Optical sensors for checking valuable documents

Abstract

The invention relates to an optical sensor for checking valuable documents. At a certain point in time before checking the valuable documents, a self-test of the optical sensor is performed, during which the light source of the optical sensor is switched on and the intensity of the light incident on each monitoring element by the light source assigned to the specific monitoring element during the self-test is detected using monitoring elements. During the checking of valuable documents after the self-test, the light source of the optical sensor is switched on to illuminate the valuable documents and the corresponding measured values ​​of the valuable documents are recorded. The recorded measured values ​​of the valuable documents are then corrected with the aid of the light intensity detected by the monitoring elements during the self-test to take into account changes in the intensity of the light emitted by the light source during the operating time of the light source.

Description

Technical Field

[0001] The invention relates to an optical sensor for checking value documents. Background Art

[0002] It is known from the prior art that valuable documents can be irradiated and the optical reflectivity of the respective valuable documents can be checked. Alternatively, the valuable documents can also be optically excited with the irradiation light and their measured luminescence values ​​can be checked. The term "luminescence" is to be understood as a general term for radiation (e.g. fluorescence or phosphorescence) emitted back by a valuable document after optical excitation. The light emitted from the valuable document due to the irradiation is detected by a photodetector and the detected measured values ​​are subsequently evaluated. For example, in order to distinguish between an authentic valuable document and a counterfeit valuable document, it can be checked whether the measured reflection value or the measured luminescence value of a specific part of the checked valuable document is above or below a specific threshold value.

[0003] It is also known that the intensity of the light emitted by a light source generally decreases gradually during the service life of the light source. Therefore, the intensity of the emitted light is monitored by means of a monitoring diode arranged near the light source. In order to compensate for the decrease in the intensity of the light source, it is often necessary to readjust the light source current. Alternatively, when evaluating the measured values ​​of the acquired valuable document, the decrease in the intensity of the light source can be taken into account and eliminated in a computationally retroactive manner. It is assumed here that the acquired measured values ​​decrease in proportion to the decrease in the light intensity of the light source. However, if a plurality of light sources adjacent to each other are used, which are switched on simultaneously to illuminate the valuable document and the light emitted by them may even be spatially superimposed on the valuable document, computational elimination in a retroactive manner is difficult. This is because the measured values ​​acquired from the valuable document by the corresponding photodetector depend on the light intensities of a plurality of light sources, the emitted light intensities of which develop differently during the service life. Summary of the invention

[0004] It is therefore an object of the invention to propose a solution for retrospectively correcting value-document measurement values ​​acquired by a photodetector in the event that the intensity of the light emitted by a plurality of light sources develops differently during the service life of the light sources.

[0005] This object is achieved by the features defined in the independent claims. In the claims dependent on these advantageous embodiments, further developments of the invention are specified.

[0006] For checking the valuable document, it is introduced into the acquisition range of an optical sensor suitable for checking the valuable document. The optical sensor comprises a row of photodetectors, which has K detector elements arranged side by side and configured to detect the intensity of light emitted from a detection zone of the valuable document. In addition, the optical sensor comprises a row of light sources, which has N multiple light sources arranged side by side and configured to illuminate the valuable document. In addition, the optical sensor has a row of monitoring detectors, which has a plurality of photosensitive monitoring elements (e.g. configured as photodetectors) arranged side by side, for example N monitoring elements. However, more than N monitoring elements can also be used. Each monitoring element is assigned to one of the light sources, preferably precisely to one of the light sources, and is configured to detect the intensity of light emitted by the light source and irradiated onto the corresponding monitoring element. For example, in the optical sensor, there is a 1:1 assignment relationship between light sources and monitoring elements, so that there is exactly one monitoring element for each light source.

[0007] At a certain point in time before checking the value document, a self-test of the optical sensor is performed, which is performed, for example, by a control device of the optical sensor. During the self-test, the N light sources are switched on and the respective light intensities MS of the light sources assigned to the respective monitoring elements, which shine on the respective monitoring elements during the self-test, are detected by means of the N monitoring elements. j (j=1...N). As part of the self-test, the light intensity MS detected by the monitoring element during the self-test can be verified. j (j=1...N). During verification, for example, attention is paid to the light intensity MS detected by the corresponding monitoring element. j (j=1 . . . N) is sufficient to check the value document, for example by comparison with a threshold value, and if the intensity detected by the monitoring element during the self-test is too low, an error message is output.

[0008] For optical inspection of a value document, the value document to be inspected is introduced into the acquisition range of the optical sensor so that the value document can be illuminated by light from a light source and light emitted from the value document can be detected by the detector element. The light emitted from the value document may be reflected illumination light or illumination light transmitted through the value document, but may also be luminescence emitted by the value document due to radiation suitable for optical excitation that may be emitted by the light source.

[0009] In particular, the value documents are fed past the optical sensor in a transport direction and in each case a detection zone of the value document is introduced into the acquisition range. The row of photodetectors is arranged transversely (e.g. in a vertical or inclined manner) to the transport direction of the value documents, wherein the respective value document is fed past the optical sensor for inspection. The detection zones of the value documents arranged adjacently in the transport direction are acquired sequentially in time. The detection zones of the value documents acquired sequentially can each correspond to a pixel of an image recorded by the row of photodetectors.

[0010] The N light sources of the optical sensor are switched on simultaneously to illuminate the respective value document introduced into the acquisition range with the light of the light sources. The N light sources may be all light sources contained in the row of light sources or only a subset of these light sources (light sources located at the edge of the row of light sources may not be utilized). With the aid of detector elements, a measurement value of the respective value document is recorded, which measurement value corresponds to the intensity of the light emitted from the respective value document as a result of the illumination. The K detector elements of the row of photodetectors each record at least one measurement value D of the value document. i (i=1...K). The K detector elements may be all detector elements contained in the row of photodetectors or only a subset of these detector elements (detector elements located at the edge of the row of detector elements may not be utilized). For example, the respective detector element detects only one measurement value D for a specific value document area. i or multiple measurements D i , for example when scanning a value document along the transport direction. For example, switching on the light source and recording the measured value of the value document is caused by the control device of the optical sensor, which drives the light source or the detector element accordingly. The corresponding measured value can be detected while the illumination pulse of the light source is irradiating (in the case of reflection measurement or transmission measurement) or after the end of the illumination pulse of the light source (in the case of luminescence measurement).

[0011] With the aid of the light intensity MS detected by the monitoring element during the self-test j (j=1...N) for the recorded measured values ​​D i (i=1...K) is calibrated to take into account (in particular to retrospectively eliminate) any changes in the intensity of the light emitted by the light source that occurred during the service life of the light source. During the calibration, the measured values ​​D of the individual detector elements are respectively i Make corrections, in particular multiply by the correction factor FK i , which correction factor is calculated individually for the respective detector element, i.e. based on the light intensity MS detected by the plurality of monitoring elements during the self-test j Calculation is performed, for example, based on the light intensity MS detected by at least two monitoring elements during a self-test jCalculation is performed based on the use of the corresponding correction factor FK i Corrected measurement value D i * Check the corresponding valuable documents.

[0012] Correction measurement value D i The calculation of *(i=1 . . . K) and the checking of the value document can be performed by an evaluation device. The evaluation device is connected to the row of photodetectors or detector elements to receive the measured value D of the detected value document. i (and possibly the measured value DA of the calibration medium i , see below). The evaluation device can output the result of the value-document inspection to an operator and / or a value-document processing apparatus equipped with an optical sensor.

[0013] With the aid of the light intensity MS detected by the monitoring element during the self-test j (j=1...N) measured value D for the value document i A correction is performed to correct or retroactively calculate variations in the intensity of the light emitted by the light source during its service life from the detected measured values. By using a plurality of monitoring elements assigned to different light sources, it is achieved that the intensity of the light emitted by the respective light source in each case is acquired separately during the self-test. By means of the light intensity MS detected by the monitoring elements during the self-test j (j=1 . . . N) Correction of the measured values ​​of the value document can also take into account different developments in the intensity of the light emitted by the individual light sources.

[0014] For example, during a self-test a correction table is created in which the correction factors FK calculated for the detector elements of the row of photodetectors are entered. i The correction factor FK can be calculated for each detector element of the row of photodetectors i , or the correction factor FK is calculated only for the detector elements which actually pick up the light emanating from the checked value document i In the latter case, for example for detector elements located at the edge of the row of photodetectors, the calculation of the correction factors is omitted.

[0015] In particular, based on the light intensity MS detected by the monitoring element during the self-test j (j=1...N) calculate the correction factor FK for the corresponding i-th detector element individually i , taking into account in each case the scaling factor A ij , the scaling factor A ijSpecifies the ratio of the intensity of the light irradiated by the respective j-th light source (j=1...N) onto the respective i-th detector element (i=1...K) due to the beam path between the light source and the detector element. The individual light intensity values ​​MS detected by the monitoring element during the self-test j (j=1...N) are respectively subject to the scaling factor A ij (j=1...N) is combined in a restricted manner with the corresponding i-th correction factor FK i (i=1...K). In order to calculate the corresponding correction factor FK of multiple or all of the N light sources i , the corresponding scaling factor A ij Multiply by the light intensity MS detected by the monitoring element during self-test j (j=1...N).

[0016] For example, the factor A can be determined based on experience before the value document is checked. ij In particular, a reference area, for example a calibration medium with homogeneous optical properties, is placed in the acquisition range of the detector element instead of the value document, the individual light sources are switched on in sequence to illuminate the reference area, and the intensity of the light emanating from the reference area and impinging on the detector element is detected in each case.

[0017] Alternatively, the scaling factor A can be calculated before the value-document inspection by numerical simulation based on a model of the beam paths from N light sources to K detector elements. ij The numerical simulation is carried out, for example, by the sensor manufacturer before the inspection of the value document. The result of this numerical simulation is then used to calculate the measured value D recorded by the detector element from the corresponding value document. i Correction factor FK i In a numerical simulation of a beam path from N light sources to K detector elements, a transfer matrix A is calculated, for example, whose matrix elements are related to the scaling factors A ij (i=1...K,j=1...N) correspond to each other. Then use the transfer matrix A (especially the scaling factor A ij (i=1...K,j=1...N) corresponding to the transfer matrix) corrected measurement value D i .

[0018] In calculating the corresponding correction factor FK for the corresponding i-th detector element (i=1...K) i When the scale factor A is set for multiple or all of the N light sources, ij Multiply by the light intensity MS j (j=1...K) to calculate the correction factor FK for each of the K detector elements (i=1...K) i, the correction factor FK i The measured value D for correcting the corresponding i-th detector element i .

[0019] Prior to the self-test of the optical sensor, a calibration process of the optical sensor is preferably carried out in order to check the possibly different sensitivities of the individual detector elements of the optical sensor. The calibration process is carried out at an earlier point in time before the self-test, for example before the optical sensor is delivered by the manufacturer of the optical sensor. However, alternatively or additionally, the calibration process may be carried out after the optical sensor has been delivered to the customer, for example by a service staff of the manufacturer. During the calibration process, a calibration medium with uniform optical properties is introduced into the acquisition range of the optical sensor and a light source is simultaneously switched on in order to illuminate the calibration medium. During the calibration process, each monitoring element of the optical sensor detects the light intensity MA j (j=1...N), which light intensity corresponds to the intensity of the light emitted by the corresponding light source during the calibration process. During the calibration process, the detector elements of the optical sensor respectively detect the intensity DA of the light emitted from the calibration medium i (i=1...K). In particular, the control device of the optical sensor may be adapted to cause the required detection during the calibration, for example detection of the light intensity MA by the detector element j (j=1...N) and / or the light intensity DA detected by the detector element i (i=1...K).

[0020] Then, in order to calculate the corresponding correction factor FK for the corresponding i-th detector element i (i=1...K), preferably also taking into account the light intensity MA of the light source detected by the monitoring element within the scope of the calibration process j (j=1...N). The light intensity MS detected by the monitoring element during the self-test j (j=1...N) and these light intensities MA j (j=1...N) to quantitatively determine the change in light intensity of the light source since the calibration process.

[0021] To correct the measured value D of the value document i , the corresponding measured value D i Multiply by the corresponding correction factor FK i (i=1...K). In the first exemplary embodiment, in order to calculate the corresponding correction factors FK for a plurality of or all of the N light sources, i , the corresponding scaling factor A ij Multiply by the corresponding light intensity MS j (j=1...N), and add the results of these multiplications to calculate the corresponding correction factor FK i.

[0022] In a second exemplary embodiment, for a plurality or all of the N light sources, in order to calculate the corresponding correction factor FK for the corresponding i-th detector element i (i=1...K), forming in each case the light intensity MS detected by the monitoring element during the self-test j (j=1...N) and the light intensity MA detected by the monitoring element at an earlier point in time (especially during the calibration process) j The corresponding ratio MS j / MA j Multiply by the corresponding scaling factor A ij (j=1...N), and add the results of these multiplications to calculate the corresponding correction factor FK i .

[0023] If necessary, the light intensity DA detected by the detector element from the calibration medium within the scope of the calibration process is also used. i Correction of the measured value D detected from the value document i (i=1 . . . K), where the measured value D of the corresponding detector element recorded from the value document i Multiply by another correction factor F i , the correction factor F i The light intensity DA detected from the calibration medium by means of the corresponding detector element during the calibration of the optical sensor i The reciprocal of .

[0024] In addition to the aging of the light source during its service life, the temperature dependency of the monitoring element (e.g., the temperature-dependent sensitivity) can also be corrected when correcting the measured values ​​of the valuable document. For example, the temperature of the monitoring element during the self-test and the temperature of the monitoring element during the calibration process can be measured with the help of a temperature sensor installed in the optical sensor. In order to correct the temperature dependency of the monitoring element, the light intensity MS detected by the monitoring element during the self-test can be multiplied by a temperature dependency factor, which is determined based on the temperature of the monitoring element during the self-test. Similarly, the light intensity MA detected by the monitoring element during the calibration process can be multiplied by a temperature dependency factor, which is determined based on the temperature of the monitoring element during the calibration process.

[0025] The valuable documents checked using the method of the present invention are, for example, banknotes, tickets, vouchers, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be described below by way of example with reference to the accompanying drawings. The description of the accompanying drawings is as follows:

[0027] Figure 1The structure of an optical sensor for checking a value document is shown;

[0028] Figure 2 A schematic diagram showing the distribution of the illumination light emitted by the light source in the plane of the value document;

[0029] Figure 3 shows the scaling factor A determined using the transfer matrix ij Grayscale image of

[0030] Figure 4 shows the light intensity (MA) detected by the monitoring element during the calibration process j ) and the light intensity detected during self-test (MS j );

[0031] Figure 5 The correction factor FK calculated for the i-th detector element (i=1...K) is shown. i ;

[0032] Figure 6 shows the measured value D of the value document recorded by the i-th detector element (i=1 . . . K) i and the corrected measured value D i * = FK i ·D i . DETAILED DESCRIPTION

[0033] Figure 1 The structure of an optical sensor 100 suitable for checking value documents is shown, which optical sensor 100 can be installed in a value document processing device, for example. In order to illuminate the value document 10 introduced into the acquisition range of the optical sensor 100, a row of light sources is used, which has a plurality of light sources 1, for example N=12 light sources, arranged side by side in the y direction on a light source mounting 11. The light source 1 is, for example, an ultraviolet light emitting diode (UVLED) suitable for cold light excitation of the value document. The light emitted by the light source 1 is guided via a plane mirror 2 to a photosensitive monitoring element 3 (for example a photodiode), with the help of which the intensity of the light emitted by the light source 1 can be verified, which light intensity usually gradually decreases during the service life of the light source. For each of the N light sources 1, exactly one monitoring element 3 is provided (for example N=12 monitoring elements). By means of an aperture 15, it is achieved that each monitoring element 3 only captures the light of one of the light sources 1: the monitoring element 31 captures the light from the light source 11, the monitoring element 32 captures the light from the light source 12, and so on. The monitoring element 3 is attached to the monitoring element mounting bracket 13 .

[0034] As an alternative to reflection on a plane mirror 2, the monitoring elements 3 can also be arranged in the irradiation area of ​​the respective light source 1, so that they directly capture part of the light emitted by the light source. Alternatively, a partially transmissive plane mirror can be arranged between the light source and the value document, which allows most of the irradiation light to pass through and reflects part of the irradiation light onto the monitoring elements. Alternatively, the monitoring elements can also detect the irradiation light scattered from the light source back to the reference area.

[0035] A portion of the light emitted by the light source 1 is directed directly onto the value document 10 to be checked, another portion is directed onto the value document 10 via the elliptical mirror 6 and passes through the measuring window 8 of the optical sensor 100 and the filter 7, which blocks the visually visible part of the irradiation light. In addition, another filter of this type can be arranged in the light path in front of the monitoring element 3 to block the visually visible part of the irradiation light again here. Alternatively, the filter 7 can also be arranged directly after the light source to spectrally filter the light irradiating onto the value document and the light irradiating onto the monitoring element 3.

[0036] The luminescent light emitted by the value document 10 is imaged by a row of self-focusing lenses 5 provided with UV protection filters onto a plurality of detector elements 4 of a row of photodetectors, which are arranged side by side in the y direction on a detector element mounting 14 and have, for example, K=112 detector elements. It is possible to use only a single row of photodetectors on which the luminescent light is imaged. With the aid of additional lenses, it is also possible to image the luminescent light onto multiple rows of photodetectors which are offset from one another in the x direction and which capture different spectral ranges of the luminescent light and have corresponding filters. Alternatively, it is also possible to use multiple rows of photodetectors of a two-dimensional image sensor on which the light emitted by the value document can be imaged.

[0037] The value document 10 can be statically introduced into the acquisition range of the optical sensor 100 for inspection. However, it is preferred to feed the value document past the optical sensor in the x direction in order to scan the individual parts of the value document in sequence with the optical sensor. The feeding is realized, for example, by means of suitable conveying means, for example, by means of conveyor belts and / or conveyor rollers for conveying value documents in the value-document processing apparatus.

[0038] The control device 30 of the optical sensor 100 is connected to the light source 1, the monitoring element 3 and the detector element 4 and has suitable hardware and software which ensure that the light source 1 is switched on simultaneously and that the detector element 4 acquires the luminescence measurement value D of the value document 10 simultaneously and / or after the irradiation. i The measured value D of the value document acquired by the detector element 4 i (i=1 . . . K) are transmitted to an evaluation device 20 connected to the detector elements.

[0039] The control device 30 is suitable for performing a self-test of the optical sensor. This self-test can be performed at a certain point in time before checking a valuable document, for example when starting the valuable document processing device, or in the time interval between two valuable documents to be checked being fed past the sensor. During the self-test, the control device switches on N light sources 1 simultaneously and the corresponding light intensity MS of the light sources 1 assigned to the corresponding monitoring elements j (j=1...N) is detected. For example, during a self-test, verify that the light intensity MS detected by the monitoring element during the self-test j (j=1...N) exceeds a certain threshold value which is necessary for illuminating the value document, for example for optical excitation of measurable luminescence. The control device 30 is also connected to the evaluation device 20 in order to transmit the light intensity MS detected by the monitoring element during the self-test j (j=1...N) to the evaluation device 20 and possibly also the light intensity MA of the calibration medium detected by the monitoring element during the calibration process j (j=1...N).

[0040] The evaluation device 20 uses the correction factor FK i Correction of the measured value D of the corresponding value document i , in which the light intensity MS detected by the monitoring element 3 during the self-test is incorporated. j (j=1...N) to calculate the corrected measurement value.

[0041] D i * = FK i ·D i (i=1...K) (1)

[0042] The evaluation device 20 then uses the corrected measured value D i * Checking the value document, for example checking the authenticity, condition or type of the value document. For example, to check the value document, the corrected measured value D i * Comparison with at least one reference value, which is an expected value for the corresponding value document or a corresponding part of the value document. Before the comparison with the corresponding reference value, the corrected measured values ​​can also be aggregated for a part of the value document (region of interest, ROI), for example averaged.

[0043] When manufacturing the optical sensor, preferably N light sources are selected that are as identical as possible, the emission intensities of which are hardly different, and the row of photodetectors is preferably selected such that the sensitivity of the individual detector elements is hardly different. In this case, preferably according to formula (1) with the aid of the factor FK i Individually corrected measurement value D i .

[0044] Otherwise, if differences in individual light sources and / or individual detector elements are unavoidable in the manufacture of the optical sensor, then before delivery of the optical sensor, the sensor manufacturer preferably carries out a calibration process for the optical sensor, in which the possibly different luminous intensities of the light sources and the possibly different sensitivities of the detector elements are checked and quantitatively determined. In the calibration process, a calibration medium with uniform optical properties (e.g. a uniform white surface) is introduced into the acquisition range of the optical sensor 100. Then, N light sources 1 are switched on simultaneously to illuminate the calibration medium, and the intensity DA of the reflected light emitted from the calibration medium is detected by the detector element 4 of the optical sensor. i (i=1...K).

[0045] If it is found during the calibration process that the individual light sources 1 and / or the individual detector elements 4 are actually significantly different, then a further correction factor F is used. i Measurement value D for value documents i Further correction is performed, and the correction factor F i Multiply the measured value D by the same i :

[0046] D i *=F i ·FK i ·D i (2)

[0047] The other correction factor F i The detector element DA obtained from the calibration medium is calculated during calibration according to the following formula: i The measurements are given by:

[0048] F i =c / DA i (3)

[0049] It is assumed that c is a fixed value.

[0050] During the calibration process, the intensity MA of the light emitted by the N light sources 1 and irradiated on the N monitoring elements 3 is also detected by means of the monitoring element 3. j (j=1...N). In determining the correction factor FK i These values ​​are used as the intensity MS of the light emitted by the N light sources 1 and incident on the N monitoring elements 3 during the self-test. j (j=1...N) reference value.

[0051] If necessary, after delivery of the optical sensor, the calibration process can also be repeated from time to time (e.g. every month or every six months) in order to obtain the variation in the light intensity of the light source during its service life. However, since the calibration process is expensive (it requires the use of service personnel), it is advantageous to use the self-test of the optical sensor as an alternative means of obtaining the variation in the light intensity of the light source during its service life, rather than repeating the calibration process frequently. During the self-test of the optical sensor, the light intensity MS of the light source 1 incident on the monitoring element 3 is regularly detected (e.g. before each start-up of the value-document processing device). j (j=1 . . . N) Since the self-test is carried out at a very close time to the inspection of the value document, the light intensity of the respective light source recorded during the respective self-test corresponds quite accurately to the light intensity of the respective light source 1 occurring during the inspection of the value document.

[0052] In the following, the light intensity MS detected by the monitoring element 3 during the self-test will be described. j (j=1...N) Determine the correction factor FK for K detector elements (i=1...K) i The correction factor can be determined within the scope of a self-check, for example immediately before checking a value document.

[0053] To determine the correction factor FK for K detector elements i (I=1...K), and the ratio of the intensity of the light emitted by the corresponding j-th light source (j=1...N) and the intensity of the light irradiated on the corresponding i-th detector element (i=1...K) is determined for each detector element 4, taking into account the beam path between the light source and the detector element. Each of these ratios is determined by the proportionality factor A ij express.

[0054] In the scale factor A ij (j=1...N) combines the position of the corresponding j-th light source in the row of light sources and the position of the corresponding i-th detector element in the row of photodetectors and / or their relative positions along the direction of the row (y direction) or their distance, as well as the radiation angle of the light source (which is approximately the same for all light sources), the reception angle of the detector element (which is approximately the same for all detector elements), and the distance of the light source 1 and the detector element 4 from the valuable document plane of the valuable document introduced into the acquisition range of the optical sensor.

[0055] Figure 2 It shows the light sources 11, 12, ..., 1 N An example of the distribution of the emitted illumination light. The acquisition ranges E1, E2, ..., E KSchematically input into the valuable document plane, which is generated by optical imaging of the row of self-focusing lenses 5. It can be seen that the light contribution radiated by the light source (e.g. light source 11) arranged at the left edge of the row of light sources to the acquisition range (e.g. E1, E2, E3) of the detector element 4 located at the left edge of the row of photodetectors is much greater than its contribution to the acquisition range (e.g. E1, E2, E3) of the detector element 4 located at the right edge of the row of photodetectors. K ) of the light contribution. The proportionality factor A of the light source 11 on the detector element located at the left edge of the row of photodetectors 11 , A 21 , A 31 The proportionality factor A with respect to the detector elements located at the right edge of the row of photodetectors K1 Relatively larger.

[0056] The proportionality factor A can be quantitatively calculated with the help of numerical simulation based on the beam path model ij , in which the beam paths from N light sources of the optical sensor to K detector elements of the optical sensor are modeled.

[0057] For the optical simulation, in the case of the optical sensor 100, it is assumed, for example, that the light source has Lambertian radiation, that the distance h of the light source 1 from the plane of the value document is 30 mm, that the spacing of the light sources 1 is 10.5 mm, and that the spacing of the detector elements 4 is 1 mm (in the y direction). In the numerical simulation, for example, a transfer matrix A is calculated, the matrix elements of which have the scaling factors A ij (i=1...K, j=1...N).

[0058] The proportionality factor A calculated by numerical simulation of the beam path of the optical sensor 100 is ij exist Figure 3 The scale factor A is shown in the form of grayscale values. ij is for N=12 (j=1...12) light sources arranged from left to right and K=112 detector elements (i=1...112) arranged from top to bottom. Figure 2 As expected from the above description, the maximum scaling factor A ij lead to Figure 3 As an alternative to numerical simulation of the beam path, the proportionality factor A can also be determined by measuring the individual light intensities emitted by the light source and impinging on the corresponding detector elements. ij .

[0059] The radiation intensity B that is decisive for the corresponding i-th detector element (i=1...K) i Usually, the scaling factor A is used toij The light intensity L of the weighted light source j (j=1...N) is obtained by adding, thus obtaining:

[0060]

[0061] Its matrix form is as follows:

[0062]

[0063] Assuming that each monitoring element detects only a certain fixed proportion 1 / β of the light intensity emitted by the light source, then the formula L j =β·MS j From the light intensity MS detected by the monitoring element during the self-test j Calculate the intensity L of the light emitted by the light source j (j=1...N). During the calibration process, it is assumed that L j =β·MA j .

[0064] For the self-test moment, the light intensity MS detected by the monitoring element during the self-test is j (j=1...N) respectively according to the proportional factor A ij After weighting, the relevant illumination intensity BS of the corresponding i-th detector element (i=1...K) is obtained. i .

[0065]

[0066] For the moment of the calibration process, the following results are obtained accordingly:

[0067]

[0068] In a first exemplary embodiment, the correction factor FK for each detector element (i=1 . . . K) is i It is calculated from two irradiation intensity values ​​BS according to the following formula i and BA i Calculated:

[0069] Where i = 1...K, j = 1...N. (8)

[0070] Therefore, the corresponding correction factor FK i The irradiation intensity present during the calibration and the irradiation intensity present during the self-test The correction factor FK according to formula (8) is iThe calculation of is an approximate calculation, in which, for simplicity, it is assumed that all monitoring elements have the same sensitivity and the optical image on each monitoring element is the same.

[0071] In a second exemplary embodiment, the correction factor FK for each detector element (i=1...K) is i It is calculated according to the following formula:

[0072] Where i = 1…K, j = 1…N. (9)

[0073] The correction factor FK according to formula (9) i The calculation of is also an approximate calculation, in which it is assumed that the light source 1 emits approximately the same light intensity during the calibration process.

[0074] Correction factor FK i can be calculated by the control device within the scope of a self-test and thus forwarded to the evaluation device 20. However, the correction factor FK i It can also be calculated first by an evaluation device which receives the corresponding light intensity MS detected by the control device during the self-test. j (j=1...N). The radiation intensity MS detected during the calibration process and the self-test j and MA j (j=1...N) According to formula (8) or (9) and using Figure 3 The scale factor A shown ij The correction factor FK is determined i Afterwards, the evaluation device 20 can evaluate the measured value D of the value document according to formula (1): i (i=1...K) for correction.

[0075] For example, in Figure 4 The illumination intensity MS of all N=12 light sources 1 detected by N=12 monitoring elements 3 during the calibration process and self-test of the optical sensor is shown in FIG. j and MA j (j=1...N). During the self-test, it was noted that the light intensity of the second light source (j=2) dropped significantly, by about half compared to the light intensity measured earlier in the calibration process. For all K=112 detector elements 4, from MS j and MA j These values ​​(j=1...N) (using Figure 3 The scale factor A shown ij ) According to formula (8) or (9), we can get Figure 5 The correction factor FK shown i .

[0076] use Figure 5 Correction factor FK i , according to formula (1) for a single detector element 4 (see Figure 6 ) The measured value D of the value document obtained i A correction (eg multiplication) is performed in order to calculate from the detected measured values ​​the light intensity which has changed during the service life, in particular the light intensity of the second light source which has significantly decreased since the calibration process. Figure 6 The corrected measured value D in the example considered is shown i * = FK i ·D i For detector elements i=1 to about i=45, this results in a measured value D i A clear upward correction.

[0077] In addition to the aging of the light source 1 during its service life, the temperature dependency of the monitoring element 3 can also be corrected if necessary. For this purpose, the temperature sensor 16 installed near the monitoring element 3 (for example, installed on the monitoring element mounting 13) is used to measure the temperature at the self-test T S The temperature of the component is monitored and corrected when the measured value D i If the temperature of the monitoring element remains approximately the same during the calibration process performed by the sensor manufacturer, it is not necessary to take into account the temperature measured during the calibration process. Otherwise, if the calibration process is performed under conditions where the temperature of the monitoring element varies greatly, it is preferred to also take into account the temperature T of the monitoring element measured during the calibration process. A stored in the optical sensor and used in the correction of the measured value D i Take this temperature value into account.

[0078] For example, a table can be stored in the evaluation device 20 of the optical sensor in which a correction factor t is assigned in each case to different temperatures T of the monitoring element as a function of the temperature T. In order to correct the temperature dependence of the monitoring element 3, the light intensity MS detected by the monitoring element during the self-test is replaced in formula (8) or (9) by j Replaced by MS j '=t(T S )·MS j , where t(T S ) is the temperature T measured during the self-test according to the above table S Assuming the correction factor t(T S ) is substantially the same for all monitoring elements 3. Similarly, the light intensity MA detected by the monitoring element during the calibration process j Can be made by MA j '=t(T A )·MA j Instead, where t(TA ) is the temperature T measured during the calibration process according to the above table A Alternatively, the measured values ​​D of the value document acquired by the individual detector elements 4 may simply be i Multiply by the additional factor t(T A ) / t(T S ).

[0079] Furthermore, the temperature dependency of the detector element can optionally be measured during the self-test with a corresponding additional temperature sensor and, if the detector element has a correspondingly large temperature dependency, then the measured value D i Any temperature changes since the calibration process can be recorded.

Claims

1. A method for checking a value document (10) introduced into the acquisition range of an optical sensor (100), wherein the optical sensor (100) has: a row of photodetectors having K detector elements (4) arranged next to one another and configured to detect the intensity of light emanating from a detection zone of the value document (10), a row of light sources having a plurality of N light sources (1) arranged next to one another, which are configured to illuminate the value document (10), - a row of monitoring detectors having a plurality of light-sensitive monitoring elements (3) arranged side by side, wherein each monitoring element is assigned to one of the light sources (1) and is configured to detect the intensity of light emitted by the light source and impinging on the corresponding monitoring element, The following steps are performed in the method: - a self-test of the optical sensor (100) is carried out at a certain point in time before checking the valuable document (10), wherein the N light sources (1) are switched on during the self-test and the respective light intensity MS of the light sources (1) assigned to the respective monitoring element is detected by means of the monitoring element (3) j , the light intensity is irradiated on the corresponding monitoring element during self-test to verify the light intensity MS detected by the monitoring element during self-test j , where j = 1...N, - introducing the value document (10) into the acquisition range of the optical sensor (100), - simultaneously switching on the N light sources (1) to illuminate the value document (10) introduced into the acquisition range with the light of the light sources and recording measured values ​​of the respective value document by means of the detector elements (4), wherein the recorded measured values ​​correspond to the intensity of the light emitted by the respective value document as a result of the illumination, and wherein the K detector elements (4) of the row of photodetectors each record at least one measured value D of the value document i , where i = 1...K, - Based on the light intensity MS detected by the monitoring element (3) during the self-test j Correction measurement value D i , where at least one measured value D of the corresponding detector element (4) i During calibration, the correction factor FK is used in each case. i Corrected, the correction factor FK i is based on the light intensity MS detected by a plurality of monitoring elements (3) during self-test j is calculated individually for the corresponding detector element (4), and - Based on the use of the corresponding correction factor FK i Corrected measurement value (D i *) Checking valuable documents (10), in, The light intensity values ​​MS detected by the monitoring element (3) during the self-test j The corresponding correction factor FK is included in the calculation i In the example, each light intensity value is subject to the scaling factor A. ij Limit, the correction factor FK i Indicates the ratio of the light intensity of the light emitted by the respective light source impinging on the respective detector element due to the beam path from the light source to the detector element.

2. The method according to claim 1, characterized in that The scaling factor A ij is determined empirically before the inspection of the value document, for example by placing a reference area instead of the value document in the acquisition range of the detector element, successively switching on the light source to illuminate the reference area and detecting in each case the intensity of the light emanating from the reference area and impinging on the detector element.

3. The method according to claim 1 or 2, characterized in that The scaling factor A ij is calculated prior to the inspection of the value document using a numerical simulation based on a model of the beam paths from N light sources to K detector elements.

4. The method according to claim 3, characterized in that In the numerical simulation of the beam path from N light sources to K detector elements, a transfer matrix (A) is calculated whose matrix elements are proportional to the scaling factor A. ij Corresponding, and using the transfer matrix (A), especially the transfer matrix A and the scaling factor A ij The corresponding matrix elements are used to correct the measured value D i .

5. The method according to claim 3, characterized in that In calculating the corresponding correction factor FK for the corresponding detector element i When the proportional factor A is set to ij The light intensity MS detected by the corresponding monitoring element during the self-test j Multiply.

6. The method according to claim 1 or 2, characterized in that: In order to calculate the corresponding correction factor FK of several or all of the N light sources i , the corresponding scaling factor A ij Multiply by the corresponding light intensity MS detected by the monitoring element during self-test j , and add the results of these multiplications to calculate the corresponding correction factor FK i .

7. The method according to claim 6, characterized in that To correct the measured value D of the value document i , the corresponding measured value D i Multiply by the corresponding correction factor FK i , where the correction factor FK i It is calculated according to the following formula:

8. The method according to claim 1 or 2, characterized in that: In order to calculate the corresponding correction factor FK for the corresponding detector element i , in each case for a plurality or all of the N light sources, a light intensity MS detected by the monitoring element during the self-test is formed j The light intensity MA detected by the monitoring element at an earlier point in time j The ratio between the corresponding ratio MS j / MA j Multiply by the corresponding scaling factor A ij , and add the results of these multiplications to calculate the corresponding correction factor FK i .

9. The method according to claim 8, characterized in that To correct the measured value D of the value document i , the corresponding measured value D i Multiply by the corresponding correction factor FK i , where the correction factor FK i It is calculated according to the following formula:

10. The method according to claim 1 or 2, characterized in that: A calibration process is performed before the self-test, in which a calibration medium is introduced into the acquisition range of the optical sensor (100), and the N light sources (1) are turned on to illuminate the calibration medium, wherein - The monitoring elements of the optical sensor respectively detect the light intensity MA j , the light intensity MA j corresponds to the intensity of the light emitted by the corresponding light source during the calibration process, where j=1...N, and / or - The detector elements of the optical sensor respectively detect the intensity DA of the light emitted from the calibration medium i , where i=1...K.

11. The method according to claim 10, characterized in that In order to calculate the corresponding correction factor FK for the corresponding detector element i , also taking into account the light intensity MA of the light source that the monitoring element has detected within the scope of the calibration process j , where the corresponding light intensity MA is in each case j Substitute into the light intensity MS detected by the corresponding monitoring element during the self-test j In the relationship between, where j=1...N.

12. The method according to claim 10, characterized in that The light intensity DA detected by the detector element from the calibration medium within the scope of the calibration process is used i Correction of the measured value D detected from the value document i , where the measured value D of the corresponding detector element recorded from the value document is i Multiply by another correction factor F i , the correction factor F i The light intensity DA detected from the calibration medium by means of the corresponding detector element during the calibration of the optical sensor i The reciprocal of .

13. The method according to claim 1 or 2, characterized in that: In the correction measurement value D i The temperature dependence of the monitoring element (3) is also taken into account, in particular by means of a temperature sensor (16) installed in the optical sensor during the self-test (T S ) and based on the temperature measured in each case (T S ) determines the temperature dependence factor, and uses the temperature dependence factor to correct the light intensity MS detected by the monitoring element during self-test j .

14. An optical sensor (100) for checking a value document introduced into a capture range of the optical sensor for checking, wherein the optical sensor has: a row of photodetectors having K detector elements (4) arranged next to one another and configured to detect in each case the intensity of light emanating from a detection zone of a value document (10), a row of light sources having a plurality of N light sources (1) arranged next to one another, which are configured to illuminate the value document, - a row of monitoring detectors having a plurality of light-sensitive monitoring elements (3) arranged side by side, wherein each monitoring element (3) is assigned to one of the light sources (4) and is configured to detect the intensity MS of the light emitted by the light source and impinging on the corresponding monitoring element j , - a control device (30) adapted to: - for optical inspection of a value document, N light sources (1) are switched on simultaneously to illuminate the value document with light from the light sources, and the detector elements (4) of the row of photodetectors are controlled so that they record a measured value of the value document, the measured value corresponding to the intensity of the light emitted from the value document (10) as a result of the illumination, wherein the K detector elements (4) Record the measured value D of the value document separately i , where i = 1...K, and - performing a self-test of the optical sensor, during which the N light sources (1) are switched on and the respective light intensities MS of the light emitted by the light sources (1) assigned to the respective monitoring elements and irradiating the respective monitoring elements during the self-test are detected by means of the monitoring elements (3) j , to verify the light intensity MS detected by the monitoring element during self-test j , where j = 1...N, - an evaluation device (20) adapted to: - Based on the light intensity MS detected by the monitoring element (3) during the self-test j Correction of the measured value D of the value document i , wherein during calibration, at least one measured value D of the corresponding detector element i In each case the correction factor FK is used i Corrected, the correction factor FK i is based on the light intensity MS detected by a plurality of monitoring elements (3) during self-test j is calculated individually for the corresponding detector element (4), and - Based on the use of the corresponding correction factor FK i Corrected measurement value (D i *) Checking valuable documents, in, The light intensity values ​​MS detected by the monitoring element (3) during the self-test j The corresponding correction factor FK is included in the calculation i In the example, each light intensity value is subject to the scaling factor A. ij Limit, the correction factor FK i Indicates the ratio of the light intensity of the light emitted by the respective light source impinging on the respective detector element due to the beam path from the light source to the detector element.

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