WO2024012634A1

WO2024012634A1 - Sensor and method for checking valuable documents having at least one reflective security element

Info

Publication number: WO2024012634A1
Application number: PCT/DE2023/100521
Authority: WO
Inventors: Thomas Happ; Wolfgang Rauscher
Original assignee: Giesecke+Devrient Currency Technology Gmbh
Priority date: 2022-07-14
Filing date: 2023-07-12
Publication date: 2024-01-18
Also published as: DE102022002581A1

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

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 which have at least one reflective security element which has an appearance that depends on a viewing angle in the visible spectral range. To increase security against forgery, documents of value, 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 that 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 document of value. In foil-based OV security elements, in addition to holograms and micromirrors, stacks of thin layers are also used to create angle-dependent interference colors or color changes. It is an object of the present invention to enable reliable machine checking of valuable documents, in particular banknotes, provided with reflective, for example optically variable, security elements. This task is solved by a sensor and a method for checking valuable documents according to the independent claims and a valuable document processing system with such a sensor. According to a first aspect of the present disclosure, a sensor for checking valuable documents which have at least one reflective security element which, in particular in the visible spectral range, has one of a has an appearance dependent on the viewing angle, on: an irradiation device which is set up to irradiate one or more different locations on a document of value with infrared radiation; a detection device which is set up to detect the infrared radiation remitted from the respective location on the document of value, the sensor being set up to detect the remitted infrared radiation in a spectrally resolved manner, with a remission spectrum being obtained in each case; and a testing device for checking the document of value with regard to a reflective security element, which has an appearance dependent on the viewing angle, which is set up to use the remission spectrum obtained for the respective location to determine whether there is something at the respective location a reflective, in particular optically variable, security element with an appearance that depends on the viewing angle is located on the document of value in order to determine one or more locations at which such (ie with an appearance that depends on the viewing angle) is reflected - the security element (the same or possibly different) is located, and the value document is checked based on the location(s) determined (with regard to a reflective security element with an appearance that depends on the viewing angle). According to a second aspect of the present disclosure, a method for checking valuable documents which have at least one reflective security element, which has an appearance that depends on a viewing angle, particularly in the visible spectral range, has the following steps: irradiating one or more different locations on a valuable document - ment with infrared radiation and spectrally resolved detection of the infrared radiation remitted from the respective location on the document of value, whereby a remission spectrum is obtained in each case; Using the remission spectrum obtained for the respective location, determine whether there is a reflected light at the respective location. of the, in particular optically variable, security element with an appearance dependent on the viewing angle is located on the document of value in order to determine one or more locations at which such a reflective security element (ie with an appearance dependent on the viewing angle) is located located; and checking the document of value based on the location(s) determined, for which it was determined that there is a reflective, in particular optically variable, security element (with an appearance dependent on the viewing angle) on the document of value at the respective location. A third aspect of the present disclosure relates to a valuable document processing system with at least one sensor according to the first aspect of the present disclosure and one or more processing devices which are set up to process, in particular to separate, transport, check, valuable documents sort, stack and/or destroy. A fourth aspect of the present disclosure relates to a computer program comprising instructions that, when the program is 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 the computer program transmits according to the fourth aspect of the present disclosure. Aspects of the present disclosure are preferably based on the approach of checking a document of value at several different locations, which in the context of the present disclosure are also referred to as “measuring locations”, “measuring pixels” or “pixels”, each carry out a spectrally resolved measurement of the infrared radiation remitted from the respective location and determine the location based on the remission spectra obtained or to identify those locations on the document of value at 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 reflectance spectrum obtained in each case is analyzed, for example with regard to its spectral profile and/or other properties. A reflectance spectrum in the sense of the present disclosure can be a spectrum which contains reflectance values at several, possibly even at a large number, or even at just a few, for example at three or two, different wavelengths. The use of infrared radiation to test a security element, which in the visible spectral range has an appearance that depends on the viewing angle, offers several advantages over (machine) testing with visible light: The security against forgery is increased because of the visual appearance of the security element cannot be deduced from its infrared signature. The security against counterfeiting is further increased because the test is carried out using invisible light and can therefore remain unnoticed by a potential counterfeiter. Furthermore, the test is more reliable because a test with infrared light is less affected by contamination of the banknote being tested than a test with visible light. The reflective security element, which has an appearance dependent on a viewing angle in the visible spectral range, is, for example, an optically variable security element, in particular an optically variable security element with a color change dependent on the viewing angle. The reflective one Security element that has an appearance dependent on a viewing angle in the visible spectral range can be both a specular (directional or specular) reflective security element and a diffusely reflective 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 document of value. Preferably, the irradiation device and the detection device are arranged such that the detection device detects diffusely reflected infrared radiation from the document of value, but not directed reflected infrared radiation. It is preferably checked 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 location on the document of value. If this is fulfilled, the presence of such a reflective or optically variable security element, possibly with certain properties, is confirmed and/or the document of value is classified as “genuine” or “fit” (acceptable condition). A determined location at which such a reflective security element is located can - with a correspondingly large detection area - be a single location which, for example, covers the entire area of the security element in question. A determined location at which such a reflective security element is located can (with a correspondingly smaller detection area) also be one of several locations that lie on the security element in question. The latter can, but does not have to, be subjected to spectrally resolved detection or the test described above based on their remission spectrum. The identified locations subject to the review described above or on which such a reflective or optically variable security element is located, can all lie on the same reflective or optically variable security element of the respective document of value, or on different such reflective or optically variable security elements of the respective document of value. Preferably, exactly one remission spectrum is recorded from the respective location. In particular, a reflectance spectrum is recorded from several locations on the respective valuable document, with preferably exactly one reflectance 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 document of value based on the remission spectrum obtained, it is not absolutely necessary to take into account the specific design and/or or to know specific properties of the document of value to be checked, such as the expected position of such a reflective or optically variable security element on the document of value, and to incorporate them into the test. In particular, a so-called adaptation of the sensor or method with regard to these properties is not absolutely necessary. In addition, it is not absolutely necessary to know the denomination and orientation of the respective document being examined at the time of the document inspection. On the other hand, an even more reliable identification of the relevant locations and/or examination of the document of value can of course be achieved if additional specific properties of the document of value are taken into account, in particular with regard to the position and/or size of the reflective or optically variable security element or denomination and Orientation of the value document must be taken into account. The described approach for identifying the locations lying on such a reflective or optically variable security element is based on the surprising finding that many reflective security elements, e.g. interference-based OV security elements (both film-based and printing pigment-based) have intrinsically specific spectral signatures not only in the visible spectral range (in which OV security elements are primarily tested) but also in the infrared (IR) spectral range. These can therefore be used for a mechanical inspection of the OV security elements in IR. Sensors that were developed or are used for spectral measurement of IR absorption pigments can advantageously even be used if the signal evaluation or testing used there is adapted to the special features of the spectral detection of interference colors. Overall, the present disclosure thus enables reliable and simple machine checking of valuable documents that are provided with one or more reflective or optically variable security elements with an appearance that depends on the viewing angle, in particular of such banknotes. The approach disclosed here is fundamentally suitable for testing or detecting reflective security elements with an appearance that depends on the viewing angle, but is particularly suitable for testing or detecting optically variable security elements, which, for example, have a color change that depends on the viewing angle, for example Interference is generated on thin layers. The optically variable security element can be in the form of an optically variable film element (e.g. so-called film patches or film strips), an optically variable (wide) security thread or an optically variable printing feature (e.g. with when printing before Hardening magnetically aligned interference pigments). The sensor is set up to record the infrared radiation remitted from the respective location in a spectrally resolved manner. For example, the detection device itself can be set up to spectrally decompose the remitted infrared radiation and to record it in a spectrally resolved manner. The spectral resolution is then on the detection side and the irradiation device then preferably has a spectral broadband in the infrared. Alternatively and particularly preferably, the detection device is a (simpler) detection device that is not designed for the spectral decomposition of light, for example the remitted infrared radiation. The spectral resolution then takes place through the successive irradiation of the respective location with infrared radiation of at least two or at least three different wavelengths/ranges and corresponding successive detection by means of the detection device at (at least approximately) the same location. In particular, the detection device is set up and/or arranged in such a way that the infrared radiation remitted from the different locations at a (vertical or oblique) detection angle is detected and the irradiation device is set up and/or arranged in such a way that the infrared radiation is absorbed to direct the document of value so that it hits the document of value at a (vertical or oblique) angle of incidence. If the irradiated infrared radiation or the detected infrared radiation has an angular variation, the average angle of the respective angular distribution is considered as the irradiation angle or detection angle. Preferably, either the angle of incidence or the detection angle is perpendicular and the respective other angle is oriented obliquely to the document of value. Preferably, the detection angle and the angle of incidence are of different sizes. It is therefore preferred that when measuring on the different Places the law of reflection (size of the angle of reflection = size of the angle of incidence) is not adhered to in order to enable even more reliable identification of the locations lying on such a reflective or optically variable security element. Advantageously, for the irradiation of the document of value with infrared radiation and the detection of the infrared radiation remitted by the document of value, a geometry is chosen which preferably does not meet the classic reflection condition, since then there are fewer artifacts due to a so-called specular reflection (directed reflection or reflection) occur at individual wavelengths. For example, with reflection OV elements, despite the micromirror arrays often used here, which produce moving images or patterns when tilted, no interference (e.g. overcontrol of the detection device) occurs in the measurement, and there is precisely no exact orientation Banknote required for sensor. Instead, micromirrors or surface areas of such an OV security element that are statistically just aligned always contribute to the signal, so that a reliable measurement is possible even with wrinkled or folded banknotes. Such a reflectance measurement already delivers characteristic and stable signals, even with a sensor with only a few spectral support points (ie measurement wavelengths), which surprisingly can be used for reliable banknote checking. Preferably, the detection angle and/or the incidence angle is essentially the same for all locations on the document of value from which the respective remitted infrared radiation is detected. The infrared radiation, in particular for all wavelengths, is preferably directed at the document of value in (exactly or approximately) a single angle of incidence from the (possibly a single) lighting device and/or, in particular for all wavelengths, in (exactly or approximately) one detected at a single detection angle by the (possibly a single) detection device. Preferably, the testing device of the sensor is set up to determine the location or locations of the respective document of value at which such a reflective security element is located on the document of value, only in (exactly or approximately) a single one To use infrared radiation directed at the document of value and detected in (exactly or approximately) a single detection angle. In other words, the reflection behavior of the document of value is preferably only recorded under a (single) angular condition, ie under a single detection angle and a single irradiation angle, at the different locations. Surprisingly, it was found that many reflective security elements with an appearance that depends on the viewing angle, in particular those with a color change that depends on the viewing angle, already have a characteristic signature in the infrared spectral range when measured under a uniform angle condition, which makes them reliable can be identified. Repeated testing at different irradiation and/or detection angles can therefore be dispensed with. The sensor can therefore be produced with correspondingly less effort, for example with only a single irradiation device and/or with a single detection device. In addition, there are no moving parts, which means that measurements can also be carried out on quickly moving documents of value and there is no wear and tear caused by mechanical movement. Approximately a single angle of incidence is also considered if, due to the beam divergence of the irradiation device, there are slightly different angles of incidence, in particular with a variation of at most +/-15°, preferably at most +/-12°. Approximately a single detection angle also applies if the aperture of the detection device allows slightly different detection angles, in particular with a variation of at most +/-18°, preferably at most +/-10°. Approximately a single angle of incidence also applies if the irradiation device has several (e.g. spectrally different) IR light sources that are immediately adjacent to one another or are arranged so close to one another that they emit their infrared radiation at approximately the same angle of incidence ( in particular with a variation of a maximum of +/-5°, preferably a maximum of +/-1°) towards the document of value. Approximately a single detection angle also applies if the detection device comprises several discrete detection elements which are arranged so close to one another that they detect the remitted infrared radiation at approximately the same detection angle (in particular with a variation of a maximum of +/-5°, preferably a maximum of + /-1°) from the value document. In the event that the exact spectral profile of interference pigments is influenced by manufacturing fluctuations and therefore spectral variations occur, it is preferred to take these spectral variations into account when identifying the locations on such a reflective or optically variable security element to be taken into account. Local fluctuations in the angle of incidence, for example due to wrinkling of the banknote, can also lead to variations in the reflectance spectrum obtained, 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 interference condition seen can vary with the respective location (and possibly the bend/wrinkle 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 set up to irradiate the respective location on the document of value (simultaneously or successively) with infrared radiation, which has 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 set up 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 or only slightly overlap with one another (either spectrally resolved or not spectrally resolved). For the spectrally resolved detection of the remitted infrared radiation, the irradiation device is preferably set up to successively irradiate the respective location on the document of value with infrared radiation of at least two, in particular at least three, different wavelengths and/or wavelength ranges, which in particular do not interact with each other overlap, and the detection device is set up 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 spectrally resolved IR remission is measured at one or more measuring locations on the document of value to be checked, in which an area of the document of value 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 a different wavelength and the respective remitted intensity is detected. Three, five or more IR sensors are preferred. Light sources with different wavelengths, preferably LEDs, with narrow-band spectra that, in particular, do not overlap each other spectrally, are used. The light from all IR light sources is preferably irradiated onto the document of value at approximately the same angle(s) of incidence. Preferably, a transport device is provided which is set up to transport the document of value to be checked in a transport direction relative to the irradiation device and detection device. Preferably, the infrared radiation emitted from several locations on the document of value along one or more measurement tracks running parallel to the transport direction is recorded in a spectrally resolved manner. Using the reflectance spectra of the locations of one or more measurement tracks running parallel to the transport direction, the testing device then checks the document of value with regard to a reflective security element with an appearance that depends on the viewing angle. The transport device is preferably set up to transport the document of value 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 set up and/or can be controlled depending on the transport speed in such a way that, particularly even at high transport speeds, the measuring locations or areas of the valuable document irradiated by different light sources at different wavelengths differ by at least 80%, preferably more than 90%, overlap. This ensures that the reflectance spectrum obtained for the respective measurement location comes from essentially the same area on the document of value. In a preferred embodiment, the detection device has only one or more discrete detection elements, but no image sensor. For example, one detection element of the detection device detects the Infrared radiation from exactly one of the measurement tracks and/or the remitted infrared radiation from each measurement track is detected by exactly one detection element. Preferably, the testing device is set up to check the document of value based on a number and/or location of the determined location(s) at which it was determined that such a reflective security element is located on the document of value. The situation is checked, for example, based on at least one region of interest (ROI) on the value document. Checking the document of value based on the number/location of the locations determined has the advantage that the sensor does not need to take an image of the document of value and does not need to carry out a (usually complicated) comparison of images of the document of value. The sensor and the testing device therefore require less effort. In addition, the check of the valuable document is quicker and easier to understand because there is no need to compare images. Preferably, the testing device is not designed to compare different images of the document of value with one another. When checking, it is preferably checked based on a number and/or location of the location(s) determined whether the number of locations determined (e.g. in total on the document of value) is greater than or equal to a predetermined first minimum number and/or whether the number within a predetermined region of interest (ROI) on the value document is greater than or equal to a predetermined second minimum number and/or whether the number is located outside the/a (the same or a different) predetermined region of interest (ROI) The determined locations lying on the document of value are less than or equal to a predetermined maximum number, in particular equal to zero. If necessary, this can also be done for one or more additional ROIs on the same value document. In particular, for the purpose of checking the authenticity and/or condition of the document of value, the location(s) identified on the document of value based on the reference spectra obtained are checked using one or more test criteria with regard to their number and/or location. In particular, it is checked whether in one or more, in particular denomination-dependent and/or transport position-dependent, ROIs there is a minimum number of locations at which a reflective security element with an appearance that depends on the viewing angle was determined. The testing device of the sensor can be set up to select the one or more ROI depending on the denomination and/or depending on the transport position of the respective document of value. For example, the document of value is classified as “genuine” or “fit” if the number of locations determined (at which it was determined that there is a reflective security element with an appearance that depends on the viewing angle on the document of value), is greater than or equal to a predetermined first minimum number, and/or if the number of determined locations located within a predetermined region of interest (ROI) on the document of value is greater than or equal to a predetermined second minimum number, and otherwise as “false” or classified as “suspect of counterfeit” or “unfit”. Alternatively or additionally, the value document is classified as “genuine” or “fit” (if applicable only) if only the maximum number of determined locations was determined in the ROI (the same or one or more other) or none at all Location with such a reflective safety element was determined. Alternatively or additionally, it is checked whether there is a total of a minimum number of locations where the security element was detected (corresponds to a special case of the first case, in which the ROI is given by the entire value document). As a result, counterfeits with a missing security element are reliably detected without the need for information about the expected position and/or shape of the security element or the location of the document of value to be checked. Alternatively or additionally, in particular in addition to the first case, it can be checked whether there is only a maximum number of locations outside the ROIs (preferably no location at all) where the OV security element was detected. All of the above-mentioned versions or cases enable, alone or in combination, a particularly reliable check, in particular authenticity check and/or condition check, of valuable documents. The location and size of the ROI or the second minimum number can be selected based on the location and size of the security element to be checked, which is known for the document of value. Preferred evaluation methods for determining the locations on the value document located on an OV security element and/or for checking the value document based on the locations determined are described below, each of the evaluation methods including preferred developments both on its own and in combination with at least one other evaluation methods can be used. Each of these evaluation methods in itself makes it possible to use the respective reflectance spectrum to determine whether or not there is a reflective, in particular optically variable, security element with an appearance that depends on the viewing angle on the document of value at the respective location. Evaluation method 1 Preferably, the testing device is set up to determine whether there is a reflective security element with an appearance on the document of value that depends on the viewing angle at the respective location, in the remission spectrum obtained for the respective location at least two , in particular at least three, different wavelengths O _i (i = 1, 2, 3, ...) and / or wavelength ranges to determine reflectance values r (O _i ), from these to derive at least one first characteristic value that characterizes the respective reflectance spectrum and the first characteristic value to compare with at least one predetermined first comparison parameter. The number and/or position of the different wavelengths O _i are preferably chosen so that characteristic points in the reference spectrum are detected, for example 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 O _i to be selected are preferably adapted. This makes it possible to reliably identify the locations located on such a reflective or optically variable security element in a particularly simple manner. The reflectance values are preferably determined at at least three different wavelengths in the infrared spectral range. However, for some security elements or spectral curves, it may be sufficient or possible to use one measuring point in the red VIS range and only two wavelength measuring points in the IR spectral range to determine the locations on the OV security element, which is particularly important for use with Cost-effective sensors in cash deposit systems (so-called cash-in) are a particular advantage. The first characteristic value is preferably a difference quotient, which is determined from the reflectance values determined in the reflectance spectrum at three or four different wavelengths O ₁ to O ₄

is formed, especially according to this way

A first characteristic value of the respective reflectance spectrum that is particularly meaningful with regard to the presence of an OV security element is obtained from the reflectance values obtained at different wavelengths. 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 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 set up to measure at least one of the determined reflectance values,

and/or at least one second characteristic value derived from at least part of the determined reflectance values r(O _i ),

|, to be compared with at least one predetermined second comparison characteristic value, for example threshold S ₀ or S ₁ . For example, a reflectance spectrum obtained from a single measuring location is evaluated as OK or the relevant measuring location is identified as a location located on an OV safety element if the reflectance spectrum has a minimum absorption, for example by r(O ₀ ) ^ S ₀ (second comparison characteristic ) applies, and the difference quotient (r(O ₁ ) - r(O ₂ ))/(r(O ₃ ) - r(O ₄ )) within a specified interval (ie between an upper and lower comparison threshold value). To ensure the numerical stability of the difference quotient, it can preferably also be required that the amount of the denominator |r(O ₃ ) - r(O ₄ )| ^ S ₁ (second comparison parameter). With this simple mechanical process, color-changing OV security elements (so-called color shift colors), whose color depends on the viewing angle, can be reliably distinguished from conventional printing inks and IR absorber colors. In simplified embodiments, O ₁ and O ₃ can be identical, that is, reflectance values at only three different wavelengths are used to calculate the difference quotient or determine the locations located on the OV security element. Furthermore, O ₀ can coincide with one of the wavelengths O ₁ to O ₄ . Alternatively or additionally, r(O ₀ ) can represent an average of several reflectance values from r(O ₁ ) to r(O ₄ ). In a further embodiment, the remission spectrum can be checked even more precisely by not only determining or testing one difference quotient, but also by determining and testing two or more difference quotients by adding additional wavelengths. Overall, this evaluation method is particularly suitable for small, relatively inexpensive sensors, since in the simple case only three different wavelengths (including possibly only two different wavelengths in the IR) are required for scanning, and the evaluation requires relatively little numerical effort. Evaluation method 2 Preferably, the testing device is set up to determine whether there is a reflective security element on the document of value at the respective location with an appearance that depends on the viewing angle, in particular a distance between that obtained for the respective location standardized, remission spectrum 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, any type of normalization can be used for the preferably carried out normalization of the respective reflectance spectrum or reference spectrum, which ensures that fluctuations caused by the respective measurement or comparison measurement (for the reference spectrum), for example in the intensity of the Infrared radiation striking the respective location and/or in the sensitivity of the detector device during the detection of the infrared radiation remitted from the location can be essentially eliminated. Preferably, the reflectance spectrum to be tested and the associated reference spectrum are normalized, for example, by a linear transformation that maps the minimum to 0 and the maximum to 1. For example, such normalization 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 represented by the reflectance values r ₁ , ... , r _n is given and “min” is the minimum of all r _i and “max” is the maximum of all r _i . Alternatively, such 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 is the standard deviation of all r _i . Preferably, the distance between the two standardized spectra (measured reflectance spectrum vs. reference spectrum) is determined, for example, using a p-norm of their difference and then checked to see whether this is smaller than a predetermined threshold. If the normalized reflectance spectrum is given by the reflectance values r ₁ ^norm , ..., r _n ^norm and the normalized reference spectrum is given by the values R ₁ , ... R _n , the point-wise difference x _i = r _i ^norm -R _i is first determined. The p-norm of the _{difference is then given by the formula} ^{^σ^} ^ _ୀ^ ^| _^^ ^{|^ ^^/^} _{defined. For p = 2 it corresponds} to the Euclidean distance. By appropriately selecting the threshold, the spectral variations that occur are absorbed and the OV security elements on the banknotes of all production batches are recognized as fit or genuine. This evaluation method is particularly suitable for cases in which the spectrum was sampled at several, in particular more than three or four, wavelengths, so that a complete spectral curve is available. In these cases, despite very simple adaptation, it offers a complete and very precise examination of the entire spectral curve. Evaluation method 3 Preferably, the testing device is set up to determine whether there is a reflective security element with an appearance on the document of value that depends on the viewing angle at the respective location, a difference between the remission spectrum obtained for the respective location and to determine a comparison spectrum adapted to the remission spectrum by means of a compensation calculation (fitting) and to check whether the determined difference is smaller than a predetermined comparison difference, the comparison spectrum preferably being given by a linear combination of at least two predetermined reference spectra. With the given references The reflection spectra are preferably representative reference reflection spectra, which can occur, for example, in extreme cases due to production fluctuations and/or due to different pigment alignment within a banknote. Preferably, the reflectance spectrum obtained (to be tested) from a location is determined using the reference reflectance spectra A and B, for example by the linear combination c ₁ A + c ₂ B, the so-called fit spectrum, with the fit parameters c ₁ and c ₂ fitted. In a preferred embodiment, the ranges of the coefficients c ₁ and c ₂ that are permissible for the adjustment are subject to predefined restrictions such as c ₁ >0 and c ₂ >0. It is then checked how well this fit fits, for example by determining a p-norm of the difference between the measured remission spectrum and the fit spectrum, and comparing the p-norm with a correspondingly specified comparison difference. A fit using the linear combination c ₀ + c ₁ A + c ₂ B with an additional fit parameter c ₀ , the offset, is even more precise. Preferably, it should be ruled out that with c ₁ = c ₂ = 0 every constant spectrum could be fitted exactly by only taking into account pixels or measurement locations whose fit, for example |c ₁ | + |c ₂ | is above a predetermined 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 predetermined threshold. This method can be further improved if an additional preselection of the reflectance spectra to be tested is carried out. Preferably, it can be provided that only those pixels or measurement locations are taken into account when checking the document of value whose reflectance spectrum in the IR range differs sufficiently from a constant curve, for example in which the difference between maximum and minimum exceeds a predetermined threshold . An IR absorption feature is presumably present at these locations. Accordingly, the testing device is preferably set up to take into account when checking the document of value only those locations for which a reflectance spectrum was obtained which fulfills at least one of the following conditions: i) the course of the reflectance spectrum is not constant and/or points opposite a predetermined minimum difference in a constant course and/or ii) the difference between a maximum and a minimum of the remission spectrum is greater than a predetermined minimum difference. Such a preselection can optionally preferably also be used 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 scanned at several 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 suitable for OV security elements with printed, locally differently aligned interference pigments. Evaluation method 4 Preferably, the testing device is set up to determine whether there is a reflective security element on the document of value at the respective location with an appearance that depends on the viewing angle, using a derivation curve by deriving the, in particular standardized, one obtained for the respective location to form a reflectance spectrum according to the wavelength and to check whether the derivative curve lies within a specified tolerance range and/or a difference to a specified target derivative. curve which is smaller than a predetermined maximum difference. Preferably, the reflectance spectrum to be tested is first standardized, for example by a linear transformation, through which the minimum is mapped to 0 and the maximum to 1 (see formula above), or by a linear transformation so that (μ, ǔ) = (0, 1) (see formula above). The derivative curve is then preferably formed (1st derivative according to the wavelength). For example, for a reflectance spectrum that is in the form of n intensity values (r(^ _i )) _i=1,..,n , the derivative can be calculated as (r'(^ _i )) _i=2,…,n = ((r(^ _i ) – r(^ _i-1 )) /( ^ _{i –} ^ _i-1 )) _i=2,..,n . It is then checked 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 value and the standard deviation for each wavelength. If the aim is not only to check whether the remission spectrum obtained fits, but also how well it fits, an alternative or additional difference curve to the target derivative curve can be used, e.g. the difference curve to the target derivative curve weighted “channel-wise” (i.e. wavelength-wise) with the reciprocal width of the tolerance range , calculated and reduced to a value, for example with a p-norm. This value can then be compared with an upper threshold, with the presence of an OV safety element being confirmed if the upper threshold is not exceeded. This method is particularly suitable for cases in which the respective reflection spectrum was sampled at several (particularly preferably: many) wavelengths, so that a complete spectral curve is available. In these cases, it offers particularly precise separation of spectra of different OVI elements despite low adaptation effort. Evaluation method 5 Preferably, the testing device is set up to determine whether there is a reflective security element on the document of value at the respective location with an appearance that depends on the viewing angle, in the reflectance spectrum obtained for the respective location the wavelengths of to determine two or more local extrema, in particular of at least one local maximum and at least one local minimum, and to check whether the determined wavelengths lie within wavelength intervals which are specified for the respective extrema, and / or whether within one There is only one local extremum within the wavelength interval specified for the respective extremum. The testing device is preferably set up to form a quotient from two determined wavelengths and to check whether the quotient lies within a predetermined tolerance range. Here, the wavelengths of the local extrema (possibly interpolated) of the remission spectrum are first determined. Then, using wavelength intervals defined in advance for the minima and maxima, it is checked whether there is a, in particular exactly one, local minimum or maximum in each of the wavelength intervals. Alternatively or additionally, a selection of pairs of the predetermined wavelength intervals is defined and it is checked whether the quotients of the wavelengths of the associated extrema lie within predetermined tolerance ranges. Preferably, wavelength intervals are specified for one to three minimums and one to three maximums. Preferably one maximum and two minimums are in the IR range. For example, wavelength intervals are defined for one maximum and one minimum, one maximum and two minimums, two maximums and one minimum, two maximums and two minimums, two maximums and three minimums, three maximums and two minimums or three maximums and three minimums . This method is particularly suitable for cases in which the reflectance spectrum was sampled at several individual wavelengths. It is not absolutely necessary that a complete spectral curve is present. It offers a particularly low adaptation effort, since it is possible to recognize many different interference-based OV color impressions at the same time with a single adaptation and to reliably distinguish these from absorption pigments. Further advantages, features and possible applications of the present invention result from the following description in connection with the figures. Shown are: FIG. 1 an example of a sensor for checking valuable documents; 2 shows a first example of a reflectance spectrum; 3 shows a first example of a diagram showing the numerator and denominator of difference quotients; 4 shows a second example of a diagram showing the numerator and denominator of difference quotients; Fig. 5 Examples of reflectance spectra before (left) and after (middle, right) normalization; Fig. 6 examples of reference spectra; 7 Examples to illustrate an adaptation of a reflection spectrum by a first linear combination of the reference spectra (left) and a second linear combination of the reference spectra (right); 8 Examples of locations determined with one-dimensional or two-dimensional adjustment; 9 shows an example of a normalized reflectance spectrum; 10 shows an example of a derivative curve calculated from the normalized reflection spectrum shown in FIG. 9 and of a predetermined tolerance range; and FIG. 11 shows an example of a reflectance spectrum, in which distributions of the respective spectral position of local minima or maxima of several measured reflectance spectra are drawn. 1 shows a schematic representation of an example of a sensor 10 for checking valuable 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 over the entire width of the document of value 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, the security element 2 can have any other shape and/or based on other functional principles to produce an appearance that depends on the viewing angle, in particular in the visible one Spectral range, such as in the form of foil elements, patches, wide security threads or printed, magnetically aligned interference pigments. In film-based OV security elements, in addition to micromirrors, stacks of thin layer 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 set up to irradiate several 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 “measuring locations”, “measuring pixels” or “pixels” in connection with the present disclosure. The infrared radiation remitted from the different locations 3, 3' is detected by a detection device 12, with a remission spectrum being obtained for each of the locations 3, 3'. The irradiation device 11 and/or the detection device 12 can be designed in various ways in order to obtain a reflectance spectrum from the locations 3, 3'. In the general case, it can be provided, for example, that the irradiation device 11 irradiates the document of value 1 with broadband infrared radiation and the detection device 12 detects the remitted infrared radiation in a spectrally resolved manner at a large number of different wavelengths. In a particularly preferred, easy-to-implement variant, 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 set up to to detect the infrared radiation remitted successively 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 reflectance spectrum is recorded from several locations 3, 3 'on the document of value 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 the locations 3, 3' can be smaller, possibly significantly smaller, or even larger, depending on the minimum spatial resolution required by the geometric expansion of the security elements to be tested. In principle, the locations 3, 3' or the respective areas can have any shape. In the case of a detection device 12 with one or more rectangular, in particular square, detector elements, the 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, the locations 3, 3' preferably have a substantially oval or round shape. Preferably, a transport device 15, indicated only schematically in the example shown, is provided, which is set up to transport the document of value 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 several locations 3, 3′ along one or more measuring tracks M running parallel to the transport direction T is successively detected. For reasons of clarity, in the present example the locations 3, 3' are only shown along a single measurement track M. In principle, the irradiation device 11 and/or the detection device 12 can also be designed in such a way that reflectance spectra can be recorded from two, preferably six, or more locations from two, preferably six, or more measurement tracks M that run parallel to one another . Alternatively or additionally, it is preferred that the irradiation device 11 is set up to irradiate a linear region of the document of value 1 that runs essentially perpendicular to the transport direction T with infrared radiation, and the detection device 12 has a so-called sensor line with a plurality of detector elements arranged perpendicular to the transport direction T has, through which the infrared radiation remitted from the irradiated linear area of the document of value 1 can be detected in a spatially resolved manner. In this embodiment, only a few different infrared wavelengths, in particular two different infrared wavelengths or three different infrared wavelengths, are preferably used alternately. By successively recording the infrared radiation emitted during the transport of the document of value 1 in the transport direction T, a spectrally resolved reflection image of the entire document of value 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 (based on the plumb line on the document of value 1) at a mean angle of incidence D, and the infrared radiation remitted from the location 3 is transmitted by the detection device 12 is detected at a detection angle E, which is preferably different from the average angle of incidence D. Furthermore, it is preferred that all examined locations 3, 3' on the document of value 1 are measured for all wavelengths under only a single angular condition, ie a single irradiation angle D and a single detection angle E. The sensor 10 also has a testing device 13, which is set up to use the reflectance spectra obtained for the different locations 3, 3 'to determine whether the location 3, 3' in question is at or on the reflecting optical variable (OV) safety element 2 is located. This is done, for example, by processing and/or analyzing the reflectance spectrum obtained in each case and/or comparing it with one or more predetermined spectra. This will be 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, in particular the number and/or location of the locations 3' identified or classified as “on the OV security element”. Preferably, it is checked whether the number of locations 3' on the value document that are determined or classified as "on the OV security element" is greater than or equal to a predetermined first minimum number, which can depend, for example, on the size of the security element 2. In this way, an individual security element 2 on the document of value 1 can be reliably detected. Alternatively or additionally, it can be checked, for example, whether the number of ROIs within at least one predetermined region of interest lies determined or classified as “on the OV security element” locations 3' is greater than or equal to a predetermined second minimum number. This means that not only the presence of a security element 2 on the valuable document 1 can be reliably proven, 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 document of value 1. Optionally, it can also be checked whether the number of locations 3' determined or classified as "on the OV security element", which lie outside the predetermined region of interest ROI, is less than or equal to a predetermined maximum number, in particular equal to 0 , is. This ensures that there are little 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 or are affirmed, the valuable document 1 is classified, for example, as “genuine” and/or “fit”. Otherwise, the valuable document 1 is classified as a “counterfeit” and/or “unfit”. Preferably, the sensor 10 is used in a valuable document processing system 100, which in the present example is only indicated in a highly schematized manner and has one or more processing devices 101 to 103, which are set up to process valuable documents 1, in particular to separate, transport, inspect, sort, stack and/or destroy. Below, with reference to the figures, preferred evaluation methods for determining or classifying the locations 3′ located on the security element 2 or for checking the document of value 1 based on the determined locations 3′ will be explained in more detail by way of example. Evaluation method 1 In this preferred method, the reflectance r(ǌ _i ) is first measured at the respective location 3, 3' at (at least) three different wavelengths ǌ _i . These are preferably chosen so that characteristic points in the spectrum are detected which are known to be characteristic of the reflectance spectrum of the OV security element to be tested, for example at which it has a maximum or a minimum. The wavelengths to be evaluated are preferably adjusted for different OV color tones. A reflection 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(ǌ ₀ ) ^ threshold S ₀ ), and the difference quotient (r(ǌ ₁ ) - r(ǌ ₂ )) / (r(ǌ ₃ ) - r(ǌ ₄ )) lies within a specified interval, and (because of the numerical stability of the quotient ) the amount of the denominator |r(ǌ ₃ ) - r(ǌ ₄ )| ^ Threshold S is ₁ . With this simple method, for example, reflective optically variable security elements, whose color depends on the viewing angle and are also referred to as “color-changing 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, ǌ ₁ and ǌ ₃ can coincide identically. ǌ ₀ can coincide with one of the wavelengths ǌ ₁ to ǌ ₄ , or r(ǌ ₀ ) can represent an average of several reflectance values, selected from r(ǌ ₁ ) to r(ǌ ₄ ). In a further preferred embodiment, the reflectance spectrum can still be can be checked in more detail by not only determining and testing one difference quotient, but also by determining and testing two or more difference quotients by adding additional wavelengths. For some spectral curves, it is even possible to carry out 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 inexpensive so-called cash-in sensors. Figures 2 to 4 illustrate how a reflective optically variable security element, the color of which depends on the viewing angle, is tested by (r(1100nm) - r(1520nm))/(r(1100nm) - r(770nm)) ^ [ 0.3, 0.8] is separated from conventional colors. Figure 2 shows a first example of a reflectance spectrum of an OV film strip. By evaluating characteristic reflectance values (at the marked wavelengths O ₁ =O _3, O _2, O ₄ ) and calculating the difference quotient from them, a characteristic IR signature of the film strip can be clearly recognized or determined. When selecting the spectral values used for the evaluation, preference is given to using wavelengths at which the reflectance curve has a local minimum or maximum. Figure 3 shows a first example of a diagram with a representation of the numerator and denominator of difference quotients of different measuring pixels 3' (small dots) of an OV film strip in comparison to different IR absorber pigments (larger individual dots). The remission intensities r(ǌ ₁ ) to r(ǌ ₄ ) at the same wavelengths ǌ ₁ to ǌ ₄ were used. The IR signature of the OV film strip - characterized by the difference quotient, which corresponds to the original angle in the representation chosen here - can be clearly distinguished 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 of different production batches of an OV film strip. The remission intensities r(ǌ ₁ ) to r(ǌ ₄ ) at the same wavelengths ǌ ₁ to ǌ ₄ were used. Preferably, the spread of the calculated values caused by production fluctuations is taken into account by choosing a suitable test interval, which is preferably predetermined by a lower comparison threshold value S _min and an upper comparison threshold value S _max . By appropriately choosing this interval, the spectral variations that occur due to 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 recognized. In this way, the influence of production fluctuations on the testing of the OV safety elements is reliably eliminated or at least kept to a minimum. The respective difference quotient can be found in FIGS. 3 and 4 by mentally forming the quotient y/x for a point (x, y). The respective difference quotient corresponds to the slope of the origin line through the point in question. The two drawn original lines with slopes 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, each sampled at n wavelengths, is determined by the remis- sion values (r ₁ , …, r _n ) is given. In a first step, the reflectance spectrum of the OV film strip to be tested is chosen from that described above Example and a reference spectrum 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” is 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 is the standard deviation of all r _i ). Figure 5 shows examples of reflectance spectra from different measurement locations of an OV film element before (left) and after (middle, right) normalization. As can be seen, the original reflectance spectra (left) show a strong variation, which is significantly reduced by linear normalization (middle, right), so that a quantitative comparison with a normalized reference spectrum is possible. A distance between the two standardized spectra (measured reflection spectrum vs. reference spectrum) is then determined, for example with a p-norm of their difference, and it is checked whether this is smaller than a predetermined threshold, which is also the case in connection with the present disclosure is referred to as the “comparison distance”. By appropriately choosing this threshold, the spectral variations that occur are absorbed so that OV security elements of valuable documents or banknotes of all production batches can be reliably recognized. Evaluation method 3 In this preferred method it is assumed that the measured or to be measured reflectance spectra of OV security elements differ, for example for production fluctuations that occur in extreme cases and/or due to different Spectral variations occurring within a banknote due to pigment alignment can be approximated using representative reference reflectance spectra A and B. Preferably, the reflectance spectrum obtained in each case to be tested with the reference reflectance spectra A and B is approximated by a first linear combination c1 *A + c2 *B, the fit spectrum with the fit parameters c ₁ and c ₂ (so-called fit) and checked how well the fit fits, for example with a p-norm of the difference between the measured reflectance spectrum and the fit spectrum, which in the context of the present disclosure is also referred to as the (adjusted) “comparison spectrum”. Optionally, the fit is even more precise using a second linear combination c ₀ + c ₁ *A + c ₂ *B with an additional fit parameter c ₀ , the offset. It should preferably be taken into account here that with c ₁ = c ₂ = 0 any constant spectrum could be fitted exactly. To rule out this case, only pixels or locations whose fit, for example |c ₁ |, are preferably taken into account + |c ₂ | is above a predetermined threshold, or whose respective remission spectrum is sufficiently non-constant, so that, for example, the standard deviation or difference between maximum and minimum is above a predetermined threshold. Figure 6 shows examples of two reference reflectance spectra, the linear combination of which is adapted to a measured reflectance spectrum. Figure 7 shows examples to illustrate an adjustment (fit, dashed line) of a measured reflectance spectrum (solid line) by the first linear combination of the reference reflectance spectra described above (left) and the second linear combination of the reference reflectance spectra described above with an offset ( right). The method can be further improved if an additional preselection of the reflectance spectra to be tested is carried out by only using those pixels or locations in the evaluation whose reflectance curve differs sufficiently from a constant curve in the IR range, for example 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 above-described 2-dimensional adjustment of a reflectance spectrum to be tested is carried out by the two reference spectra A and B (with the fit parameters c ₁ and c ₂ and optionally c ₀ ) for a value number “printed with magnetically aligned interference pigments”. 100” and compared with the case of an adjustment using a single reference remission spectrum (1-dimensional adjustment). Figure 8 shows a comparison of the locations or measuring pixels of a lettering “100” printed with aligned interference pigments that were correctly recognized with a 1-dimensional adjustment (top) and with a 2-dimensional adjustment (bottom). In the pixel images shown 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 adapted spectrum (fit spectrum or comparison spectrum) and the measured remission spectrum is smaller as a test threshold, which is also referred to as a “comparison difference” in the context of the present disclosure. To assess authenticity, it is checked whether a minimum number of pixels or locations meet this criterion. Additionally or alternatively, a comparison with the expected shape of the “100” is possible if the measurement has sufficient spatial resolution. With 1-dimensional fitting, relatively few valid pixels are obtained compared to 2-dimensional fitting. Since in this printed image the alignment of the interference pigments varies systematically over the height, in the 1-dimensional case the pixels matching the reference are concentrated in a narrow area in height. This should preferably be taken into account for sensors with only a few (or possibly only one) measurement tracks by adjusting their position as precisely as possible to this narrow area in the case of an evaluation with 1-dimensional adjustment. In order to keep possible influences due to so-called acceleration/deceleration variations during the transport of the document of value to be checked as low as possible, an evaluation with 2-dimensional adjustment is preferred, as this relates to the position of the respective measurement track or the track height is much more tolerant than the evaluation with 1-dimensional adjustment. Evaluation method 4 In this preferred method, the reflectance spectrum to be tested is first standardized, for example 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 so that (μ, ǔ) = (0,1), for example according to the formula r _i ^norm = ((r _i – μ) / ǔ) _i=1…n , (see above). Then the derivative curve (1st derivative according to the wavelength) of the normalized reflectance spectrum is formed and it is checked whether the derivative curve lies within a predetermined tolerance range. Preferably, the tolerance range is determined or specified by using standardized reflectances for a large number of real, ie measured on real banknotes or reference patterns. on spectra, the derivative curves are calculated and the mean value and standard deviation are determined 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 normalized reflectance spectrum. Figure 10 shows an example of a derivative curve calculated from this (solid line) and a predetermined tolerance range (dashed lines). If the aim is not only to check whether the respective remission spectrum fits, but also how well it fits, an alternative can be a difference curve between the respective derivative curve and a predetermined one, weighted channel by channel (ie for the individual wavelengths) with the reciprocal width of the tolerance range The target derivative curve can be calculated and then reduced to a value, for example with a p-norm. This value can then be compared with an upper threshold, which is also referred to as a “maximum difference” in the context of the present disclosure. Evaluation method 5 In this preferred method, the wavelengths of the local extrema (possibly interpolated) of the remission spectrum obtained are first determined. Furthermore, intervals are determined or specified in advance for the minima and maxima, which are also referred to as “wavelength intervals” in the context of the present disclosure. It is then checked whether there is exactly a local minimum or maximum in each of these intervals. Optionally, a selection of pairs of these intervals can also be defined or specified and it can be checked whether the quotients of the wavelengths of the associated extrema lie within predetermined tolerance ranges. Preferably Such wavelength intervals are defined for one to three minima and one to three maxima. Preferably one maximum and two minimums are in the IR range. For example, wavelength intervals are defined for one maximum and one minimum, one maximum and two minimums, two maximums and one minimum, two maximums and two minimums, two maximums and three minimums, three maximums and two minimums, or three maximums and three minimums. Figure 11 shows an example of a reflectance spectrum in which frequency distributions of the respective spectral position of local minima “min” or maxima “max” of several measured reflectance spectra are plotted. In general, color-changing 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-changing 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, e.g. [600nm, 1000nm] and [1000nm, 1700nm], are selected, then the quotient of the wavelengths of the associated minima is checked , for example, whether it lies in the interval [1.6, 1.75], or even more precisely, whether it lies in a narrower interval depending on the type. This does not check whether there is a color-changing OV security element with a specific color, but only whether such an OV security element is present with any color. Nevertheless, in this way, a missing color-changing OV security element and/or normal colors and IR absorber colors can be reliably recognized and the corresponding banknote can be rejected or classified as a counterfeit.

Claims

P a t e n t a n s r ü c h e 1. Sensor (10) 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, with: - an irradiation device ( 11), which is set up to irradiate one or more different locations (3, 3') on a document of value (1) with infrared radiation, - a detection device (12), which is set up to receive from the respective location ( 3, 3') to detect infrared radiation remitted on the document of value (1), the sensor being set up to detect the remitted infrared radiation in a spectrally resolved manner in order to obtain a remission spectrum at the respective measuring location, and - a testing device (13 ) for checking the document of value with regard to a reflective security element (2), which has an appearance that depends on the viewing angle, which is set up to determine whether on the basis of the remission spectrum obtained for the respective location (3, 3 '). a reflective security element (2) with an appearance that depends on the viewing angle is located on the document of value (1) at the respective location (3, 3') in order to determine one or more locations at which one is located reflective security element (2) is located on the document of value (1), and to check the document of value (1) based on the determined location(s) (3').

2. Sensor (10) according to claim 1, wherein the detection device (12) is set up 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 set up and/or arranged for this purpose To direct infrared radiation onto the document of value in such a way that it hits the document of value (1) at an angle of incidence (D), the detection angle (E) and the angle of incidence (D) preferably being of different sizes.

3. Sensor (10) according to claim 1 or 2, wherein the testing device (13) is set up to determine whether there is a reflective security element with an appearance that depends on the viewing angle at the respective location of the valuable document To use only the infrared radiation directed at the document of value at exactly or approximately a single angle of incidence and detected at exactly or approximately at a single detection angle.

4. 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 set up to successively detect the respective location (3, 3 ') on the valuable document (1). to irradiate 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 one another, and the detection device (12) is set up to receive the radiation from the respective location (3, 3') to successively detect remitted infrared radiation in the at least two, in particular at least three, different wavelengths (O _i ) and/or wavelength ranges in order to obtain the respective reflection spectrum.

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

6. Sensor (10) according to claim 5, wherein the testing device (13) is set up 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 within one predetermined region of interest (ROI) on the valuable document (1) determined locations (3 ') is greater than or equal to a predetermined second minimum number and / or whether the number of outside a predetermined region of interest (ROI) on the Determined locations (3') lying on the value document are less than or equal to a predetermined maximum number, in particular equal to zero.

7. Sensor (10) according to one of the preceding claims, wherein the testing device (13) is set up to determine whether there is a reflective security element (2) with a at the respective location (3, 3 '). The appearance on the document of value (1), which is dependent on the viewing angle, is located in the remission spectrum obtained for the respective location (3, 3') at 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 determined from the reflectance values (r(O ₁ ) to r) determined in the reflectance spectrum at three or four different wavelengths (O ₁ to O ₄ ). (O ₄ )) is formed ((r(O ₁ ) – r(O ₂ ))/(r(O ₃ )- r(O ₄ ))).

9. Sensor (10) according to one of claims 7 to 8, wherein the testing device (13) is set up to at least one of the determined reflectance values (r (O _i ), r (O ₀ )) and / or at least one of at least one Part of the identified to compare the second characteristic value (r(O ₀ ),|r(O ₃ ) - r(O ₄ )|) derived from remission values with at least one predetermined second comparison characteristic value (S ₀ , S ₁ ).

10. Sensor (10) according to one of the preceding claims, wherein the testing device (13) is set up to determine whether there is a reflective security element (2) with a at the respective location (3, 3 '). The appearance on the document of value (1), which is dependent on the viewing angle, determines and checks a distance between the, in particular standardized, remission spectrum obtained for the respective location (3, 3') and at least one predetermined, in particular standardized, reference spectrum whether the determined distance is smaller than a specified comparison distance.

11. Sensor (10) according to one of the preceding claims, wherein the testing device (13) is set up to determine whether there is a reflective security element (2) with a at the respective location (3, 3 '). The appearance on the document of value (1), which depends on the viewing angle, is to determine and to determine a difference between the reflectance spectrum obtained for the respective location (3, 3') and a comparison spectrum (fit) adapted to the reflectance spectrum by means of a compensation calculation check whether the determined difference is smaller than a predetermined comparison difference, the comparison spectrum preferably being 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 set up to take into account only those locations (3 ') for which a reflectance spectrum is available when checking the document of value. 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 - on spectrum is greater than a specified minimum difference.

13. Sensor (10) according to one of the preceding claims, wherein the testing device (13) is set up to determine whether there is a reflective security element (2) with a at the respective location (3, 3 '). The appearance on the document of value (1), which depends on the viewing angle, is to form a derivation curve by deriving the, in particular standardized, remission spectrum obtained for the respective location (3, 3') and to check whether the derivation curve is within a predetermined tolerance range lies and/or has a difference to 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 set up to determine whether there is a reflective security element (2) with a at the respective location (3, 3 '). The appearance on the document of value (1), which depends on the viewing angle, contains the wavelengths of two or more local extrema, in particular of at least one local maximum (“max”) and at least, in the remission spectrum obtained for the respective location (3, 3'). a local minimum (“min”) to determine and check whether the determined wavelengths lie within wavelength intervals specified for the respective extremes, and/or whether within a wavelength interval specified for the respective extremum only a local extreme lies.

15. Method for checking documents of value (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, with the following steps: - Irradiating one or more different locations (3 , 3') on a document of value (1) with infrared radiation and spectrally resolved detection of the infrared radiation remitted from the respective location (3, 3') on the document of value (1), whereby a remission spectrum is obtained in each case, - determining based on the for the respective The remission spectrum obtained at the location (3, 3') determines whether there is a reflective security element (2) with an appearance that depends on the viewing angle on the document of value (1) at the respective location (3, 3'). or several locations to determine where such a reflective security element (2) is located on the document of value (1), and - checking the document of value (1) based on the location(s) determined (3' ).