, by arranging the grid structure on a micromirror array. The sub-areas are respectively NIR-absorbing, however, their absorption behavior in the NIR range differs, in particular spectrally differently.

[0014] An unexpected security effect is thereby achieved, which is only visible when irradiated with radiation in the NIR range, but hidden when irradiated with light, i.e. from the visible wavelength spectrum. The visual impression is not changed between the sub-areas by the IR-effective structure.

[0015] Preferably, the sub-areas realize a hidden coding in the NIR spectrum. The production of these codings is due to the fact that some sub-areas are absorptive when illuminated with wavelengths from the partial spectrum of the NIR range, thereby appearing in transmission, while others do not. The sub-areas can also change at different NIR wavelengths, since each sub-area preferentially absorbs radiation of different wavelengths or different wavelength ranges. For example, a sub-area that appears at a wavelength of 800 nm can disappear again at a wavelength of 1000 nm. Another sub-area that is not visible when illuminated with a wavelength of 800 nm can become visible, for example, at 1000 nm. Furthermore, these effects can be different in these sub-areas in reflection when illuminated with radiation of specific wavelengths in the NIR range, but the same in transmission, and vice versa.

[0016] Preferably, the security element shows a view, for example a pattern, an image, text, etc., which is visible to the naked eye in the sub-areas superimposed in plan view. This view is visible when illuminated in the visible wavelength range, and under these conditions is not changed by the grid structure, which works in principle only in the NIR.

[0017] Preferably, the grid structure is absorptive in at least one sub-area for radiation in the NIR partial spectrum from 780 nm to 1100 nm. Preferably, a plurality of sub-areas absorb in the NIR partial spectrum from 780 nm to 1100 nm, respectively, but the spectral absorption properties of these sub-areas differ in this spectral range. The change in at least one grid structure parameter in the sub-area is such that the view visible to the naked eye when illuminated with light in the visible wavelength range does not produce an effect on all sub-areas. The sub-area appears, for example, as a unit, as long as no further printing, etc., is provided. When illuminated with radiation from the NIR range, the sub-areas absorb differently in the NIR spectral range. The sub-area is able to realize a binary transparency coding in the sense of, for example, s / w perspective views in the NIR, without affecting the visible impression.

[0018] The sub-areas can in particular absorb the NIR radiation differently spectrally. The sub-areas are then able to realize different codings at different NIR wavelengths, for example in the sense of multicolor perspective views.

[0019] The grid structure can be both a one-dimensional periodic sub-wavelength structure with grid bars and grid gaps, for example a double-line grid, and also a two-dimensional sub-wavelength structure, in particular a two-dimensional periodic sub-wavelength structure with periodically arranged elevations or depressions. Both variants of the grid structure show a contrast difference when the grid parameters, in particular the modulation height, are varied. The period of the one-dimensional periodic sub-wavelength structure also has an influence on the reflection behavior in the NIR range. The period of the one-dimensional periodic sub-wavelength structure can therefore also be selected as a grid structure parameter, as is explained in more detail by means of the diagrams in the examples.

[0020] One-dimensional periodic sub-wavelength structures with grating bars and grating gaps and two-dimensional periodic sub-wavelength structures with periodically arranged protrusions or recesses are both suitable to produce a good contrast between sub-regions in the NIR range, while the color appearance in the visible light remains unchanged between the regions. Each sub-region can be set such that it spectrally selectively absorbs radiation in another wavelength in the NIR range.

[0021] It is particularly preferred that the parameters of the grating structures in the sub-regions differ from each other include the modulation height h of the grating structures. It has been determined and is demonstrated in the examples by graphs that the reflection of the sub-regions in the NIR range decreases significantly with increasing modulation height h. Thus for two-dimensional periodic sub-wavelength structures, for example a modulation height h = 300 nm shows a high contrast in the NIR range, while a modulation depth of for example h = 250 nm shows hardly any difference in contrast.

[0022] The grating structure parameters can also be set in the sub-regions such that the sub-regions spectrally selectively absorb radiation in the NIR range. The described grating structures thus form an improvement over known IR-absorbing printing inks, which do not allow sub-regions to have different diffuse properties when illuminated with different wavelengths. In comparison to spectrally selective printing inks, the grating structures also have the advantage of being able to be applied to the security element with a lateral positional accuracy that is significantly higher than the printing process. Furthermore, the printing process for applying the IR-absorbing properties to the security element can be completely omitted, so that the manufacturing effort is also significantly reduced due to the saving of the entire process step.

[0023] The described grating structures can be combined with known optically embossed structures such as holograms, micromirrors or known grating structures that produce a color effect when illuminated with the visible wavelength spectrum. The grating structures can be applied both as a transfer element to the value document and also directly integrated into the printed image of the value document.

[0024] As grid structures, metal wire grids and / or slit grids and also metal nanopore arrays or nanodot arrays can also be used. Such grid structures are known from the literature and likewise show optical resonance effects in the NIR range. See the literature: H. Lochbihler, “Surface polaritons on metallic wire gratings studied via power losses.” Physical Review B 53.15 (1996): 10289; J. A. Porto et al., “Transmission resonances on metallic gratings with very narrow slits.” Physical review letters 83.14 (1999): 2845; T. W. Ebbesen et al., “Extraordinary optical transmission through sub-wavelength hole arrays.” nature 391.6668 (1998): 667-669; H. Lochbihler, “Sicherheitselement mit farbfilterendem Gitter”, AZ 10 2015 008 655.3.

[0025] A method for producing a security element for value documents, such as banknotes, checks or the like, is also disclosed. The security element has a front side and a back side. In the method, a substrate is provided. A grid structure extending over a region is introduced in the substrate, and a reflective layer is applied on the grid structure. The grid structure shows at least one color in the visible spectral range in plan view and / or in transmission, and subregions are introduced in the grid structure in which the grid structure is respectively uniformly configured within the subregion. Between the subregions, the grid structure differs in at least one grid structure parameter, so that the subregions are not distinguishable in the visible spectral range for the at least one color, but are different in the near-infrared range with regard to their absorption and / or reflection.

[0026] The security elements are preferably manufactured using a photolithography process, in particular in an electron beam device. Devices based on two-photon absorption are also a feasible alternative. Other known methods for manufacturing are interference lithography or mechanical ruling. Known manufacturers of such devices are, for example, Nanoscribe GmbH & Co. KG, Hermann-von-Helmholtz-Platz 6, 76344 Eggenstein-Leopoldshafen, Germany.

[0027] In the manufacturing method, first an exposure master is produced. These exposure masters are replicated by electroforming or using photopolymers (e.g. Ormocere) after development of the photoresist. Such masters can also be combined with other master structures, for example by a nanoimprint process, in exact register. This method is particularly suitable for combining with other known structures such as embossed holograms, micromirror arrays, moth-eye structures or known imprinted nanostructures to produce structural colors. The masters thus produced are replicated by an electroforming or nanoimprint process. Furthermore, a multiple arrangement of the master pattern is required by thermal imprinting or by nanoimprinting on a template in order to manufacture an imprinted cylinder for subsequent replication. Such an imprinted cylinder finally allows the replication of the master structure onto a film in a roll-to-roll process on a UV lacquer in succession. Here, either thermal imprinting or nano-casting in the UV lacquer can be employed. Finally, the structured film is metallized. Commonly used metallization processes are electron beam evaporation, thermal evaporation or sputtering. Particularly suitable as a metallization are Al, Ag, Au, Ni or Cr and alloys of these metals. The thickness of the metallization ranges from approximately 10 nm to approximately 150 nm. Alternatively, the grid structure can also be metal-printed, for example coated with Supersilber. 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 the environment thereafter. The security elements thus produced can be applied to value documents in different ways.

[0028] The security elements can be designed as transfer elements, i.e. film elements, which additionally have an adhesive layer on the back in order to be applied to value documents. Such transfer elements are, for example, security threads, LEAD strips or patches and are mainly used as features which can be perceived by the human eye. Known optically variable effects of these features are formed by embossed holograms, micromirror arrays, microlens arrays or color-shifting coatings. These transfer elements are difficult to carry out a machine authenticity check when illuminated with light of the visible wavelength spectrum. This disadvantage is exploited by counterfeiters in practice, so that, in the present invention, the known optical transfer elements are supplemented by the above-mentioned grid structure as a security feature which can be recognized in the NIR range in order to achieve a machine authenticity check and to change the optical appearance of the transfer element when illuminated with different NIR wavelength radiation.

[0029] The security element can also have a substrate which forms 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 in the printed area of a banknote. In this case, the security element can also be overprinted with a printing ink.

[0030] It is also known for value documents to be nanostructured over the entire surface, which are known, for example, from the already mentioned DE 102 014 018 551 A1, the optical appearance, i.e. the printed image, of which is formed entirely by nanostructures. These value documents can advantageously be equipped with the above-mentioned grid structure as a security feature in the NIR range to ensure machine authentication of the thin-film element and thus to increase the security against forgery. In the thin-film composite, the front side of the security element faces inwards, the back side of the security element faces outwards.

[0031] The application is explained in more detail below with reference to the drawings, which likewise disclose essential features of the application. The examples are merely intended to illustrate the application and should not be interpreted as being restrictive. For example, the description of an example with a plurality of elements or components should not be interpreted as meaning that all these elements or components are essential for the implementation. Rather, other examples can also comprise alternative elements and components, fewer elements or components or additional elements or components. The elements or components of different examples can be combined with one another, unless stated otherwise. Changes and modifications described for one example can also be applied to other examples. In order to avoid repetitions, identical or corresponding elements are denoted by the same reference signs in different figures and are not explained again. In the drawings:

[0032] Figure 1 a value document is shown when illuminated with light in the visible spectral range;

[0033] Figure 2 a value document according to Figure 1 is shown when illuminated with radiation in the near-infrared range;

[0034] Figure 3 a value document in another example is shown when illuminated with light in the visible spectral range;

[0035] Figure 4 a value document according to Figure 3 is shown when illuminated with radiation in the near-infrared range;

[0036] Figure 5 a value document in another example is shown when illuminated with light in the visible spectral range;

[0037] Figure 6 a value document according to Figure 3 is shown when illuminated with radiation in the near-infrared range;

[0038] Figure 7 A one-dimensional periodic grid structure is shown in a cross-sectional view;

[0039] Figure 8A A two-dimensional periodic grid structure with three different sub-regions is shown in a top view;

[0040] Figure 8B A two-dimensional periodic grid structure with three different sub-regions is shown in an oblique view;

[0041] Figure 9A A measurement geometry of an optical sensor under diffuse reflection is shown;

[0042] Figure 9B A measurement geometry of an optical sensor under transmission is shown;

[0043] Figure 10A Emission spectra of two different IR-LEDs, transmission curves of two IR filters and quantum efficiency of a silicon detector are shown;

[0044] Figure 10B Emission spectra of two different IR-LEDs and transmission curves of two IR filters are shown;

[0045] Figure 11A Reflection of a two-dimensional periodic grid structure as a function of wavelength for different modulation heights is shown;

[0046] Figure 11B Reflection of two different two-dimensional periodic grid structures as a function of modulation height is shown;

[0047] Figure 12 Color perception of a two-dimensional periodic grid structure in the CIE color space in reflection is shown;

[0048] Figures 13A to 13C Color perception of a two-dimensional periodic grid structure in the LCH color space in reflection is shown;

[0049] Figure 14 Reflection of a one-dimensional periodic grid structure as a function of wavelength for different periods is shown;

[0050] Figure 15 Emission spectra of two different IR-LEDs and transmission curves of two IR filters are shown;

[0051] Figure 16 Reflection of two different one-dimensional periodic grid structures as a function of period is shown;

[0052] Figure 17 Color perception of a one-dimensional periodic grid structure in the CIE color space in reflection is shown;

[0053] Figures 18A to 18C Color perception of a one-dimensional periodic grid structure in reflection in the LCH color space is shown;

[0054] Figure 19 Relationship of reflection of a one-dimensional periodic grid structure to modulation depth is shown;

[0055] Figure 20 Relationship of transmission of a one-dimensional periodic grid structure to modulation depth is shown;

[0056] Figures 21A to 21C Color perception of a one-dimensional periodic grid structure in reflection in the LCH color space is shown;

[0057] Figures 22A to 22C Color perception of a one-dimensional periodic grid structure in transmission in the LCH color space is shown;

[0058] Figure 23 Valuable document with security element designed as a shifting element is shown;

[0059] Figure 24 Valuable document with embedded security element is shown;

[0060] Figure 1 Valuable document 1 is shown, to the front side 2 of which a security element 4 is applied. A first printed element 6 and a second printed element 8 are also applied to the front side 2. Figure 1 The valuable document 1 is shown in the case of observation in reflection and / or transmission when illuminated with light of the visible wavelength spectrum. The area in which the security element 4 is applied presents a uniform color to the outside observer.

[0061] Figure 2 The same valuable document 1 is shown in the case of illumination with radiation of wavelengths in the near-infrared range (NIR range). The security element 4 is applied to an area of the front side 2 of the valuable document 1. This area has a first subarea 10, a second subarea 12 and a third subarea 14. These subareas 10 to 14 can only be recognized in reflection when illuminated with radiation of wavelengths in the NIR range. Likewise, the subareas 10 to 14 can also appear in transmission when illuminated with radiation of wavelengths in the NIR range. When illuminated with light of the visible wavelength spectrum, the subareas 10 to 14 appear to the outside observer as shown in Figure 1 they produce a uniform color impression and thus cannot be recognized in either transmission or reflection.

[0062] Each of the sub-regions 10 to 14 exhibits a different absorption behavior in the NIR range than the other sub-regions 10 to 14. For example, the first sub-region 10 absorbs radiation of a first wavelength in the NIR range, sub-region 12 absorbs radiation of a second wavelength in the NIR range, and sub-region 14 absorbs radiation of other different wavelengths in the NIR range. Therefore, the safety element displays different patterns under reflection or transmission when illuminated with different NIR wavelengths, and these patterns are hidden again when illuminated with other NIR wavelengths. When illuminated with light of the visible wavelength spectrum, sub-regions 10 to 14 are invisible, and the safety element 4 appears as a single color.

[0063] Figure 3 Another embodiment of the valuable document 1 is shown, wherein the security element 4 is divided into a first region 16, a second region 18, and a third region 20. When illuminated with light of the visible wavelength spectrum, an observer viewing the front 2 from above and / or under transmission sees according to... Figure 3 The appearance of the first area 16 is presented to an external observer as a first color, the second area 18 as a second color, and the third area 20 as a third color, so that the safety element 4 presents a color gradient, pattern, or similar effect when illuminated with visible light.

[0064] Figure 4 It shows Figure 3 The valuable document 1 in the implementation method describes the situation when irradiated with radiation of wavelengths in the NIR range. For example... Figure 2 As shown, the first region 16 is divided into the first sub-region 10, the second sub-region 12, and the third sub-region 14. However, the second region 18 is also additionally divided into the fourth sub-region 22, the fifth sub-region 24, and the sixth sub-region 26. Similarly, the third region 20 is divided into the seventh sub-region 28, the eighth sub-region 30, and the ninth sub-region 32.

[0065] Each sub-region 10 to 14 and 22 to 32 may exhibit different absorption behavior in the NIR range than other sub-regions 10 to 14 and 22 to 32, but one or more sub-regions 10 to 14 and 22 to 32 may also be identical. All sub-regions 10 to 14 and 22 to 32 may absorb radiation of different wavelengths in the NIR range; some sub-regions 10 to 14 and 22 to 32 may also absorb light of the same wavelength. Therefore, different patterns are displayed when illuminated with different wavelengths in the NIR range, and these patterns disappear when illuminated with other wavelengths in the NIR range. Preferably, multiple sub-regions 10 to 14 and 22 to 32 may exhibit the same diffuse when illuminated with light of the same wavelength in the NIR range. When illuminated with wavelengths of the visible light spectrum, sub-regions 10 to 14 and 22 to 32 are not visible, and the safety element 4 presents a pattern according to... Figure 3 Color gradients or patterns.

[0066] In Figure 5 and Figure 6 , similar to Figure 1 and Figure 2 and Figure 3 and Figure 4 another embodiment of the value document 1 is shown, in which as security element 4 an image element 34 is applied to the front side 2 of the value document 1 Figure 5 . This image element 34 disappears upon irradiation with radiation in the NIR range and a QR code 36 appears, as shown in Figure 6 . It is likewise possible that the QR code 36 only appears at specific wavelengths in the NIR range, while at other wavelengths a different pattern is revealed in reflection and / or transmission. Furthermore, the pattern / coding / image can be different in transmission and reflection upon irradiation with radiation at a specific wavelength in the NIR range, while it can be the same upon irradiation with radiation at another wavelength in the NIR range.

[0067] The described different behavior of the security element 4 upon irradiation with light in the visible spectral wavelength range and light in the NIR range enables a machine authenticity check of the value document 1. An unexpected anti-counterfeiting effect is created, making the replicability of the value document 1 difficult.

[0068] The optical appearance upon irradiation with visible light and the behavior in the NIR range is created by a metallized grid structure, as exemplarily shown in Figure 7 , 8a and 8b. As grid structure it can be a linear periodic subwavelength structure as shown in Figure 7 , but also a two-dimensional periodic grid as shown in Figures 8a and 8b.

[0069] Figure 7The security element 4 is shown in a sectional view. The security element 4 has a double wire grid 38 embedded in a substrate 37 as an example of a one-dimensional periodic grid structure. The first wire grid structure of the double wire grid 38 consists of first grid bars 40 having a width a, which extend in the longitudinal direction perpendicular to the plane of the drawing. Between the first grid bars 40 there are first grid gaps 42 having a width b. The thickness of the first grid bars 40 is denoted by t. At a modulation height h above the first grid bars 40 there is a second wire grid structure having second grid bars 44; the second wire grid structure has a width b. The second wire grid structure is phase-shifted relative to the first wire grid structure, so that the second grid bars 44 are located above the first grid bars 40. At the same time, the second grid gaps 46 present between the second grid bars 44 are located above the first grid bars 40. The thickness t is smaller than the modulation height h, so that no continuous film is formed by the grid bars 40 and 44. It is important that the modulation height h, i.e. the height difference between the first wire grid structure and the second wire grid structure, is greater than the sum of the thicknesses of the grid bars 40 and 44, otherwise no separation between the two wire grid structures would exist. Figure 7 The security element 4 reflects the incident radiation as reflected radiation R. In addition, part of the radiation is transmitted as transmitted radiation T. The reflection and transmission properties depend on the angle of incidence Θ, as explained in more detail with reference to WO 2013 / 053435 A1.

[0070] Figure 8A and 8B The security element 4 is shown, which has a two-dimensional periodic grid structure, the grid structure having elevations 50 raised relative to a flat area 48. Both the flat area 48 and the elevations 50 are provided with metallizations 52.

[0071] Figure 8A The security element 4 is shown in a plan view. The grid structure 49 can be seen, which is divided into sub-areas 10 to 14. Between the sub-areas, the grid structure parameters such as the modulation height, the period, the area fill factor, etc. change.

[0072] Figure 8B The change in the modulation height h is exemplarily shown in an isometric view. In the sub-area 10 of the security element 4, the elevations 50 have a first modulation height hi, in the second sub-area 12 a second modulation height h2, and in the third sub-area 14 a third modulation height h3. From the first sub-area 10 to the third sub-area 14, the modulation height decreases, i.e. h3> h2> hi. Preferably, by such a change, a uniform color impression of the sub-areas 10 to 14 is produced under reflection and / or transmission when illuminated with light of the visible wavelength spectrum, but the optical impression in the sub-areas 10 to 14 is different when illuminated with radiation in the NIR range.

[0073] Figure 9A and 9BThe measurement geometry of an optical sensor is shown. Figure 9A An optical sensor for measuring diffuse reflection is shown, Figure 9B An optical sensor for measuring transmission is shown. The optical sensor consists of a radiation source 54, an optical filter 56 and a detector 58. Furthermore, in Figure 9A and 9B A value document 1 to be examined is shown. In such a measurement geometry, the absorption properties of the grid structure in the NIR range are investigated. Thus, in Figure 9A and 9B In the embodiment of the optical sensor, the radiation source 54 is an IR-LED, the optical filter 56 is an IR filter and the detector 58 is a silicon line detector.

[0074] In order to be able to estimate the measurement signal of such an IR sensor, the emission spectrum of the radiation source 54, the spectral emission of the filter 56 and the spectral sensitivity of the detector 58 need to be known. Figure 10A and 10B The diagram in and shows the radiation intensity of the wavelength on the y-axis, the wavelength on the x-axis in pm. Figure 10A The curve of an optical sensor with a filter with uniform spectral emission is shown. The emission spectrum of a first radiation source 60, here an IR-LED, the transmission curve of a first filter 62, here an IR filter, and the quantum efficiency of a detector 64, here a silicon detector, are shown. Suitable as a radiation source is an IR-LED with a centroid wavelength in the NIR range. Figure 10B The curve of two spectrally different radiation sources 54 with corresponding optical filters 56 (edge filters) is shown. In addition to the emission spectrum of the first radiation source 60 and the transmission curve of the first filter 62, the emission spectrum of a second radiation source 66, here a second IR-LED, and the transmission curve of a second filter 68, here a second IR filter, are shown. The latter embodiment offers the possibility of an IR contrast of the analytical feature relative to the first variant. In a diffuse arrangement, the measurement signal I of the sensor can be estimated as follows:

[0075] I = ∫ E(λ) * Q(λ) * T F (λ) * R G (λ) dλ

[0076] Or for a transmission geometry:

[0077] I = ∫ E(λ) * Q(λ) * T F (λ) * T G (λ) dλ,

[0078] where E(λ) is the spectral emission of the IR-LED, Q(λ) is the quantum efficiency of the detector, T F(λ) is the transmittance of the edge filter, R G (λ) or T G (λ) is the reflectance or transmittance of the grating structure. In Figure 10B case, the sensor layout consists of two radiation sources with different emission spectra E1(λ) and E2(λ). The detector consists of two detector rows, wherein the first row is equipped with an edge filter with the transmission characteristic T F1 (λ) and the second row is equipped with an edge filter with the transmission characteristic T F2 (λ). This sensor layout provides two different signals in the respective detector rows, which for a diffuse layout are given, for example, by

[0079] I1=∫E1(λ)*Q(λ)*T F1 (λ)*R G (λ)dλ

[0080] I2=∫E2(λ)*Q(λ)*T F2 (λ)*R G (λ)dλ.

[0081] By analyzing the ratio I1 / I2, the measurement accuracy of the sensor layout can be improved.

[0082] The thus determined values are used to further estimate the IR signal.

[0083] First, the spectral reflection of a two-dimensional periodic grating as shown in Figure 8A and 8B is investigated.

[0084] Figure 11A The spectral reflection of a two-dimensional periodic grating with constant period d = 240 nm in the NIR range as a function of the wavelength is shown for different modulation heights h from 240 nm to 300 nm at an angle of incidence of 8° (reflectance in % is shown on the y-axis and the wavelength in pm is shown on the x-axis). The incident radiation is unpolarized. The parameters of the two-dimensional periodic grating are b = 120 nm, n = 1.52 and coated with aluminum with a thickness t = 40 nm. The modulation heights hi to h7 are shown in the diagram, wherein the modulation height h decreases by 10 nm from the first modulation height hi to the seventh modulation height h7, wherein hi = 300 nm and h7 = 240 nm.

[0085] It can be seen from Figure 11A that the reflectance in the NIR range decreases significantly with increasing modulation depth h. The peak of the resonant light absorption moves from the red into the infrared region. This absorption is equivalent to the excitation of surface plasmons (see above). The signal of the optical sensor according to Figure 9A and 9B can be derived by means ofFigure 10A or the spectral curve of 10B is calculated by convolving the spectral curve with Figure 11A the reflectance of 10A according to the above formula.

[0086] The results of the above calculation are shown in Figure 11B for two different LED and filter curves of Figure 10B . Figure 11B The modulation height in pm on the x-axis is plotted against the reflected signal in % on the y-axis measured by the sensor layout. The first curve kl is for the emission spectrum of the first radiation source 66 and the transmission curve of the first filter 68, the second curve k2 is for the emission spectrum of the second radiation source 60 and the transmission curve of the second filter 62, wherein curve kl corresponds to the spectral properties of curves 66, 68 in Figure 10B and curve k2 corresponds to the spectral properties of curves 60, 62 in Figure 10B . It can be seen that the modulation depth h = 300 nm shows a high contrast in the NIR range with respect to the used radiation sources, whereas the modulation height h = 250 nm shows almost no difference.

[0087] It has been set out that a further requirement of the security element 4 is that it is not visible, or at least hardly perceptible, when observed in the visible spectral range. Therefore, Figure 12 and 13A Figs. 1 to 13C show the color perception in the CIE and LCh color space for a two-dimensional periodic grid structure with a modulation height varying between 250 nm and 300 nm. For this variation of the modulation height, the contrast in the NIR range when using different radiation sources and filters has also been investigated before.

[0088] Figure 12 shows the color coordinates x and y in the CIE color space, Figures 13A to 13C shows the individual color values in the LCH color space in relation to the modulation height h on the x-axis. Figure 13A shows the modulation height h on the x-axis in pm against the luminance in % on the y-axis, Figure 13B shows the modulation height h on the x-axis in pm against the chroma in % on the y-axis, Figure 13C shows the modulation height h on the x-axis in pm against the hue as the angle of the color on the y-axis. It can be seen that the analyzed two-dimensional periodic structure appears greenish in reflection. The variation of the modulation height hardly changes the color appearance. Thus, although structured in the NIR range, the same color, i.e. green, is maintained in the visible spectral range.

[0089] This is merely an example of a grating structure; by changing the grating structure parameters, the grating structure exhibits different IR characteristics while maintaining its visible light appearance. Several other variations of the two-dimensional periodic grating structure can be considered to achieve similar results. Other grating structure parameters, such as the period d or the area fill factor, can also be changed without altering the modulation height h.

[0090] Figure 14 In the NIR range, for periods d = 270–370 nm, reflectance (y-axis) in units of % as a function of wavelength (x-axis) in units of μm is shown for a one-dimensional periodic grating structure coated with aluminum with a constant modulation height h = 300 nm, where the width b = d / 2, t = 30 nm, n = 1.52, and the angle of incidence is 8°. Curves are shown for the first period d1 = 270 nm, the second period d2 = 290 nm, the third period d3 = 310 nm, the fourth period d4 = 330 nm, the fifth period d5 = 350 nm, and the sixth period d6 = 370 nm. The corresponding one-dimensional periodic grating structure is shown below. Figure 7 As shown.

[0091] As can be seen, the minimum intensity shifts towards longer wavelengths as the period d increases. The minimum intensity coincides with the maximum light absorption, which is due to the excitation of surface plasmon resonances.

[0092] Figure 15 The optical characteristics of a sensor layout with two detector rows and two radiation sources with different spectral characteristics are shown. The view is similar to... Figure 10B The only difference is that the spectral characteristics of the IR-LED or edge filter are changed and used for subsequent signal calculations.

[0093] Figure 16 and Figure 11B Similarly, it shows the targeting Figure 15 The calculation results for the two different LED and filter curves have been explained. Figure 16 The period on the x-axis, in μm, corresponds to the reflectance on the y-axis, in % of the time. Figure 16 The sensor measures the value of the reflection. Figure 14 The IR intensity of the one-dimensional periodic grid parameters in the NIR range is a function of the period d = 270-370 nm. The third curve k3 is generated from the emission spectrum of the third radiation source 70 and the transmission curve of the third filter 72, and the fourth curve k4 is generated from the emission spectrum of the fourth radiation source 74 and the transmission curve of the fourth filter 76. It can be seen that, for a period d = 280 nm, the two detector rows exhibit good contrast in the NIR range. In contrast, the two detector rows provide approximately the same signal for a period d = 330 nm.

[0094] Figure 17 The color perception in the CIE color space is shown for Figure 14 parameters of a one-dimensional periodic grid in reflection for variable periods d = 270 - 370 nm in the visible wavelength range. Figures 18A to 18C Analogously to Figures 13A to 13C , the color perception in the LCH color space is shown, with the difference that on the x-axis the period d is used instead of the modulation height h. The perceived color range goes from neutral shades via green, yellow to reddish colors. In contrast, the color brightness remains approximately constant Figure 18A .

[0095] Furthermore, in Figure 19 the IR signal (y-axis, %) in reflection for a one-dimensional periodic grid structure at constant period d = 250 nm is investigated as a function of the modulation height h (x-axis) from 100 nm to 450 nm. The remaining parameters correspond to those of the grid of Figure 14 . Figure 20 The results measured in transmission (y-axis, %) under the same conditions are shown.

[0096] It can be seen that the IR signal can be changed significantly by a corresponding selection of the modulation height h. At h ~ 250 nm, the detector rows show a maximum in reflection, wherein the reflection of the second detector row has a minimum. In contrast, in transmission both detector rows have a good contrast at h ~ 400 nm. This can be used to produce a contrast effect in the NIR range.

[0097] Figures 21A to 21C The color perception in the LCH color space in reflection is shown for a one-dimensional periodic grid with constant period d = 250 nm and variable modulation height h = 100 to 450 nm. Figures 22A to 22C The case in transmission is shown. The scale on the y-axis is the same as in Figures 13A to 13C .

[0098] It is shown that the visible brightness change is relatively small, both in reflection and in transmission. For reflection, the hue is in the green, yellow region, for transmission the hue is in the blue region. This shows that, for example, the two grid parameters, in particular the modulation height, can be selected such that there is a good contrast in the NIR range and the visible color appearance is hardly changed.

[0099] Finally, Figure 23 and 24 a value document 1 with a security element 4 is shown in a sectional view. In Figure 23In the middle, the security element 4 is applied as a transfer element onto the front side 2 of the substrate 77 of the value document 1. On the back side 78 of the substrate 77, a first printed layer 80 is applied. On the front side 2, a second printed layer 82 is applied. The security element 4 is also overprinted with the second printed layer 82.

[0100] Figure 24 The security element 4 is shown, which is directly processed into the substrate 77 of the value document 1. One embodiment for this is a value document structured over the entire surface, the visual appearance of which is generated without the use of printed inks. Other embodiments are value documents on which the structures described here are directly applied to the base material, for example by a transfer process, and subsequently overprinted with a pigment ink.

[0101] List of reference signs

[0102] 1 value document

[0103] 2 front side

[0104] 4 security element

[0105] 6 first printed element

[0106] 8 second printed element

[0107] 10 first subregion

[0108] 12 second subregion

[0109] 14 third subregion

[0110] 16 first region

[0111] 18 second region

[0112] 20 third region

[0113] 22 fourth subregion

[0114] 24 fifth subregion

[0115] 26 sixth subregion

[0116] 28 seventh subregion

[0117] 30 eighth subregion

[0118] 32 ninth subregion

[0119] 34 image element

[0120] 36 QR code

[0121] 37 substrate

[0122] 38 double-line grid

[0123] 40 first grid bar

[0124] 42 first grid gap

[0125] 44 second grid bar

[0126] 46 second grid gap

[0127] 48 flat area

[0128] 50 protrusion

[0129] 52 metallization layer

[0130] 54 radiation source

[0131] 56 optical filter

[0132] 58 detector

[0133] 60 emission spectrum of first radiation source

[0134] 62 transmission curve of first filter

[0135] 64 quantum efficiency of detector

[0136] 66 emission spectrum of second radiation source

[0137] 68 transmission curve of second filter

[0138] 70 emission spectrum of third radiation source

[0139] 72 transmission curve of third filter

[0140] 74 emission spectrum of fourth radiation source

[0141] 76 transmission curve of fourth filter

[0142] 77 substrate

[0143] 78 back side

[0144] 80 first printed layer

[0145] 82 second printed layer

[0146] d period

[0147] d1 first period

[0148] d2 second period

[0149] d3 third period

[0150] d4 fourth period

[0151] d5 fifth period

[0152] d6 sixth period

[0153] h modulation height

[0154] h1 first modulation height

[0155] h2 second modulation height

[0156] h3 third modulation height

[0157] h4 fourth modulation height

[0158] h5 fifth modulation height

[0159] h6 sixth modulation height

[0160] h7 seventh modulation height

[0161] k1 first curve

[0162] k2 second curve

[0163] k3 third curve

[0164] k4 fourth curve

[0165] L longitudinal

[0166] R reflected radiation

[0167] T transmitted radiation

[0168] Θ angle of incidence

Claims

1. A security element for manufacturing valuable documents such as banknotes, checks, or the like, said security element - It has a front (2) and a back (78), - It has a substrate (37) having a grid structure (38, 49) extending over a region, and a reflective layer (52) on the grid structure, and - The grating structure (38, 49) displays at least one color in the visible spectrum under top view and / or transmission, characterized in that, - The grating structure has sub-regions (10-14, 22-32) in which grating structures (38, 49) are uniformly constructed, wherein the grating structures (38, 49) differ between the sub-regions (10-14, 22-32) in at least one grating structure parameter (h, d), such that the sub-regions (10-14, 22-32) are indistinguishable in terms of at least one color in the visible spectrum, but differ in terms of absorption and / or reflection in the near-infrared range.

2. The safety element according to claim 1, wherein the safety element displays a view that can be seen by the naked eye from the front (2).

3. The safety element according to any one of the preceding claims, characterized in that, At least one sub-region (10-14, 22-32) absorbs radiation in the wavelength range of 780 nm to 1100 nm.

4. The safety element according to any one of the preceding claims, characterized in that, The grid structure parameters that differ from each other in the grid structures (38, 49) in the sub-regions (10-14, 22-32) include the modulation height (h) of the grid structures (38, 49).

5. The safety element according to any one of the preceding claims, characterized in that, The at least two sub-regions (10-14, 22-32) absorb radiation in the wavelength range of 780 nm to 1100 nm, but the at least two sub-regions have different spectral absorption characteristics in the wavelength range.

6. The safety element according to any one of the preceding claims, characterized in that, The grating structure has a one-dimensional periodic subwavelength structure (38) in at least one sub-region (10-14, 22-32), the one-dimensional periodic subwavelength structure having grating strips (40, 44) and grating gaps (42, 46).

7. The safety element according to any one of the preceding claims, characterized in that, The grid structure has a two-dimensional periodic subwavelength structure (49) in at least one sub-region (10-14, 22-32), the two-dimensional periodic subwavelength structure having protrusions (50) and flat regions (48).

8. The safety element according to any one of the preceding claims, characterized in that, The security element additionally has an adhesive layer on its back side (78) so as to be applied to the valuable document (1) as a transfer element.

9. The safety element according to any one of the preceding claims, characterized in that, The substrate (37) is part of the valuable document (1), and the security element (4) is directly introduced into the substrate (77) of the valuable document (1).

10. The safety element according to any one of the preceding claims, characterized in that, The substrate (37) is part of a thin film composite, wherein the front side (2) of the safety element (4) faces inward in the thin film composite, and the back side (78) of the safety element (4) faces outward.

11. A method for manufacturing a security element for valuable documents such as banknotes, checks, or the like, wherein... - The safety element (4) has a front (2) and a back (78), - Provide matrix (37), - A grid structure (38, 49) extending over the region is introduced into the substrate (37). - A reflective layer (52) is applied to the grid structure (38, 49), and - The grid structures (38, 49) display at least one color in the visible spectrum when viewed from above and / or transmitted light. Its features are, Sub-regions (10-14 and 22-32) are introduced within the grating structures (38, 49), in which the grating structures (38, 49) are uniformly constructed, wherein the grating structures (38, 49) differ in at least one grating structure parameter (h, d) between the sub-regions (10-14 and 22-32), such that the sub-regions (10-14 and 22-32) are indistinguishable in terms of at least one color in the visible spectrum, but differ in terms of their absorption and / or reflection in the near-infrared range.

12. A valuable document having a security element (4) according to any one of claims 1 to 10.

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