Method for determining the layer thickness of an at least partially transparent surface coating

Abstract

The invention relates to a method for determining the layer thickness of an at least partially transparent surface coating on the surface of a substrate, by means of a measuring device for emitting focused radiation. The invention is characterized in that the distance between the surface of the substrate and the measuring device for emitting focused radiation is changed by a relative movement between the measuring device and the substrate, wherein the image of the radiation is captured in different relative positions, wherein, in the case of an image having a clear contour edge, focusing on the interface between the surface coating and the substrate is deduced and the current relative position is stored as a first position, and wherein, in the case of an image surrounded by scattering effects, focusing on the interface between the surface coating and the air is deduced and the current relative position is stored as a second position, whereupon the layer thickness of the at least partially transparent surface coating is deduced from the difference between the first and second positions, taking into account the refractive index of the transparent surface coating.

Description

[0001] Mercedes-Benz Group AG Dr. Michael Ehrmann

[0002] November 21, 2025

[0003] Method for determining the layer thickness of an at least partially transparent surface coating

[0004] The invention relates to a method for determining the layer thickness of an at least partially transparent surface coating on the surface of a substrate, using a measuring device for emitting focused radiation.

[0005] DE 196 32 763 A1 describes a measuring head for observing the photoresist development on silicon wafers, which are used to structure integrated circuits coated with a thin layer of photoresist.

[0006] A structural pattern is then exposed onto the photoresist layer and developed. During development, a critical decrease in layer thickness can occur, which can be observed and measured using the measuring head. This head comprises a converging lens and a ring-shaped detector in its focal plane, with its optical axis perpendicular to the photoresist layer. Monochromatic light is focused into the inner opening of the ring-shaped detector, projected by the converging lens as a collimated beam onto the photoresist layer, and reflected back onto itself. The reflected light falls directly back as long as the surface is unstructured. However, structures that form in the resist layer during development diffract the light, causing some of the light to reach the ring detector. The signal on the ring detector thus records the temporal progression of the photoresist development.

[0007] The measuring head is unfortunately unsuitable for measuring the thickness of a fully cured surface coating, such as a partially transparent paint layer or clear coat. However, the German patent application filed at the same time by the same applicant, entitled "Method for producing a structure in the volume of a partially transparent material using a laser beam," describes a method for producing an optical structure in the volume of a partially transparent material, such as a surface coating, using a laser beam. Therefore, if such structures are to be precisely positioned in all three dimensions within a surface coating, such as the clear coat of a vehicle, then measuring the thickness and / or topography of the surface coating would be necessary.

[0008] The object of the present invention is therefore to create an improved method for determining the layer thickness of an at least partially transparent surface coating, which allows for simple, reliable and process-safe measurement, which can in particular be integrated into a manufacturing process.

[0009] According to the invention, this problem is solved by a method with the features in claim 1, and in particular in the characterizing part of claim 1. Advantageous embodiments of the method according to the invention are set forth in the dependent claims.

[0010] The method according to the invention is thus used to determine the layer thickness of an at least partially transparent surface coating on the surface of a substrate. It utilizes a measuring device for emitting focused radiation. Unlike the prior art mentioned above, the distance between the surface of the substrate and the measuring device for emitting focused radiation is changed by a relative movement between the measuring device and the substrate. The image of the radiation is then captured in various relative positions. In the case of an image with a clear outline, e.g., an elliptical or circular image, focusing at the interface between the surface coating and the substrate is inferred, and the current relative position is stored as the first position.If, however, an image is captured that is surrounded by scattering effects, then focusing on the interface between the surface coating and the air is inferred, and the current relative position is stored as a second position. The layer thickness can then be determined from the difference between the first and second positions. Since the propagation speed of light varies in different media, the path difference by which the focusing optics have moved relative to the substrate surface does not directly correspond to the thickness of the surface coating, but must be multiplied by the refractive index of the intervening material, in this case, the refractive index of the surface coating. Therefore, if the refractive index of the surface coating is known, the desired thickness can be easily calculated from the path difference.Preferably, a well-known image processing software is used to quickly and efficiently capture and differentiate the images in the first and second positions.

[0011] A particularly advantageous embodiment of the invention further provides that the topography of the surface coating is inferred from the changing distance between the first and / or second position and the measuring device along a travel path transverse to the surface coating. In addition to the pure layer thickness, the course of the layer or its surface in space can thus also be recorded.

[0012] In a highly advantageous further development of the method, laser radiation is used as the focused beam. A laser makes it very simple and efficient to generate focused radiation with sufficient brightness for reliable image capture. The method according to the invention can be used for any type of layer thickness measurement of surface coatings. It can be used on both very small and very large objects, such as vehicles, to measure the thickness of a clear coat layer. The measuring device can be moved, for example, by a multi-axis robot to or over the area where the layer thickness is to be measured. This can be done either while the measuring device is moving, i.e., "on-the-fly," or after the measuring device has been moved into a measuring position and remains there during the measurement.

[0013] According to a very favorable further development of the invention, the layer thickness and / or the topography of the surface coating in a predetermined area of ​​the surface can be recorded and used directly for further processing processes and / or stored for later processing processes.

[0014] In addition to simply measuring the layer thickness, e.g. for quality control or assurance, it is now particularly interesting if, according to an advantageous further development of the method according to the invention, the measured layer thickness and / or topography of the surface coating in the specified area is used to introduce a specified structuring into the inner volume of the surface coating, given the now known position of the surface coating in space.

[0015] According to a preferred further development, the specified structuring can be realized as an optical structuring comprising modification bubbles formed in a uniform, periodic, or random arrangement in at least one plane within the volume. Such structures or structurings can modify the optical properties, such as the colors, reflections, and / or transparency of the surface coating.

[0016] Such structures are evident from the two unpublished older German applications with file numbers 102024 138 585.5 and

[0017] Patent No. 102024 138 590.1 is known. It describes the process of providing materials, such as surface coatings or varnishes, with optical structures that enable graphic representation and color changes.

[0018] For this purpose, modification bubbles are formed in the material in a uniform, periodic, or random arrangement in at least one plane within the volume. This allows, for example, the creation of iridescent colors in the case of periodic structures with a spacing on the order of the wavelength of visible light. The structures then act like an optical grating.

[0019] Furthermore, it can be provided that the modification bubbles are introduced into the surface coating via a focused laser beam from a structuring device moving relative to the surface. They are therefore preferably introduced into the inner volume of the surface coating by means of a focused laser beam, in particular a focused ultrashort pulse laser.

[0020] It is particularly advantageous if the structuring device is further configured to emit focused radiation to determine the layer thickness and / or topography of the surface coating and to infer its layer thickness and / or topography. The structuring device and the measuring device can thus be combined. This reduces the complexity and enables excellent correlation between the measured parameters and their use in processing, since the two devices are in a fixed, unchanging position relative to each other and are moved together. The inventive method is preferably used for a surface coating that is formed as a paint layer, particularly a clear coat, on a vehicle or on a component for a vehicle.

[0021] Further advantageous embodiments will also become clear from the following exemplary embodiment, which is described with reference to the figures.

[0022] This shows:

[0023] Fig. 1 shows a schematic representation of the system for carrying out the method for determining the layer thickness of an at least partially transparent surface coating and for structuring the internal volume of the surface coating;

[0024] Fig. 2 shows an image of the measuring device in a first position relative to the surface coating and a schematic view of how it is formed;

[0025] Fig. 3 shows an image in a second position of the measuring device in relation to the surface coating and a schematic view of how it is formed;

[0026] Fig. 4 shows a cross-section through a substrate with a transparent lacquer layer and a structure inside the lacquer layer;

[0027] Fig. 5 is a top view of the representation according to Fig. 4 in a first embodiment a) and a second embodiment b); Fig. 6 is a cross-section through a substrate with a transparent lacquer layer and an alternative configuration of the structure inside the lacquer layer; and

[0028] Fig. 7 is a photographic illustration of a surface coating according to the invention on a section of a rim.

[0029] Figure 1 shows a device 10 for determining the layer thickness of an at least partially transparent surface coating 1 on the surface of a vehicle 11, using a measuring device 16 for emitting a focused measuring beam M. The device 10 can also be used to produce an optical structure 3 in the inner volume of the surface coating 1 of the vehicle 11, in this case its clear coat, as will be described in more detail later.

[0030] To determine the layer thickness and topography of the clearcoat 1, a focused visible light beam in the form of a laser beam is preferably used as the measuring beam M. In principle, radiation in a different wavelength range would also be conceivable, provided that a suitable device is used to capture the required image. The process engineering solution for the automated determination of the layer thickness and topography of the clearcoat 1 is implemented via an optical evaluation of the focal points / laser spots. For this purpose, the distance between the measuring device 16 and the surface coating 1 is varied. This results in different images of the clearcoat 1, which are shown on the left in Figures 2 and 3.

[0031] In the illustration of Figure 2, the focus F of the measuring beam M is located on the underside of the clear coat 1, i.e., an interface 17 between the clear coat 1 and a substrate 2 supporting it, here, for example, the body panel or a base coating thereof. The result in the image on the left, which can be captured by a camera 18, is a single focal point F. This can have any shape, e.g., an ellipse. However, it always has a clear outline edge that is not surrounded by scattering effects. The measuring device 16 is located at a distance e from a visible surface 4 of the clear coat 1.

[0032] The measuring device 16 is moved until the measuring beam M is focused on the visible surface 4 of the clear coat 1. This is the interface between the clear coat 1 and the surrounding air. Without changing the focusing distance of the measuring beam M itself, the image shown on the left in Figure 3 is then produced. The focal point F, now located on the visible surface 4, remains visible. However, the central focal point F is now surrounded by scattering ST, which originates from the back reflection at the interface 17 indicated on the right in Figure 3. The change in the distance from e to e' between the measuring device 16 and the visible surface 4 then allows the layer thickness d of the clear coat 1 to be determined.

[0033] As explained above, the speed of light propagation varies in different media. Therefore, the path difference (e'-e) does not directly correspond to the layer thickness d of clear coat 1. Rather, d = (e'-e)*n, where n is the refractive index of the clear coat. However, if the refractive index n of clear coat 1 is known, which is typically the case, the layer thickness d of clear coat 1 can be easily calculated from the path difference (e'-e).

[0034] The images captured, for example, by the camera 18 indicated in Figure 1, can then be evaluated to determine the layer thickness and, by considering the position of the measuring device in space, also the topography of the clearcoat layer. This can be achieved with automated focus position determination through the integration of image recognition software. The two relevant focus positions in Figures 2 and 3 for determining the layer thickness d of the clearcoat 1 are the visible surface 4 and the interface 17 between the substrate 2 and the clearcoat 1. Structures 3 (see Figure 4) can now be introduced into the internal volume of the clearcoat 1 as volume modifications. The measurement can be performed either before or during the manufacturing process for the structures 3.

[0035] If this measurement takes place before the manufacturing process, it can be performed particularly easily immediately before the production of the structures 3 at the respective location, or at every nth location, depending on the tolerance of the clear coat layer 1 in space. However, the entire clear coat layer 1 can also be measured in a first step in order to then carry out the manufacturing process for the structures 3. To verify the material present in the (partially transparent) clear coat layer 1, an inline verification analysis of the clear coat is performed.

[0036] The fabrication of the structures 3 can be carried out using a laser beam S, preferably using a focused ultrashort pulse laser.

[0037] A structuring device 13 serves to emit at least one focused laser beam S. The structuring device 13 can include a laser scanner to move the laser beam S precisely within the clear coat 1 and a lens to focus it. This allows the predefined structure 3 to be formed from modification bubbles 5, which will be described in more detail later. In laser processing and laser welding, the term "laser scanner" typically refers to a processing optic with at least one laser that can be directed to different points and / or along different paths via movable mirrors.

[0038] The structuring device 13 is moved relative to the vehicle 11 by an industrial robot 14 with up to 6 axes. Additionally, the vehicle 11 can be mounted on an assembly frame, e.g., in a flow production line.

[0039] Slide 15 is moved. Typically, however, it will remain stationary during the production of structure 3, and only the industrial robot 14 moves the structuring device 13 into various required positions relative to 2023P04262WQ

[0040] 10 the vehicle 11. This allows the desired structures to be introduced into the clear coat 1, preferably using an ultrashort pulse laser. In the illustration of Figure 1, for example, a logo 12 is created by the structures 3, which is indicated here by way of example as a circle.

[0041] The measuring beam M of the measuring device 16 and the laser beam S used to produce the structures 3 can preferably originate from the same laser source. For this purpose, another beam / light source, e.g., from the visible spectrum, can be coaxially coupled into the beam-guiding optics of the laser beam S used for processing (or, if necessary, provided by a separate focusing optic) to obtain the measuring beam M. Ideally, a measuring device 16 is used instead that can detect the wavelength generated by the laser beam S itself. Then the laser beam S can also be used for the measurement. Inaccuracies due to a misalignment of the laser beam S on the one hand and the radiation source for the measuring beam on the other can thus be avoided.

[0042] Figure 4 shows a schematic and greatly enlarged cross-section of the logo 12. The clear coat 1 is applied to a substrate designated 2, in this case, for example, the body panel or a base coating thereof. Within the thickness of the clear coat 1, the structures 3 are arranged in three planes, one behind the other, starting from a visible surface 4 of the clear coat 1. Each of these planes can contain a dot matrix.

[0043] In the two top views shown in Figure 5 a) and b), these dot matrices can be seen in two possible embodiments. In Figure 5 a), it can be seen that individual lines 6, each formed from a series of modification bubbles 5 (described in more detail in Figures 6 and 7), create a regular rectangular grid. This grid acts as an optical grating and can very precisely and homogeneously alter the optical properties, particularly the transmission and / or reflection within the clear coat 1, across its entire extent.

[0044] The lines 6 are formed by a series of modification bubbles 5, which is indicated here by the dotted representation of the lines 6. With a spacing x of the lines 6 of a few tens of pm, e.g., approximately 30 pm, but in any case above the order of magnitude of visible light, black, white, and shades of gray can be generated. This allows for a high degree of design flexibility. The somewhat more complex optical gratings enable the achievement of very homogeneous colors or color gradients. If this is not essential, good results can also be achieved with the alternative shown in Figure 3 b). Here, individual modification bubbles 5, distributed in a random pattern, replace the periodically repeating lines 6. The color nuances then arise solely from the average area density of the modification bubbles 5. This is less computationally intensive and requires less planning.However, the resulting grey tone may appear slightly less homogeneous than in the implementation according to Figure 5 a).

[0045] It is particularly interesting when, in an optical grating as shown in Figure 5a), the distance between the modification bubbles 5 of each individual line 6 is on the order of 400 to 780 nm. The distance x between the individual lines 6 is, for example, more than 30 pm. This then allows, as described in the older German application with the file numbers

[0046] 102024 138 585.5 explains in detail how to achieve an iridescent color effect, i.e., a shimmering in rainbow colors.

[0047] With both variants, structures 3 can be subsequently introduced into an already applied layer of clear coat 1 via the structuring device 13, e.g., in the form of the brand logo 12. If different layers are provided with different grids, different distances between the modification bubbles 5 and / or distances x between the lines 6 – and thus colors in the sense mentioned above – can also be used, which further increases the design possibilities. In addition, the individual layers can now also be changed in their angle to each other and to the visible surface 4. This can be seen in the illustration of Figure 6, which is otherwise analogous to Figure 4.

[0048] The modification bubbles 5 themselves are droplet-shaped. Their diameter ranges from 0.1 to 10 pm, typically from approximately 2 to 8 pm. These modification bubbles alter the refractive index of the clear lacquer 1 used. According to current understanding, this is likely due to a reorganization / rapid solidification of the molten clear lacquer 1, as well as a local change in chemical composition caused by irradiation.

[0049] Finally, Figure 7 shows a photograph of a section of a rim 7 for the vehicle 11. The rim 7 is sealed with clear lacquer 1. Structures 3 are introduced into this clear lacquer 1 in at least one plane, in particular parallel to the visible surface 4 of the clear lacquer 1. These structures represent a logo designated 12, in the image the trademark "Maybach" (corresponding to a registered trademark of one of the co-applicants). The entire optical effect that makes the trademark logo 12 visible as a structure 3 within the clear lacquer 1 is based on modification bubbles 5, which are formed in a lattice structure.

Claims

Mercedes-Benz Group AG Dr. Michael Ehrmann November 21, 2025 Patent claims 1. Method for determining the layer thickness (d) of an at least partially transparent surface coating (1) on the surface (17) of a substrate (2) by means of a measuring device (16) for emitting focused radiation (M), characterized in that the distance (e) between the surface (17) of the substrate and the measuring device (16) for emitting focused radiation (M) is changed by a relative movement between the measuring device (16) and the substrate (17), wherein the image of the radiation is recorded in different relative positions, wherein in the case of an image with a clear outline edge, a focus on the interface (17) between the surface coating (1) and the substrate (2) is inferred and the current relative position is stored as the first position (e), and wherein in the case of an image which is surrounded by scattering effects (ST),a focus is inferred on the interface (4) between the surface coating (1) and the air, and the current relative position is stored as the second position (e'), after which the layer thickness (d) of the at least partially transparent surface coating (1) is inferred from the difference (e'-e) between the first and second positions, taking into account the refractive index (n) of the transparent surface coating (1).

2. Method according to claim 1, characterized in that a topography of the surface coating (1) is inferred from the changing distance (e, e') between the first and / or the second position and the measuring device (16) along a travel path transverse to the surface coating (1).

3. Method according to claim 1 or 2, characterized in that laser radiation is used as the focused radiation (M).

4. Method according to claim 1, 2 or 3, characterized in that the layer thickness (d) of the surface coating (1) is detected in a predetermined area of ​​the surface and is used directly for further processing processes and / or stored for a later processing process.

5. Method according to claim 2 or 3, characterized in that the topography of the surface coating (1) is recorded in a predetermined area of ​​the surface and is used directly for further processing processes and / or stored for a later processing process.

6. Method according to claim 4 or 5, characterized in that the detected layer thickness (d) and / or topography of the surface coating (1) is used to selectively introduce a predetermined structure (3) into the inner volume of the surface coating (1).

7. Method according to claim 6, characterized in that the predetermined structure (3) is realized as an optical structure (3) comprising modification bubbles (5) which are formed in a uniform periodic or random arrangement in at least one plane in the volume.

8. Method according to claim 7, characterized in that the modification bubbles (5) are introduced via a focused laser beam (S) of a structuring device (13) moving relative to the surface of the surface coating (1).

9. Method according to claim 8, characterized in that the structuring device (13) is further configured to emit the focused radiation (M) for determining the layer thickness (d) and / or topography of the surface coating (1).

10. Method according to one of claims 1 to 9 characterized in that the measuring device (16) and / or the structuring device (13) are moved relative to the surface coating (1) by means of a multi-axis robot (14).

11. Method according to one of claims 1 to 10 characterized by its use on a coating layer, in particular a clear coat (1), on or in a vehicle (11) or on a component (7) for a vehicle (11).

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