Multi-stage procedure for examining surfaces and corresponding device

Abstract

Multi-stage procedure for determining surface properties with the following steps: - Determining at least one topographic property of at least one area (B1, B2) of a surface (10) to be investigated, wherein the surface (10) is selected from a group of surfaces which includes plastic surfaces, painted surfaces and natural leather surfaces, and wherein this surface (10) is illuminated from at least two directions and the surface (10) is illuminated from a direction which forms an angle between 40° and 75° with a direction perpendicular to the surface (10), wherein the topographic property is selected from a group of properties which includes a height position of areas of the surface (10), a curvature of areas of the surface (10) and / or a change in curvature of the areas of the surface (10), wherein certain topographic properties are defined and then individual areas (B1, B2) of the surface (10) are assigned to these areas and / or wherein at least two areas (B1, B2) of the surface are defined which differ in their curvature and / or a change in curvature or a plurality of areas (B1, B2) of the surface which differ in the height position of the surface are defined and the topographic property is determined for each of these areas (B1, B2), - Determining an optical property of at least this area (B1, B2) of the surface (10) to be investigated and outputting at least one value in a second process step that is characteristic of this optical property, wherein, for the purpose of determining the optical property, a further radiation device (6) shines light onto the surface (10), wherein a spatially resolved image of the light reflected or scattered from the surface (10) is recorded, wherein When determining the value that is characteristic of this optical property, the topographic property of the area (B1, B2) is taken into account, and the optical properties are selected or determined depending on this area (B1, B2), and only those areas (B1, B2) of the surface (10) are evaluated whose topographic properties correspond to the conditions specified in a previous procedure step.

Description

[0001] The present invention relates to a method and a device for examining surfaces, particularly with regard to optically perceptible properties. Numerous methods for examining surface properties are known in the prior art. These methods often aim to obtain the most realistic or objective impression of a surface possible, for example, also taking into account influences such as the incident light. In some of these devices, light is therefore shone onto the surface at a predetermined angle, and the light scattered and / or reflected by the surface is observed with an image acquisition device, such as a camera.

[0002] Structured surfaces are also known from the prior art; that is, surfaces which exhibit a certain structure, in particular a topography, such as roughness, imprinted three-dimensional structures, or curvatures, perpendicular to their longitudinal direction. While such a structured surface can also be evaluated optically, the problem sometimes arises that the optical evaluation cannot distinguish whether certain measured values ​​or phenomena actually result from color or gloss differences of the observed surface, or whether these impressions result from the structure or topography of the surface, for example, as a consequence of shadows and shading or from areas of the device observed from different directions.When images are taken of curved or structured surfaces, the results of an image analysis can also be derived from this curvature or structure of the surface under investigation. The visually perceived structure, i.e., the overall impression of the surface, is composed of its reflectivity and topography. Reflectivity, in turn, is subdivided into specular reflection (gloss, DOI, haze, orange peel) and non-specific reflection (color).

[0003] From DE 10 2012 104 282 A1, a method and a device for inspecting the surfaces of an object under investigation are known. In this method, a surface is illuminated with a lighting device, an image is captured using an area image sensor, and the captured images are then supplied to an image analysis device.

[0004] US patent 2004 / 0119833 A1 describes a three-dimensional scanner and a method. A pattern is captured, and the areas that need to be scanned at a higher resolution are identified.

[0005] DE 100 63 293 A1 describes a method and a device for multi-channel surface inspection in a continuous process. Two light sources are used, which are switched alternately synchronously with a camera's recording cycle.

[0006] A picture acquisition system is known from US patent 2010 / 0026850 A1. This system includes a light source for illuminating areas of an object and a camera for recording the illuminated object.

[0007] The subsequently published EP 2 956 084 A1 discloses a 3D scanner for scanning objects, especially teeth.

[0008] The present invention is therefore based on the objective of achieving the most realistic possible evaluation of surfaces and, in particular, of enabling observation that can also take into account or differentiate effects caused by surface topography. These objectives are achieved according to the invention by the subject matter of the independent claims. Advantageous embodiments and further developments are the subject matter of the dependent claims.

[0009] In a multi-stage method according to the invention for determining surface properties, in a first step at least one structural property and / or topographic property of at least one region of a surface to be investigated is determined. In a further process step, an optical property of at least this region of the surface to be investigated is determined and at least one value characteristic of this optical property is output.

[0010] According to the invention, the structural or topographical property of this area is taken into account when determining the value characteristic of this optical property. This consideration can be carried out in various ways. For example, the value to be determined could depend on the determined topographical property, such as a curvature of the surface, and this curvature could be included in a calculation. Alternatively, as explained in more detail below, the surface could first be divided into several areas with different topographical properties, and these topographical properties could then be considered separately for further evaluation.

[0011] Taking into account means that the aforementioned topographical properties are taken into account, in particular by using at least one measured value that is characteristic of these properties, or a value derived from them, in the evaluation, for example by calculation.

[0012] For example, the curvature of a surface can be determined, or a contour line can be derived, particularly in a specific area or at a specific point. This value can then be mathematically related to a specific optical property determined at that point, for instance, by using both this value and the optical property to determine or calculate a result. Knowing the geometry of the measuring device allows the angle-dependent reflection behavior to be determined without a goniometric setup. Here, the radiation directions of the incident and reflected light are preferably measured with respect to the surface normal of the individual surface areas or points. A surface point is a very small surface area. It is advantageous for the evaluation if the observation direction and the direction of the incident light coincide.Instead of curvature, the (absolute or relative) height of the point in question can also be used. From this, further surface properties can be derived, such as the difference in reflectance between higher and lower surface segments.

[0013] Preferably, in a further process step, at least two regions of this surface are defined that differ in this topographic property. In a further process step, an optical property of at least one of these regions of the surface under investigation is preferably determined. It would also be possible to evaluate these surface regions separately. The properties of the different surface regions can also be correlated (for example, within the framework of a statistical analysis).

[0014] This approach is particularly suitable for rough, uneven, or structured surfaces. In the first step, the "valleys" and "peaks" of this surface can be distinguished, especially if it is a three-dimensional structure exhibiting this peak-and-valley behavior. These peaks and peaks can then be treated separately in a subsequent step. In the following analysis, these areas can then be evaluated and measured separately. If the surface to be measured has a specific curvature, this curvature, determined in the first step, can be incorporated into the optical measurement and / or the analysis. This can be done using software.

[0015] This measurement method is particularly suitable for use in quality assurance, defect detection, and / or characterization. Suitable surfaces for examination include, in particular, plastic surfaces, painted surfaces, natural surfaces such as wood or leather, woven fabrics, paper, embossed and etched structures on metal surfaces, and similar materials.

[0016] Tolerance ranges can be defined in the various application areas. For example, tolerance ranges can be defined within which the surfaces are still considered visually identical.

[0017] Therefore, a two-stage method is proposed within the scope of the invention. In a first step, the surface to be examined is analyzed with regard to its topographic properties; in particular, the height h(x, y) (where x and y are two mutually perpendicular directions of extension of this surface) and / or curvature and / or change in curvature of individual areas of the surface can be analyzed. Preferably, at least two images of the surface are taken to determine the topography, more preferably at least three images, and more preferably at least four images. Preferably, the surface is illuminated from at least partially different directions in these different images.

[0018] These areas can be regions of the surface that are offset from other regions in a direction perpendicular to the surface. These can be sub-regions of any size. It would even be possible to distinguish between the smallest, resolvable regions, for example, optically or visually without optical aids. In one implementation, it would be possible to first define topographic properties, such as a first and a second elevation range, and then assign individual regions of the surface to these elevation ranges.

[0019] In the second step of the process, the optical properties can then be selected or determined depending on this area. For example, only those areas of the surface whose topographic properties correspond to the conditions specified in the first step, such as a specific elevation or elevation range, can be evaluated in a second step. In this way, the aforementioned topographic properties can also be taken into account when evaluating the surface.

[0020] In another preferred method, the topographic property is selected from a group of properties that includes the elevation position of areas of the surface, the elevation profile of areas of the surface, the curvature of areas of the surface, the curvature profile of areas of the surface, the change in curvature of areas of the surface, the curvature change profile of areas of the surface, and the like. In particular, the elevation positions and the curvatures of the surface are especially relevant for the evaluation of the topographic properties.

[0021] In a preferred method, a measuring range for the investigation of the optical property is defined as a function of at least one measured value that is characteristic of the topographic property of the surface.

[0022] In other words, in the first step of the process, the surface is subdivided into at least two elevation ranges (for example, by defining an elevation limit). Subsequently, based on the measured values ​​for the topographic property (such as elevation), at least two groups are formed, assigned to these two elevation ranges, and distinguished from each other in the second step. As mentioned, this topographic parameter, or the measured value for the topographic parameter, is advantageously a profile of the surface in a direction perpendicular to the surface.

[0023] In another preferred method, it would be possible to define several different regions. For example, a first region could be defined with surface section heights ranging from 0 to 5 µm, another region between 5 and 10 µm, and a further region between 10 and 20 µm. Advantageously, this allows the topographic properties of the surface under investigation to be determined with spatial resolution. This means that a measurement value for the topographic property is output for a multitude of sub-regions or positional sections of the surface. Preferably, in a further step, the topographic properties of the sub-regions or positional sections are correlated, and a measurement value is generated or calculated from this correlation.

[0024] In another preferred method, radiation is directed onto the surface using a radiation device to determine the optical properties, and radiation reflected and / or scattered from the surface is detected by a radiation detector device. Advantageously, the method for determining the optical properties of the surface is an optical measurement method in which radiation reflected and / or scattered from the surface is recorded. Advantageously, the radiation detector device acquires a spatially resolved image of the surface. Advantageously, this spatially resolved image is further subdivided according to the topographical properties of the first method step.To determine the optical properties of an image, specific pixels or pixel ranges can be extracted from the captured image that correspond to specific topographical features, and it would be possible to consider only these areas. For example, when determining the optical properties, only those image sections, such as pixels, that yielded specific topographical values ​​during the initial measurement could be considered.

[0025] Advantageously, the radiation detector device detects radiation that is scattered and / or reflected substantially perpendicular to the plane of the surface under investigation. In this embodiment, the radiation detector device is therefore preferably arranged substantially perpendicular to the surface under investigation. However, other detection directions are also conceivable. Observation in the perpendicular direction offers the advantage that the observation can be carried out independently of the sample orientation.

[0026] Advantageously, to determine the optical properties, the radiation device shines light onto the surface at an angle of less than 30°, preferably less than 20°, and preferably less than 10°, with respect to a normal or a direction perpendicular to the surface. It is possible that the radiation emitted by the first radiation device is coupled into a perpendicular beam direction, via a coupling device such as a partially reflective mirror, through which the radiation travels from the surface to the radiation detector device. Thus, the illumination and detection directions are identical.

[0027] In another embodiment, light of different wavelengths can be successively shone onto the surface. A spatially resolved and / or monochrome detector can then be used to perform a spatially resolved color measurement. Alternatively, the color determination can be carried out using a spatially resolved color sensor, such as an RGB camera, and a polychrome light source.

[0028] Illumination for investigating optical properties such as color can be achieved at a specific angular arrangement using parallel, convergent, or divergent beam paths. Spatially diffuse illumination, generated, for example, by an integrating sphere or a diffuser device, is also conceivable.

[0029] Various illumination-detector arrangements are also conceivable. The angle between illumination and detection can take values ​​between 0° and 90°, preferably between 30° and 60°, and particularly preferably between 40° and 50°.

[0030] One embodiment may include polychrome illumination and a spectrally resolved detector; in another embodiment, monochromatic light of different wavelengths may be used for illumination and a non-spectral-resolving detector. Combinations of these approaches are also conceivable.

[0031] In a further advantageous method, the radiation detector device acquires at least two images of the surface under investigation to determine its optical properties. This increases the dynamic range of the radiation detector device. Preferably, the surface can be imaged multiple times with different exposure times. These images can then preferably be processed in such a way that low-reflection and high-reflection areas are compared, thereby increasing the dynamic range of the overall image. In this way, a composite image can be calculated, which is composed of the individual images, particularly those taken of the same surface area.

[0032] For individual images with different exposure times, certain permissible brightness limits can be defined.

[0033] The intensity values ​​of the individual pixels in the overall image can be calculated, for example, by averaging. Over- and underexposed areas can be disregarded when combining the individual images. The resulting composite image has a higher dynamic range in brightness reproduction than the individual images.

[0034] Preferably, the optical property of the surface to be determined is selected from a group of properties which includes a reflectivity of the surface, a gloss of the surface, a color of the surface and the like.

[0035] Preferably, at least partially the same surface areas of the surface under investigation are observed or evaluated in the first and second process steps. Advantageously, but not necessarily, the same areas of the surface under investigation are examined in the first and second process steps. This makes it possible and advantageous for the respective surface areas to be recorded using the same radiation detection device.

[0036] To analyze and align the same surface areas in the first and second processing steps, it is possible to project markers, such as projection points, onto the surface. This allows the images to be superimposed pixel by pixel. This is particularly useful when different detectors are used. In other words, individual pixels of an image can be identified and assigned to specific points on the surface. Based on this assignment, when a second image is captured, it can be aligned with the first. This method also allows for analysis even if slightly different areas of the surface were captured in the first and second processing steps. Therefore, the precise position of the image acquisition device relative to the surface does not need to be maintained throughout the entire process.Rather, a (deliberate or unintentional) change in position can be taken into account.

[0037] In this way, images can not only be brought into pixel alignment by moving them, but image processing functions such as zooming, tilting, perspective changes, and distortion correction can also be applied to achieve this goal.

[0038] In another advantageous method, radiation is also directed onto the surface by means of a radiation device to determine the topographic properties, and radiation reflected and / or scattered from the surface is detected by means of a radiation detector device. Advantageously, this radiation detector device also provides a spatially resolved image of the incident radiation.

[0039] For this purpose, the radiation detector device may, for example, include a CCD or CMOS chip, a camera, or the like.

[0040] According to the invention, to determine the topographical properties of the surface, this surface is illuminated from at least two, and preferably several, different directions. In a further preferred method, the surface is illuminated from a direction that forms an angle with the surface normal or a direction perpendicular to the surface, with an angle between 40° and 90°, preferably between 45° and 85°, preferably between 45° and 80°, and preferably between 45° and 75°. In the preferred geometry, the radiation is directed onto the surface at a relatively shallow angle. The exposure time of the radiation detector device is preferably adapted to the optical properties of the surface. Over- and underexposed areas are preferably to be avoided.

[0041] A variable angle setting is also conceivable. This allows the optimal angle for illuminating and subsequently evaluating the sample to be selected based on the surface amplitude and / or reflectance of the sample being examined. It is also conceivable that different illumination geometries could be implemented in a single device and / or selectively controlled.

[0042] By illuminating the surface from at least two directions, it is possible to measure the surface using photometric analysis. This allows for a shading-based reconstruction of the surface under investigation. Preferably, this reconstruction is based on the surface's photometric properties.

[0043] Other methods such as white light interferometry, confocal techniques or triangulation methods are also possible.

[0044] Alternatively, the entire surface could be illuminated (including directions perpendicular to the surface). Another option would be to illuminate the surface with pulsed light. The image acquisition could then be synchronized to this pulsed light illumination.

[0045] The reconstructed topography and other surface properties can be encoded as an image. For example, in the topography, the elevation is represented as a grayscale value in the black and white image. This type of data processing allows for analysis using modern image analysis techniques.

[0046] Preferably, at least two images of the surface under investigation are taken to determine its topographical properties. In this way, several irradiances (the total power of the incoming electromagnetic energy striking a surface, relative to the area) can be assigned to a single imaged surface point, particularly without a correspondence problem arising.

[0047] Preferably, an image of the same surface area to be observed is evaluated at least twice, with the first evaluation focusing on the topographic properties and the second evaluation focusing on the optical properties. It would be possible to take only one image and evaluate it for both topographic and optical properties. However, preferably at least two images of the surface under investigation, and preferably two images of the same area of ​​the surface under investigation, are evaluated.

[0048] It is possible that the radiation device used to evaluate the surface for its topographical properties shines the light or radiation onto the surface at a different angle than the radiation device used to evaluate the optical properties.

[0049] As mentioned above, the same radiation detector is preferably used to determine both the optical and topographic properties. Advantageously, the position of the measuring device relative to the surface is maintained for both determinations. This ensures that the same image of the surface under investigation is captured at least twice.

[0050] Advantageously, a spatially resolved image of the surface under investigation is acquired to determine its topographic properties, and / or a spatially resolved image of the surface under investigation is acquired to determine its optical properties. Advantageously, the time interval between acquiring the spatially resolved image for investigating the topographic properties and acquiring the spatially resolved image for investigating the optical properties is less than 3 seconds, preferably less than 2 seconds, preferably less than 1 second, and most preferably less than 200 milliseconds.

[0051] A short acquisition time guarantees a more precise spatial assignment of all image elements from the topographic and optical measurement to their corresponding surface area, as shifts in the measurement area can be more easily avoided. In this way, such a measuring device can also be designed as a handheld instrument, which is also a preferred application.

[0052] It would be possible to first save (at least) an image after the measurement of the topographic properties and analyze it with regard to these properties. The measured values ​​resulting from the spatially resolved image can then be assigned to specific groups. In a further step, the recorded image can potentially be divided pixel by pixel into areas exhibiting the first set of topographic properties and areas exhibiting the second set. In the second step, the image can then be evaluated area by area, meaning that the areas with the first set of structural properties and those with the second set of structural properties can be evaluated separately.Instead of or alongside this separate evaluation, it would also be possible to weight certain areas with specific topographical properties differently compared to other areas during image analysis, or to compare them to one another. A histogram analysis or other statistical analysis is also conceivable, whereby one or more optical properties are assigned to the various areas with different topographical structures.

[0053] In another preferred method, the spatially resolved image is divided into a multitude of image segments, and at least a proportion of the image segments are assigned to the first area and at least a proportion of the image segments to the second area. As mentioned above, the image segments can be pixels at the smallest case or groups of pixels, depending on the desired measurement resolution.

[0054] The present invention further relates to a device for investigating surface properties. This device comprises a structural or topographic measuring device for detecting a (particularly topographic) structure or surface structure, in particular a device for detecting the topography of this surface, which in turn comprises a radiation device that shines radiation onto a surface to be investigated, and a first radiation detector device that detects radiation shining onto the surface and scattered and / or reflected from the surface.In addition, the device has an evaluation unit which determines at least one topographical property of the surface to be examined based on the radiation recorded by the first radiation detector device and outputs at least one measured value that is characteristic of the topography of the surface to be examined.

[0055] Furthermore, the device includes a second measuring unit, which also comprises a radiation device that shines radiation onto the surface under investigation, and a radiation detector that records the radiation incident on the surface and scattered and / or reflected from it. This radiation detector allows for spatially resolved recording and evaluation of the incident radiation.

[0056] According to the invention, the device has a processor unit which outputs a value characteristic of the optical property of the surface, taking into account the measured value and the spatially resolved recording, and / or a value characteristic of this recording, preferably with reference to the segmented areas determined by the topography measuring device.

[0057] It is therefore also proposed on the device side that the topographic properties of the surface under investigation be taken into account during the optical evaluation.

[0058] In a further advantageous embodiment, the device includes a control unit which causes the first radiation device to first shine onto the surface, and the radiation detector to detect the radiation emitted onto the surface and scattered and / or reflected by the surface, and subsequently the second radiation device to shine onto the surface, and the radiation detector to detect the radiation emitted onto the surface and scattered and / or reflected by the surface. Preferably, the radiation detector that is part of the structural measurement device is the same radiation detector that is also part of the second measurement device.

[0059] Furthermore, the device preferably includes an assignment device which, during the measurement, assigns sub-areas of the image acquired by the radiation detector device to a specific structural property of the surface. Preferably, the radiation devices have light sources that shine light onto the surface under investigation. These can be, for example, light-emitting diodes (LEDs) that shine light onto the surface. In another advantageous embodiment, the first radiation device shines light onto the surface at a different angle than the second radiation device.

[0060] Preferably, optical elements are found in a beam path between the respective radiation devices and the radiation detector device, which are selected from a group of elements that includes filters, lenses, mirrors, beam splitters, monochromators, spectrometers, apertures, scattering elements and the like.

[0061] In a preferred embodiment, the lighting and detector arrangement, as well as the control and computing unit, are housed in a portable enclosure. The measuring device can preferably be battery-operated. Measurements can be performed and evaluated on-site using operating elements and a display. Advantageously, the device features a data interface that enables communication with an external computer. This interface also allows for robot-controlled measurement in online mode.

[0062] However, the evaluation can also be carried out on the external computer; the calculated result can then be sent to the device or display.

[0063] Further advantages and embodiments can be seen from the attached drawings:

[0064] It shows: Fig. 1. A representation of a surface to illustrate the problem underlying the invention; Fig. 2 a rough schematic representation of a device according to the invention; Fig. 3. A schematic representation to illustrate image analysis; and Fig. 4 Another schematic representation for evaluation on curved surfaces.

[0065] Fig. Figure 1 shows a possible surface 10, which can be observed with appropriate optical instruments. It can be seen that this surface is uneven and has a multitude of protrusions. Depending on the angle of the light, these protrusions can, for example, cast shadows. A simple analysis based on intensity would reveal different light-dark contrasts on this surface, depending on the angle of the light.

[0066] The invention therefore proposes, as mentioned above, to take the surface texture into account in a two-stage process.

[0067] Fig. Figure 2 shows a schematic representation of a device according to the invention. Here again, a surface 10 to be observed is shown. The device 1 has a radiation detector or image acquisition device 4, which captures images of this surface. This radiation detector device can, in particular, be a color or black-and-white camera or a CCD chip. The radiation detector device can also consist of a spatially resolved spectrometer. For reasons of symmetry, the radiation detector device is arranged centrally above the observation area. The observation direction is 0° with respect to the normal of the observation plane.

[0068] In a first step, light is shone onto a lighting device or radiation device 2, more precisely at a shallow angle (relative to the surface). The angle of incidence, relative to the surface normal, lies between 45° and 85°, preferably between 65° and 75°. Advantageously, the lighting device 2 is a circumferential lighting device with multiple light sources, thus illuminating the surface 10 from several sides (arrow P1). The light scattered or reflected from the surface travels along arrow P2 to the observation device 4, which can thereby also detect the topography of the surface 10.

[0069] In this process, radiation source 2 is preferably switched sequentially, meaning that several images are taken one after the other, with the direction of illumination being changed. A height profile can then be calculated from the various images.

[0070] Reference numeral 6 designates a second lighting device, which serves for the optical evaluation of the surface 10. The light is directed at the Fig. In the embodiment shown in Figure 2, the light is directed onto the surface via a semi-transparent deflecting mirror 18, and the reflected light also reaches the observation device in this way. This device can then perform an optical evaluation of the surface, preferably also taking into account data obtained from the preceding topographic measurement.

[0071] Reference numeral 12 designates a control unit that manages the entire evaluation process. This control unit 12 ensures that, in the first process step, the surface topography is determined, and in the second process step, the actual image evaluation takes place, taking the topography into account. For this purpose, the device may also include an image output unit 14, which outputs an image of the recorded surface to the user. Furthermore, the device may include a storage unit 16, in which, for example, reference data for recorded images are stored. Measured values, such as topographic data, can also be stored in this storage unit.

[0072] Reference numeral 22 designates a processor unit used for evaluating or calculating the optical values. As mentioned above, this processor unit 22 can also take topographic data into account, such as (local) surface curvature or local elevation positions. Reference numeral 24 designates an evaluation unit that determines at least one topographic property of the surface under investigation based on the radiation recorded by the first radiation detector unit. The evaluation can be performed more precisely using the image data acquired by the radiation detector unit.

[0073] Fig. Figure 3 shows an example of a possible image evaluation. The solid line 10 refers to a surface that exhibits a specific roughness or surface structure, i.e., deviations in the Z direction. The dashed horizontal line H can denote a threshold value used to differentiate the values ​​into a region B1 and a region B2. In one embodiment of the invention, it would be possible to examine separately those components of the surface located in region B1 and also those surface areas located in region B2. In this way, influences arising solely from these surface irregularities can be taken into account.

[0074] Preferably, in the method described here, both the first process step, in which the surface topography is determined, and the process step, in which the optical evaluation, e.g., with regard to gloss, is carried out on the same surface section. In particular, however, the same surface segments that were captured by the image acquisition device are compared with each other.

[0075] Fig.Figure 4 shows another example of determining the surface properties of a curved surface. If, as in this case, the radiation detector is positioned vertically above the surface under investigation, light would only be reflected directly to the detector from horizontally oriented areas. Light from differently curved areas (see dashed lines), however, would be deflected in other directions. The reference symbol T denotes a tangent in the area where the radiation strikes the surface. This deflection would result in a lower intensity in the image. The invention proposes that such curvatures also be taken into account when evaluating an image or when evaluating measured values. For example, a measured intensity value could be corrected by a factor that accounts for the curvature of the surface.Measuring the gloss intensity over the angle range of 0° to 90° with respect to the surface normal is also conceivable.

[0076] The applicant reserves the right to claim all features disclosed in the application documents as essential to the invention, provided that they are novel individually or in combination compared to the prior art. Reference symbol list 1 Device 2 Radiation device / first measuring device 4 Radiation detector device / first measuring device 6 Radiation device 10 surface 12 Control unit 14 Image output device 16 Storage setup 18 deflecting mirrors 22 Processor setup 24 Evaluation unit B1, B2 areas of the surface P1, P2 Radiation direction T tangent

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