DE102022002581A1 - Sensor and method for checking valuable documents with at least one reflective security element - Google Patents

Abstract

The invention relates to a sensor (10) and a method for checking valuable 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. The sensor (10) has: an irradiation device (11), which is designed 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 detect the infrared radiation emitted from the different locations (3, 3') on the document of value (1), the sensor recording a remission spectrum from each location; and a testing device (13), which is set up to use the respective reflectance spectrum to determine whether there is a reflective security element at the respective location with an appearance that depends on the viewing angle, in order to determine one or more of the locations (3 '). which or which such a reflective security element (2) is located on the document of value (1), and to check the document of value (1) based on the location or locations (3 ') determined.

Description

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, 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 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 produce 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 an appearance that depends on a viewing angle, has: an irradiation device which is set up for this purpose, one or more to irradiate 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 determine, based on the remission spectrum obtained for the respective location, whether there is a reflective, in particular optically variable, at the respective location , 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 (i.e. with an appearance dependent on the viewing angle) reflective security element (the same or possibly different) is located, and that Valuable document to be 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 documents of value which have at least one reflective security element, which has an appearance dependent on a viewing angle, particularly in the visible spectral range, has the following steps: irradiating one or more different locations on a document of value 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; Determine based on the remission spectrum obtained for the respective location whether there is a reflective, in particular optically variable, security element with an appearance that depends on the viewing angle on the document of value, in order to determine one or more locations at which one is located such a reflective security element (i.e. with an appearance that depends on the viewing angle) is 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 valuable documents, in particular to separate, transport, check, sort and stack valuable documents 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 is stored according to the fourth aspect of the present disclosure. A sixth aspect of the present disclosure relates to a disk 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 carrying out a spectrally resolved measurement of the value at several different locations on a document of value to be checked, which are also referred to as “measuring locations”, “measuring pixels” or “pixels” in the context of the present disclosure to carry out infrared radiation remitted at the respective location and, based on the remission spectra obtained, to identify the location or 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 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 a large number, or only at a few, for example 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 it cannot be determined from the visual appearance of the security element its infrared signature can be closed. 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 security element, which has an appearance dependent on a viewing angle in the visible spectral range, can be both a specularly (directed or specularly) 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 of 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 met, 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 where 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 where 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 determined locations that are subjected to the test described above or where such a reflective or optically variable security element is located can all be on the same reflective or optically variable security element of the respective document of value, or on different such reflective or optically variable ones Security elements of the respective valuable document.

Preferably, exactly one remission spectrum is recorded from the respective location. In particular, a reflectance spectrum is recorded from several locations on the respective document of value, 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 reflectivity spectrum obtained, it is not absolutely necessary to take into account the specific design and/or specific properties of the of the document of value to be checked, for example reflecting the expected situation of such a document the or optically variable security element on the document of value, and to incorporate it 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 valuable 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, in particular with regard to the position and/or size of the reflective or optically variable security element or denomination and orientation of the document of value, be taken into account.

The approach described 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 is dependent on the viewing angle, for example due to interference on thin layers is produced. 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 interference pigments magnetically aligned during printing before curing). .

The sensor is set up to record the infrared radiation emitted 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 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 means of the detection device at (at least approximately) the same location.

In particular, the detection device is set up and/or arranged so 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 to direct the infrared radiation onto the document of value in such a way 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 incidence angle 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 different. It is therefore preferred that the reflection law (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 an 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 mirroring) in individual cases wavelengths occur. This is how Refle xion-OV elements, despite the micromirror arrays often used here, which provide moving images or patterns when tilted, there are no interferences (e.g. overdrive of the detection device) in the measurement, and no exact orientation of the banknote to the sensor is required. 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 crumpled or bent 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 inspection.

Preferably, the detection angle and/or the angle of incidence is essentially the same for all locations on the document of value from which the respective remitted infrared radiation is detected.

Preferably, the infrared radiation, in particular for all wavelengths, is directed onto 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) a single detection angle detected 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 angle of incidence onto the document of value directed infrared radiation detected at (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, i.e. 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, through which they can be reliably identified. Repeated testing at different irradiation and/or detection angles can therefore be dispensed with. The sensor can therefore be manufactured 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 irradiation angle also applies if, due to the beam divergence of the irradiation device, there are slightly different irradiation angles, 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 maximum +/-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 reflect 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 document of value.

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 accordingly when identifying the locations on such a reflective or optically variable security element. Local fluctuations in the angle of incidence, e.g. 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 folding/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 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). capture.

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 overlap with one another, and the detection device for this purpose 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 with the same area is illuminated by 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 light sources with different wavelengths, preferably LEDs, with narrow-band spectra that, in particular, do not overlap each other spectrally, are preferably used. The light from all IR light sources is preferably irradiated onto the document of value at approximately the same angle(s) of incidence.

A transport device is preferably 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 document of value irradiated by different light sources at different wavelengths change 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 of exactly one of the measurement tracks and/or the remitted infrared radiation of 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 location is checked, for example, using at least one region of interest (ROI) on the document of value. 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, checking the document of value is quicker and easier to understand - due to the unnecessary image comparison. 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 or equal a predetermined first minimum number and/or whether 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/or whether the number of locations outside of the (the same or one). other) predetermined region of interest (ROI) on the document of value is 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 dependent on the viewing angle on the document of value) is greater or is equal to a predetermined first minimum number, and/or if the number of locations determined 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 “suspect of forgery” or classified as “unfit”. Alternatively or additionally, the value document is (possibly only) classified as “genuine” or “fit” if only the maximum number of identified locations was determined in the (the same or one or more other) ROIs/no location at all such a reflective safety element was determined.

Alternatively or additionally, it is checked whether there is a minimum number of locations in total where the security element was recognized (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 procedure 1

Preferably, the testing device is set up to determine whether there is a reflective security element with an appearance dependent on the viewing angle on the document of value at the respective location in the reflectance spectrum obtained for the respective location at at least two, in particular at least three, different wavelengths λ _i , (i = 1, 2, 3, ...) and/or wavelength ranges to determine reflectance values r(λ _i ), to derive from these at least one first characteristic value characterizing the respective reflectance spectrum and to assign the first characteristic value to at least one predetermined first comparison characteristic value compare. The number and/or position of the different wavelengths λ _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 colors for different OV security elements, they are selected end wavelengths λ _i are preferably adapted. This enables reliable identification of 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 useful for use with inexpensive sensors is particularly advantageous in cash deposit systems (so-called cash-in).

The first characteristic value is preferably a difference quotient, which is formed from the reflectance values r(λ ₁ ) to r(λ ₄ ) determined in the reflectance spectrum at three or four different wavelengths λ ₁ to λ ₄ , in particular according to ((r( λ ₁ ) - r(λ ₂ ))/ ((r(λ ₃ ) - r(λ ₄ )). In this way, the reflectance values obtained at different wavelengths become a particularly meaningful first characteristic value with regard to the presence of an OV security element respective remission spectrum.

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 test at least one of the determined reflectance values, for example r(λ ₀ ), and/or at least one second characteristic value, for example |r(λ ₃ ), derived from at least a part of the determined reflectance values _r (λ _i ). - r(λ ₄ )|, 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(λ ₀ ) ≤ S ₀ (second comparison characteristic value). , and the difference quotient (r(λ ₁ ) - r(λ ₂ ))/(r(λ ₃ ) - r(λ ₄ )) lies within a predetermined interval (ie between an upper and lower comparison threshold). To ensure the numerical stability of the difference quotient, it can preferably also be required that the amount of the denominator is |r(λ ₃ ) - r(λ 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, λ ₁ and λ ₃ can be identical, ie reflectance values at only three different wavelengths are used to calculate the difference quotient or determine the locations on the OV security element. Furthermore, λ ₀ can coincide with one of the wavelengths λ ₁ to λ ₄ . Alternatively or additionally, r(λ ₀ ) can represent an average of several reflectance values from r(λ ₁ ) to r(λ 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 procedure 2

Preferably, the testing device is set up to determine whether there is a reflective security element with an appearance dependent on the viewing angle on the document of value at the respective location, a distance between the, in particular standardized, remission spectrum obtained for the respective location and at least one predetermined, in particular to determine the 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 a normalization is carried out according to the following formula ri ^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; is. 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 ri ^norm = ((r _i - µ) / σ) _i=1...n , where µ the mean of all r; and σ is the standard deviation of all r; is.

definiert. Für p = 2 entspricht sie dem Euklidischen Abstand. Durch geeignete Wahl der Schwelle werden die auftretenden spektralen Variationen aufgefangen, und die OV-Sicherheitselemente auf den Banknoten aller Produktionschargen als fit bzw. echt erkannt.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 ri ^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 ${\left({\displaystyle {\sum }_{i=1}^n{\left|x_i\right|}^p}\right)}^{1/p}$

Are 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 present. In these cases, despite very simple adaptation, it offers a complete and very precise examination of the entire spectral curve.

Evaluation procedure 3

Preferably, the testing device is set up to determine whether there is a reflective security element with an appearance that depends on the viewing angle on the document of value at the respective location, a difference between the remission spectrum obtained for the respective location and one based on the remission spectrum by means of a compensation calculation (fitting ) to determine the adapted comparison spectrum 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. The specified reference spectra are preferably representative reference reflectance 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 (to be tested) obtained from a location is fitted 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 . 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 measuring locations are taken into account when checking the document of value whose reflectance spectrum in the IR range differs sufficiently from a constant curve, e.g. 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 towards a constant one Course a predetermined minimum difference and / or ii) the difference between a maximum and a minimum of the remission spectrum is greater than a specified 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 procedure 4

Preferably, the testing device is set up to determine whether there is a reflective security element with an appearance dependent on the viewing angle on the document of value at the respective location, using a derivation curve by deriving the, in particular standardized, remission spectrum obtained for the respective location according to the wavelength form and 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 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, σ) = ( 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(λ _i )) _i=1,..,n , the derivative can be calculated as (r'(λ _i )) _{i=2,..., n} = ((r(λ _i ) - r(λ _i-1 )) /(λ _i - λ _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, a difference curve to the target derivative curve can alternatively or additionally be calculated, 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 and these are reduced to a value, e.g. 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 reflectance spectrum was sampled at several (particularly preferably: many) wavelengths, so that a complete spectral curve is present. In these cases, it offers particularly precise separation of spectra of different OVI elements despite low adaptation effort.

Evaluation procedure 5

Preferably, the testing device is set up to determine whether there is a reflective security element with an appearance dependent on the viewing angle on the document of value at the respective location, in the remission spectrum obtained for the respective location, the wavelengths of two or more local extrema, in particular of at least one local maximum and at least one local minimum, to determine and check whether the determined wavelengths lie within wavelength intervals that are specified for the respective extrema, and / or whether only one local one within a wavelength interval specified for the respective extremum extremum lies. The testing device is preferably set up to form a quotient from two determined wavelengths and to check whether the quotient is 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 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 are defined.

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 simultaneously recognize many different interference-based OV color impressions with a single adaptation and to reliably distinguish them from absorption pigments.

• 1 ein Beispiel eines Sensors zum Prüfen von Wertdokumenten;
• 2 ein erstes Beispiel eines Remissionsspektrums;
• 3 ein erstes Beispiel eines Diagramms mit einer Darstellung des Zählers und Nenners von Differenzenquotienten;
• 4 ein zweites Beispiel eines Diagramms mit einer Darstellung des Zählers und Nenners von Differenzenquotienten;
• 5 Beispiele von Remissionsspektren vor (links) und nach (Mitte, rechts) einer Normierung;
• 6 Beispiele von Referenzspektren;
• 7 Beispiele zur Veranschaulichung einer Anpassung eines Remissionsspektrums durch eine erste Linearkombination der Referenzspektren (links) und eine zweite Linearkombination der Referenzspektren (rechts);
• 8 Beispiele von mit eindimensionaler bzw. zweidimensionaler Anpassung ermittelten Orten;
• 9 ein Beispiel eines normierten Remissionsspektrums;
• 10 ein Beispiel einer aus dem in 9 gezeigten normierten Remissionsspektrum berechneten Ableitungskurve sowie eines vorgegebenen Toleranzbereichs; und
• 11 ein Beispiel eines Remissionsspektrums, in welches Verteilungen der jeweiligen spektralen Lage lokaler Minima bzw. Maxima mehrerer gemessener Remissionsspektren eingezeichnet sind.

Further advantages, features and possible applications of the present invention result from the following description in connection with the figures. Show it:

• 1 an example of a sensor for checking valuable documents;
• 2 a first example of a remission spectrum;
• 3 a first example of a diagram showing the numerator and denominator of difference quotients;
• 4 a second example of a diagram showing the numerator and denominator of difference quotients;
• 5 Examples of reflectance spectra before (left) and after (middle, right) normalization;
• 6 Examples of reference spectra;
• 7 Examples to illustrate an adjustment of a reflectance spectrum by a first linear combination of the reference spectra (left) and a second linear combination of the reference spectra (right);
• 8th Examples of locations determined using one-dimensional or two-dimensional fitting;
• 9 an example of a normalized reflectance spectrum;
• 10 an example of one from the in 9 shown standardized remission spectrum calculated derivative curve and a predetermined tolerance range; and
• 11 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, for example, applied 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 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, for example, in the form of film elements, patches, wide security threads or printed, magnetically aligned ones interference pigments. In film-based OV security elements, in addition to micromirrors, stacks of thin layer thickness-controlled layers are 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 preferred here set up to detect the infrared radiation remitted successively at different wavelengths from the respective location 3, 3', whereby a reflectance spectrum with reflectance 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 extent 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, which is only indicated 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 M can be recorded from two, preferably six, or more locations from two, preferably six, or more measurement tracks running 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, 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 detecting the infrared radiation remitted during the transport of the document of value 1 in the transport direction T, a spectrally resolved remission 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 α, and the infrared radiation remitted from the location 3 is detected by the detection device 12 at a detection angle β, which is preferably from average angle of incidence α is different. 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, i.e. a single incidence angle α and a single detection angle β.

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 on or on the reflective optically variable (OV- )Security 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, for example, depends on the size of the Security element 2 can depend. 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 locations 3' located within at least one predetermined region of interest ROI or classified as "on the OV security element" 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 document of value 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. 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 are affirmed, the value document 1 is classified, for example, as “genuine” and/or “fit”. Otherwise, the valuable document 1 is classified as a “counterfeit” and/or “unfit”.

The sensor 10 is preferably used in a valuable document processing system 100, which is only indicated in a highly schematic manner in the present example and has one or more processing devices 101 to 103, which are set up to process valuable documents 1, in particular to separate them, transport them, check them, to 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' are explained in more detail by way of example.

Evaluation procedure 1

In this preferred method, the reflectance r(λ _i ) is first measured at the respective location 3, 3' at (at least) three different wavelengths λ _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 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 (eg r(λ ₀ ) ≤ threshold S ₀ ), and the difference quotient (r(λ ₁ ) - r(λ ₂ )) / (r(λ ₃ ) - r(λ ₄ )) lies within a given interval, and (because of the numerical stability of the quotient) the magnitude of the denominator | r(λ ₃ ) - r(λ ₄ )|≥ threshold S 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, λ ₁ and λ ₃ can coincide identically. λ ₀ may coincide with one of the wavelengths λ ₁ to λ ₄ , or r(λ ₀ ) may represent an average of several reflectance values selected from r(λ ₁ ) to r(λ ₄ ). In a further preferred embodiment, the reflectance spectrum can be checked even more precisely by not only determining and checking one difference quotient, but also by determining and checking two or more difference quotients with the addition of further 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.

Based on 2 until 4 illustrates how a reflective optically variable security element, whose color depends on the viewing angle, is tested by testing (r(1100nm) - r(1520nm))/ (r(1100nm) - r(770nm)) ∈ [0.3, 0.8] of conventional Colors are separated.

2 shows a first example of a reflectance spectrum of an OV film strip. By evaluating characteristic reflectance values (at the marked wavelengths λ ₁ =λ ₃ , λ ₂ , λ ₄ ) and calculating the difference quotient therefrom, a characteristic IR signature of the film strip can be clearly recognized or determined. For the 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.

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 in comparison to different IR absorber pigments (larger individual dots). The reflectance intensities r(λ ₁ ) to r(λ ₄ ) at the same wavelengths λ ₁ to λ 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.

4 shows a second example of a diagram with a representation of the numerator and denominator of difference quotients of different measuring pixels of different production batches of an OV film strip. The reflectance intensities r(λ ₁ ) to r(λ ₄ ) at the same wavelengths λ ₁ to λ ₄ were used. The spread of the calculated values caused by production fluctuations is preferably 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 is the 3 and 4 can be found by mentally forming the quotient y / x for a point (x, y). The respective difference quotient corresponds to the slope of the straight line through the point in question. The two original lines drawn with slopes 0.3 and 0.8 represent the limits [0.3, 0.8] of the permitted range for the difference quotients.

Evaluation procedure 2

In this preferred method it is assumed that a reflectance spectrum to be tested, each sampled at n wavelengths, is given by the reflectance values (r ₁ , ..., r _n ). In a first step, the reflectance spectrum to be tested of the OV film strip from the example described above and a reference spectrum are normalized, 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 , 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 The mean is 0 and the standard deviation is 1, for example according to the formula r _i ^norm = ((r _i - µ) / σ) _i=1...n , where µ is the mean of all r _i and σ is the standard deviation of all r _i ) .

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 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 reflectance spectrum vs. reference spectrum) is then determined, e.g. using a p-norm of their difference, and it is checked whether this is smaller than a predetermined threshold, which in the context of the present disclosure is also referred to as a “comparison distance”. referred to as. 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 procedure 3

In this preferred method, it is assumed that the measured or to-be-measured reflectance spectra of OV security elements can be approximated by means of representative reference reflectance spectra A and B, for example for production fluctuations that occur in extreme cases and/or spectral variations that occur due to different pigment orientations within a banknote.

Preferably, the reflectance spectrum obtained in each case to be tested with the reference reflectance spectra A and B is approximated (so-called fit) by a first linear combination c1 *A + c2 *B, the fit spectrum with the fit parameters c ₁ and c ₂ , and tested , how well the fit fits, for example with a p-norm of the difference between the measured reflectance spectrum and the fit spectrum, which is also referred to as the (adjusted) “comparison spectrum” in the context of the present disclosure.

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 any constant spectrum could be fitted exactly with c ₁ = c ₂ = 0. In order to exclude this case, preferably only pixels or locations are taken into account whose fit, for example |c ₁ |+ |c ₂ | is above a predetermined threshold, or whose respective reflectance spectrum is sufficiently non-constant, so that, for example, the standard deviation or difference between maximum and minimum is above a predetermined threshold.

6 shows examples of two reference reflectance spectra, the linear combination of which is adapted to a measured reflectance spectrum. 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 the reflectance spectra to be tested are additionally preselected by only using those pixels or locations in the evaluation whose reflectance curve differs sufficiently from a constant curve in the IR range, e.g. in which the difference of 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 demonstrated 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 “100” printed with magnetically aligned interference pigments and compared with the case of fitting by a single reference reflectance spectrum (1-dimensional fitting).

8th 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 reflectance spectrum is smaller than one Test threshold, which is also referred to as “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 ramp-up/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 is much more tolerant with regard to the position of the respective measuring track or the track height is than the evaluation with 1-dimensional adjustment.

Evaluation procedure 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, σ) = (0,1), for example according to the formula r _i ^norm = ((r _i -µ ) / σ) _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. The tolerance range is preferably determined or specified by calculating the derivative curves for a large number of real, i.e. measured on real banknotes or reference patterns, standardized reflectance spectra and determining the mean value and the standard deviation for each wavelength. The tolerance range then corresponds to the area around the spectral profile of the determined mean values, which is delimited by the spectral profile of the determined standard deviations.

9 shows an example of a normalized remission spectrum. 10 shows an example of a derivative curve calculated from this (solid line) and a specified tolerance range (dashed lines).

If the aim is not only to check whether the respective remission spectrum fits, but also how well it fits, a difference curve weighted channel by channel (i.e. for the individual wavelengths) with the reciprocal width of the tolerance range between the respective derivative curve and a predetermined target derivative curve can alternatively be calculated and these are then reduced to a value, e.g. 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 procedure 5

In this preferred method, the wavelengths of the local extrema (possibly interpolated) of the reflectance 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. Such wavelength intervals are preferably 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.

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 drawn.

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 can be used with less adaptation effort 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 to see whether For example, 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, a missing color-changing OV security element and/or normal colors and IR absorber colors can be reliably recognized in this way and the corresponding banknote can be rejected or classified as a counterfeit.

Claims (15)

Sensor (10) for checking valuable documents (1), which have at least one reflective security element (2) which has an appearance dependent on a viewing angle in the visible spectral range, 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 detect the infrared radiation remitted from the respective location (3, 3') 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 reflectance spectrum at the respective measurement location, and - a testing device (13) for checking the document of value with regard to a reflective security element (2), which has an appearance dependent on the viewing angle, which is set up to determine based on the remission spectrum obtained for the respective location (3, 3 '), 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') in order to determine one or more locations at which such a reflective element is located Security element (2) is located on the document of value (1), and the document of value (1) is to be checked based on the determined location(s) (3'). sensor (10). 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 (β) and/or the irradiation device is set up and/or arranged for this purpose, to direct the infrared radiation onto the valuable document in such a way that it hits the valuable document (1) at an irradiation angle (α), the detection angle (β) and the irradiation angle (α) preferably being of different sizes. sensor (10). 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 document of value, only the one directed at the document of value at exactly or approximately a single angle of incidence and in exactly or to use infrared radiation detected approximately at a single detection angle. 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 illuminate the respective location (3, 3 ') on the document of value (1) with infrared radiation from at least two, in particular to irradiate at least three different wavelengths (λ _i ) and/or wavelength ranges, which in particular do not overlap with one another, and the detection device (12) is set up to irradiate from the respective location (3, 3 ') in the at least two, in particular at least to successively detect remitted infrared radiation at three different wavelengths (λ _i ) and/or wavelength ranges in order to obtain the respective remission spectrum. 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') at which the determination is made was that there is a reflective security element (2) with an appearance that depends on the viewing angle on the document of value (1). sensor (10). 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 a predetermined region of interest (ROI) on the document of value ( 1) determined locations (3') located are greater than or equal to a predetermined second minimum number and/or whether the number of determined locations (3') located outside a predetermined region of interest (ROI) on the document of value 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 set up to determine whether there is a reflective security element (2) with an appearance dependent on the viewing angle at the respective location (3, 3 '). Document of value (1) is to determine reflectance values (r;, r(Xi)) at least two, in particular at least three, different wavelengths (λ _i ) in the reflectance spectrum obtained for the respective location (3, 3 '), from these at least to derive a first characteristic value that characterizes the respective remission spectrum and to compare the first characteristic value with at least one predetermined first comparison characteristic value. sensor (10). Claim 7 , where the first characteristic value is a difference quotient, which is formed from the reflectance values (r(λ ₁ ) to r( _λ4 )) determined in the reflectance spectrum at three or four different wavelengths (Xi to λ 4 ) ((r(λ ₁ ) - r(λ ₂ ))/ (r(λ ₃ )- r(λ ₄ ))). Sensor (10) according to one of the Claims 7 until 8th , wherein the testing device (13) is set up to measure at least one of the determined reflectance values (r(λ _i ), r(λ ₀ )) and/or at least one second characteristic value (r(λ ₀ )) derived from at least a portion of the determined reflectance values. , |r(λ ₃ ) - r(λ ₄ )|) with at least one predetermined second comparison characteristic value (S ₀ , S ₁ ). 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 an appearance dependent on the viewing angle at the respective location (3, 3 '). Document of value (1) is to determine 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 and to check whether the determined distance is smaller than a predetermined one Comparison distance. 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 an appearance dependent on the viewing angle at the respective location (3, 3 '). Value document (1) is 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 using a compensation calculation and to check whether the determined difference is smaller than a predetermined comparison difference , whereby the comparison spectrum is preferably given by a linear combination of at least two predetermined reference spectra. Sensor (10) according to one of the preceding claims, wherein the testing device (13) is set up to take into account when checking the document of value only those locations (3 ') for which a reflectance spectrum 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. 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 an appearance dependent on the viewing angle at the respective location (3, 3 '). Value document (1) 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 and / or a difference to a predetermined target derivation curve which is smaller than a predetermined maximum difference. 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 an appearance dependent on the viewing angle at the respective location (3, 3 '). Document of value (1), in the remission spectrum obtained for the respective location (3, 3'), the wavelengths of two or more local extrema, in particular of at least one local maximum (“max”) and at least one local minimum (“min”) , to determine and check whether the determined wavelengths lie within wavelength intervals which are specified for the respective extrema, and/or whether only one local extremum lies within a wavelength interval specified for the respective extremum. Method for checking documents of value (1) which have at least one reflective security element (2) which has an appearance dependent on a viewing angle in the visible spectral range, with the following steps: - Irradiating one or more different locations (3, 3') on a valuable document (1) with infrared radiation and spectrally resolved detection of the infrared radiation remitted from the respective location (3, 3') on the valuable document (1), each obtaining a remission spectrum becomes, - Determine based on the remission spectrum obtained for the respective location (3, 3') whether there is a reflective security element (2) with an appearance dependent on the viewing angle on the document of value (1) at the respective location (3, 3'). to determine one or more locations at which 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').

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