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

Abstract

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) or by irradiation with at least one radiation source (5-5b; 5c, 19; 5d-g) and inductively, 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 based on the emitted thermal radiation, characterized in that the at least one surface region is provided for heating with at least one superluminescent diode, at least one thermal radiator and / or at least one Quantum cascade laser irradiation.

Description

The invention relates to a 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 by irradiation with at least one radiation source or by irradiation and inductively and thermal radiation emitted by the at least one surface region is detected by a detection device and the layer thickness is determined on the basis of the emitted thermal radiation. The invention further relates to a device for determining a layer thickness of a layer applied to a substrate.Such a method referred to as photothermy is known from DE 195 20 788 A1, DE 100 13 173 A1, DE 197 49 984 A1, WO 2001 / 031293 A1, DE 199 07 804 C1 D2, U.S. Pat. No. 4,875,175 A and Petry, Harald: Online measurement of lacquer layer thicknesses with thermal waves.tm. Technical Measurement Vol. 65 (1998), No. 11, pp. 396-399. A coated substrate is irradiated with a laser at one location. There, the layer is heated and emits heat radiation, in particular depending on a layer thickness and a coating material, which heat radiation is detected by a detector. The emission of the thermal radiation takes place with a time offset with respect to the irradiation with a so-called phase shift φ. By ascertaining Φ, the emitted thermal radiation, in particular the wavelength thereof, and using known calibration curves, the layer thickness can be determined.DE 103 31 070 B4 discloses a method in which a metallic substrate with an electrically non-conductive coating is inductively heated, a temperature distribution on the layer surface is recorded with an IR camera and a layer thickness is determined therefrom.EP 0 427 943 A1 discloses a photo-thermal method in which no emitted thermal radiation is detected in order to determine material properties, but the property to be examined is determined by a "photo-thermal induced beam deflection", that is to say by a deflection of a sample beam which passes through a sample. Due to local heating by the sample beam and by an additional excitation beam, refractive index gradients are formed which cause the deflection of the sample beam. A determination of a layer thickness using the method known from EP 0 427 943 A1 is not described.EP 3 086 087 A1 and EP 2 975 360 A1 describe photo-thermal hand-held measuring devices for determining a layer thickness, in which LEDs are used as excitation light source.The object of the present invention is to further develop a method of the type mentioned at the beginning which extends the field of application of the photohermia for determining layer thicknesses.According to the invention, the object is achieved in that the at least one surface region is irradiated with at least one superluminescent diode, at least one thermal radiator and / or at least one quantum cascade laser for heating.A QCL laser (= cascade laser) can emit a wavelength between 4 and 13 μm.A superluminescent diode emits electromagnetic radiation in a broad wavelength range with simultaneously high spatial coherence. Advantageously, a single radiation source may be sufficient to be able to determine a layer thickness of differently colored layers using a single device.Thermal radiators can be designed, for example, as radiant heaters.To determine the layer thickness, a surface to be measured can be completely detected or divided into a plurality of surface regions to be measured individually, the respective average surface region layer thicknesses of which can be joined to form a layer thickness profile of the surface. The at least one surface region is illuminated and heated by the at least one radiation source and / or is inductively heated. Thermal radiation emitted by the surface region is characteristic of a specific layer thickness and is detected by the detection device and transmitted to the evaluation device. By comparing a recorded heat radiation profile and / or a parameter determined from the heat 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. If heating is effected by irradiation and inductively, particularly good and rapid heating can advantageously be effected.If heating takes place inductively, a determination of a layer thickness between 2 and 10 μm is possible particularly accurately.Expediently, 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. A radiation source can be designed, for example, as a laser having a wavelength between 200 and 1000 nm. It is also conceivable to use a CO 2- laser for a wavelength of 10.6.mu.m.If electromagnetic radiation of a specific wavelength range is intended to be used, it is conceivable to use one or more, in particular colored, for example blue, light emitting diodes (LEDs). This is advantageous in particular when the lacquer layer of a protective cover or a housing for a mobile telephone is colored. By suitably selecting a radiation source, a particularly good absorption of the radiation and thus a particularly good heating, which leads to a detectable emission of thermal radiation, is achieved. The better the absorption, the better is a measurement result, on the basis of which the layer thickness is determined.The radiation source preferably has a wavelength or a wavelength range in the near IR range, in particular between 800 and 1600 nm. The wavelength or wavelength range used for heating 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 can be operated without shielding or antiglare means. A construction is thereby simplified and a particularly secure workplace for a person operating the device is created.In one embodiment of the invention, the at least one surface region is irradiated with a plurality of identical or mutually different radiation sources. Advantageously, a particularly high power for heating can be provided. Furthermore advantageously, an existing measuring device can be supplemented by one or more further radiation sources in order to enable a layer thickness measurement on a further, in particular different coated substrate. For example, a thickness of an uppermost lacquer layer applied to a railway car can be determined by irradiation with light-emitting diodes, while a thickness of an underlying filling layer can be determined by irradiation with halogen lamps.Expediently, 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 radiation sources different from one another are used, irradiation with a particularly broad wavelength range can be effected. For example, a red, a green and a blue LED can be used for irradiation, in particular simultaneously. Advantageously, in particular a layer thickness determination can be determined independently of a color of a layer to be used. This allows a flexible use of a device for carrying out the method. It is conceivable that the wavelength ranges of the plurality of radiation sources do not have any overlap regions.In a further embodiment of the invention, the irradiation of the at least one surface region takes place periodically modulated, in particular with a frequency between 0.01 and 2000 Hz, preferably 20 to 800 Hz. Preferably, a temperature change of the surface region is detected for each period. By means of periodic irradiation, a layer thickness value per period or a thickness of a layer of the layer per period can be determined by an evaluation device. By averaging over all periods, a particularly accurate value for the layer thickness can be determined.A frequency of 10 to 800 Hz, preferably between 140 and 500 Hz, has proven particularly advantageous for a plastic substrate.For determining a layer thickness of a powder-coated substrate, frequencies between 0.5 and 10 Hz are advantageous, while paint layer thickness measurements in metallic motor vehicle components or motor vehicle bodies require frequencies between 2 and 120 Hz. In particular, the method according to the invention makes it very easy to determine the measurement of a thickness of a powder layer which has been applied, for example, to an electrodeposition coating (ETL).If a thickness of a layer applied to a strip is to be determined, frequencies between 5 and 500 Hz are advantageous.In a further embodiment of the invention, the irradiation of the at least one surface region takes place once by a light pulse. Advantageously, a high surface power is possible. Such a light pulse preferably has a wavelength or a wavelength range in the infrared range (IR) and can be generated by a pulse radiation source.In one embodiment of the invention, a plurality of, preferably adjacent, surface regions are successively irradiated by the at least one radiation source and a layer thickness profile, which can preferably be graphically displayed, of a surface to be measured is created by an evaluation device. As a result, a surface of large substrates can be divided into a plurality of smaller surface regions, which are heated and measured successively in order to determine a layer thickness profile for the entire surface therefrom. It is also conceivable that a single surface area corresponds to the entire surface to be measured. A graphically representable layer thickness profile can be displayed directly to an operator on a display screen, for example. By suitable selection of layer thickness limit values, it can be immediately indicated whether these limit values are exceeded or undershot at some points. This allows simple quality control. Incorrectly coated substrates can be sorted out.In a particular embodiment of the invention, all surface regions which together form an overall surface are irradiated successively or simultaneously in a pulsed or periodic manner with the at least one radiation source. If the irradiation is periodically modulated, continuous measurement and continuous measurement data acquisition are possible. It is possible to use an apparatus according to the invention in a continuous process, for example for measuring the thickness of the paint layer in the case of automobile production or in the case of production of coated strips or films.For the quality control of coated components of large surface area, for example painted bumpers for a motor vehicle, it is conceivable that the at least one radiation source, a device for inductive heating and / or the detection device is or are movable and is or are preferably attached to an industrial robot. This allows it to pass the component. An existing production line can advantageously be supplemented by the device.Expediently, the at least one surface region has a size, in particular a diameter, of 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 a surface area to be examined can be. For example, in a protective sheath or in a housing for a mobile telephone, a surface area can comprise the entire surface facing a radiation source or can be between 1 and 2 μm in size, while 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.In one configuration of the invention, the at least one surface region is illuminated obliquely or parallel to a surface region normal. Whereas in an interferometric measurement of a surface in order to determine its, for example, optical properties, an irradiation must take place parallel to a surface normal, this is not necessary in the method according to the invention. A layer thickness can be reliably determined even if the irradiation takes place obliquely 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, a device for inductive heating and / or the detection device is not necessary.Expediently, a substrate comprising an electrically conductive, in particular metallic material and / or a layer comprising an electrically conductive, in particular metallic material is inductively heated. If inductive heating is to take place, the layer or the substrate must comprise an electrically conductive material. It is also conceivable that heating takes place simultaneously or successively inductively and by irradiation, or that some surface regions are heated by irradiation and others are inductive.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 a component is being measured, in particular during quality control, an additional, optionally simultaneous determination of the layer thickness can be carried out. Coated substrates can be painted components such as vehicle exterior mirrors or bumpers for motor vehicles or painted housings or painted protective covers for mobile telephones or laptop computers.In a special embodiment of the method, the coated substrate is preheated before the beginning of irradiation and / or inductive heating. Advantageously, a more accurate determination of the layer thickness is possible by the preliminary heating.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.In addition, the method according to the invention can reliably determine a layer thickness on a particularly rough substrate, for example a sand blasted substrate.The invention is explained in more detail below with reference to exemplary embodiments and the appended drawings relating to the exemplary embodiments. The following are shown: FIG. 1 shows a perspective view of a photohermic method with a schematically illustrated device, FIG. 2 shows an embodiment of a schematically illustrated device according to the invention in a perspective view, FIG. 3 shows a perspective view of a further embodiment of a schematically illustrated device according to the invention, FIG. 4 shows an embodiment of a schematically illustrated device in a perspective view, FIG. 5 shows an embodiment of a schematically illustrated device in a perspective view, FIG. 6 shows an embodiment of a schematically illustrated device according to the invention in a perspective view, FIG. 7 shows a perspective view of a further embodiment of a schematically illustrated device according to the invention, FIG. 8 shows a further embodiment of a schematically illustrated device according to the invention in a side view.A device (1) shown schematically in a perspective view in FIG. 1 for determining a paint layer (3) applied to a mobile phone protective shell (2) comprises a radiation source (5) comprising a plurality of light emitting diodes (LEDs) (4) emitting in the IR range, the irradiation cone (6) of which shown schematically with dashed lines is provided for irradiating a paint surface (7) facing the light source (5). Although pulse-like flashing of the lacquer surface (7) is conceivable, in this embodiment a periodic irradiation with a frequency of 200 Hz is provided.Furthermore, the apparatus ( 1) comprises a detection device ( 8) which is designed as a bolometer camera and which can capture the entire paint surface ( 7). A detection region ( 9) is represented by a two-dot, one-dot chain line and comprises the entire paint surface ( 7) in this exemplary embodiment. Each image point 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, three measurement points ( 10- 12) are shown in each case in FIGS. 1 to 6 by way of example.An evaluation device ( 13) determines a thickness of the lacquer layer ( 3) applied to the mobile phone protection shell ( 2) at the respective measurement point ( 10- 12) on the basis of the measurement signal. For this purpose, calibration curves are stored in the evaluation device ( 13), on the basis of which calibration curves a layer thickness in the measurement point can be assigned to a measurement signal for the respective measurement point ( 10- 12).Furthermore, the evaluation device ( 13) is connected to a screen ( 14) on which an individual measured value or a layer thickness profile, i.e. a layer thickness distribution over a plurality of measurement points, can be displayed.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 by displaying surface regions coated too thick, for example yellow, displaying red coated too thin and displaying those which lie within a required layer thickness range green. Advantageously, surface areas painted too thin or too thick can be quickly recognized by an operator of the device ( 1).Although the detection device ( 8) is designed as a bolometer camera in this exemplary embodiment, a design as an IR camera or other sensor array for detecting thermal radiation is conceivable.Although it is also conceivable that the device (1) is movable three-dimensionally, it is movable vertically and horizontally two-dimensionally in the direction of arrows (15, 16) in this and the following exemplary embodiments, wherein a distance to the surface regions is preferably constant at 10 cm.It is also conceivable that a module formed from radiation source (5), detection device (8) and / or evaluation device (13) is arranged in a fixed position and a mobile phone protection shell (2) is guided past to determine a paint layer thickness. This can be done either manually by an operator of the device or mechanically.Reference is now made to FIG. 2, in which identical or identically acting parts are denoted by the same reference numeral as in FIG. 1 and the respective reference numeral is in each case appended with the letter a.A device ( 1 a) schematically shown in FIG. 2 comprises a radiation source ( 5 a) having a superluminescent light emitting diode ( 4 a), which is provided for the pulsed or periodic irradiation of a paint surface ( 7 a) with light of a wavelength range from 500 to 800 nm at a frequency of 150 Hz.Reference is now made to FIG. 3, in which identical or identically acting parts are denoted by the same reference numeral as in FIGS. 1 and 2 and the respective reference numeral is respectively appended with the letter b.A device ( 1 b) shown in a schematic view in FIG. 3 is provided for determining the thickness of a powder coating ( 3 b) applied to a metallic component ( 2 b). In this example, a paint surface ( 7 b) is heated by irradiation with a radiation source ( 5 b) and inductively by an induction device ( 18) mounted on a side ( 17) of the component ( 2 b) facing away from a detection device ( 8 b). The detection device ( 8 b) is configured in this exemplary embodiment as a bolometer camera, which can detect a powder coating surface ( 7 b).An exclusively inductive heating of the paint surface ( 7 b) is conceivable.Reference is now made to FIG. 4, in which identical or identically acting parts are denoted by the same reference numeral as in FIGS. 1, 2 and 3 and the respective reference numeral is respectively appended with the letter c.An apparatus (1c) shown in Fig. 4 differs from those shown in Figs. 1 to 3 in that two radiation sources (5c, 19) different from each other are provided, which irradiate a paint surface (7c). In this exemplary embodiment, a first radiation source ( 5 c) comprises a plurality of blue light-emitting diodes ( 4 c), and a second radiation source ( 19) comprises a thermal emitter ( 20) having an irradiation cone ( 21) which irradiates the entire paint surface ( 7 c).Reference is now made to FIG. 5, in which identical or identically acting parts are denoted by the same reference numeral as in FIGS. 1 to 4 and the respective reference numeral is in each case appended with the letter d.An apparatus (1d) shown in FIG. 5 for determining a layer thickness of a layer (3d) differs from those shown in FIGS. 1 to 4 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 successively irradiated with a laser ( 4 d) of a radiation source ( 5 d) once or periodically for heating and emitted thermal radiation is detected by a detection device ( 8 d). Although a size of an excitation spot, i.e. a diameter of a laser beam impinging on the surface ( 7 d) 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 ( 13 d) determines the layer thickness of the layer ( 3 d) for each individual surface region ( 22- 25) and can either display thickness values on a display screen ( 14 d) or determine and display a layer thickness profile of the coating surface ( 7 d) by inter- and extrapolation of the layer thickness values of the surface regions ( 22- 25).Reference is now made to FIG. 6, in which identical or identically acting parts are denoted by the same reference numeral as in FIGS. 1 to 5 and the respective reference numeral is in each case appended with the letter e.A device ( 1 e) shown in FIG. 6 adiffers from those shown in FIGS. 1 to 5 in that a beam path of a radiation source ( 5 e) and that of a detection device ( 8 e) are parallel in sections. For this purpose, the radiation source ( 5 e) for irradiating a surface ( 7 e) and a detection device ( 8 e) are arranged perpendicular to one another. Light of the radiation source ( 5 e) is deflected by a dichroic beam splitter ( 26) in the direction of a surface ( 7 e), while thermal radiation emitted by heating the surface ( 7 e) can pass through the beam splitter ( 26) in the direction of the detection device ( 8 e). This embodiment advantageously makes it possible to determine the layer thickness accurately independently of a distance of the radiation source ( 5 e) from the surface ( 7 e). Although in this exemplary embodiment a determination of the layer thickness takes place in specific surface regions ( 22 e- 25 e), it is conceivable that the entire surface ( 7 e) is irradiated.It is also conceivable that a beam splitter ( 26) shown in FIG. 6 bis 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 specular part of the perforated mirror in the direction of a detection device. A perforated mirror is advantageous in particular when a laser or a radiation source with high spatial coherence, such as a superluminescent diode, is used as the radiation source ( 5 e).Reference is now made to FIG. 7, in which identical or identically acting parts are denoted by the same reference numeral as in FIGS. 1 to 6 and the respective reference numeral is in each case appended with the letter f.A device (1f) shown in FIG. 7 differs from those shown in FIGS. 1 to 6 in that the device (1f) is configured to determine a layer thickness of a coated tape (2f).A radiation source (5f) irradiates - modulated pulse-like or periodically - a narrow surface region (27) of the coated tape (2f) which moves past the device (1f) in the direction of an arrow (28) and extends parallel to a direction of movement of the tape (2f). For moving the band ( 2 f), a conveying means, not shown in FIG. 7, which can comprise a reel, for example, can be provided.A stationary detection device ( 8 f) detects radiated heat of the narrow, moving surface region ( 27) and determines an average layer thickness from a plurality of determined layer thicknesses by averaging.Reference is now made to FIG. 8, in which identical or identically acting parts are denoted by the same reference numeral as in FIGS. 1 to 7 and the respective reference numeral is in each case appended with the letter g.A device ( 1 g) shown in a side view in FIG. 8 differs from those shown in FIGS. 1 to 7 in that a radiation source ( 5 g) is provided for forming an irradiation cone ( 6 g), the beams of which impinge on a coating surface ( 7 g) obliquely to a normal ( 29). Because a time delay with which thermal radiation is emitted is independent of an angle of incidence of excitation radiation of a radiation source on a coating surface ( 7 g), a perpendicular incidence of beams of the radiation cone ( 6 g) is not required for determining a layer thickness. Advantageously, with the method according to the invention, a layer thickness can be determined on curved, coated substrates such as body add-on parts, laptop housings, mobile telephone housings or mobile telephone protective shells without repeated realignment of the radiation source (5g), a device for inductive heating and / or the detection device (8g).It is conceivable that a device ( 1- 1 g) is movable and preferably attached to an industrial robot. As a result, a plurality of surface regions ( 7; 7 a; 7 b; 7 c; 22- 25; 22 e- 25 e; 27; 7 g) can be detected in succession in an automated manner.It is also conceivable that an optical system, which can comprise a lens, for example, is introduced into an optical path formed by an irradiation cone (6-g).Furthermore, a surface region ( 22- 25; 22 e- 25 e; 27) heated inductively or by irradiation can have a smaller size than a size of the irradiation cone ( 6-g) impinging on the surface ( 7; 7 a; 7 b; 7 c; 7 g) or can be at most just as large.

Claims

Method for determining a layer thickness of a layer applied to a substrate (2-2g), in particular of 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 and inductively by irradiation with at least one radiation source (5-5b; 5c, 19; 5d-g) or by irradiation with at least one radiation source (5-5b; 5c, 19; 5d-g) and of the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7 g) are detected by a detection device (8-8 g) and the layer thickness is determined on the basis of the emitted thermal radiation, characterized in that the at least one surface region is irradiated with at least one superluminescent diode, at least one thermal radiator and / or at least one quantum cascade laser for heating.Method according to claim 1, characterised 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.Method according to claim 1 or 2, characterised in that the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) is irradiated with a plurality of identical or mutually different radiation sources (5c, 19).Method according to any 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 configured to generate electromagnetic radiation of a specific wavelength range or a specific wavelength.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) takes place in a periodically modulated manner, in particular with a frequency between 0.01 and 2000 Hz, preferably 20 to 800 Hz.Method according to one of Claims 1 to 5, characterized in that a plurality of, preferably adjacent, surface regions (7-7c; 22-25; 22e-25e; 27; 7g) are successively irradiated by the at least one radiation source (5-5b; 5c, 19; 5d-g) and a layer thickness profile, which can preferably be graphically represented, of a surface to be measured is produced by an evaluation device (14-14g).Method according to one of claims 1 to 6, characterised in that the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) has a size, in particular a diameter, of between 0.2 μm and 200 cm, preferably between 1 and 20 μm or 0.2 and 2 cm.Method according to one of Claims 1 to 7, characterized in that the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) is illuminated obliquely or parallel to a surface normal (29).Method according to one of Claims 1 to 8, characterized in that preheating of the coated substrate (2-2g) takes place before the start of the irradiation and / or of the inductive heating.Device (1-1g) for determining a layer thickness of a layer applied to a substrate (2-2g), in particular of a lacquer layer, the 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) by irradiation or 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) by irradiation and a device (18) for inductively heating the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g), at least one detection device (8-8g) for detecting from the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g) has emitted thermal radiation and an evaluation device (14-14g) for determining a layer thickness, characterized in that the at least one radiation source (5-5b; 5c, 19; 5d-g) comprises at least one superluminescent diode, at least one thermal radiator and / or at least one quantum cascade laser.Device according to claim 10, characterised in that the substrate (2-2g) and / or the layer is or are formed from an electrically conductive, in particular metallic material and / or comprise an electrically conductive, in particular metallic material and can be inductively heated.Device according to claim 10 or 11, characterised in that the at least one radiation source (5-5b; 5c, 19; 5d-g) comprises at least one light emitting diode and / or at least one polariton laser, and is or are in particular configured for periodic irradiation of the at least one surface region (7-7c; 22-25; 22e-25e; 27; 7g).Apparatus according to one of Claims 10 to 12, characterized in that the detection device (8-8g) has an IR camera and / or a bolometer camera.Device according to one of Claims 10 to 13, characterized in that the detection device (8-8g) is set up for simultaneously or successively detecting a plurality of surface regions (7-7c; 22-25; 22e-25e; 27; 7g).Device according to one of Claims 10 to 14, characterized in that the at least one radiation source (5-5b; 5c, 19; 5d-g), the device (18) for inductive heating and / or the detection device (8-8g) is or are arranged movably and is or are designed to be guided past a plurality of, in particular adjacent, surface regions (7-7c; 22-25; 22e-25e; 27; 7g).Apparatus according to one of Claims 10 to 15, characterized in that the at least one radiation source (5-5b; 5c, 19; 5d-g), the device (18) for inductive heating and / or the detection device (8-8g) is or are arranged in a fixed manner, and a conveying means is provided which is designed 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), the device (18) for inductive heating and / or 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.

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