Apparatus and method for inspecting flat samples

Abstract

The invention relates to a device and a method for inspecting a flat sample, in particular a value document or a semi-finished product for producing a value document, comprising an optical sensor module which is designed to record a measurement signal of the flat sample when the flat sample is in or at least approximately in a defined measurement plane, and optical inspection is carried out on the flat sample. A window is arranged between the sensor module and the flat sample, and the window distance between the window and the specified measurement plane is selected to be small, so that the retroreflective effect of the measurement signal of the flat sample in the specified measurement plane through the window is increased. The sensor module is arranged at a module distance from the defined measurement plane, which module distance is selected to be small. In this way, the signal variation of the measurement signal of the flat sample as a function of the measurement distance deviation in the region defining the measurement plane is reduced as a result of the retroreflective effect of the window compared to the signal variation occurring without the retroreflective effect.

Description

[0001] The invention relates to a device and a method for checking flat samples, in particular value documents or semi-finished products for producing value documents, for example to check the authenticity of the value documents or to check the quality of the value documents or semi-finished products when producing value documents.

[0002] In order to protect value documents, it is known to embed or apply machine-testable security features in value documents or on value documents. For this purpose, for example, certain luminescent materials or diffusely reflecting pigments can be used.

[0003] When producing such valuable documents, quality checks are usually provided, for example to ensure that the finished valuable documents contain security features of a predetermined number and / or predetermined characteristics. To this end, it can be provided that valuable documents or semi-finished products for producing valuable documents, such as paper or paper webs, are checked at least in a random manner in a production device or a quality control device separate from the production device, and only valuable documents or semi-finished products that meet the pre-set standards can be delivered as circulated, while all other valuable documents or semi-finished products are classified as waste and destroyed if necessary.

[0004] Furthermore, value documents already in circulation are usually subjected from time to time to an optical authenticity check with the aid of value-document processing devices in order to identify and sort out possible forged or suspected forged value documents.

[0005] In both the optical quality check and the optical authenticity check, the value document can be illuminated by light and the optical radiation emitted by the value document, in particular the diffusely reflected light or the luminous light, can be detected and evaluated by means of the sensor module in order to check the security feature.

[0006] In a manufacturing device or quality inspection device for valuable documents or in a valuable document processing device, the anti-counterfeiting features are usually optically inspected by means of a fixed optical sensor module, the valuable document or semi-finished product is transported past the module, and the module is arranged in the corresponding device at a distance from the valuable document or semi-finished product to be inspected. When the valuable document or semi-finished product is transported past the sensor module, the valuable document or semi-finished product may slightly protrude or float from the expected transport plane, which may cause distance fluctuations relative to the optical sensor module. If the measurement signal of the optical sensor module depends on the distance of the valuable document or semi-finished product to be inspected, the protrusion or float may distort the measurement signal. Therefore, in the past, when performing quality inspection or authenticity inspection, it was necessary to allow deviations from the expected measurement signal or allow a certain acceptance range. A larger acceptance range leads to a less strict quality inspection or authenticity inspection, so there is a risk that actually unacceptable quality deviations or some forged valuable documents may be overlooked.

[0007] It is known to reduce the protrusion or floating of value documents out of their transport plane by providing a mechanical restriction, such as a guide element, for the transport path of the value documents or semi-finished products. However, in the above-mentioned devices, such mechanical restrictions cannot be arranged arbitrarily near the transport path, because this could lead to transport faults or damage to the value documents or semi-finished products, especially when the value documents or semi-finished products are transported past the sensor module at high speed.

[0008] The object of the present invention is to provide a method and a device for testing flat samples, such as value documents or semi-finished products for producing value documents, by which a more stable measurement signal can be achieved.

[0009] This object is achieved by a method and a device for testing flat samples according to the independent claims.

[0010] The flat sample to be inspected is, in particular, a value document or a semi-finished product for producing a value document.

[0011] The device has an optical sensor module and is designed for optically checking corresponding flat samples, for example, for authenticity or quality, with the aid of the optical sensor module, wherein the device provides or defines a predetermined measurement plane for the flat sample, into which the flat sample can be placed for optical checking thereof. Preferably, the flat sample is placed in the predetermined measurement plane for optical checking thereof, as viewed along the sample normal, such that a detection region of the flat sample detected during recording of a measurement signal (e.g., located on the sample surface) is (at least approximately) located in the predetermined measurement plane. In particular, the optical sensor module is arranged in the device or embedded in the device. The predetermined measurement plane is arranged outside the sensor module and, for example, is located within the device or is at least arranged in the vicinity of the device in such a way that the sensor module of the device can record a measurement signal of the flat sample.

[0012] For optical inspection of the flat sample, the optical sensor module is designed to record a measurement signal of the flat sample when the flat sample is located in a predetermined measuring plane or at least approximately in the predetermined measuring plane, in particular while the flat sample is transported past the sensor module, for example in the device along a transport path located in the predetermined measuring plane or at least approximately in the predetermined measuring plane, for example by means of a transport device of the device. The optical sensor module is designed to perform an optical inspection of the flat sample, for example an authenticity inspection or a quality inspection, based on the measurement signal and other measurement signals of the flat sample recorded in this way.

[0013] The sensor module is arranged at a module distance from a prescribed measuring plane or transport path. A window is arranged between the sensor module and the flat sample, through which illumination light / excitation light incident on the flat sample (by a light source of the sensor module or by a light source of the device) and optical radiation reaching the sensor module from the flat sample, in particular diffusely reflected light or luminous light emitted from the flat sample due to the illumination light / excitation light, are transmitted.

[0014] The window is arranged at a window distance from the predetermined measurement plane, and the window distance is selected to be so small that the measurement signal of the flat sample located in the predetermined measurement plane is preferably increased by at least 10% due to the retroreflection effect of the window compared to a corresponding measurement signal of a flat sample located in the predetermined measurement plane that is generated or can be recorded without the retroreflection effect, or compared to a corresponding device whose window does not cause the retroreflection effect, for example due to a greater distance from the flat sample. The retroreflection effect is based on the law of optical reflection and will be explained in detail below.

[0015] The window arranged between the sensor module and the flat sample is preferably at least 0.3 mm, preferably at least 0.5 mm, particularly preferably at most 3 mm, for example at most 2 mm away from the window of the prescribed measurement plane. The window is designed as a transparent solid, for example a glass plate. The illumination light / excitation light incident on the flat sample by the sensor module and the light radiation reaching the sensor module from the flat sample, in particular the diffusely reflected light or luminous light reaching the sensor module from the flat sample, are all transmitted through the same window.

[0016] The module distance of the sensor module to the prescribed measuring plane is selected to be so small that the signal change of the measurement signal of the flat sample as a function of the measurement distance deviation in the area of ​​the prescribed measuring plane is reduced due to the retroreflection effect of the window compared to a signal change of the measurement signal of the flat sample as a function of the measurement distance deviation in the area of ​​the prescribed measuring plane that occurs in the absence of retroreflection effects, or compared to a corresponding device in which the window does not produce any retroreflection effects or produces only a negligibly small retroreflection effect, for example due to its greater distance from the flat sample.

[0017] By utilizing the retroreflection effect of the window, the module distance between the sensor module and the predetermined measuring plane is reduced in a targeted manner, so that the signal change of the measurement signal as a function of the measurement distance deviation in the region of the predetermined measuring plane is significantly reduced.

[0018] This results in a more stable measurement signal which depends only slightly on the exact distance of the flat sample from the sensor module, but still achieves a sufficiently high measurement signal so that even weak measurement signals can be measured with a good signal-to-noise ratio.

[0019] The module distance of the sensor module from the defined measuring plane is preferably at least 2 mm.

[0020] In order to compensate for manufacturing tolerances, the module distance can be set individually for each sensor module (e.g., for each individual sensor module in a sensor series), for example, in order to minimize the signal change of the measurement signal as a function of the measurement distance deviation (individually for the sensor). The respective sensor module of each device has, for example, an individual module distance that differs from other devices of the same series. In particular, the device can be provided for adjusting the module distance of the sensor modules by fastening the sensor modules on a moving stand or in the device by means of slots.

[0021] To record the measurement signal, the optical sensor module has at least one detector device, which is configured to detect light radiation emitted by the flat sample, in particular diffuse reflected light or luminous light, and optionally at least one light source, which is configured to irradiate illumination light or excitation light onto the flat sample. The detector device is preferably arranged such that its optical axis extends perpendicularly to a predetermined measuring plane or perpendicularly to a sample normal of the flat sample.

[0022] The sensor module, in particular the detector device and / or the light source, and the window are arranged and designed, for example, in such a way that a retroreflective effect of the window is promoted, so that

[0023] - part of the illumination light / excitation light is reflected or scattered at the flat sample toward the window, and then, in particular at a large angle of incidence at the window (greater than about 50° relative to the sample normal), is partially reflected from the window back to the flat sample due to the law of reflection, and causes the flat sample to emit additional light radiation, in particular additional diffusely reflected light or luminescent light, which is reflected or scattered in the direction of the window so that it can be detected by the sensor module or its detector device / detector devices through the window and contribute to the measurement signal, and / or

[0024] - Light radiation emitted from the flat sample, in particular diffusely reflected light or luminescent light caused by illumination light / excitation light (for example, isotropically emitted from the flat sample in all directions) strikes the window, and in particular at a large angle of incidence (approximately greater than 50° relative to the sample normal), part of the light radiation, in particular the diffusely reflected light or luminescent light, is reflected back to the flat sample at the window due to the law of reflection, and then reflected or scattered again in the direction of the window at the flat sample, so that it can be detected by the sensor module or its detector device / multiple detector devices through the window and contribute to the measurement signal.

[0025] The sensor module is preferably designed to record one or more local measurement signals of the flat sample at one or more discrete locations on the flat sample, for example along one or more measurement tracks spaced apart from one another. The sensor module in particular does not include an image sensor or is not designed to record an image of the flat sample.

[0026] The sensor module or the device is particularly designed so that the illumination light / excitation light (of the light source) impinges on the flat sample in the form of converging or focusing rays (through the window), wherein at least some of the converging rays of the illumination light / excitation light extend toward each other at an angle of at least 10°. For example, rays outside the illumination light / excitation light extend toward each other at an angle of at least 10°, for example in a cone shape. In particular, the illumination light / excitation light does not impinge on the window and the flat sample in parallel beam paths.

[0027] The device or sensor module is designed, for example, to irradiate the flat sample with illumination light / excitation light (of the light source) at an angle that deviates from the sample normal (i.e., perpendicular to the sample). The illumination light / excitation light is then not irradiated in the direction of the sample normal, but is irradiated obliquely through the window onto the flat sample.

[0028] The flat sample or the detection region of the flat sample detected when recording the measurement signal can be located at a measurement position that is spaced apart from the prescribed measurement plane by a measurement distance deviation when recording the measurement signal. The device is particularly designed to introduce the flat sample into the prescribed measurement plane or the region of the prescribed measurement plane so that when recording the measurement signal, the flat sample or the detection region of the flat sample detected when recording the measurement signal can be located at a measurement position that is spaced apart from the prescribed measurement plane by a measurement distance deviation. Thus, the measurement position of the flat sample (or the detection region of the flat sample detected when recording the measurement signal) can have an actual measurement distance from the sensor module that is different from the module distance when recording the measurement signal. The actual measurement distance a of the flat sample corresponds to the sum of the module distance d and the measurement distance deviation y: a=d+y. The device can be particularly designed to introduce the flat sample inaccurately into the prescribed measurement plane or the region of the prescribed measurement plane so that the flat sample can be located slightly outside the prescribed measurement plane (at least partially or sectionally) when recording the measurement signal. Inaccurate introduction is usually unavoidable, for example when the flat sample is transported past the sensor module for testing.

[0029] For example, in the case of a flat sample being moved, the flat sample is usually transported by means of a transport device, so that when the sample is measured, the measuring position of the flat sample may deviate from the specified measuring plane or the actual measuring distance may fluctuate. The flat sample is transported past the sensor module in such a way that the flat sample (or the detection region of the flat sample detected when recording the measuring signal) can be located in a measuring position that is outside / spaced from the specified measuring plane or has a measuring distance deviation from the specified measuring plane when recording the measuring signal. The actual measuring distance may fluctuate, for example, due to the floating of the flat sample.

[0030] The measurement signal of the flat sample (which can be detected taking into account retroreflection effects) has in particular a maximum as a function of the measuring distance deviation y, and the module distance of the sensor module to the prescribed measuring plane is selected to be so small that the maximum value m of the measurement signal is or will be reached at a maximum measurement position pm of the flat sample, which is located outside the prescribed measuring plane, in particular behind the prescribed measuring plane, i.e. on the side of the prescribed measuring plane away from the sensor module.

[0031] The measuring distance of the flat sample at which the measurement signal (which can be detected taking into account the retroreflection effect) reaches a maximum value deviates from the module distance d, in particular by at least 0.2 mm. The maximum measurement position pm of the flat sample, i.e. the maximum measurement position pm at which the maximum value m of the measurement signal is reached as a function of the measuring distance deviation, is preferably at least 0.2 mm further from the sensor module than the prescribed measuring plane. In other words, the measuring distance of the flat sample at which the maximum value of the measurement signal (which can be detected taking into account the retroreflection effect) is reached is greater than the module distance d, in particular by at least 0.2 mm.

[0032] Additionally or alternatively, the module distance between the sensor module and a specified measuring plane is selected to be so small that a maximum value of the measurement signal that can be detected of the flat sample without taking into account retroreflection effects is reached at a measuring position of the flat sample that is located outside the specified measuring plane, in particular behind the specified measuring plane, i.e. on the side of the specified measuring plane remote from the sensor module, wherein the measuring position is preferably at least 0.3 mm further from the sensor module than the specified measuring plane.

[0033] The measurement signal of the flat sample changes as a function of the measurement distance deviation y from the defined measurement plane. In order to reduce or keep it low, the module distance d of the sensor module from the defined measurement plane is expediently selected to be small.

[0034] The module distance of the sensor module to the specified measuring plane is preferably selected to be so small that the signal change of the measurement signal as a function of the measurement distance deviation y in the area of ​​the specified measuring plane (observed in the direction of the sample normal or perpendicular to the window, within a range of + / - 1 mm around the specified measuring plane E) (due to the retroreflection effect of the window) is reduced by at least 50% compared to the signal change that occurs in the absence of the retroreflection effect, or compared to the signal change of the measurement signal of a flat sample as a function of the measurement distance deviation in the area of ​​the specified measuring plane in the absence of the retroreflection effect of the window.

[0035] Alternatively or additionally, the module distance between the sensor module and the defined measurement plane is selected to be so small that for those measurement positions of the flat sample whose measurement distance deviations (viewed in the direction of the sample normal / perpendicular to the window) are within a range of + / -1.0 mm around the defined measurement plane, the measurement signal of the flat sample as a function of the measurement distance deviation (due to the retroreflection effect of the window) has a signal change of a maximum of 10% relative to the measurement signal in the defined measurement plane. In other words, the module distance between the sensor module and the defined measurement plane is preferably selected to be so small that if the measurement position of the flat sample deviates from the defined measurement plane by no more than 1.0 mm in both directions (y=+ / -1.0 mm), the measurement signal of the flat sample has a signal change of a maximum of 10% relative to the measurement signal in the defined measurement plane.

[0036] Alternatively or additionally, the module distance between the sensor module and the prescribed measuring plane is selected to be so small that for a measuring position of the flat sample in the entire section between the window and the prescribed measuring plane, the measurement signal of the flat sample as a function of the measurement distance deviation (due to the retroreflection effect of the window) has a signal change of a maximum of 10% relative to the measurement signal in the prescribed measuring plane.

[0037] In some embodiments, the transport path of the flat sample in the device is mechanically limited on both sides (in the direction normal to the sample / perpendicular to the window) at least in the region of the measurement position of the flat sample, and the transport path (in the direction normal to the sample / perpendicular to the window) has a transport path width B, within which the measurement position of the flat sample can vary. It is provided that the measurement plane is located within the transport path, for example in the center.

[0038] In order to reduce or keep low the signal change of the measurement signal as a function of the measurement distance deviation, the module distance of the sensor module from the predetermined measurement plane is preferably selected to be so small that the signal change of the measurement signal of the flat sample as a function of the measurement distance deviation (due to the retroreflection effect of the window) is a maximum of 15%, preferably a maximum of 10% over the entire transport path width relative to the measurement signal in the predetermined measurement plane. The transport path of the flat sample is limited, for example, by the window on the side facing the sensor module and by a mechanical limit or another window on the side facing away from the sensor module. The transport path width (viewed in the direction of the sample normal / perpendicular to the window) results from the distance between the window and the mechanical limit or another window.

[0039] Typically, when the sensor module is installed in the device, a predetermined module distance d0 relative to the specified measurement plane is correspondingly assigned to the sensor module, in which the sensor module will provide a maximum measurement signal of the flat sample without considering the retroreflection effect of the window, taking into account the beam path of the illumination light / excitation light from the sensor module through the window to the flat sample, and based on the beam path of the light radiation, especially the diffuse reflection light or the luminous light, from the flat sample through the window to the sensor module, that is, the measurement signal will provide a maximum value as a function of the measurement distance deviation. However, in order to reduce or keep low the signal change of the measurement signal as a function of the measurement distance deviation, the module distance d of the sensor module from the specified measurement plane is selected to be at least 0.3 mm smaller than the predetermined module distance d0.

[0040] In some embodiments, the device has a further window behind the prescribed measurement plane, i.e. on the side of the prescribed measurement plane remote from the window, by which the illumination light / 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 prescribed measurement plane, and the further window distance is selected to be so small that the measurement signal of the flat sample is increased by the additional retroreflection effect of the further window, in particular by at least 2% compared to the measurement signal that occurs or can be recorded without the additional retroreflection effect of the further window.

[0041] The illumination light / excitation light transmitted through the flat sample can, for example, be reflected at other windows, thereby irradiating the flat sample again and causing it to emit light radiation, especially diffuse reflection light or luminescent light. Alternatively or additionally, the light radiation, especially diffuse reflection light or luminescent light emitted by the flat sample in response to the illumination light / excitation light in the direction of other windows can be reflected back to the flat sample at other windows, transmitted through the flat sample and the window and reaching the sensor module, detected by the sensor module, and contribute to the measurement signal of the sensor module.

[0042] The sensor module can have an evaluation device which is designed to check the flat sample, for example an optical security feature of the flat sample, in particular its authenticity or quality, based on one or more recorded measurement signals of the flat sample.

[0043] The device can be a valuable document processing device for checking a flat sample designed as a valuable document by means of an optical sensor module, in particular for checking the authenticity or quality of the checked valuable document, for example for checking the optical security features of the valuable document. The device can also be configured to classify the checked valuable documents.

[0044] Alternatively, the above-mentioned device can also be designed for producing valuable documents or semi-finished products used in the production of valuable documents, in which device or by which the valuable documents or semi-finished products are checked by means of an optical sensor module, for example, a device for producing a base material web for a valuable document substrate, or a device for producing a valuable document web comprising a plurality of valuable documents or semi-finished products. The device is, for example, a device for producing a paper web from which a valuable document substrate can be produced, or a web printing device for producing a valuable document web comprising a plurality of valuable documents or semi-finished products. The device is configured to check, in particular quality check, valuable documents or semi-finished products by means of an optical sensor module, for example to check an optical security feature of the checked valuable document or semi-finished product or an optical security feature provided for the checked valuable document or semi-finished product in the production device. The optical security feature can be introduced into the valuable document or semi-finished product or applied to the valuable document or semi-finished product.

[0045] The measurement signal of the flat sample can be output by the sensor module or the device, for example, displayed at an external location or displayed to an operator, and / or corresponding information about the inspection result (for example, whether the valuable document or semi-finished product is "qualified" or "unqualified") can be output by the device or the sensor module, for example, displayed to an external location or displayed to an operator.

[0046] The invention also relates to a method for testing a flat sample by means of an optical sensor module, which is designed for optical testing of a flat sample. The method can be performed by one of the above-mentioned devices or by another device having a sensor module and providing a defined measuring plane.

[0047] In the method, the sensor module records a measurement signal of the flat sample for optical inspection of the flat sample when the flat sample is located in a defined measurement plane outside the sensor module or at least approximately (e.g. at a distance of at most + / -1 mm from the defined measurement plane), in particular when the flat sample is transported past the sensor module along a transport path (located in the defined measurement plane or at least approximately in the defined measurement plane). The measurement signal is used to inspect the flat sample. The sensor module is spaced a module distance from the transport path or the defined measurement plane. A window is arranged between the sensor module and the flat sample, through which illumination light / excitation light incident on the flat sample and optical radiation from the flat sample to the sensor module, in particular diffuse reflection light or luminescence light emitted from the flat sample (due to the illumination light / excitation light), is transmitted. The flat sample (or a detection region of the flat sample detected when recording the measurement signal) can be located at a measurement position that is spaced a measurement distance from the defined measurement plane when recording the measurement signal.

[0048] In the method, the window is arranged at a window distance from the defined measurement plane, and the window distance is selected to be so small that the measurement signal of the flat sample is increased by the retroreflection effect of the window, in particular by at least 10% compared to the corresponding measurement signal of the flat sample located in the defined measurement plane that occurs without the retroreflection effect. In addition, the module distance of the sensor module to the defined measurement plane is selected to be so small that the signal change of the measurement signal of the flat sample as a function of the measurement distance deviation in the region of the defined measurement plane is reduced due to the retroreflection effect of the window compared to the signal change of the measurement signal of the flat sample that occurs without the retroreflection effect (as a function of the measurement distance deviation in the region of the defined measurement plane), i.e., compared to the case where the window does not generate a retroreflection effect or generates only a negligibly small retroreflection effect, for example due to its greater distance from the flat sample.

[0049] The flat sample is, for example, a valuable document or a semi-finished product used in the manufacture of a valuable document, in particular a valuable document substrate that can be used to manufacture a valuable document, or a valuable document page comprising a plurality of valuable documents, or a base material web for a valuable document substrate, for example a paper web that can be used to manufacture a valuable document substrate.

[0050] The recorded measurement signal preferably characterizes at least one of the following optical properties of the flat sample, in particular a semi-finished product or a value document: diffuse reflection, luminescence (fluorescence, phosphorescence), Raman scattering, in particular surface enhanced Raman scattering (SERS), absorption or transmission.

[0051] When checking the flat sample, for example the flat sample, the value document or the semi-finished product is checked on the basis of the measurement signal for the presence and / or type and / or number of optical security features, in particular optical security features.

[0052] The sensor module, in particular its evaluation device, is preferably designed to determine at least one characteristic property of a security feature introduced into or applied to a value document or semi-finished product based on the measurement signal 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 reliable conclusions to be drawn as to whether a certain or desired security feature is present in the semi-finished product or the value document.

[0053] For example, the sensor module, in particular its analysis device, checks whether a measurement signal corresponding to the intensity of the light radiation is greater than a predetermined threshold value and / or is within a preset acceptance range. This makes it possible to draw conclusions in a simple manner whether a certain or desired number of desired security features are present in / on the semi-finished product or the value document.

[0054] The characteristic or preset characteristic of the security feature to be tested is, for example, at least one of the following characteristics of the light radiation emitted by the security feature of the semi-finished product or the valuable document: i) spectral characteristics, such as the intensity in a certain spectral range (fingerprint), the position, intensity or width, absolute value or relative value of the spectral maximum, minimum or shoulder (Schulter); ii) temporal characteristics, such as the absolute or relative intensity at a certain time point relative to the irradiated excitation pulse, the decay or decay time, the time-resolved intensity change trend or functional form (fitting parameters), the position or intensity of the intensity maximum over time; iii) a combination of spectral and temporal characteristics (such as the decay time of multiple spectral channels, the emission spectrum at multiple measurement time points); iv) characteristics after complex excitation achieved by irradiation (such as multiple excitation wavelengths, complex temporal modulation of the excitation light).

[0055] In ambient light, the security feature is preferably not visible to the naked eye. Preferably, a security feature is used for which the light radiation (attenuation, luminescence) emitted by the security feature in response to the illumination light / excitation light during the test is 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 light / excitation light can be in the visible spectral range as well as in the invisible spectral range.

[0056] Other advantages, features and applications of the present invention may be derived from the following description with reference to the accompanying drawings. In the accompanying drawings:

[0057] Figure 1a -c shows the value document processing equipment ( Figure 1a ), a local area of ​​a manufacturing device for a valuable document substrate or a valuable document ( Figure 1b ) and sensor modules designed for multi-track inspection ( Figure 1c ),

[0058] Figure 2a shows the variation of the measurement signal of a flat sample with the deviation y of the measurement distance from the prescribed measurement plane E when the sensor modules are arranged at a module distance d0 from the prescribed measurement plane and there is no retroreflection effect,

[0059] Figure 2b The schematic diagram shows the previous arrangement of sensor modules at a predetermined module distance d0 from a defined measuring plane E of a flat sample.

[0060] Figure 2c An example of the variation of the measurement signal of a flat sample as a function of the deviation y of the measurement distance from a prescribed measurement plane E is shown in the case of a sensor module arranged at a module distance d0 with a retroreflection effect,

[0061] Figure 3a -c shows the results for different window distances g1( Figure 3a ) and g2( Figure 3b ) Example extension of the light beam during the retroreflection effect and the increase in the measurement signal due to the retroreflection effect ( Figure 3c ),

[0062] Figure 4a shows the variation of the measurement signal of a flat sample as a function of the deviation y of the measurement distance from a predetermined measurement plane E in the case of an arrangement of the sensor modules according to the invention at a distance d from the modules with a retroreflection effect,

[0063] Figure 4b shows an example of an arrangement according to the invention with only one window, where the sensor module is spaced a small distance d from a defined measuring plane E of a flat sample,

[0064] Figure 5a It shows that in the case of an additional retroreflection effect due to other windows, the measured signal increases due to the retroreflection effect.

[0065] Figure 5b A schematic diagram of an arrangement according to the invention showing a sensor module at a small distance d' from a predetermined measuring plane E in the presence of a further window,

[0066] Figure 5cshows the variation of the measurement signal of a flat sample as a function of the deviation y of the measurement distance from a predetermined measurement plane E in the case of a sensor module arranged according to the invention at a distance d′ from the module and with the retroreflection effect of two windows,

[0067] Figure 5d The variation of the measurement signal of a flat sample as a function of the deviation y of the measurement distance from a predetermined measurement plane E is shown for different module distances of the sensor module in the case of a retroreflection effect with two windows.

[0068] Figure 1a The schematic diagram of a valuable document processing device 1 for valuable documents is shown, which is used to check individual valuable documents 10, for example to check the authenticity or quality of valuable documents. The valuable documents 10 are provided in the form of a valuable document stack 3 in an input rack 2 of the valuable document processing device 1. The valuable documents 10 are removed from the input rack individually and one by one by means of a separation device 8, and are transported by means of a transport device, such as rollers and / or a conveyor belt, along a transport direction x past a sensor module 24, which is designed to perform an optical authenticity check or quality check on the optical security features of the valuable documents. Depending on whether the corresponding valuable document meets the standard applied during the optical check, the corresponding valuable document can be transported to a first output rack 30 or a second output rack 31. The control device 40 of the device controls the switch 11, 12 of the device accordingly, so as to classify the valuable documents. Alternatively, the valuable documents 10 can also be transported to other devices of the valuable document processing device 1 via a transport section 13.

[0069] Figure 1b The schematic diagram of a production device 100 which can be used when producing value documents is shown, which is designed to produce a paper web 10 for a value document substrate. The sensor module 24 is designed to optically inspect the quality of the paper web 10 for optical security features. Depending on whether the respectively inspected section of the paper web meets the standard used in the optical inspection, a corresponding quality grade can be assigned to the respective section of the paper web, whereby sections of the paper web that do not meet the standard can be sorted out. If necessary, the respective section of the paper web that does not meet the standard can be marked for this purpose.

[0070] However, the manufacturing device 100 can also be a page printing device for manufacturing valuable document pages 10, which contain a plurality of valuable documents or semi-finished products in a matrix-like manner. The page printing device is, for example, a printing press for valuable document pages, in which the optical security features are printed on the valuable documents to be manufactured of the corresponding valuable document pages by means of a printing device. The sensor module 24 is arranged in the printing press after the printing device in order to perform a print check on the security features printed on the page. In the page printing device, the valuable document page 10 can be connected to the valuable document page 10. Figure 1a The valuable documents in the stack are similarly separated from the stack and transported and inspected. The corresponding printed pages 10 can be assigned quality grades accordingly, thereby sorting out the corresponding printed pages that do not meet the standards. If necessary, the corresponding printed pages that do not meet the standards can be marked for this purpose.

[0071] exist Figure 1a The sensor module 24 is shown in the example of FIG. Figure 1b As shown, a second sensor module 25 is arranged on the opposite side of the transport path. Furthermore, the device 1 can also contain additional sensor modules for further checks of value documents, for example image sensors or magnetic sensors.

[0072] exist Figure 1b In the example of FIG. 1 , when a one-sided inspection of the optical security feature is sufficient, only one of the sensor modules 24 can be used instead of two oppositely disposed sensor modules 24 , 25 .

[0073] In the corresponding apparatus 1, 100, the flat sample 10 is transported in a transport direction x by means of a suitable transport device past the sensor module 24. Ideally, the corresponding value document is located in a predetermined measuring plane E. The sensor module 24 is provided for detecting light radiation emitted by the flat sample when the flat sample is moved past the sensor module 24. The sensor module 24 has at least one light source 22, which is provided for irradiating illumination light or excitation light onto the flat sample, and at least one detector device 21, which is provided for detecting light radiation emitted by the flat sample.

[0074] The optical radiation emitted by the flat sample in response to illumination light / excitation light incident on the flat sample is, for example, diffusely reflected light, luminescent light, or Raman scattered light.

[0075] At one or more points in time, when the flat sample 10 is located in a defined measurement plane E within the detection area of ​​the sensor module 24, the sensor module records one or more measurement signals of the flat sample 10, which measurement signals correspond to the intensity of the optical radiation of the flat sample, for example the intensity of the diffusely reflected light or the luminescent light. In the example described, it is assumed that the optical radiation is the diffusely reflected light or the luminescent light of the flat sample.

[0076] Figure 1c A schematic top view of a sensor module 24 is shown, below which is located a flat sample 10 to be inspected, which, depending on the purpose of the sensor module 24, can be a value document, a value document web or a printed web or paper web. For the sake of clarity, the example schematically shows a flat sample 10 designed as a value document.

[0077] The sensor module 24 can be designed for single-track or multi-track measurement of a flat sample, so as to detect 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 detector device 21 is correspondingly assigned to one of the tracks SP1 to SP5. The detector devices 21 are, for example, respectively configured to detect light radiation emitted by the flat sample in one or more spectral ranges or spectral channels K1, K2, ..., and send corresponding signals to the inspection device 23, where these signals are further processed or inspected. In the example described, the first light source 22 and the optionally additional second light source 22' are configured to simultaneously apply light radiation to all tracks SP1 to SP5 on the flat sample. However, it can also be alternatively provided that each track SP1 to SP5 is provided with its own light source 22 and optionally 22'.

[0078] Figure 2a The results of a simulation calculation of the conventional beam path of a sensor module 24 are shown, which show the expected change in the measurement signal of a flat sample 10 located in a predetermined measuring plane E (the predetermined measuring plane is located at y=0) that can be detected by means of the sensor module 24 through the window 4 as a function of the measurement distance deviation y from the predetermined measuring plane E. The influence of the window 4 on the direct beam path, the shape of the illumination light / excitation light and the detection area (focal position, divergence, direction) and their overlap are taken into account, which are adapted to one another in the sensor module. The simulation calculation results in a change profile that has the expected maximum value of the measurement signal at a determined distance from the sensor module 24, which is referred to below as the predetermined module distance d0. In order to detect the maximum possible measurement signal of the sensor module 24, the sensor module 24 has hitherto usually been arranged at a distance of this module distance d0 from the predetermined measuring plane E (preset by the device 1, 100 for the flat sample), at which module distance d0 the measurement signal of the flat sample reaches a maximum when the flat sample is located in its predetermined measuring plane E, see Figure 2a In this simulation calculation, the reverberation effect of the window 4 , which will be explained in greater detail below, was not taken into account, since the influence of this reverberation effect on the signal profile of the measurement signal was not recognized until the preparation stage of the present invention.

[0079] Figure 2bThe corresponding previous arrangement of the sensor module 24 at a predetermined module distance d0 from a predefined defined measuring plane E of the flat sample 10 is shown. The window 4 is located between the sensor module and the defined measuring plane E at a position that corresponds to a measuring distance deviation y=−g, wherein g is, for example, in the range of 0.3 mm to 3 mm. In the case of transport of the flat sample 10 perpendicular to the y direction, the window 4 defines, for example, the transport path of the flat sample 10.

[0080] However, when the flat sample is introduced into the predetermined measuring plane E, for example when it is transported past the sensor module by means of a transport device, it may happen that the flat sample is positioned slightly outside the desired predetermined measuring plane E, for example due to floating movements, which may cause fluctuations in the distance relative to the sensor module 24. For this purpose, Figure 2b , the position of a flat sample 10 (dashed line) is shown as an example, which is spaced apart from the defined measurement plane E by a measurement distance deviation y=dy and correspondingly has an actual measurement distance a from the sensor module, which deviates from the predetermined module distance d0. If the actual measurement distance a of the flat sample deviates from the predetermined module distance d0, i.e. the sample is located outside the defined measurement plane E, then even without retroreflection effects, the measurement signal will have a significant signal change, for example a change b0=17% in the range from +g to -g around the defined measurement plane E (see Figure 2a ).

[0081] In the preparation of the present invention it has been recognized that if the sensor module is provided with a window very close to the defined measuring plane, then depending on the exact position of the flat sample two effects contribute to the observed signal change of the measuring signal:

[0082] First, the sensor module itself has an intrinsic distance dependence i0, see Figure 2a Typically, the distance dependence is such that the measurement signal has a local maximum as a function of the distance from the sample, ie a maximum measurement signal is detected from the sample in its predetermined module distance.

[0083] Secondly, if the distance between the window 4 and the specified measurement plane is very small, the measurement signal of the flat sample very close to the window is increased by the retroreflection effect of the window 4. The window can be a coated glass plate or an uncoated glass plate, for example. In order to illustrate this retroreflection effect, Figure 3a and Figure 3b The window 4 and the flat sample 10 are shown in FIG. Figure 3a ) and g2=0.5mm( Figure 3b) The detection area 20 of the flat sample 10 extends from -1 mm to +1 mm along the direction x perpendicular to the optical axis or on the sample surface.

[0084] Figure 3a and Figure 3b The light shown by the solid line is emitted from the sample surface at the center of the detection area 20, reflected at the window 4, and then falls into the detection area 20 of the flat sample 10 and can contribute to the measurement signal. The light shown by the dotted line is irradiated outside the detection area 20 and does not contribute to the measurement signal. It can be seen that when the distance between the sample and the window is small (see Figure 3b ), compared with the case where the distance is larger (see Figure 3a ), reflections from a larger range of angles contribute to the measurement.

[0085] For diffuse reflection measurements, this retroreflection effect is produced in the following way: The illumination light passes through the window and strikes the flat sample. On the sample surface, the illumination light is scattered in all directions and strikes the window again at different angles. At larger angles (approximately >50°), a considerable portion of the illumination light is reflected at the window 4 in the direction of the flat sample 10 according to the law of reflection at the transition between the optically sparser medium (air) and the optically denser medium (window). This illumination light can then be scattered at the sample surface in the direction of the sensor module 24. If the distance between the flat sample and the window is small, the offset when the illumination light strikes the sample again is small even at large angles. If the offset is still within the detection area 20, the illumination light reflected back at the window 4 and scattered again on the sample also contributes to the measurement signal of the sensor module 24.

[0086] For luminescence measurements, two effects occur at window 4:

[0087] First, the excitation light is scattered at the flat sample, and then reflected back to the flat sample at the window 4 as described above, and excites luminescence there again. Secondly, the luminescence excited in the flat sample is emitted isotropically in all directions and illuminates the window 4 at different angles. When the angle is large (about >50°), a large part of the light is reflected at the window 4, still in the direction of the flat sample. These light rays may still be scattered at the sample surface in the direction of the sensor module 24. Only when the light is irradiated (impacted) in the detection area 20 on the sample after being reflected at the window 4, do these two effects contribute to the measured luminescence measurement signal. When the distance g1 between the flat sample and the window is small, the angular range is larger than when the distance g1 is large, thereby increasing the measured luminescence measurement signal.

[0088] Figure 3cThe increase in the measurement signal due to the retroreflection effect at a small measuring distance is shown, see curve r1. Other windows 5 arranged at the rear boundary of the measuring range or transport path also produce similar retroreflection effects, see Figure 1b and Figure 5a -d, thereby contributing to further increasing the measurement signal.

[0089] Figure 2c The variation of the measurement signal of a flat sample with a deviation y of the measurement distance from the specified measurement plane E is shown in the case where the sensor modules 24 are arranged at a predetermined module distance d0 with a retroreflection effect. The curve marked with i0 is the inherent distance dependency of the sensor module when the sensor module 24 is positioned at a distance d0 from the specified measurement plane E, see Figure 2a The curve marked with r1 represents the Figure 3c The resulting curve t0 shows that, taking into account the reflection effect, the signal change of the measurement signal for the hitherto customary predetermined module distance d0 is even greater than without the reflection effect (see Figure 2a For example, the change b1=33% in the range of +g to -g around the specified measurement plane E is b0=17%.

[0090] The positioning of the sensor modules at a predetermined module distance d0 corresponds to the conventional arrangement to date, since this positioning also produces a purely empirical optimization of the measured height or the measurable height of the measurement signal, regardless of the intensity or presence of the retroreflection effect. To this end, a stationary flat sample has been positioned precisely in a defined measurement plane E of the sensor module to date. The module distance has then been varied to date in such a way that, other conditions being equal, the measurement signal of the flat sample is maximized. The module distance thus set corresponds exactly to the predetermined module distance d0 of the sensor module.

[0091] It has been found that the intrinsic distance dependency i0 of the sensor module 24 and the signal increase due to the reflection effects r1, r2 of the windows 4, 5 can at least partially compensate for each other if the sensor module 24 is positioned at different distances from the defined measurement plane E. The module distance d at which the sensor module 24 is positioned is preferably closer to the defined measurement plane E than the previously customary predetermined module distance d0, which is determined by the intrinsic distance dependency i0 and its maximum value, see Figure 4a , 4b .

[0092] exist Figure 4a , Figure 4bIn the exemplary embodiment, only one window 4 is used between the sensor module 24 and the predetermined measuring plane E, without using a rear window 5. However, a rear boundary of the transport path can also optionally be provided, which boundary is, for example, opaque and has no or only negligible retroreflection. In this exemplary embodiment, the sensor module 24 is positioned at a module distance d = 4.9 mm, which is closer to the predetermined measuring plane E than the predetermined module distance d0 = 5.7 mm of the sensor module, see Figure 4b .

[0093] Figure 4a The curve marked with i in FIG. 1 shows the intrinsic distance dependency i of the sensor module when the module distance d between the sensor module 24 and the predetermined measuring plane E is . Figure 2a and Figure 2c Compared with the inherent distance dependence i0 when the module is positioned at a predetermined module distance d0, the inherent distance dependence i is shifted to the right. Figure 4a The curve marked with r1 in the figure represents the measurement signal increased due to the retroreflection effect, and the curve marked with t represents the superposition or sum of the curves i and r1. The resulting curve t shows that the reduction in the measurement signal due to the deviation of the module distance from the predetermined module distance d0 can well compensate for the increase in intensity due to retroreflection at the window 4.

[0094] Since the module distance d from the defined measurement plane E is selected to be smaller than the usual predetermined module distance d0, the maximum measurement position pm at which the maximum measurement signal m is reached is located behind the defined measurement plane E, i.e. on the side of the defined measurement plane E facing away from the sensor module 24, see Figure 4a In this exemplary embodiment, the maximum measurement position is located at pm=+0.7 mm. However, compared to the maximum measurement signal, the measurement signal of a flat sample located in the defined measurement plane E is only slightly weakened and is still sufficiently high in terms of its signal-to-noise ratio.

[0095] By means of a targeted variation of the module distance d in combination with the retroreflective effect of the window, the signal variation of the measurement signal as a function of the measurement distance deviation y in the region of the defined 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 over the entire width of the transport path (from -g to +g) is only b=5%. In contrast, Figure 4a The curve i and Figure 2a The curve i0 shown in FIG. 5 with a signal change of b0=17% shows that the signal change of the measurement signal would be significantly increased without the interaction between the changed module distance d and the retroreflection effect of the window.

[0096] The signal change of b=5% is significantly lower than the signal change for other module distances and compared to the case of identical sensor modules without window or without window reverberation effect. The lower signal change makes it possible, for example, to select a significantly narrower acceptance range for evaluation as authenticity during a plausibility check, thus making the plausibility check more stringent and more reliable.

[0097] The reduction of the signal variation achieved according to the invention by optimizing the sensor distance can be used for all known luminescence sensors and diffuse reflection sensors with limited illumination and / or detection ranges. The illumination and detection beam paths can be arranged in particular parallel to one another or at an angle to one another, and any optical means can be used to concentrate the illumination or detection in a limited area, in particular using collimated and focused beams in the beam paths by means of lenses, concave mirrors and other optical components. The method can be used for any illumination wavelength and detection wavelength.

[0098] In a second embodiment, the back side of the transport path is delimited by a further window 5 at a distance g from the defined measuring plane E, which likewise produces a retroreflection of the illumination light or the excitation light, see Figure 5b However, the window 5 can also be arranged at a different distance from the predetermined measuring plane E than the window 4 .

[0099] Even in the case of two windows 4, 5, the usual optimization of the height of the measurement signal at a stationary sample results in the position of the sensor module being located at the predetermined module distance d0 as described above. When the sensor module 24 is usually positioned at the predetermined module distance d0, Figure 5a The curve i0' shown in represents the inherent distance dependence of the sensor module 24, r1 and r2 represent the increase in the measurement signal caused by the reflection effect at the two windows 4 and 5, and the curve t0' represents the superposition or sum of the curves i0', r1 and r2. In the entire range between the windows, the signal change of the curve t0' is b1'=17%, see Figure 5a .

[0100] The additional retroreflection effect of the rear window 5 results in different preferred module distances d' of the sensor modules 24, in which the reduction in the measurement signal due to the deviation of the measurement distance from the specified measurement plane can well compensate for the increase in intensity due to retroreflection at the windows 4 and 5. In order to keep the resulting signal change of the measurement signal as small as possible, the sensor modules in the described embodiment are positioned at a module distance of d'=5.2 mm, which is between the predetermined module distance d0=5.7 mm and the embodiment with only one window 4 (see Figure 4a , b) the module distance d = 4.9 mm, see Figure 5b .

[0101] Figure 5c The variation of the measurement signal of a flat sample 10 with the retroreflection effect of two windows 4, 5 arranged at a module distance d from the specified measurement plane E is shown. The maximum value of the inherent distance dependency i' is closer to the specified measurement plane E for the case of two windows 4, 5 than for the case of only one window 4. For the case of a module distance d'=5.2 mm, the remaining signal variation of the curve t' consisting of i', r1 and r2 in the entire area between the windows is only about b'=3%, see Figure 5c .

[0102] Figure 5d The dependence of the measurement signal on the measurement distance deviation y and the signal change of the measurement signal in the range between the windows are shown for two windows 4, 5 when the distance of the sensor modules 24 changes by 0.5 mm from the optimized module distance d'=5.2 mm forwards or backwards to d1'=4.7 mm and d2'=d0=5.7 mm. Each curve is here individually normalized to the measurement signal in a predetermined measurement plane E (100%). At a module distance d1'=4.7 mm and a module distance d2'=d0=5.7 mm, a large signal change of more than 20% of the measurement signal occurs, which is significantly higher than the signal change (only about 3%) at the module distance d'=5.2 mm optimized for the case of two windows 4, 5.

[0103] The sensor module 24 may optionally have a plurality of measuring tracks, see Figure 1c , and the number of detector devices 21 corresponding to the number of tracks. If the distances between the detector devices 21 can be set independently, it is possible to select the optimal measurement distance for each detector device 21 in terms of the signal change of the measurement signal as described above. Due to the manufacturing tolerances of the detector devices, this may result in each detector device 21 in the sensor module having a different distance from the prescribed measurement plane E. The detector devices of the sensor module 24 have, for example, respective relative positions that are different from the detector devices of other sensor modules of the same series. However, if the distance of the detector device 21 from the prescribed measurement plane E cannot be set independently for the sensor module 24, the module distance of the (entire) sensor module 24 can be set as follows, so that the module distance is optimized in terms of the signal change of the measurement signal for a determined measurement track (or track), for example, the most important measurement track related to the flat sample inspection. Alternatively, the module distance of the sensor module 24 can also be optimized in terms of the signal change of the measurement signal based on the (optionally weighted) average value of the measurement signals of multiple or all measurement tracks of the sensor module 24.

Claims

1. A device (1, 100) for testing a flat sample (10), wherein: The device - providing a defined measuring plane (E) for a flat sample, into which the flat sample can be introduced in order to inspect it, and - an optical sensor module (24) for optically checking a flat sample (10), the optical sensor module being designed to record a measurement signal of the flat sample when the flat sample is located in a predetermined measuring plane (E) or at least approximately in the predetermined measuring plane, in particular while the flat sample is transported past the sensor module along a transport path, the measurement signal corresponding to the intensity of light radiation, in particular diffuse reflected light or luminous light, of the flat sample, The sensor module (24) is arranged at a module distance (d) from the specified measurement plane (E), and a window (4) is arranged between the sensor module and the flat sample, through which illumination light / excitation light incident on the flat sample and light radiation from the flat sample (10) reaching the sensor module, in particular diffusely reflected light or luminous light, are transmitted. wherein the flat sample can be located at a measuring position which is a measuring distance deviation (y) from a defined measuring plane (E) when recording the measuring signal, It is characterized in that the window (4) is arranged at a window distance (g) from the predetermined measurement plane (E), and the window distance (g) is selected to be so small that the measurement signal of the flat sample in the predetermined measurement plane is increased by the retroreflection effect of the window compared to the corresponding measurement signal of the flat sample located in the predetermined measurement plane which would occur without the retroreflection effect of the window, and -The module distance (d) of the sensor module to the prescribed measuring plane is selected to be so small that the signal change of the measurement signal of the flat sample as a function of the measurement distance deviation (y) in the area of ​​the prescribed measuring plane (E) is reduced due to the retroreflection effect of the window compared to the signal change of the measurement signal of the flat sample that occurs in the absence of retroreflection effect.

2. The device according to claim 1, wherein The module distance (d) of the sensor module to the specified measuring plane is selected to be so small that the signal change of the measurement signal as a function of the measurement distance deviation (y) in the area of ​​the specified measuring plane (E) within a range of + / -1 mm around the specified measuring plane is reduced, in particular by at least 50%, due to the retroreflection effect of the window compared to the signal change that occurs in the absence of the retroreflection effect.

3. The device according to any one of the preceding claims, wherein: The sensor module and the window are arranged and designed to facilitate the retroreflective effect of the window, so that - a portion of the illumination light / excitation light is reflected or scattered at the flat sample toward the window, and then reflected from the window back to the flat sample, causing the flat sample to emit additional light radiation, in particular additional diffusely reflected light or luminescent light, which is reflected or scattered in the direction of the window so that it can be detected by the sensor module through the window and contribute to the measurement signal, and / or - Light radiation emitted from the flat sample, especially diffusely reflected light or luminescent light caused by illumination light / excitation light, hits the window, and part of the light radiation, especially diffusely reflected light or luminescent light, is reflected back to the flat sample at the window, and then reflected or scattered again in the direction of the window at the flat sample, so that it can be detected by the sensor module through the window and contribute to the measurement signal.

4. The device according to any 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 the module distance (d) of the sensor module from a predetermined measurement plane is selected to be so small that the maximum value (m) of the measurement signal is reached at a maximum measurement position (pm) of the flat sample, which is located outside the predetermined measurement plane and in particular on a side of the predetermined measurement plane remote from the sensor module (24).

5. The device according to any one of the preceding claims, wherein: The device or the sensor module is designed to impinge the flat sample with illumination / excitation light in the form of converging light beams, wherein at least some of the converging light beams of illumination / excitation light extend towards one another at an angle of at least 10°.

6. The device according to any one of the preceding claims, wherein: The device or the sensor module is designed to cause the illumination light / excitation light to impinge on the flat sample at an angle deviating from the sample normal of the flat sample.

7. The device according to any one of the preceding claims, wherein: The module distance (d) between the sensor module and the defined measuring plane (E) is selected to be so small that for the following measuring positions of the flat sample, i.e., the measuring distance deviation (y) of these measuring positions is in the range of + / -1.0 mm around the defined measuring plane, the measuring signal of the flat sample as a function of the measuring distance deviation (y) has a signal change of a maximum of 10% relative to the measuring signal in the defined measuring plane.

8. The device according to any one of the preceding claims, wherein The module distance (d) between the sensor module and the predetermined measuring plane (E) is selected to be so small that, for a measuring position of the flat sample over the entire section between the window (4) and the predetermined measuring plane (E), the measurement signal of the flat sample has a signal change of a maximum of 10% relative to the measurement signal in the predetermined measuring plane as a function of the measurement distance deviation (y).

9. The device according to any of the preceding claims, wherein: - for recording the measurement signal, the flat sample is transported along a transport path past the sensor module, and the transport path of the flat sample is mechanically limited on both sides at least in the region of the measuring position of the flat sample and has a transport path width (B), and The module distance (d) of the sensor module to the predetermined measuring 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 change over the entire transport path width (B) of a maximum of 15%, preferably a maximum of 10%, relative to the measurement signal in the predetermined measuring plane.

10. The device according to any of the preceding claims, wherein: A predetermined module distance (d0) relative to a specified measuring plane (E) is correspondingly assigned to the sensor module, at which the sensor module will provide a maximum measurement signal for the flat sample without taking into account the retroreflection effect of the window (4), and the module distance between the sensor module and the specified measuring plane is selected to be at least 0.3 mm smaller than the predetermined module distance.

11. The device according to any of the preceding claims, wherein the device has a further window (5) on the side of the defined measurement plane (E) remote from the window (4), from which the 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 prescribed measuring plane (E), and the further window distance (h) is selected to be so small that the measuring signal of the flat sample is increased by the additional retroreflection effect of the further window (5) compared with the measuring signal that occurs without the additional retroreflection effect of the further window (5), in particular by at least 2%.

12. The device according to any of the preceding claims, wherein: The device is a value-document processing device (1) which is configured to check a flat sample (10) designed as a value document by means of an optical sensor module (24), in particular to check the authenticity or quality of the checked value document, for example for checking an optical security feature of the corresponding value document.

13. The device according to any one of the preceding claims, wherein The device (100) is designed to manufacture valuable documents or semi-finished products used in the manufacture of valuable documents, in which the valuable documents or semi-finished products can be inspected with the help of an optical sensor module (24). The device is, for example, a device for manufacturing a base material web for a valuable document substrate, or a device for manufacturing a valuable document page web comprising a plurality of valuable documents or semi-finished products.

14. A method (1, 100) for testing a flat sample (10) by means of an optical sensor module (24), the optical sensor module being designed for optical testing of the flat sample (10), wherein: The method is carried out in particular by means of a device according to one of the preceding claims, Therein, for optical inspection of a flat sample, the optical sensor module (24) records a measurement signal of the flat sample when the flat sample is located in a predetermined measurement plane (E) provided for the flat sample or at least approximately in the predetermined measurement plane, in particular while the flat sample is transported past the sensor module along a transport path, the measurement signal corresponding to the intensity of light radiation, in particular diffuse reflected light or luminous light, of the flat sample. The sensor module (24) is arranged at a module distance (d) from the specified measurement plane (E), and a window (4) is arranged between the sensor module and the flat sample, through which illumination light / excitation light incident on the flat sample and light radiation from the flat sample (10) reaching the sensor module, in particular diffusely reflected light or luminous light, are transmitted. wherein the flat sample can be located at a measuring position which is a measuring distance deviation (y) from a defined measuring plane (E) when recording the measuring signal, It is characterized in that The window (4) is arranged at a window distance (g) from the predetermined 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 the retroreflection effect of the window compared to the corresponding measurement signal of the flat sample located in the predetermined measurement plane without the retroreflection effect of the window, and The module distance (d) of the sensor module to a predetermined measuring plane is selected to be so small that the signal change of the measurement signal as a function of the measurement distance deviation (y) in the area of ​​the predetermined measuring plane (E) is reduced due to the retroreflection effect of the window compared to the signal change of the measurement signal of the flat sample that occurs in the absence of retroreflection effect.

15. The method according to claim 14, wherein: The flat sample (10) - is a valuable document, or - is a semi-finished product used in the production of valuable documents, in particular a valuable-document substrate that can be used to produce valuable documents, or a valuable-document page web comprising a plurality of valuable documents or semi-finished products, or a base material web for a valuable-document substrate, for example a paper web that can be used to produce a valuable-document substrate.

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