Value document and method for checking a value document

Abstract

The invention relates to a security document, such as a banknote. It comprises a substrate body (2) on the front (3) of which a foil element (8) is arranged, a first IR absorber (6) which is located below the foil element (8) and which has a first reflection profile R1(λ), ​​a first spectrum complementary to R1(λ), ​​and an IR absorption maximum at a wavelength λ1, and a second IR absorber (10) which is applied to or embedded in the foil element (8), is located at least partially above the first IR absorber (6), and which has a second reflection profile R2(λ), a second spectrum complementary to R2(λ), and an absorption maximum at a wavelength λ2. The first IR absorber (6) has a remission R1(λ1) of 30% to 80% at wavelength λ1.The second IR absorber (10) has an opacity O2(λ2) of 1.1 to 10 at wavelength λ2, and the first spectrum differs from the second spectrum in such a way that there is a cosine similarity between the two spectra in the range of 0.1 to 0.9.

Description

[0001] The invention relates to a value document, such as a banknote, a check, a credit or other payment card, an identification card or the like, comprising: a substrate body having a front side and a back side and on the front side of which a film element is arranged, a first IR absorber which, as viewed towards the front side, is located below the film element and which has a first, spectrally dependent remission curve R1(λ), ​​a first spectrum which is complementary to the remission curve R1(λ) and is thus defined by 1 - R1(λ), ​​and an IR absorption maximum A1max at a wavelength λ1, and a second IR absorber which is applied to the film element or embedded in the film element, is located at least partially above the first IR absorber as viewed towards the front side, and which has a second, spectrally dependent remission curve R2(λ), a second spectrum,which is complementary to the remission curve R2(λ) and is thus defined by 1 - R2(λ) and has an absorption maximum A2max at a wavelength λ2.

[0002] Such security elements, which comprise film elements with printed IR absorbers that make these film elements machine-readable for IR sensors, are known from WO 2018 / 147966 A1.

[0003] Optically variable foil-based security elements (OVI foils), such as transfer security foil strips, hologram strips, micromirror strips, or patches, are widely used on valuable documents, such as banknotes, as a proven security feature. However, as a security element that can be identified with the naked eye without any additional aids (so-called Level 1 features), they are often subject to so-called harvesting attacks, in which a counterfeiter removes the security element from a genuine banknote and applies it to a counterfeit, such as a photocopy, in order to enhance the security by applying the genuine security element.

[0004] DE 10 2019 007 417 A1 describes a security element with two different IR absorbers. If such a security element is removed in a harvesting attack and applied to another substrate, it is not detectable that it has been transferred. This creates precisely the problem described above for a common Level 1 security feature.

[0005] EP 3 922 476 A1 describes a security document with a substrate body containing a specially IR-luminescent substance and a foil element with an IR absorber. A transfer process would be detected here; however, this requires special sensors for IR luminescence, which are generally not available in banks, ATMs, etc.

[0006] DE 10 2021 003 697 A1 describes a security document in which the protection of a substrate body and a security element, which is located as a foil element on the substrate body, are combined to make a transfer process of the foil element detectable. The use of IR absorbers whose absorption spectra are spectrally disjoint, i.e., non-overlapping, is discussed. However, IR absorber pigments that absorb spectrally narrowband and completely disjointly and simultaneously meet the requirements for banknote use with regard to stability, non-toxicity, printability, etc., cannot be found in reality, so the implementation of this concept is not or hardly possible.

[0007] In reality, IR absorbers with spectrally overlapping absorption spectra are typically used. However, due to the subtractive spectral mixing inherent in overlaying or overprinting, these can generally no longer be detected independently of one another. An example from the visible spectral range can illustrate this: Overprinting a red ink layer with a green ink layer generally produces a dark brown / gray color impression as a mixed color without a usable spectral signature by which the individual color layers could still be identified. The situation is analogous for spectrally overlapping IR absorbers.

[0008] EP 2 513 875 A1 describes an inverse spectral sensor for measuring individual spectra.

[0009] The invention is based on the object of specifying a security element and a testing method therefor, wherein the security element uses IR absorbers that are superimposed in plan view and yet can be detected individually.

[0010] The invention is defined in independent claims 1 and 10. The dependent claims relate to preferred developments.

[0011] A valuable document, such as a banknote, a check, a credit or other payment card, an identification card, or the like, is provided. The valuable document comprises a substrate body having a front and a back. Polymer substrates, such as PET substrates, paper substrates, or combined paper and polymer substrates, can be used as the substrate body.

[0012] A foil element is arranged on the front side of the substrate body. The foil element preferably comprises a dielectric made of a polymer, preferably PET. It can be a single-piece or multi-piece, consisting of several foil sub-elements.

[0013] The foil elements can be designed as transfer elements, i.e., a transfer foil is removed after being transferred to the valuable document. Furthermore, the security elements can contain a foil that is transparent at least in the wavelength range of visible light, which remains in the security element structure even after being applied to the valuable document. The latter security elements are preferably used to cover through-openings in valuable documents. Both security elements frequently use diffractive (holograms), reflective (micromirrors), or refractive (microlenses) optical elements to create optically variable effects.

[0014] Viewed from the front side of the substrate body, a first IR absorber is located beneath the film element. It has a first, spectrally dependent remission curve R1(λ), ​​a first spectrum complementary to the remission curve R1(λ) and thus defined by 1 - R1(λ), ​​and an IR absorption maximum A1max at a wavelength λ1. The first spectrum can also be referred to as the complementary spectrum to the remission spectrum R1(λ) and can equally be referred to as the B1(λ) spectrum, with B1(λ) = 1 - R1(λ).

[0015] The first IR absorber is firmly bonded to the substrate body. This is achieved, for example, by printing or coating the substrate body with a suitable printing ink, which, in the case of a paper substrate, bonds well to cotton fibers or, in the case of a polymer substrate, to a polymer surface or ink-receiving layer. If the first IR absorber is a printing ink, it contains, for example, IR pigments or IR dyes. Examples of suitable IR pigments or IR dyes are, in particular, iron(II) and / or copper(II) compounds with an Fe2+ ion and / or a Cu2+ ion in a suitable chemical compound, where a suitable chemical compound is, for example, a phosphate ion and / or a polyphosphate ion or, more generally, a phosphorus and / or oxygen-containing group. These IR-absorbing substances absorb light broadband in a sub-range of the near infrared range, at wavelengths from 750 nm to 1100 nm.Preferably, the IR-absorbing substances can also be absorbing at higher wavelengths in the NIR range (from 750 nm to 3000 nm).

[0016] The IR-absorbing substances are preferably transparent in the visible range (400 nm to 750 nm) of the electromagnetic spectrum, preferably having at most a yellowish, greenish, or bluish tint. Suitable IR pigments or IR dyes for the first IR absorber are specified in WO 2007 / 060133 A1. Other suitable IR pigments or IR dyes are: LUMOGEN-S from BASF Corporation, 100 Park Avenue, Florham Park, NJ 07932, the IR-absorbing materials disclosed in GB 2 168 372, which are invisible or transparent in the visible range of the electromagnetic spectrum, and the IR absorbing materials disclosed in US Pat. No. 6,926,764. These IR absorbers are substituted phthalocyanines, naphthalocyanines, metal-containing phthalocyanines or poly-substituted phthalocyanines.

[0017] Thiophenol-substituted copper phthalocyanines, in particular para-toluenethiol-persubstituted copper phthalocyanines, are preferred.

[0018] The printing ink, which is preferably used for the first IR absorber, can additionally contain color pigments that are effective in the visible spectral range. Different visible color pigments can also be used in certain regions, for example to print a visible image with several different colors, but which exhibits a uniform spectral absorption profile across the NIR range or part of it, e.g., in the wavelength range from 750 nm to 2500 nm. The printing ink can be applied as a conventional printing ink using printing processes such as offset printing, gravure printing, flexographic printing, or screen printing, or, e.g., in gravure printing, it can be applied as a primer layer for later application of the film element.

[0019] Spatially, the first IR absorber is arranged beneath the film element. The first IR absorber can also be arranged beneath the film element and simultaneously extend laterally beyond the film element—in this case, the first IR absorber is located beneath and also adjacent to the film element on the substrate body.

[0020] The second IR absorber is firmly bonded to the film element. In principle, the same pigments as for the first IR absorber can be used for the second IR absorber. However, different pigments with the properties specified in the invention must be selected for each of the two IR absorbers so that the two IR absorbers can be distinguished from one another. The second IR absorber can be applied by printing onto the film element or onto a partial film layer on the top or inside of the film element.

[0021] The second IR absorber can also be incorporated onto or into the film element by being incorporated into a top or bottom layer of film, lacquer, or adhesive. The second IR absorber can be applied to the film element as a transfer element or embedded into the film element. The second IR absorber is located at least partially above the first IR absorber when viewed from the front side and has a second, spectrally dependent reflectance curve R2(λ), a second spectrum that is complementary to the reflectance curve R2(λ) and is thus defined by 1 - R2(λ), and an absorption maximum A2max at a wavelength λ2. The second spectrum can equally be referred to as a complementary spectrum or B2(λ) spectrum, with B2(λ) = 1 - R2(λ).

[0022] It is important for reliable security functionality that the second IR absorber is removed or transferred together with the film element during harvesting. This ensures a security check of the film element with the first IR absorber, as well as the substrate body with the first IR absorber on it, and also their combination.

[0023] For harvesting detection, several situations must be reliably distinguished. In a first situation, a genuine value document exists on which the first IR absorber and the second IR absorber are located one above the other, i.e., they are physically superimposed. In a second situation, the substrate body is genuine, but the foil element has been removed. Only the first IR absorber is (still) present. In a third situation, the substrate body is counterfeit, onto which an original foil element has been transferred. Only the second IR absorber is present on the value document. The second and third situations occur in the case of a counterfeit.

[0024] In order to reliably detect counterfeits in the case of spectrally overlapping IR absorbers, both the first IR absorber and the second IR absorber must have specific properties.

[0025] A first essential criterion for reliable detectability of the two IR absorbers and their combined spectrum is that the first spectrum differs from the second spectrum to such an extent that a similarity x between the first and second spectra lies in the range of 0.1 to 0.9. The similarity x is a cosine similarity between the two 1-R(λ) spectra and provides a measure of the spectral overlap of the two IR absorbers. The calculation of the similarity x will be explained in more detail below using an example.

[0026] To specify the overlap, the experimentally more easily accessible remission spectra R(λ), i.e. the spectral distribution of the non-directionally reflected IR radiation, are used instead of the absorption spectra of the two IR absorbers. For this purpose, the two 1 - R(λ) spectra, i.e. the complementary spectra, are first calculated from R(λ) spectra measured against a white background using the equation B(λ) = 1 - R(λ). This results in the first spectrum (1 - R1(λ)) of the first IR absorber and the second spectrum (1 - R2(λ)) of the second IR absorber. Subsequently, the similarity x between the first and second spectrum is calculated, which is given by the cosine similarity of the two spectra over a subsequent spectral range with a specified interpolation point spacing.

[0027] The cosine similarity is a measure of the similarity between two vectors. When referring to a vector in the following, this will be used to

[0028] Illustration in bold. To calculate the cosine similarity, specific wavelengths are defined as support points at which the respective 1-R(λ) value is determined. The determined 1-R(λ) values ​​are summarized as a vector in an n-tuple. This yields a vector v for the first IR absorber and a vector w for the second IR absorber. The cosine of an angle between the two vectors is then determined. The angle between the two vectors v and w can be derived using the scalar product and the Euclidean norm. The abbreviated notation v : = v 2 applies: v ⋅ w = v w cos θ

[0029] The cosine similarity of the two vectors v and w is the cosine of the included angle cos θ = v ⋅ w v w = ∑ i = 1 n v i ⋅ w i ∑ i = 1 n v i 2 ⋅ ∑ i = 1 n w i 2

[0030] The cosine similarity lies between -1 (exactly opposite) and 1 (exactly the same). A value of zero usually means independence (orthogonality). Consequently, the similarity x can assume a value between -1 and 1. According to the invention, a spectral range from 750 nm to 1600 nm with a sampling point spacing of 10 nm between adjacent sampling points in the spectral range is used to calculate the similarity x. For an overlap according to the invention, the similarity x between the first and second spectrum is at least 0.1 and is not higher than 0.9, preferably not higher than 0.8. If the spectral overlap is too strong, a combination spectrum R3(λ) of the two IR absorbers would be too similar to one (or even both) of the reflectance spectra R1(λ) or R2(λ) and could no longer be stably and reliably identified.If the spectrally dependent IR absorptions of the two IR absorbers are completely disjoint, the 1-R(λ) spectra are orthogonal and the similarity x is zero.

[0031] The following calculation example serves to illustrate this: The first IR absorber has n=86 reflectance values ​​R1(750 nm; ...; 1600 nm) for the wavelengths 750 nm; 760 nm; 770 nm; ...; 1590 nm; 1600 nm at a sampling point spacing of 10 nm. These are the following values, for example: R1(750 nm; ...; 1600 nm) = (0.8883; 0.8503; 0.8091; 0.7748; 0.7414; 0.7087; 0.6749; 0.6399; 0.6052; 0.5773; 0.5577; 0.5424; 0.5316; 0.5238; 0.5261; 0.5431; 0.6420; 0.7767; 0.8112; 0.8526; 0.8904; 0.9243; 0.9472; 0.9591; 0.9618; 0.9650; 0.9682; 0.9713; 0.9739; 0.9760; 0.9776; 0.9786; 0.9794; 0.9803; 0.9812; 0.9819; 0.9828; 0.9836; 0.9845; 0.9853; 0.9861; 0.9870; 0.9878; 0.9883; 0.9888; 0.9892; 0.9896; 0.9898; 0.9901; 0.9903; 0.9906; 0.9908; 0.9907; 0.9904; 0.9899; 0.9891; 0.9880; 0.9870; 0.9859; 0.9849; 0.9839; 0.9828; 0.9818; 0.9808; 0.9797; 0.9788; 0.9773; 0.9748; 0.9715; 0.9674; 0.9624; 0.9571; 0.9520; 0.9469; 0.9417; 0.9366) From this, the respective 1 - R1(λ i ) values, i.e. the respective complementary values, calculated by replacing each value R1(λ i ) by 1 - R1(λ i ): 1 - R1(750 nm; ...; 1600 nm) = (1 - R1(750 nm); ...; 1 - R1(1600 nm)) This yields the following vector v = (v1, ..., v86) T< : v = (0.1117; 0.1497; 0.1909; 0.2252; 0.2586; 0.2913; 0.3251; 0.3601; 0.3948; 0.4227; 0.4423; 0.4576; 0.4684; 0.4762; 0.4739; 0.4415; 0.3230; 0.2233; 0.1888; 0.1474; 0.1096; 0.0757; 0.0528; 0.0409; 0.0382; 0.0350; 0.0318; 0.0287; 0.0261; 0.0240; 0.0224; 0.0214; 0.0206; 0.0197; 0.0188; 0.0181; 0.0172; 0.0164; 0.0155; 0.0147; 0.0139; 0.0130; 0.0122; 0.0117; 0.0112; 0.0108; 0.0104; 0.0102; 0.0099; 0.0097; 0.0094; 0.0092; 0.0093; 0.0096; 0.0101; 0.0109; 0.0120; 0.0130; 0.0141; 0.0151; 0.0161; 0.0172; 0.0182; 0.0192; 0.0203; 0.0212; 0.0227; 0.0252; 0.0285; 0.0326; 0.0376; 0.0429; 0.0480; 0.0531; 0.0583; 0.0634) T< The second IR absorber has a support point distance of 10 nm between 750 nm and 1600 nm n=86 Remission values ​​R2(750 nm; ...; 1600 nm) for the wavelengths 750 nm; 760 nm; 770 nm; ...; 1590 nm; 1600 nm: R2(750 nm; ...; 1600 nm) = (0.9627; 0.9598; 0.9566; 0.9531; 0.9492; 0.9450; 0.9403; 0.9351; 0.9294; 0.9231; 0.9161; 0.9083; 0.8997; 0.8902; 0.8679; 0.8251; 0.7691; 0.7014; 0.6312; 0.6103; 0.5921; 0.5773; 0.5666; 0.5603; 0.5586; 0.5612; 0.5679; 0.5779; 0.5906; 0.6053; 0.6213; 0.6381; 0.6551; 0.6719; 0.6884; 0.7043; 0.7194; 0.7337; 0.7471; 0.7597; 0.7715; 0.7825; 0.7927; 0.8021; 0.8109; 0.8191; 0.8267; 0.8337; 0.8402; 0.8463; 0.8520; 0.8572; 0.8622; 0.8667; 0.8710; 0.8750; 0.8788; 0.8823; 0.8855; 0.8886; 0.8915; 0.8942; 0.8968; 0.8992; 0.9014; 0.9036; 0.9056; 0.9075; 0.9093; 0.9110; 0.9126; 0.9141; 0.9155; 0.9169; 0.9181; 0.9194; 0.9205) From these, the respective 1 - R2(λ i ) values, i.e. the respective complementary values calculated by replacing each value R2(λ i ) by 1 - R2(λ i ): 1 - R2(750 nm; ...; 1600 nm) = (1 - R2(750 nm); ...; 1 - R2(1600 nm)) This yields the following vector w = (wi, ..., w 86 ) T< : w = (0.0373; 0.0402; 0.0434; 0.0469; 0.0508; 0.0550; 0.0597; 0.0649; 0.0706; 0.0769; 0.0839; 0.0917; 0.1003; 0.1098; 0.1321; 0.1749; 0.2309; 0.2986; 0.3688; 0.3897; 0.4079; 0.4227; 0.4334; 0.4397; 0.4414; 0.4388; 0.4321; 0.4221; 0.4094; 0.3947; 0.3787; 0.3619; 0.3449; 0.3281; 0.3116; 0.2957; 0.2806; 0.2663; 0.2529; 0.2403; 0.2285; 0.2175; 0.2073; 0.1979; 0.1891; 0.1809; 0.1733; 0.1663; 0.1598; 0.1537; 0.1480; 0.1428; 0.1378; 0.1333; 0.1290; 0.1250; 0.1212; 0.1177; 0.1145; 0.1114; 0.1085; 0.1058; 0.1032; 0.1008; 0.0986; 0.0964; 0.0944; 0.0925; 0.0907; 0.0890; 0.0874; 0.0859; 0.0845; 0.0831; 0.0819; 0.0806; 0.0795) T< .

[0032] Now the similarity x of the two IR absorbers is calculated from the vectors v and w using the above equation (1).

[0033] The result in the example is a similarity x = 0.4642.

[0034] A second criterion for reliable detectability is that the second IR absorber has an opacity O2(λ2) of 1.1 to 10, preferably 1.2 to 4, at a wavelength λ2. The opacity of the second IR absorber can be determined experimentally by a transmission measurement. For this purpose, the diffuse transmission spectrum of the second IR absorber is measured on a transparent substrate, such as the PET film of the film element, in a commercially available transmission spectrometer, e.g., a Perkin Elmer Lambda 1050+. The sample is placed directly at the entrance of an integrating sphere, which also carries the detector, in order to capture not only the directed transmitted light but also the light components scattered by the absorber particles during the measurement. The opacity then results directly from O2(λ2) = 1 / T2(λ2), where T2(λ2) is the transmission of the second IR absorber at wavelength λ2.An opacity O2(λ2) in the range of 1.1 to 10 ensures that the first IR absorber "below" is sufficiently visible through the second IR absorber "above." Only then does a combination spectrum R3(λ) differ sufficiently from both the first remission spectrum R1(λ) and the second remission spectrum R2(λ).

[0035] As a third criterion for reliable detectability, the first IR absorber has a remission R1(λ) of 30% to 80% at wavelength λ1. The remission R1(λ) is measured in a commercially available remission spectrometer, e.g., a Perkin Elmer Lambda 1050+, as the diffuse reflection spectrum of the first IR absorber on a white substrate, such as banknote paper. For this purpose, the sample is placed directly at the exit of the integrating sphere, which also carries the detector, at an 8° tilt to the input beam in order to capture not only the specularly reflected light but also the diffusely reflected light components during the measurement. The uncoated substrate (e.g., banknote paper) serves as a reference and is set to R(λ) = 1.

[0036] This third criterion also ensures the distinguishability of the combination spectrum R3(λ) of the two superimposed IR absorbers from the respective remission spectra R1(λ) and R2(λ). The above-mentioned value of remission at wavelength λ1 is set by the intensity with which the first IR absorber is applied, which is influenced by the pigment concentration and / or ink quantity and / or pressure. If the remission were less than 30%, the contribution of the IR pigment of the first IR absorber would be lost in the combination spectrum. If the remission were too strong, i.e., greater than 80%, the dynamic range available for detection would be excessively reduced ("black on black").

[0037] The three criteria mentioned above together achieve a reliable differentiation of the combined spectrum R3(λ) of the two superimposed IR absorbers from the first remission spectrum R1(λ) and the second remission spectrum R2(λ). This makes numerous IR dyes available. The spectral range from 750 nm to 1600 nm is used to specify the IR absorbers with regard to the overlap of the remission spectra (calculated above). Using IR absorber bodies specified in this way, different IR spectral ranges can be used for security testing, depending on the IR sensor used. Silicon-based detectors (photodiodes, CMOS pixels, CCD pixels, etc.) are often used for more cost-effective IR sensors. These are limited to a spectral range of <1100 nm due to the silicon band gap, so that testing is practically carried out over an IR spectral range of 750 nm to 1050 nm.Alternatively, IR sensors with InGaAs-based detector elements can be used, which allow testing over an extended IR spectral range from 750 nm to 1600 nm. Furthermore, other detector technologies (extended InGaAs, InAs, PbS, etc.) can be used, which can detect even broader spectral ranges up to longer wavelengths. The similarity x is specified over the spectral range from 750 nm to 1600 nm. However, the actual testing of the value document can also be carried out in a smaller spectral range, for example from 750 nm to 1050 nm, or at a different sampling point spacing, such as 50 nm, with fewer sampling points. One is not necessarily bound to the spectral range and sampling point spacing established for the specification of the IR absorbers. The specification is chosen so that even with a different, e.g.A narrower spectral range and a smaller or larger sampling point spacing are all factors that allow verification. This is completely unproblematic if a narrower spectral range and / or a larger sampling point spacing is chosen in the values ​​given in the numerical example. The technical background to this is that the remission behavior is spectrally continuous.

[0038] Preferably, the first IR absorber is applied over the entire surface and the second IR absorber is applied in a grid pattern. Alternatively, both the first IR absorber and the second IR absorber can be applied in a grid pattern. Both IR absorbers can also be applied over the entire surface. Reduced surface coverage of the second IR absorber reduces its opacity and thus, in particular, also its opacity at the absorption maximum. This means that even strongly absorbing and / or strongly scattering IR absorbers can be used as a second IR absorber for reliable harvesting detection, even if they would have too high an opacity if applied over the entire surface as a second IR absorber. The grid pattern of the second IR absorber on top reduces its opacity to such an extent that sufficient spectral signature of the first IR absorber on the bottom still appears in the combination spectrum.

[0039] If reflective metal layers are used in the valuable document, for example, in reflection holograms or thin interference layer structures (color shift), larger areas with millimeter dimensions are provided as perforated, transparent openings (clear text). In these areas, both the second IR absorber in or on the foil element and the first IR absorber below the foil element on the valuable document are visible from the outside. In these areas, an optical inspection of the machine-readable IR marking can be performed. For transparent or translucent foil elements, such as volume holograms or microlens-based moiré magnifiers, the first IR absorber and the second IR absorber can be inspected across the entire surface.

[0040] Furthermore, a method for checking a value document is provided, in which the value document is illuminated and remitted light is measured spectrally resolved as a combination spectrum R3(λ). From the combination spectrum R3(λ), the remission spectrum R1(λ) of the first IR absorber and the remission spectrum R2(λ) of the second IR absorber are determined and, based on the intensity of the first remission spectrum R1(λ) and the second remission spectrum R2(λ), it is checked whether the first IR absorber and the second IR absorber are present on the value document. The specific inspection of the value document can, for example, be carried out in the spectral range from 750 nm to 1600 nm. Alternatively, the inspection can also be carried out in a smaller spectral range, for example from 750 nm to 1050 nm, or in a larger spectral range, for example from 780 nm to 1700 nm.The distance between the support points can be 10 nm or greater, such as 50 nm, so that fewer support points are used. Furthermore, non-equidistant support points are also possible when verifying valuable documents.

[0041] To determine the proportions of the first IR absorber and the second IR absorber in the remitted combination spectrum R3(λ), the measured curve of the combination spectrum R3(λ) is fitted with a linear least-square fit, using the stored remission curves R1(λ) of the first IR absorber and R2(λ) of the second IR absorber as a basis.

[0042] In order to demonstrate the formulas for the quality of fit as clearly as possible, a linear superposition of R1(λ) and R2(λ) is used as an idealized numerical example, whereby the first IR absorber has an intensity of 0.3 (30% of the target application strength) and the second IR absorber has an intensity of 0.6 (60% of the target application strength). In actual measurements in real applications, the measured curves are somewhat noisy and, when applied on top of each other, are no longer superimposed in a perfectly linear manner. For the valuable document to be tested, for example, one measures n=86 remission values ​​R3(750 nm; ...; 1600 nm) for the wavelengths 750 nm; 760 nm; 770 nm; ...; 1590 nm; 1600 nm. The following reflectance values ​​are examples: R3 (750 nm; ...; 1600 nm) = (0.9441; 0.9310; 0.9167; 0.9043; 0.8919; 0.8796; 0.8667; 0.8530; 0.8392; 0.8271; 0.8170; 0.8077; 0.7993; 0.7919; 0.7702; 0.7197; 0.7117; 0.7095; 0.7110; 0.7135; 0.7172; 0.7203; 0.7229; 0.7253; 0.7302; 0.7372; 0.7458; 0.7554; 0.7656; 0.7761; 0.7866; 0.7970; 0.8071; 0.8169; 0.8262; 0.8351; 0.8433; 0.8512; 0.8585; 0.8653; 0.8717; 0.8776; 0.8830; 0.8881; 0.8928; 0.8971; 0.9011; 0.9048; 0.9083; 0.9115; 0.9146; 0.9172; 0.9197; 0.9220; 0.9240; 0.9258; 0.9274; 0.9289; 0.9304; 0.9317; 0.9329; 0.9341; 0.9351; 0.9361; 0.9370; 0.9377; 0.9380; 0.9381; 0.9378; 0.9372; 0.9364; 0.9357; 0.9349; 0.9342; 0.9333).

[0043] From this, the respective 1 - R3(λ i ) values, i.e. the respective complementary values, are calculated by replacing each value R3(λ i ) with 1 - R3(λ i ): 1 - R3(750 nm; ...; 1600 nm) = (1 - R3(750 nm); ...; 1 - R3(1600 nm))

[0044] Dies liefert folgenden Vektor z = (z 1 , ..., zn ) T< : z = (0.0559; 0.0690; 0.0833; 0.0957; 0.1081; 0.1204; 0.1334; 0.1470; 0.1608; 0.1730; 0.1923; 0.2581; 0.2298; 0.2761; 0.2803; 0.2847; 0.2883; 0.2905; 0.2890; 0.2865; 0.2828; 0.2797; 0.2771; 0.2747; 0.2698; 0.2628; 0.2543; 0.2447; 0.2344; 0.2239; 0.2134; 0.2030; 0.1929; 0.1831; 0.1738; 0.1649; 0.1567; 0.1488; 0.1415; 0.1347; 0.1283; 0.1224; 0.1170; 0.1119; 0.1072; 0.1029; 0.0989; 0.0952; 0.0917; 0.0885; 0.0854; 0.0828; 0.0803; 0.0780; 0.0760; 0.0742; 0.0726; 0.0711; 0.0696; 0.0683; 0.0671; 0.0659; 0.0649; 0.0639; 0.0630; 0.0623; 0.0620; 0.0619; 0.0622; 0.0628; 0.0636; 0.0643; 0.0651; 0.0658; 0.0667) T<

[0045] For the linear least-squares fit, a design matrix A must be created. This consists of the vectors v = 1 - [R1(750 nm; ...; 1600 nm)] T< and w = 1 - [R2(750 nm; ...; 1600 nm)] T<, written side by side in columns. From this, the pseudoinverse A = (AT< A) -1< AT< , a 2xn matrix, is calculated.

[0046] The fit coefficients are then A + z = 0,3000 0,6000

[0047] These are the estimated intensities of the IR absorbers included in the measurement. This means that the first IR absorber is estimated to be present at c 1 = 0.3, i.e., at 30% of its reference intensity, and the second IR absorber is estimated to be present at c 2 = 0.6, i.e., at 60% of its reference intensity. For comparison: if both IR absorbers had been applied in finely rastered strips with a 1:1 duty cycle at their reference intensity, the value would be c 1 = c 2 = 0.5.

[0048] Next, it is checked whether c 1 and c 2 both exceed a threshold. The threshold is at least 0.1, preferably at least 0.15. In this case, c 1 and c 2 are both above the threshold. In a harvesting attack, c 1 or c 2 would theoretically be zero, i.e., below the threshold, and the value document would be rejected.

[0049] In addition, the fit is checked by determining the fit curve y: y = (y 1 ;...; yn ) T< = c 1 v + c 2 w = (c 1 v 1 + c 2 w 1 ;...; c 1 vn + c 2 wn ) T< = y(750 nm; ... ; 1600 nm) = (y(750 nm); ... ; y(1600 nm) T< = c 1 (1 - R1(750 nm; ... ; 1600 nm)) T< + c 2 (1 - R2(750 nm; ... ; 1600 nm)) T< = (c 1 R1(750 nm) + c 2 R2(750 nm); ...; c 1 R1(1600 nm) + c 2 R2(1600 nm)) T< = (0,0559; 0,0690; 0,0833; 0,0957; 0,1081; 01,1204; 0,1334; 0,1470; 0,1608; 0,1730; 0,1830; 0,1923; 0,2007; 0,2081; 0,2151; 0,2223; 0,2298; 0,2378; 0,2453; 0,2523; 0,2589; 0,2650; 0,2693; 0,2725; 0,2761; 0,2803; 0,2847; 0,2883; 0,2905; 0,2890; 0,2856; 0,2828; 0,2797; 0,2771; 0,2747; 0,2698; 0,2628; 0,2543; 0,2447; 0,2344; 0,2239; 0,2134; 0,2030; 0,1929; 0,1831; 0,1738; 0,1649; 0,1567; 0,1488; 0,1415; 0,1347; 0,1283; 0,1224; 0,1170; 0,1119; 0,1072; 0,1029; 0,0989; 0,0952; 0,0917; 0,0885; 0,0854; 0,0828; 0,0803; 0,0780; 0,0760; 0,0742; 0,0726; 0,0711; 0,0696; 0,0683; 0,0671; 0,0659; 0,0649; 0,0639; 0,0630; 0,0623; 0,0620; 0,0619; 0,0622; 0,0628; 0,0636; 0,0643; 0,0651; 0,065; 0,0667) T< .

[0050] From this vector, representing a fit curve, an effective value, the so-called RMS value, is determined to check whether other or additional IR absorbers were used. This checks whether the fit is sufficiently good (e.g., the average squared distance between the curve and the fit RMS value): RMS = 1 n ∑ i = 1 n z i − y i 2 and it is checked whether this RMS value is below a threshold, e.g., 0.05. In the numerical example, the fit curve y matches the curve R3(λ) perfectly, because the RMS value is zero and below a predefined threshold of 0.05.

[0051] Normally, the measured values ​​are noisy. The following shows an example of a valuable document to be tested with noisy measured values. Between 750 nm and 1600 nm, n=86 reflectance values ​​R3(750 nm; ...; 1600 nm) were measured for the wavelengths 750 nm; 760 nm; 770 nm; ...; 1590 nm; 1600 nm at a sampling point spacing of 10 nm: R3(750 nm; ...); 1600 nm) = (0.8959; 0.9429; 0.8822; 0.9057; 0.8964; 0.9033; 0.8488; 0.8449; 0.8124; 0.7627; 0.7936; 0.7934; 0.7605; 0.8158; 0.8314; 0.7478; 0.6980; 0.6967; 0.6847; 0.6938; 0.6800; 0.7445; 0.7140; 0.7194; 0.7297; 0.7201; 0.7535; 0.7169; 0.7482; 0.8338; 0.8028; 0.8167; 0.8277; 0.7996; 0.8418; 0.8198; 0.8106; 0.8500; 0.8721; 0.8415; 0.8478; 0.8942; 0.9108; 0.8549; 0.8629; 0.8831; 0.9101; 0.8769; 0.8696; 0.8962; 0.9104; 0.8522; 0.9167; 0.8912; 0.8854; 0.9582; 0.9404; 0.9248; 0.9516; 0.8967; 0.9434; 0.9553; 0.8974; 0.9058; 0.9943; 0.9598; 0.9481; 0.9691; 1.0050; 0.8654; 0.9562; 0.9884; 0.8799; 0.9441; 0.8930).

[0052] From this, the respective 1 - R3(λ i ) values, i.e. the respective complementary values, are calculated by replacing each value R3(λ i ) with 1 - R3(λ i ): 1 - R3(750 nm; ...; 1600 nm) = (1 - R3(750 nm); ...; 1 - R3(1600 nm))

[0053] This yields the following vector z = (z 1 , ..., zn ) T< : z = (0.1041; 0.0571; 0.1178; 0.0943; 0.1036; 0.0967; 0.1513; 0.1551; 0.1876; 0.2374; 0.2064; 0.2066; 0.2395; 0.1842; 0.1660; 0.1686; 0.1584; 0.2304; 0.2614; 0.2603; 0.2886; 0.2773; 0.2948; 0.2367; 0.2595; 0.2522; 0.2087; 0.3020; 0.3033; 0.3153; 0.3062; 0.3201; 0.2555; 0.2860; 0.2806; 0.2703; 0.2799; 0.2466; 0.2831; 0.2518; 0.1662; 0.1972; 0.1833; 0.1723; 0.2004; 0.1582; ​​0.1802; 0.1894; 0.1500; 0.1279; 0.1585; 0.1522; 0.1058; 0.0892; 0.1451; 0.1371; 0.1169; 0.0899; 0.1231; 0.1304; 0.1038; 0.0896; 0.1478; 0.0833; 0.1088; 0.1146; 0.0418; 0.0596; 0.0752; 0.0484; 0.1033; 0.0566; 0.0447; 0.1026; 0.0942; 0.0057; 0.0402; 0.0519; 0.0309; -0.0050; 0.1346; 0.0438; 0.0116; 0.1201; 0.0559; 0.1070) T<

[0054] In this case, the fit coefficients c 1 and c 2 are: A + z = 0,2943 0,6096

[0055] Hieraus ergibt sich die Fitcurve: y = (y 1 ;...; yn ) T< = c 1 v + c 2 w = (c 1 v 1 + c 2 w 1 ;...; c 1 vn + c 2 wn ) T< = y(750 nm; ... ; 1600 nm) = (y< ...75); (1 - R1(750 nm; ... ; 1600 nm)) T< + c 2 (1 - R2(750 nm; ... ; 1600 nm)) T< = (c 1 R1(750 nm) + c 2 R2(750 nm); ...; c 1 R1(1600 nm) + c 2 R2(1600 nm)) T< = (0.0556; 0.0686; 0.0826; 0.0949; 0.1071; 0.1193; 0.1321; 0.1455; 0.1592; 0.1713; 0.1813; 0.1906; 0.2065; 0.2730; 0.2818; 0.2865; 0.2905; 0.2931; 0.2920; 0.2899; 0.2865; 0.2836; 0.2811; 0.2787; 0.2737; 0.2667; 0.2580; 0.2483; 0.2379; 0.2272; 0.2165; 0.2061; 0.1957; 0.1858; 0.1764; 0.1674; 0.1590; 0.1510; 0.1436; 0.1367; 0.1302; 0.1242; 0.1187; 0.1136; 0.1088; 0.1044; 0.1004; 0.0966; 0.0931; 0.0898; 0.0867; 0.0840; 0.0815; 0.0792; 0.0771; 0.0753; 0.0736; 0.0721; 0.0706; 0.0692; 0.0680; 0.0668; 0.0658; 0.0647; 0.0638; 0.0631; 0.0627; 0.0626; 0.0629; 0.0634; 0.0641; 0.0648; 0.0656; 0.0663; 0.0671) T< .

[0056] This yields an RMS value of 0.0317, which is still below the previously defined threshold of 0.05.

[0057] Depending on the application, better results can be achieved by adding (1;...;1) T< as a third basis vector to the basis. Adding this third basis vector makes the method more robust in the event of white balance inaccuracies. Furthermore, if the security document is dirty, the remission curves are slightly attenuated, so that the absorption curves have a base that does not come from the IR absorber. This is better compensated for with this variant.

[0058] The idealized numerical example from above is used again, but this time the reflectance curves are assumed to be attenuated by 10% due to inaccuracies in white balance. For the security document to be tested, measurements are again taken between 750 nm and 1600 nm at a sampling point spacing of 10 nm. n=86 reflectance values ​​R3(750 nm; ...; 1600 nm) for the wavelengths 750 nm; 760 nm; 770 nm; ...; 1590 nm; 1600 nm: R3(750 nm; ...); 1600 nm) = (0.8497; 0.8379; 0.8250; 0.8139; 0.8027; 0.7916; 0.7800; 0.7677; 0.7553; 0.7443; 0.7353; 0.7269; 0.7194; 0.7127; 0.6932; 0.6729; 0.6548; 0.6406; 0.6386; 0.6399; 0.6422; 0.6455; 0.6483; 0.6506; 0.6527; 0.6572; 0.6635; 0.6712; 0.6798; 0.6890; 0.6985; 0.7080; 0.7173; 0.7264; 0.7352; 0.7436; 0.7516; 0.7590; 0.7661; 0.7726; 0.7788; 0.7845; 0.7898; 0.7947; 0.7993; 0.8035; 0.8074; 0.8110; 0.8143; 0.8175; 0.8204; 0.8231; 0.8255; 0.8277; 0.8298; 0.8316; 0.8332; 0.8347; 0.8360; 0.8373; 0.8385; 0.8396; 0.8407; 0.8416; 0.8425; 0.8433; 0.8439; 0.8442; 0.8442; 0.8440; 0.8435; 0.8428; 0.8422; 0.8414; 0.8407; 0.8400).

[0059] From this, the respective 1 - R3(λ i ) values, i.e. the respective complementary values, are calculated by replacing each value R3(λ i ) with 1 - R3(λ i ): 1 - R3(750 nm; ...; 1600 nm) = (1 - R3(750 nm); ...; 1 - R3(1600 nm))

[0060] This yields the following vector z = (z 1 , ..., zn ) T< : z = (0.1503; 0.1621; 0.1750; 0.1861; 0.1973; 0.2084; 0.2200; 0.2323; 0.2447; 0.2557; 0.2647; 0.2731; 0.2806; 0.2873; 0.2936; 0.3001; 0.3068; 0.3140; 0.3208; 0.3271; 0.3330; 0.3385; 0.3424; 0.3452; 0.3485; 0.3523; 0.3562; 0.3594; 0.3614; 0.3601; 0.3579; 0.3545; 0.3517; 0.3494; 0.3473; 0.3428; 0.3365; 0.3288; 0.3202; 0.3110; 0.3015; 0.2920; 0.2827; 0.2736; 0.2648; 0.2564; 0.2484; 0.2410; 0.2339; 0.2274; 0.2212; 0.2155; 0.2102; 0.2053; 0.2007; 0.1965; 0.1926; 0.1890; 0.1857; 0.1825; 0.1796; 0.1769; 0.1745; 0.1723; 0.1702; 0.1684; 0.1668; 0.1653; 0.1640; 0.1627; 0.1615; 0.1604; 0.1593; 0.1584; 0.1575; 0.1567; 0.1561; 0.1558; 0.1558; 0.1560; 0.1565; 0.1572; 0.1578; 0.1586; 0.1593; 0.1600) T<

[0061] Using the above pseudoinverse A +< we get the fit coefficients c 1 and c 2 : A + z = 0,4095 0,8481

[0062] Hieraus ergibt sich die Fitcurve: y = (y 1 ;...; yn ) T< = c 1 v + c 2 w = (c 1 v 1 + c 2 w 1 ;...; c 1 vn + c 2 wn ) T< = y(750 nm; ... ; 1600 nm) = (y< ...75); (1 - R1(750 nm; ... ; 1600 nm)) T< + c 2 (1 - R2(750 nm; ... ; 1600 nm)) T< = (c 1 R1(750 nm) + c 2 R2(750 nm); ...; c 1 R1(1600 nm) + c 2 R2(1600 nm)) T< = (0.0774; 0.0954; 0.1150; 0.1320; 0.1490; 0.1659; 0.1837; 0.2025; 0.2215; 0.2383; 0.2523; 0.2651; 0.2871; 0.3179; 0.3500; 0.3920; 0.3987; 0.4042; 0.4078; 0.4063; 0.4034; 0.3985; 0.3945; 0.3911; 0.3878; 0.3808; 0.3710; 0.3589; 0.3454; 0.3310; 0.3161; 0.3013; 0.2867; 0.2723; 0.2585; 0.2454; 0.2329; 0.2212; 0.2101; 0.1998; 0.1901; 0.1811; 0.1728; 0.1652; 0.1580; 0.1514; 0.1453; 0.1397; 0.1344; 0.1295; 0.1250; 0.1206; 0.1169; 0.1133; 0.1101; 0.1072; 0.1047; 0.1024; 0.1002; 0.0982; 0.0963; 0.0946; 0.0929; 0.0915; 0.0901; 0.0887; 0.0877; 0.0872; 0.0871; 0.0875; 0.0882; 0.0892; 0.0901; 0.0912; 0.0922; 0.0934) T< and from this the RMS value 0.0463.

[0063] If we now add T< as the third basis vector to the design matrix (1;...;1), we obtain with the pseudoinverse A +< = (AT< A) -1< AT< as fit coefficients c 1 , c 2 , c 3 : A + z = 0,2700 0,5400 0,1000

[0064] Here, c 1 = 0.2700 and c 2 = 0.5400 are the feature intensities. These are attenuated by 10% compared to the intensities of 0.3 and 0.6 in the first example (exact white balance).

[0065] Man erhält als Fitkurve: y = (y 1 ;...; y n ) T< = c 1 v + c 2 w = (c 1 v 1 + c 2 w 1 ;...; c 1 v n + c 2 w n ) T< = y(750 nm; ... ; 1600 nm) = (y(750 nm); ... ; y(1600 nm) T< = c 1 (1 - R1(750 nm; ... ; 1600 nm)) T< + c 2 (1 - R2(750 nm; ... ; 1600 nm)) T< + c 3 = (c 1 R1(750 nm) + c 2 R2(750 nm) + c 3 ; ...; c 1 R1(1600 nm) + c 2 R2(1600 nm) + c 3 ) T< = (0.1503; 0.1621; 0.1750; 0.1861; 0.1973; 0.2084; 0.2200; 0.2323; 0.2447; 0.2557; 0.2647; 0.2873; 0.3424; 0.3485; 0.3523; 0.3562; 0.3594; 0.3614; 0.3601; 0.3579; 0.3545; 0.3517; 0.3494; 0.3473; 0.3428; 0.3365; 0.3288; 0.3202; 0.3110; 0.3015; 0.2920; 0.2827; 0.2736; 0.2648; 0.2564; 0.2484; 0.2410; 0.2339; 0.2274; 0.2212; 0.2155; 0.2102; 0.2053; 0.2007; 0.1965; 0.1926; 0.1890; 0.1857; 0.1825; 0.1797; 0.1769; 0.1745; 0.1723; 0.1702; 0.1684; 0.1668; 0.1653; 0.1640; 0.1627; 0.1615; 0.1604; 0.1593; 0.1584; 0.1575; 0.1567; 0.1561; 0.1558; 0.1558; 0.1560; 0.1565; 0.1572; 0.1578; 0.1586; 0.1593; 0.1600) T< .

[0066] The fit curve y (750 nm; ...; 1600 nm) perfectly matches the curve z = 1 - R3(750 nm; ...; 1600 nm). Therefore, the RMS value here is zero and below the threshold.

[0067] Theoretically, the following additional scenario could occur: With a very strongly absorbing first IR absorber, the remission spectrum could be distorted, resulting in ghost components appearing in the fit even though the second IR absorber is missing. To counteract this, one can additionally check that the measured IR absorption is not too high (e.g., by measuring average absorption or absorption at a single wavelength, or by checking whether the remission curve is within a given tube).

[0068] The invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings, which also disclose features essential to the invention. These exemplary embodiments are for illustrative purposes only and are not to be interpreted as restrictive. For example, a description of an exemplary embodiment with a large number of elements or components should not be interpreted as meaning that all of these elements or components are necessary for implementation. Rather, other exemplary embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary 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 in a first embodiment in a sectional view, Fig. 2 a value document in a second embodiment in a sectional view, Fig. 3 a spectral curve of remission, transmission and complementary remission, i.e. 1-R, of an IR absorber, Fig. 4 l-R(λ) spectra of two distinguishable IR absorbers, Fig. 5 l-R(λ) spectra of two indistinguishable IR absorbers, Fig. 6 R(λ) remission spectra of two IR absorber layers and a combination spectrum thereof, Fig. 7 R(λ) remission spectra of two IR absorbers and their combination spectrum, Fig. 8 R(λ) remission spectra of two IR absorbers, wherein an IR absorber is applied both over the entire surface and in a rasterized manner, and a combination spectrum, Fig. 9R(λ) reflectance spectra of two IR absorbers and their combination spectrum, Fig. 10R(λ) reflectance spectra of two IR absorbers and their combination spectrum.

[0069] Fig. 1shows a value document 1 with a substrate body 2, which has a front side 3 and a back side 4. On the front side 3 of the substrate body 2, a first IR absorber 6 is applied in a partial area. This first IR absorber 6 is firmly connected to the substrate body 2. Such a firm connection between the substrate body 2 and the first IR absorber 6 is achieved by printing or varnishing with a printing ink. The printing ink of the first IR absorber 6 contains IR pigments or IR dyes. In addition to the IR pigments or IR dyes, it can also contain color pigments in the visible spectral range. Different visible color pigments can also be used in certain areas, for example to print a visible image with several different colors, which, however, have a uniform spectral profile in the IR spectral range. The first IR absorber 6 is applied using conventional printing processes, such asOffset printing, gravure printing, flexographic printing or screen printing, or can be applied, for example, in gravure printing as a primer layer for the subsequent application of a film element 8.

[0070] In case of Fig. 1 The first IR absorber 6 is applied in a partial region of the substrate body 2, with the film element 8, which is formed as a single piece, being applied to the first IR absorber 6. The first IR absorber 6 is applied partly below and partly next to the film element 8.

[0071] A second IR absorber 10 is applied to the film element 8 and is printed in a raster pattern.

[0072] Fig. 2shows a further embodiment of a value document 1, in which the first IR absorber 6 is again applied to a partial area of ​​the front side 3 of the substrate body 2. The film element 8 is arranged above the first IR absorber 6. In this embodiment, however, it is composed of several parts, a first film sub-element 12 and a second film sub-element 14. Between the first film sub-element 12 and the second film sub-element 14 is the second IR absorber 10, which is applied over the entire surface.

[0073] Both versions of the Fig. 1 and 2have spectrally overlapping IR absorber layers 6, 10, so that several specific criteria must be met for reliable detectability of R1(λ) / R2(λ) spectra of the individual IR absorbers 6, 10 as well as a combination spectrum R3(λ). Detectable R1(λ) spectra as well as the combination spectrum R3(λ) ensures that both the substrate body 2, the film element 8, and the entire valuable document 1 can be verified for authenticity using a single measurement.

[0074] A first criterion for the reliable detectability of the value document 1 is that the first IR absorber and the second IR absorber spectrally overlap. The spectral overlap must be selected such that the combination spectrum R3(λ) of the two R(λ) spectra can be stably and reliably identified, i.e., clearly distinguishable from both the R1(λ) spectrum and the R2(λ) spectrum. This is achieved by appropriately selecting the two IR absorbers 6, 10, so that a similarity x of the 1 - R1(λ) spectrum with the 1 - R2(λ) spectrum, i.e., the cosine similarity described in detail in the general section, lies in the range of 0.1 to 0.9, preferably in the range of 0.1 to 0.8.

[0075] A second criterion for the reliable detectability of the value document 1 is that the second IR absorber 10 has an opacity O2(λ2) of 1.1 to 10, preferably 1.2 to 4, at a wavelength λ2 of a second IR absorption maximum A2max of the second IR absorber 10. Only then is it guaranteed that the resulting combination spectrum R3(λ) differs sufficiently from both the R1(λ) spectrum of the first IR absorber 6 and the R2(λ) spectrum of the second IR absorber 10.

[0076] As a third criterion for the reliable detectability of the value document 1, the first IR absorber 6 exhibits a remission R1(λ1) between 30% and 80% at a wavelength λ1 of a first absorption maximum A1max. This also represents the distinguishability of the combination spectrum R3(λ) of the two superimposed IR absorbers 6, 10 from the respective R1(λ) and R2(λ) spectra. The first IR absorber 6 is applied with such an intensity (pigment concentration, ink quantity / pressure) that the aforementioned remission R1(λ1) between 30% and 80% is achieved. If the remission is too weak (<30%), the dynamic range available for detection may be excessively reduced ("black on black"). If the remission is too strong (>80%), the contribution of the IR pigment of the first IR absorber 6 is lost in the combination spectrum R3(λ).

[0077] The three criteria described will now be analyzed in more detail using the following examples.

[0078] Fig. 3shows, in a coordinate system 16, a spectral profile of a remission R1(λ) of an IR absorber 6 printed on a white substrate body 2 in a remission curve 18, a spectral profile of a transmission T1(λ) of the IR absorber 6 printed on a transparent substrate body 2 in a transmission curve 20, and a 1-R1(λ) remission B1(λ) complementary to the remission R1(λ) of the IR absorber 6 printed on a white substrate body 2 in a curve 22, which is the complement (supplement to one) of the remission curve 18. In the coordinate system 16, the wavelength λ in nanometers on the x-axis is compared with the remission / transmission / complementary remission, ie 1-R, on the y-axis for one and the same IR absorber layer 6. The relationship between the spectra R1(λ) for remission, T1(λ) for transmission, and 1-R(λ) for complementary remission is evident. The relationship 1 = R1(λ) + T1(λ) + A1(λ) applies.

[0079] Fig. 4 shows the spectral B(λ) curves, i.e. the complementary curves 24, 26 to the R(λ) curves (B1(λ) = 1 - R1(λ) and B2(λ) = 1 - R2(λ)), of two suitable, distinguishable IR absorber layers 6, 10 in a coordinate system 16. The wavelength λ in nanometers on the x-axis is compared to the 1- R(λ) remission on the y-axis. The first complementary curve 24 shows the B1(λ) spectrum of the first IR absorber 6 and the second complementary curve 26 shows the second B2(λ) spectrum of the second IR absorber 10. The similarity x of the two B(λ) spectra is 0.46 when a wavelength range from 750 nm to 1600 nm is examined.

[0080] The absorbers 6, 10 shown meet the first necessary criterion for reliable differentiation of the combination spectrum R3(λ) of the two superimposed absorbers 6, 10. The similarity x of the two B(λ) spectra B1(λ) and B2(λ) must be between 0.1 and 0.9, preferably up to 0.8, for reliable differentiation of the combination spectrum R3(λ) of the two IR absorbers 6, 10 in the intended spectral range. If the spectral overlap is too strong, the combination spectrum R3(λ) becomes too similar to one or even both of the reflectance spectra R1(λ) or R2(λ) and can no longer be stably and reliably identified.

[0081] Fig. 5shows, in a comparative example, the spectrally dependent 1-R(λ) curves (equivalent to the B(λ) curves) of two unsuitable IR absorbers, which are indistinguishable when superimposed, in a coordinate system 16. The wavelength λ in nanometers on the x-axis is compared with the 1-R spectra on the y-axis. The first complementary curve 24 again represents the course of the 1-R1(λ) spectrum of the first IR absorber, and the second complementary curve 26 represents the course of the 1-R2(λ) spectrum of the second IR absorber. The similarity x of the two 1-R(λ) spectra is 0.92 when a wavelength range from 750 nm to 1600 nm is examined. Due to the excessive overlap of the absorption ranges, this combination of IR absorbers is not suitable for reliable detection of the harvesting situation.

[0082] Fig. 6As an exemplary embodiment, in a coordinate system 16, it shows a first remission spectrum R1(λ) of the first IR absorber 6 in a first remission curve 28, a second remission spectrum R2(λ) of the second IR absorber 10 in a second remission curve 30, and a third remission spectrum R3(λ) in a third remission curve 32; it represents a combination spectrum of the second IR absorber 10 over the first IR absorber 6. The wavelength λ in nanometers on the x-axis is compared with the remission on the y-axis. The wavelength range from 750 nm to 1600 nm is examined.

[0083] To produce the value document 1, the first IR absorber 6 is printed flat in the area below the film element 8 and combined with the second IR absorber 10 printed flat in the film element 8. The pressure of the second IR absorber 10 is adjusted such that the opacity O2(λ2) of the second IR absorber 10 at a wavelength λ2 of the second absorption maximum A2max(λ2) is equal to 1.5.A suitable selection of the first absorber 6 R1(λ) at a wavelength λ1 of the first absorption maximum A1max(λ1) of 53% is also present, so that the combination spectrum R3(λ), shown in the third remission curve 32, is a combination of the first remission spectrum R1(λ), ​​shown in the first remission curve 28, with the second remission spectrum R2(λ), shown in the second remission curve 30, with good dynamics, which can be clearly distinguished from both the first remission spectrum R1(λ) of the first IR absorber 6 and the second remission spectrum R2(λ) of the second IR absorber 10.

[0084] Another comparison example is now in Fig. 7 shown. Fig. 7shows in a coordinate system the first remission curve 28, which in turn represents the first remission spectrum R1(λ) of the first IR absorber 6, the second remission curve 30, which represents the second remission spectrum R2(λ) of the second IR absorber 10, and the third remission curve 32, which represents the third remission spectrum R3(λ) (combination spectrum) of a combination of the first remission spectrum R1(λ) with the second remission spectrum R2(λ). The wavelength λ in nanometers on the x-axis is compared to the remission on the y-axis. The wavelength range from 750 nm to 1600 nm is investigated. In this embodiment, the same materials as in the embodiment of Fig. 6The first IR absorber 6 has a remission of 53% at its absorption maximum, but the second IR absorber 10 in film element 8 was now printed more heavily over its entire surface, so that its opacity at the absorption maximum A2max(λ2) at approximately 1080 nm is now 15.6. Thus, the second IR absorber 6 was printed too heavily – the layer is too opaque. The opacity criterion (second criterion) is not met.

[0085] It can be seen that the combination spectrum R3(λ) (third remission curve 32) can be clearly distinguished from the first remission spectrum R1(λ) (first remission curve 28) of the first IR absorber 6, but can no longer be reliably distinguished from the second remission spectrum R2(λ) (second remission curve 30) of the second IR absorber 10. Thus, reliable harvesting detection is no longer possible.

[0086] Fig. 8now shows a further exemplary embodiment in a coordinate system 16, in which the second IR absorber 10 was printed in a rasterized manner. The first remission spectrum R1(λ) of the first IR absorber 6 can be seen, again shown in the first remission curve 28, the second remission spectrum R2(λ) of the second IR absorber 10, again shown in the second remission curve 30, a fourth remission spectrum R4(λ) of the rasterized second IR absorber 10, shown in a fourth remission curve 34, and a fifth remission spectrum R5(λ) of a combination of the rasterized second IR absorber 10 over the first IR absorber 6, shown in a fifth remission curve 36. The wavelength λ in nanometers on the x-axis is compared with the remission on the y-axis in the coordinate system 16. The wavelength range from 750 nm to 1600 nm is investigated.

[0087] The same colours and pressures are used as in the example according to Fig. 6 was chosen, but instead of full-surface printing of the second absorber 10 on top, it was raster printed with an area coverage of only 30%. This reduces the opacity of the second IR absorber layer 10 at the absorption maximum A2max(λ2) to O2(λ2) = 2.0, which again enables reliable harvesting detection.

[0088] The screening of the upper second absorber 10 reduces its opacity to such an extent that sufficient spectral signature of the lower first absorber 6 (R 900mn = 53%) still appears in the fifth remission spectrum R5(λ). The fifth remission spectrum R5(λ) (fifth remission curve 36) of the combination of the screened second IR absorber 10 above the first IR absorber 6 can be clearly distinguished from the first remission spectrum R1(λ) (first remission curve 28) of the first IR absorber 6 and from the fourth remission spectrum R4(λ) (fourth remission curve 34) of the screened second IR absorber 10. Reliable detectability is ensured.

[0089] Fig. 9 shows a further embodiment in a coordinate system 16, in which, as in the embodiment according to Fig. 7 the materials reversed in the sequence are used for the first IR absorber 6 and the second IR absorber 10. Fig. 9shows the first remission spectrum R1(λ) (first remission curve 28) of the first IR absorber 6, the second remission spectrum R2(λ) (second remission curve 30) of the second IR absorber 10 at an opacity O2(λ2)=1.67, and the third remission spectrum R3(λ) (combination spectrum - third remission curve 32) of a combination of the first two remission spectra R1(λ) and R2(λ) at an opacity O2(λ2)=1.27. The wavelength λ in nanometers on the x-axis is compared to the remission on the y-axis in the coordinate system 16. The first IR absorber 6 is printed flat on the foil element 8, while the second IR absorber 10 is printed flat on the substrate body 2, which in turn is designed as a paper substrate, beneath the foil element 8. An InGaAs-based detector operating in the wavelength range 750 nm to 1600 nm is used for the measurement.

[0090] The opacity of the second IR absorber 10 located at the top at the absorption maximum is O2(λ2) = 1.27. The first IR absorber 6 located at the bottom has a remission of 56% at the absorption maximum A1max(λ1) at approximately 1080 nm. It can be seen that the third remission spectrum R3(λ) (third remission curve 32) can be clearly distinguished from the first remission spectrum R1(λ) (first remission curve 28) and the second remission spectrum R2(λ) (second remission curve 30).

[0091] Last shows Fig. 10 Another counterexample, where the opacity O2(λ2) of the upper second IR absorber 10 is too low. The structure is analogous to Fig. 9 used, but the pressure strength of the second IR absorber 10 located at the top of the film element 8 is reduced to such an extent that the opacity O2(λ2) in the absorption maximum A2max(λ2) at approximately 900 nm is only equal to 1.05.

[0092] Fig. 10again shows the reflectance spectrum R1(λ) of the first IR absorber 6 (first reflectance curve 28), the reflectance spectrum R2(λ) of the second IR absorber 10 at an opacity of O2(λ2)=1.67 (second reflectance curve 30), and the combination spectrum R3(λ) of the two IR absorbers 6, 10 with reduced pressure of the second IR absorber 10 located at the top of the film element 8 with an opacity of O2(λ2)=1.05 (third reflectance curve 32). The wavelength λ in nanometers in the wavelength range 750 nm to 1600 nm is plotted in the x-direction, and the reflectance is plotted in the y-direction in the coordinate system 16.

[0093] It can be clearly seen that due to the insufficient opacity of the second IR absorber 10 located on top, its spectral characteristic in the remission spectrum R3(λ) (third remission curve 32) of the combination is reduced to such an extent that, although it can still be clearly distinguished from the second remission spectrum (second remission curve 30), a reliable distinction from the first remission spectrum R2(λ) (first remission curve 28) is no longer possible. List of reference symbols

[0094] 1Value document 2Substrate body 3Front 4Back 6First IR absorber 8Foil element 10Second IR absorber 12First foil sub-element 14Second foil sub-element 16Coordinate system 18Remission curve 20Transmission curve 22Curve 24First complementary curve 26Second complementary curve 28First remission curve 30Second remission curve 32Third remission curve 34Fourth remission curve 36Fifth remission curve

Claims

1. A value document, such as a banknote, a check, a credit or other payment card, an identification card or the like, comprising: - a substrate body (2) having a front side (3) and a back side (4), - a first IR absorber (6) located on the front side (3) and having a first, spectrally dependent remission curve R1(λ), ​​a first spectrum that is complementary to the remission curve R1(λ) and is thus defined by 1 - R1(λ), ​​and an IR absorption maximum A1max at a wavelength λ1, and - a second IR absorber (10) located at least partly above the first IR absorber (6) as viewed towards the front side, and having a second, spectrally dependent remission curve R2(λ), a second spectrum that is complementary to the remission curve R2(λ) and is thus defined by 1 - R2(λ) and has an absorption maximum A2max at a wavelength λ2, characterized in that- the first IR absorber (6) has a remission R1(λ1) of 30% to 80% at the wavelength λ1, - the second IR absorber (10) has an opacity O2(λ2) of 1.1 to 10 at the wavelength λ2, and - the first spectrum differs from the second spectrum in such a way that a similarity x between the first and the second spectrum lies in the range from 0.1 to 0.9, the similarity x being given by the cosine similarity, in the calculation of which the first and the second spectrum are each included as a vector over a spectral range from 750 nm to 1600 nm and with a support point spacing of 10 nm.

2. Value document according to claim 1, characterized in that the similarity x is in a range from 0.1 to 0.

8.

3. A value document according to any one of the above claims, characterized in that the second IR absorber (10) has an opacity O2(λ2) of 1.2 to 4 at the wavelength λ2.

4. A value document according to any one of the above claims, characterized in thatthe first IR absorber (6) is applied in a grid pattern and the second IR absorber (10) is formed over the entire surface.

5. A value document according to any one of the above claims, characterized in that the first and / or the second IR absorber (6, 10) is present in a printing ink.

6. Value document according to claim 5, characterized in that the printing ink comprises one or more of the following compounds in a suitable chemical environment: - iron (II) compounds containing an Fe2+ ion, - copper (II) compounds containing a Cu2+ ion, wherein the suitable chemical compound is a phosphorus and / or oxygen-containing group.

7. A value document according to one of claims 5 or 6, characterized in that the printing ink contains color pigments in the visible spectral range.

8. A valuable document according to any one of the above claims, characterized in that the first IR absorber (6) is present in a primer layer.

9. A valuable document according to any one of the above claims, characterized in thata film element (8) is arranged on the front side (3), the first IR absorber (6) is located below the film element (8) when viewed on the front side (3), and the second IR absorber (10) is applied to the film element (8) or embedded in the film element (8).

10. Value document according to claim 9, characterized in that the film element has optically variable elements, preferably selected from holograms, interference layer stacks, micromirrors and microlenses.

11. A method for checking a value document according to one of claims 1 to 10, wherein - the value document (1) is illuminated and remitted IR radiation is measured spectrally resolved as a combination spectrum R3(λ), characterized in that- intensity components of the first remission spectrum R1(λ) of the first IR absorber (6) and of the second remission spectrum R2(λ) of the second IR absorber (10) are determined, which are present in the combination spectrum R3(λ), and - based on the intensity components of the first remission spectrum R1(λ) and the second remission spectrum R2(λ), it is checked whether the first IR absorber (6) and the second IR absorber (10) are present on the value document (1).

12. Method according to claim 11, characterized in that to determine the intensity components, a fit curve is fitted to the combination spectrum R3(λ) using a linear least-square fit, wherein the remission curves R1(λ) of the first IR absorber (6) and R2(λ) of the second IR absorber (10) are used.

13. Method according to claim 12, characterized in that For the linear least-square fit, a design matrix A is set up, from which a pseudoinverse A + = (A T A) -1 A Twhere a fit coefficient is calculated by calculating the pseudoinverse A + is multiplied by the combination spectrum R3(λ) as a vector, and it is checked whether the fit coefficient lies above a certain threshold.

14. Method according to one of claims 11 to 13, characterized in that an RMS value is determined to check whether other or additional IR absorbers were used.

15. Method according to one of claims 13 or 14, characterized in that for the design matrix A a basis vector (1; ...;1) T is added.

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