Sensor and method for checking valuable documents having at least one reflective security element

Abstract

The invention relates to a sensor (10) and to a method for checking valuable documents (1) which have at least one reflective security element (2) which, in the visible spectral range, has an appearance that depends on a viewing angle. The sensor (10) has: an irradiation device (11) which is designed to irradiate one or more different locations (3, 3') on a valuable document (1) with infrared radiation; a detection device (12) which is designed to detect the infrared radiation reflected from the different locations (3, 3') on the valuable document (1) in each case, the sensor recording a reflectance spectrum from each location; and a checking device (13) which is designed to determine, on the basis of each reflectance spectrum, whether there is a reflective security element having an appearance that depends on the viewing angle at each location in order to determine one or more of the locations (3') at which such a reflective security element (2) is located on the valuable document (1) and to check the valuable document (1) on the basis of the location or locations (3') determined.

Description

[0001]Sensor and method for checking valuable documents with at least one reflective security element. The invention relates to a sensor and a method for checking valuable documents that have at least one reflective security element that has an appearance in the visible spectral range that depends on the viewing angle. To increase counterfeit security, valuable documents, in particular banknotes, are provided with so-called optically variable (OV) security elements, for example in the form of foil elements, so-called patches, wide security threads, or printed, magnetically aligned interference pigments.which are reflective. For example, in remission, these have an appearance that depends on the viewing angle and can therefore be checked with the naked eye by simply tilting the value document. In foil-based OV security elements, in addition to holograms and micromirrors, stacks of thin layers are also used to produce angle-dependent interference colors or color changes. It is an object of the present invention to enable reliable machine checking of value documents, in particular banknotes, provided with reflective, e.g. optically variable, security elements. This object is achieved by a sensor and a method for checking value documents according to the independent claims, as well as a value document processing system with such a sensor. According to a first aspect of the present disclosure, a sensor for checking value documents,which have at least one reflective security element which, in particular in the visible spectral range, has an appearance dependent on a viewing angle, comprising: an irradiation device which is configured to irradiate one or more different locations on a value document with infrared radiation; a detection device which is configured to detect the infrared radiation remitted from the respective location on the value document, wherein the sensor is configured to detect the remitted infrared radiation in a spectrally resolved manner, whereby a remission spectrum is obtained in each case; and a testing device for testing the value document with respect to a reflective security element which has an appearance dependent on the viewing angle, which is configured to determine, based on the remission spectrum obtained for the respective location,whether a reflective, in particular optically variable, security element with a viewing angle-dependent appearance is located on the value document at the respective location, in order to determine one or more locations at which such a reflective security element (i.e. with a viewing angle-dependent appearance) is located (the same or possibly different), and to check the value document based on the determined location(s) (with regard to a reflective security element with a viewing angle-dependent appearance). According to a second aspect of the present disclosure, a method for checking value documents which have at least one reflective security element which has a viewing angle-dependent appearance, in particular in the visible spectral range,The method comprises the following steps: irradiating one or more different locations on a valuable document with infrared radiation and spectrally resolving the infrared radiation remitted from the respective location on the valuable document, whereby a remission spectrum is obtained in each case; determining, based on the remission spectrum obtained for the respective location, whether a reflective, in particular optically variable, security element with an appearance dependent on the viewing angle is located on the valuable document at the respective location, in order to determine one or more locations at which such a reflective security element (i.e. with an appearance dependent on the viewing angle) is located; and checking the valuable document based on the determined location(s), for which it was determined that a reflective, in particular optically variable,A security element (with an appearance dependent on the viewing angle) is located on the value document. A third aspect of the present disclosure relates to a value document processing system having at least one sensor according to the first aspect of the present disclosure and one or more processing devices configured to process value documents, in particular to separate, transport, check, sort, stack, and / or destroy them. A fourth aspect of the present disclosure relates to a computer program comprising instructions which, when executed by a computer, cause the computer to carry out the method according to the second aspect of the present disclosure. A fifth aspect of the present disclosure relates to a computer-readable data carrier,on which the computer program according to the fourth aspect of the present disclosure is stored. A sixth aspect of the present disclosure relates to a data carrier signal that transmits the computer program according to the fourth aspect of the present disclosure. Aspects of the present disclosure are preferably based on the approach of performing a spectrally resolved measurement of the infrared radiation remitted from the respective location at several different locations on a value document to be checked, which in the context of the present disclosure are also referred to as "measurement locations", "measurement pixels" or "pixels", and of identifying the location or locations on the value document based on the remission spectra obtained.on which such a reflective or optically variable security element is present or which are located on a reflective or optically variable security element. For this purpose, the respective remission spectrum obtained is analyzed, for example with regard to its spectral profile and / or other properties. A remission spectrum within the meaning of the present disclosure can be a spectrum that contains remission values ​​at several, possibly even a large number, or even just a few, for example, three or two, different wavelengths. The use of infrared radiation to test a security element that has an appearance in the visible spectral range that depends on a viewing angle offers several advantages over (mechanical) testing with visible light: The security against counterfeiting is increased,because the visual appearance of the security element cannot be used to determine its infrared signature. Counterfeit security is further increased because the verification is performed using non-visible light and can therefore go unnoticed by a potential counterfeiter. Furthermore, the verification is more reliable because a verification with infrared light is less affected by contamination of the banknote being verified than a verification with visible light. The reflective security element, which has an appearance in the visible spectral range that depends on the viewing angle, is, for example, an optically variable security element, in particular an optically variable security element with a viewing-angle-dependent color change. The reflective security element, which has an appearance in the visible spectral range that depends on the viewing angle,can be either a specular (directed or mirrored) reflecting security element or a diffusely reflecting security element. The infrared radiation remitted from the respective location, which is detected by the detection device, can be directed and / or diffusely reflected infrared radiation of the value document. Preferably, the irradiation device and the detection device are arranged such that the detection device detects diffusely reflected infrared radiation of the value document, but not directed reflected infrared radiation. Preferably, a check is carried out to determine whether the location(s) determined in this way meet at least one predetermined test criterion, for example with regard to their number and / or their position on the value document. If this criterion is met, the presence of such a reflective or optically variable security element, optionally with specific properties,affirmed and / or the valuable document is classified as "genuine" or "fit" (acceptable condition). A determined location where such a reflective security element is located can - with a sufficiently large detection range - be a single location, e.g., encompassing the entire surface of the security element in question. A determined location where such a reflective security element is located can (with a correspondingly smaller detection range) also be one of several locations on the security element in question. The latter can, but do not have to, be subjected to spectrally resolved detection or the test described above based on their remission spectrum. The determined locations that are subjected to the test described above or at which such a reflective or optically variable security element is located,can all be located on the same reflective or optically variable security element of the respective value document, or on different such reflective or optically variable security elements of the respective value document. Preferably, exactly one remission spectrum is recorded from the respective location. In particular, a remission spectrum is recorded from several locations on the respective value document, with preferably exactly one remission spectrum being recorded from each of the locations. When determining the location(s) at which such a reflective or optically variable security element is located on the value document based on the remission spectrum obtained in each case, it is not absolutely necessary to take into account the specific design and / or specific properties of the value document to be checked.For example, the expected location of such a reflective or optically variable security element on the valuable document, and to incorporate this into the verification. In particular, this eliminates the need for so-called adaptation of the sensor or method with regard to these properties. Furthermore, it is not absolutely necessary to know the denomination and orientation of the valuable document to be verified at the time of the valuable document verification. On the other hand, an even more reliable identification of the relevant locations and / or verification of the valuable document can of course be achieved if specific properties of the valuable document, in particular with regard to the location and / or size of the reflective or optically variable security element or the denomination and orientation of the valuable document, are also taken into account.The described approach for identifying the locations on such a reflective or optically variable security element is based on the surprising finding that many reflective security elements, e.g., interference-based optical security elements (both film-based and printing pigment-based), have intrinsically specific spectral signatures not only in the visible spectral range (in which optical security elements are primarily tested) but also in the infrared (IR) spectral range. These can thus be used for machine testing of the optical security elements in the IR range. Advantageously, even sensors that were developed or are used for spectral measurement of IR absorption pigments can be used.if the signal evaluation or testing used there is adapted to the specifics of spectral detection of interference colors. Overall, the present disclosure thus enables reliable and simple machine testing of value documents provided with one or more reflective or optically variable security elements with a viewing angle-dependent appearance, in particular of such banknotes. The approach disclosed here is generally suitable for testing or detecting reflective security elements with a viewing angle-dependent appearance, but is particularly suitable for testing or detecting optically variable security elements that, for example, exhibit a viewing angle-dependent color change.which is generated, for example, by interference on thin layers. The optically variable security element can be in the form of an optically variable foil element (e.g., so-called foil patches or foil strips), an optically variable (wide) security thread, or an optically variable printing feature (e.g., with interference pigments magnetically aligned during printing before curing). The sensor is configured for the spectrally resolved detection of the infrared radiation remitted from the respective location. For example, the detection device itself can be configured to spectrally decompose the remitted infrared radiation and detect it with spectral resolution. The spectral resolution then lies on the detection side, and the irradiation device is then preferably spectrally broadband in the infrared. Alternatively, and particularly preferably, the detection device is a (simpler) detection device,which is not designed for the spectral decomposition of light, e.g., the remitted infrared radiation. Spectral resolution is then achieved by successively irradiating the respective location with infrared radiation of at least two or at least three different wavelengths / ranges and corresponding successive detection by the detection device at (at least approximately) the same location. In particular, the detection device is configured and / or arranged to detect the infrared radiation remitted from the different locations at a (vertical or oblique) detection angle, and the irradiation device is configured and / or arranged to direct the infrared radiation onto the value document such that it strikes the value document at a (vertical or oblique) angle of incidence. If the irradiated infrared radiation or the detected infrared radiation exhibits an angular variation,The mean angle of the respective angular distribution is considered the angle of incidence or detection angle. Preferably, either the angle of incidence or the detection angle is oriented perpendicular to the value document, and the other angle is oriented obliquely. Preferably, the detection angle and the angle of incidence are of different sizes. It is therefore preferable that the law of reflection (size of the angle of reflection = size of the angle of incidence) is not observed when measuring at the different locations, in order to enable even more reliable identification of the locations on such a reflective or optically variable security element. Advantageously, a geometry is selected for irradiating the value document with infrared radiation and detecting the infrared radiation remitted by the value document, which preferably does not fulfill the classic reflection condition.This is because fewer artifacts occur due to so-called specular reflection (directed reflection or mirroring) at individual wavelengths. For example, with reflection OV elements, despite the micromirror arrays frequently used, which create moving images or patterns when tilted, no interference (e.g., overdriving of the detection device) occurs in the measurement, and no precise orientation of the banknote to the sensor is required. Instead, statistically precisely aligned micromirrors or surface areas of such an OV security element always contribute to the signal, so that reliable measurement is possible even with crumpled or folded banknotes. Such a remission measurement already delivers characteristic and stable signals even with a sensor with only a few spectral support points (i.e., measurement wavelengths).which can surprisingly be used for reliable banknote verification. Preferably, the detection angle and / or the angle of incidence is substantially the same for all locations on the value document from which the respective remitted infrared radiation is detected. Preferably, the infrared radiation, in particular for all wavelengths, is directed at the value document by the (possibly a single) illumination device at (exactly or approximately) a single angle of incidence and / or, in particular for all wavelengths, is detected by the (possibly a single) detection device at (exactly or approximately) a single detection angle. Preferably, the sensor's verification device is configured to determine the location(s) on the respective value document at which such a reflective security element is located on the value document.to use only the infrared radiation directed at the value document at (exactly or approximately) a single angle of incidence and detected at (exactly or approximately) a single angle of incidence. In other words, the reflection behavior of the value document is preferably only recorded under one (single) angular condition, i.e., under a single detection angle and a single angle of incidence, at the different locations. Surprisingly, it has been found that many reflective security elements with a viewing angle-dependent appearance, especially those with a viewing angle-dependent color change, already exhibit a characteristic signature in the infrared spectral range when measured under a uniform angular condition.by which they can be reliably identified. Repeated testing under different angles of incidence and / or detection can therefore be dispensed with. The sensor can therefore be manufactured with correspondingly less effort, e.g., with only a single irradiation device and / or with a single detection device. In addition, no moving parts are provided, which means that measurement is also possible on fast-moving valuable documents and thus no wear occurs due to mechanical movement. Approximately a single angle of incidence is also considered to exist if, due to the beam divergence of the irradiation device, slightly different angles of incidence are present, in particular with a variation of at most + / -15°, preferably at most + / -12°. Approximately a single detection angle is also considered to exist if the aperture of the detection device has slightly different detection angles,in particular with a variation of at most + / -18°, preferably at most + / -10°. Approximately a single angle of incidence is also considered if the irradiation device has several (e.g. spectrally different) IR light sources that are directly adjacent to one another or are arranged so close to one another that they direct their infrared radiation at approximately the same angle of incidence (in particular with a variation of at most + / -5°, preferably at most + / -1°) onto the value document. Approximately a single detection angle is also considered if the detection device comprises several discrete detection elements that are arranged so close to one another that they detect the remitted infrared radiation from the value document at approximately the same detection angle (in particular with a variation of at most + / -5°, preferably at most + / -1°). In the eventBecause the exact spectral profile of interference pigments is influenced by manufacturing variations, for example, and therefore spectral variations occur, it is preferable to take these spectral variations into account when identifying the locations on such reflective or optically variable security elements. Local fluctuations in the angle of incidence, e.g., due to creasing of the banknote, can also lead to variations in the resulting remission spectrum, which are preferably taken into account when determining the respective locations. The same applies to magnetically aligned printing pigments if differently oriented interference pigments are applied at different printing locations, so that the viewing angle and thus the perceived interference condition can vary with the respective location (and possibly the bending / creasing angle). Accordingly, variations in the measured spectral shape can also occur here.which are preferably taken into account when determining the relevant locations based on the respective recorded reflection spectra. Preferably, the irradiation device is configured to irradiate the respective location on the value document (simultaneously or successively) with infrared radiation having at least two, in particular at least three, different wavelengths and / or wavelength ranges, which in particular do not overlap with one another. Alternatively or additionally, the detection device is configured to detect the infrared radiation remitted from the respective location in at least two, in particular at least three, different wavelengths and / or wavelength ranges, which in particular do not overlap or only slightly overlap with one another.(either spectrally resolved or non-spectral resolved). Preferably, for the spectrally resolved detection of the remitted infrared radiation, the irradiation device is configured to successively irradiate the respective location on the value document with infrared radiation of at least two, in particular at least three, different wavelengths and / or wavelength ranges, which in particular do not overlap with one another, and the detection device is configured to successively detect the infrared radiation remitted from the respective location in the at least two, in particular at least three, different wavelengths and / or wavelength ranges in order to obtain the respective remission spectrum. For example, the measurement of the spectrally resolved IR remission takes place at one or more measuring locations on the value document to be checked,in which an area of ​​the value document is successively illuminated at the respective measuring location with a first IR light source and the remitted first intensity is detected, then the same area is illuminated with a second IR light source with a different wavelength and the remitted second intensity is detected, and optionally the same area is illuminated with one or more further IR light sources with different wavelengths and the respective remitted intensity is detected. Preferably, three, five or more IR light sources with different wavelengths, preferably LEDs, with narrow-band spectra, which in particular do not overlap spectrally, are used. The light from all IR light sources is preferably irradiated onto the value document at approximately the same angle(s) of incidence. Preferably, a transport device is provided which is configured toto transport the respective value document to be checked relative to the irradiation device and detection device in a transport direction. Preferably, the infrared radiation remitted from several locations on the value document along one or more measuring tracks running parallel to the transport direction is recorded in a spectrally resolved manner. Based on the remission spectra of the locations of one or more measuring tracks running parallel to the transport direction, the checking device then checks the value document for a reflective security element with an appearance dependent on the viewing angle. Preferably, the transport device is configured toto transport the value document during the measurements at a transport speed of approximately 2 to 11 m / s relative to the irradiation and detection device. Preferably, the irradiation device and the detection device are configured and / or controllable depending on the transport speed such that, in particular even at high transport speeds, the measuring locations or areas of the value document irradiated by different light sources at different wavelengths overlap by at least 80%, preferably more than 90%. This ensures that the remission spectrum obtained for the respective measuring location originates from essentially the same area on the value document. In a preferred embodiment, the detection device has only one or more discrete detection elements,but no image sensor. For example, each detection element of the detection device detects the infrared radiation of exactly one of the measuring tracks and / or the remitted infrared radiation of each measuring track is detected by exactly one detection element. Preferably, the testing device is configured to check the value document based on the number and / or location of the determined location(s) at which it was determined that such a reflective security element is located on the value document. The location is checked, for example, based on at least one region of interest (ROI) on the value document. Checking the value document based on the number / location of the determined locations has the advantageThat the sensor does not need to take an image of the valuable document and does not need to perform a (usually complicated) comparison of images of the valuable document. The sensor and the verification device therefore require less effort. Furthermore, the verification of the valuable document is faster and easier to understand because no image comparison is required. Preferably, the verification device is not designed to compare different images of the valuable document with each other. Preferably, the verification is based on the number and / or location of the determined location(s).whether the number of locations determined (e.g., total on the value document) is greater than or equal to a predefined first minimum number and / or whether the number of locations determined within a predefined region of interest (ROI) on the value document is greater than or equal to a predefined second minimum number and / or whether the number of locations determined outside of the (same or another) predefined region of interest (ROI) on the value document is less than or equal to a predefined maximum number, in particular equal to zero. If necessary, this can also be carried out for one or more additional ROIs on the same value document. In particular, for the purpose of authenticating and / or verifying the condition of the value document,The location(s) on the value document identified based on the reference spectra obtained are checked against one or more test criteria with regard to their number and / or location. In particular, it is checked whether a minimum number of locations exists in one or more, in particular denomination-dependent and / or transport-position-dependent, ROIs at which a reflective security element with an appearance dependent on the viewing angle was detected. The sensor's test device can be configured to select the one or more ROIs depending on the denomination and / or the transport position of the respective value document. For example, the value document is classified as "genuine" or "fit" if the number of determined locations (at which it was determined that a reflective security element with an appearance dependent on the viewing angle is located on the value document)is greater than or equal to a predefined first minimum number, and / or if the number of locations determined within a predefined region of interest (ROI) on the value document is greater than or equal to a predefined second minimum number, and otherwise classified as "false" or "suspected of being counterfeit" or "unfit". Alternatively or additionally, the value document is classified (if applicable only) as "genuine" or "fit" if only the maximum number of locations determined was determined in the (same or one or more other) ROIs / no location with such a reflective security element was determined at all. Alternatively or additionally, a check is carried out to determine whether a minimum number of locations exists in total at which the security element was detected (corresponds to a special case of the first case,where the ROI is defined by the entire value document). This reliably detects counterfeits with a missing security element without requiring information about, for example, the expected position and / or shape of the security element or the location of the value document to be checked. Alternatively or additionally, particularly in addition to the first case, it can be checked whether there is only a maximum number of locations (preferably no location at all) outside the ROIs where the OV security element was detected. All of the above-mentioned embodiments or cases, either alone or in combination, enable particularly reliable testing, in particular authenticity testing and / or condition testing.of valuable documents. The position and size of the ROI or the second minimum number can be selected based on the position and size of the security element to be checked, which is known for the valuable document. Preferred evaluation methods for determining the locations on an OV security element on the valuable document and / or for checking the valuable document based on the locations determined in this way are described below. Each of the evaluation methods, including preferred further developments, can be used both alone and in combination with at least one other evaluation method. Each of these evaluation methods makes it possible to determine, based on the respective remission spectrum, whether a reflective, in particular optically variable,Security element with an appearance dependent on the viewing angle is located on the value document or not. Evaluation method 1 Preferably, the testing device is configured to determine whether a reflective security element with an appearance dependent on the viewing angle is located on the value document at the respective location, in the remission spectrum obtained for the respective location at at least two, in particular at least three, different wavelengths O, i (i = 1, 2, 3, …) and / or wavelength ranges remission values ​​r( O i ), to derive from these at least one first characteristic value characterizing the respective remission spectrum, and to compare the first characteristic value with at least one predetermined first comparison characteristic value. The number and / or position of the different wavelengths O iare preferably selected so that characteristic points in the reference spectrum are detected, e.g., by specifically scanning a local maximum and / or a local minimum. Depending on the application, for example, in the case of different color tones for different OV security elements, the wavelengths to be selected are O ipreferably adapted. This enables a particularly simple, reliable identification of the locations located on such a reflective or optically variable security element. Preferably, the remission values ​​are determined at at least three different wavelengths in the infrared spectral range. However, for some security elements or spectral profiles, it may be sufficient or possible to use one measurement point in the red VIS range and only two wavelength measurement points in the IR spectral range to determine the locations on the OV security element, which is particularly advantageous for use with cost-effective sensors in cash deposit systems (so-called cash-in). Preferably, the first characteristic value is a difference quotient calculated from the remission values ​​determined in the remission spectrum at three or four different wavelengths O1 to O4. is formed, in particular in accordance with In this way From the remission values ​​obtained at different wavelengths, a particularly meaningful first characteristic value of the respective remission spectrum is obtained with regard to the presence of an OV security element. Preferably, the at least one predetermined first comparison characteristic value has a lower comparison threshold value and / or an upper comparison threshold value, with which the first characteristic value, in particular the difference quotient, is compared. In the case of a predetermined lower and upper comparison threshold value, the at least one first comparison characteristic value is thus given in the form of an interval within which the first characteristic value, in particular the difference quotient, must lie in order to affirm the presence of an OV security element. Alternatively or additionally, the testing device is configured to and / or at least one of at least some of the determined remission values ​​r(O i ) derived second characteristic value, |, with at least one predetermined second comparison value, e.g. threshold S0 or S1. For example, a reflectance spectrum obtained from a single measurement location is rated as OK or the measurement location in question is identified as a location on an OV security element if the reflectance spectrum has a minimum absorption, e.g. r(O0) ^ S0 (second comparison value), and the difference quotient (r(O1) - r(O2)) / (r(O3) - r(O4)) lies within a predetermined interval (i.e. between an upper and lower comparison threshold value). Preferably, to ensure the numerical stability of the difference quotient, it can additionally be required that the absolute value of the denominator is |r(O3) - r(O4)| ^ S1 (second comparison value). With this simple mechanical process, for example, color-changing OV security elements (so-calledColor-shift inks), whose color depends on the viewing angle, can be reliably distinguished from conventional printing inks and IR-absorber inks. In simplified embodiments, O1 and O3 can be identical, i.e., reflectance values ​​at only three different wavelengths are used to calculate the difference quotient or determine the locations on the OV security element. Furthermore, O0 can coincide with one of the wavelengths O1 to O4. Alternatively or additionally, r(O0) can represent an average of several reflectance values ​​from r(O1) to r(O4). In a further embodiment, the reflectance spectrum can be tested even more precisely by not only determining or testing one difference quotient, but also by determining and testing two or more difference quotients using additional wavelengths.Overall, this evaluation method is particularly suitable for small, relatively inexpensive sensors, since in the simple case only three different wavelengths (of which possibly only two different wavelengths in the IR) are necessary for scanning, and the evaluation requires relatively little numerical effort. Evaluation method 2 Preferably, the testing device is set up to determine whether a reflective security element with an appearance dependent on the viewing angle is located on the value document at the respective location, to determine a distance between the, in particular standardized, remission spectrum obtained for the respective location and at least one predetermined, in particular standardized, reference spectrum and to check whether the determined distance is smaller than a predetermined comparison distance. In principle, for the preferably carried out standardization of the respective remission spectrum orAny type of standardization can be used to determine the reference spectrum, ensuring that fluctuations caused by the respective measurement or comparison measurement (for the reference spectrum), such as in the intensity of the infrared radiation incident on the respective location and / or in the sensitivity of the detector device during detection of the infrared radiation remitted from the location, are essentially eliminated. Preferably, the remission spectrum to be tested and the associated reference spectrum are standardized, for example, by a linear transformation that maps the minimum to 0 and the maximum to 1. For example, such standardization is carried out according to the following formula r. i norm = ((r i – min) / (max – min)) i=1…n , where the reflectance spectrum to be tested, sampled at n wavelengths, is determined by the reflectance values ​​r1, …, r n is given and “min” is the minimum of all r iand “max” the maximum of all r i Alternatively, such a normalization can also be carried out, for example, by a linear transformation in which the mean is 0 and the standard deviation is 1, for example according to the following formula r i norm = ((r i – μ) / ǔ) i=1…n , where μ is the mean of all r i and ǔ the standard deviation of all r i Preferably, the distance between the two standardized spectra (measured remission spectrum vs. reference spectrum) is determined, e.g., using a p-norm of their difference, and then checked to see whether this is smaller than a specified threshold. If the standardized remission spectrum is determined by the remission values ​​r1 norm , …, r n norm and the normalized reference spectrum is given by the values ​​R1, … R n , first the pointwise difference x i = r i norm -R iThe p-norm of the difference is then determined by the formula ^σ^ ^ ୀ^ | ^^ |^ ^^ / ^defined. For p = 2, it corresponds to the Euclidean distance. By selecting the appropriate threshold, the spectral variations that occur are absorbed, and the OV security elements on banknotes from all production batches are recognized as correct or authentic. This evaluation method is particularly suitable for cases in which the spectrum was sampled at several, especially more than three or four, wavelengths, so that an entire spectral curve is available. In these cases, it offers a complete and very precise examination of the entire spectral profile despite very simple adaptation.Evaluation method 3 The testing device is preferably set up to determine whether a reflective security element with an appearance dependent on the viewing angle is located on the value document at the respective location, by determining a difference between the reflectance spectrum obtained for the respective location and a comparison spectrum adapted to the reflectance spectrum by means of a fitting calculation and to check whether the determined difference is smaller than a predetermined comparison difference, wherein the comparison spectrum is preferably given by a linear combination of at least two predetermined reference spectra. The predetermined reference spectra are preferably representative reference reflectance spectra which, for example, can occur in extreme cases due to production fluctuations and / or due to different pigment orientation within a banknote.Preferably, the reflectance spectrum (to be tested) obtained from a location is fitted using the reference reflectance spectra A and B, e.g., by the linear combination c1A + c2B, the so-called fit spectrum, with the fit parameters c1 and c2. In a preferred embodiment, the permissible ranges of the coefficients c1 and c2 for the fit are subject to predefined restrictions such as c1>0 and c2>0. The fit is then tested, e.g., by determining a p-norm of the difference between the measured reflectance spectrum and the fit spectrum, and comparing the p-norm with a correspondingly specified comparison difference. A fit using the linear combination c0 + c1A + c2B with an additional fit parameter c0, the offset, is even more precise. Preferably, it should be ruled out that every constant spectrum could be fitted exactly with c1=c2=0, by only considering pixels or measurement locations whose fit, e.g.,|c1| + |c2| is above a predefined threshold or whose remission spectrum is sufficiently non-constant so that, for example, the standard deviation or difference between maximum and minimum is above a predefined threshold. This method can be further improved if a preselection of the remission spectra to be tested is also carried out. Preferably, it can be provided that only those pixels or measurement locations are considered when testing the value document whose remission spectrum differs sufficiently from a constant curve in the IR range, e.g., where the difference between maximum and minimum exceeds a predefined threshold. An IR absorption feature is presumably present at these locations.Accordingly, the testing device is preferably configured to consider only those locations when testing the value document for which a reflectance spectrum was obtained that fulfills at least one of the following conditions: i) the course of the reflectance spectrum is not constant and / or has a predetermined minimum difference compared to a constant course and / or ii) the difference between a maximum and a minimum of the reflectance spectrum is greater than a predetermined minimum difference. Such preselection can optionally also be used, preferably, in the other evaluation methods 1, 2, 4, or 5. This evaluation method is also particularly suitable for applications in which the reflectance spectrum was sampled at multiple wavelengths, so that an entire spectral curve is present in one or more measurement tracks.In these cases, it offers particularly robust and stable detection of the OV security elements. Furthermore, the evaluation method is particularly well suited for OV security elements with printed, locally differently aligned interference pigments. Evaluation method 4 Preferably, the testing device is set up to determine whether a reflective security element with an appearance dependent on the viewing angle is located on the value document at the respective location, to form a derivative curve by deriving the remission spectrum obtained for the respective location, in particular a standardized one, with respect to the wavelength and to check whether the derivative curve lies within a predetermined tolerance range and / or has a difference from a predetermined target derivative curve which is smaller than a predetermined maximum difference.Preferably, the remission spectrum to be tested is first normalized, for example by a linear transformation by which the minimum is mapped to 0 and the maximum to 1 (formula see above), or by a linear transformation such that (μ, ǔ) = (0, 1) (formula see above). The derivative curve is then preferably formed (1st derivative with respect to the wavelength). For example, for a remission spectrum which is in the form of n intensity values ​​(r(^. i )) i=1,..,n is present, the derivative can be calculated as (r'(^ i )) i=2,…,n = ((r(^ i ) – r(^ i-1 )) / ( ^ i – ^ i-1 )) i=2,..,n. A check is then carried out to determine whether this derivative curve lies within the specified tolerance range. The specified tolerance range can, for example, be determined in advance by calculating the derivative curve for a large number of standardized reflectance spectra obtained from (comparison) value documents and determining the mean and standard deviation for each wavelength. If the aim is not only to check whether the respective reflectance spectrum obtained fits, but also how well it fits, a difference curve to the target derivative curve can be calculated alternatively or additionally, e.g. the difference curve to the target derivative curve weighted “channel by channel” (i.e. wavelength by wavelength) with the reciprocal width of the tolerance range, and this can be reduced to a value, e.g. with a p-norm. This value can then be compared with an upper threshold, whereby the presence of an OV security element is affirmed if the upper threshold is not exceeded.This method is particularly suitable for cases where the respective remission spectrum has been sampled at several (preferably many) wavelengths, resulting in a complete spectral curve. In these cases, it offers particularly precise separation of spectra of different OVI elements despite minimal adaptation effort.Evaluation method 5 The testing device is preferably set up to determine whether a reflective security element with an appearance dependent on the viewing angle is located on the value document at the respective location, by determining the wavelengths of two or more local extrema, in particular of at least one local maximum and at least one local minimum, in the remission spectrum obtained for the respective location and to check whether the determined wavelengths lie within wavelength intervals which are predetermined for the respective extrema, and / or whether only one local extremum lies within a wavelength interval predetermined for the respective extremum. The testing device is preferably set up to form a quotient from each two determined wavelengths and to check whether the quotient lies within a predetermined tolerance range.In this case, the wavelengths of the local extrema of the remission spectrum are first determined (possibly interpolated). Then, based on wavelength intervals defined in advance for the minima and maxima, a check is made as to whether one, in particular exactly one, local minimum or maximum lies in each of the wavelength intervals. Alternatively or additionally, a selection of pairs of the specified wavelength intervals is defined, and a check is made as to whether the quotients of the wavelengths of the corresponding extrema lie within specified tolerance ranges. Preferably, wavelength intervals are specified for one to three minima and one to three maxima. Preferably, one maximum and two minima lie in the IR range. For example, wavelength intervals are defined for one maximum and one minimum, one maximum and two minima, two maxima and one minimum, two maxima and two minima, two maxima and three minima, three maxima and two minima, or three maxima and three minima.This method is particularly suitable for cases in which the remission spectrum was scanned at several individual wavelengths. It is not absolutely necessary for an entire spectral curve to be available. It offers particularly low adaptation effort, since it is possible to simultaneously detect very many different interference-based OV color impressions with a single adaptation and to reliably distinguish these from absorption pigments. Further advantages, features, and possible applications of the present invention will become apparent from the following description in conjunction with the figures. They show: Fig. 1 an example of a sensor for checking valuable documents; Fig. 2 a first example of a remission spectrum; Fig. 3 a first example of a diagram with a representation of the numerator and denominator of difference quotients; Fig.Fig. 4 shows a second example of a diagram with a representation of the numerator and denominator of difference quotients; Fig. 5 shows examples of remission spectra before (left) and after (middle, right) normalization; Fig. 6 shows examples of reference spectra; Fig. 7 shows examples to illustrate an adjustment of a remission spectrum by a first linear combination of the reference spectra (left) and a second linear combination of the reference spectra (right); Fig. 8 shows examples of locations determined with one-dimensional and two-dimensional adjustment; Fig. 9 shows an example of a normalized remission spectrum; Fig. 10 shows an example of a derivative curve calculated from the normalized remission spectrum shown in Fig. 9, as well as a specified tolerance range; and Fig. 11 shows an example of a remission spectrum in which distributions of the respective spectral position of local minima or maxima of several measured remission spectra are plotted.Figure 1 shows a schematic representation of an example of a sensor 10 for checking value documents 1, in particular banknotes, which have at least one such reflective, optically variable (OV) security element 2. In the present example, the security element 2 is designed as a so-called lead strip, which extends across the entire width of the value document 1. Such a lead strip is applied, for example, to a paper primer and provided with an integrating overprint, typically by means of intaglio embossing. In principle, however, the security element 2 can have any other shape and / or be based on other functional principles for generating an appearance that depends on the viewing angle, in particular in the visible spectral range, such as in the form of foil elements, patches, wide security threads or printed, magnetically aligned interference pigments.Thus, in film-based OV security elements, in addition to micromirrors, stacks of thin, thickness-controlled layers are also used to produce angle-dependent interference colors or color changes. The sensor 10 has an irradiation device 11 which is designed to irradiate a plurality of different locations 3, 3' with infrared radiation. This illumination of the different locations preferably takes place sequentially in time. The different locations 3, 3' are also referred to as "measurement locations", "measurement pixels" or "pixels" in the context of the present disclosure. The infrared radiation remitted by the different locations 3, 3' is detected by a detection device 12, wherein a remission spectrum is obtained for each of the locations 3, 3'. The irradiation device 11 and / or the detection device 12 can orcan be designed in various ways in order to obtain a remission spectrum from each of the locations 3, 3'. In the general case, it can be provided that, for example, the irradiation device 11 irradiates the value document 1 with broadband infrared radiation and the detection device 12 detects the remitted infrared radiation at a plurality of different wavelengths in a spectrally resolved manner. In a particularly preferred variant that is easy to implement, it can also be provided that the irradiation device 11 has two or more different radiation sources 11a to 11c which emit infrared radiation at different wavelengths or in different wavelength ranges, with which the respective location 3, 3' is successively irradiated.The detection device 12 is preferably configured to detect the infrared radiation successively remitted at different wavelengths from the respective location 3, 3', whereby a remission spectrum with remission values ​​at, in the present example, three different wavelengths is obtained for each location 3, 3'. For reasons of clarity, only two locations 3, 3' are shown here. However, it is preferred that a remission spectrum is recorded from each of several locations 3, 3' on the value document 1. Furthermore, for reasons of clarity, the locations 3, 3' or the corresponding areas that are irradiated with infrared radiation or from which the remitted infrared radiation is detected are shown relatively large.In principle, the areas at locations 3, 3' can be smaller, possibly significantly smaller, or even larger, depending on the minimum spatial resolution required by the geometric extent of the security elements to be tested. In principle, locations 3, 3' or the respective areas can have any desired shape. In the case of a detection device 12 with one or more rectangular, in particular square, detector elements, locations 3, 3' preferably have a substantially rectangular or square shape. In the case of a detection device 12 with one or more round detector elements, locations 3, 3' preferably have a substantially oval or round shape.Preferably, a transport device 15 is provided, which is only indicated schematically in the example shown and is designed to transport the value document 1 to be checked relative to the irradiation device 11 and detection device 12 in a transport direction T. Preferably, the infrared radiation remitted from a plurality of locations 3, 3' along one or more measuring tracks M running parallel to the transport direction T is successively recorded. For reasons of clarity, the locations 3, 3' are only shown along a single measuring track M in the present example. In principle, the irradiation device 11 and / or the detection device 12 can also be designed such that remission spectra can be recorded from two, preferably six, or more locations from two, preferably six, or more measuring tracks M running parallel to one another.Alternatively or additionally, it is preferred that the irradiation device 11 is configured to irradiate a linear region of the value document 1 running substantially perpendicular to the transport direction T with infrared radiation, and that the detection device 12 has a so-called sensor line with a plurality of detector elements arranged perpendicular to the transport direction T, by means of which the infrared radiation remitted by the irradiated linear region of the value document 1 can be detected in a spatially resolved manner. In this embodiment, preferably only a few different infrared wavelengths are used, in particular two different infrared wavelengths or three different infrared wavelengths alternately.By successively detecting the infrared radiation remitted in each case during the transport of the value document 1 in the transport direction T, a spectrally resolved remission image of the entire value document 1 is obtained, the image points (pixels) of which are formed by the individual locations 3, 3'. The infrared beam generated by the irradiation device 11 strikes the respective location 3 at an average angle of incidence D (relative to the normal on the value document 1), and the infrared radiation remitted from the location 3 is detected by the detection device 12 at a detection angle E, which is preferably different from the average angle of incidence D. Furthermore, it is preferred that all locations 3, 3' examined on the value document 1 are measured for all wavelengths under only a single angular condition, i.e. a single angle of incidence D and a single detection angle E.The sensor 10 further comprises a testing device 13 which is configured to determine, based on the reflectance spectra obtained for the different locations 3, 3', whether the location 3, 3' in question is located on or at the reflective optically variable (OV) security element 2. This is done, for example, by processing and / or analyzing the respective reflectance spectrum obtained and / or comparing it with one or more predetermined spectra. This is explained in more detail below using preferred evaluation methods. Based on the locations 3, 3' determined or classified in this way (e.g., "not on the OV security element" or "on the OV security element"), the value document 1 is then checked, with particular reference to the number and / or location of the locations 3' determined or classified as "on the OV security element".Preferably, a check is carried out to determine whether the number of locations 3' on the value document that have been determined or classified as "on the OV security element" is greater than or equal to a predetermined first minimum number, which may depend, for example, on the size of the security element 2. In this way, an individual security element 2 on the value document 1 can be reliably detected. Alternatively or additionally, a check can be carried out, for example, to determine whether the number of locations 3' that have been determined or classified as "on the OV security element" within at least one predetermined region of interest ROI is greater than or equal to a predetermined second minimum number. This allows not only the presence of a security element 2 on the value document 1 to be reliably detected, but also its correct position (in the ROI) and / or minimum size (second minimum number).Furthermore, it is also possible to check this for two or more different reflective optically variable security elements on the value document 1. Optionally, it is also possible to check whether the number of locations 3' determined or classified as "on the OV security element" which lie outside the specified region of interest ROI is less than or equal to a specified maximum number, in particular equal to 0. This ensures that there are hardly any or no locations classified as "on the OV security element" outside the region of interest ROI. If one or more of these test criteria is affirmed, the value document 1 is classified, for example, as "genuine" and / or "fit". Otherwise, the value document 1 is classified as "forgery" and / or "unfit".The sensor 10 is preferably used in a value document processing system 100, which is only indicated very schematically in the present example and has one or more processing devices 101 to 103 which are designed to process value documents 1, in particular to separate, transport, check, sort, stack and / or destroy them. In the following, preferred evaluation methods for determining or classifying the locations 3' located on the security element 2 or for checking the value document 1 based on the determined locations 3' are explained in more detail by way of example with reference to the figures. Evaluation Method 1 In this preferred method, the remission r(nj. i ) at the respective location 3, 3' at (at least) three different wavelengths nj imeasured. These are preferably chosen so that characteristic points in the spectrum are recorded which are known to be characteristic of the reflectance spectrum of the OV security element to be tested, e.g. at which it has a maximum or a minimum. For different OV color tones, the wavelengths to be evaluated are preferably adapted. A reflectance spectrum recorded in this way at a single measuring location 3, 3' is preferably classified as "on the OV security element" if it has a minimum absorption at one or more wavelengths (e.g. r(nj0) ^ threshold S0), and the difference quotient (r(nj1) - r(nj2)) / (r(nj3) - r(nj4)) lies within a predetermined interval, and (due to the numerical stability of the quotient) the absolute value of the denominator is |r(nj3) - r(nj4)| ^ threshold S1. With this simple procedure you can, for example,Reflective optically variable security elements, whose color depends on the viewing angle and are also referred to as "color-changeable OV security elements" in the context of the present disclosure, can be distinguished from normal banknote and IR absorber colors. In further simplified preferred embodiments, nj1 and nj3 can coincide identically. nj0 can coincide with one of the wavelengths nj1 to nj4, or r(nj0) can represent an average of several remission values ​​selected from r(nj1) to r(nj4). In a further preferred embodiment, the remission spectrum can be tested even more precisely by not only determining and testing one difference quotient, but also by determining and testing two or more difference quotients using additional wavelengths.For some spectral profiles, it is even possible to perform this test with one measuring point in the red visible (VIS) spectral range and only two wavelength measuring points in the IR spectral range, which is particularly relevant for cost-effective so-called cash-in sensors. Figures 2 to 4 illustrate how a reflective optically variable security element, whose color depends on the viewing angle, is separated from conventional colors by the test (r(1100nm) - r(1520nm)) / (r(1100nm) - r(770nm)) ^ [0.3, 0.8]. Figure 2 shows a first example of a reflectance spectrum of an OV film strip. By evaluating characteristic reflectance values ​​(at the marked wavelengths O1=O. 3, O 2,O4) and calculating the difference quotient from it, a characteristic IR signature of the film strip can be clearly identified or determined. For the selection of the spectral values ​​used for evaluation, wavelengths at which the remission curve has a local minimum or maximum are preferred. Figure 3 shows a first example of a diagram with a representation of the numerator and denominator of difference quotients of different measurement pixels 3' (small dots) of an OV film strip compared to different IR absorber pigments (larger individual dots). In each case, the remission intensities r(nj1) to r(nj4) at the same wavelengths nj1 to nj4 were used. The IR signature of the OV foil strip – characterized by the difference quotient, which in the presently chosen representation corresponds to the angle of origin – is clearly distinguishable from all IR absorber colors.Figure 4 shows a second example of a diagram with a representation of the numerator and denominator of difference quotients of different measurement pixels from different production batches of an OV film strip. In each case, the remission intensities r(nj1) to r(nj4) were used at the same wavelengths nj1 to nj4. The scatter of the calculated values ​​caused by production fluctuations is preferably compensated for by selecting a suitable test interval, which is preferably defined by a lower comparison threshold S. min and an upper comparison threshold S maxis taken into account. By appropriately selecting this interval, the spectral variations caused by production fluctuations can easily be absorbed, so that not only a single banknote, but the OV security elements of banknotes from all production batches can be reliably detected. The influence of production fluctuations on the testing of the OV security elements is thus reliably eliminated or at least minimized. The respective difference quotient can be seen in Figures 3 and 4 by mentally forming the quotient y / x for a point (x, y). The respective difference quotient therefore corresponds to the gradient of the straight line through the origin through the point in question. The two drawn straight lines through the origin with gradients of 0.3 and 0.8 represent the limits [0.3, 0.8] of the permitted range for the difference quotients.Evaluation method 2 In this preferred method it is assumed that a remission spectrum to be tested, scanned at n wavelengths, is determined by the remission values ​​(r1, ..., r. n ) is given. In a first step, the reflectance spectrum of the OV film strip to be tested from the example described above and a reference spectrum are normalized, e.g. by a linear transformation that maps the minimum to 0 and the maximum to 1, for example according to the formula r i norm = ((r i – min) / (max – min)) i=1…n , where “min” is the minimum of all r i and “max” the maximum of all r i or by a linear transformation so that the mean is 0 and the standard deviation is 1, for example according to the formula r i norm = ((r i – μ) / ǔ) i=1…n , where μ is the mean of all r i and ǔ the standard deviation of all r iFigure 5 shows examples of reflectance spectra from various measurement locations of an OV foil element before (left) and after (center, right) standardization. As can be seen, the original reflectance spectra (left) show a strong variation, which is significantly reduced by linear standardization (center, right), so that a quantitative comparison with a standardized reference spectrum is possible. Subsequently, a distance between the two standardized spectra (measured reflectance spectrum vs. reference spectrum) is determined, e.g., using a p-norm of their difference, and it is checked whether this distance is smaller than a predetermined threshold, which in the context of the present disclosure is also referred to as the "comparison distance." By appropriately selecting this threshold, the occurring spectral variations are absorbed, so that OV security elements of value documents or banknotes from all production batches can be reliably detected.Evaluation method 3 In this preferred method it is assumed that the measured or to be measured reflectance spectra of OV security elements, e.g. for extreme cases of production fluctuations and / or spectral variations occurring due to different pigment orientation within a banknote, can be approximated by means of representative reference reflectance spectra A and B. Preferably, the respectively obtained reflectance spectrum to be tested is combined with the reference reflectance spectra A and B by a first linear combination c1 * A + c2 * B, the fit spectrum with the fit parameters c1 and c. 2,approximated (so-called fit) and checked how well the fit fits, e.g. with a p-norm of the difference between the measured remission spectrum and the fit spectrum, which in the context of the present disclosure is also referred to as an (adjusted) “comparison spectrum”. Optionally, the fit is even more precise using a second linear combination c0 + c1*A + c2*B with an additional fit parameter c0, the offset. It should preferably be taken into account here that with c1 = c2 = 0 any constant spectrum could be fitted exactly. To exclude this case, preferably only pixels or locations are taken into account for which the fit, e.g. |c1| + |c2|, lies above a predetermined threshold, or whose respective remission spectrum is sufficiently non-constant so that, e.g. the standard deviation or difference between maximum and minimum lies above a predetermined threshold.Figure 6 shows examples of two reference reflectance spectra whose linear combination is fitted to a measured reflectance spectrum. Figure 7 shows examples illustrating a fit (dashed line) of a measured reflectance spectrum (solid line) using the above-described first linear combination of the reference reflectance spectra (left) and the above-described second linear combination of the reference reflectance spectra with an offset (right). The method can be further improved by additionally preselecting the reflectance spectra to be tested, by using only those pixels or locations in the evaluation whose reflectance curve differs sufficiently from a constant curve in the IR range, e.g., where the difference between maximum and minimum exceeds a predetermined threshold. An IR absorption feature is presumably present at these points.This preselection can optionally also be used for the other evaluation methods. In the following example, the 2-dimensional fitting of a reflectance spectrum to be tested using the two reference spectra A and B (with the fitting parameters c1 and c2 and optionally c0) for a value of “100” printed with magnetically aligned interference pigments is demonstrated and compared with the case of a fitting using a single reference reflectance spectrum (1-dimensional fitting). Figure 8 shows a comparison of the locations or measurement pixels of a lettering “100” printed with aligned interference pigments that were correctly identified using a 1-dimensional fitting (top) and a 2-dimensional fitting (bottom). In the pixel images reproduced here, those measurement locations are shown in black where the distance (e.g. p-norm with p=2, i.e. the Euclidean distance) between the fitted spectrum (fit spectrum orComparison spectrum) and the measured remission spectrum is smaller than a test threshold, which in the context of the present disclosure is also referred to as a "comparison difference". For the authenticity assessment, it is checked whether a minimum number of pixels or locations fulfills this criterion. Additionally or alternatively, a comparison with the expected shape of the "100" is possible with sufficient spatial resolution of the measurement. With a 1-dimensional adjustment, only relatively few valid pixels are obtained compared to a 2-dimensional adjustment. Since the alignment of the interference pigments in this printed image systematically varies over the height, in the 1-dimensional case the pixels matching the reference are concentrated in a narrow height area. This is preferably the case with sensors with only a few (or possiblyonly one) measuring tracks are taken into account by adjusting their position as precisely as possible to this narrow range in the case of an evaluation with 1-dimensional adaptation. In order to keep possible influences due to so-called run-up / run-down variations during the transport of the value document to be checked as low as possible, an evaluation with 2-dimensional adaptation is preferred because this is much more tolerant with regard to the position of the respective measuring track or the track height than the evaluation with 1-dimensional adaptation. Evaluation method 4 In this preferred method, the reflectance spectrum to be checked is first normalized, e.g. by a linear transformation that maps the minimum to 0 and the maximum to 1, for example according to the formula r. i norm = ((r i – min) / (max – min)) i=1…n (see above), or by a linear transformation such that (μ, ǔ) = (0,1), for example according to the formula ri norm = ((r i – m) / ǔ) i=1…n, (see above). The derivative curve (1st derivative with respect to the wavelength) of the standardized reflectance spectrum is then created, and a check is carried out to determine whether the derivative curve lies within a predefined tolerance range. The tolerance range is preferably determined or specified by calculating the derivative curves for a large number of genuine, i.e., standardized reflectance spectra measured on genuine banknotes or reference samples, and by determining the mean value and standard deviation for each wavelength. The tolerance range then corresponds to the area around the spectral curve of the determined mean values, which is delimited by the spectral curve of the determined standard deviations. Figure 9 shows an example of a standardized reflectance spectrum. Figure 10 shows an example of a derivative curve calculated from this (solid line) and a predefined tolerance range (dashed lines).If it is not just necessary to check whether the respective remission spectrum fits, but also how well it fits, a difference curve between the respective derivative curve and a predetermined target derivative curve can be calculated channel by channel (i.e. for the individual wavelengths) with the reciprocal width of the tolerance range and this difference curve can then be reduced to a value, e.g. with a p-norm. This value can then be compared with an upper threshold, which in the context of the present disclosure is also referred to as the "maximum difference". Evaluation method 5 In this preferred method, the wavelengths of the local extremes (possibly interpolated) of the respective obtained remission spectrum are first determined. Furthermore, intervals are determined or predetermined in advance for the minima and maxima, which in the context of the present disclosure are also referred to as "wavelength intervals".It is then checked whether exactly one local minimum or maximum lies in each of these intervals. Optionally, a selection of pairs of these intervals can also be defined or specified, and a check can be made as to whether the quotients of the wavelengths of the corresponding extremes lie within specified tolerance ranges. Such wavelength intervals are preferably defined for one to three minima and one to three maxima. Preferably, one maximum and two minima lie in the IR range. For example, wavelength intervals are defined for one maximum and one minimum, one maximum and two minima, two maxima and one minimum, two maxima and two minima, two maxima and three minima, three maxima and two minima, or three maxima and three minima. Figure 11 shows an example of a remission spectrum in which frequency distributions of the respective spectral position of local minima “min” or maxima “max” of several measured remission spectra are plotted.In general, color-changeable OV security elements in different colors are provided for different currencies and denominations, and possibly even at different locations within a banknote. In order to keep the adaptation effort as low as possible, a simplified, albeit somewhat less precise, test of such color-changeable OV security elements can preferably be carried out. For this purpose, a special case of the evaluation described above with less adaptation effort can be used as follows: First, two generous intervals for minima are selected, e.g. [600 nm, 1000 nm] and [1000 nm, 1700 nm]. Then the quotient of the wavelengths of the corresponding minima is checked to see whether it lies, for example, in the interval [1.6, 1.75], or more precisely, whether it lies in a type-dependent narrower interval.Although this does not check whether a color-changeable OV security element with a specific color is present, it only checks whether such an OV security element with any color is present. Nevertheless, a missing color-changeable OV security element and / or normal colors and IR-absorber colors can be reliably detected in this way, and the corresponding banknote can be rejected or classified as a counterfeit.

Claims

Patent Claims 1. Sensor (10) for checking value documents (1) which have at least one reflective security element (2) which has an appearance in the visible spectral range that depends on a viewing angle, comprising: - an irradiation device (11) which is configured to irradiate one or more different locations (3, 3') on a value document (1) with infrared radiation, - a detection device (12) which is configured to detect the infrared radiation remitted from the respective location (3, 3') on the value document (1), wherein the sensor is configured to detect the remitted infrared radiation in a spectrally resolved manner in order to obtain a remission spectrum at the respective measurement location, and - a testing device (13) for checking the value document with respect to a reflective security element (2) which has an appearance that depends on the viewing angle,which is configured to determine, based on the remission spectrum obtained for the respective location (3, 3'), whether a reflective security element (2) with an appearance dependent on the viewing angle is located on the value document (1) at the respective location (3, 3'), in order to determine one or more locations at which such a reflective security element (2) is located on the value document (1), and to check the value document (1) based on the determined location(s) (3').

2. Sensor (10) according to claim 1, wherein the detection device (12) is configured and / or arranged to detect the infrared radiation remitted from the respective location (3, 3') at a detection angle (E) and / or the irradiation device is configured and / or arranged to, To direct infrared radiation onto the value document such that it strikes the value document (1) at an angle of incidence (D), wherein the detection angle (E) and the angle of incidence (D) are preferably different.

3. Sensor (10) according to claim 1 or 2, wherein the testing device (13) is configured to use only the infrared radiation directed onto the value document at exactly or approximately a single angle of incidence and detected at exactly or approximately a single detection angle to determine whether a reflective security element with an appearance dependent on the viewing angle is located at the respective location on the value document.Sensor (10) according to one of the preceding claims, wherein for the spectrally resolved detection of the remitted infrared radiation, the irradiation device (11) is designed to successively illuminate the respective location (3, 3') on the value document (1) with infrared radiation of at least two, in particular at least three, different wavelengths (O. i ) and / or wavelength ranges which in particular do not overlap with each other, and the detection device (12) is designed to detect the radiation emitted by the respective location (3, 3') in the at least two, in particular at least three, different wavelengths (O i) and / or wavelength ranges in order to successively detect the infrared radiation remitted in each case in order to obtain the respective remission spectrum.

5. Sensor (10) according to one of the preceding claims, wherein the testing device (13) is configured to test the value document (1) based on a number and / or position of the determined location(s) (3') at which it was determined that a reflective security element (2) with an appearance dependent on the viewing angle is located on the value document (1).

6. Sensor (10) according to claim 5, wherein the checking device (13) is configured to check whether the number of determined locations (3') is greater than or equal to a predetermined first minimum number and / or whether the number of determined locations (3') lying within a predetermined region of interest (ROI) on the value document (1) is greater than or equal to a predetermined second minimum number and / or whether the number of determined locations (3') lying outside a predetermined region of interest (ROI) on the value document is less than or equal to a predetermined maximum number, in particular equal to zero.Sensor (10) according to one of the preceding claims, wherein the testing device (13) is configured to determine whether a reflective security element (2) with an appearance dependent on the viewing angle is located on the value document (1) at the respective location (3, 3'), in the remission spectrum obtained for the respective location (3, 3') at least two, in particular at least three, different wavelengths (O. i ) Remission values ​​(r i , r(O i)), to derive from these at least one first characteristic value characterizing the respective remission spectrum and to compare the first characteristic value with at least one predetermined first comparison characteristic value.

8. Sensor (10) according to claim 7, wherein the first characteristic value is a difference quotient which is formed from the remission values ​​(r(O1) to r(O4)) determined in the remission spectrum at three or four different wavelengths (O1 to O4) ((r(O1) - r(O2)) / (r(O3) - r(O4))).

9. Sensor (10) according to one of claims 7 to 8, wherein the testing device (13) is configured to compare at least one of the determined remission values ​​(r(O i ), r(O0)) and / or at least one of at least a part of the determined to compare the second characteristic value (r(O0),|r(O3) - r(O4)|) derived from the reflectance values ​​with at least one predetermined second comparison characteristic value (S0, S1).

10. Sensor (10) according to one of the preceding claims, wherein the testing device (13) is configured to determine whether a reflective security element (2) with an appearance dependent on the viewing angle is located on the value document (1) at the respective location (3, 3'), to determine a distance between the reflectance spectrum obtained, in particular standardized, for the respective location (3, 3') and at least one predetermined, in particular standardized, reference spectrum, and to check whether the determined distance is smaller than a predetermined comparison distance.Sensor (10) according to one of the preceding claims, wherein the testing device (13) is configured to determine whether a reflective security element (2) with an appearance dependent on the viewing angle is located on the value document (1) at the respective location (3, 3'), to determine a difference between the remission spectrum obtained for the respective location (3, 3') and a comparison spectrum (Fit) adapted to the remission spectrum by means of a compensation calculation, and to check whether the determined difference is smaller than a predetermined comparison difference, wherein the comparison spectrum is preferably given by a linear combination of at least two predetermined reference spectra.

12. Sensor (10) according to one of the preceding claims, wherein the testing device (13) is configured to take into account only those locations (3') for which a remission spectrum is respectively available when checking the value document. rum was obtained which fulfills at least one of the following conditions: - the course of the remission spectrum is not constant and / or has a predetermined minimum difference compared to a constant course and / or - the difference between a maximum and a minimum of the remission spectrum is greater than a predetermined minimum difference. 13.Sensor (10) according to one of the preceding claims, wherein the testing device (13) is configured to determine whether a reflective security element (2) with an appearance dependent on the viewing angle is located on the value document (1) at the respective location (3, 3'), to form a derivative curve by deriving the remission spectrum obtained, in particular standardized, for the respective location (3, 3') and to check whether the derivative curve lies within a predetermined tolerance range and / or has a difference from a predetermined target derivative curve which is smaller than a predetermined maximum difference. 14.Sensor (10) according to one of the preceding claims, wherein the testing device (13) is configured to determine whether a reflective security element (2) with an appearance dependent on the viewing angle is located on the value document (1) at the respective location (3, 3'), to determine the wavelengths of two or more local extrema, in particular of at least one local maximum ("max") and at least one local minimum ("min"), in the remission spectrum obtained for the respective location (3, 3') and to check whether the determined wavelengths lie within wavelength intervals which are predetermined for the respective extremes, and / or whether only one local extreme lies within a wavelength interval predetermined for the respective extremum.

15. A method for checking value documents (1) which have at least one reflective security element (2) which has an appearance in the visible spectral range that depends on a viewing angle, comprising the following steps: - irradiating one or more different locations (3, 3') on a value document (1) with infrared radiation and spectrally resolving the infrared radiation remitted from the respective location (3, 3') on the value document (1), wherein a remission spectrum is obtained in each case, - determining, based on the remission spectrum obtained for the respective location (3, 3'), whether a reflective security element (2) with an appearance that depends on the viewing angle is located on the value document (1) at the respective location (3, 3'), in order to determine one or more locations at which such a reflective security element (2) is located on the value document (1),and - checking the value document (1) based on the determined location(s) (3').,

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