Measuring method and measuring device for determining transmission and / or reflection properties

Abstract

Optical measuring method for measuring the transmission and reflection properties of, in particular, large-area translucent objects (1), preferably applicable for process and quality control in the production of surface-coated substrates, wherein - the measurement is carried out with two illuminating devices (7, 9) emitting diffuse light and two photodetectors (4, 14) by either: - a first large area (3) of the object (1) is illuminated with a first of the illumination devices (9) and at the same time the total transmission is measured with a first photodetector (14) and the reflection at this large area (3) is measured with the second photodetector (4), and then - the second large surface (2) of the object (1) opposite the first is illuminated with the second illumination device (7) and at the same time the diffuse transmission is measured with the second photodetector (4) and the reflection at this large surface (2) is measured with the first photodetector (14), or sequentially in any desired order: - a first large area (3) of the object (1) is illuminated with a first of the illumination devices (9) and the total transmission is measured with a first photodetector (14), - the second large surface (2) of the object (1) opposite the first is illuminated with the second illumination device (7) and the diffuse transmission is measured with the second photodetector (4).

Description

[0001] The invention relates to an optical measuring method for determining the transmission and / or reflection properties, preferably of large-area translucent objects. It is particularly applicable for process and quality control in the production of surface-coated substrates. The invention further relates to an optical measuring device suitable for implementing the method according to the invention.

[0002] In the production of coated translucent materials, such as substrates for photovoltaics provided with electrically conductive layers, the control of the transparent and reflective properties of the substrate is particularly important with regard to achieving optimal efficiency in the conversion of solar radiation into electricity.

[0003] Haze, also known as large-angle scattering or HAZE, is an important parameter. According to the international standard ASTM D 1003, haze is defined as the percentage of light passing through a translucent object that is deflected by more than 2.5° in its propagation direction and is therefore scattered from a directed beam of light as it passes through the object, for example, due to surface roughness.

[0004] According to ASTM D 1003, the turbidity of translucent objects can be determined by placing the object in front of the light entrance of an integrating sphere and illuminating it from the side opposite the integrating sphere so that the illuminating light passes through the object and enters the integrating sphere. A photodetector is integrated into the integrating sphere, which receives the penetrating light and converts it into a measurement signal. The detection direction of this photosensor forms a predetermined angle with the normal to the object surface, which also forms the measurement axis.

[0005] In a different measurement method, also specified in the ASTM D 1003 standard, the integrating sphere itself is used as the illumination device. The integrating sphere illuminates the object positioned in front of its light exit aperture with diffuse light. In this case, the photodetector is located on the other side of the object, opposite the integrating sphere.

[0006] In the latter case, a light trap is provided within the integrating sphere. This can be activated and, when active, blocks the directed portion of the light so that it does not reach the photodetector. By alternately activating and deactivating the light trap, two different transmission values ​​are determined. One of these values ​​provides a measure of the diffuse transmission Tdiffus, where the light is scattered by the influence of the object at an angle > 2.5°; the other value provides a measure of the total transmission Ttotal, where no light scattering by the object or only scattering at an angle ≤ 2.5° occurs. From these two values, the haze of the object is determined using the function HAZE = Tdiffus / Ttotal*100%.

[0007] DE 27 57 196 A1 describes an optical measurement method for measuring the transmission and reflection properties of objects using two photometer spheres. The document describes an optical measurement method for measuring the transmission and reflection properties of objects using two partial beams of a light source, but without determining the diffuse transmission.

[0008] DE 100 10 213 B4 describes a "measuring device, particularly for quality monitoring in continuous processes," which operates according to the principle of spectroscopy. This measuring device has a measuring head with an integrating sphere that serves to diffusely illuminate the measurement object. This arrangement allows the total transmission of the illuminating light through the object and the reflection of the illuminating light from a surface of the object to be measured sequentially.

[0009] DE 10 2009 040 642 B3 describes a method for measuring optical parameters of transparent, scattering measurement objects and a device for carrying out this method, in particular intended for the inline measurement of various transmission and reflection values ​​of plate- or strip-shaped substrates which are provided with transparent layers in a coating system.

[0010] In the latter method, the measurement object is illuminated with diffuse light using an integrating sphere. The light passing through the measurement object is simultaneously detected by two photodetectors with diverging detection directions. In one of the two detection directions, the radiation directed directly at the photodetector is suppressed by a light trap. This method and the device used to implement it make it possible to simultaneously determine the diffuse transmission Tdiffus and the total transmission Ttotal. Two additional photodetectors are also present, one of which detects the diffuse light reflection from one of the object surfaces, the other the total light reflection.

[0011] In the context of process and quality control during the production of large-area coated substrates, there is often a requirement to determine both the diffuse transmission and the total transmission, as well as the light reflection at both surfaces of the substrate—that is, both the reflection at the coated substrate surface and the reflection at the uncoated substrate surface—with minimal technical effort. This requirement cannot be met with the methods and devices available at the state of the art.

[0012] The invention is therefore based on the object of specifying a method and at least one device which no longer have the above-described disadvantages of the prior art.

[0013] According to the invention, the measurement of the transmission and reflection properties of large-area translucent objects, in particular the inline process and quality control in the production of large-area coated substrates, is carried out according to the method steps specified in claim 1.

[0014] Accordingly, in a first variant of the method according to the invention, the measurement is carried out with only two illumination devices emitting diffuse light and two photodetectors, in that: - a first large area of ​​the object is illuminated with a first of the illumination devices and at the same time the total transmission is measured with a first photodetector located opposite the object and the reflection on this large area is measured with a second photodetector located on the side of this large area, and then - the second large surface of the object opposite the first is illuminated with the other illumination device and at the same time the diffuse transmission is measured with the second photodetector and the reflection on this large surface is measured with the first photodetector.

[0015] In an alternative variant of the method according to the invention, the measurement is also carried out with only two illumination devices emitting diffuse light and two photodetectors, in that: - a first large area of ​​the object is illuminated with a first of the illumination devices and the total transmission is measured with a first photodetector, and - the second large surface of the object opposite the first is illuminated with the second illumination device and the diffuse transmission is measured with the second photodetector.

[0016] Additional, optional measurements are carried out by - the first large area of ​​the object is illuminated with the first illumination device and the reflection on this large area is measured with the second photodetector, - the second large area of ​​the object is illuminated with the second illumination device and the reflection on this large area is measured with the first photodetector.

[0017] In both variants, the sequence of two or four process steps constitutes a measurement cycle, with which the transmission and reflection properties are determined relative to a region of a large-area translucent object. The order of the process steps within a measurement cycle can be changed as desired and adapted to a specific measurement task. It is also within the scope of the invention to create measurement cycles that relate solely to the determination of the two transmission values ​​Tdiffus and Ttotal or the determination of the reflection properties of the opposing surfaces of the object.

[0018] The measured values ​​recorded during a measuring cycle are transmitted to a computer with evaluation software, which is connected to an information output which provides the determined values ​​for both the diffuse and total transmission as well as for the reflection properties of both object surfaces for evaluation or further processing, such as the aforementioned linking of T diffuse / T total*100% to a HAZE value for the turbidity.

[0019] A significant advantage of the method according to the invention is that the transmission and reflection properties can be determined with a lower technical effort compared to the prior art, which applies to both method variants.

[0020] The two illumination devices and the two photosensors are fixedly positioned relative to one another. With respect to the object, one of the illumination devices and one of the photodetectors is spatially assigned to a first large surface of the object, while the other illumination source and the second photodetector are spatially assigned to the opposite large surface, such that one illumination device and one photodetector are located opposite one another on one side of the object, and the other illumination device and the second photodetector are located opposite one another on the other side of the object. The illumination devices are preferably located on a common axis, referred to in the invention as the measuring axis, which is a normal to the large surfaces of the object.

[0021] The lighting devices are always switched on alternately, so that only one of the two large surfaces is illuminated for transmission measurement or one for reflection measurement.

[0022] Direction-sensitive photodetectors are used as photodetectors. To achieve or enhance directional sensitivity, direction-sensitive optics can be placed upstream of the photodetectors.

[0023] The detection directions of the two photodetectors are aligned at an angle to each other and to the measurement axis. In a preferred embodiment of the method, the two detection directions and the measurement axis are inclined relative to each other such that they intersect at a single point. This intersection point can advantageously be positioned within the object, between the two large surfaces of the object.

[0024] The detection direction of one of the photodetectors is aligned in such a way that it always points to a large area of ​​the object for the purpose of reflection measurement and to a light trap opposite the object for the purpose of measuring the diffuse transmission, which filters out the directly directed portion of the illumination light that hits the object from the illumination device on the opposite side of the object and penetrates it.

[0025] The detection direction of another photodetector is aligned in such a way that it always points to the other large surface of the object for the purpose of reflection measurement and to a surface opposite the object that reflects the illuminating light and does not mask out the directly directed portion of the illuminating light for the purpose of measuring the total transmission.

[0026] Furthermore, the measurement axis and the detection direction of the first photodetector lie in a first measurement plane, while the measurement axis and the detection direction of the second photodetector lie in a second measurement plane. According to the invention, these two measurement planes are positioned such that they preferably enclose an angle α ≠ 180°, particularly preferably an angle α = 90°, in order to prevent the detection direction of the first photodetector from being directed toward the light trap during the reflection measurement and therefore only measuring the reflection of the diffuse component of the illumination light. The angle α = 180° is therefore only chosen for measurement tasks in which a value for the reflection of the diffuse component is of interest.

[0027] Integrating spheres, each with an integrated light source, are preferred as illumination devices. The integrating spheres are positioned centrally on the measurement axis on either side of the object. The detection directions of the two photodetectors are directed in opposite directions, into the respective opposite integrating sphere.

[0028] In addition to more general applications for various types of objects, the method according to the invention is particularly advantageous for quality control directly during the production of surface-coated substrates. For this purpose, the substrate is moved past the two illumination devices. In successive, periodically repeated measurement cycles, the described optical parameters of substrate areas are obtained. Their spatial extent, viewed in the direction of movement, is determined by the sum of the illumination durations in the individual measurement phases, the sum of the time periods between the individual measurement phases, and the speed at which the substrate is moved.In order to keep this spatial extent of the substrate area to be measured as small as possible, the illumination of the substrate within each measurement cycle is preferably carried out in a stroboscopic manner, so that an optimal relationship between the duration of the measurement and the speed of the substrate movement is achieved.

[0029] The method according to the invention can be used to measure the transmission properties according to the specifications of the international standard ASTM D 1003.

[0030] The invention further relates to an optical measuring device for implementing the method according to the invention. This measuring device essentially comprises: - a first illumination device in the form of an integrating sphere, from which diffuse light is directed through a light exit opening onto a first large area of ​​the object, - a second illumination device in the form of an integrating sphere, from which diffuse light is directed through a light exit opening onto the second large surface of the object, which is parallel to the first, - a photodetector, the detection direction of which points through both light exit openings to a light trap located on the inner wall of the second illumination device, which filters out the directly directed portion of the illumination light, - a further photodetector, the detection direction of which points through both light exit openings to the light-reflecting inner wall of the first illumination device.

[0031] Each integrating sphere has a light source that can be switched on and off, and both light sources are connected to a control circuit for alternating switching on, whereby - each lighting device has a light source that can be switched on and off and is connected to a control circuit for alternating switching on, wherein - to measure the diffuse transmission and / or to measure the reflection on the large surface, the light source in the first lighting device is switched on, - to measure the total transmission and / or to measure the reflection on the large surface, the light source in the second lighting device is switched on, and wherein - the photodetectors are connected to an evaluation circuit at least during the switching-on time of the light sources.

[0032] The photodetectors are preferably integrated into the inner wall of the integrating sphere assigned to them.

[0033] In a particularly preferred embodiment, reference photodetectors are present in the integrating spheres, the detection directions of which are directed toward the reflective inner surface of the respective integrating sphere for the purpose of obtaining reference signals. Furthermore, shielding means are provided within the integrating spheres to prevent light from the light source from radiating directly into the photodetectors for obtaining measured values, as well as into the photodetectors for obtaining reference values.

[0034] Furthermore, the detection direction of the two photodetectors and the normal of the large surfaces are advantageously inclined relative to each other such that they have a common intersection point located between the two large surfaces within the object. The normal forms a measurement axis with respect to the two detection directions.

[0035] Preferably, the detection directions of the two photodetectors each enclose an angle of 8° with the normal or the measuring axis, so that the requirement of the ASTM D 1003 standard is met.

[0036] In particular with regard to obtaining the reflection values, the detection directions of the first and the second photosensor are located in different measuring planes, for example in measuring planes that are offset by 90° around the normal to each other, in order to prevent the detection direction of the first photodetector from being directed towards the light trap during the reflection measurement and therefore only measuring the reflection of the diffuse portion of the illumination light, as will be explained below using an exemplary embodiment.

[0037] In particular for the embodiments of the invention in which a relative movement between the object and the measuring device is provided, the light exit openings of the integrating spheres are arranged at distances from the object that allow an unhindered relative movement.

[0038] The invention is explained in more detail below using an exemplary embodiment. The accompanying drawings show: Fig. 1 a schematic diagram of the measuring device according to the invention in section through a plane in which the measuring axis, the detection direction of one of two photodetectors and the direction of movement of a large-area, single-sided coated substrate are located, for which the turbidity and the reflection properties of both the coated and the uncoated large area are to be measured periodically, Fig. 2 the principle representation of the measuring device according to the invention according to Fig. 1, but in section through a plane in which the measuring axis and the detection direction of the other photodetector lie, while the direction of movement of the substrate runs perpendicular into the plane of the drawing.

[0039] In Fig. 1 symbolically depicts a band-shaped, translucent object 1, here, for example, a substrate, which is coated with a transparent, electrically conductive layer on its large surface 2 in an inline coating system. The object 1 is continuously moved in the direction R and, following the coating process, passes Fig. 1 shows the measuring device according to the invention.

[0040] In the example chosen here, quality control should refer to the measurement and assessment of the transmission properties of the coated object and the measurement of the reflectivity of both the coated large surface 2 and the uncoated large surface 3 of object 1. It is performed periodically while the substrate strip continuously moves past the measuring device.

[0041] To determine the transmission properties, the measurement method according to the international standard ASTM D 1003 must be applied. The total transmitted component Ttotal, which includes both the directed and diffuse transmission of the illumination light, and the diffuse transmitted component Tdiffuse, which includes only the diffuse component of the illumination light, are measured. The transmission values ​​Tdiffuse and Ttotal thus determined can then be further processed into a HAZE value for turbidity using the function Tdiffuse / Ttotal*100%.

[0042] The reflection properties for both large surfaces 2, 3 are to be evaluated based on the reflected illumination light.

[0043] Fig. Figure 1 shows the measuring device according to the invention in section through a plane containing a photodetector 4, its detection direction D4, the measuring axis 5, which is also a normal to the large surfaces 2, 3, and the direction of movement R of the object 1. The detection direction D4 preferably forms an angle β = 8° with the measuring axis 5, thus fulfilling the requirements of the international standard. Different angle dimensions are of course possible, but they will produce different measurement results.

[0044] At a distance a1 from the large surface 2, the light exit opening 6 of an illumination device 7, designed as an integrating sphere, is arranged, which emits diffuse light and serves to illuminate the object 1 from the coated large surface 2. Opposite the object 1, at a distance a2 from the large surface 3, is arranged the light exit opening 8 of another illumination device 9, also designed as an integrating sphere, which emits diffuse light. This illumination device is intended to illuminate the object 1 from the large surface 3. The distances a1 and a2 are kept as small as possible. They are only large enough to allow unhindered movement of the object 1 relative to the two illumination devices 7, 9.

[0045] A light source 10 is integrated into the illumination device 7, and a light source 11 is located in the illumination device 9. Both light sources 10, 11 are designed, for example, as high-performance LEDs that can be switched on briefly and are connected to a control circuit (not shown in the drawing) for the purpose of switching on. A light trap 15 is provided in the illumination device 7, toward which the detection direction D4 of the photodetector 4 is directed.

[0046] In order to be able to evaluate and incorporate into the measurement results any distortions in the characteristics of the illumination light emerging from the illumination devices 7, 9 and striking the respective large area 2, 3, which may be caused, for example, by extraneous light interference due to the distances a1, a2, reference photodetectors 12 and 13 are present in the two illumination devices 7, 9. Their detection directions D12, D13 are directed toward the reflective inner wall of the respective illumination device 7, 9. The signal outputs of the reference photodetectors 12, 13, like the signal outputs of the photodetectors 4, 14, are connected to an evaluation circuit (not shown).

[0047] The photodetector 4 and the reference photodetectors 12, 13 are positioned so that no light from the light sources 10, 11 can enter from a direct direction. Shielding means may be provided to prevent such direct light incidence and thus the falsification of the measurement results.

[0048] Fig. 2 shows a section AA from Fig. 1. From Fig. 2 again shows the object 1, but here in its bandwidth, the measuring axis 5, the illumination devices 7 and 9 arranged symmetrically to the measuring axis 5, and the distances a1, a2 between the object 1 and the light exit openings 6 and 8. The direction of movement of the object 1 runs perpendicularly into the plane of the drawing. For the sake of clarity, the illustrations of the photodetector 4 and the light source 11, which are actually visible in the illumination device 9, as well as the reference photodetector 12, which is actually visible in the illumination device 7, have been omitted.

[0049] Can be seen in Fig. 2, however, a photodetector 14 integrated into the illumination device 7 in this plane and its detection direction D14, which points to the inner surface of the opposite illumination device 9.

[0050] As can be seen from the summary of Fig. 1 and Fig. 2, the measuring axis 5 and the detection direction D4 are in the Fig. 1, the measuring axis 5 and the detection direction D14, however, in the plane shown in Fig. 2. Both planes form measuring planes that are rotated by 90° around the measuring axis 5. The reference detection directions D12 and D13 are only exemplary in the plane of the Fig. 1.

[0051] The measuring method according to the invention is carried out in its first variant with this device as follows, whereby the sequence of the two method steps mentioned can be specified as desired: - the large surface 3 of the object 1 is illuminated with the first of the illumination devices 9, whereby the total transmission is measured with the photodetector 14 and the reflection is measured with the photodetector 4 on this large surface 3, - the second large surface 2 of the object 1 opposite the first is illuminated with the second illumination device 7, wherein at the same time the diffuse transmission is measured by the second photodetector 4 and the reflection on this large surface 2 is measured by the photodetector 14.

[0052] The second variant of the measuring method according to the invention can be carried out with this device as follows, whereby the sequence of the four method steps mentioned can be specified as desired, whereby the last two, concerning the measurement of the reflection, are optional: - the object 1 is illuminated by means of a first illumination device 9 and the total transmission is measured with a first photodetector 14, - the object 1 is illuminated with the second illumination device 7 and the diffuse transmission is measured with the second photodetector 4, - the first large area 3 of the object 1 is illuminated with the first illumination device 9 and the reflection on this large area 3 is measured with the second photodetector 4, - the second large area 2 of the object 1 is illuminated with the second illumination device 7 and the reflection on this large area 2 is measured with the first photodetector 14.

[0053] As described above, process steps in both process variants each constitute a measurement cycle. The measured values ​​acquired by the two photodetectors 4 and 14 during the measurement cycles are transmitted to a computer with evaluation software, where the determined values ​​for the diffuse and total transmission, as well as for the reflection properties of both large surfaces 2 and 3, are evaluated or further processed, for example, by combining T diffuse / T total*100% to form a HAZE value for turbidity. List of reference symbols 1 object 2 large area 3 large area 4 Photodetector 5 measuring axis 6 Light exit opening 7 Lighting equipment 8 Light exit opening 9 Lighting equipment 10 Light source 11 Light source 12 Reference photodetector 13 Reference photodetector 14 Photodetector 15 Light trap

Claims

[1] Optical measuring method for measuring the transmission and reflection properties of, in particular, large-area translucent objects (1), preferably applicable for process and quality control in the production of surface-coated substrates, wherein - the measurement is carried out with two illuminating devices (7, 9) emitting diffuse light and two photodetectors (4, 14) by either: - a first large area (3) of the object (1) is illuminated with a first of the illumination devices (9) and at the same time the total transmission is measured with a first photodetector (14) and the reflection at this large area (3) is measured with the second photodetector (4), and then - the second large surface (2) of the object (1) opposite the first is illuminated with the second illumination device (7) and at the same time the diffuse transmission is measured with the second photodetector (4) and the reflection at this large surface (2) is measured with the first photodetector (14), or sequentially in any desired order: - a first large area (3) of the object (1) is illuminated with a first of the illumination devices (9) and the total transmission is measured with a first photodetector (14), - the second large surface (2) of the object (1) opposite the first is illuminated with the second illumination device (7) and the diffuse transmission is measured with the second photodetector (4). [2] Optical measuring method according to claim 1, wherein in the case of a sequential measurement additionally - the first large area (3) of the object (1) is illuminated with the first illumination device (9) and the reflection on this large area (3) is measured with the second photodetector (4), and / or - the second large area (2) of the object (1) is illuminated with the second illumination device (7) and the reflection on this large area (2) is measured with the first photodetector (14). [3] Optical measuring method according to claim 1 or 2, in which - the two lighting devices (7, 9) and the two photosensors (4, 14) are fixedly positioned with respect to each other, - in relation to the object (1), a lighting device (9) and a photodetector (4) are spatially assigned to a first large area (3) of the object (1) and the other lighting source (7) and the second photodetector (14) are spatially assigned to the opposite large area (2), and - the lighting devices (7, 9) are always switched on alternately so that only one of the two large surfaces (2, 3) is illuminated at a time. [4] Optical measuring method according to one of the preceding claims, in which direction-sensitive photodetectors (4, 14) are used or a direction-sensitive optic is arranged upstream of the photodetectors (4, 14). [5] Optical measuring method according to one of the preceding claims, in which integrating spheres each with an integrated light source (10, 11) are used as illumination devices (7, 9). [6] Optical measuring method according to one of the preceding claims, in which - the object (1) is moved past the two lighting devices (7, 9), - the spatial extent of an object area measured during a measurement is determined in the direction of movement R - by the respective lighting duration, - the time intervals between the individual measurements, and - the speed of movement of the object (1), and - the measurements are repeated periodically to measure a variety of object areas. [7] Optical measuring method according to one of the preceding claims, in which the measurement of the transmission properties is carried out according to the specifications of the international standard ASTM D 1003. [8] Optical measuring method according to one of the preceding claims, in which a HAZE value for the turbidity of the object (1) is determined from the measurement results for the total and diffuse transmission according to the function HAZE = Tdiffus / Ttotal*100%. [9] Optical measuring device for determining the transmission and reflection properties of large-area translucent objects (1), preferably in the form of coated substrates, which are continuously moved past the measuring device, comprising: - a first illumination device (9), designed as an integrating sphere, from which light is directed through a light exit opening (8) onto a first large surface (3) of the object (1), - a second illumination device (7), designed as an integrating sphere, from which light is directed through a light exit opening (6) onto the large surface (2) of the object (1) opposite the first, - a direction-sensitive photodetector (4), the detection direction (D4) of which points through both light exit openings (6, 8) to a light trap (15) located on the inner wall of the second illumination device (7), - a direction-sensitive photodetector (14), the detection direction (D14) of which points through both light exit openings (6, 8) to the reflecting inner wall of the first illumination device (9), wherein - the detection directions (D4, D14) are in different measuring planes. [10] Optical measuring device according to claim 9, in which - each lighting device (7, 9) has a light source (10, 11) which can be switched on and off and is connected to a control circuit for alternating switching on, wherein - to measure the diffuse transmission and / or to measure the reflection on the large surface (3), the light source (11) in the first lighting device (9) is switched on, - for measuring the total transmission and / or for measuring the reflection on the large surface (2), the light source (10) in the second illumination device (7) is switched on, and wherein - the photodetectors (4, 14) are connected to an evaluation circuit at least during the switching-on time of the light sources (10, 11). [11] Optical measuring device according to claim 10, wherein the photodetector (4) is integrated into the inner wall of the first illumination device (9) and the photodetector (14) is integrated into the inner wall of the second illumination device (7). [12] Optical measuring device according to claim 10 or 11, in which - a reference photodetector (12, 13) is provided in each of the illumination devices (7, 9) for the purpose of obtaining a reference signal for the illumination light, and - shielding means are provided within the lighting devices (7, 9) to prevent direct light irradiation from the respective light source (10, 11) into the photodetectors (4, 12, 13, 14). [13] Optical measuring device according to one of claims 9 to 12, wherein the detection direction (D4) of the photodetector (4), the detection direction (D14) of the photodetector (14) and the normal of the large surfaces (2, 3) are inclined relative to one another, preferably having a common intersection point which is located between the two large surfaces (2, 3) within the object (1). [14] Optical measuring device according to claim 13, wherein the detection directions (D4, D14) of the two photodetectors (4, 14) each enclose an angle of β = 8° with the normal. [15] Optical measuring device according to claim 13 or 14, in which the different measuring planes in which the detection directions (D4, D14) lie are offset from one another by an angle α = 90° around the normal. [16] Optical measuring device according to one of claims 9 to 15, in which the light exit openings (6, 8) of the illumination devices (7, 9) are arranged at distances (a1, a2) from the object (1) which allow an unhindered relative movement between the object (1) and the illumination devices (7, 9).

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