Method and device for determining the thickness of a layer applied to a substrate

Description

[0001] The invention relates to a photothermal method for determining a layer thickness of a layer applied to a substrate, in particular a lacquer layer, in which at least one surface region of the coated substrate is heated exclusively by irradiation with at least one radiation source, and thermal radiation emitted by the at least one surface region is detected by a detection device, and the layer thickness is determined based on the emitted thermal radiation. Furthermore, the invention relates to a device for determining a layer thickness of a layer applied to a substrate.

[0002] EP 3 086 087 A1 describes a photothermal handheld measuring device for determining the thickness of a layer applied to a substrate 5, the radiation source of which comprises several near-IR LEDs (4). The handheld measuring device is designed to determine the layer thickness at individual points on a surface.

[0003] DE 103 31 070 A1 does not disclose a photothermal method, but rather an eddy current method for determining a layer thickness, in which a metallic substrate or a metallic layer is heated exclusively inductively and a layer thickness is determined on the basis of emitted thermal radiation.

[0004] US 2013 / 0230072 A1 describes a photothermal lock-in method for determining a component thickness of a fuel cell or for crack detection by means of excitation by LEDs, in which inductive heating is also carried out.

[0005] From DE 199 60 880 A1 a photothermal measuring method is known in which a layer thickness can be determined by irradiation with a solid-state laser at a single point on a substrate.

[0006] DE 10 2016 014 967 A1 describes another photothermal method for determining a layer thickness, in which a laser beam 12 is used for excitation and emitted thermal radiation is detected with an IR camera 18.

[0007] The article "Advantages of multi-pulse thermography" by Daniel Hoffmann et al. describes a pulsed lock-in thermography method for layer thickness measurement using a flash lamp and a bolometer camera.

[0008] A process known as photothermal technology is known from the prior art. A coated substrate is irradiated with a laser at one point. There, the layer is heated and emits thermal radiation, depending on the layer thickness and coating material, which is detected by a detector. The emission of the thermal radiation occurs with a time shift Φ relative to the irradiation, with a so-called phase shift Φ. The layer thickness can be determined by determining Φ, the emitted thermal radiation, in particular its wavelength, and using known calibration curves.

[0009] The present invention is based on the object of developing a photothermal method of the type mentioned above, with which a layer thickness on a large coated component, for example a bumper for a motor vehicle, can be reliably determined, thereby enabling simple quality control.

[0010] According to the invention, the object is achieved by the features of independent claims 1 and 9, and inter alia in that several adjacent surface areas are irradiated simultaneously or successively by the at least one radiation source and a graphically representable layer thickness profile of a surface to be measured is created by an evaluation device.

[0011] To determine the layer thickness, a surface to be measured can be divided into several individually measured surface areas, the average surface area layer thicknesses of which can be combined to form a layer thickness profile of the surface. The at least one surface area is irradiated and heated by the at least one radiation source. Thermal radiation emitted by the surface area is characteristic of a specific layer thickness and is detected by the detection device and transmitted to the evaluation device. By comparing a detected thermal radiation profile and / or a parameter determined from the thermal radiation profile, in particular the phase shift Φ, with a calibration curve, the evaluation device can determine the layer thickness and output a measured value or a layer thickness profile.

[0012] If several, preferably adjacent surface areas are irradiated one after the other by the at least one radiation source and a graphically displayable layer thickness profile of a surface to be measured is created by an evaluation device, a surface of large substrates can be divided into several smaller surface areas, which are heated and measured one after the other in order to determine a layer thickness profile for the entire surface.

[0013] It is also conceivable that a single surface area corresponds to the entire surface to be measured.

[0014] A graphically displayable coating thickness profile can, for example, be directly displayed to an operator on a screen. By appropriately selecting coating thickness limits, it is possible to immediately indicate whether these limits are exceeded or not met at certain points. This enables simple quality control. Defectively coated substrates can be sorted out.

[0015] Thermal radiators can be, for example, radiant heaters.

[0016] Conveniently, the at least one surface region is irradiated with a monochromatic and / or coherent radiation source or with electromagnetic radiation of a specific wavelength range, preferably between 200 nm and 15 µm, in particular between 200 and 750 nm, between 800 and 3500 nm, or between 4 and 13 µm. According to the invention, the radiation source comprises a QCL laser (= quantum cascade laser), in particular for a wavelength between 4 and 13 µm.

[0017] If electromagnetic radiation of a specific wavelength range is to be irradiated, the additional use of one or more light-emitting diodes (LEDs), particularly colored ones, such as blue, is conceivable. This is particularly advantageous if the lacquer coating of a protective cover or a mobile phone casing is colored. By selecting a suitable radiation source, particularly good absorption of the radiation and thus particularly good heating, which leads to detectable emission of thermal radiation, is achieved. The better the absorption, the better the measurement result, which is used to determine the layer thickness.

[0018] It is also conceivable to use light-emitting diodes that emit in the ultraviolet or infrared wavelength range. The radiation source preferably has a wavelength or wavelength range in the near IR range, in particular between 800 and 1600 nm. The wavelength or wavelength range used to heat a coated substrate to be measured is not in the wavelength range visible to an operator. Advantageously, the device can have an open housing and be operated without shielding or glare protection. This simplifies the design and creates a particularly safe workplace for the person operating the device.

[0019] In one embodiment of the invention, the at least one surface region is irradiated with a plurality of identical or different radiation sources. Identical radiation sources emit identical wavelengths or identical wavelength ranges, while different radiation sources emit different wavelengths or wavelength ranges. Wavelength ranges are different from one another if central wavelengths are different from one another. Advantageously, using a plurality of identical radiation sources can provide a particularly high heating power. It is also conceivable that a particularly large surface section, which may be several square centimeters or several square meters, is irradiated to determine the layer thickness.

[0020] Further advantageously, an existing measuring device can be supplemented by one or more, preferably different, additional radiation sources to enable layer thickness measurement on a further, in particular differently coated, substrate. For example, the thickness of a top layer of paint applied to a railway carriage can be determined by irradiation with light-emitting diodes, while the thickness of an underlying filler layer can be determined by irradiation with halogen lamps.

[0021] It is also conceivable that the multiple radiation sources are housed in a single housing. These could, for example, be several different light-emitting diodes (LEDs).

[0022] Advantageously, the at least one surface region is irradiated with a plurality of radiation sources, each of which is configured to generate electromagnetic radiation of a specific wavelength range or a specific wavelength. If a plurality of different radiation sources are used, irradiation can occur over a particularly broad wavelength range. For example, a red, a green, and a blue LED can be used, in particular for simultaneous irradiation. Advantageously, in particular, a layer thickness determination can be determined independently of the color of a layer to be measured. This allows flexible use of a device for carrying out the method.

[0023] It is conceivable that the wavelength ranges of the multiple radiation sources do not have any overlapping areas.

[0024] Another possibility is the use of a superluminescent diode, which emits electromagnetic radiation over a broad wavelength range while maintaining high spatial coherence. Advantageously, a single radiation source may be sufficient to determine the thickness of differently colored layers using a single device.

[0025] In a further embodiment of the invention, the irradiation of the at least one surface region is periodically modulated, in particular at a frequency between 0.01 and 2000 Hz, preferably 20 to 800 Hz. Preferably, a temperature change of the surface region is recorded for each period. Through periodic irradiation, an evaluation device can determine a layer thickness value per period or a thickness of a layer of the layer per period. By averaging over all periods, a particularly precise value for the layer thickness can be determined.

[0026] For a plastic substrate, a frequency of 10 to 800 Hz, preferably between 140 and 500 Hz, has proven to be particularly advantageous.

[0027] Frequencies between 0.5 and 10 Hz are advantageous for determining the coating thickness of a powder-coated substrate, while paint coating thickness measurements on metallic automotive components or vehicle bodies require frequencies between 2 and 120 Hz. In particular, the method according to the invention is very well suited for measuring the thickness of a powder coating applied, for example, to an electrocoating (ETC).

[0028] If the thickness of a layer applied to a strip is to be determined, frequencies between 5 and 500 Hz are advantageous.

[0029] In a further embodiment of the invention, the at least one surface area is irradiated once with a light pulse. Advantageously, a high surface performance is possible. Such a light pulse preferably has a wavelength or wavelength range in the infrared (IR) range and can be generated by a pulsed radiation source.

[0030] In a particular embodiment of the invention, all surface areas that together form an overall surface are irradiated successively or simultaneously with the at least one radiation source in a pulsed or periodically modulated manner. If the irradiation is periodically modulated, continuous measurement and continuous measurement data acquisition are possible. Use of a device according to the invention in a continuous process, for example, for paint layer thickness measurement in automobile production or in the manufacture of coated strips or films, is possible.

[0031] For quality control of large-area coated components, such as painted bumpers for a motor vehicle, it is conceivable that the at least one radiation source and / or the detection device is / are movable and is / are preferably attached to an industrial robot. This allows the device to be moved past the component. An existing production line can advantageously be supplemented with the device.

[0032] The at least one surface area expediently has a size, in particular a diameter, between 0.2 µm and 200 cm, preferably between 1 and 20 µm or 0.2 and 2 cm. The smaller the substrate to be examined, the smaller the surface area to be examined can be. For example, in a protective cover or a housing for a mobile phone, a surface area can comprise the entire surface facing a radiation source or can be between 1 and 2 µm in size, whereas in a coated bumper for a motor vehicle, a surface area can correspond to the entire surface facing a radiation source or can be between 0.5 mm and 2 cm in size.

[0033] In one embodiment of the invention, the at least one surface region is irradiated obliquely or parallel to a surface normal. While in an interferometric measurement of a surface to determine its optical properties, for example, irradiation must be parallel to a surface normal, this is not necessary with the method according to the invention. A layer thickness can be reliably determined even if the irradiation is oblique to the surface normal of a surface region. Advantageously, a layer thickness can be reliably determined in the case of curved, coated substrates. Repositioning of a radiation source and / or the detection device is not required.

[0034] In one embodiment of the invention, the device according to the invention is integrated into an optical and / or mechanical coordinate measuring system. Advantageously, when measuring a component, particularly during quality control, an additional, possibly simultaneous, coating thickness determination can be performed. Coated substrates can be painted components such as exterior mirrors or bumpers for motor vehicles, or painted housings or protective covers for mobile phones or laptop computers.

[0035] The substrate can be formed from various materials, for example from a metallic, an organic and / or an inorganic material, in particular from steel, aluminum or magnesium or a plastic such as polypropylene, polyurethane or silicone or a ceramic such as aluminum oxide or zirconium oxide.

[0036] In addition, the method according to the invention can be used to reliably determine a layer thickness on a particularly rough substrate, for example a sandblasted substrate.

[0037] The invention is explained in more detail below with reference to exemplary embodiments and the accompanying drawings relating to the exemplary embodiments. They show: Fig. 1 shows a first embodiment of a photothermal method and a schematically illustrated device for the photothermal determination of a layer thickness in a perspective view, Fig. 2 shows a second embodiment of a schematically illustrated device for the photothermal determination of a layer thickness in a perspective view, Fig. 3 shows a further device for the photothermal determination of a layer thickness in a perspective view, Fig. 4 shows an embodiment of a schematically illustrated device for the photothermal determination of a layer thickness in a perspective view, Fig. 5 shows a first embodiment of the invention of a schematically illustrated device for the photothermal determination of a layer thickness in a perspective view, Fig.Fig. 6 shows a second embodiment of a schematically illustrated device for the photothermal determination of a layer thickness in a perspective view, Fig. 7 shows a further embodiment of a schematically illustrated device for the photothermal determination of a layer thickness in a perspective view, Fig. 8 shows a further embodiment of a schematically illustrated device for the photothermal determination of a layer thickness in a side view.

[0038] One in Fig. 1A device (1) for determining a lacquer layer (3) applied to a mobile phone protective shell (2), shown schematically in a perspective view, comprises a radiation source (5) having a plurality of light-emitting diodes (LEDs) (4) emitting in the IR range, the radiation cone (6) of which, shown schematically in dashed lines, is provided for irradiating a lacquer surface (7) facing the light source (5). Although a pulsed flash of the lacquer surface (7) is conceivable, periodic irradiation with a frequency of 200 Hz is provided in this exemplary embodiment.

[0039] Furthermore, the device (1) comprises a detection device (8), which is designed according to the invention as a bolometer camera and which can detect the entire paint surface (7). A detection area (9) is represented by a double-dotted, single-dashed line and, in this exemplary embodiment, comprises the entire paint surface (7). Each pixel of the bolometer camera (8) generates a measurement signal, i.e., a temporal temperature profile in a single measurement point (10, 11, 12) on the paint surface (7). For reasons of clarity, the Fig. 1 to 6 Three measuring points (10-12) are shown as examples.

[0040] An evaluation device (13) uses the measurement signal to determine the thickness of the lacquer layer (3) applied to the mobile phone protective shell (2) at the respective measurement point (10-12). For this purpose, calibration curves are stored in the evaluation device (13), which can be used to assign a layer thickness at the measurement point to a measurement signal for the respective measurement point (10-12).

[0041] Furthermore, the evaluation device (13) is connected to a screen (14) on which a single measured value or a layer thickness profile, i.e. a layer thickness distribution over several measuring points, can be displayed.

[0042] It is particularly advantageous if limit values ​​for a layer thickness are stored in the evaluation device (13) and the layer thickness profile is displayed in color, for example, by displaying surface areas that are coated too thickly in yellow, those that are coated too thinly in red, and those that lie within a required layer thickness range in green. Advantageously, surface areas that are coated too thinly or too thickly can be quickly identified by an operator of the device (1).

[0043] Although the detection device (8) is designed as a bolometer camera according to the invention, in variants not claimed a design as an IR camera or other sensor array for detecting thermal radiation is conceivable.

[0044] Although it is also conceivable that the device (1) is three-dimensionally movable, in this and the following embodiments it is two-dimensionally movable vertically and horizontally in the direction of arrows (15, 16), wherein a distance to the surface areas is preferably constant at 10 cm.

[0045] It is also conceivable that an assembly consisting of a radiation source (5), a detection device (8), and / or an evaluation device (13) is arranged in a stationary manner, and a mobile phone protective cover (2) is guided past it to determine the thickness of the paint layer. This can be done either manually by an operator of the device or mechanically. Fig. 2 Reference is made where identical or equivalent parts are identified by the same reference number as in Fig. 1 and the relevant reference number is followed by the letter a.

[0046] One in Fig. 2The device (1a) shown schematically comprises a radiation source (5a) having a superluminescent light-emitting diode (4a), which is intended for the pulsed or periodic irradiation of a paint surface (7a) with light of a wavelength range from 500 to 800 nm at a frequency of 150 Hz.

[0047] It will now Fig. 3 Reference is made where identical or equivalent parts are identified by the same reference number as in Fig. 1 and 2 and the relevant reference number is followed by the letter b.

[0048] One in Fig. 3A device (1b) not according to the invention, shown in a schematic view, is provided for determining the thickness of a powder coating (3b) applied to a metallic component (2b). In this example, heating of a paint surface (7b) takes place by irradiation with a radiation source (5b) and inductively by an induction device (18) mounted on a side (17) of the component (2b) facing away from a detection device (8b). The detection device (8b) is designed as a bolometer camera that can detect a powder coating surface (7b).

[0049] An exclusively inductive heating of the paint surface (7b) is conceivable, but not according to the invention.

[0050] It will now Fig. 4 Reference is made where identical or equivalent parts are identified by the same reference number as in Fig. 1, 2 and 3 and the relevant reference number is followed by the letter c.

[0051] One in Fig. 4 The device shown (1c) differs from those in Fig. 1 to 3 shown in that two mutually different radiation sources (5c, 19) are provided, which irradiate a paint surface (7c). In this exemplary embodiment, a first radiation source (5c) comprises a plurality of blue light-emitting diodes (4c), and a second radiation source (19) comprises a thermal radiator (20) with an irradiation cone (21) that irradiates the entire paint surface (7c).

[0052] It will now Fig. 5 Reference is made where identical or equivalent parts are identified by the same reference number as in Fig. 1 to 4 and the relevant reference number is followed by the letter d.

[0053] One in Fig. 5 The device (1d) shown for determining a layer thickness of a layer (3d) differs from those in Fig. 1 to 4shown in that not the entire coating surface (7d) is irradiated, but individual, discrete surface regions (22, 23, 24, 25). The surface regions (22-25) are irradiated one after the other with a laser (4d) from a radiation source (5d) for heating purposes, either once or periodically, and emitted thermal radiation is detected by a detection device (8d). Although the size of an excitation spot, i.e., the diameter of a laser beam impinging on the surface (7d) in the respective surface region (22-25), corresponds to the size of the preferably round, irradiated surface region (22-25), it is conceivable that a measurement spot is smaller than the excitation spot.An evaluation device (13d) determines the layer thickness of the layer (3d) for each individual surface area (22-25) and can either display thickness values ​​on a display screen (14d) or determine and display a layer thickness profile of the coating surface (7d) by interpolating and extrapolating the layer thickness values ​​of the surface areas (22-25).

[0054] It will now Fig. 6 Reference is made where identical or equivalent parts are identified by the same reference number as in Fig. 1 to 5 and the relevant reference number is preceded by the letter e.

[0055] One in Fig. 6a The device shown (1e) differs from those in Fig. 1 to 5shown in that a beam path of a radiation source (5e) and that of a detection device (8e) are partially parallel. For this purpose, the radiation source (5e) for irradiating a surface (7e) and a detection device (8e) are arranged perpendicular to one another. Light from the radiation source (5e) is deflected towards a surface (7e) by a dichroic beam splitter (26), while thermal radiation emitted by heating the surface (7e) can pass through the beam splitter (26) in the direction of the detection device (8e). Advantageously, this embodiment enables a precise determination of the layer thickness regardless of the distance between the radiation source (5e) and the surface (7e). Although in this exemplary embodiment the layer thickness is determined in certain surface regions (22e-25e), it is conceivable that the entire surface (7e) is irradiated.

[0056] It is also conceivable that a Fig. 6bThe beam splitter (26) shown is designed as a perforated mirror, in which light from a radiation source passes through a hole in the perforated mirror, and emitted thermal radiation is deflected by a reflective part of the perforated mirror toward a detection device. A perforated mirror is particularly advantageous when a laser or a radiation source with high spatial coherence, such as a superluminescent diode, is used as the radiation source (5e).

[0057] It will now Fig. 7 Reference is made where identical or equivalent parts are identified by the same reference number as in Fig. 1 to 6 and the relevant reference number is followed by the letter f.

[0058] One in Fig. 7 The device shown (1f) differs from those in Fig. 1 to 6 shown in that the device (1f) is arranged to determine a layer thickness of a coated strip (2f).

[0059] A radiation source (5f) irradiates - in pulsed or periodically modulated fashion - a narrow surface area (27) of the coated strip (2f) moving past the device (1f) in the direction of an arrow (28), which extends parallel to a direction of movement of the strip (2f). For moving the strip (2f), a Fig. 7 conveying means not shown, which may, for example, comprise a reel, may be provided.

[0060] A stationary detection device (8f) detects radiated heat from the narrow, moving surface area (27) and determines an average layer thickness from several determined layer thicknesses by averaging.

[0061] It will now Fig. 8 Reference is made where identical or equivalent parts are identified by the same reference number as in Fig. 1 to 7 and the relevant reference number is followed by the letter g.

[0062] One in Fig. 8The device shown in a side view (1g) differs from those in Fig. 1 to 7 shown in that a radiation source (5g) is provided to form an irradiation cone (6g), the rays of which impinge on a coating surface (7g) at an angle to a normal (29). Because a time delay with which thermal radiation is emitted is independent of an angle of incidence of excitation radiation from a radiation source on a coating surface (7g), a perpendicular incidence of rays of the irradiation cone (6g) is not necessary to determine a layer thickness. Advantageously, the method can be used to determine a layer thickness on curved, coated substrates such as bodywork components, laptop housings, mobile phone housings or mobile phone protective covers without repeatedly realigning the radiation source (5g) and / or the detection device (8g).

[0063] It is conceivable that a device (1-1g) is movable and preferably attached to an industrial robot. This allows several surface areas (7; 7a; 7b; 7c; 22-25; 22e-25e; 27; 7g) to be automatically scanned one after the other.

[0064] It is also conceivable that an optical system, which may comprise a lens, for example, is introduced into a beam path formed by an irradiation cone (6-g).

[0065] Furthermore, a surface area (22-25; 22e-25e; 27) heated by irradiation can have a smaller size than a size of the irradiation cone (6-g) impinging on the surface (7; 7a; 7b; 7c; 7g) or can be at most the same size.

Claims

1. Photothermal method for determining a layer thickness of a layer applied to a substrate (2-2g), in particular a lacquer layer, in which at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) of the coated substrate (2-2g) is heated by irradiation with at least one radiation source (5-5b; 5c, 19; 5d-g) and thermal radiation emitted by the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) is detected by a detection device (8- 8g), and the layer thickness is determined on the basis of the emitted thermal radiation, wherein a plurality of adjacent surface regions (22-25; 22e-25e) are irradiated simultaneously or successively by the at least one radiation source (5-5b; 5c, 19; 5d-g) and an evaluation device (14-14g) is used to create a graphically displayable layer thickness profile of a surface to be measured, characterized in that the at least one radiation source (5-5b; 5c, 19; 5d-g) comprises a quantum cascade laser, and the thermal radiation emitted by the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) is detected by a bolometer camera (8-8g).

2. Photothermal method according to claim 1, characterized in that the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) is irradiated with a monochromatic and / or coherent radiation source (5-5b; 5c, 19; 5d-g) or with electromagnetic radiation of a specific wavelength range, preferably between 200 nm and 15 µm, in particular between 200 and 750 nm, between 800 and 3500 nm or between 4 and 13 µm.

3. Photothermal method according to claim 1 or 2, characterized in that the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) is irradiated with a plurality of radiation sources (5c, 19) which are the same or different from one another.

4. Photothermal method according to one of claims 1 to 3, characterized in that the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) is irradiated with a plurality of radiation sources (5c, 19), each of which is set up to generate electromagnetic radiation of a specific wavelength range or a specific wavelength.

5. Photothermal method according to one of claims 1 to 4, characterized in that the irradiation of the at least one surface region (7-7c; 22-25; 22e- 25e; 27; 7g) is periodically modulated, in particular with a frequency between 0,01 and 2000 Hz, preferably 20 to 800 Hz.

6. Photothermal method according to one of claims 1 to 5, characterized in that the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) has a size, in particular a diameter, between 0,2 µm and 200 cm, preferably between 1 and 20 µm or 0,2 and 2 cm.

7. Photothermal process according to one of claims 1 to 6, characterized in that the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) is irradiated obliquely or parallel to a surface normal (29).

8. Photothermal method according to one of claims 1 to 7, characterized in that a preheating of the coated substrate takes place before the start of the irradiation.

9. Device (1-1 g) for photothermally determining a layer thickness of a layer applied to a substrate (2-2g), in particular a lacquer layer, which has at least one radiation source (5-5b; 5c, 19; 5d-g) for heating at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) solely by irradiation, at least one detection device (8-8g) for detecting thermal radiation emitted by the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) and an evaluation device (14-14g) for determining a layer thickness, wherein the detection device (8-8g) is set up for the simultaneous or successive detection of several surface regions (7-7c; 22-25; 22e-25e; 27; 7g), and wherein a graphically displayable layer thickness profile of a surface to be measured may be created by the evaluation device (14-14g), characterized in that the at least one radiation source (5-5b; 5c, 19; 5d-g) comprises a quantum cascade laser and the detection device (8-8g) has a bolometer camera.

10. Device according to claim 9, characterized in that the at least one radiation source (5-5b; 5c, 19; 5d-g) is set up for periodically irradiating the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g).

11. Device according to claim 9 or 10, characterized in that the at least one radiation source (5-5b; 5c, 19; 5d-g) and / or the detection device (8-8g) is or are arranged movably and is or are set up to be guided past a plurality of, in particular, adjacent surface regions (7-7c; 22-25; 22e-25e; 27; 7g).

12. Device according to any one of claims 9 to 11, characterized in that the at least one radiation source (5-5b; 5c, 19; 5d-g) and / or the detection device (8-8g) is or are arranged in a fixed position and a conveying means is provided, which is set up to guide the coated substrate (2-2g) with the surface regions (7-7c; 22-25; 22e-25e; 27; 7g) to be heated past the radiation source (5-5b; 5c, 19; 5d-g) and / or past the detection device (8-8g) in such a way that heating and detection of thermal radiation emitted by the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) can take place.