# Patent

## Title
Sensor and method for operating the sensor

## Abstract

A sensor for checking value documents is arranged to carry out a self-test for testing its functionality. The sensor in the self-test reacts to at least one malfunction that is ascertained during the self-test and hinders the check of the value documents by the sensor automatically employing for checking the value documents, instead of the operating mode provided for checking the value documents. A modified operating mode employs at least one other measured value of the sensor for checking the value documents than is determined in the operating mode provided for checking the value documents. In contrast to the hitherto customary function failure of the sensor, the sensor can continue being operated for checking the value documents despite the malfunction.

## Background
BACKGROUND
This invention relates to a method for operating a sensor which is configured for checking value documents, and to a sensor which is configured for carrying out this method.
SUMMARY
For checking value documents there are usually employed sensors with which the kind of value document is determined and/or with which the value documents are checked for authenticity and/or for their state. Such sensors are employed e.g. for checking bank notes, checks, identity cards, credit cards, check cards, tickets, vouchers and the like. The value documents are usually checked in an apparatus for value-document processing which contains one or several sensors depending on the value-document properties to be checked. For the check of the value documents, the latter are transported past the sensor along a transport path individually using a transport system.
For testing the function of a sensor which is arranged along the transport path of the value documents, a test medium is usually brought into the capture region of the sensor in order to detect a measured value of the test medium with the sensor. For this purpose, the value-document check is interrupted and—instead of a value document—the test medium is brought into the capture region of the sensor. This method is disadvantageous in that a test medium must be provided and its association with the sensor must be ensured. In the case of several sensors or upon replacement of the test medium, e.g. due to degradation of the test medium, confusion can easily arise, which can lead to false test results. Further, it has been proposed to install a test medium in the sensor itself, e.g. in the housing of the sensor. For testing the sensor function, the check of the value documents is interrupted and the test medium is swiveled into the capture region of the sensor in order to detect measured values thereof. It is disadvantageous here, too, that the check of the value documents must be interrupted for testing the sensor function.
Upon the functional check of a sensor it has hitherto been tested whether measured values that the sensor detects from the test medium brought into its capture region exceed predetermined minimum values. As long as the actual measured values are above the minimum value, the sensor is functional. If an undershooting of a minimum value is ascertained, however, this results in a function failure of the sensor. Such function failures lead to interruptions of the operation of the sensor and the appurtenant apparatus checking the value documents, thereby reducing the throughput of the value-document check. Function failures usually require an intensive testing of the sensor and/or involve manual interventions, e.g. by service staff, and thus increase the effort for operating the sensor.
An object of the present invention is hence to reduce the effort for operating a sensor configured for checking value documents.
This object is achieved by the subject matter of the independent claims. Claims dependent thereon state advantageous developments and configurations of the invention.
The method according to the invention relates to a sensor which is configured for checking value documents. The sensor can be a sensor for checking optical or magnetic or electrical or mechanical properties of the value documents, in order to check the value documents for their authenticity, their kind, their state or their quality. For checking the value documents there is provided in the sensor at least one operating mode of the sensor in which it is determined which measured values the sensor is to detect for checking the value documents and how the detected measured values are to be evaluated. The sensor has e.g. a data memory in which there is stored the provided operating mode or information on the provided operating mode employed by the sensor for checking the value documents. The sensor can have provided therein one or also several operating modes, e.g. for checking one or several kinds of value document.
For ascertaining any malfunctions, the sensor carries out a self-test by which it tests its functionality autonomously. The sensor can be configured for carrying out one or several self-tests. If no malfunction is ascertained in the self-test of the sensor that has been carried out, the sensor employs for checking the value documents the operating mode that is provided for checking the value documents, and carries out a check of the value documents in the provided operating mode.
In the self-test it is provided that the sensor reacts to at least one malfunction that is ascertained during the self-test and would hinder the check of the value documents by the sensor employing a modified operating mode for checking the value documents, instead of the operating mode provided for checking the value documents. In the modified operating mode there is employed for checking the value documents at least one other measured value of the sensor than is determined in the operating mode provided for checking the value documents. In contrast to the hitherto customary function failure of the sensor, the sensor can hence continue being operated for checking the value documents despite the malfunction. Although the malfunction would hinder the check of the value documents, the sensor only fails when there is no possibility for the sensor to circumvent the malfunction using the modified operating mode. In many cases the sensor will be able to circumvent the malfunction, so that—instead of failing—it can continue being operated and can carry out a check of the value documents despite the malfunction. By employing a modified operating mode in which another measured value is employed, it is achieved that the number of function failures of the sensor is reduced. Employing another measured value moreover has the advantage that the evaluation of the measured values must only be slightly changed, because the provided measured value merely needs to be replaced by the other measured value, while the evaluation can otherwise remain the same.
The other measured value is e.g. a measured value that is derived from measured values of the sensor that are employed in the provided operating mode but are not affected by the malfunction. For example, the other measured value can be a measured value interpolated or extrapolated from the detected measured values. The other measured value can also be a measured value of the sensor that is not at all determined for checking the value documents in the provided operating mode. The other measured value can e.g. be an additionally detected measured value that is not detected in the provided operating mode but is detected and evaluated in the modified operating mode, or it can be an additionally evaluated measured value that is detected but not evaluated in the provided operating mode. Employing an additional measured value has the advantage that measuring information lost through the malfunction can be at least partly compensated by the additional measured value.
In the modified operating mode, a measured value that is affected by the malfunction and was to be employed in the provided operating mode can be omitted. For example, the affected measured value is also detected in the modified operating mode but is not taken into consideration in the evaluation of the measured values, and the evaluation is carried out on the basis of the remaining measured values which are not affected by the malfunction. Or the measured value affected by the malfunction is not at all detected in the modified operating mode. In the case of an optical sensor, the relevant light source in an illumination sequence can e.g. be omitted, or the relevant measured value not detected or not taken into consideration in the evaluation. In a sensor having several measuring tracks transverse to a transport direction of the value documents, the measured value of the relevant measuring track can be omitted, e.g. ignored upon the evaluation.
While in the provided operating mode the value documents would be checked on the basis of one or several measured values that are affected by the malfunction, in the modified operating mode the value documents are preferably checked exclusively on the basis of measured values of the sensor whose generation is not affected by the malfunction. The check of the value documents is carried out e.g. on the basis of those measured values already provided hitherto that are not affected by the malfunction, and on the basis of one or several additional measured values that are not affected by the malfunction. The modified operating mode differs e.g. by an excitation that is modified in comparison to the provided operating mode, e.g. by employing another light source in the case of an optical sensor.
In the self-test it can be provided that the sensor reacts to one or several malfunctions in this way. In the case of different such malfunctions the sensor can react in the same or in a different way. Furthermore, there can also be further malfunctions of the sensor that result in a function failure of the sensor, e.g. when the sensor has no modified operating mode available for this malfunction in order to circumvent the malfunction.
Upon ascertainment of a malfunction the sensor can store information about the ascertained malfunction in an error memory, in order for the information on the ascertained malfunction to be available later. If the sensor ascertains a malfunction and the ascertained malfunction would not hinder the check of the value documents, the sensor carries out the check of the value documents in the provided operating mode. In this case no circumvention of the malfunction is necessary and the ascertained malfunction can be ignored in the check of the value documents.
If the sensor ascertains during its self-test a malfunction that would hinder the check of the value documents and the malfunction is circumventable, the sensor reacts by replacing the provided operating mode by the modified operating mode or suitably modifying the provided operating mode for checking the value documents, and carrying out the check of the value documents in the modified operating mode. Moreover, it can be provided that the sensor reacts to the same malfunction in certain other cases when the malfunction is not circumventable by the sensor outputting an error message indicating a non-functionality of the sensor or of a certain function of the sensor. The sensor can display the error message itself and/or send it via the communication interface to the apparatus in order to display the error message and/or process it further.
The self-test is carried out in particular by a sensor already installed in an apparatus for checking value documents. The sensor carries out the self-test e.g. in the interim between the check of value documents to be successively checked. Additionally or alternatively, the sensor can also carry out the self-test before the onset of the value-document check, e.g. when the sensor or the apparatus is started up. The information on how the sensor can react to the respectively ascertained malfunction is stored e.g. in the data memory of the sensor. It is advantageous here that the sensor can perform its self-test fully autonomously and the sensor e.g. requires no data exchange with its environment for this purpose. Alternatively, the information on the different reactions can also be fed to the sensor from outside, however, e.g. through the above-mentioned apparatus.
When the sensor is an optical sensor, the self-test comprises e.g. a test of the function of at least one light source of the sensor and/or of at least one photodetector of the sensor. For testing the function of the light source and/or of the photodetector, a portion of the light of the light source that is reflected on a window of the sensor is detected by the photodetector while no value document is present in the capture region of the sensor. Because this self-test requires no test medium and no value document, the self-test of the sensor is already possible before the onset of the value-document check. Moreover, this self-test can also test measuring tracks of the sensor that lie outside the value document to be checked. The hitherto customary employment of a test medium does not enable such edge measuring tracks to be tested, in contrast. For testing the function of the light sources, the portion of the light of the light source that is reflected on a window of the sensor is detected by that photodetector that also detects the light emanating from the value document for checking the value documents. Hence, no additional detector needs to be provided for the purpose of checking the light sources during the self-test. The self-test of the sensor by which the function of the light sources and/or of the photodetectors is tested can be carried out in the gap between two value documents transported successively past the sensor. In particular, the self-test can be carried out in each of these gaps or regularly after a certain time or number of value documents, or the self-test can be carried out before a change to other value documents.
When the self-test of the sensor is carried out in this way, it comprises not only a test of the function of the light sources, but automatically also a test of the function of the photodetector. Using logical analyses it can be found out which of the light sources and/or of the photodetectors are affected by the malfunction. When e.g. the light of several light sources is detected successively with a certain photodetector, and the photodetector detects an insufficient signal upon the switch-on of each of these light sources, one can infer a malfunction of the photodetector or of the electronic circuit connected thereto. When the photodetector only detects an insufficient signal for one of these light sources, however, one infers a malfunction of this light source or its power supply or drive. A malfunction can already be ascertained on the basis of one insufficient measured value, or only through several measured values that indicate a malfunction. The sensor can additionally or alternatively also carry out different kinds of self-tests and identify malfunctions using other methods. Depending on which malfunction is ascertained and whether or not it is circumventable, the sensor might employ one of its modified operating modes for checking the value documents.
When the sensor is an optical sensor that detects the light emanating from the value documents at several wavelengths, there can be employed for checking the value documents in the modified operating mode at least one measured value that is detected at another wavelength than the measured values that are provided in the provided operating mode for checking the value documents. In particular, the measured value detected upon illumination with another wavelength can be employed. In the case of spectrally different light sources, this can be obtained e.g. by a spectrally different illumination and, where applicable, an accordingly adapted evaluation. In the case of spectrally different photodetectors, a measured value can be detected and evaluated at another wavelength with identical illumination. Advantageously, upon the check of a spectrally broad-band feature of the value documents, the function failure of a light source or of a photodetector can be avoided when a spectrally adjacent light source or a spectrally adjacent photodetector is also suitable for checking the feature.
When the sensor is an optical sensor that detects the light emanating from the value documents at several wavelengths, there can be employed for checking the value documents in the modified operating mode at least one derived measured value, e.g. instead of the measured value affected by the malfunction. The employment of a derived measured value, e.g. interpolated or extrapolated measured value, has the advantage that the evaluation can remain substantially the same, because the deriving step only needs to be added before the evaluation while the evaluation can otherwise remain the same. The interpolated measured value is interpolated e.g. from the detected measured values that are detected on both sides spectrally adjacent to the measured value affected by the malfunction. For example, in the case of a malfunction of one of the light sources there is employed, instead of the measured value that is detected upon illumination with the malfunctioning light source, an interpolated measured value that is interpolated from measured values that are detected upon illumination with light sources that are spectrally adjacent to the malfunctioning light source. In the case of spectrally resolved detection, the measured values of spectrally adjacent photodetectors are accordingly interpolated.
An optical sensor having several light sources can employ for checking the value documents in the modified operating mode, in the case of a malfunction of one of the light sources, one or several other light sources than is determined in the provided operating mode. The illumination can for this purpose be changed over to one or several other light sources. The check of the value documents is carried out e.g. exclusively using those light sources that are not affected by the malfunction. Instead of the light source affected by the malfunction there can be employed the spectrally identical wavelength, if present in the sensor. Otherwise there can also be employed one or several light sources of another wavelength whose spectrum differs from the light source provided in the provided operating mode.
If the sensor is a sensor having several measuring tracks transverse to a transport direction of the value documents along which the value documents are transported past the sensor for their check, there can be employed for checking the value documents in the modified operating mode, in the case of a malfunction of one of the measuring tracks, a derived measured value instead of the measured value of the malfunctioning measuring track. The derived value can be interpolated from the measured values of the measuring tracks adjacent to the malfunctioning measuring track. Advantageously, the evaluation can remain substantially the same in this case, too, because only an interpolation step before the evaluation is necessary. The sensor can employ for checking the value documents in the modified operating mode, in the case of a malfunction of one of the measuring tracks, instead of the measured value of the malfunctioning measuring track, the measured value of another measuring track that is most closely adjacent to the malfunctioning measuring track. This makes it possible to avoid function failures of the sensor when checking spatially extensive features of the value documents.
The invention also relates to a sensor which is configured for checking value documents and which is configured, e.g. programmed, for carrying out the self-test according to the invention in which the sensor tests its functionality. In the self-test of the sensor it is provided that the sensor reacts to a malfunction that is ascertained during the self-test and would hinder the check of the value documents by the sensor employing for checking the value documents, instead of the provided operating mode, a modified operating mode in which at least one other measured value of the sensor is employed for checking the value documents than is determined in the operating mode provided for checking the value documents. The sensor is moreover so programmed to subsequently carry out the check of the value documents in the modified operating mode. In a data memory of the sensor there can be stored one or several provided operating modes or information thereon as well as one or several modified operating modes or information thereon from which the sensor can take or derive how to react to the respective malfunction. The data memory can be integrated in the housing of the sensor, or the data memory is a data memory present outside, e.g. data memory of the apparatus to which the sensor is connected.
To enable a check of different features, e.g. authentication features, of value documents or of different kinds of value document, different operating modes can be provided in the sensor. For this purpose, the data memory of the sensor has for example stored therein for each of the different features a respective provided operating mode or information on the respective provided operating mode which the sensor employs for checking the respective feature. Before the sensor carries out its self-test, at least one of the features that is to be checked by the sensor can be selected. Upon the self-test of the sensor it can be provided that the sensor rates a malfunction ascertained in the self-test differently in dependence on the selected feature and that the sensor reacts to the malfunction ascertained in the self-test differently in dependence on the selected feature. There can furthermore also be malfunctions for which it is determined that the sensor always reacts in the same way, independently of the selected feature. In the case of an optical sensor, the sensor can rate the malfunction differently and react to the malfunction differently for example in dependence on the spectral properties of the selected feature, in particular in dependence on the spectral position and/or the spectral pattern of the feature. If the sensor is a sensor having several measuring tracks transverse to a transport direction of the value documents, the sensor can rate the ascertained malfunction differently and react differently to the ascertained malfunction in dependence on the position of the respective feature on the value document. The information on the different reactions that the sensor is to perform upon the ascertained malfunction, in dependence on the selected feature, is stored e.g. in the data memory of the sensor. From this information the sensor can derive or take the different reactions.
In the self-test of the sensor it can be provided that the sensor rates a malfunction ascertained during its self-test differently in dependence on the selected kind of value document and that the sensor reacts to the malfunction ascertained during the self-test differently in dependence on the selected kind of value document. Kinds of value document are understood to be e.g. bank notes, checks, identity cards, credit cards, check cards, tickets, vouchers or a certain sort or version of the same. The kind of value document can also be a selection of several different sorts of value documents, e.g. value documents with certain features or value documents with certain size specifications. In the case of bank notes, the kind of value document can be the denomination, the currency, the emission or a statement about a selection of different denominations and/or currencies.



BRIEF DESCRIPTION OF THE DRAWINGS
Hereinafter the invention will be explained by way of example with reference to the following figures. There are shown:
FIG. 1a a sensor for checking value documents which carries out a self-test,
FIG. 1b a kind of value document W equipped with two features, and two kinds of value document W1, W2 which are respectively equipped with one feature,
FIG. 2a spectral distribution of the light emanating from a feature of a value document, for two features,
FIG. 2b four malfunctions and appurtenant different reactions of the sensor, for two different features,
FIG. 3 flowchart on the run of the self-test.



DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS
FIG. 1 shows a sensor which carries out a self-test according to the invention. As an example there was chosen an optical sensor 100 which has measuring elements 4, including one or several light sources 41 and one or several photodetectors 43, as well as, where applicable, further optical elements such as e.g. lenses, filters, etc. A value document 10 to be checked is checked while it is being transported past the sensor 100 along a transport direction T. For the check of the value document 10 the latter is, when it is located in the capture region of the sensor, illuminated by the light emitted by the light source(s) 41, and the light that is sent off by the value document as a result of the illumination is detected using the photodetector(s) 43. What is detected is e.g. the luminescent light or remitted light of the value document 10. The optical sensor 100 is configured in this example for detecting the light sent off by the value documents at several different wavelengths from λ1 to λ7, cf. FIG. 2a. For this purpose, the detector has e.g. several light sources with different emission spectra or several photodetectors with spectrally different sensitivities, e.g. photodetectors equipped with different filters.
FIG. 2a shows for two features M1 and M2 of value documents, e.g. authentication features, the respective spectral intensity distribution of the light that is sent off by a value document having the respective feature. FIG. 1b top shows by way of example a kind of value document W which is equipped with the two features M1, M2. FIG. 1b middle shows another kind of value document W1 which has only the feature M1, and FIG. 1b bottom shows a further other kind of value document W2 which is equipped only with the feature M2. Because the two features M1, M2 are present at different positions on the respective value document, different measuring tracks are relevant for the check of the two features M1 and M2.
The sensor 100 has a control device 3, e.g. a processor, which controls the measuring elements 4 for carrying out the self-test as well as for checking the features and evaluates the thereby detected measured values according to the respective operating mode. The control device 3 is connected to a data memory 5 of the sensor which has stored therein information on the different operating modes of the sensor 100 for one or several features Mn=M1, M2 . . . . These include the wavelengths and measuring tracks provided for the check of the respective feature Mn as well as information on the evaluation that is to be applied for checking the respective feature.
In the data memory 5 there is contained for example the information on the operating modes from Table 6, cf. FIG. 2b, through which the features M1, M2 have associated therewith the operating modes B1, B2 that the sensor is to employ upon the check of the respective feature. The information on the operating mode comprises here the wavelengths and measuring tracks to be evaluated upon the check of the respective feature, and the evaluation to be employed. In this example, measured values for the same wavelengths λ1-λ7 and measuring tracks L1-L10 are to be detected in both operating modes B1 and B2, but different evaluations carried out. In the operating mode B1 for checking the feature M1, the wavelengths λ1, λ3, λ5, λ6 and λ7 are provided for evaluation, namely in the measuring tracks L8 to L10. And in the operating mode B2 for checking the feature M2, the wavelengths λ1 to λ5 are provided for evaluation, namely in the measuring tracks L3 to L10. Further, Table 6 also contains information on the reactions R1, R2, . . . of the sensor to ascertained malfunctions. Furthermore, the data memory 5 can contain the information of Table 9 through which the sensor can establish from a selected kind of value document Wn the appurtenant features Mn. In addition, the data memory 5 can also store further information for checking the features, e.g. reference data of the respective feature with which the detected measured values are compared upon the check of the feature.
The sensor 100 further has a communication interface 2 via which it can receive and output information. To initiate the check of a certain feature, information can be fed to the sensor 100 before the value-document check via the communication interface 2 about which of the different features Mn or which of the kinds of value document Wn is to be checked by the sensor. For example, it is fed to the sensor via the communication interface 2 that it is to check the feature M1. To inform the sensor of the features to be checked, it suffices for the kinds of value document W1 and W2 to inform the sensor only of the kind of value document. For this purpose, only the information is e.g. fed to the sensor about which kind of value document Wn it is to check. For example, the sensor is informed that the kind of value document W1 is to be checked. Using the information 9 deposited in the data memory the sensor can unambiguously establish from this kind of value document W1 the feature M1 to be checked, and analogously M2 from W2.
Alternatively, the sensor can also carry out the value-document check without a previous selection of a feature Mn or of a kind of value document Wn, e.g. when the sensor 100 has provided therein only one operating mode in which the sensor checks one or several certain features Mn. For example, only the operating mode B1 is provided, so that there is no selection of the feature M1. Upon certain malfunctions for which a circumvention is provided, a modified operating mode BP is then employed, cf. FIG. 2b. 
At the time represented in FIG. 1a, there is no value document 10 located in the capture region of the sensor 100. In this gap between the value documents 10, the function of the light sources 41 of the sensor is e.g. tested during the self-test. For this purpose, the light sources 41 are switched on individually one after the other in the gap between two value documents 10, and the light of the light sources partly reflected back on the window 8 of the sensor is respectively detected using the photodetector 43. On the basis of the light intensity detected from the individual light sources the sensor 100 ascertains whether or not a malfunction of the respective light source 41 is present. A malfunction of a light source is ascertained e.g. when the detected light intensity of the light source undershoots a certain minimum value. Analogously, the function of the photodetectors 43 can also be tested. The self-test can additionally or alternatively comprise a test of electronic components of the sensor, e.g. by checking an electrical voltage. The sensor can also employ the respective modified operating mode upon a malfunction of a component on whose function the light source or the photodetector depends.
For carrying out the self-test of the sensor 100 one can proceed e.g. according to the flowchart represented in FIG. 3. Before the self-test is carried out the sensor can, if this is provided, be informed via the communication interface 2 of the feature Mn to be checked or of the kind of value document Wn to be checked. This can be effected before or during the value-document check. The sensor 100 then carries out the self-test before or during the value-document check, e.g. in the gap between two value documents. In the checking step S10 the sensor decides on the result of the self-test: If the sensor passes the self-test, the check of the selected feature Mn is carried out on the relevant value documents having the feature Mn. If the sensor ascertains a malfunction F, however, the self-test is not passed. A non-passing of the self-test does not automatically lead to a non-functionality of the sensor, however. For the sensor checks whether or not the ascertained malfunction F is relevant for checking the selected feature Mn (checking step S20). Corresponding information 6 is deposited in the data memory 5, cf. FIGS. 1a and 2b. If the malfunction F does not hinder the check of the feature Mn, the check of the selected feature is carried out as provided. However, a corresponding entry is written to the error memory 7 of the sensor. If the sensor ascertains in the checking step S20 that the ascertained malfunction F hinders the check of the selected feature Mn, the sensor distinguishes between the two cases of whether or not the malfunction F is circumventable for the selected feature Mn (checking step S30). For this purpose, the sensor 100 tests whether its data memory 5 contains information for the selected feature Mn about how to deal with the ascertained malfunction F in the case of the feature Mn, e.g. whether for the selected feature Mn information is contained for a modified operating mode for circumventing the ascertained malfunction F. If no modified operating mode is provided for the selected feature Mn for circumventing the malfunction F, the sensor ascertains that it is not functional for checking the feature Mn and emits a corresponding error message e.g. via the communication interface 2 to the outside and writes a corresponding entry to the error memory 7. For example, the check of the value documents is thereupon stopped. Upon the check of different kinds of value document, the value documents in which this feature Mn is to be checked can also be rejected upon the check (reject stacking), while the remaining kinds of value document are checked as provided. However, if the sensor 100 finds in the Table 6 information on a modified operating mode through which the ascertained malfunction F is circumventable for the selected feature Mn, it chooses this modified operating mode. In this way the sensor circumvents the ascertained malfunction F, and the check of the selected feature Mn is carried out in the modified operating mode.
The data memory 5 of the sensor contains e.g. the information stated in Table 6 on the reactions of the sensor to a malfunction ascertained during the self-test, cf. FIG. 2b. On the basis of this information the sensor decides how to react to the ascertained malfunction for the respectively selected feature Mn. FIG. 2b specifies four examples of malfunctions F1-F4 and respective information on how the sensor is to rate one and the same malfunction differently in dependence on the feature M1 or M2 and respectively react differently thereto:
Malfunction F1:
In the self-test of the sensor a malfunction F1 of the light source λ3 is ascertained. If the feature M1 was selected, the sensor ascertains on the basis of the information on the operating mode B1 that this malfunction F1 would hinder the functionality of the sensor. On the basis of the information of Table 6, however, the sensor ascertains that the malfunction F1 can be circumvented for the feature M1 by employing for checking the feature M1—instead of the measured value detected upon illumination with the malfunctioning light source λ3—another measured value, namely the measured value detected upon illumination with the functional light source λ4, cf. FIG. 2a. In the case of the feature M2 the sensor takes from Table 6 that the malfunction F1 is relevant for the check of this feature M2 and not circumventable. The sensor thereupon emits an error message that it is not functional for checking the feature M2 or the relevant value documents due to the malfunction F of the light source λ3.
Malfunction F2:
In the self-test of the sensor a malfunction F2 of the light source λ5 is ascertained. If the feature M1 was selected, the sensor ascertains on the basis of the information on the operating mode B1 that this malfunction F2 would hinder the functionality of the sensor. On the basis of the information of Table 6 the sensor further ascertains that the malfunction is not circumventable for this feature M1. The sensor thereupon emits an error message that it is not functional for checking the feature M1 or the relevant value documents due to the malfunction F2 of the light source λ5. In the case of the feature M2, however, the malfunction F2 can be circumvented by the measured value detected upon illumination with the malfunctioning light source λ5 not being employed for checking the feature M2, as provided in the operating mode B2, but rather the relevant measured value being dispensable, cf. FIG. 2a. 
Malfunction F3:
In the self-test of the sensor a malfunction F3 of the light source λ6 is ascertained. If the feature M1 was selected, the sensor ascertains on the basis of the information on the operating mode B1 that this malfunction F3 would hinder the functionality of the sensor. On the basis of the information of Table 6, however, the sensor ascertains that the malfunction is circumventable for this feature M1 by employing another measured value, namely by interpolating the measured values that are detected at λ5 and λ7. The measured value detected upon illumination with λ6 is then, for the evaluation, replaced by the measured value λ6* calculated by means of interpolation. In the case of the feature M2 the sensor takes from the information on the operating mode B2 that the check of the feature M2 is not hindered by the malfunction F3. The check of the feature M2 or of the relevant value documents can be carried out with the provided operating mode B2.
Malfunction F4:
In the self-test of the sensor a malfunction F4 of the measuring track L5 is ascertained, e.g. a malfunction of the photodetector of the measuring track L5. If the feature M1 was selected, the sensor ascertains on the basis of the information on the operating mode B1 that this malfunction F4 does not hinder the check of the feature M1. The check of the feature M1 or of the relevant value documents can be carried out with the provided operating mode B1. In the case of the feature M2 the sensor takes from Table 6 that the malfunction F4 would hinder the functionality of the sensor, but is circumventable for this feature M2 by employing another measured value, namely by interpolating the measured values that are detected in measuring track L4 and in measuring track L6. The measured value from measuring track L5 is then, for the evaluation, replaced by the measured value L5* calculated by means of interpolation.
During the self-test there are always tested for example, independently of the selected feature, all light sources or photodetectors of the sensor. However, the results of the self-test are rated differently in dependence on the kind of value document, cf. FIG. 2b. To accelerate the self-test, different self-tests can also be carried out in dependence on the selected feature. By the self-test there can be tested e.g. only those respective light sources or photodetectors that are provided for checking the respectively selected feature. If the feature M1 was selected, the self-test can be limited to the light sources of the wavelengths λ1, λ3, λ5, λ6 and λ7 in the measuring tracks L8 to L10. And if the feature M2 was selected, the self-test can be limited to the light sources of the wavelengths λ1 to λ5 in the measuring tracks L3 to L10.

## Claims

1. A method for checking value documents using an optical sensor configured to detect light emanating from the value documents, a plurality of variables being measurable by the optical sensor, one of the variables being a preferred variable to measure for checking the value documents, the method comprising:
performing a self-test of the optical sensor;
when the self-test indicates a malfunction of the optical sensor that would hinder an ability of the optical sensor to check value documents, using the preferred variable, using a second one of the variables to check the value documents by measuring, by the optical sensor, the second variable as the value documents pass by the optical sensor;
otherwise using the preferred variable to check the value documents by measuring, by the optical sensor, the preferred variable as the value documents pass by the sensor.

2. The method according to claim 1, wherein the second variable is derivable from measured values of the first variable that are not affected by the malfunction.

3. The method according to claim 1, wherein the second variable is not derivable from measured values of the first variable.

4. The method according to claim 1, wherein the optical sensor has several light sources and is configured to detect the light emanating from the value documents at several wavelengths, and wherein the preferred and second variables respectively correspond to measurements obtained from the value documents using different ones of the light sources.

5. The method according to claim 1, wherein the optical sensor is configured to detect the light emanating from the value documents at several wavelengths, and wherein the preferred and second variables respectively correspond to measurements obtained from the value documents at different wavelengths.

6. The method according to claim 1, wherein the optical sensor is configured to detect the light emanating from the value documents at several wavelengths, and wherein the preferred and second variables respectively correspond to measurements obtained from the value documents at wavelengths that are spectrally adjacent to each other.

7. The method according to claim 1, wherein the self-test comprises a testing of the function of at least one light source of the sensor and/or of at least one photodetector of the sensor while there is no value document present in the capture region of the sensor, and performing the self-test comprises:
reflecting a portion of the light of the light source on a window of the sensor; and
detecting the reflected light by the photodetector.

8. The method according to claim 7, wherein the self-test is carried out in a gap between two value documents transported successively past the sensor.

9. The method according to claim 1, wherein the sensor has several measuring tracks transverse to a transport direction of the value documents along which the value documents are transported past the sensor for their check, and wherein the second variable corresponds to derived measured values derived from measurements obtained from the value documents using measuring tracks that are adjacent to each other.

10. The method according to claim 1, wherein the sensor has several measuring tracks transverse to a transport direction of the value documents along which the value documents are transported past the sensor for their check, and wherein the preferred and second variables respectively correspond to measurements obtained from the value documents using measuring tracks that are adjacent to each other.

11. The method according to claim 1, wherein the sensor is configured for checking different features of value documents, and that, before the self-test is performed, at least one of the features that is to be checked by the sensor is selected, and that the sensor rates a malfunction ascertained in the self-test differently in dependence on the selected feature and reacts to the ascertained malfunction differently in dependence on the selected feature.

12. The method according to claim 11, wherein the sensor rates the ascertained malfunction differently and reacts differently to the ascertained malfunction in dependence on spectral properties of the selected feature.

13. The method according to claim 11, wherein the sensor has several measuring tracks transverse to a transport direction of the value documents along which the value documents are transported past the sensor for their check, and that the sensor rates the ascertained malfunction differently, in dependence on the position of the selected feature on the value document, and reacts to the ascertained malfunction differently, in dependence on the position of the selected feature on the value document.

14. An optical sensor configured to detect light emanating from value documents, the optical sensor comprising:
a window;
at least one light source configured to emit light towards a value document through the window to illuminate the value document; and
at least one photodetector configured to detect light from the value document through the window as a result of the illumination,
wherein a plurality of variables are measurable by the optical sensor, one of the variables being a preferred variable to measure for checking value documents, and wherein the optical sensor is configured to perform a self-test for testing its functionality, such that:
when the self-test indicates a malfunction of the optical sensor that would hinder an ability to check value documents using the preferred variable, using a second one of the variables to check the value documents, by measuring the second variable as the value documents pass by the optical sensor;
otherwise using the preferred variable to check the value documents by measuring the preferred variable as the value documents pass by the sensor.