OPTICAL SENSOR FOR VERIFICATION OF VALUABLE DOCUMENTS

Description

[0001] The invention relates to an optical sensor for checking valuable documents.

[0002] It is known from the prior art to illuminate valuable documents and to check their optical reflectance. Alternatively, the valuable documents can also be optically stimulated with the illuminating light and their luminescence measurements checked. The term luminescence is understood as a generic term for the radiation emitted back from the valuable document after optical stimulation, e.g. fluorescence or phosphorescence. The light emitted by the valuable document as a result of the illumination is detected by a photodetector and the detected measurements are then evaluated. To distinguish genuine valuable documents from counterfeits, for example, a check is carried out to determine whether the reflectance measurements or the luminescence measurements of a specific section of an examined valuable document exceed or fall below certain threshold values.

[0003] An optical measurement of valuable documents is discussed in DE 102 43 051 A1.

[0004] Furthermore, it is known that the light intensity emitted by a light source usually decreases over the course of its operating life. Therefore, the emitted light intensity is monitored using a monitor diode arranged near the light source. In order to compensate for the decreasing intensity of the light source, the light source current is often adjusted. Alternatively, it is possible to take the decreasing intensity of the light source into account when evaluating the recorded measured values ​​of the value document and to subsequently calculate it out. It is assumed that the recorded measured values ​​decrease proportionally to the decreasing light intensity of the light source. However, subsequent calculation is difficult when several adjacent light sources are used which are switched on simultaneously to illuminate the value document and whose emitted light may even spatially overlap on the value document.This is because the measured values ​​that the respective photodetector detects from a value document depend on the light intensity of several light sources, whose emitted light intensity can develop differently over the course of their operating life.

[0005] It is therefore an object of the invention to provide a solution for the subsequent correction of the value document measurement values ​​detected by the photodetectors in the event that the emitted light intensity of several light sources develops differently over the course of the operating life of the light sources.

[0006] This object is achieved by the features of the independent claims. The dependent claims specify advantageous embodiments and further developments of the invention.

[0007] To check a value document, it is placed into a detection area of ​​an optical sensor that is designed to check value documents. The optical sensor comprises a photodetector row that has a number K of multiple detector elements arranged next to one another, each designed to detect the intensity of the light emanating from a detection area of ​​the value document. In addition, the optical sensor comprises a light source row that has a number N of multiple light sources arranged next to one another, each designed to illuminate the value document. In addition, the optical sensor has a monitor detector row that has a plurality of photosensitive monitor elements (e.g. designed as photodetectors) arranged next to one another, e.g. a number N of monitor elements. However, more than N monitor elements could also be used.Each of the monitor elements is assigned to one of the light sources, preferably exactly one of the light sources, and is designed to detect the intensity of the light emitted by this light source and incident on the respective monitor element. For example, in the optical sensor, there is a 1:1 mapping between the light sources and the monitor elements, so that exactly one monitor element is present for each light source.

[0008] At a time before the valuable documents are checked, a self-test of the optical sensor is carried out, for example, by a control device of the optical sensor. During the self-test, the N light sources are switched on and, with the help of the N monitor elements, the respective light intensity MS j (j = 1 ... N) of the light source assigned to the respective monitor element that hits the respective monitor element at the time of the self-test is detected. During the self-test, the light intensities MS j (j = 1 ... N) detected by the monitor elements at the time of the self-test can be checked. During the check, it is determined, for example, whether the light intensity MS j (j = 1 ... N) detected by the respective monitor element is sufficient to check the valuable document, e.g. by comparing it with a threshold, and an error message is issued if a monitor element has detected too low an intensity during the self-test.

[0009] For the optical inspection of a valuable document, the valuable document to be inspected is placed within the detection range of the optical sensor so that the valuable document can be illuminated by the light from the light sources, and the light emitted by the valuable document can be detected by the detector elements. The light emitted by the valuable document can be remitted or transmitted illumination light, but it can also be luminescent light emitted by the valuable document as a result of illumination suitable for optical excitation, which the light sources may emit.

[0010] In particular, the valuable document is transported past the optical sensor along a transport direction, and in each case, a detection area of ​​the valuable document is brought into the detection area. The photodetector array is arranged transversely, e.g., perpendicularly or diagonally, to the transport direction of the valuable document, in which the respective valuable document is transported past the optical sensor for inspection. Detection areas of the valuable document arranged adjacent to the transport direction are detected sequentially. The successively detected detection areas of the valuable document can each correspond to a pixel of an image captured by the photodetector array.

[0011] In order to illuminate the respective valuable document placed in the detection area with the light from the light sources, the N light sources of the optical sensor are switched on simultaneously. The N light sources can be all or only a subset of the light sources contained in the light source row (light sources at the edge of the row may not be used). The detector elements record measured values ​​of the respective valuable document that correspond to the light intensity emanating from the respective valuable document as a result of the illumination. The K detector elements of the photodetector row each record at least one measured value D i (i=1...K) of the valuable document. The K detector elements can be all or only a subset of the detector elements contained in the photodetector row (elements at the edge of the row may not be used).For example, the respective detector element detects only one measured value Di of a specific value document area or multiple measured values ​​D i , for example, during scanning of the value document along the transport direction. The switching on of the light sources and the recording of the measured values ​​of the value document are initiated, for example, by the control unit of the optical sensor, which controls the light sources or detector elements accordingly. The respective measured value can be detected simultaneously with the illumination (in the case of a remission measurement or transmission measurement) or after the end of an illumination pulse of the light sources (in the case of a luminescence measurement). The recorded measured values ​​D i (i=1...K) are compared using the light intensities MS j (j=1...N) in order to take into account, in particular to subsequently calculate out, any change in the light intensity emitted by the light sources that occurs over the course of their operating life. During the correction, the measured values ​​D i of the respective detector element are each corrected, in particular multiplied, by a correction factor FK i that is calculated individually for the respective detector element, specifically on the basis of the light intensities MS j detected by several of the monitor elements at the time of the self-test, e.g. on the basis of the light intensities MS j detected by at least two of the monitor elements at the time of the self-test. The respective value document is checked on the basis of the measured values ​​D i * corrected using the respective correction factor FK i.

[0012] The calculation of the corrected measured values ​​D i * (i=1...K) and the verification of the value document are performed by an evaluation device. The evaluation device is connected to the photodetector array or detector elements to receive the detected measured values ​​D i (and, if applicable, measured values ​​DA i of a comparison medium, see below) of the value document. The evaluation device can output the result of the value document verification to an operator and / or to a value document processing device in which the optical sensor is installed.

[0013] The correction of the measured values ​​D i of the valuable document using the light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test is carried out in order to correct any change in the light intensity emitted by the light sources that occurs over the course of the operating life of the light sources or to subsequently calculate this change from the detected measured values. By using multiple monitor elements that are assigned to different light sources, it is achieved that the emitted light intensity of each light source is individually recorded during the self-test. By correcting the measured values ​​of the valuable document using the light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test, it can therefore also be taken into account if the emitted light intensity of the various light sources develops differently.

[0014] For example, during the self-test, a correction table is created in which the correction factors FK i calculated individually for the detector elements of the photodetector array are entered. The correction factor FK i can be calculated for each detector element of the photodetector array or only for those detector elements that actually detect the light emanating from the value document being tested. In the latter case, for example, the correction factors are not calculated for those detector elements located at the edge of the photodetector array.

[0015] In particular, the correction factor FK i is calculated individually for the respective i-th detector element based on the light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test, in each case taking into account a proportion factor A ij , which indicates which proportion of the light intensity emitted by the respective j-th light source (j=1...N) hits the respective i-th detector element (i=1...K) due to the optical beam path between the light sources and the detector elements. The respective light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test are each limited by the proportion factor A ij (j=1...N) and included in the respective i-th correction factor FK i (i=1...K).To calculate the respective correction factor FK i for several or all of the N light sources, the respective component factors A ij are multiplied by the respective light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test.

[0016] For example, the component factors A ij can be determined empirically prior to the valuable document verification. Specifically, a reference surface (e.g., a calibration medium with homogeneous optical properties) is placed in place of the valuable document within the detection range of the detector elements. The light sources are switched on one after the other to illuminate the reference surface, and the light intensity emanating from the reference surface and striking the detector elements is detected.

[0017] Alternatively, the proportion factors A ij can be calculated before the valuable document verification by means of a numerical simulation based on a model of the optical beam path from the N light sources to the K detector elements. The numerical simulation is carried out before the valuable document verification, e.g. by the sensor manufacturer. The result of this numerical simulation is then used to calculate the correction factors FK i for the measured values ​​D i that the detector elements have recorded from the respective valuable document. During the numerical simulation of the optical beam path from the N light sources to the K detector elements, for example, a transfer matrix A is calculated whose matrix elements correspond to the proportion factors A ij , (i=1...K, j=1...N). The transfer matrix A, in particular the matrix elements of the transfer matrix A corresponding to the proportion factors A ij (i=1...K, j=1...N), are then used to correct the measured values ​​Di.

[0018] When calculating the respective correction factor FK i of the respective i-th detector element (i=1...K), the component factors A ij are multiplied for several or all of the N light sources by the light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test in order to calculate the correction factor FK i for each of the K (i=1...K) detector elements, which is used to correct the measured value D i of the respective i-th detector element.

[0019] Preferably, an optical sensor calibration procedure is performed before the optical sensor's self-test to check the individual detector elements of the optical sensor for any differences in sensitivity. The calibration procedure is performed at an earlier point in time, prior to the self-test, e.g., before the optical sensor is delivered by the manufacturer. Additionally or alternatively, it can also be performed after the optical sensor has been delivered to a customer, e.g., by the manufacturer's service personnel. During the calibration procedure, a calibration medium with homogeneous optical properties is brought into the detection range of the optical sensor, and the light sources are switched on simultaneously to illuminate the calibration medium. During the calibration procedure, the monitor elements of the optical sensor each detect a light intensity MA j (j=1...N), which corresponds to the light intensity emitted by the respective light source during the calibration procedure. During the calibration procedure, the detector elements of the optical sensor each detect a light intensity DA i (i=1...K) emanating from the calibration medium. In particular, the control device of the optical sensor can be configured to initiate the detections required during the calibration procedure, e.g., the detection of the light intensities MA j (j=1...N) by the monitor elements and / or the detection of the light intensity DA i (i=1...K) by the detector elements.

[0020] To calculate the respective correction factor FK i of the respective i-th detector element (i=1...K), the light intensities MA j (j=1...N) of the light sources detected by the monitor elements during the calibration procedure are preferably also used. The light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test are then related to these light intensities MA j (j=1...N) in order to quantitatively determine the change in the light intensity of the light sources since the calibration procedure.

[0021] To correct the measured values ​​D i of the value document, the respective measured value D i is multiplied by the respective correction factor FK i (i=1...K). In a first embodiment, to calculate the respective correction factor FK i for several or all of the N light sources, the respective component factors A ij are multiplied by the respective light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test, and the results of these multiplications are summed to calculate the respective correction factor FK i.

[0022] In a second embodiment, to calculate the respective correction factor FK i of the respective i-th detector element (i=1...K), the ratio between the light intensity MS j (j=1...N) detected by the monitor elements at the time of the self-test and the light intensity MA j (j=1...N) detected by the monitor elements at an earlier time, in particular at the time of the calibration procedure, is formed for several or all of the N light sources. The respective ratio MS j / MA j is multiplied by the respective component factor A ij (j=1...N), and the results of these multiplications are summed to calculate the respective correction factor FK i.

[0023] The light intensities DA i detected by the detector elements during the calibration procedure from the calibration medium can also be used, if necessary, to correct the measured values ​​D i (i=1...K) detected from the value document, wherein the measured values ​​D i of the respective detector element recorded from the value document are multiplied by a further correction factor F i which corresponds to the inverse of the light intensity DA i detected by the respective detector element during the calibration procedure of the optical sensor from the calibration medium.

[0024] In addition to the aging of the light sources over the course of their operating life, the temperature dependence of the monitor elements (e.g., temperature-dependent sensitivity) can also be corrected when correcting the measured values ​​of the value document. For example, using a temperature sensor built into the optical sensor, the temperature of the monitor elements can be measured at the time of the self-test and, if necessary, also at the time of the calibration procedure. To correct the temperature dependence of the monitor elements, the light intensities MS detected by the monitor elements at the time of the self-test can be multiplied by a temperature-dependent factor determined based on the temperature of the monitor elements at the time of the self-test.Analogously, the light intensities MA detected by the monitor elements at the time of the calibration procedure can be multiplied by a temperature-dependent factor that is determined based on the temperature of the monitor elements at the time of the calibration procedure.

[0025] The valuable documents that are checked using the method according to the invention are, for example, banknotes, tickets, vouchers, etc.

[0026] The invention is described below by way of example with reference to the accompanying drawings. They show: Fig. 1 Structure of an optical sensor for checking value documents, Fig. 2 Schematic sketch of the distribution of the illumination light emitted by the light sources in the value document plane, Fig. 3 Grayscale representation of the proportion factors A ij determined with the transfer matrix, Fig. 4 Light intensities detected by the monitor elements at the time of the adjustment procedure (MA j ) and at the time of the self-test (MS j ), Fig. 5 Correction factors FK i calculated for the i=1...K detector elements, Fig. 6 Measured values ​​D i of a value document recorded by the i=1...K detector elements and corrected measured values ​​D i *= FK i ·D i .

[0027] In Figur 1 The structure of an optical sensor 100 configured for valuable document verification is shown, which can be installed, for example, in a valuable document processing device. To illuminate a valuable document 10 introduced into the detection area of ​​the optical sensor 100, a light source row is used, which has a plurality of light sources 1 arranged next to one another along the y-direction on a light source holder 11, for example N=12 light sources. The light sources 1 are, for example, UV LEDs, which are suitable for exciting the luminescence of the valuable document. The light emitted by the light sources 1 is directed via a mirror 2 onto photosensitive monitor elements 3 (e.g., photodiodes), by means of which the light intensity emitted by the light sources 1 can be checked, which light intensity typically decreases over the course of the light sources' operating life. For each of the N light sources 1, exactly one monitor element 3 is present (e.g., N=12 monitor elements).Apertures 15 ensure that each monitor element 3 only detects the light from exactly one of the light sources 1: the monitor element 3 1 detects the light from the light source 1 1 , the monitor element 3 2 detects the light from the light source 1 2 , etc. The monitor elements 3 are attached to a monitor holder 13.

[0028] As an alternative to reflection by a mirror 2, the monitor elements 3 could also be arranged in the radiation area of ​​the respective light source 1 so that they directly detect a portion of the light emitted by the light sources. Alternatively, a partially transparent mirror could be arranged between the light sources and the valuable document, transmitting most of the illuminating light and reflecting a portion of the illuminating light onto the monitor elements. Alternatively, the monitor elements could also detect the illuminating light from the light sources that is scattered back by a reference surface.

[0029] The light emitted by the light sources 1 is directed partly directly and partly via an elliptical mirror 6 onto the valuable document 10 to be checked. It is transmitted through the measuring window 8 of the optical sensor 100 and through a spectral filter 7, which blocks the visually visible portion of the illumination light. Additionally, another such spectral filter could be provided in the beam path in front of the monitor elements 3 in order to block the visually visible portion of the illumination light. Alternatively, the spectral filter 7 could also be arranged directly after the light sources in order to spectrally filter both the light striking the valuable document and the light striking the monitor elements 3.

[0030] The luminescent light emanating from the valuable document 10 is imaged by a row of Selfoc lenses 5 provided with a UV-blocking filter onto a plurality of detector elements 4 of a photodetector row, which are arranged next to one another along the y-direction on a detector element holder 14 and have, for example, K=112 detector elements. Only a single photodetector row can be used onto which the luminescent light is imaged. With the aid of additional lenses, the luminescent light can also be imaged onto a plurality of photodetector rows offset from one another along the x-direction, which capture different spectral ranges of the luminescent light and have corresponding spectral filters. Alternatively, a plurality of photodetector rows of a two-dimensional image sensor can be used, onto which the light emanating from the valuable document can be imaged.

[0031] For its inspection, the valuable document 10 can be statically placed within the detection range of the optical sensor 100. Preferably, however, the valuable document is transported past the optical sensor along the x-direction in order to scan the various valuable document sections sequentially with the optical sensor. The transport is achieved, for example, by means of appropriate transport means, e.g., transport belts and / or transport rollers, which are used in a valuable document processing device to transport the valuable documents.

[0032] A control device 30 of the optical sensor 100 is connected to the light sources 1, the monitor elements 3, and the detector elements 4 and has corresponding hardware and software that ensures that the light sources 1 are switched on simultaneously and that the detector elements 4, simultaneously and / or after illumination, are triggered to capture luminescence measurement values ​​D i of the value document 10. The measurement values ​​D i (i=1...K) of the value document captured by the detector elements 4 are transmitted to an evaluation device 20 connected to the detector elements.

[0033] The control device 30 is configured to perform a self-test of the optical sensor. The self-test can be performed at a time immediately before the verification of the value documents, e.g., when starting the value-document processing device or in the gap between two value documents to be verified that are transported past the sensor. During the self-test, the control device causes the N light sources 1 to be switched on simultaneously, and the N monitor elements 3 to detect the respective light intensity MS j (j=1...N) of the light source 1 assigned to the respective monitor element. For example, during the self-test, the light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test are checked to determine whether they exceed a specific threshold value necessary to illuminate the value document, e.g., to optically excite a measurable luminescence.The control device 30 is also connected to the evaluation device 20 in order to transmit the light intensities MS j (j=1...N) detected by the monitor elements at the time of the self-test to the evaluation device 20, as well as, if applicable, the light intensities MA j (j=1...N) of a calibration medium detected by the monitor elements during the calibration procedure.

[0034] The evaluation device 20 corrects the measured values ​​D i of the respective value document with the aid of correction factors FK i , which include the light intensities MS j (j=1...N) detected by the monitor elements 3 at the time of the self-test, in order to obtain corrected measured values D i * = FK i ⋅ D i i = 1 … K to calculate. The evaluation device 20 then checks the value document using the corrected measured values ​​D i *, e.g., its authenticity, condition, or the type of value document. To check the value document, the corrected measured values ​​D i * are compared, e.g., with at least one reference value that is expected for the respective value document or the respective section of the value document. The corrected measured values ​​can also be summarized, e.g., averaged, over a section (region of interest, ROI) of the value document before the comparison with the respective reference value is carried out.

[0035] When manufacturing the optical sensor, N light sources are preferably selected that are as similar as possible and barely differ in their emitted light intensity. The photodetector array is preferably selected so that the sensitivities of the individual detector elements barely differ. In this case, a correction of the measured values ​​D i using only the factors FK i according to formula (1) is preferred.

[0036] Otherwise, if it is unavoidable during the manufacture of the optical sensor that the individual light sources or the individual detector elements are different, the sensor manufacturer preferably carries out a calibration procedure for the optical sensor before delivery of the optical sensor, during which the possibly different emission intensities of the light sources and the possibly different sensitivities of the detector elements are tested and quantified. During the calibration procedure, a calibration medium with homogeneous optical properties, e.g., a homogeneous white surface, is brought into the detection range of the optical sensor 100. The N light sources 1 are then switched on simultaneously to illuminate the calibration medium, and the reflected light intensity DA i (i=1...K) emanating from the calibration medium is detected by the detector elements 4 of the optical sensor.

[0037] If the adjustment procedure reveals that the individual light sources 1 or the individual detector elements 4 are indeed significantly different, the measured values ​​D i of the value documents are subjected to a further correction using a further correction factor F i , which is also multiplied by the measured values ​​D i : D i * = F i ⋅ FK i ⋅ D i

[0038] The further correction factor F i results from the measured values ​​of the detector elements DA i recorded by the calibration medium during the calibration procedure according to the formula F i = c / DA i where c is assumed to be a fixed numerical value.

[0039] During the calibration procedure, the light intensities MA j (j=1...N) emitted by the N light sources 1 and incident on the N monitor elements 3 are also detected with the help of the monitor elements 3. These are used to determine the correction factor FK i as reference values ​​for the light intensities MS j (j=1...N) emitted by the N light sources 1 and incident on the N monitor elements 3 during the self-test.

[0040] If necessary, the calibration procedure can be repeated from time to time even after delivery of the optical sensor (e.g., monthly or every six months) in order to record the change in the light intensity of the light sources over the course of its operation. However, since the calibration procedure is complex (requiring the use of service personnel), it is advantageous to use a self-test of the optical sensor as an alternative to frequently repeating the calibration procedure in order to record the change in the light intensity of the light sources over the course of its operation. During the self-tests of the optical sensor, the light intensities MS j (j=1...N) of the light sources 1 striking the monitor elements 3 are detected regularly (e.g., before each start-up of the value document processing device).Since the self-test is carried out shortly after the security document verification, the light intensity of the respective light source determined during the respective self-test corresponds almost exactly to the light intensity of the respective light source 1 present during the security document verification.

[0041] The following describes the determination of the correction factors FK i for the K detector elements (i=1...K) using the light intensities MS j (j=1...N) detected by the monitor elements 3 at the time of the self-test. The correction factors can be determined during the self-test, e.g., immediately before the verification of the valuable documents.

[0042] To determine the correction factors FK i for the K detector elements (i=1...K), the fraction of the light intensity emitted by the respective jth light source (j=1...N) that hits the respective i-th detector element (i=1...K) is determined individually for each detector element 4, taking into account the optical beam path between the light sources and the detector elements. Each of these fractions is represented by a fraction factor A ij.

[0043] The proportion factor A ij , j=1...N includes, for example, the position of the respective j-th light source along the light source row and the position of the respective i-th detector element along the photodetector row or their relative position or their distance along the row direction (y-direction), as well as the radiation angles of the light sources (approximately the same for all light sources), the reception angles of the detector elements (approximately the same for all detector elements) and the distance of the light sources 1 and the detector elements 4 from the valuable document plane of a valuable document introduced into the detection range of the optical sensor.

[0044] Fig. 2 shows an example of the distribution of the illuminating light emitted by the light sources 1 1 , 1 2 , ..., 1 N. In the value document plane, the detection areas E 1 , E 2 , ..., EK of the individual detector elements (i=1...K) are schematically shown, which result from the optical imaging by means of the row of Selfoc lenses 5. It can be seen that the light contribution that a light source arranged at the left edge of the light source row (e.g., light source 1 1 ) radiates onto the detection areas (e.g., E 1 , E 2 , E 3 ) of the detector elements 4 located at the left edge of the photodetector row is much greater than their light contribution to the detection area (e.g., EK ) of a detector element 4 located at the right edge of the photodetector row.The contribution factors A 11 , A 21 , A 31 of the light source 1 1 to the detector elements located at the left edge of the photodetector row are correspondingly large compared to their contribution factor A K1 to the detector element located at the right edge of the photodetector row.

[0045] The contribution factors A ij can be calculated quantitatively by means of a numerical simulation based on a model of the optical beam path in which the optical beam path from the N light sources of the optical sensor to the K detector elements of the optical sensor is modeled.

[0046] For the optical simulation, in the case of the optical sensor 100, it is assumed, for example, that the light sources have Lambertian radiation, the distance h of the light sources 1 from the value document plane is 30 mm, the light sources 1 are spaced 10.5 mm apart, and the detector elements 4 are spaced 1 mm apart (in the y-direction). In the numerical simulation, for example, a transfer matrix A is calculated whose matrix elements are the component factors A ij , (i=1...K, j=1...N).

[0047] In Fig. 3 The contribution factors A ij , which were calculated by the numerical simulation of the optical beam path of the optical sensor 100, are shown in the form of grayscale values. The contribution factors A ij are shown for the N=12 (j=1...12) light sources from left to right and for the K=112 detector elements (i=1...112) from top to bottom. As described above, Fig. 2 As expected, the largest contribution factors A ij are found in the "diagonal" of the Fig. 3 . As an alternative to the numerical simulation of the optical beam path, the component factors A ij can also be determined by measuring the individual light intensities emitted by the light sources and incident on the respective detector element.

[0048] The illumination intensity B i , which is relevant for the respective i-th detector element (i=1...K), is generally obtained by summing the light intensities L j (j=1...N) emitted by the light sources, each weighted by the component factor A ij , to B i = ∑ j = 1 N A ij ⋅ L j and in matrix notation B 1 B 2 … B K = A 1 , 1 ⋯ A 1 , N ⋮ ⋱ ⋮ A K , 1 ⋯ A K , N ⋅ L 1 L 2 … L N .

[0049] Assuming that the monitor elements each detect only a certain, fixed proportion 1 / β of the light intensity emitted by the light sources, the light intensity emitted by the light sources (L jj=1...N) can be calculated from the light intensities MS j detected by the monitor elements during the self-test by L j = β · MS j . At the time of the adjustment procedure, L j = β · MA j assumed.

[0050] For the time of the self-test, the relevant illumination intensity BS i for the respective i-th detector element (i=1...K) is obtained by summing the light intensities MS j (j=1...N) detected by the monitor elements during the self-test, each weighted by the proportion factor A ij , to BS i = ∑ j = 1 N A ij ⋅ β ⋅ MS j and for the timing of the adjustment procedure, the following results BA i = ∑ j = 1 N A ij ⋅ β ⋅ MA j .

[0051] In a first embodiment, the correction factors FK i of the individual detector elements (i=1...K) are calculated from the two illumination intensities BS i and BA i according to the following formula: FK i = BA i BS i = ∑ j = 1 N A ij ⋅ MA j ∑ j = 1 N A ij ⋅ MS j , wobei i = 1 … K , j = 1 … N .

[0052] The respective correction factor FK i is therefore calculated from the quotient of the illumination intensity present during the adjustment procedure BA i = ∑ j = 1 N A ij ⋅ MA j and the lighting intensity present during the self-test BS i = ∑ j = 1 N A ij ⋅ MS j . The calculation of the correction factors FK i according to formula (8) is an approximation in which it is assumed for the sake of simplicity that all monitor elements have the same sensitivity and that the optical image on each of the monitor elements is the same.

[0053] In a second embodiment, the correction factors FK i of the individual detector elements (i=1...K) are calculated according to the following formula: FK i = ∑ j = 1 N A ij ∑ j = 1 N A ij ⋅ MS j MA j , wobei i = 1 … K , j = 1 … N .

[0054] The calculation of the correction factors FK i according to formula (9) is also an approximation, assuming that the light sources 1 emit approximately the same light intensity at the time of the adjustment procedure.

[0055] The correction factors FK i can either be calculated by the control device during the self-test and forwarded by it to the evaluation device 20. However, the correction factors FK i can also be calculated by the evaluation device, which receives the respective light intensities MS j (j=1...N) detected during the self-test from the control device. After the correction factors FK i have been calculated according to formulas (8) and (9) from the light intensities MS j and MA j (j=1...N) detected during the calibration procedure and the self-test using the Fig. 3 shown proportion factors A ij have been determined, the evaluation device 20 can carry out a correction of the measured values ​​D i (i=1...K) of a value document according to formula (1).

[0056] As an example, Fig. 4 The illumination intensities MS j and MA j (j=1...N) detected by the N=12 monitor elements 3 during the calibration procedure and the illumination intensities MS j and MA j (j=1...N) detected during a self-test of the optical sensor for all N=12 light sources 1 are shown. During the self-test, a greatly reduced light intensity of the second light source (j=2) was detected, which is reduced by about half compared to the light intensity measured earlier during the calibration procedure. From these values ​​for MS j and MA j (j=1...N) the following results are obtained (using the values ​​in Fig. 3 shown proportion factors A ij ) from formula (8) or (9) which are shown in Fig. 5 shown correction factors FK i for all K=112 detector elements 4.

[0057] With the correction factors FK i from Fig. 5 the measured values ​​D i of the value document recorded by the individual detector elements 4 (cf. Fig. 6 ) according to formula (1), e.g. multiplied, in order to calculate out of the detected measured values ​​the light intensity which has changed over the course of the operating time, in particular the light intensity of the second light source which has been greatly reduced since the calibration procedure. Fig. 6 shows the corrected measured values ​​D i * = FK i ·D i in the example considered. For detector elements i=1 to approximately i=45, this results in a significant upward correction of the measured values ​​D i.

[0058] In addition to the aging of the light sources 1 over the course of their operating life, the temperature dependence of the monitor elements 3 can also be corrected if necessary. For this purpose, the temperature of the monitor elements at the time of the self-test TS is measured with the help of a temperature sensor 16 installed near the monitor elements 3 (e.g. on the monitor holder 13) and taken into account when correcting the measured values ​​D i. If the temperature of the monitor elements is always approximately the same at the time of an adjustment procedure carried out by the sensor manufacturer, it is not necessary to take into account the temperature measured at the time of the adjustment procedure. Otherwise, if an adjustment procedure is carried out under significantly changed temperature of the monitor elements, the temperature of the monitor elements TA measured during the adjustment procedure is preferably also stored in the optical sensor and taken into account when correcting the measured values ​​D i.

[0059] For example, a table can be stored in the evaluation device 20 of the optical sensor in which a correction factor t is assigned to different temperatures T of the monitor elements as a function of the temperature T. To correct the temperature dependence of the monitor elements 3, the light intensities MS j detected by the monitor elements at the time of the self-test are then replaced in equations (8) or (9) by MS j ' = t(TS ) ·MS j , where t(TS ) results from the temperature TS measured during the self-test according to the table mentioned above. It is assumed that the correction factors t(TS ) are approximately the same for all monitor elements 3. Analogously, the light intensities MA j detected by the monitor elements at the time of the calibration procedure can also be replaced by MA j ' = t(TA ) ·MA j , where t(TA ) results from the temperature TA measured during the calibration procedure according to the table mentioned above.Alternatively, the measured values ​​D i of the value document recorded by the individual detector elements 4 could simply be multiplied by an additional factor t(TA ) / t(TS ).

[0060] In addition, the temperature dependence of the detector elements can optionally be measured during the self-test using additional temperature sensors and - if the detector elements have a sufficiently large temperature dependence - any temperature changes since the calibration procedure can be taken into account when correcting the measured values ​​D i.

Claims

1. Method for checking a valuable document (10) brought into an acquisition range of an optical sensor (100), wherein the optical sensor (100) comprises: - a photodetector line comprising a number K of multiple adjacent detector elements (4) which are designed to detect in each case the intensity of the light emanating from a detection region of the valuable document (10), - a light source line comprising a number N of multiple adjacent light sources (1) which are designed to illuminate the valuable document (10), - a monitor detector line comprising multiple adjacent photosensitive monitor elements (3), with each monitor element being assigned to one of the light sources (1) and being designed to detect the intensity of the light emitted by this light source and incident on the respective monitor element, - an evaluation device (20), wherein the following steps are performed in the method: - performing a self-test of the optical sensor (100) at a time prior to the check of the valuable document (10), wherein the N light sources (1) are switched on during the self-test and the respective light intensity MSj of the light source (1) assigned to the respective monitor element and incident on the respective monitor element at the time of the self-test is detected by means of the monitor elements (3) in order to check the light intensities MSj detected by the monitor elements at the time of the self-test, where j=1...N, - introducing a valuable document (10) into the acquisition range of the optical sensor (100), - simultaneously switching on the N light sources (1) in order to illuminate the valuable document (10) introduced into the acquisition range with the light from the light sources, and recording measured values of the respective valuable document by means of the detector elements (4), wherein the measured values recorded correspond to the light intensity emanating from the respective valuable document as a consequence of the illumination, and wherein the K detector elements (4) of the photodetector line each record at least one measured value Di of the valuable document, where i=1...K, - using the evaluation device to correct the measured values Di on the basis of the light intensities MSj detected by the monitor elements (3) at the time of the self-test, wherein the at least one measured value Di of the respective detector element (4) is in each case corrected during the correction using a correction factor FKi that is calculated on an individual basis for the respective detector element (4) using the light intensities MSj detected at the time of the self-test by multiple of the monitor elements (3), and - checking the valuable document (10) by way of the evaluation device using the measured values (Di*) corrected with the aid of the respective correction factor FKi.

2. Method according to Claim 1, characterized in that the respective light intensities MSj detected by the monitor elements (3) at the time of the self-test are included by calculation in the respective correction factor FKi in each case limited by a proportion factor Aij that indicates the proportion of the light intensity emitted by the respective light source that on account of the optical beam path from the light sources to the detector elements is incident on the respective detector element.

3. Method according to Claim 2, characterized in that the proportion factors Aij are determined empirically before the valuable document check, for example by virtue of placing a reference surface rather than the valuable document into the acquisition range of the detector elements, the light sources being switched on individually one after the other in order to illuminate the reference surface and the light intensity emanating from the reference surface and incident on the detector elements being detected in each case.

4. Method according to either of Claims 2 or 3, characterized in that the proportion factors Aij are calculated prior to the valuable document check by means of a numerical simulation on the basis of a model of the optical beam path from the N light sources to the K detector elements.

5. Method according to Claim 4, characterized in that, in the numerical simulation of the optical beam path from the N light sources to the K detector elements, a transmission matrix (A) is calculated, the matrix elements of which correspond to the proportion factors Aij, and the transmission matrix (A), in particular the matrix elements of the transmission matrix A that correspond to the proportion factors Aij, is / are used to correct the measured values Di.

6. Method according to either of Claims 4 and 5, characterized in that, when calculating the respective correction factor FKi of the respective detector element, the proportion factors Aij for multiple or all of the N light sources are multiplied by the light intensities MSj detected by the respective monitor elements at the time of the self-test.

7. Method according to any of Claims 2 to 4, characterized in that, in order to calculate the respective correction factor FKi, for multiple or all of the N light sources, the respective proportion factors Aij are multiplied by the respective light intensities MSj detected by the monitor elements at the time of the self-test, and the results of these multiplications are summed up to calculate the respective correction factor FKi.

8. Method according to Claim 7, characterized in that, in order to correct the measured values Di of the valuable document, the respective measured value Di is multiplied by the correction factor FKi, wherein the correction factors FKi are calculated according to the following formula: FK i = ∑ j = 1 N A ij ⋅ MA j ∑ j = 1 N A ij ⋅ MS j .

9. Method according to any of Claims 2 to 6, characterized in that, in order to calculate the respective correction factor FKi of the respective detector element, for multiple or all of the N light sources, the ratio is formed in each case between the light intensity MSj detected by the monitor elements at the time of the self-test and the light intensities MAj detected by the monitor elements at an earlier time, the respective ratio MSj / MAj is multiplied by the respective proportion factor Aij and the results of these multiplications are summed up to calculate the respective correction factor FKi.

10. Method according to Claim 9, characterized in that, in order to correct the measured values Di of the valuable document, the respective measured value Di is multiplied by the respective correction factor FKi, wherein the correction factors FKi are calculated according to the following formula: FK i = ∑ j = 1 N A ij ∑ j = 1 N A ij ⋅ MS j MA j .

11. Method according to any of the preceding claims, characterized in that the self-test is preceded by a calibration procedure in which a calibration medium is brought into the acquisition range of the optical sensor (100), and the N light sources (1) are switched on in order to illuminate the calibration medium, wherein - the monitor elements of the optical sensor each detect a light intensity MAj that corresponds to the light intensity emitted by the respective light source during the calibration procedure, where j=1...N, and / or - the detector elements of the optical sensor each detect a light intensity DAi emanating from the calibration medium, where i=1...K.

12. Method according to Claim 11, characterized in that the light intensities MAj of the light sources detected by the monitor elements within the scope of the calibration procedure are also used to calculate the respective correction factor FKi of the respective detector element, wherein the respective light intensity MAj in each case is related to the light intensity MSj detected by the respective monitor element at the time of the self-test, where j=1...N.

13. Method according to Claim 11 or 12, characterized in that the light intensities DAi detected within the scope of the calibration procedure from the calibration medium by the detector elements are used to correct the measured values Di detected from the valuable document, wherein the measured values Di of the respective detector element that are recorded from the valuable document are multiplied by a further correction factor Fi, which corresponds to the inverse of the light intensity DAi detected in the calibration procedure for the optical sensor from the calibration medium by means of the respective detector element.

14. Method according to any of the preceding claims, characterized in that the temperature dependence of the monitor elements (3) is also taken into account in the correction of the measured values Di, wherein a temperature of the monitor elements at the time of the self-test (TS) is measured, in particular by means of a temperature sensor (16) installed in the optical sensor, and the measured temperature (TS) is used to determine a temperature-dependent factor that is used to correct the light intensities MSj detected by the monitor elements at the time of the self-test.

15. Optical sensor (100) for checking a valuable document that for checking purposes is brought into an acquisition range of the optical sensor, wherein the optical sensor comprises: - a photodetector line comprising a number K of multiple adjacent detector elements (4) which are designed to detect in each case the intensity of the light emanating from a detection region of the valuable document (10), - a light source line comprising a number N of multiple adjacent light sources (1) which are designed to illuminate the valuable document, - a monitor detector line comprising multiple adjacent photosensitive monitor elements (3), with each monitor element (3) being assigned to one of the light sources (4) and being designed to detect the intensity MSj of the light emitted by this light source and incident on the respective monitor element, - a control device (30) that is configured - to switch on the N light sources (1) simultaneously for the optical check of the valuable document, in order to illuminate the valuable document with the light from the light sources, and to control the detector elements (4) of the photodetector line in such a way that these record measured values of the valuable document that correspond to the light intensity emanating from the valuable document (10) as a result of the illumination, wherein the K detector elements (4) each record a measured value Di of the valuable document, where i=1...K, and - to perform a self-test of the optical sensor, in which the N light sources (1) are switched on and the respective light intensity MSj of the light source (1) assigned to the respective monitor element and incident on the respective monitor element at the time of the self-test is detected by means of the monitor elements (3) in order to check the light intensities MSj detected by the monitor elements at the time of the self-test, where j=1...N, - an evaluation device (20) that is configured - to correct the measured values Di of the valuable document on the basis of the light intensities MSj detected by the monitor elements (3) at the time of the self-test, wherein the at least one measured value Di of the respective detector element is in each case corrected during correction using a correction factor FKi that is calculated on an individual basis for the respective detector element (4) using the light intensities MSj detected at the time of the self-test by multiple of the monitor elements (3), and - to check the valuable document using the measured values (Di*) that have been corrected with the respective correction factor FKi.