Device and method for testing flat samples
Patent Information
- Application Number
- EP2023736602
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-06-29
- Filing Date
- 2023-06-23
- Publication Date
- 2025-05-07
Smart Images
Figure 1.1
Abstract
Description
[0001]Device and method for testing flat samples The invention relates to a device and a method for testing flat samples, in particular value documents or semi-finished products used to produce the value documents, e.g. for testing the authenticity of value documents or for testing the quality of the value documents or semi-finished products during the production of value documents. To secure value documents, it is known to incorporate machine-verifiable security features into the value documents or to apply them to them. For this purpose, certain luminescent substances or reflectance inks can be used, for example. During the production of such value documents, a quality check is generally provided, e.g. to ensure that the finished value documents contain the security feature in a predetermined quantity and / or with predetermined properties.For this purpose, it may be provided that the valuable documents or semi-finished products used to produce them, such as paper sheets or webs, are at least randomly inspected in a production facility or in a separate quality control facility and are only released as fit for circulation if they meet specified criteria. All other valuable documents or semi-finished products are sorted out as rejects and, if necessary, destroyed. Furthermore, valuable documents already in circulation are usually subjected to an optical authenticity check from time to time using valuable document processing devices in order to identify and sort out any counterfeit or suspected counterfeit valuable documents.In both an optical quality check and an optical authenticity check, the value document can be irradiated with light and the optical radiation emanating from the value document, in particular remission or luminescence light, can be detected and analyzed by a sensor module in order to check the security feature. In production devices for value documents or in quality control devices or in value document processing devices, the optical inspection of the security features is usually carried out by means of a stationary optical sensor module, past which the value documents or semi-finished products are transported and which is arranged in the respective device at a distance from the value document or semi-finished product to be checked. When the value documents or semi-finished products are transported past the sensor module, it can happen that the value documents orSemi-finished products may lift or flutter slightly out of the desired transport plane, which can lead to distance fluctuations in relation to the optical sensor module. If the measurement signal of the optical sensor module depends on the distance between the valuable documents or semi-finished products being checked, the lifting or fluttering can falsify the measurement signal. Therefore, during quality inspection or authenticity testing, a deviation from the expected measurement signal or a specific acceptance range must be permitted. A large acceptance range leads to a less stringent quality inspection or authenticity testing and therefore carries the risk that otherwise unacceptable quality deviations or some counterfeit valuable documents could be overlooked. It is known that the fluttering or lifting of valuable documents from their transport plane can be reduced by providing the transport path of the valuable documents or semi-finished products with mechanical limitations, e.g., guide elements.However, such mechanical limitations may not be arranged arbitrarily close to the transport path in the above-mentioned devices, as this could lead to transport disruptions or damage to the value documents or semi-finished products, particularly when they are transported past the sensor module at high speed. It is an object of the invention to provide a method and a device for testing flat samples, such as value documents or semi-finished products used to produce the value documents, whereby a more stable measurement signal can be achieved. This object is achieved by a method and a device for testing the flat samples according to the independent claims. The flat samples to be tested are, in particular, value documents or semi-finished products used to produce the value documents. The device has an optical sensor module and is suitable for optical testing, e.g.for authenticity or quality testing, of the respective flat sample by means of the optical sensor module, wherein the device provides or defines a target measuring plane for the flat sample, in which the flat sample can be introduced for its optical testing. Preferably, the flat sample is introduced into the target measuring plane for its optical testing - viewed along the sample normal - in such a way that a detection area of the flat sample recorded when the measurement signal is recorded (e.g. located on the sample surface) lies (at least approximately) in the target measuring plane. In particular, the optical sensor module is arranged in the device or built into it. The target measuring plane is arranged outside the sensor module and lies, for example, within the device or at least adjacent to it such that the sensor module of the device can record measurement signals from the flat sample.The optical sensor module is designed for the optical inspection of the flat sample to record a measurement signal from the flat sample that corresponds to the intensity of optical radiation, in particular remission or luminescence light, of the flat sample when the sample is located in the target measurement plane or at least approximately in the target measurement plane, in particular while the flat sample (e.g. in the device) is transported past the sensor module along a transport path that lies in or at least approximately in the target measurement plane, e.g. by means of a transport device of the device. The optical sensor module is designed to carry out the optical inspection, e.g. authenticity or quality inspection, of the flat sample based on the measurement signal and, if applicable, further measurement signals from the flat sample recorded in this way. The sensor module is arranged at a module distance from the target measurement plane or from the transport path.A window is arranged between the sensor module and the flat sample, through which window both the illumination / excitation light radiated onto the flat sample (from a light source of the sensor module or from a light source of the device) and the optical radiation reaching the sensor module from the flat sample, in particular remission or luminescence light which emanates from the flat sample as a result of the illumination / excitation light, are transmitted. The window is arranged at a window distance from the desired measurement plane, and the window distance is selected to be so small that the measurement signal of the flat sample in the desired measurement plane is increased by a backreflection effect of the window, preferably by at least 10%, compared to a corresponding measurement signal occurring or recordable without the backreflection effect from the flat sample or sample located in the desired measurement plane.in comparison to a corresponding device in which the window - e.g. due to its greater distance from the flat sample - does not cause a back reflection effect. The back reflection effect is based on the law of optical reflection and is explained in detail below. Preferably, the window arranged between the sensor module and the flat sample is arranged at a window distance from the desired measuring plane of at least 0.3 mm, preferably of at least 0.5 mm, and particularly preferably of at most 3 mm, e.g. of at most 2 mm. The window is designed as a transparent solid, e.g. as a glass plate. Both the illumination / excitation light radiated from the sensor module onto the flat sample and the optical radiation reaching the sensor module from the flat sample, in particular the remission or luminescence light reaching the sensor module from the flat sample, are transmitted through the same window.The module distance of the sensor module from the target measuring plane is selected to be so small that a signal variation in the measurement signal of the flat sample as a function of the measurement distance deviation in the area of the target measuring plane is reduced by the back reflection effect of the window compared to a signal variation in the measurement signal of the flat sample occurring without the back reflection effect as a function of the measurement distance deviation in the area of the target measuring plane or compared to a corresponding device in which the window - e.g. due to its greater distance from the flat sample - causes no or only a negligible back reflection effect. By specifically reducing the module distance of the sensor module from the target measuring plane, and utilizing the back reflection effect of the window, the signal variation of the measurement signal as a function of the measurement distance deviation in the area of the target measuring plane is significantly reduced.This results in a more stable measurement signal that depends only slightly on the exact distance between the flat sample and the sensor module. Nevertheless, a sufficiently high measurement signal is achieved to enable measurement of even weak measurement signals with a good signal-to-noise ratio. Preferably, the module distance of the sensor module from the target measurement plane is at least 2 mm. To compensate for manufacturing tolerances, the module distance can be individually set for each sensor module (e.g. for the individual sensor modules of a sensor series), for example, to minimize the signal variation of the measurement signal as a function of the measurement distance deviation (sensor-specific). For example, each device has an individual module distance for the respective sensor module that differs from other devices of the same series.In particular, the device can be configured to adjust the module spacing of the sensor module, for example, by mounting the sensor module on a travel table or using elongated holes in the device. The optical sensor module has at least one detector device for receiving the measurement signal, which is configured to detect the optical radiation emanating from the flat sample, in particular remission or luminescence light, and optionally at least one light source configured to radiate the illumination or excitation light onto the flat sample. Preferably, the detector device is arranged such that its optical axis runs perpendicular to the desired measurement plane or perpendicular to the sample normal of the flat sample. The sensor module, in particular the detector device(s) and / or the light source(s), and the window are, for example,arranged and designed in such a way that the back-reflection effect of the window is contributed to - that a part of the illumination / excitation light is reflected or scattered by the flat sample towards the window and then, in particular at a large angle of incidence at the window (approx. > 50° to the sample normal), is partially reflected back from the window to the flat sample due to the law of reflection and causes the flat sample to emit additional optical radiation, in particular additional remission or luminescence light, which is reflected or scattered in the direction of the window in such a way that it can be detected by the sensor module or its detector device(s) through the window and contributes to the measurement signal, and / or - that optical radiation emanating from the flat sample, in particular remission or luminescence light caused by the illumination / excitation light (which e.g.is emitted isotropically from the flat sample in all directions) hits the window and, particularly at a large angle of incidence (approx. > 50° to the sample normal), a portion of the optical radiation, in particular the remission or luminescence light, is reflected back to the flat sample due to the law of reflection at the window and is then reflected or scattered by the flat sample in the direction of the window in such a way that it can be detected by the sensor module or its detector device(s) through the window and contributes to the measurement signal. Preferably, the sensor module is designed to record one or more local measurement signals from the flat sample at one or more discrete positions on the flat sample, e.g. along one or more spaced-apart measurement tracks. In particular, the sensor module does not comprise an image sensor orthe sensor module is not designed to record an image of the flat sample. In particular, the sensor module or the device is designed to direct the illumination / excitation light (of the light source) in the form of converging or focusing light beams (through the window) onto the flat sample, wherein at least some of the converging light beams of the illumination / excitation light converge at an angle of at least 10°. For example, the outer light beams of the illumination / excitation light converge at an angle of at least 10°, e.g., conically. In particular, the illumination / excitation light is not directed onto the window and the flat sample using a parallel beam path.For example, the device or the sensor module is designed to direct the illumination / excitation light (of the light source) onto the flat sample at an angle that deviates from the sample normal (direction perpendicular to the sample). The illumination / excitation light is then not directed in the direction of the sample normal, but obliquely through the window onto the flat sample. The flat sample or the detection area of the flat sample recorded when the measurement signal is recorded can be located at a measurement position that is spaced from the target measurement plane by a measurement distance deviation when the measurement signal is recorded. In particular, the device is designed to introduce the flat sample into the target measurement plane or into the area of the target measurement plane in such a way that the flat sample orthe detection area of the flat sample recorded when the measurement signal is recorded can be located at a measurement position that is spaced from the target measurement plane by a measurement distance deviation. The measurement position of the flat sample (or the detection area of the flat sample recorded when the measurement signal is recorded) can therefore have an actual measurement distance from the sensor module when the measurement signal is recorded that deviates from the module distance. The actual measurement distance a of the flat sample corresponds to the sum of the module distance d and a measurement distance deviation y: a=d+y. In particular, the device can be designed to introduce the flat sample into the target measurement plane or into the region of the target measurement plane so inaccurately that the flat sample can be located slightly outside the target measurement plane (at least temporarily or in sections when the measurement signal is recorded).Inaccurate insertion is often unavoidable, e.g., when the flat sample is transported past the sensor module for testing. For example, in the case of a moving flat sample, the transport of the flat sample by means of a transport device is usually such that the measuring position of the flat sample can deviate from the target measuring plane or the actual measuring distance can fluctuate when measuring the sample. The flat sample is transported past the sensor module in such a way that the flat sample (or the detection area of the flat sample recorded when the measuring signal is recorded) can be at a measuring position when the measuring signal is recorded that lies outside the target measuring plane / is at a distance from the target measuring plane or has a measuring distance deviation from the target measuring plane. The actual measuring distance can fluctuate, e.g., due to fluttering movement of the flat sample.In particular, the measurement signal of the flat sample (which can be detected taking into account the back reflection effect) has a maximum value as a function of the measurement distance deviation y, and the module distance of the sensor module from the target measurement plane is selected to be so small that the maximum value m of the measurement signal is or would be reached at a maximum measurement position pm of the flat sample that lies outside the target measurement plane, in particular that lies behind the target measurement plane, i.e. on the side of the target measurement plane facing away from the sensor module. The measurement distance of the flat sample at which the maximum value of the measurement signal (detectable taking into account the back reflection effect) would be reached deviates from the module distance d, in particular by at least 0.2 mm.The maximum measurement position pm of the flat sample, at which the maximum value m of the measurement signal would be reached as a function of the measurement distance deviation, is preferably at least 0.2 mm further away from the sensor module than the target measurement plane. In other words, the measurement distance of the flat sample at which the maximum value of the measurement signal (detectable taking into account the back reflection effect) would be reached is, in particular, at least 0.2 mm greater than the module distance d. Additionally or alternatively, the module distance of the sensor module from the target measurement plane is selected to be so small that the maximum value of the measurement signal of the flat sample that can be detected without taking the back reflection effect into account would also be reached at a measurement position of the flat sample that lies outside the target measurement plane, in particular behind the target measurement plane, i.e.on the side of the target measuring plane facing away from the sensor module, whereby this measuring position is preferably at least 0.3 mm further away from the sensor module than the target measuring plane. The measuring signal of the flat sample is subject to a signal variation as a function of the measuring distance deviation y from the target measuring plane. In order to reduce or keep this low, the module distance d of the sensor module from the target measuring plane is selected to be suitably small. Preferably, the module distance of the sensor module from the target measuring plane is selected to be so small that the signal variation of the measuring signal as a function of the measuring distance deviation y in the area of the target measuring plane (viewed in the direction of the sample normal or perpendicular to the window in the range of + / - 1 mm around the target measuring plane E (due to the back reflection effect of the window) is reduced by at least 50% compared to a signal variation occurring without the back reflection effect.compared to a signal variation in the measurement signal of the flat sample as a function of the measurement distance deviation in the area of the target measurement plane that occurs without the back-reflection effect of the window. Alternatively or additionally, the module distance of the sensor module from the target measurement plane is selected to be so small that the measurement signal of the flat sample as a function of the measurement distance deviation for measurement positions of the flat sample whose measurement distance deviation (in the direction of the sample normal / perpendicular to the window) lies within a range of + / -1.0 mm around the target measurement plane (due to the back-reflection effect of the window) exhibits a signal variation of a maximum of 10% with respect to the measurement signal in the target measurement plane.In other words, the module distance of the sensor module from the target measuring plane is preferably chosen to be so small that the signal variation of the measurement signal of the flat sample, which can result if the measurement position of the flat sample deviates from the target measuring plane by up to 1.0 mm in both directions (y = + / - 1.0 mm), is a maximum of 10% with respect to the measurement signal in the target measuring plane. Alternatively or additionally, the module distance of the sensor module from the target measuring plane is chosen to be so small that the measurement signal of the flat sample, as a function of the measurement distance deviation for measurement positions of the flat sample, has a signal variation of a maximum of 10% with respect to the measurement signal in the target measuring plane over the entire section between the window and the target measuring plane (due to the back-reflection effect of the window).In some embodiments, the transport path of the flat sample in the device is mechanically limited on both sides (in the direction of the sample normal / perpendicular to the window) - at least in the area of the measurement position of the flat sample - and the transport path has a transport path width B (in the direction of the sample normal / perpendicular to the window) within which the measurement position of the flat sample can vary. The target measurement plane lies within, e.g., in the center, of the transport path. In order to reduce orTo keep this distance as low as possible, the distance between the sensor module and the target measurement plane is preferably selected to be so small that the measurement signal of the flat sample, as a function of the measurement distance deviation, has a signal variation of a maximum of 15%, preferably a maximum of 10%, relative to the measurement signal in the target measurement plane across the entire transport path width (due to the back reflection effect of the window). For example, the transport path of the flat sample is limited by the window on the side facing the sensor module and by a mechanical limitation or another window on the side facing away from the sensor module. The transport path width is then determined (in the direction of the sample normal / perpendicular to the window) from the distance between the window and the mechanical limitation or the another window.Typically, the sensor module - in the event that it is installed in the device - is assigned a target module distance d0 to the target measurement plane, at which the sensor module - taking into account the optical beam path of the illumination / excitation light from the sensor module through the window to the flat sample and based on the optical beam path of the optical radiation passing from the flat sample through the window to the sensor module, in particular the remission or luminescence light - without including the back-reflection effect of the window, would deliver a maximum measurement signal from the flat sample, i.e. the measurement signal would deliver a maximum as a function of the measurement distance deviation. In order to reduce or keep the signal variation of the measurement signal as a function of the measurement distance deviation low, the module distance d of the sensor module from the target measurement plane is selected to be at least 0.3 mm smaller than the target module distance d0.In some embodiments, the device has a further window behind the target measurement plane, i.e., on the side of the target measurement plane facing away from the window, from which illumination / excitation light transmitted through the flat sample can be reflected back onto the flat sample. The further window is arranged at a further window distance from the target measurement plane, and the further window distance is selected to be so small that the measurement signal of the flat sample is increased by an additional back-reflection effect of the further window, in particular by at least 2%, compared to a measurement signal occurring or recordable without the additional back-reflection effect of the further window.For example, the illumination / excitation light transmitted through the flat sample can be reflected back at the further window and thus be directed again onto the flat sample, causing it to emit optical radiation, in particular remission or luminescence light. Alternatively or additionally, optical radiation emitted by the flat sample in response to the illumination / excitation light, in particular remission or luminescence light, which is emitted in the direction of the further window, can be reflected back at the further window towards the flat sample, transmitted through the latter and the window, and reach the sensor module, be detected by the sensor module, and contribute to the measurement signal of the sensor module. The sensor module can have an evaluation device designed to analyze the flat sample based on one or more recorded measurement signals from the flat sample, e.g.an optical security feature of the planar sample, in particular its authenticity or quality. The above-mentioned device can be a value document processing device that is configured to check planar samples designed as value documents by means of the optical sensor module, in particular to check the authenticity or quality of the checked value documents, e.g., to check an optical security feature of the value documents. The device can also be configured to sort the checked value documents. Alternatively, the above-mentioned device can be a device configured to produce value documents or to produce semi-finished products used in the production of value documents, in which or by which the value documents or semi-finished products are checked by means of the optical sensor module, e.g.,a device for producing a substrate web for value document substrates or a device for producing value document sheets comprising a plurality of value documents or semi-finished products. For example, it is a device for producing a paper web from which value document substrates can be produced, or a sheet-fed printing device for producing value document sheets comprising a plurality of value documents or semi-finished products. The device is set up to test, in particular quality test, the value documents or semi-finished products using the optical sensor module, e.g. to test an optical security feature which the tested value documents or semi-finished products have or with which the tested value documents or semi-finished products were provided in the production device. The optical security feature can be incorporated into or applied to the value documents or semi-finished products. The measurement signal of the flat sample can be from the sensor module orbe output by the device, e.g. to an external location or displayed for an operator and / or corresponding information on the result of the test (e.g. valuable document or semi-finished product "OK" or "NOT OK") can be output by the device or the sensor module, e.g. to an external location or for an operator. The invention also relates to a method for testing the flat sample by means of the optical sensor module, which is designed for optical testing of the flat sample. The method can be carried out by one of the devices described above or another device which has the sensor module and provides the desired measuring plane.In the method, the sensor module for optically testing the flat sample records a measurement signal from the flat sample that corresponds to the intensity of optical radiation, in particular remission or luminescence light, from the flat sample when the sample is located in the target measurement plane or at least approximately in (e.g. at a distance of at most + / - 1 mm from) the target measurement plane outside the sensor module, in particular while the flat sample is transported past the sensor module along a transport path (lying in or at least approximately in the target measurement plane). The measurement signal is used to test the flat sample. The sensor module is arranged at a module distance from the transport path or from the target measurement plane.A window is arranged between the sensor module and the flat sample, through which both the illumination / excitation light radiated from the sensor module onto the flat sample and the optical radiation reaching the sensor module from the flat sample, in particular remission or luminescence light (which emanates from the flat sample as a result of the illumination / excitation light), are transmitted. The flat sample (or the detection area of the flat sample captured during the recording of the measurement signal) can be located at a measurement position that is spaced from the target measurement plane by a measurement distance deviation when the measurement signal is recorded.In the method, the window is arranged at a window distance from the target measurement plane, and the window distance is selected to be so small that the measurement signal of the flat sample is increased by the back reflection effect of the window, in particular by at least 10%, compared to a corresponding measurement signal of the flat sample located in the target measurement plane that occurs without the back reflection effect. In addition, the module distance of the sensor module from the target measurement plane is selected to be so small that a signal variation of the measurement signal of the flat sample as a function of the measurement distance deviation in the area of the target measurement plane is reduced by the back reflection effect of the window compared to a signal variation of the measurement signal of the flat sample that occurs without the back reflection effect (as a function of the measurement distance deviation in the area of the target measurement plane), i.e., compared to the case where the window – e.g.due to its greater distance from the flat sample - causes no or only a negligible backreflection effect. The flat sample is, for example, a valuable document or a semi-finished product used in the production of valuable documents, in particular a valuable document substrate that can be used to produce a valuable document, or a valuable document sheet comprising several valuable documents or a substrate web for valuable document substrates, e.g. a paper web that can be used to produce valuable document substrates. Preferably, the recorded measurement signal is characteristic of at least one of the following optical properties of the flat sample, in particular of the semi-finished product or valuable document: remission, luminescence (fluorescence, phosphorescence), Raman scattering, in particular surface-enhanced Raman scattering (SERS), absorption or transmission. When testing the flat sample, the measurement signal is used, for example, toan optical security feature of the flat sample or the value document or the semi-finished product is checked, in particular its presence and / or its type and / or its quantity. Preferably, the sensor module, in particular its evaluation device, is designed to use the measurement signal to determine at least one characteristic property of the security feature introduced or applied into or onto the value document or semi-finished product and to check whether the determined characteristic property of the security feature matches or is at least similar to at least one predetermined property. This allows a reliable conclusion to be drawn about the presence of a specific or desired security feature in the semi-finished product or in the value document.For example, the sensor module, in particular its evaluation device, checks whether the measurement signal corresponding to the intensity of the optical radiation is greater than a specified threshold value and / or within a specified acceptance range. This makes it easy to conclude whether a specific or desired amount of a desired security feature is present in / on the semi-finished product or value document. The tested characteristic property of the security feature or the specified property is, for example, at least one of the following properties of the optical radiation emanating from the security feature of the semi-finished product or value document: i) spectral properties, such as the intensity in specific spectral ranges (fingerprint), the position, intensity or width of spectral maxima, minima or shoulders, absolute or relative to one another; ii) temporal properties, such asthe intensity at specific times relative to an excitation pulse of the irradiation, absolute or relative to each other, a decay or rise time, a curve or a functional form (fit parameter) of the time-resolved intensity, the position or intensity of a temporal intensity maximum; iii) combinations of spectral and temporal properties (e.g. decay times in multiple spectral channels, emission spectrum at multiple measurement times); iv) properties after complex excitation by the irradiation (e.g. multiple excitation wavelengths, complex temporal modulation of the excitation light). Preferably, the security feature is invisible to the naked eye in ambient light. Preferably, a security feature is used in which the optical radiation (remission, luminescence) emanating from the security feature during testing in response to the illumination / excitation light lies in the invisible spectral range.The invisible spectral range includes, for example, the infrared and ultraviolet spectral range, preferably between 100 nm and 380 nm and between 780 nm and 100 µm, in particular between 780 nm and 3 µm. The illumination / excitation light can be in the visible or invisible spectral range. Further advantages, features, and possible applications of the present invention will become apparent from the following description in conjunction with the figures. They show: Fig. 1a-c schematic representations of a value document processing device (Fig. 1a), a section of a production device for value document substrates or for value documents (Fig. 1b), and a sensor module designed for multi-track testing (Fig. 1c), Fig.2a an example of the course of the measurement signal of the flat sample as a function of the measurement distance deviation y from the target measurement plane E for a sensor module arranged at the target module distance d0 to the target measurement plane without back reflection effect, Fig. 2b schematic representation of the previous arrangement of the sensor module at the target module distance d0 to the target measurement plane E of the flat sample, Fig. 2c example of the course of the measurement signal of the flat sample as a function of the measurement distance deviation y from the target measurement plane E for a sensor module arranged at the target module distance d0 with back reflection effect, Fig. 3a-c exemplary course of light rays with the back reflection effect for different window distances g1 (Fig. 3a) and g2 (Fig. 3b) and increase in the measurement signal due to the back reflection effect (Fig. 3c), Fig.4a Example of the course of the measurement signal of the flat sample as a function of the measurement distance deviation y from the target measurement plane E for a sensor module arranged according to the invention at a distance d with a back-reflection effect, Fig. 4b Example of an inventive arrangement of the sensor module at a smaller distance d from the target measurement plane E of the flat sample in the case of only one window, Fig. 5a Increase in the measurement signal due to the back-reflection effect with an additional back-reflection effect through the further window, Fig. 5b Schematic representation of the inventive arrangement of the sensor module at a smaller distance d' from the target measurement plane E in the case of an additional window, Fig. 5c Example of the course of the measurement signal of the flat sample as a function of the measurement distance deviation y from the target measurement plane E for a sensor module arranged according to the invention at a distance d' with a back-reflection effect of both windows, Fig.5d Curve of the measurement signal of the flat sample as a function of the measurement distance deviation y from the target measurement plane E with the back-reflection effect of both windows for different module distances of the sensor module. Figure 1a shows a schematic representation of a value document processing device 1 for value documents, which is designed to check individual value documents 10, e.g., the authenticity or quality of value documents. The value documents 10 are provided in an input compartment 2 of the value document processing device 1 in the form of a value document stack 3.The value documents 10 are withdrawn one after the other from the input compartment by means of a separating device 8 and transported by means of a transport device, for example rollers and / or belts, along a transport direction x past a sensor module 24, which is designed for optical authenticity or quality testing of an optical security feature of the value documents. Depending on whether or not the respective value document meets the criteria applied during the optical test, the respective value document can be transported to a first output compartment 30 or to a second output compartment 31. A control device 40 of the device controls the switches 11, 12 of the device accordingly in order to sort the value documents. Alternatively, the value documents 10 can also be transported via the transport section 13 to other devices of the value document processing device 1.Figure 1b shows a schematic representation of a production device 100 which can be used in the production of value documents and which is designed to produce a paper web 10 for value document substrates. The sensor module 24 is designed for the optical quality inspection of the paper web 10 with regard to an optical security feature. Depending on whether or not the section of the paper web being inspected meets the criteria applied during the optical inspection, a corresponding quality classification can be assigned to the respective section of the paper web so that the respective section of the paper web which does not meet the criteria can be sorted out. If necessary, the respective section of the paper web which does not meet the criteria can be marked for this purpose. The production device 100 can, however, also be a sheet-fed printing device for producing value document sheets 10 which, in matrix form, prints a plurality of value documents orSemi-finished products included. The sheet-fed printing device is, for example, a printing machine for security document sheets, in which an optical security feature is printed by means of a printing device onto the security documents to be produced on the respective security document sheet. The sensor module 24 is arranged in the printing machine downstream of the printing device in order to carry out a print inspection of the security feature printed on the sheet. In the sheet-fed printing device, the security document sheets 10 - analogous to the security documents in Fig. 1a - can be separated from the stack, transported and checked. A quality classification can be assigned to the respective printed sheet 10 so that the respective printed sheet that does not meet the criteria can be sorted out. If necessary, the respective printed sheet that does not meet the criteria can be marked for this purpose. In the example in Fig.1a shows a sensor module 24, but a second sensor module 25 can also be arranged on the opposite side of the transport path, as shown in Fig. 1b. Furthermore, the device 1 can also contain further sensor modules for further checks of the value documents, e.g., an image sensor or a magnetic sensor. In the example in Fig. 1b, instead of the two opposing sensor modules 24, 25, only one of the sensor modules 24 can be used if a one-sided check of the optical security feature is sufficient. The flat sample 10 is transported in the respective device 1, 100 by means of a suitable transport device in the transport direction x past the sensor module 24. Ideally, the respective value document is located in a desired measuring plane E.The sensor module 24 is configured to detect optical radiation emanating from the flat sample as the flat sample is moved past the sensor module 24. The sensor module 24 has at least one light source 22 configured to radiate the illumination or excitation light onto the flat sample, as well as at least one detector device 21 configured to detect the optical radiation emanating from the flat sample. The optical radiation emitted by the flat sample in response to the illumination / excitation light radiated onto the flat sample is, for example, reflected light, luminescence light, or Raman scattered light.At one or more times, when the flat sample 10 is located in the target measurement plane E in the detection range of the sensor module 24, the sensor module records one or more measurement signals from the flat sample 10 that correspond to the intensity of the optical radiation, e.g., remission or luminescence light, of the flat sample. In the present example, it is assumed that the optical radiation is remission or luminescence light from the flat sample. Figure 1c shows a schematic plan view of the sensor module 24, beneath which there is a flat sample 10 to be tested, which, depending on the use of the sensor module 24, can be a value document, a value document or printed sheet, or a paper web. For clarity, the present example schematically shows a flat sample 10 designed as a value document.The sensor module 24 can be designed for a single-track or multi-track measurement of the flat sample in order to detect the measurement signals of the flat sample along one or more tracks SP1 to SP5. For this purpose, the sensor module 24 has a number of detector devices 21 corresponding to the number of tracks, wherein each of the detector devices 21 is assigned to one of the tracks SP1 to SP5. For example, the detector devices 21 are each configured to detect the optical radiation emanating from the flat sample in one or more spectral ranges or spectral channels K1, K2, ... and to forward the corresponding signals to a testing device 23, in which they are further processed or tested. A first light source 22 and optionally an additional second light source 22' are configured in the present example to simultaneously expose all tracks SP1 to SP5 on the flat sample to optical radiation.Alternatively, however, it may also be provided to provide a separate light source 22 and, if applicable, 22' for each of the tracks SP1 to SP5. Fig. 2a shows the result of a simulation calculation of the previously customary optical beam path of the sensor module 24, which shows the expected course of a measurement signal of the flat sample 10 located in the target measurement plane E, which can be detected by the sensor module 24 through the window 4, as a function of the measurement distance deviation y from the target measurement plane E (which is at y=0). The influence of the window 4 on the direct beam path, the shape (focus position, divergence, direction) of the illumination / excitation light and the detection area, as well as their overlap, which are coordinated with one another in the sensor module, are taken into account.The simulation calculation results in a curve with a maximum of the expected measurement signal at a specific distance from the sensor module 24, which is referred to below as the target module distance d0. In order to detect the largest possible measurement signal from the sensor module 24, it has been customary to position the sensor module 24 at this target module distance d0 from the target measurement plane E (specified for the flat sample by the device 1, 100), in which the measurement signal of the flat sample, when it is in its target measurement plane E, reaches its maximum, see Fig. 2a. In this simulation calculation, the back-reflection effect of window 4, explained in more detail below, was not taken into account, since its influence on the signal curve of the measurement signal was only recognized prior to the present invention. Fig. 2b shows the corresponding previous arrangement of the sensor module 24 at the target module distance d0 to the specified target measuring plane E of the flat sample 10.Between the sensor module and the target measuring plane E, the window 4 is located at the position that corresponds to the measuring distance deviation y=-g, where g is, for example, in the range from 0.3 mm to 3 mm. In the case of a flat sample 10 that is transported perpendicular to the y-direction, the window 4 limits, for example, the transport path of the flat sample 10. When introducing the flat sample into the target measuring plane E, for example when transporting the flat sample past the sensor module using a transport device, it can happen that the flat sample is positioned slightly outside the desired target measuring plane E, for example due to fluttering movements, so that distance fluctuations in relation to the sensor module 24 can occur. An example of this is shown in Fig.Fig. 2b shows the position of a flat sample 10 (dashed line) that is spaced from the target measurement plane E by a measurement distance deviation y=dy and correspondingly has an actual measurement distance a from the sensor module that deviates from the target module distance d0. If the actual measurement distance a of the flat sample deviates from the target module distance d0, ie, the sample lies outside the target measurement plane E, a significant signal variation of the measurement signal occurs—even without a backreflection effect—e.g., by b0=17% in the range +g to –g around the target measurement plane E, cf. Fig. 2a. Prior to the invention, it was recognized that in a sensor module with a window in front, which is located very close to the target measurement plane, two effects can contribute to the observed signal variation of the measurement signal depending on the exact position of the flat sample: Firstly, the sensor module itself has an intrinsic distance dependence i0, see Fig.2a. This is usually such that the measurement signal has a local maximum as a function of the sample distance, namely at its target module distance, at which a maximum measurement signal is detected from a sample. Secondly, if the window distance of window 4 from the target measurement plane is very small, a back-reflection effect of window 4 results in an increase in the measurement signal for flat samples that are then very close to the window. The window is, for example, a coated or uncoated glass plate. To explain this back-reflection effect, Figs. 3a and 3b show, as an example, the path of light rays during the back-reflection effect for different distances g1 = 1 mm (Fig. 3a) and g2 = 0.5 mm (Fig. 3b) of window 4 from the flat sample 10. The detection area 20 of the flat sample 10 extends in the x-direction perpendicular to the optical axis or on the sample surface from -1 mm to +1 mm. The area shown in Fig.The light rays shown in solid lines in Figures 3a and 3b originate from the sample surface at the center of the detection area 20, are reflected by the window 4 and fall back into the detection area 20 of the flat sample 10 and can contribute to the measurement signal. The light rays shown in dashed lines, on the other hand, arrive outside the detection area 20 and do not contribute to it. As can be seen, in the case of a small distance between the sample and window (cf. Fig. 3b), reflections from a larger angular range contribute to the measurement than in the case of a larger distance (cf. Fig. 3a). For a remission measurement, this backreflection effect arises as follows: The illumination light falls through the window onto the flat sample. At the sample surface, the illumination light is scattered in all directions and hits the window again at different angles. At large angles (approx.> 50°), according to the law of reflection, at the transition between the optically thinner medium (air) and the optically denser medium (window), a significant portion of the illumination light is reflected back at window 4 toward the flat sample 10. This illumination light can then be scattered at the sample surface toward the sensor module 24. If the distance between the flat sample and the window is small, the offset when the illumination light re-impacts the sample remains small, even at very large angles. If the offset is still within the detection range 20, the illumination light reflected back at window 4 and scattered again by the sample also contributes to the measurement signal of the sensor module 24. For a luminescence measurement, two effects occur at window 4: Firstly, excitation light is scattered at the flat sample, reflected back to the flat sample at window 4 as described above, and can excite luminescence there again.Secondly, luminescence excited in the flat sample is emitted isotropically in all directions and strikes window 4 at different angles. At large angles (approx. >50°), a significant portion of the light is reflected by window 4, back toward the flat sample. This light can be scattered at the sample surface back toward the sensor module 24. Both effects only contribute to the measured luminescence signal if the light, after reflection at window 4, strikes the sample within the detection area 20. For a small distance g1 between the flat sample and window, this is the case for a larger angular range than for a larger distance g1, whereby the measured luminescence signal increases. Fig. 3c shows the increase in the measurement signal caused by the back-reflection effect for short measurement distances, see curve r1.An analogous backreflection effect can also occur at a further window 5 arranged at the rear boundary of the measuring area or transport path, see Fig. 1b and Fig. 5a-d, and contribute to a further increase in the measurement signal. Fig. 2c shows the course of the measurement signal of the flat sample as a function of the measurement distance deviation y from the target measurement plane E for a sensor module 24 arranged at the target module distance d0 with a backreflection effect. The curve labeled i0 is the intrinsic distance dependence of the sensor module 24 when it is positioned at the target module distance d0 from the target measurement plane E, see Fig. 2a. The curve labeled r1 shows the increase in the measurement signal due to the backreflection effect from Fig. 3c and the curve labeled t0 shows the superposition or sum of the curves i0 and r1.The resulting curve t0 shows that, taking the back reflection effect into account, the previously customary target module spacing d0 results in an even greater signal variation in the measurement signal than without the back reflection effect (cf. b0=17% in Fig. 2a), e.g., by b1=33% in the range +g to -g around the target measurement plane E. Positioning the sensor module at the target module spacing d0 corresponds to the previously customary procedure, since it results – regardless of the strength or presence of the back reflection effect – even with a purely empirical optimization of the measured or measurable height of the measurement signal. For this purpose, a stationary flat sample is positioned exactly in the target measurement plane E of the sensor module. Subsequently, the module spacing is varied such that – all other conditions being equal – the measurement signal of the flat sample is maximized. The module spacing set in this way corresponds exactly to the target module spacing d0 of the sensor module.It was discovered that the intrinsic distance dependence i0 of the sensor module 24 and the signal increase due to the back-reflection effect r1, r2 of the window(s) 4, 5 can at least partially compensate for each other if the sensor module 24 is positioned at a different distance from the target measurement plane E. For this purpose, the sensor module 24 is preferably positioned at a module distance d that is closer to the target measurement plane E than the previously customary target module distance d0, which is predetermined by the intrinsic distance dependence i0 and its maximum, see Fig. 4a, 4b. In the embodiment of Fig. 4a, 4b, only one window 4 is used between the sensor module 24 and the target measurement plane E, but no rear window 5. However, optionally there can still be a rear boundary of the transport path, which is, for example, opaque and has no or only a negligible back reflection.In this exemplary embodiment, the sensor module 24 is positioned at a module spacing d=4.9 mm, which is closer to the target measuring plane E than the target module spacing d0=5.7 mm specified for the sensor module, see Fig. 4b. The curve labeled i in Fig. 4a shows the intrinsic distance dependence i of the sensor module 24 when it is positioned at the module spacing d to the target measuring plane E. Compared to the intrinsic distance dependence i0 when positioned at the target module spacing d0 from Fig. 2a, 2c, this intrinsic distance dependence i is shifted to the right. The curve labeled r1 in Fig. 4a shows the increase in the measurement signal due to the backreflection effect, and the curve labeled t shows the superposition or sum of the curves i and r1.The resulting curve t shows that the decrease in the measurement signal due to the module spacing deviating from the target module spacing d0 compensates the increase in intensity due to the back reflection at window 4, the sd, quite well. Since the module spacing d from the target measuring plane E is selected to be smaller than the usual target module spacing d0, the maximum measuring position pm, in which the maximum measurement signal m would be reached, is now located behind the target measuring plane E, i.e. on the side of the target measuring plane E facing away from the sensor module 24, see Fig. 4a. In this exemplary embodiment, this maximum measuring position is at pm=+0.7 mm. However, the measurement signal of a flat sample located in the target measuring plane E is only slightly reduced compared to the maximum measurement signal and is still sufficiently high in terms of its signal-to-noise ratio.Due to the deliberately changed module spacing d in conjunction with the back-reflection effect of the window, the signal variation of the measurement signal as a function of the measurement distance deviation y in the area of the target measurement plane E is significantly reduced. In this exemplary embodiment, the signal variation of the measurement signal as a function of the measurement distance deviation y is only b=5% across the entire width of the transport path (from -g to +g). In comparison, curve i in Fig. 4a and curve i0 shown in Fig. 2a with the signal variation b0=17% show that without the interaction between the changed module spacing d and the back-reflection effect of the window, a significantly larger signal variation of the measurement signal would occur. This signal variation of b=5% is significantly lower than for other module spacings and compared to the same sensor module without a window or without the back-reflection effect of a window.The low signal variation makes it possible, for example, for the acceptance range assessed as authentic in an authenticity test to be selected much more narrowly, making the authenticity test more stringent and reliable. The reduction in signal variation achieved by optimizing the sensor distance can be used with all known luminescence and remission sensors with limited illumination and / or detection ranges. In particular, the illumination and detection beam paths can be arranged parallel or at an angle to one another, and any optical arrangement for concentrating the illumination or detection on a limited area can be used, in particular collimated and focused beams with lenses, concave mirrors, and other optical components in the beam path. The method is applicable to any illumination and detection wavelength.In a second embodiment, the rear boundary of the transport path is formed by a further window 5 at a distance g from the target measurement plane E, which also causes a back reflection of the illumination or excitation light, see Fig. 5b. However, the window 5 can also be arranged at a different distance from the target measurement plane E than the window 4. Even in the case of two windows 4, 5, the usual optimization of the height of the measurement signal on a stationary sample results in a position of the sensor module at the target module distance d0, as described above. With the usual positioning of the sensor module 24 at the desired module distance d0, the curves i0' shown in Fig. 5a result for the intrinsic distance dependence of the sensor module 24, r1 and r2 for the increase in the measurement signal due to the back reflection effect at the two windows 4 and 5 and the curve t0' is the superposition or sum of the curves i0', r1 and r2.The signal variation of the curve t0' over the entire area between the windows is then b1'=17%, see Fig. 5a. Due to the additional back-reflection effect of the rear window 5, another preferred module spacing d' results for the sensor module 24, in which the decrease in the measurement signal due to the measurement distance deviating from the target measurement plane well compensates for the increase in intensity due to the back reflections at windows 4 and 5. In order to keep the resulting signal variation of the measurement signal as low as possible, the sensor module in this exemplary embodiment is positioned at a module spacing d'=5.2 mm, which lies between the target module spacing d0=5.7 mm and the module spacing d=4.9 mm selected in the exemplary embodiment with only one window 4 (see Fig. 4a, b), see Fig. 5b.Fig. 5c shows the course of the measurement signal of the flat sample 10 as a function of the measurement distance deviation y from the target measurement plane E for the sensor module arranged at a module distance d'=5.2 mm with back reflection effect of both windows 4, 5. The maximum of the intrinsic distance dependence i' is placed closer to the target measurement plane E in the case of two windows 4, 5 than in the case of only one window 4. For the module distance of d'=5.2 mm, the remaining signal variation of the curve t' resulting from i', r1 and r2 over the entire area between the windows is only approx. b'=3%, see Fig. 5c. Figure 5d shows how, in the case of the two windows 4, 5, the dependence of the measurement signal on the measurement distance deviation y and the signal variation of the measurement signal over the area between the windows changes when the distance of the sensor module 24 is varied from the optimized module distance d'=5.2 mm by 0.5 mm forwards or backwards to d1'=4.7 mm and d2'=d0=5.7 mm.Each curve was individually normalized to the measurement signal in the target measurement plane E (100%). Both at a module spacing of d1'=4.7 mm and at a module spacing of d2'=d0=5.7 mm, a large signal variation of over 20% of the measurement signal results, which is significantly higher than in the case of the module spacing d'=5.2 mm optimized for the case of both windows 4, 5 (only approximately 3%). The sensor module 24 can optionally also have several measurement tracks, see Fig. 1c, and a number of detector devices 21 corresponding to the number of tracks. If the distances between the detector devices 21 can be individually adjusted, a measurement distance optimized with regard to the signal variation of the measurement signal can be selected for each of the detector devices 21, as described above.Due to manufacturing tolerances of the detector devices, a sensor module may result in which each detector device 21 has an individually different distance from the target measurement plane E. For example, the detector devices of a sensor module 24 have individual relative positions that differ from those of other sensor modules of the same series. However, if the distances of the detector devices 21 from the target measurement plane E cannot be individually adjusted for the sensor module 24, the module spacing of the (entire) sensor module 24 can be adjusted such that it is optimized for a specific measurement track, e.g., the most important measurement track for testing the flat sample, with regard to the signal variation of the measurement signal.Alternatively, the module spacing of the sensor module 24 can also be optimized with regard to the signal variation of the measurement signal on the basis of an (optionally weighted) average value of the measurement signals of several or all measurement tracks of the sensor module 24.
Claims
Patent Claims 1. Device (1, 100) for testing a flat sample (10), wherein the device - provides a target measurement plane (E) for the flat sample, in which the flat sample can be introduced for testing, and - has an optical sensor module (24) for optically testing the flat sample (10), which is designed to record a measurement signal of the flat sample that corresponds to the intensity of optical radiation, in particular remission or luminescence light, of the flat sample when the latter is located in the target measurement plane (E) or at least approximately in the target measurement plane, in particular while the flat sample is transported past the sensor module along a transport path, and wherein the sensor module (24) is arranged at a module distance (d) from the target measurement plane (E) and a window (4) is arranged between the sensor module and the flat sample,through which both the illumination / excitation light irradiated onto the flat sample and the optical radiation, in particular remission or luminescence light, reaching the sensor module from the flat sample (10) are transmitted, wherein the flat sample can be located at a measuring position that is spaced from the target measuring plane (E) by a measuring distance deviation (y) when recording the measuring signal, characterized in that - the window (4) is arranged at a window distance (g) from the target measuring plane (E) and the window distance (g) is selected to be so small that the measuring signal of the flat sample in the target measuring plane is increased by a back-reflection effect of the window compared to a corresponding measuring signal of the flat sample located in the target measuring plane occurring without the back-reflection effect, and, - that the module distance (d) of the sensor module from the target measurement plane is selected to be so small that a signal variation of the measurement signal of the flat sample as a function of the measurement distance deviation (y) in the region of the target measurement plane (E) is reduced by the back-reflection effect of the window compared to a signal variation of the measurement signal of the flat sample occurring without the back-reflection effect.
2. Device according to claim 1, wherein the module distance (d) of the sensor module from the target measurement plane is selected to be so small that the signal variation of the measurement signal as a function of the measurement distance deviation (y) in the region of the target measurement plane (E) in the range of + / - 1 mm around the target measurement plane is reduced by the back-reflection effect of the window, in particular by at least 50%, compared to a signal variation occurring without the back-reflection effect. 3.Device according to one of the preceding claims, wherein the sensor module and the window are arranged and designed in such a way that the back-reflection effect of the window contributes to - that a part of the illumination / excitation light is reflected or scattered by the flat sample towards the window and is then reflected back from the window to the flat sample and causes the flat sample to emit additional optical radiation, in particular additional remission or luminescence light, which is reflected or scattered in the direction of the window in such a way that it can be detected through the window by the sensor module and contributes to the measurement signal, and / or - that optical radiation emanating from the flat sample, in particular remission or luminescence light caused by the illumination / excitation light, strikes the window and a portion of the optical radiation. in particular the remission or luminescence light, is reflected back at the window to the flat sample and then reflected or scattered by the flat sample in the direction of the window in such a way that it can be detected by the sensor module through the window and contributes to the measurement signal.
4. Device according to one of the preceding claims, wherein the measurement signal of the flat sample has a maximum value (m) as a function of the measurement distance deviation (y), and wherein the module distance (d) of the sensor module from the target measurement plane is selected to be so small that the maximum value (m) of the measurement signal would be reached at a maximum measurement position (pm) of the flat sample that lies outside the target measurement plane, and which lies in particular on the side of the target measurement plane facing away from the sensor module (24).Device according to one of the preceding claims, wherein the device or the sensor module is designed to direct the illumination / excitation light in the form of converging light beams onto the flat sample, wherein at least some of the converging light beams of the illumination / excitation light converge at an angle of at least 10°.
6. Device according to one of the preceding claims, wherein the device or the sensor module is designed to direct the illumination / excitation light onto the flat sample at an angle deviating from the sample normal of the flat sample.
7. Device according to one of the preceding claims, wherein the module distance (d) of the sensor module from the desired measuring plane (E) is selected to be small. is that the measurement signal of the flat sample, as a function of the measurement distance deviation (y), for measurement positions of the flat sample whose measurement distance deviation (y) lies within a range of + / -1.0 mm around the target measurement plane, has a signal variation of a maximum of 10% with respect to the measurement signal in the target measurement plane.
8. Device according to one of the preceding claims, wherein the module distance (d) of the sensor module from the target measurement plane (E) is selected to be so small that the measurement signal of the flat sample, as a function of the measurement distance deviation (y), for measurement positions of the flat sample over the entire section between the window (4) and the target measurement plane (E), has a signal variation of a maximum of 10% with respect to the measurement signal in the target measurement plane.Device according to one of the preceding claims, wherein - the flat sample is transported past the sensor module along a transport path to record the measurement signal, and the transport path of the flat sample is mechanically limited on both sides at least in the region of the measurement position of the flat sample and has a transport path width (B), and - the module distance (d) of the sensor module from the target measurement plane is selected to be so small that the measurement signal of the flat sample, as a function of the measurement distance deviation (y), has a signal variation of a maximum of 15%, preferably a maximum of 10%, with respect to the measurement signal in the target measurement plane over the entire transport path width (B).Device according to one of the preceding claims, wherein the sensor module is assigned a desired module distance (d0) to the desired measuring plane (E), in which the sensor module would deliver a maximum measuring signal of the flat sample without taking into account the back reflection effect of the window (4), and the. The module distance of the sensor module from the target measurement plane is selected to be at least 0.3 mm smaller than the target module distance.
11. Device according to one of the preceding claims, wherein - the device has a further window (5) on the side of the target measurement plane (E) facing away from the window (4), from which window illumination / excitation light transmitted through the flat sample can be reflected back onto the flat sample, and - the further window (5) is arranged at a further window distance from the target measurement plane (E), and the further window distance (h) is selected to be so small that the measurement signal of the flat sample is increased by an additional back-reflection effect of the further window (5), in particular by at least 2%, compared to a measurement signal occurring without the additional back-reflection effect of the further window (5).Device according to one of the preceding claims, wherein the device is a value document processing device (1) which is configured to check flat samples (10) designed as value documents by means of the optical sensor module (24), in particular to check the authenticity or the quality of the checked value documents, e.g. to check an optical security feature of the respective value document.
13. Device according to one of the preceding claims, wherein the device (100) is configured to produce value documents or to produce semi-finished products used in the production of value documents, in which the value documents or semi-finished products can be checked by means of the optical sensor module (24), e.g. a device for producing a substrate web for value document substrates or a device for producing value document sheets comprising a plurality of value documents or semi-finished products.
14. A method (1, 100) for testing a flat sample (10) by means of an optical sensor module (24) designed for optical testing of the flat sample (10), wherein the method is carried out in particular by a device according to one of the preceding claims, wherein the sensor module (24) for optical testing of the flat sample records a measurement signal of the flat sample that corresponds to the intensity of optical radiation, in particular remission or luminescence light, of the flat sample when the latter is located in a target measurement plane (E) specified for the flat sample or at least approximately in the target measurement plane, in particular while the flat sample is transported past the sensor module along a transport path, wherein the sensor module (24) is arranged at a module distance (d) from the target measurement plane (E) and a window (4) is arranged between the sensor module and the flat sample,through which both the illumination / excitation light irradiated onto the flat sample and the optical radiation, in particular remission or luminescence light, reaching the sensor module from the flat sample (10) are transmitted, wherein the flat sample can be located at a measurement position that is spaced from the target measurement plane (E) by a measurement distance deviation (y) when recording the measurement signal, characterized in that the window (4) is arranged at a window distance (g) from the target measurement plane (E), and the window distance (g) is selected to be so small that the measurement signal of the flat sample is increased by a back-reflection effect of the window compared to a corresponding measurement signal of the flat sample located in the target measurement plane, occurring without the back-reflection effect, and that the module distance (d) of the sensor module from the target measurement plane is selected to be so small,that the signal variation of the measurement signal as a function of, The measurement distance deviation (y) in the region of the target measurement plane (E) is reduced by the back-reflection effect of the window compared to a signal variation of the measurement signal of the flat sample occurring without the back-reflection effect.
15. The method according to claim 14, wherein the flat sample (10) - is a value document or - is a semi-finished product used in the production of value documents, in particular a value document substrate that can be used to produce a value document, or is a value document sheet comprising several value documents or semi-finished products, or is a substrate web for value document substrates, e.g. a paper web, that can be used to produce value document substrates.