Sensor and method for checking a value document with at least one reflective security element

Abstract

The invention relates to a sensor (10) and a method for inspecting a value document (1) having at least one reflective security element (2) which has an appearance dependent on the viewing angle in the visible spectral range. The sensor (10) has: an irradiation device (11), which is arranged to irradiate one or more different locations (3, 3 ') on the value document (1) with infrared radiation; a detection device (12) which is arranged to detect infrared radiation respectively reflected from different positions (3, 3 ') on the value document (1), the sensor recording a reflection spectrum from each position; and an inspection device (13), which is designed to determine, on the basis of the respective reflection spectrum, whether the reflective security element having an appearance dependent on the viewing angle is in the respective position in order to determine one or more positions (3 ') of such a reflective security element (2) on the value document (1), and to inspect the value document (1) on the basis of the one or more determined positions (3').

Description

[0001] The invention relates to a sensor and a method for checking a value document having at least one reflective security element which has an appearance in the visible spectral range that depends on the viewing angle.

[0002] In order to increase security against counterfeiting, value documents, in particular banknotes, are provided with so-called optically variable (OV) security elements, for example in the form of film elements, so-called patches, wide security threads or printed, magnetically oriented interference pigments, which are reflective. For example, in reflection they have an appearance that depends on the viewing angle and can therefore be checked with the naked eye by simply tilting the value document. In the case of film-based OV security elements, in addition to holograms and micromirrors, stacks of thin layers are used in order to produce angle-dependent interference colors or color changes.

[0003] The object of the present invention is to enable a reliable machine-based inspection of value documents, in particular banknotes, which are provided with a reflective, for example optically variable, security element.

[0004] This object is achieved by a sensor and a method for checking value documents as well as a value-document processing system having such a sensor according to the independent claims.

[0005] According to a first aspect of the disclosure, a sensor for checking a valuable document is proposed, the valuable document having at least one reflective security element, which has an appearance that depends on the viewing angle, in particular in the visible spectral range, the sensor comprising: an irradiation device, which is arranged to irradiate one or more different positions on the valuable document with infrared radiation; a detection device, which is arranged to detect the infrared radiation reflected from the corresponding positions on the valuable document, wherein the sensor is arranged to detect the reflected infrared radiation in a spectrally resolved manner, wherein a reflection spectrum is respectively obtained; an inspection device for checking the valuable document with respect to the reflective security element with an appearance that depends on the viewing angle, the inspection device being arranged to determine, based on the reflection spectrum obtained for the corresponding position, whether a reflective, in particular optically variable security element with an appearance that depends on the viewing angle is located on the valuable document at the corresponding position, so as to determine one or more positions at which such a reflective security element (i.e., with an appearance that depends on the viewing angle) is (the same or, if necessary, different) and to inspect the valuable document based on the one or more determined positions (with respect to the reflective security element with an appearance that depends on the viewing angle).

[0006] According to a second aspect of the present disclosure, a method for inspecting a valuable document is proposed, wherein the valuable document has at least one reflective security element, which has an appearance that depends on the observation angle, especially in the visible spectral range, and the method has the following steps: irradiating one or more different positions on the valuable document with infrared radiation and spectrally resolving the infrared radiation reflected from the corresponding positions on the valuable document, wherein reflection spectra are respectively obtained; determining whether a reflective, especially optically variable security element with an appearance that depends on the observation angle is located on the valuable document at the corresponding position based on the reflection spectra obtained for the corresponding position, so as to determine one or more positions at which such a reflective security element (i.e., having an appearance that depends on the observation angle) is located; and inspecting the valuable document based on the one or more determined positions, for which it has been determined that the reflective, especially optically variable security element (having an appearance that depends on the observation angle) is located on the valuable document at the corresponding position.

[0007] A third aspect of the present disclosure relates to a valuable document processing system, which has at least one sensor according to the first aspect of the present disclosure and one or more processing devices, which are configured to process valuable documents, in particular to separate, transport, inspect, sort, stack and / or destroy valuable documents.

[0008] A fourth aspect of the present disclosure relates to a computer program comprising instructions which, when executed by a computer, cause the computer to perform a method according to the second aspect of the present disclosure. A fifth aspect of the present disclosure relates to a computer-readable data carrier having a computer program according to the fourth aspect of the present disclosure stored thereon. A sixth aspect of the present disclosure relates to a data carrier signal which transmits a computer program according to the fourth aspect of the present disclosure.

[0009] The various aspects of the disclosure are preferably based on the means that at a plurality of different positions on the value document to be checked (also referred to in the context of the disclosure as "measurement positions", "measurement pixels" or "pixels") a spectrally resolved measurement of the infrared radiation reflected from the respective position is performed and one or more positions on the value document are identified based on the reflection spectrum obtained thereon, at which such a reflective and / or optically variable security element is present and / or the one or more positions are located on the reflective and / or optically variable security element. For this purpose, the respectively obtained reflection spectrum is analyzed, for example, with regard to its spectral curve and / or other characteristics. A reflection spectrum in the sense of the disclosure can be a spectrum containing reflection values ​​at a plurality, if necessary even at many, or at a small number, for example three or two, of different wavelengths.

[0010] Compared to (machine) inspection by visible light, the use of infrared radiation to inspect security elements with an appearance in the visible spectral range that depends on the viewing angle offers several advantages: The anti-counterfeiting security is increased, since the infrared signature of the security element cannot be inferred from its visual appearance. The anti-counterfeiting security is further increased, since the inspection is carried out by invisible light and therefore goes unnoticed by a potential counterfeiter. Furthermore, the inspection is more reliable, since inspection by infrared light is less adversely affected by soiling of the banknote to be inspected than inspection with visible light.

[0011] The reflective security element having an appearance in the visible spectral range that depends on the viewing angle is, for example, an optically variable security element, in particular an optically variable security element having a color change that depends on the viewing angle. The reflective security element having an appearance in the visible spectral range that depends on the viewing angle can be a specularly (directional or retroreflective) reflecting security element or a diffusely reflecting security element.

[0012] The infrared radiation reflected from the corresponding position and detected (or collected) by the detection device can be directional and / or diffuse reflected infrared radiation of the valuable document. Preferably, the irradiation device and the detection device are arranged so that the detection device detects diffuse reflected infrared radiation of the valuable document, but does not detect directional reflected infrared radiation.

[0013] It is preferably checked whether one or more of the positions determined in this way meet at least one predetermined check criterion, for example in terms of their number and / or position on the value document. If so, the presence of such a reflective and / or optically variable security element (optionally with certain properties) is confirmed and / or the value document is classified as "authentic" or "fit" (acceptable state).

[0014] The position determined at which such a reflective security element is located (when the detection range is correspondingly large) can be, for example, a single position which includes the entire surface of the relevant security element. However, the position determined at which such a reflective security element is located (when the detection range is correspondingly small) can also be one of a plurality of positions located on the relevant security element. The latter can, but need not, be subjected to spectrally resolved detection or the above-mentioned examination according to its reflection spectrum. The positions determined to be subjected to the above-mentioned examination or the positions at which such a reflective or optically variable security element is located can all be located on the same reflective or optically variable security element of the respective value document, or on different such reflective or optically variable security elements of the respective value document.

[0015] Preferably, exactly one reflection spectrum is recorded from the respective position. In particular, reflection spectra are respectively recorded from a plurality of positions on the respective value document, wherein preferably exactly one reflection spectrum is respectively recorded from each position.

[0016] When determining the position of such a reflective or optically variable security element on a valuable document, it is not necessarily necessary to identify the specific design and / or specific properties of the valuable document to be checked respectively, such as the expected condition of such a reflective or optically variable security element on the valuable document, and to include them in the check, based on the respectively obtained reflection spectrum. In particular, a so-called adaptation of the sensor or the method with respect to these properties is not necessarily necessary. Furthermore, it is not necessarily necessary to identify the denomination and orientation of the valuable document to be checked respectively at the time of checking the valuable document. On the other hand, if the specific properties of the valuable document are additionally taken into account here, in particular with respect to the condition and / or size of the reflective or optically variable security element and / or the denomination and orientation of the valuable document, then of course a more reliable identification of the relevant position and / or a more reliable check of the valuable document can be achieved.

[0017] The described means for identifying positions on such a reflective or optically variable security element is based on the surprising knowledge that many reflective security elements, for example interference-based OV security elements (film-based and printed pigment-based), have an inherent characteristic spectral signature not only in the visible spectral range (where OV security elements are primarily checked) but also in the infrared (IR) spectral range. This can therefore be used for machine inspection of OV security elements in the infrared. Even sensors that have been developed or specified for spectral measurement of infrared absorbing pigments can be advantageously used here, if the signal evaluation or examination used here is adapted to the specificities of the spectral identification of interference colors.

[0018] Overall, the present disclosure thus enables reliable and simple machine checking of value documents, in particular such banknotes, which have one or more reflective or optically variable security elements, the appearance of which depends on the viewing angle.

[0019] The means disclosed herein are basically suitable for detecting or verifying reflective security elements having an appearance that depends on the viewing angle, but are particularly suitable for detecting or verifying optically variable security elements that have, for example, a color change that depends on the viewing angle, which is produced, for example, by interference on a thin layer. The optically variable security element can be in the form of an optically variable film element (e.g. a so-called film block or film strip), an optically variable (broad) security thread or an optically variable printed feature (e.g. with interference pigments that are magnetically oriented during printing before curing).

[0020] The sensor is arranged for spectrally resolved detection of the infrared radiation reflected from the respective position. For example, the detection device itself can be arranged to spectrally resolve the reflected infrared radiation and to detect it spectrally resolved. The spectral resolution is located on the detection side, while the irradiation device is preferably spectrally broadband in the infrared. Alternatively and particularly preferably, the detection device is a (simpler) detection device, which is not designed for spectral resolution of light, for example spectral resolution of reflected infrared radiation. The spectral resolution is then performed by sequentially irradiating the respective positions with at least two or at least three infrared radiations of different wavelengths / ranges and by the detection device correspondingly detecting in sequence at (at least approximately) the same position.

[0021] In particular, the detection device is configured and / or arranged to detect infrared radiation reflected from different positions at a (vertical or oblique) detection angle, and the irradiation device is configured and / or arranged to direct the infrared radiation onto the value document so that it reaches the value document at a (vertical or oblique) angle of incidence. If the incident infrared radiation or the detected infrared radiation has an angular variation, the average angle of the corresponding angular distribution is regarded as the angle of incidence or the detection angle. Preferably, the angle of incidence or the detection angle is oriented vertically with respect to the value document and the corresponding other angle is oriented obliquely with respect to the value document.

[0022] Preferably, the detection angle and the angle of incidence are not the same size. Therefore, preferably, the reflection law (the size of the exit angle = the size of the incident angle) is not followed when measuring at different positions, so that the position located on such a reflective or optically variable security element can be more reliably identified. Therefore, advantageously, in order to irradiate the valuable document with infrared radiation and detect the infrared radiation reflected by the valuable document, a geometry that preferably does not meet the classical reflection condition is selected, because this will reduce the artifacts that appear at individual wavelengths caused by the so-called mirror reflection (directional reflection or reflection). Therefore, for example, in the case of a reflective OV element, although a micromirror array is often used here, which is responsible for the moving image or pattern when tilted, there will be no measurement interference (such as excessive control of the detection device), and the precise orientation of the banknote relative to the sensor is not required. On the contrary, the micromirrors or surface areas of this OV security element that are just properly oriented statistically always contribute to the signal, so that even if the banknote is wrinkled or bent, reliable measurements can be performed. Even in the case of a sensor with only a small number of spectral support positions (i.e., the measurement wavelength), this reflection measurement already provides a representative and stable signal, which can be unexpectedly used for reliable banknote inspection.

[0023] Preferably, the detection angle and / or the angle of incidence are substantially the same for all positions on the value document, from which the respectively reflected infrared radiation is detected.

[0024] Preferably, the infrared radiation, in particular for all wavelengths, is directed onto the valuable document at a (exactly or approximately) unique angle of incidence by a (possibly unique) illumination device and / or, in particular for all wavelengths, is detected by a (possibly unique) detection device at a (exactly or approximately) unique detection angle.

[0025] Preferably, the inspection device of the sensor is arranged so that, in order to determine the one or more positions of such a reflective security element of the corresponding valuable document on the valuable document, only infrared radiation is used which is directed to the valuable document at a (just or approximately) unique angle of incidence and detected at a (just or approximately) unique detection angle. In other words, preferably only under (unique) angle conditions, i.e., under a unique detection angle and a unique angle of incidence, the reflection behavior of the valuable document is detected at different positions. Surprisingly, it has been found that many reflective security elements with an appearance that depends on the viewing angle, in particular such security elements with a color change that depends on the viewing angle, have a characteristic mark in the infrared spectrum when measured under uniform angle conditions, which can be reliably identified by the mark. Therefore, repeated inspections at different angles of incidence and / or detection can be omitted. Therefore, the sensor can be manufactured with correspondingly less expenditure, for example, only with a unique irradiation device and / or with a unique detection device. In addition, no moving parts are provided, so that measurements can also be made on rapidly moving valuable documents, and thus no wear caused by mechanical movement will occur.

[0026] As an approximately unique angle of incidence, it is also applicable that due to the beam divergence of the illumination device, slightly different angles of incidence exist, in particular with a variation of up to + / -15°, preferably up to + / -12°. As an approximately unique detection angle, it is also applicable that the aperture of the detection device enables slightly different detection angles, in particular with a variation of up to + / -18°, preferably up to + / -10°.

[0027] As an approximately unique angle of incidence, it is also applicable if the illumination device has a plurality of (e.g. spectrally different) infrared light sources which are arranged directly or closely adjacent to one another so that they direct their infrared radiation onto the value document at approximately the same angle of incidence (in particular with a variation of at most + / -5°, preferably at most + / -1°). As an approximately unique angle of detection, it is also applicable if the detection device comprises a plurality of discrete detection elements which are arranged closely adjacent to one another so that they detect the infrared radiation reflected by the value document at approximately the same detection angle (in particular with a variation of at most + / -5°, preferably at most + / -1°).

[0028] In the case where the precise spectral curve in the interference pigment is, for example, affected by manufacturing fluctuations and spectral changes occur as a result, it is preferred that these spectral changes are taken into account accordingly when identifying the position on such a reflective or optically variable security element. Local fluctuations in the angle of incidence, for example caused by wrinkles in banknotes, can also lead to changes in the respectively obtained reflection spectrum, which are preferably taken into account when determining the corresponding position. This applies similarly to magnetically oriented printing pigments, if differently oriented interference pigments are applied to different printing positions, so that the observation angle and therefore the interference conditions seen can vary with the corresponding position (and possible bending / wrinkle angle). Correspondingly, changes in the measured spectral shape will also occur, which are preferably taken into account when determining the relevant position based on the respectively recorded reflection spectrum.

[0029] Preferably, the irradiation device is configured to irradiate the corresponding positions on the value document (simultaneously or successively) with infrared radiation having at least two, in particular at least three different wavelengths and / or wavelength ranges, the wavelength ranges in particular not overlapping with each other. Alternatively or additionally, the detection device is configured to detect (spectrally resolved or non-spectrally resolved) the infrared radiation reflected from the corresponding positions at at least two, in particular at least three different wavelengths and / or wavelength ranges, the wavelength ranges in particular not overlapping or only slightly overlapping.

[0030] Preferably, for the spectrally resolved detection of reflected infrared radiation, the irradiation device is arranged to sequentially irradiate corresponding positions on the valuable document with infrared radiation of at least two, in particular at least three different wavelengths and / or wavelength ranges that in particular do not overlap with each other, and the detection device is arranged to sequentially detect infrared radiation reflected from the corresponding positions with at least two, in particular at least three different wavelengths and / or wavelength ranges, respectively, so as to obtain a corresponding reflection spectrum (Remissionsspektrum).

[0031] For example, a spectrally resolved IR reflection is measured at one or more measuring positions on the value document to be checked, wherein at the respective measuring positions, a region of the value document is successively illuminated by a first IR light source and a first intensity of the reflection is detected, then the same region is illuminated by a second IR light source with a different wavelength and a second intensity of the reflection is detected, and optionally the same region is illuminated by one or more further IR light sources with different wavelengths and the respective intensities of the reflection are detected. Preferably, three, five or more infrared light sources with different wavelengths and narrowband spectra, preferably LEDs, are used, in particular without spectrally overlapping one another. Preferably, the light of all IR light sources impinges on the value document at approximately the same angle of incidence or angles of incidence.

[0032] Preferably, a transport device is provided, which is arranged to transport the valuable documents to be checked respectively relative to the irradiation device and the detection device in the transport direction. Preferably, the infrared radiation reflected respectively from a plurality of positions of the valuable document along one or more measurement tracks extending parallel to the transport direction is detected in a spectrally resolved manner. Based on the reflection spectra of the positions of the one or more measurement tracks extending parallel to the transport direction, the inspection device then checks the valuable document with respect to the reflective security element having an appearance that depends on the viewing angle. Preferably, the transport device is arranged to transport the valuable document during the measurement at a transport speed of about 2 to 11 m / s relative to the irradiation and detection device. Preferably, the irradiation device and the detection device are arranged for this and / or can be controlled according to the transport speed so that, in particular at high transport speeds, the measurement positions or areas of the valuable document, which are respectively irradiated by different light sources at different wavelengths, overlap by at least 80%, preferably by more than 90%. It is thereby ensured that the reflection spectra obtained for the respective measurement positions come from essentially the same area on the valuable document.

[0033] 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 respectively detects the infrared radiation of exactly one of the measurement tracks and / or the reflected infrared radiation of each measurement track is detected by exactly one detection element.

[0034] Preferably, the inspection device is configured to inspect the valuable document based on the number and / or condition of one or more determined positions, at which it has been determined that such a reflective security element is located on the valuable document. The condition is checked, for example, based on at least one region of interest (ROI) on the valuable document. Inspection of the valuable document based on the number / condition of the determined positions has the advantage that the sensor does not need to record an image of the valuable document and a (usually complex) comparison of images of the valuable document is also not required. As a result, the sensor and the inspection device require little effort. In addition, since no image comparison is required, the inspection of the valuable document can be carried out more quickly and easily. Preferably, the inspection device is not configured to compare different images of the valuable document with each other.

[0035] Preferably, during the check, it is checked, depending on the number and / or the situation of the determined positions, whether the number of determined positions (e.g., in total on the value document) is greater than or equal to a predetermined first minimum number and / or whether the number of determined positions located 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 of determined positions located outside the / one (the same or another) predetermined region of interest (ROI) on the value document is less than or equal to a predetermined maximum number, in particular equal to zero. If necessary, this can also be performed for one or more further ROIs on the same value document.

[0036] In particular, for checking the authenticity and / or the condition of the value document, one or more locations on the value document identified according to the respectively acquired reference spectrum are checked according to one or more check criteria with regard to their quantity and / or condition.

[0037] In particular, it is checked whether in one or more ROIs, in particular depending on the denomination and / or on the transport conditions, there is a minimum number of positions at which a reflective security element with an appearance dependent on the viewing angle is determined. The checking device of the sensor can be designed to select one or more ROIs depending on the denomination and / or on the transport conditions of the respective value document.

[0038] For example, a value document is classified as "authentic" or "suitable" if the number of determined positions at which a reflective security element with an appearance dependent on the viewing angle is determined to be located on the value document is greater than or equal to a predetermined first minimum number and / or if the number of determined positions located within a predetermined region of interest (ROI) on the value document is greater than or equal to a predetermined second minimum number, otherwise it is classified as "false" or "suspected false" or "unsuitable". Alternatively or additionally, a value document is (if necessary) classified as "authentic" or "suitable" if only the maximum number of determined positions is determined in the / one (the same or one or more other) ROI / no positions with such a reflective security element are determined.

[0039] Alternatively or additionally, it is checked whether a minimum number of positions on which the security element is recognized as a whole is present (corresponding to a special case of the first case, in which the ROI is given by the entire value document). Forgeries lacking a security element are thus reliably detected without requiring information, for example, about the expected position and / or shape of the security element or the condition of the respective value document to be checked. Alternatively or additionally, in particular in addition to the first case, it can be checked whether only a maximum number of positions on which the OV security element is recognized is present outside the ROI (preferably no positions at all). All of the above-mentioned designs or situations, respectively, individually or in combination, enable a particularly reliable check of the value document, in particular an authenticity check and / or a status check.

[0040] The condition and size of the ROI or the second minimum number can be selected based on the condition and size of the security element to be checked which are known for the value document.

[0041] Preferred evaluation methods for determining a position on a value document located on an OV security element and / or for checking a value document based on the position determined therein are described below, wherein each evaluation method, including preferred refinements, can be used alone or in combination with at least one other evaluation method. Each of these evaluation methods itself enables the determination based on the corresponding reflection spectrum whether a reflective, in particular optically variable, security element having an appearance that depends on the viewing angle is located on the value document at the corresponding position.

[0042] Evaluation Method 1

[0043] Preferably, the checking device is arranged to detect whether a reflective security element having an appearance depending on the viewing angle is located on the value document at the respective position at at least two, in particular at least three different wavelengths λ i (i=1, 2, 3, ...) and / or the reflectance value r(λ i ), thereby deriving at least one first characteristic value characterizing the corresponding reflection spectrum, and comparing the first characteristic value with at least one predetermined first comparison characteristic value. i The number and / or condition of the wavelength λ to be selected is preferably adjusted, depending on the application, for example in the case of different shades of different OV security elements, so that a characteristic point in the reference spectrum is detected thereby, for example by targeted scanning of local maxima and / or local minima. i As a result, reliable detection of positions on such a reflective or optically variable security element is achieved in a particularly simple manner.

[0044] Preferably, the reflection values ​​are determined at at least three different wavelengths in the infrared spectral range. However, in some security elements or spectral curves, it is sufficient or feasible to use a measuring point in the red VIS range and only two wavelength measuring points in the IR spectral range to determine the position on the OV security element, which is particularly advantageous for low-cost sensors used in cash payment systems (so-called cash-in).

[0045] Preferably, the first characteristic value is a difference quotient obtained by measuring the difference between the two parameters at three or four different wavelengths λ. 1 To 4 The reflectance value r(λ 1 ) to r(λ 4 ) is formed, especially according to ((r(λ 1 )–r(λ 2 )) / ((r(λ 3 )-r(λ 4 )). In this way, first characteristic values ​​of the respective reflection spectrum are obtained from the reflection values ​​obtained at different wavelengths, which are particularly indicative of the presence of an OV security element.

[0046] Preferably, at least one predetermined first comparison characteristic value has a lower comparison threshold and / or an upper comparison threshold, with which the first characteristic value, in particular the difference quotient, is compared. Thus, in the case of a predetermined lower and upper comparison threshold, 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 confirm the presence of the OV security element.

[0047] Alternatively or additionally, the checking device is configured to determine at least one of the reflection values, for example r(λ 0 ), and / or from the determined reflection value r(λ i ) derived from at least one second eigenvalue of at least a portion of |r(λ 3 )–r(λ 4 )|, and at least one predetermined second comparison characteristic value, such as a threshold value S 0 or S 1 , for comparison.

[0048] For example, the reflection spectrum obtained from a single measurement position is evaluated as OK, or the relevant measurement position is identified as a position located on the OV security element, provided that the reflection spectrum has a minimum absorption, for example by means of r(λ 0 )≤S 0 (Second comparison eigenvalue), and the difference quotient (r(λ 1 )-r(λ 2 )) / (r(λ 3 )-r(λ 4 )) is within a predetermined interval (i.e., between the upper and lower comparison thresholds). Preferably, in order to ensure the numerical stability of the difference quotient, it is additionally required that the value of the denominator |r(λ 3 )-r(λ 4 )|≥S 1 (Second comparison eigenvalue).

[0049] By means of this simple machine-based method, for example color-shifting OV security elements whose color depends on the viewing angle (so-called Color-Shift colors) can be reliably distinguished from conventional printed colors and infrared-absorbing colors.

[0050] In a simplified implementation, λ 1 and λ 3 can be the same, that is, only the reflection values ​​at three different wavelengths are used to calculate the difference quotient or determine the position on the OV security element. In addition, λ 0 Can be related to wavelength λ 1 To 4 Alternatively or additionally, r(λ 0 ) can be expressed from r(λ 1) to r(λ 4 ) is an average of a plurality of reflection values ​​of the wavelength. In another embodiment, the reflection spectrum can be checked more precisely by determining or checking not only one difference quotient but two or more difference quotients by adding further wavelengths.

[0051] Overall, this evaluation method is particularly suitable for small, relatively cost-effective sensors, since in simple cases only three different wavelengths are required for scanning (of which only two different wavelengths are required in the IR if necessary) and the evaluation requires a relatively low numerical effort.

[0052] Evaluation Method 2

[0053] Preferably, the checking device is arranged to determine, in order to determine whether a reflective security element having an appearance depending on the observation angle is located at a corresponding position on the valuable document, a distance between a reflection spectrum obtained for the corresponding position, in particular normalized, and at least one predetermined, in particular normalized reference spectrum, and to check whether the determined distance is smaller than a predetermined comparison distance.

[0054] Basically, any type of normalization can be used for the normalization of the respective reflection spectrum or reference spectrum, which is preferably performed, by which it is ensured that fluctuations caused by the respective measurement or comparative measurement (for the reference spectrum), such as fluctuations in the intensity of the infrared radiation occurring at the respective location during the detection of the infrared radiation reflected from this location and / or in the sensitivity of the detection device, are substantially eliminated. Preferably, the reflection spectrum to be checked and the associated reference spectrum, respectively, are normalized, for example, by a linear transformation, which maps the minimum values ​​to 0 and the maximum values ​​to 1. For example, such normalization is performed according to the following formula r i norm =((r i –min) / (max–min)) i＝1…n The reflectance spectrum to be examined is scanned at n wavelengths and is represented by the reflectance value r 1 ,…,r n Given, and "min" is all r i The minimum value of "max" is the value of all r i Alternatively, this normalization can also be performed, for example, by a linear transformation, with a mean value of 0 and a standard deviation of 1, for example according to the following formula r i norm =((r i –μ) / σ) i＝1…n , μ is the sum of all r i The average value of all r i The standard deviation of .

[0055] Preferably, the distance between two normalized spectra (measured reflection spectrum and reference spectrum) is determined, for example, by the p-norm of their difference, and it is then checked whether the distance is smaller than a predetermined threshold value. 1 norm ,…,r n norm is given by, and the normalized reference spectrum is given by the value R 1 ,…R n Given, first determine the point-wise difference x i =r i norm -R i The p-norm of the difference is given by the formula Definition. For p=2 it corresponds to the Euclidean distance. The spectral variations that occur are captured by appropriate choice of the threshold and the OV security elements on banknotes of all production batches are identified as being proper or genuine.

[0056] This evaluation method is particularly suitable for scanning the spectrum at multiple, especially more than three or four wavelengths, so that a complete spectral curve exists. In these cases, although the adaptation is very simple, a complete and very accurate examination of the entire spectral curve is provided.

[0057] Evaluation Method 3

[0058] Preferably, the checking device is configured to determine whether a reflective security element having an appearance that depends on the viewing angle is located at the corresponding position on the value document by determining the difference between the reflection spectrum obtained for the corresponding position and a comparison spectrum adapted to the reflection spectrum by fitting, and to check whether the determined difference is smaller than a predetermined comparison difference, wherein the comparison spectrum is preferably given by a linear combination of at least two predetermined reference spectra. The predetermined reference spectra are preferably representative reference reflection spectra, which may occur, for example, in extreme cases due to production fluctuations and / or due to different pigment orientations in the banknote. Preferably, the radiation spectra (to be checked) obtained from one position in each case are fitted using the reference reflection spectra A and B, for example by a linear combination c 1 A+c 2 B, the so-called fitted spectrum, the fitting parameter c 1 and c 2 In a preferred embodiment, the coefficient c for the adaptation permission is 1 or c 2 The range is determined by predefined constraints, such as c 1 >0 and c 2 >0. The degree of fit is then checked, for example by determining the p-norm of the difference between the measured reflectance spectrum and the fitted spectrum and comparing the p-norm with a corresponding predetermined comparison difference.0 The linear combination of (offsets) c 0 +c 1 A+c 2 The fit of B is more accurate. It should preferably be excluded here that 1 =c 2 = 0 can accurately fit each constant spectrum by considering only the following pixels or measurement positions, whose fit is, for example, |c 1 |+|c 2 | is above a predetermined threshold or its reflection spectrum is sufficiently non-constant so that, for example, the standard deviation or difference between the maximum and minimum values ​​is above a predetermined threshold.

[0059] The method can be further improved if a preselection of the reflection spectrum to be checked is additionally carried out. Preferably, provision is made here that only those pixels or measurement positions are taken into account when checking the value document, whose reflection spectrum deviates sufficiently from a constant curve in the IR range, for example in which the difference between the maximum and the minimum exceeds a predetermined threshold value. IR absorption features may be present at these positions.

[0060] Accordingly, the checking device is preferably arranged so that, when checking the valuable document, only the following positions are taken into account for which reflection spectra have been respectively obtained, which satisfy at least one of the following conditions: i) the curve of the reflection spectrum is not constant and / or has a predetermined minimum difference compared to a constant curve, and / or ii) the difference between the maximum and the minimum value of the reflection spectrum is greater than a predetermined minimum difference.

[0061] This preselection can optionally be used preferably in the other evaluation methods 1 , 2 , 4 or 5 .

[0062] This evaluation method is also particularly suitable for applications in which the reflection spectrum is scanned at multiple wavelengths, so that the entire spectral curve is present in one or more measurement tracks. In these cases, a particularly robust and stable identification of OV security elements is provided. In addition, the evaluation method is particularly well suited for OV security elements with printed, locally differently oriented interference pigments.

[0063] Evaluation Method 4

[0064] Preferably, the checking device is configured to determine whether a reflective security element having an appearance dependent on the viewing angle is located at the corresponding position on the valuable document by forming a derivative curve (Ableitungskcurve) by the derivative of the reflection spectrum obtained for the corresponding position, in particular normalized, according to the wavelength, and to check whether the derivative curve lies within a predetermined tolerance range and / or has a difference with a predetermined target derivative curve that is smaller than a predetermined maximum difference. Preferably, the reflection spectrum to be checked in each case is first normalized, for example by a linear transformation, whereby the minimum value is mapped to 0 and the maximum value to 1 (see the above formula), or by a linear transformation such that (μ, σ)=(0,1) (see the above formula). Preferably, a derivative curve (first-order derivative according to the wavelength) is then formed. For example, for a reflection spectrum with n intensity values ​​(r(λ i )) i＝1,..,n The reflectance spectrum exists in the form of, and 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 the derivative curve is within a predetermined tolerance range. The predetermined tolerance range can be determined in advance, for example, by calculating the derivative curve for a plurality of normalized reflection spectra obtained on the (comparison) value documents and determining the mean value and the standard deviation for each wavelength.

[0065] If it is to be checked not only whether the respectively obtained reflection spectra are adapted, but also how well they are adapted, a difference curve with the target derivative curve can be calculated alternatively or additionally, for example a difference curve weighted by the inverse width of the tolerance range relative to the target derivative curve "channel-wise" (i.e. wavelength-wise) and reduced to a certain value, for example by the p-norm. This value can then be compared with an upper threshold value, wherein if the upper threshold value is not exceeded, the presence of an OV security element is confirmed.

[0066] This method is particularly suitable for the following cases, wherein the corresponding reflection spectrum is scanned at a plurality of (particularly preferably: many) wavelengths, so that a complete spectral curve is present. In these cases, a particularly precise separation of the spectra of different OVI components is provided despite the low adaptation effort.

[0067] Evaluation Method 5

[0068] Preferably, the checking device is configured to determine, in order to determine whether the reflective security element with an appearance dependent on the viewing angle is located at the respective position on the value document, the wavelengths of two or more local extrema, in particular the wavelengths of at least one local maximum and at least one local minimum, in the reflection spectrum obtained for the respective position, and to check whether the determined wavelengths are located within a wavelength interval predetermined for the respective extrema and / or whether only one local extrema in each case is located within a wavelength interval predetermined for the respective extrema. Preferably, the checking device is configured to form a quotient from each of the two determined wavelengths and to check whether the quotient is within a predetermined tolerance range.

[0069] At first determine the wavelength of the local extreme value (interpolatable) of the reflection spectrum.Then according to the wavelength interval defined for the minimum and maximum values ​​in advance, check whether there is especially just a local minimum and / or maximum in each wavelength interval.Alternate or additionally define the selection of the predetermined wavelength interval, and check whether the quotient of the wavelength of the relevant extreme value is within the predetermined tolerance range.Preferably one to three minimum values ​​and one to three maximum predetermined wavelength intervals.Preferably a maximum and two minimum values ​​are located in the IR range.For example, define the wavelength interval for a maximum and a minimum value, a maximum and two minimum values, two maximum values ​​and a minimum value, two maximum values ​​and two minimum values, two maximum values ​​and three minimum values, three maximum values ​​and two minimum values ​​or three maximum values ​​and three minimum values.

[0070] This method is particularly suitable for scanning the reflection spectrum at a plurality of individual wavelengths. In this case, it is not necessary for the complete spectral curve to be present. In this case, a particularly low adaptation effort is provided, since a number of different interference-based OV color imprints can be identified simultaneously by a single adaptation and reliably distinguished from absorbing pigments.

[0071] Other advantages, features and possible applications of the present invention are apparent from the following description in conjunction with the accompanying drawings.

[0072] Figure 1 An example of a sensor for checking a value document is shown;

[0073] Figure 2 A first example of a reflectance spectrum is shown;

[0074] Figure 3 A first example of a graph representing the numerator and denominator of a difference quotient is shown;

[0075] Figure 4 A second example of a graph representing the numerator and denominator of a difference quotient is shown;

[0076] Figure 5 Examples of reflectance spectra before (left) and after (middle, right) normalization are shown;

[0077] Figure 6 An example of a reference spectrum is shown;

[0078] Figure 7 An example of adapting a reflectance spectrum by a first linear combination of reference spectra (left) and a second linear combination of reference spectra (right) is shown;

[0079] Figure 8 shows examples of positions determined by one-dimensional or two-dimensional fitting;

[0080] Fig. 9 An example of a normalized reflectance spectrum is shown;

[0081] Fig.10 Shown by Fig. 9 An example of a calculated derivative curve of a normalized reflectance spectrum and a predetermined tolerance range is shown; and

[0082] Fig.11 An example of a reflection spectrum is shown, wherein a distribution of the corresponding spectral conditions of local minima and / or maxima of a plurality of measured reflection spectra is shown.

[0083] Figure 1 The schematic diagram shows an example of a sensor 10 for checking a value document 1 , in particular a banknote, having at least one such reflective, optically variable (OV) security element 2 .

[0084] In the present example, the security element 2 is designed as a so-called lead strip extending over the entire width of the value document 1. Such a lead strip is applied, for example, to a paper primer and provided by an integral overprint, usually by intaglio printing.

[0085] Basically, however, the security element 2 can have any other arbitrary shape and / or be based on other functional principles, in particular to produce an appearance that depends on the viewing angle in the visible spectral range, for example in the form of film elements, patches, wide security threads or printed magnetic directional interference pigments. In the case of film-based OV security elements, in addition to micromirrors, stacks of thin layers of controlled thickness are therefore used in order to produce angle-dependent interference colors or color changes.

[0086] The sensor 10 has an irradiation device 11, which is arranged to irradiate a plurality of different positions 3, 3' with infrared radiation. Preferably, this irradiation of the different positions takes place sequentially in time. In the context of the present disclosure, the different positions 3, 3' are also referred to as "measurement positions", "measurement pixels" or "pixels". The infrared radiation reflected from the different positions 3, 3' is detected by a detection device 12, wherein a reflection spectrum is obtained for each position 3, 3'.

[0087] The irradiation device 11 and / or the detection device 12 can be designed in various ways in order to obtain the reflection spectrum from the positions 3, 3', respectively. Thus, in general, it can be provided that, for example, the irradiation device 11 irradiates the value document 1 with broadband infrared radiation and the detection device 12 detects the reflected infrared radiation spectrally resolved at a plurality of different wavelengths.

[0088] However, 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 different wavelength ranges, by which the corresponding positions 3, 3' are successively irradiated. The detection device 12 is preferably configured to detect infrared radiation reflected from the corresponding positions 3, 3' in sequence at different wavelengths, thereby obtaining a reflection spectrum with reflection values ​​at three, different wavelengths in this example for each position 3, 3'.

[0089] For the sake of clarity, only two positions 3, 3' are shown here. However, it is preferred that the reflection spectra are recorded from a plurality of positions 3, 3' on the value document 1. Furthermore, for the sake of clarity, the positions 3, 3' or the corresponding areas irradiated with infrared radiation or thus detected reflected infrared radiation are shown relatively large. Basically, the area at the position 3, 3' can be smaller, if necessary much smaller, or also larger, depending on the minimum spatial resolution required for the geometrical dimensions of the security element to be checked.

[0090] Basically, the positions 3, 3' or the corresponding areas can have any arbitrary shape. In the case of a detection device 12 with one or more rectangular, in particular square detection elements, the positions 3, 3' preferably have a substantially rectangular or square shape. In the case of a detection device 12 with one or more circular detection elements, the positions 3, 3' preferably have a substantially elliptical or circular shape.

[0091] Preferably, a transport device 15 , which is shown only schematically in the example shown, is provided and which is provided to transport the value document 1 to be checked in a transport direction T relative to the illumination device 11 and the detection device 12 .

[0092] In this case, infrared radiation reflected from a plurality of positions 3 , 3 ′ is preferably detected successively along one or more measuring tracks M extending parallel to the transport direction T. For the sake of clarity, the positions 3 , 3 ′ are shown only along a single measuring track M in the present example.

[0093] Basically, however, the irradiation device 11 and / or the detection device 12 can also be designed such that the reflection spectra can be detected from two, preferably six or more positions respectively from two, preferably six or more measurement tracks M extending parallel to one another.

[0094] Alternatively or additionally, it is preferred that the irradiation device 11 is configured to irradiate a linear region of the value document 1 extending substantially perpendicularly to the transport direction T with infrared radiation, and that the detection device 12 has a so-called sensor row having a plurality of detection elements arranged perpendicularly to the transport direction T, by which the infrared radiation reflected by the irradiated linear region of the value document 1 can be detected in a spatially resolved manner. In this embodiment, preferably only a few different infrared wavelengths, in particular two different infrared wavelengths or three different infrared wavelengths, are used alternately. By sequentially detecting the infrared radiation respectively reflected there during the transport of the value document 1 in the transport direction T, a spectrally resolved reflection image of the entire value document 1 is obtained, whose pixels are formed by individual positions 3, 3'.

[0095] The infrared radiation generated by the irradiation device 11 is incident on the respective position 3 (relative to the vertical on the value document 1) at an average angle of incidence α, and the infrared radiation reflected from the position 3 is detected by the detection device 12 at a detection angle β, which preferably differs from the average angle of incidence α. It is also preferred that all checked positions 3, 3' on the value document 1 are measured for all wavelengths only under a single angle condition, i.e. a single angle of incidence α and a single detection angle β.

[0096] The sensor 10 also has a checking device 13, which is configured to determine, based on the reflection spectra obtained for different positions 3, 3', whether the relevant position 3, 3' is located at or on the reflective optically variable (OV) security element 2. This is done, for example, by processing and / or analyzing the reflection spectra obtained respectively and / or by comparison with one or more predetermined spectra. This is explained in more detail below based on a preferred evaluation method.

[0097] Depending on the positions 3, 3' determined or classified in this way (for example "not on the OV security element" or "on the OV security element"), the valuable document 1 is then checked, wherein in particular the number and / or the condition of the positions 3' determined or classified as "on the OV security element" are used.

[0098] It is preferably checked here whether the number of positions 3' determined on the value document or classified as "on an 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, individual security elements 2 on the value document 1 can be reliably authenticated.

[0099] Alternatively or additionally, it is possible, for example, to check whether the number of positions 3' determined or classified as "on an OV security element" within at least one respectively predetermined region of interest ROI is greater than or equal to a respectively predetermined second minimum number. In this way, not only the presence of the security element 2 on the value document 1 can be reliably proven, but also its correct position (in the ROI) and / or minimum size (second minimum number). In addition, a check can also be performed here for two or more different reflective, optically variable security elements on the value document 1.

[0100] Optionally, it can also be checked whether the number of positions 3' determined or classified as "on the OV security element" 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 almost no or no positions classified as "on the OV security element" outside the region of interest ROI.

[0101] If one or more of these check criteria are positive, the value document 1 is classified, for example, as “authentic” and / or “suitable.” Otherwise the value document 1 is classified as “fake” and / or “unsuitable.”

[0102] The sensor 10 is preferably used in a value-document processing system 100 which is shown only highly schematically in the present example and has one or more processing devices 101 to 103 which are provided for processing, in particular separating, transporting, checking, sorting, stacking and / or destroying the value documents 1 .

[0103] A preferred evaluation method for determining or classifying a position 3 ′ located on a security element 2 or for checking a value document 1 on the basis of a determined position 3 ′ is explained in more detail below by way of example with reference to the figures.

[0104] Evaluation Method 1

[0105] In the preferred method, firstly at (at least) three different wavelengths λ i Measure the reflection r(λ at the corresponding positions 3 and 3' i ). This is preferably selected so that a characteristic point in the spectrum is detected thereby, which is known to be characteristic in the reflection spectrum of the OV security element to be checked, for example it has a maximum or minimum here. For different OV hues, the wavelength to be evaluated is preferably adjusted.

[0106] If there is a minimum absorption at one or more wavelengths (e.g. r(λ 0 )≤Threshold S 0 ), and the difference quotient (r(λ 1 )-r(λ 2 )) / (r(λ 3 )-r(λ 4)) is within a predetermined interval, and (due to the numerical stability of the quotient) the denominator |r(λ 3 )-r(λ 4 )|≥threshold S 1 , the reflection spectra recorded in this way at the individual measurement locations 3 , 3 ′ are preferably classified as “on the OV security element”.

[0107] By 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 ordinary banknotes and IR-absorbing colors. 1 and λ 3 can overlap identically. 0 Can be related to wavelength λ 1 To 4 One of them coincides, or r(λ 0 ) can be expressed as selected from r(λ 1 ) to r(λ 4 ) is an average of a plurality of reflection values. In another preferred embodiment, the reflection spectrum can be checked more precisely by determining and checking not only one difference quotient but two or more difference quotients by adding further wavelengths.

[0108] In some spectral curves, this check can even be performed with a measuring point in the red visible (VIS) spectral range and only two wavelength measuring points in the IR spectral range, which is important in particular for low-cost so-called cash payment sensors.

[0109] according to Figures 2 to 4 It is shown how a reflective optically variable security element whose colour depends on the viewing angle can be separated from conventional colours by examining (r(1100 nm)–r(1520 nm)) / (r(1100 nm)–r(770 nm))∈[0.3,0.8].

[0110] Figure 2 A first example of a reflectance spectrum of an OV film strip is shown. By evaluating the representative reflectance values ​​(at the marked wavelength λ 1 =λ 3 , 2 , 4 By calculating the difference quotient from this, a characteristic IR signature of the film strip can be clearly identified or determined. For the spectral values ​​selected for evaluation, wavelengths at which the reflectance curve has a local minimum or maximum are preferably used.

[0111] Figure 3A first example of a graph is shown, which represents the numerator and denominator of the difference quotient of different measurement pixels 3' (smaller dots) of an OV film strip compared to different IR absorbing pigments (larger individual dots). 1 To 4 The reflection intensity r(λ 1 ) to r(λ 4 ). The IR signature of the OV film strip—characterized by the difference quotient corresponding to the origin angle in the representation described here—can be clearly distinguished from all IR-absorbing colors.

[0112] Figure 4 A second example of a graph is shown, which represents the numerator and denominator of the difference quotient of different measured pixels of different production batches of OV film strips. 1 To 4 The reflection intensity r(λ 1 ) to r(λ 4 ). The dispersion of the calculated values ​​caused by production fluctuations is preferably taken into account by selecting a suitable check interval, which is preferably determined by the lower comparison threshold S min and the upper comparison threshold S max Predetermined. By appropriately selecting this interval, spectral changes caused by production fluctuations can be easily captured, so that not only individual banknotes but also the OV security elements of banknotes of all production batches can be reliably identified. The influence of production fluctuations on the OV security element inspection is reliably eliminated or at least kept small in this way.

[0113] The corresponding difference quotient can be obtained from Figure 3 and Figure 4 The way to see this is to imagine forming the quotient y / x for the point (x, y). The corresponding difference quotient thus corresponds to the slope of the origin line through the relevant point. The two origin lines shown with slopes 0.3 and 0.8 represent the limits of the range allowed for the difference quotient [0.3, 0.8].

[0114] Evaluation Method 2

[0115] In the preferred method, it is assumed that the reflection spectrum to be examined, which is scanned at n wavelengths, is composed of the reflection values ​​(r 1 ,…,r n In a first step, the reflectance spectrum of the OV film strip to be inspected and the reference spectrum from the above example are normalized, for example by a linear transformation that maps the minimum value to 0 and the maximum value to 1, for example according to the formula r i norm =((r i –min) / (max–min)) i＝1…n , where "min" is the value of all r i The minimum value of "max" is the value of all ri The maximum value of , or through 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) / σ) i＝1…n , where μ is the sum of all r i The average value of all r i standard deviation of ).

[0116] Figure 5 Examples of reflectance spectra of different measurement positions of an OV film strip are shown before (left) and after (middle, right) normalization. As can be seen, the original reflectance spectrum (left) shows strong variations, which are greatly reduced by linear normalization (middle, right), allowing quantitative comparison with the normalized reference spectrum.

[0117] The distance of the two normalized spectra (measured reflection spectrum and reference spectrum) is then determined, for example by the p-norm of their difference, and a check is made as to whether this distance is less than a predetermined threshold value, which in the context of the present disclosure is also referred to as the "comparison distance". By appropriately selecting this threshold value, the spectral changes that occur are captured, making it possible to reliably identify OV security elements of value documents or banknotes of all production batches.

[0118] Evaluation Method 3

[0119] In the preferred method it is assumed that the measured or to be measured reflection spectrum of the OV security element can be approximated by representative reference reflection spectra A and B, for example for production fluctuations occurring in extreme cases and / or spectral variations due to different pigment orientations within the banknote.

[0120] Preferably, the reflection spectra to be checked, respectively obtained here, are approximated with the reference reflection spectra A and B by a first linear combination c1*A+c2*B, i.e. a fitted spectrum with fitting parameters c1 and c2 (the so-called Fit), and the degree of fit is checked, for example, using the p-norm of the difference between the measured reflection spectrum and the fitted spectrum, which is also referred to as a (fitted) "comparison spectrum" in the context of the present disclosure.

[0121] Optionally, by having an additional fitting parameter c 0 The linear combination of (offsets) c 0 +c 1 *A+c 2 *B is more accurate. It should preferably be taken into account here that by c 1 =c 2 = 0 can accurately fit any constant spectrum. To exclude this situation, it is preferred to only consider the following pixels or positions, whose fitting, for example, 1 |+|c 2| is above a predetermined threshold or its corresponding reflection spectrum is sufficiently non-constant, so that, for example, the standard deviation or difference between the maximum and minimum values ​​is above a predetermined threshold.

[0122] Figure 6 An example of two reference reflection spectra is shown, whose linear combination is fitted to the measured reflection spectrum. Figure 7 An example of an adaptation (fit, dashed line) of a measured reflection spectrum (solid line) is shown by illustrating the above first linear combination of the reference reflection spectra (left) and the above second linear combination of the reference reflection spectra with a shift (right).

[0123] The method can be further improved if a preselection of the reflection spectrum to be examined is additionally performed by using only those pixels or positions in the evaluation whose reflection curve deviates sufficiently greatly from a constant curve in the IR range, for example where the difference between the maximum and minimum exceeds a predetermined threshold value. IR absorption features may be present at these points. Such a preselection can optionally be used in other evaluation methods.

[0124] In the example below, it is demonstrated that for a printed magnetically oriented interference pigment the value "100" is obtained by means of two reference spectra A and B (with the fitting parameter c 1 and c 2 and optionally c 0 ) The above-mentioned two-dimensional adaptation of the reflection spectrum to be examined in each case is carried out and compared with the case of adaptation via a single reference reflection spectrum (one-dimensional adaptation).

[0125] Figure 8 A comparison of the positions or measured pixels of the lettering "100" printed with directional interference pigments that were correctly identified by one-dimensional adaptation (upper figure) and two-dimensional adaptation (lower figure) is shown. In this reproduced pixel map, the following measurement positions are represented in black, where the distance between the adapted spectrum (fitted spectrum or comparison spectrum) and the measured reflection spectrum (e.g., the p-norm with p=2, i.e., the Euclidean distance) is less than a check threshold, which is also referred to as "relatively poor" in the context of the present disclosure.

[0126] To assess the plausibility, a check is made to see whether a minimum number of pixels or positions meets this criterion. Additionally or alternatively, a comparison can be made with the expected shape of "100" when the measurement has sufficient spatial resolution.

[0127] In the case of one-dimensional adaptation, only relatively few effective pixels are obtained compared to two-dimensional adaptation. Since the orientation of the interference pigments in the printed image varies systematically in height, the pixels adapted to the reference in one dimension are concentrated in a highly narrow region. This is preferably taken into account in sensors with only a few (or if necessary only one) measuring tracks, by adapting its conditions as accurately as possible to this narrow region in the case of evaluation by one-dimensional adaptation. In order to keep possible influences, for example due to so-called high / low travel variations (Hoch / Tieflauf-Variations) during the transport of the valuable documents to be checked respectively, as low as possible, evaluation by two-dimensional adaptation is preferred, since it is significantly more tolerant with respect to the position of the corresponding measuring track or the track height than evaluation by one-dimensional adaptation.

[0128] Evaluation Method 4

[0129] In the preferred method, the respective reflection spectrum to be examined is first normalized, for example by a linear transformation which maps the minimum value to 0 and the maximum value to 1, for example according to the formula r i norm =((r i –min) / (max–min)) i＝1…n (see above), or by a linear transformation such that (μ,σ)=(0,1), for example according to the formula r i norm =((r i –μ) / σ) i＝1…n , (see above).

[0130] A derivative curve of the normalized reflection spectrum (first derivative according to wavelength) is then formed and it is checked whether the derivative curve is within a predetermined tolerance range. The tolerance range is preferably determined or predetermined by calculating the derivative curve for a plurality of real (i.e. measured on real banknotes or reference patterns) normalized reflection spectra and determining the mean value and the standard deviation for each wavelength. The tolerance range corresponds to a range around the spectral curve of the determined mean value, which is bounded by the spectral curve of the determined standard deviation.

[0131] Fig. 9 An example of a normalized reflectance spectrum is shown. Fig.10 An example of a derivative curve calculated thereby (solid line) and a predetermined tolerance range (dashed line) is shown.

[0132] If it is to be checked not only whether the corresponding reflection spectrum is adapted, but also how well it is adapted, it is also possible, as an alternative, to calculate a difference curve between the corresponding derivative curve and a predetermined target derivative curve, weighted by the inverse width of the tolerance range in a channel manner (i.e. in the case of a single wavelength), and then reduce it to a certain value, for example by means of a p-norm. This value is then compared with an upper threshold value, which is also referred to as the “maximum difference” in the context of the present disclosure.

[0133] Evaluation Method 5

[0134] In this preferred method, first determine the wavelength (interpolation) of the local extreme value of the corresponding reflection spectrum obtained. In addition, predetermine or predetermine intervals for minimum and maximum values, which are also referred to as "wavelength intervals" in the context of the present disclosure. Then check whether there is just a local minimum and / or maximum value in each of these intervals. Optionally, the selection of these interval pairs can be additionally defined or predetermined, and check whether the quotient of the wavelength of the relevant extreme value is within a predetermined tolerance range. Preferably, one to three minimum values ​​and one to three maximum values ​​define this wavelength interval. Preferably, a maximum and two minimum values ​​are located in the IR range. For example, define the wavelength interval for a maximum and a minimum value, a maximum and two minimum values, two maximum values ​​and a minimum value, two maximum values ​​and two minimum values, two maximum values ​​and three minimum values, three maximum values ​​and two minimum values ​​or three maximum values ​​and three minimum values.

[0135] Fig.11 An example of a reflection spectrum is shown, wherein a frequency distribution of corresponding spectral conditions of local minima “min” and / or maxima “max” of a plurality of measured reflection spectra is shown.

[0136] Usually, for different currencies and denominations, and possibly even at different positions within the banknote, a color-changing OV security element of different colors is provided. In order to keep the adaptation effort as low as possible, it is preferred to perform a simple, but slightly less accurate check on such a color-changing OV security element. For this purpose, a special case of the above evaluation can be used with less adaptation effort as follows: First, two wide intervals are selected for the minimum values, such as [600nm, 1000nm] and [1000nm, 1700nm], and then a check is made as to whether the quotient of the wavelength of the relevant minimum value is located, for example, in the interval [1.6, 1.75], or more precisely, in a narrower interval depending on the type. It is thus not checked whether a color-changing OV security element with a certain hue exists, but only whether such an OV security element with any hue exists. However, in this way, missing color-changing OV security elements and / or normal colors and IR absorber colors can be reliably identified, and the corresponding banknotes can be rejected or classified as counterfeit.

Claims

1. A sensor (10) for checking a valuable document (1) having at least one reflective security element (2) which has an appearance in the visible spectral range that depends on the viewing angle, the sensor comprising: - an irradiation device (11) arranged to irradiate one or more different locations (3, 3') on the value document (1) with infrared radiation; a detection device (12) arranged to detect infrared radiation reflected from a corresponding position (3, 3') on the value document (1), wherein the sensor is arranged to detect the reflected infrared radiation in a spectrally resolved manner in order to obtain a reflection spectrum at the corresponding measuring position; - an inspection device (13) for inspecting a valuable document with respect to a reflective security element (2) having an appearance that depends on the viewing angle, the inspection device being arranged to determine, based on the reflection spectrum obtained for the respective position (3, 3'), whether a reflective security element (2) having an appearance that depends on the viewing angle is located on the valuable document (1) at the respective position (3, 3'), so as to determine one or more positions at which such a reflective security element (2) is located on the valuable document (1), and to inspect the valuable document (1) based on the one or more determined positions (3').

2. The sensor (10) according to claim 1, wherein: The detection device (12) is configured and / or arranged to detect infrared radiation reflected from the corresponding position (3, 3') at a detection angle (β), and / or the irradiation device is configured and / or arranged to direct the infrared radiation onto the valuable document so that it reaches the valuable document (1) at an angle of incidence (α), wherein the detection angle (β) and the angle of incidence (α) are preferably not the same size.

3. The sensor (10) according to claim 1 or 2, wherein: The checking device (13) is designed to use only infrared radiation directed onto the value document at an exactly or approximately unique angle of incidence and detected at an exactly or approximately unique angle of detection in order to determine whether a reflective security element having an appearance that depends on the viewing angle is located at a corresponding position on the value document.

4. The sensor (10) according to one of the preceding claims, wherein: For spectrally resolved detection of the reflected infrared radiation, the illumination device (11) is arranged to illuminate the reflected infrared radiation with at least two, in particular at least three, different wavelengths (λ i ) and / or infrared radiation of wavelength ranges that do not overlap with one another successively irradiate corresponding positions (3, 3') on the value document (1), and the detection device (12) is arranged to successively detect infrared radiation emitted from the corresponding positions (3, 3') at at least two, in particular at least three different wavelengths (λ i ) and / or wavelength range respectively, so as to obtain the corresponding reflection spectrum.

5. The sensor (10) according to one of the preceding claims, wherein: The checking device (13) is configured to check the valuable document (1) based on the number and / or condition of one or more determined locations (3') at which it has been determined that a reflective security element (2) having an appearance dependent on the viewing angle is located on the valuable document (1).

6. The sensor (10) according to claim 5, wherein: The checking device (13) is arranged to check whether the number of determined positions (3') is greater than or equal to a predetermined first minimum number and / or whether the number of determined positions (3') located within a predetermined region of interest (ROI) on the valuable document (1) is greater than or equal to a predetermined second minimum number and / or whether the number of determined positions (3') located outside a predetermined region of interest (ROI) on the valuable document is less than or equal to a predetermined maximum number, in particular equal to zero.

7. The sensor (10) according to one of the preceding claims, wherein: The checking device (13) is configured to detect, in order to determine whether a reflective security element (2) having an appearance dependent on the viewing angle is located at a respective position (3, 3') on the value document (1), the respective position (3, 3') at at least two, in particular at least three different wavelengths (λ i ) is determined from the reflectance spectrum of i 、r(λ i )), thereby deriving at least one first characteristic value characterizing the corresponding reflection spectrum, and comparing the first characteristic value with at least one predetermined first comparison characteristic value.

8. The sensor (10) according to claim 7, wherein: The first characteristic value is a difference quotient formed from the reflection values ​​(r(λ1) to r(λ4)) determined in the reflection spectra at three or four different wavelengths (λ1 to λ4), ((r(λ1)−r(λ2)) / (r(λ3)−r(λ4))).

9. The sensor (10) according to claim 7 or 8, wherein: The checking device (13) is configured to measure at least one determined reflection value (r(λ i ), r(λ0)) and / or compare at least one second characteristic value (r(λ0), |r(λ3)-r(λ4)|) derived from at least a portion of the determined reflection value with at least one predetermined second comparison characteristic value (S0, S1).

10. The sensor (10) according to one of the preceding claims, wherein: The checking device (13) is configured to determine whether a reflective security element (2) having an appearance dependent on the viewing angle is located on the valuable document (1) at a corresponding position (3, 3'), by determining a distance between a reflection spectrum obtained for the corresponding position (3, 3'), in particular normalized, and at least one predetermined, in particular normalized reference spectrum, and to check whether the determined distance is smaller than a predetermined comparison distance.

11. The sensor (10) according to one of the preceding claims, wherein: The checking device (13) is arranged to determine whether a reflective security element (2) having an appearance depending on the viewing angle is located on the valuable document (1) at a corresponding position (3, 3') by determining a difference between a reflection spectrum obtained for the corresponding position (3, 3') and a comparison spectrum (fit) adapted to the reflection spectrum by means of an adjustment calculation and to check whether the determined difference is smaller than a predetermined comparison difference, wherein the comparison spectrum is preferably given by a linear combination of at least two predetermined reference spectra.

12. The sensor (10) according to one of the preceding claims, wherein: The checking device (13) is configured to take into account, when checking the value document, only those positions (3') for which a reflection spectrum has been determined, which satisfies at least one of the following conditions: - the curve of the reflection spectrum is not constant and / or has a predetermined minimum difference compared to a constant curve, and / or - The difference between the maximum and the minimum of the reflection spectrum is greater than a predetermined minimum difference.

13. The sensor (10) according to one of the preceding claims, wherein: The checking device (13) is configured to determine whether a reflective security element (2) having an appearance dependent on the viewing angle is located on the valuable document (1) at a corresponding position (3, 3') by forming a derivative curve of a reflection spectrum obtained for the corresponding position (3, 3'), in particular a normalized derivative curve, and to check whether the derivative curve is within a predetermined tolerance range and / or has a difference with a predetermined target derivative curve which is smaller than a predetermined maximum difference.

14. The sensor (10) according to one of the preceding claims, wherein: The checking device (13) is configured to determine, in order to determine whether a reflective security element (2) having an appearance dependent on the viewing angle is located on the valuable document (1) at the respective position (3, 3'), the wavelengths of two or more local extreme values, in particular the wavelengths of at least one local maximum ("max") and at least one local minimum ("min"), in the reflection spectrum obtained for the respective position (3, 3'), and to check whether the determined wavelengths are located within a wavelength interval predetermined for the respective extreme value and / or whether only one local extreme value is located within the wavelength interval predetermined for the respective extreme value.

15. A method for checking a valuable document (1) having at least one reflective security element (2) which has an appearance in the visible spectral range that depends on the viewing angle, the method comprising the following steps: - irradiating one or more different locations (3, 3') on the value document (1) with infrared radiation and detecting spectrally resolved infrared radiation reflected from the corresponding locations (3, 3') on the value document (1), wherein a reflection spectrum is respectively obtained; - determining, based on the reflection spectra obtained for the respective positions (3, 3'), whether a reflective security element (2) having an appearance dependent on the viewing angle is located on the value document (1) at the respective position (3, 3'), in order to determine one or more positions on the value document (1) at which such a reflective security element (2) is located; and - checking the value document (1) as a function of the determined position or positions (3').

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