Method and measuring arrangement for determining the layer thickness of a thin, optically transparent layer

Abstract

The invention relates to a method and a measuring arrangement for determining the layer thickness (d) of a thin, optically transparent layer (S) in a layer stack (1) by means of a reflection measurement of light reflected at the layer stack (S), wherein reflection is measured for different colours (R, G, B, UV) and a reflection measurement value (MR, MG, MB, MUV) is captured for each of the colours (R, G, B, UV). For each colour (R, G, B, UV), a reflection curve (R(d), G(d), B(d), UV(d)) for the layer stack (S) is calculated as a function of the layer thickness (d) of the thin, optically transparent layer (S). The layer thickness (d) of the thin, optically transparent layer (S) is determined from a comparison of the reflection measurement values (MR, MG, MB, MUV) and the reflection curves (R(d), G(d), B(d), UV(d)). Reflection is measured using an optical camera (4) having a plurality of sensors (5), which stores a spatially resolved image, wherein each sensor (5) selectively measures light of the different colours (R, G, B, UV) as reflection measurement values (MR, MG, MB, MUV) and in the comparison, the difference between two reflection measurement values (MR, MG, MB, MUV) and the difference between two reflection curves (R (d), G (d), B (d), UV (d)) of the same different colours (R, G, B, UV) are compared with one another.

Description

[0001] Method and measuring setup for determining the layer thickness of a thin optically transparent layer

[0002] The invention relates to a method and a measuring arrangement for determining the thickness of a thin, optically transparent layer that is or can be located in a layer stack (in the sense of a layer structure consisting of several thin, optically transparent layers stacked on top of each other). A reflection measurement of light reflected from the layer stack is performed, wherein the layer stack comprises the at least one thin layer and is applied to a substrate (for example, optically opaque or optically very dense, but also, in principle, optically transparent). The material properties of the layer stack, in particular its structure, refractive indices, and surface properties, are known. This can also include a desired thickness of the thin layer, for example, as a range of possible thicknesses.The invention can then be used, according to a preferred application, to verify a manufacturing process for the layer stack.

[0003] Typically, the material properties of the substrate and / or the surrounding medium are known, or preferably should be known. This applies particularly to the refractive index. The material properties may be required to optically model the structure, or at least to achieve more accurate results. The refractive index of the substrate allows, in particular, the precise determination of the reflection (and, if applicable, absorption) at the underside of the thin optically transparent layer at the interface with the substrate (the bottom layer within the layer). Alternatively, this value can also be estimated, especially if the substrate is optically very dense. The same applies to the surrounding medium when light strikes the upper surface of the thin optically transparent layer (the top layer).

[0004] February 9, 2026 I 48 P 146 WOim layer stack). Provided that the surrounding medium is air, as is usually the case, a refractive index of 1 can generally be assumed with sufficient accuracy. In this case, no special knowledge of the material properties of the surrounding medium is necessary.

[0005] The proposed method, for which the proposed measuring setup is also designed, involves performing a reflectance measurement for at least three different discrete spectral ranges, referred to as colors. A reflectance measurement is recorded for each of the colors.

[0006] For each of these colors, a (theoretical, i.e., based on a physical description of reflection using the known material properties of the layer stack) reflection curve is calculated for the layer stack. Within a predefined layer thickness range, each of the reflection curves is recorded as a function of the thickness of the single thin optically transparent layer whose thickness is to be determined. The thickness of this single thin optically transparent layer is then determined by comparing the reflection measurements with the reflection curves.

[0007] For the method proposed according to the invention, a thin optically transparent layer is a layer with a thickness that is typically smaller than the wavelength of the reflected light or on the order of the wavelength of the reflected light. An optically thin layer, as defined by the invention, is a layer or a stack of layers that causes optical interference upon reflection of light. The physical principle on which the interference is based is described below. The occurrence of optical interference (and thus the suitability as a thin optically transparent layer) can, if necessary, be determined experimentally by a person skilled in the art. The optical system used for the reflection measurement

[0008] February 9, 2026 I 48 P 146 Light can preferably be in a wavelength range between approximately 300 nm (nanometers) (ultraviolet light, UV light) and 800 nm (red light) for silicon (SI)-based detectors. When using infrared (IR)-sensitive detectors, the wavelength range can also extend beyond 800 nm, for example, to approximately 1300 nm. According to a typical or preferred embodiment, the thickness of the thin optically transparent layer to be determined can be less than 300 nm, preferably less than 200 nm, or more preferably less than 150 nm. In typical applications, for example, as an antireflective coating, the thickness to be determined can be in the range between approximately 30 nm and approximately 130 nm, but applications for layer thicknesses of less than 10 nm are also possible, provided the layer optically generates interference in the described wavelength range.As long as interference is detectable in a camera, the method according to the invention is applicable. In materials with low absorptive properties, the layer thickness d can also be greater than the wavelength of the more reflective light, according to the invention.

[0009] It is known that light rays striking thin, optically transparent layers or layer stacks are reflected to varying degrees depending on their wavelength due to optical interference. This results in reflection spectra of the reflected light. The underlying physical principle is as follows:

[0010] When light strikes a thin layer (in the following text also used synonymously for optically thin layer stacks), a portion of the light is reflected at the surface and another portion enters the thin layer. After passing through the layer, the incoming light reaches its back side, is partially reflected there, and thus travels back towards the front side of the layer, where at least some of the incoming light exits. This exiting portion of the light then follows the immediately reflected portion. Since the

[0011] February 9, 2026 I 48 P 146 WO If the component of light passing through the layer and back has traveled a longer path than the directly reflected component, the two light components generally no longer have the same oscillation phases, but rather a phase shift exists. Since this phase shift, which can also be described physically as a path difference, changes with the angle of incidence of the light at a constant media thickness, or with the layer thickness at a constant angle of incidence, interference arises from the path difference (which is generally a function of the layer thickness, the (wavelength-dependent) refractive index, the angle of incidence, and the wavelength), forming the (wavelength-dependent) reflection spectrum. For known layer thicknesses, refractive indices, angles of incidence, and wavelengths, the reflection spectrum can be theoretically calculated by a person skilled in the art using physical laws.The same applies to textured surfaces or refractive indices, for example, from material libraries. Freely available programs exist for this purpose, such as the program "OPAL 2" from PV Lighthouse, Australia, which can be used to calculate the front surface of a silicon solar cell (as a possible application of the method) if the material properties of the layer stack are known. A basic description of the program can be found in McIntosh et al., "OPAL 2: Rapid Optical Simulation of Silicon Solar Cells," Proceedings of the 38th IEEE Photovoltaic Specialists Conference, Austin, 2021.

[0012] The material properties of the layer stack known according to the invention can, in particular, be a (especially wavelength-dependent) refractive index of each of the thin optically transparent layers located in the layer stack, a (nominal) layer thickness of each of the thin optically transparent layers located in the layer stack, and / or, in the case of more than one thin optically transparent layer, the relative arrangement of the thin optically transparent layers to one another on the substrate. These material properties

[0013] February 9, 2026 I 48 P 146 WOdes layer stack wavelength-dependent reflection spectra can be calculated as described.

[0014] In the proposed method, according to the invention, a reflection measurement is performed using an optical sensor, e.g., a camera, for at least three different discrete spectral ranges, referred to as colors, and a reflection measurement is recorded for each of the colors. Thus, a measured reflection value is available for each discrete spectral value (hereinafter also referred to simply as color). Colors can be, for example, red, green, and blue, which can be captured with conventional digital cameras with a known (relative to the entire spectrum) discrete spectral range. In principle, other colors from the spectrum can also be captured, for example, by using suitable filters in front of the light-active sensor elements of a camera. For the purposes of the invention, the ultraviolet or infrared spectral range is also considered a color, even though UV or IR light is not in the optically visible range.

[0015] According to the invention, for each of the colors for which a reflection measurement is recorded, i.e., the corresponding wavelength or spectral range, a theoretical reflection curve is calculated for the layer stack with the known material properties. Within a previously defined layer thickness range, each reflection curve is recorded as a function of the thickness of the thin optically transparent layer whose thickness is to be determined. The thickness of any other thin optically transparent layers that may be present in the layer stack is kept constant during the calculation of the reflection curve. The reflection curve thus shows, for the (fixed) wavelength (spectral range) belonging to the color, the variation of the (theoretically expected) reflection values ​​over a layer thickness range in which the layer thickness to be determined is determined.

[0016] February 9, 2026 I 48 P 146 WO lies in a thin optically transparent layer. This layer thickness range is defined beforehand or is known.

[0017] A typical application of the method proposed according to the invention can be in quality control during the production of solar cells, where the layer thickness of a thin optically transparent layer is to be checked with a generally known structure and known material properties of the layer (or one of possibly several layers in the layer stack).

[0018] For this purpose, the measured reflection values ​​are compared with the theoretically determined reflection curves for the different colors. The layer thickness is then read from the curve where all reflection measurements for all colors best match their respective reflection curves. Because the interference occurring at the individual wavelengths reacts differently to changes in layer thickness due to phase shifts within the measured layer stack, the proposed method represents a simple and suitable way to determine the layer thickness of thin, optically transparent individual layers or individual layers within the layer stack. The resulting interference can also lead to a change in the observed color.

[0019] Such a method is already known in the prior art. The publication by C. Choi et al., “Simple method for volumetric thickness measurement using a color camera”, Applied Optics, Vol. 57, No. 26, page 7550, September 10, 2018, describes a method for capturing an interference pattern with a camera and estimating the layer thickness from the interference color information based on the relationship between color and layer thickness. For this purpose, a point on a sample is illuminated with white light focused by a lens, and the corresponding interference pattern captured by the same lens is then measured.

[0020] February 9, 2026 I 48 P 146 WO reflected light captured by the pixel (image point or image point area) of a normal color camera (CMOS sensor with multispectral RGB capture (red, green, blue)) in a single image (see Fig. 1 of the publication). Fig.

[0021] Figure 7 shows the corresponding theoretical reflection curves for the different colors R, G, and B, along with the reflection measurements recorded for each color. These measurements were fitted to the reflection curves as closely as possible using standard mathematical fitting methods. All reflection measurements for the different colors, as boundary conditions during the fitting process, always exhibit the same, but jointly variable, value for the layer thickness. This allows the layer thickness to be determined.

[0022] CN 105937882 B describes a comparable measuring principle with a corresponding setup of a measuring arrangement.

[0023] The problem with current technology is that theoretically determined reflection curves capture the total reflection of the incident light, which in practice occurs over a wider angular range. This implies that measurements must also be angle-integrated for comparison with the reflection curves. This can be achieved, for example, using an integrating sphere: a hollow sphere with diffuse internal reflection, a light-in aperture in front of which the light source is positioned, and a light-out aperture arranged at right angles to it. The luminous flux exits through this aperture after multiple reflections off the inner wall of the sphere. This allows for the measurement of a reflection value that closely matches the theoretically calculated reflection. However, such a setup is very complex and unsuitable for quality control in a production plant.

[0024] The publication by C. Choi et al. and CN 105937882 B describe a measurement setup with optics that detects the light reflected from the surface, at least over a large beam angle range, thus enabling a

[0025] February 9, 2026 I 48 P 146 WO A significant portion of reflected radiation is detected. However, the problem here is that the optical sensor only detects a small area of ​​the sample, and the layer thickness can only be determined at a single measurement point on the sample. A comprehensive measurement of the layer thickness across large areas is either impossible or extremely complex.

[0026] To determine the layer thickness of a layer in a layer stack on a planar substrate across its entire surface, it would be necessary to move the measuring setup relatively across the entire area to be measured and to scan the area in successive measurements. This is complex and does not allow for the area-wide determination of the layer thickness of a thin, optically transparent layer in a layer stack on a planar substrate during production, especially not in a conveyor line, for example, in a production plant where a planar substrate is transported in one direction after the layer stack has been applied, as is common in the manufacture of solar cells, but also in other applications.

[0027] Against this background, the object of the invention is to simplify and accelerate the determination of the layer thickness of a thin optically transparent layer of a layer stack, especially also in the case of planar layers on a planar substrate.

[0028] This problem is solved according to the invention in the previously described method for determining the layer thickness by the features of claim 1. In particular, it is provided that the reflection measurement is performed using an optical camera with a planar sensor arrangement of several sensors, wherein the several sensors detect a planar area of ​​the layer stack and the optical camera stores the detected area of ​​the layer stack as a spatially resolved image.

[0029] February 9, 2026 I 48 P 146 WO Each individual sensor or group of individual sensors in the camera thus maps a point-like (small in area relative to the total area) section of the layer stack. The individual sensor or group of individual sensors is also referred to as a camera pixel (camera resolution).

[0030] The camera's multiple sensors (camera pixels) capture, with spatial resolution, the entire area, or at least a significant portion of the entire area, of the layer stack in which the layer thickness of a single layer is to be determined. This portion of the entire area can, for example, capture a section of the layer stack across its entire width while the stack is being transported perpendicular to the conveyor direction. A line scan camera can be used for this purpose. Alternatively, Karner's sensors (camera pixels) can be arranged in a matrix configuration oriented in two spatial directions, similar to optical digital cameras. With such a configuration (in contrast to line scan cameras with multiple adjacent sensors (camera pixels) in only one spatial direction), even larger layer stacks extending in two spatial directions can be captured in a single image.This enables a fast, spatially resolved determination of layer thicknesses and thus, according to the invention, also of layer homogeneity in a planar layer stack.

[0031] The method involves selectively measuring light of (preferably all) different colors as reflection measurements in each of the camera's multiple sensors, for which the reflection measurement is to be carried out.

[0032] February 9, 2026 I 48 P 146 WO There are various embodiments according to the invention. According to a preferred embodiment, each individual sensor (camera pixel) of the optical camera can preferably consist of at least three sensor elements, each configured to selectively measure light of one of at least three different colors, for example by means of suitable filters that each selectively transmit a discrete spectral range corresponding to a color to the sensor element. In the optically visible range, these are the colors red, green, and blue in conventional cameras, with which any visible color can be captured in a camera pixel by suitable additive mixing of the colors red, green, and blue. There are also cameras with sensor elements for colors in the ultraviolet and / or infrared spectral range, which can also be combined with the sensor elements for the visible colors, such as red, green, and blue.The method proposed according to the invention applies generally to all colors, for example, by providing suitable color filters in front of the sensor elements. In this embodiment, the layer stack is preferably illuminated with white light. White light in this sense is light that exhibits the discrete spectral value ranges (colors) for which the reflectance measurement is performed. In this embodiment, if the white light is generated simultaneously, an image can be acquired for the reflectance measurement for all spectral value ranges (colors or color channels) simultaneously in a single image. In principle, such illumination can even be achieved with ambient light. However, illumination with a dedicated lighting device is preferred in order to achieve better and more defined (i.e., in particular, temporally constant) illumination of the surface with the required colors.This improves image quality and allows for shorter exposure times, which is particularly advantageous when dealing with layer stacks transported at a transport speed during image acquisition.

[0033] February 9, 2026 I 48 P 146 WO In principle, white light can also be generated by a lighting device that sequentially produces light from one or more discrete (narrowband) spectral ranges, which are captured by the camera in successive images, with the light covering all colors for which a reflection measurement is to be performed. According to a specific embodiment, for example, a lighting device may be provided that successively generates flashes of visible light (with the spectral ranges red, green, blue) as well as of UV light and / or IR light in order to cover all spectral ranges.

[0034] In another preferred embodiment, each of the camera's multiple sensors selectively measures light of several (preferably all) different colors as reflectance values ​​for which the reflectance measurement is to be performed, wherein the optical camera can be configured as a monocular camera (if the monocular camera measures light of all different colors). A monocular camera has only one sensor element in each of its sensors, which is sensitive to all discrete spectral ranges (colors) for which a reflectance measurement is to be performed. Accordingly, a monocular camera illuminated with white light would display only a single, aggregated reflectance value for all colors.

[0035] To carry out the inventive method in this embodiment with a monocular camera (or sensor elements that simultaneously detect several colors), the invention may therefore provide that a lighting device sequentially (one after the other) generates light from each of the discrete spectral value ranges (colors) and that the cameras each capture an image for the light from each of the discrete spectral value ranges (colors). For this purpose, the lighting device can be configured in a specific manner.

[0036] February 9, 2026 I 48 P 146 WO The process involves sequentially generating flashes of light of different colors and synchronously capturing an image with the monocular camera for each flash (in one of the different colors). This allows for the sequential inclusion of multiple images with different, spectrally narrowband excitation (i.e., for different colors or discrete spectral ranges). This is particularly suitable for large-area imaging of a planar stack of layers with one or more thin, optically transparent layers. It is especially well-suited for a stationary stack of layers.

[0037] In a monocular camera, fewer sensor elements in a sensor (camera pixels) need to be read out. This results in a high possible image acquisition frequency and, with a correspondingly high flash frequency and a transport speed that is low compared to the image acquisition frequency, also enables the inventive method to be carried out with this embodiment on a moving layer stack. According to the invention, a transport speed that is low compared to the image acquisition frequency exists when the surface area captured during the image acquisition sequence for each image of each color remains the same. This applies when the surface area of ​​the layer stack captured in the first image of the image acquisition sequence and the surface area of ​​the layer stack captured in the last image of the image acquisition sequence overlap to a predefined limit of, for example, at least 80%, preferably at least 90%.

[0038] According to the invention, embodiments are also conceivable in which the sensor has several sensor elements, wherein at least one of the sensor elements detects more than one of the discrete spectral ranges (colors). For example, one sensor element can be provided for the optically visible wavelength range (red, green, blue) and one sensor element for the optically non-visible wavelength range (UV, IR).

[0039] February 9, 2026 I 48 P 146 WO This allows the person skilled in the art to select optical cameras with a suitable number and / or type of colors, depending on the material of the thin layer whose thickness is to be examined. This is particularly useful when the layers are indistinguishable or invisible with conventional digital RGB cameras in the RGB wavelength range. A suitable camera can, for example, be equipped with sensors for the colors red (R), green (G), blue (B), and optionally subviolet (UV). However, the invention is not intended to be limited to any of these preferred applications. Depending on the material, the person skilled in the art can select a particularly suitable type and / or number of colors to be detected by the optical camera.

[0040] By using an optical camera that captures a spatially resolved image of the layer stack, the reflected light is typically superimposed on a scattered component of the ambient light. Therefore, only a portion of the measured light originates from the reflected light that is actually of interest here.

[0041] This situation is similar to a "reflection" at a textured surface, which prevents the camera from simply viewing and imaging the direct reflection of light from a sample (layer stack) with an optimally smooth, reflective surface. With a textured surface, light scattered within the texture structure is captured in the camera's image pixel. This scattered light has a component that is scattered across the entire spectral range, independent of wavelength. This wavelength-independent scattering component creates a uniform offset in all color channels captured by the camera's sensor elements, similar to the effect of ambient light during measurement.

[0042] February 9, 2026 I 48 P 146 WO This proportion can be observed, for example, in solar cells with a textured surface. In practice, the surface of solar cells, i.e., their layer system or stack of layers, is optimized as an antireflective coating for optimal power output. The antireflective coating is intended to minimize the reflection of the red light component. Therefore, in bright-field (i.e., white-light) illumination, the spectrum of the light reflected by the sensor elements shows no significant red component in planar (non-textured) solar cell surface structures. Consequently, the surface of the solar cell module appears blue in the image (and when viewed with the naked eye).When a suitably textured surface of a solar cell module with an antireflective coating is illuminated with white light in an otherwise dark room, or when the surface is viewed perpendicularly in a dark field, significantly higher proportions of red are measured in the spectral range.

[0043] The proportion of ambient light is scattered almost uniformly across the entire spectrum, essentially independent of wavelength. This scattered portion of the light (in the case of white light) results in a roughly equal offset in the measured reflectance values, which are also referred to as the color channels of the optical camera. This portion of scattered light (also known as the non-spectral portion of the light) is superimposed on the portion of wavelength-dependent reflected light (also known as the spectral portion of the light).

[0044] The invention proposes comparing the difference between two reflection measurements of two different colors and the difference between two reflection curves of the same different colors when comparing reflection measurements and reflection curves. For example, in an optical camera in the visible spectral range, which measures the colors red (R), green (G), and blue (B) in one camera pixel, the differences between the

[0045] February 9, 2026 I 48 P 146 WO Color channels (RG, GB) are considered, i.e., the differences in the reflectance measurements (MR-MG, MG-MB) and the corresponding differences in the reflectance curves (R(d)-G(d), G(d)-R(d)). The difference in the spectral components of the reflectances of the color channels RG and GB eliminates the non-spectral component of the light, such as scattered light and / or ambient light during measurement. The differences contain only the spectral difference due to the wavelength-dependent reflectance of the different colors (color channels).

[0046] In contrast to the prior art, this enables the spatially resolved determination of the layer thickness of a thin optically transparent layer in a layer stack using an image of the layer stack taken by an optical camera, because scattered light influences and a superposition of the spectral component of the reflected light by, for example, ambient light are eliminated or minimized in the spatially resolved image of the layer stack.

[0047] According to a particularly preferred embodiment of the method proposed according to the invention, when comparing the reflection measurements and the reflection curves, one or more quotients of the different differences in the reflection measurements of different colors, as well as one or more quotients of the different differences in the reflection curves of the same different colors, can be compared with one another. The quotients of the different differences are hereinafter also referred to as the difference quotients of the reflection measurements and the reflection curves, respectively. A particularly advantageous aspect of this embodiment of the invention is that only ratios of measured reflection values ​​and theoretically (computationally) determined reflection curves are considered.

[0048] February 9, 2026 I 48 P 146 WO Theoretical reflection curves provide angle-integrated values ​​of the reflection. However, when a large-area image of the layer stack is captured with an optical camera, the reflected light is no longer captured angle-integrated (unlike in the prior art, where it is at least approximately realized), so that the recorded reflection measurements cannot be compared in absolute terms (i.e., as absolute values) with the theoretically determined reflection curves. This is because only the portion of the reflected light that is optically imaged in the single camera pixel is recorded as a reflection measurement. In other words, the measured reflection is only considered within a small solid angle. Therefore, regardless of the angle from which the optical camera views the sample (the layer stack), it only captures a portion of the total reflection.The absolute values ​​of the measured reflection measurements and the theoretically calculated reflection from the reflection curves therefore do not match.

[0049] However, it has been shown that, at least within the desired or required accuracy range, there is no spectral angular dependence of the reflection. Therefore, if, instead of the angle-integrated measurement common in the prior art, the reflection is considered only within a small solid angle range (captured by the camera pixel in the optical image), the ratio of the colors considered in the reflection measurement does not change within this solid angle range. The ratio of the color channels to each other remains unchanged.

[0050] This also applies to surface structures (textures) whose structure size is larger than the wavelength of the scattered light. Such a texture is also found in typical surface structure sizes of solar cells. Scattering at such a texture differs from the so-called wavelength-dependent Rayleigh scattering of light by particles whose size is small compared to the wavelength of the light. This is the case, for example, with atmospheric air particles compared to the optically visible wavelength range. The Rayleigh-

[0051] February 9, 2026 I 48 P 146 WOS scattering causes the blue hue of the sky during the day and the red coloration of the rising and setting sun due to the spectral angular dependence of light scattering.

[0052] According to this embodiment, instead of considering the absolute reflection measurements, the ratio of different reflection measurements to each other is taken into account. Thus, a relative signal quantity is considered, in the sense of a dimensionless quantity that is independent of the angular measurement range captured in the measurement. This dimensionless quantity (difference quotient of the reflection measurements) is therefore directly comparable to a corresponding dimensionless quantity (difference quotient of the theoretical reflection curves), even though the theoretical values ​​of the reflection curves actually represent the entire angle-integrated reflection. According to the invention, this simplifies a comparison of a non-angle-integrated reflection measurement with an angle-integrated (theoretical) reflection curve, without requiring an angle-integrated measurement or the acquisition of the measurement's angular range.

[0053] In combination with the scatter-corrected acquisition of reflectance values ​​by calculating the difference between two color channels, whose difference quotient is then used to compare reflectance measurements and reflectance curves, it is therefore particularly easy to determine the layer thickness using a simple measurement setup, because interference from scattering and ambient light is largely avoided by calculating the difference. This makes it possible to determine the angle-dependent spectral component of the reflected light for the individual color channels with high accuracy, even if only a fraction of the reflected light is captured in the camera pixel. Furthermore, the quotient calculation makes the measurement independent of the actual captured proportion of reflected light compared to the theoretically determined measurement curves.

[0054] February 9, 2026 I 48 P 146 WO Regardless of this, when carrying out the proposed method for the arrangement of the optical camera and white light source, it is advantageous to arrange them in the reflection angle relative to the surface of the layer stack or the sample in order to maximize the proportion of reflected light in the sensor elements of the optical camera.

[0055] The application of the inventive method for determining the layer thickness of a layer in the layer stack, as previously described in one embodiment and further explained below in accordance with other forms, represents a preferred application of the invention for determining the layer thickness of a coating (layer stack) of solar cells or solar cell modules.

[0056] According to a further preferred embodiment, in addition to a quotient derived from the different differences of various colors (both the reflected values ​​and the reflected curves), the reciprocal of the quotient can also be calculated and used for calibration. This avoids difficulties in determining layer thicknesses at poles of the denominator of the quotient, where the reflected values ​​or the values ​​of the reflected curves of the colors whose difference is calculated are equal. According to the invention, the quotient or the reciprocal of the quotient can then be selected for calibration for different reflected value ranges of the different colors, wherein, for the various reflected value ranges, regions of the quotient or the reciprocal of the quotient are selected in which the quotient or the reciprocal of the quotient of the reflected curves as a function of the layer thickness has no pole.

[0057] February 9, 2026 I 48 P 146 WO The reflection measurement ranges can be selected by a person skilled in the art depending on the material properties of the layers of the layer stack and the discrete spectral value ranges of colors, in particular based on the curves of the quotient and / or reciprocal of the quotient of the reflection curves as a function of the layer thickness. Examples of this will be explained later.

[0058] According to a particularly preferred embodiment, a person skilled in the art can determine which difference quotients of which colors lead to good results in a layer thickness range of interest within the reflection curves to be investigated. A particularly suitable selection criterion according to the invention can be that the quotient of the reflection curves exhibits a strictly monotonic profile over the layer thickness range. This facilitates the finding of unambiguous solutions for determining the layer thickness. If necessary, depending on conditions relating to the reflection measurements (such as the reflection measurement of color 1 being greater or less than the reflection measurement of color 2), several different quotients can also be selected section by section.

[0059] According to one embodiment of the invention, a white balance of the optical camera can be performed before the reflection measurement. White balancing ensures that the sensor elements of all sensors display the same measured value when the pixels of the optical camera are illuminated with white light. White light in this sense means that the intensity of the light is the same across all spectral ranges. After white balancing, the camera's sensor elements for the different colors display the same measured values ​​when illuminated with white light. In such a case, the measured values ​​in the different color channels of the individual sensor values ​​are thus aligned so that these measured values ​​are directly comparable. White balancing can be performed during camera calibration by measuring the light from a white light source and

[0060] February 9, 2026 I 48 P 146 WO Individual color channel adjustments can be performed. This applies to embodiments in which each camera sensor has a separate sensor element for capturing each selective color channel and in which the separation of the color channels is achieved through spectral filtering within the camera (e.g., by different sensor elements with different colored filters in a single sensor or camera pixel). Specifically, white balance can be performed, for example, by photographing a white sample (i.e., a white light-scattering surface). The requirement that all camera color channels should deliver the same intensity necessitates the setting of correction factors, which are applied accordingly to the camera's color channels separated by color.The only crucial factor in these methods is that the intensity ratio of the color channels determined by the camera also corresponds to the ratio of the intensity of the wavelengths under consideration in the received light.

[0061] White balance during sequential image capture under different spectral illumination (sequential capture of the various colors or color channels) can be achieved, for example, by using different exposure times for the different colors, such as by varying (adjusting) the flash duration for each color. The flash duration must be selected for each illumination within a selective spectral range (i.e., for each color channel) such that a white sample produces the same measured intensity in all spectrally selective exposure channels.

[0062] Such a white balance of the optical camera simplifies the determination of layer thickness and improves its quality. The term "white balance" is not limited to the visible color spectrum, in which the primary colors R, G, B produce white light when mixed evenly. The term "white balance" is also intended to include non-visible light, such as UV light.

[0063] February 9, 2026 I 48 P 146 WOmit. White balance ensures that the measured values ​​of the sensor elements in all used colors or color channels truly correspond to the reflection and are comparable with each other.

[0064] During camera calibration, it is also possible to eliminate or minimize the influence of dark current, for example, by using an offset value and / or cooling the individual sensor elements of the optical camera. Dark current refers to the spontaneous formation of free charge carriers in a light-sensitive semiconductor, such as a CCD image sensor, due to heat. Eliminating or minimizing dark current is possible, for example, by capturing images with the optical camera in darkness. The values ​​measured in darkness for each sensor element indicate the dark current, which can be compensated for by specifying a corresponding offset value. Since the dark currents of the various sensor elements of an optical camera are usually different, the offset values ​​can be used to align the different sensor elements.The effect of dark current is essentially eliminated by the difference formation of the measured values ​​proposed according to the invention.

[0065] Another aspect of the proposed camera calibration can concern the linearity of the sensor and sensor elements, which should ensure that the measured values ​​of the sensor element correspond scalably to the light intensity (including reflection). This can be achieved by measuring various (known) light intensities of white light with the camera and adjusting the sensitivity curve of the individual sensor elements.

[0066] Such camera calibration measures are known to experts and therefore do not need to be explained in more detail here.

[0067] February 9, 2026 I 48 P 146 WO In one embodiment of the method, the at least three different colors for the reflection measurement can be red, green, and blue, which are captured in different color channels by the optical camera designed as a digital color camera (i.e., by corresponding different sensor elements of a sensor or a camera pixel). This allows conventional digital color cameras to be used for carrying out the method. According to the invention, other and / or additional spectral ranges can also be used. The selection depends in particular on the material properties in the layer stack and can be determined by a person skilled in the art based on their expertise and / or suitable simple experiments.

[0068] In another embodiment, one of the various colors of the optical camera can be an ultraviolet spectral range, i.e., the color ultraviolet (UV), or an infrared spectral range (IR). This enables the examination of materials in a layer stack when observing colors in the visible wavelength range does not allow for an unambiguous determination of the layer thickness because a specific layer thickness is not clearly distinguishable, and / or a layer is not visible or only barely visible in the visible wavelength range due to its material properties, and / or the additional wavelength (spectral range) simplifies the determination of the layer thickness or increases its accuracy due to a different interference pattern. Recognizing such a situation is easily accomplished by a person skilled in the art, for example, from theoretical reflection curves.

[0069] According to a further embodiment, a reflection measurement can be performed for at least (or exactly) four colors. For this purpose, an optical camera with sensors, each having at least (or exactly) four sensor elements, can preferably be used according to the invention.

[0070] February 9, 2026 I 48 P 146 WO where each sensor element measures one of at least (or exactly) four colors. This generally improves accuracy. This also applies, for example, to a monocular camera (such as a black and white camera) with four sequential flashes in the respective spectral ranges (colors) or combinations of these previously described embodiments.

[0071] In this context, it can be considered that when calculating the difference between two reflectance measurements, the first difference should use the first and second of at least (or exactly) four colors, and the second difference should use the third and fourth of at least (or exactly) four colors, with each of the first, second, third, and fourth colors being distinct from the others (i.e., having a different spectral range). This generally facilitates the determination of unique solutions for the layer thickness.

[0072] According to a further embodiment of the invention, the planar sensor arrangement of the optical camera (hereinafter also referred to as a planar camera) can be a camera with one or more line sensors, i.e., sensors arranged in one or more rows. A camera with only one row of sensors can also be referred to as a line camera. This, too, constitutes a planar camera in the sense that it captures an image of a surface area (namely, a line). Preferably, the image acquisition by the optical camera can be timed with a transport of the layer stack relative to the camera such that the entire surface of the layer stack is imaged by multiple successive images. In this way, the layer thickness can be determined with the pixel resolution of the camera over the entire surface of the layer stack, and thus the layer homogeneity over the entire surface of the layer stack can be easily determined.

[0073] February 9, 2026 I 48 P 146 WO With an area scan camera equipped with multiple line sensors, a larger area of ​​the layer stack is captured at once. This allows the clock frequency for image acquisition by the optical camera to be reduced. Furthermore, by overlapping the image captures, it can be ensured that the entire surface of the layer stack is indeed captured.

[0074] In a further embodiment of the invention, the layer stack can comprise further thin optically transparent layers, wherein the layer thickness of these further thin optically transparent layers is preferably known. The further thin optically transparent layers of the layer stack can be one or more thin optically transparent layers whose layer thickness and material properties are already known, for example, due to the application technique of the further layer(s) and / or by a prior measurement of the further layer(s), such as by applying the previously described method to these layers as well, in particular stepwise after their application.

[0075] This makes it possible to determine the thickness of one of several thin optically transparent layers, even if the other optical layers contribute to the reflection and thus influence the measured reflection values. Due to the known layer thickness and material properties, these can also be taken into account in the theoretically calculated reflection curves, so that determining the thickness of the single thin optically transparent layer is possible using a comparable application of the method.

[0076] The invention also relates to a measuring arrangement with the features of claim 11 to solve the stated problem. The measuring arrangement is designed to determine the layer thickness of a thin optically transparent layer in a layer stack comprising at least one thin layer.

[0077] February 9, 2026 I 48 P 146 WOund is applied to a substrate, wherein the material properties of the layer stack and preferably also of the substrate (as described above) are known. The measuring arrangement comprises a carrier for holding the layer stack with the at least one thin optical layer and an illumination device for illuminating the surface of the layer stack (and thus the thin optically transparent layer). The illumination device is designed to generate (preferably white) light, which comprises at least three discrete spectral ranges designated as colors. This can occur simultaneously or sequentially, e.g., as flashes of light of specific spectral ranges (colors). Specific embodiments will be described below.The optical camera is configured to measure the reflection of at least three colors from the illumination device, wherein the illumination device and the optical camera are arranged in a reflection arrangement with respect to the surface of the layer stack being recorded. A reflection arrangement means that light rays from the illumination device strike the surface of the layer stack at a reflection angle and are reflected from there into the optical camera at the same reflection angle, possibly after multiple reflections, particularly if the surface permits reflection.

[0078] A control unit directs the optical camera to capture an image of the surface of the layer stack (and thus perform the reflectance measurement). The control unit can also be configured to control the lighting device, switching the light on and off, and / or setting the discrete spectral ranges of the lighting device.

[0079] Furthermore, the measuring arrangement includes a computing unit for evaluating the images of the surface of the layer stack taken by the optical camera and

[0080] February 9, 2026 I 48 P 146 WO for performing a comparison of reflection measurements of at least three colors extracted from the images with the reflection curves calculated (theoretically) as described above for the layer stack with the known material properties. Within a previously defined layer thickness range, each of the reflection curves is recorded as a function of the layer thickness of the thin optically transparent layer whose layer thickness is to be determined. The processing unit can optionally also be configured to perform a calibration of the camera and / or lighting equipment, in particular to perform a white balance, to detect the influence of a dark current and / or to linearize the camera sensors, as already explained.

[0081] The optical camera is a planar sensor arrangement with multiple sensors, wherein the optical camera is designed to store the reflection measurements of the sensors as a spatially resolved image of the layer stack.

[0082] The computing unit is equipped by suitable program code means, which, when executed on a processor of the computing unit, performs the procedure steps specified in the program code means, to compare the difference between two reflection measurements of two different colors and the difference between two reflection curves of the same different colors when comparing the reflection measurements and the reflection curves.

[0083] Further details and options for these process steps have already been described in the preceding description of the corresponding procedure and can be implemented accordingly in the computing unit (by the appropriate program code means) without repeating them here. The program code means can be designed such that, after inputting the material properties of the

[0084] February 9, 2026 I 48 P 146 WO The reflection curves of the layer stack and the properties of the measurement setup are calculated as already explained. It is also possible that the reflection curves are pre-calculated for specific layer stacks during the setup of the measurement setup and recorded as parameter files. The processing unit can then be configured via the program code to read the parameter files, thereby setting up the reflection measurement curves in the processing unit for carrying out the described procedure or parts thereof.

[0085] According to a particularly preferred embodiment, the computing unit can further be configured to compare, when comparing the reflection measurements and the reflection curves, one or more quotients of the different differences in the reflection measurements of different colors and one or more quotients of the different differences in the reflection curves of the same different colors. Further details of this process step and optional variations thereof have already been described in the preceding description of the corresponding method and can be implemented accordingly in the computing unit (by the appropriate program code) without repeating them here.

[0086] The effects and advantages achieved in connection with the method also apply accordingly to the measuring setup used to carry out the method. A repetitive description is omitted because the person skilled in the art will immediately recognize how the described, and in some cases optional, method steps can be implemented accordingly in the measuring setup.

[0087] According to the invention, the lighting device and each of the camera's sensors are designed to selectively emit light of different colors as

[0088] February 9, 2026 I 48 P 146 WOReflection measurements. This can be done in various ways according to the invention. According to one embodiment, the camera sensors can have at least one sensor element that is selectively configured to measure light of one color and / or the camera sensors can have at least one sensor element that is configured to measure light of several colors and that the lighting device is configured to generate the light of these several colors sequentially one after the other.

[0089] In one specific embodiment, each camera sensor can exclusively comprise sensor elements that are selectively configured to measure light of only one color. In this embodiment, each camera sensor has at least three or more sensor elements. The number of sensor elements can be as many as the reflectance values ​​of different colors are to be measured. Preferably, but not necessarily, the illumination device can be configured to simultaneously generate white light, i.e., all colors whose reflectance values ​​are to be measured.

[0090] In another specific embodiment, each camera sensor can have exactly one sensor element designed to measure light of all colors whose reflectance values ​​are to be measured. In this embodiment, the illumination device is then designed to generate the light of all these colors sequentially, for example, as successive flashes of different colors.

[0091] Hybrid forms of these two previously described, specific embodiments are also possible according to the invention. This has already been explained. Reference is made to this explanation.

[0092] February 9, 2026 I 48 P 146 WOThe computing unit of the control unit, for example, can also be set up for calibrating the optical camera and / or lighting device by means of white balance, elimination of dark current and / or linearization of the camera sensor with regard to light intensity and the use of the number and type of colors (discrete spectral ranges).

[0093] As explained, the optical camera can be equipped with one or more line sensors, whereby a line sensor of the optical camera can, in particular, capture an image across the entire width of the layer stack perpendicular to a transport direction of the substrate or layer stack. If necessary, it is also possible to arrange several line sensors (distributed across the width of the layer stack) side by side to capture the entire width of the layer stack. The images from these multiple line sensors can be combined into a single image (stitching the individual images to create a composite image).

[0094] The measuring arrangement can include a transport device with which the layer stack, comprising at least one thin optically transparent layer and the substrate, can be moved along a transport direction. Preferably, the transport movement, and in particular the transport speed, can be controlled by the control unit or read from the control unit via an interface. The control unit can then synchronize the image acquisition of the optical camera with the transport movement in such a way that the entire surface of the layer stack is preferably captured uniformly by the optical camera. Alternatively, the optical camera can also be designed as an area scan camera such that it captures the entire (or at least the entire surface to be examined) surface of the layer stack in a single image.

[0095] February 9, 2026 I 48 P 146 WO To clearly display the results of the layer thickness determination, the processing unit can be further configured to generate, for example, a two-dimensional spatially resolved layer thickness image from the determined layer thicknesses of the thin optically transparent layer in the spatially resolved image, which can be displayed, for example, on a screen. The layer thickness image can be designed as a false-color image in which specific color tones are assigned to specific layer thickness values. If the described method and the described measuring arrangement are used to check the layer thickness of a layer in a layer stack after the product has been manufactured, it can be advantageous according to the invention to use an intuitive color scheme, e.g., green tones for a layer thickness within the desired standard thickness range and red or blue tones for a deviation from the desired standard thickness range ("too thin" or "too thick").

[0096] With the method and measuring arrangement described according to the invention, the layer thickness of a thin optically transparent layer can be easily determined by recording reflected light in a spatially resolved image of the layer stack. The measuring method according to the invention is not sensitive to light scattered at the surface or to ambient light. Furthermore, the reflected light does not need to be measured with angular resolution. The same applies to the measuring arrangement according to the invention, which is configured to carry out the method described above and any optional features of the described method.

[0097] Further advantages, features, and applications of the invention will also become apparent from the following description of exemplary embodiments and the drawings. All described and / or illustrated features, together or in any technically sensible combination, belong to the subject matter of the invention.

[0098] February 9, 2026 I 48 P 146 WO Irrespective of their summary in described or illustrated embodiments or in the claims.

[0099] They show:

[0100] Fig. 1 schematically shows a light ray incident on and reflected from a thin optically transparent layer;

[0101] Fig. 2 shows the dependence of a reflection spectrum on the wavelength for different layer thicknesses d1 to d9 using an example;

[0102] Fig. 3 shows the dependence of reflection curves for the colors R, G, B as a function of the layer thickness d using an example;

[0103] Fig. 4 shows the curve progression of difference quotients of the reflection curves for the colors R, G, B, which, according to a preferred embodiment of the invention, are used for comparison with corresponding difference quotients from the reflection measurement, using an example;

[0104] Fig. 5 shows the curve of a difference quotient of the reflection curves for the colors UV, R, G, B, which, according to a preferred embodiment of the invention, are used for comparison with the corresponding difference quotient from the reflection measurement, using another example;

[0105] Fig. 6a schematically shows the structure of an optical camera according to one embodiment of the invention;

[0106] February 9, 2026 I 48 P 146 WOFig. 6b schematically shows the construction of an optical camera according to a further embodiment of the invention;

[0107] Fig. 6c schematically shows the construction of an optical camera according to a further embodiment of the invention;

[0108] Fig. 7 schematically shows the setup of a measuring arrangement for carrying out the method for determining the layer thickness of a thin optically transparent layer in a layer stack according to an embodiment of the invention;

[0109] The following descriptions of embodiments of the invention are based on the drawing.

[0110] Fig. 1 schematically shows a so-called "layer stack" 1, which in the example shown, for the sake of simplicity, comprises only a thin optically transparent layer S, the thickness of which d is to be determined. The layer stack 1 is applied to a substrate 2, which can also be optically transparent or non-optically transparent.

[0111] To determine the layer thickness d, optically white light according to the invention (for example, simultaneously and / or sequentially) from a lighting device 3 falls onto the layer stack 1, comprising at least three different colors in the sense of optically discrete spectral value ranges (also referred to synonymously as colors). Optically white light in the visible range can, for example, include the colors red (R), green (G), and blue (B), which can be detected in conventional digital optical cameras 4 with sensors 5. In the example described here, each sensor 5 of the camera 4 has at least three sensor elements 6, each of which detects one of the colors, for example, by means of filters in front of the light-active sensor elements. Other embodiments of cameras and

[0112] February 9, 2026 I 48 P 146 WO Lighting devices have already been described and can also be used instead of the camera 4 and lighting device 3 according to the invention.

[0113] In conventional color digital cameras for visible light, each sensor 5 comprises sensor elements 6 for the colors R, G, and B, from which any color from the visible spectral range can be generated by additive superposition at the corresponding intensity. Similarly, any color can be captured by selectively capturing the colors R, G, and B in the three sensor elements 6. There are also optical cameras with additional and / or different sensor elements, each of which can capture other discrete spectral ranges (colors) from the visible and / or non-visible wavelength range, for example, through suitable filters and / or suitable sensor-active elements. Such optical cameras 4 can also be used according to the invention. The same applies to monocular cameras 4 with exactly one sensor element 6 per sensor 5 (e.g.,A black-and-white camera in combination with an illumination device 3, which emits the different colors sequentially one after the other, with the mono camera 4 capturing an image for each of the colors. A sensor 5 with all its sensor elements 6 is also referred to as a camera pixel, for which the intensities of all multispectral components of the incident light detected by the sensor elements 6 are recorded (multispectral measurement).

[0114] Accordingly, other colors may also be present in the white light of the lighting device 3 and detected by the corresponding sensor elements in the sensors 5. These other colors can also be selectively measured by corresponding sensor elements and may lie in the non-visible wavelength range, e.g., in the ultraviolet (UV) and / or infrared (IR) wavelength range. UV and IR are also referred to here as colors.

[0115] February 9, 2026 I 48 P 146 WOThere are, for example, suitable lighting devices 3 and optical cameras 4 that generate optically white light with the discrete spectral value ranges R, G, B, UV and can detect the discrete spectral value ranges R, G, B, UV with corresponding sensor elements 6 in each of the sensors 5 (camera pixels) of the optical camera 4 (compare also Fig. 6a and 6b, Fig. 6c shows a mono camera 4 for sequential illumination of the layer stack with different colors).

[0116] The following explains the basic principles of the physical measurement principle, which are familiar to those skilled in the art, with reference to Fig. 1. A portion of the (here) white light beam L incident from the lighting device 3 onto the front surface 7 of the layer stack 1 is immediately reflected upon striking the front surface 7 and detected as a reflective light beam Lr1 by a sensor 5 of the optical camera 4.

[0117] Another part of the light beam L is refracted upon striking the front surface 7, passes through the thin optically transparent layer S as refracted light beam Lg, and is reflected at the rear surface 8 of layer S. It then exits at the front surface 7 of layer S and is also detected by the sensor 5 of the optical camera 4 as light beam Lr2.

[0118] As can be seen in Fig. 1, the light rays Lr1 and Lr2 detected in sensor 5 have different paths and therefore different transit times when measured in sensor 5. This leads to a phase shift in the light waves of the light rays Lr1 and Lr2. This phase shift is also referred to as path difference.

[0119] Because the thin layer S is a thin optically transparent layer, the superposition of the light rays Lr1 and Lr2 in the

[0120] February 9, 2026 I 48 P 146 WOSensor 5 Interference occurs, causing the light beams Lr1 and Lr2 to reinforce, weaken, or cancel each other out. This leads to different measured intensities in Sensor 5, which depend on the path difference between the light waves of the light beams Lr1 and Lr2, which is largely determined by the thickness and refractive index of the layer. Depending on the wavelength under consideration, the path difference leads to different phase differences and thus to the occurrence of wavelength-dependent interference.

[0121] Furthermore, in dispersive media, the refraction of the light beam L in the thin layer S and its propagation speed depend on the wavelength of the light. This leads, depending on the wavelength of the light, to different path lengths or transit times of the refracted light beam Lg in layer S, and thus to wavelength-dependent path differences, which in turn lead to further wavelength-dependent phase shifts. Due to the interference of the light at different path differences, fluctuations in the light intensity occur, depending on the layer thickness d, when the reflected light beams Lr1 and Lr2 are superimposed. These fluctuations are measured in the sensor 5 of the optical camera 4 and vary depending on the layer thickness and the material of the layer for different wavelengths.

[0122] Besides the wavelength of the light, the path and thus the travel time of the refracted light ray Lg also depends on the thickness d of the layer S, which is to be determined. This dependence of the intensity on the thickness d of the thin layer d and the wavelength X of the light is called the reflection spectrum, which, as already explained, can be calculated by a person skilled in the art within the scope of their expertise.

[0123] February 9, 2026 I 48 P 146 WO The dependencies of such a reflection spectrum are illustrated in Fig. 2 using a specific example. The reflection R, i.e., the intensity of the light reflected by the thin layer S, is shown in arbitrary units as a family of curves as a function of wavelength in the unit "nm". Each of the curves shows the reflection of white light in a spectral range between 350 nm (UV light) and 750 nm (red light) for different layer thicknesses. Discrete spectral value ranges (colors), for which, for example, a multispectral measurement can be carried out, are shown as vertical lines in the reflection spectrum, whereby, according to the invention, a multispectral measurement can also be carried out for only a part of the specifically shown colors and / or for other colors.The example illustration shows the colors UV (ultraviolet light at about 370 nm), IG (indigo light at about 420 nm), B (blue light at about 470 nm), G (green light at about 520 nm) and R (red light at about 640 nm).

[0124] The individual curves show the reflection at different layer thicknesses d1 (40nm), d2 (50nm), d3 (60nm), d4 (70nm), d5 (80nm), d6 (90nm), d7 (100nm), d8 (110nm), d9 (120nm).

[0125] When using a camera 4 with sensors 5, which has sensor elements 6 for specific colors (discrete spectral ranges), only reflection curves for these colors are acquired from the reflection spectrum shown in Fig. 2. According to the invention, reflection curves for the layer stack 1, whose material properties are known as described, are calculated selectively for these colors. This calculation is performed as a function of the layer thickness d of the thin optically transparent layer S from the layer stack 1, whose layer thickness d is to be determined. For each of the colors R, B, G, the calculated reflection curve R(d), G(d), B(d) can be represented accordingly as a function of the layer thickness d of layer S.

[0126] February 9, 2026 I 48 P 146 WOThis is shown in Fig. 3 for the example of the family of curves from Fig. 2 for the colors R, G, B, which are detected by the sensor elements 6 of the camera 4, for a predetermined layer thickness range of 35 nm to 115 nm, which is defined beforehand and in which the layer thickness d lies, which is to be determined by the reflection measurement.

[0127] In the example shown in Fig. 3, the sensor elements 6 of the camera 4 detect the colors R, G, B. For a certain layer thickness d, the reflection values ​​that should be detected in an optimal measurement of the reflection by the individual sensor elements 6 of the camera 4, i.e. in the associated color channels, can therefore be read from the reflection curves.

[0128] In the example shown, with a layer thickness d of 45 nm (arbitrarily chosen here), as indicated by the arrow, this would result in a reflection measurement of MR=0.033 for color R, MG=0.017 for color G, and MB=0.09 for color B.

[0129] As explained, reflection measurements are technically very complex and cannot be achieved under production conditions in a production line, for example, for solar cells. This is primarily because the surfaces of the layer stack must have optimal reflectivity (and therefore, in particular, must not exhibit any pronounced texture), because no light scattered at the surface (e.g., ambient light) may be captured by the optical camera, and because the reflected light must be captured with angle integration by the camera, which in practice prevents a uniform thickness measurement at different points on the surface in a single image. This was already explained at the beginning.

[0130] In order to ensure a corresponding assignment of measured values ​​even in the case of reflection measurements that are not optimal in the sense described above,

[0131] February 9, 2026 I 48 P 146 WO To enable the determination of the layer thickness d of the thin optically transparent layer using reflection curves, a comparison of reflection measurement values ​​MR, MG, MB and reflection curves of the colors R, G, B is proposed according to a particularly preferred embodiment, which is explained below. The reflection curves for the individual colors are referred to below as functions of the layer thickness d (in the sense of reflection curve functions) and are abbreviated as R(d), G(d), B(d) for the different colors R, G, B.

[0132] When comparing the reflectance measurements MR, MG, MB, the difference between two reflectance values ​​MR - MG, MG - MB of two different colors R and G or G and B is calculated. According to the invention, the differences between the different colors R and G, G and B can share a common color, in the example shown here, color G. By using the differences between two different colors, angle-independent light components, such as those caused by scattered (and not reflected) light and / or ambient light, are eliminated from the measured values. This ensures that the measured values ​​MR, MG, MB and the reflectance curves R(d), G(d), B(d) can be directly compared because interfering light influences are eliminated. For the reflectance curves R(d), G(d), B(d), the corresponding differences R(d) - G(d), G(d) - B(d) of the same different colors R and G, G and B are calculated according to the invention for comparison.

[0133] According to a particularly preferred embodiment of the invention, it is proposed that, for the reflectance measurements MR, MG, MB, the quotients f1, f2 are formed from the different differences MR - MG, MG - MB of the reflectance measurements of different colors. These quotients f1, f2 are also referred to as difference quotients f1, f2.

[0134] February 9, 2026 I 48 P 146 WOThe difference quotient f1 in an embodiment considered here can be calculated as

[0135] _ MG -MB

[0136] 1 - MR - MG

[0137] At the point where the measured value MR equals the measured value MG, a pole occurs in the difference quotient f1, where the difference quotient f1 is undefined and therefore an evaluation is not possible.

[0138] To circumvent the difficulty in treating this pole for MR = MG, it can further be provided to also calculate the reciprocal of the difference quotient f1 as the difference quotient f2.

[0139]

[0140] Because there is no spectral angular dependence of the reflection, at least within the scope of the desired or required accuracy, it is possible according to the invention to perform an angle-selective measurement of the reflection instead of an angle-integrated measurement of the reflection by forming the quotient.

[0141] This quotient can then be directly compared with corresponding quotients of the different differences R(d) - G(d), G(d) - B(d) of the reflection curves R(d), G(d), B(d) of the same different colors R and G, G and B.

[0142] The difference quotients f1(d) and f2(d) of the reflection curves R(d), G(d), B(d) are formed accordingly as

[0143] February 9, 2026 I 48 P 146 WO

[0144]

[0145] The curves resulting from the difference quotients f1(d) and f2(d) as a function of the layer thickness d are shown in Fig. 4.

[0146] For comparing the difference quotients f1, f2 of the measured values ​​MR, MG, MB with the difference quotients f1(d) and f2(d) of the reflection curves R(d), G(d), B(d) to determine the layer thickness d, according to a preferred embodiment of the invention, the difference quotients f1, f1(d) can be used when the reflection measurement MR is greater than MB, and the difference quotients f2, f2(d) can be used when the reflection measurement MB is greater than MR. This avoids the regions of the difference quotients with poles P1, P2. If the reflection measurement MR and MB are equal, the difference quotients have the same value (f1=f2=f1(d)=f2(d)).

[0147] For example, for a measured value MR greater than MB from a reflectance measurement, the difference quotient f1(mess) = 0.48 is calculated using the difference quotient f1. This value is then fitted to the function f1(d) in Fig. 4, or by a computational fit. This results in a layer thickness d = 45 nm, as indicated by the arrow.

[0148] More generally, according to a preferred embodiment of the invention, the ranges of measured values ​​from the reflection measurement and the quotients used in the respective range (of the reflection measurement and the reflection curves) can be selected by the person skilled in the art in such a way that the quotients have no poles in the ranges of the measured values.

[0149] February 9, 2026 I 48 P 146 WO Instead of the difference quotients previously described using specific colors as examples, differently defined quotients can also be formed, for example by swapping the colors and / or using other colors. Additional colors, in particular four colors, can also be used.

[0150] For example, the color ultraviolet (UV) can be used as an additional color alongside the colors R, G, and B. In this case, the MUV measurement is additionally detected in camera 4, and the UV(d) reflection curves are calculated.

[0151] The difference quotient f1 of the reflection measurements MUV, MG, MB, MR can then be defined, for example, as

[0152] _ MUV - MG

[0153] 1 MB - MR

[0154] The calculation of its reciprocal as the difference quotient f2 and the difference quotients f1(d) and f2(d) of the reflection curves UV(d), G(d), B(g), and R(d) is then carried out accordingly. The use of the additional color UV is advantageous for layer stack 1, where determining the layer thickness using the colors R, G, and B alone is not possible.

[0155] A curve representing the difference quotients of the reflection curves for the colors UV, R, G, and B, calculated using the predefined difference quotients (including a reflection measurement MUV), is shown in Fig. 5. The adjustment for determining the layer thickness d is performed accordingly.

[0156] In principle, it is within the expertise of a specialist to select and adjust the colors and / or difference quotients for a specific layer stack to be measured, for a specific layer thickness range of interest, so that the determination of the layer thickness leads to the desired results.

[0157] February 9, 2026 I 48 P 146 Such optimization is within the reach of the person skilled in the art and can be carried out by them, if necessary, also by means of experiments, without having to be inventive and without leaving the subject matter of the invention.

[0158] A suitable criterion proposed according to the invention is that the difference quotients f1(d) and f2(d) are strictly monotonic in the ranges of measured values ​​from the reflectance measurement for which they are used. This allows the layer thickness d to be uniquely determined in the respective measurement ranges. According to the invention, this can also be achieved by defining other and / or additional conditions for the range of measured values, e.g., MR / MB greater than 1 or MR / MB less than 1.

[0159] Figure 6a shows a top view of an optical camera 4 according to an embodiment with a planar sensor arrangement of several sensors 5. The several sensors 5 are arranged side by side in a row. With this row arrangement of sensors 5, the optical camera 4 can capture and store an image of the surface of the layer stack 1 in order to determine the spatially resolved layer thickness d of a layer S in the layer stack 1 according to the method described above. Each sensor 5 of the optical camera 4 is configured to selectively measure light of three different colors as reflected values. In the example shown here, the colors are red (R), green (G), and blue (B). For this purpose, each sensor 5 comprises three sensor elements 6, one of which is configured to measure each of the three colors, in this example, one of the different colors R, G, and B.

[0160] Fig. 6b shows another embodiment of an optical camera 4, which is fundamentally constructed in the same way as the optical camera 4 according to Fig. 6a. Therefore, the same reference numerals are used. The only difference is that each sensor 5 of the optical camera 4 has four

[0161] February 9, 2026 I 48 P 146 WOSensor elements 6, each of which is configured to measure one of four different colors. In the example shown here, this is the color ultraviolet (UV) in addition to the colors R, G, and B.

[0162] Because in the optical cameras 4 according to the embodiments shown in Fig. 6a or Fig. 6b a separate sensor element 6 is provided for each of three or four different colors, a multispectral measurement of the reflection of light can be carried out with one image, in which the four colors are present as (preferably equally intense) components (white light).

[0163] Fig. 6c shows another embodiment of an optical camera 4 in which only one sensor element 6 is arranged in each sensor 5. The camera 4 can be configured as a black-and-white camera in which the sensor element 6 measures only the light intensity (of all colors or color channels). The sensor element 6 alone is therefore not designed for color-selective measurement of individual spectral value ranges (colors). To carry out the method according to the invention, in this embodiment the illumination device 3 is configured to generate the light of several colors R, G, B, UV, IR sequentially one after the other, preferably as individual color flashes. The camera 4 is configured to capture and store each of these color flashes as a separate image. The entirety of these images then constitutes the multispectral measurement according to the invention, with which the method described according to the invention can be carried out.The camera 4 and the lighting device 3 can be controlled accordingly by the control unit 23.

[0164] Fig. 7 schematically shows a measuring arrangement 40 suitable and set up for carrying out the described method for determining the layer thickness d of a thin optically transparent layer S in a layer stack 1 with

[0165] February 9, 2026 I 48 P 146 Known material properties of the layer stack 1, which have already been described. The layer stack 1 can comprise layer S and, if applicable, further thin optically transparent layers and can be applied to a substrate 2.

[0166] The measuring arrangement 20 has a carrier 21 on which the layer stack 1 is arranged or can be arranged, wherein the front surface 7 of the layer stack 1 faces a lighting device 3 and an optical camera 4, as already described. The lighting device 3 and an optical camera 4 are arranged opposite, preferably above, the carrier 21. The lighting device 3 is configured to illuminate the surface 7 of the layer stack 1 and the thin optically transparent layer S with (preferably white) light, which comprises at least three discrete spectral ranges designated as colors R, G, B, UV, IR. The optical camera 4 is configured, as already described with reference to Figures 6a, 6c, or 6c, to measure the reflection of the at least three colors R, G, B, UV, IR from the lighting device 3.For this purpose, the lighting device 3 and the optical camera 4 are arranged in a reflection arrangement with respect to the recorded surface 7 of the layer stack 1.

[0167] A control unit 23 of the measuring arrangement 20 is designed to control the optical camera 4 for capturing images of the surface 7 of the layer stack 1 and, if necessary, to control the lighting device 3 for switching the light of the lighting device 3 on and off (possibly also as flashes of specific spectral value ranges as a temporally sequential flash sequence of different colors) and / or setting discrete spectral value ranges of the lighting device 3. This enables a spatially resolved reflectance measurement of the surface 7 of the layer stack 1 to be carried out according to the previously described method.

[0168] February 9, 2026 I 48 P 146 WOThe measuring arrangement 20 further comprises a computing unit 24 for evaluating the images of the surface 7 of the layer stack 1 recorded by the optical camera 4 and for performing a comparison of reflection measurement values ​​MR, MG, MB, MUV of the at least three colors R, G, B, UV extracted from the images with calculated (theoretical) reflection curves R(d), B(d), G(d), UV(d) for the layer stack 1 with the known material properties, wherein in a previously defined layer thickness range each of the reflection curves R(d), B(d), G(d), UV(d) is recorded as a function of the layer thickness (d) of the one thin optically transparent layer (S), whose layer thickness (d) is to be determined.

[0169] The processing unit 24 is designed to compare, when comparing the reflection measurements MR, MG, MB, MUV and the reflection curves R(d), B(d), G(d), UV(d), the difference (for example, MR - MG, MG - MB, MUV - MG and / or MB - MR) of two reflection measurements MR, MG, MB, MUV of two different colors R, G, B, UV and the difference (for example, R(d) - G(d), G(d) - B(d), UV(d) - G(d) and / or B(d) - R(d)) of two reflection curves R(d), B(d), G(d), UV(d) of the same different colors R, G, B, UV.

[0170] Preferably, the comparison process also includes the computation unit 24 being configured to compare quotients f1, f2 of the different differences (for example, MR - MG, MG - MB, MUV - MG and / or MB - MR) of the reflection measurements MR, MG, MB, MUV of different colors R, G, B, UV, IR (or other colors) and quotients f1(d), f2 of the different differences (for example, R(d) - G(d), G(d) - B(d), UV(d) - G(d) and / or B(d) - R(d)) of the reflection curves R(d), B(d), G(d), UV(d) of the same different colors R, G, B, UV, IR. The details of this have already been described. The colors and reflection curves are referred to here as concrete examples, without implying that...

[0171] February 9, 2026 I 48 P 146 WOInvention (as already explained) is limited to precisely these colors and reflection curves.

[0172] In order to perform spatially resolved reflection measurements for the entire surface 7 of the layer stack 1, the carrier 21 is part of a transport device 22, with which the layer stack 1 is moved perpendicular to its width relative to the optical camera 4 and the illumination device 3. During the movement, images of the surface 7 of the layer stack 1 can be acquired by the control unit 23, thus covering the entire surface 7.

[0173] According to a scanner, the optical camera 4 and the lighting device 3 cover the width of the layer stack 7 perpendicular to the transport direction of the transport device 22.

[0174] The computing unit 24 can be configured to compile the spatially resolved layer thickness d of layer S in the layer stack 1 obtained as a result of carrying out the method according to the invention into a spatially resolved layer thickness image 25 and to output it on a display of the measuring arrangement.

[0175] The control unit 23 and the computing unit 24 can be separate units or combined into a common unit, as shown in Fig. 7.

[0176] With a fundamentally identical structure to that in Fig. 7, the camera 4, unlike in Figs. 6a, 6b or 6c, in which the sensors 5 extend in only one line across the width of the camera 4 (line camera), can also be constructed as an arrangement of several sensor lines adjacent in the transport direction and form a large-area camera 4 that captures a large area of ​​the surface of the layer stack in one image.

[0177] February 9, 2026 I 48 P 146 WO In cameras with one or more adjacent camera arrays, the illumination device 3 can simultaneously generate light of several (or all) spectral value ranges (color) for which the reflection measurement is to be performed. In another embodiment, the illumination device 3 can sequentially generate the light of several or all spectral value ranges for one spectral value range at a time, or simultaneously for several (preferably but not all) spectral value ranges (by generating correspondingly colored flashes). The basic principle of these variants has already been explained, as has the possibility of hybrid forms.

[0178] Specific examples of implementation could be:

[0179] In a specific embodiment, a color digital camera can be used with sensors comprising three sensor elements for the colors R, G, and B, where the sensor element for color B can also be sensitive to the color UV. The illumination device can be configured to alternately generate flashes of white light and UV light, or to flash the other channel additionally with every second flash of one channel. In this way, lines are alternately captured with white light and UV light, or with white light and UV light. Thus, one obtains alternating reflection measurements with color values ​​MR, MG, MB (white light) and MR=0, MG=0 and MB=UV, or MR, MG, and MB=B+UV. If the changeover occurs faster than the advancement by one line, two images are obtained (offset by, for example, one camera pixel) containing the color information from R, G, B, and UV.These can be evaluated as if all color measurements had been taken at the same location; alternatively, the missing color information for a pixel can be interpolated from the neighboring pixels.

[0180] February 9, 2026 I 48 P 146 WO In another specific embodiment, a black-and-white digital camera (mono camera) can be used. The illumination device can sequentially generate color flashes of the colors R, G, B, and UV in a continuous sequence. The sequence of color flashes can be so rapid that flashes of all colors are generated and the corresponding images of all colors are captured before the layer stack has been advanced by one camera pixel by the transport device. Such illumination is referred to as strobe illumination. This yields four images for the different color channels, each shifted by a maximum of one camera pixel. The evaluation can be performed as if all color measurements had been taken at the same location, or the missing color information for a pixel can be interpolated from the adjacent (before / after) recorded values.

[0181] Reference symbol list

[0182] 1 layer stack

[0183] 2 Substrat

[0184] 3 Lighting equipment

[0185] 4 optical cameras

[0186] 5 Optical camera sensor (camera pixels)

[0187] 6 Sensor element

[0188] 7 front surface

[0189] 8 rear surface

[0190] 20 Measuring setup

[0191] 21 carriers

[0192] 22 Transport equipment

[0193] 23 Control unit

[0194] 24 computing units

[0195] 25 spatially resolved layer thickness image

[0196] February 9, 2026 I 48 P 146 WOS thin layer

[0197] d layer thickness

[0198] L incident light ray

[0199] Lr1 reflected light beam

[0200] Lr2 reflected light beam

[0201] Lg refracted light beam

[0202] X wavelength of light

[0203] R Red

[0204] Green

[0205] Blue

[0206] UV Ultraviolet

[0207] IG Indigo

[0208] f1 Difference quotient of reflection measurements

[0209] f2 Difference quotient of reflection measurements (reciprocal) f1 (d) Difference quotient of reflection curves

[0210] P1 Pole of the difference quotient f1 (d)

[0211] f2(d) Difference quotient of reflection curves (inverse) P2 Pole of the difference quotient f2(d)

[0212] MR, MG, MB, MUV reflectance measurements

[0213] R(d), G(d), B(d), UV(d) reflection curves

[0214] February 9, 2026 I 48 P 146 WO

Claims

Claims:

1. Method for determining the layer thickness (d) of a thin optically transparent layer (S) in a layer stack (1) by measuring the reflection of light reflected from the layer stack (S), wherein the layer stack (1) comprises the at least one thin optically transparent layer (S) and is applied to a substrate (2), and wherein the material properties of the layer stack (S) are known, wherein in the method A reflection measurement is performed for at least three different discrete spectral value ranges designated as colors (R, G, B, UV), and a reflection measurement value (MR, MG, MB, MUV) is recorded for each of the colors (R, G, B, UV); For each of the colors (R, G, B, UV) a reflection curve (R(d), G(d), B(d), UV(d)) is calculated for the layer stack (S) with the known material properties, wherein in a previously defined layer thickness range each of the reflection curves (R(d), G(d), B(d), UV(d)) is recorded as a function of the layer thickness (d) of the one thin optically transparent layer (S), whose layer thickness (d) is to be determined; the layer thickness (d) of the thin optically transparent layer (S) is determined from a comparison of the reflection measurements (MR, MG, MB, MUV) and the reflection curves (R(d), G(d), B(d), UV(d)); characterized by the fact that The reflection measurement is carried out with an optical camera (4) with a planar sensor arrangement of several sensors (5), wherein the February 9, 2026 I 48 P 146 WO several sensors (5) detect a planar area of ​​the layer stack (1) and the optical camera (4) stores the detected area of ​​the layer stack (1) as a spatially resolved image; Each of the multiple sensors (5) selectively measures light of different colors (R, G, B, UV) as reflection measurements (MR, MG, MB, MUV); When comparing the reflection measurements (MR, MG, MB, MUV) and the reflection curves (R(d), G(d), B(d), UV(d)), the difference between two reflection measurements (MR, MG, MB, MUV) of two different colors (R, G, B, UV) and the difference between two reflection curves (R(d), G(d), B(d), UV(d)) of the same different colors (R, G, B, UV) are compared.

2. Method according to claim 1, characterized in that when comparing the reflection measurement values ​​(MR, MG, MB, MUV) and the reflection curves (R(d), G(d), B(d), UV(d)), at least one quotient (f 1 ) of the different differences of the reflection measurement values ​​(MR, MG, MB, MUV) of different colors (R, G, B, UV) and at least one quotient (f1(d)) of the different differences of the reflection curves (R(d), G(d), B(d), UV(d)) of the same different colors (R, G, B, UV) are compared with each other.

3. Method according to claim 2, characterized in that, in addition to the quotient (f 1 , f1(d)) from the different differences of different colors (R, G, B, UV), the reciprocal (f2, f2(d)) of the quotient is also formed and used for comparison.

4. Method according to one of the preceding claims, characterized in that a white balance of the optical camera (4) is performed before the reflection measurement is carried out. February 9, 2026 I 48 P 146 WO5. Method according to one of the preceding claims, characterized in that the colors red, green, blue are used as the at least three different colors (R, G, B) for the reflection measurement, which are recorded by the optical camera (4) in different color channels.

6. Method according to one of the preceding claims, characterized in that an ultraviolet spectral range or an infrared spectral range is used as one of the different colors (R, G, B, UV) of the optical camera (4).

7. Method according to one of the preceding claims, characterized in that a reflection measurement is carried out for at least four colors (R, G, B, UV).

8. Method according to one of claims 3 to 6 and according to claim 7, characterized in that in the differentiation of two reflectance measurement values ​​(MR, MG, MB, MUV) a first and second of the at least four colors (R, G, B, UV) are used in a first difference and a third and fourth of the at least four colors (R, G, B, UV) are used in a second difference, wherein each of the first, second, third and fourth color (R, G, B, UV) is different from the other colors.

9. Method according to one of the preceding claims, characterized in that a line scan camera with one or more line sensors is used as the planar sensor arrangement of the optical camera (4).

10. Method according to one of the preceding claims, characterized in that the layer stack (S) further comprises thin optically transparent February 9, 2026 I 48 P 146 WO layers, preferably the layer thickness of the further thin optically transparent layers is known.

11. Measuring arrangement for determining the layer thickness (d) of a thin optically transparent layer (S) in a layer stack (1) consisting of the at least one thin optically transparent layer (S) and a substrate (2), wherein the layer stack (1) comprises the at least one thin layer (S) and is applied to the substrate (2) and wherein the material properties of the layer stack (S) are known, wherein the measuring arrangement (20) comprises: a support (21) for holding the layer stack (1) with the at least one thin optically transparent layer (S); a lighting device (3) for illuminating the surface (7) of the layer stack (1) with light which comprises at least three discrete spectral ranges designated as colors (R, G, B, UV); an optical camera (4) for measuring the reflection of the at least three colors (R, G, B, UV) of the lighting device (3), wherein the lighting device (3) and the optical camera (4) are arranged in a reflection arrangement with respect to the recorded surface (7) of the layer stack (1); a control unit (23) for controlling the optical camera (4) for taking an image of the surface (7) of the layer stack (1); a computing unit (24) for evaluating the images of the surface (7) of the layer stack (1) taken by the optical camera (4) and for performing a comparison of reflection measurement values ​​(MR, MG, MB, MUV) extracted from the images of at least three colors (R, G, February 9, 2026 I 48 P 146 WOB, UV) with calculated reflection curves (R(d), G(d), B(d), UV(d)) for the layer stack (1) with the known material properties, wherein in a previously defined layer thickness range each of the reflection curves (R(d), G(d), B(d), UV(d)) is recorded as a function of the layer thickness (d) of the thin optically transparent layer (1), whose layer thickness (d) is to be determined; characterized by the fact that the optical camera (4) is a planar sensor arrangement with several sensors (5), wherein the optical camera (4) is configured to store the reflection measurement values ​​(MR, MG, MB, MUV) as a spatially resolved image of the layer stack (1); the lighting device (4) and each of the sensors (5) are designed to selectively measure light of different colors (R, G, B, UV, IR) as the reflection measurement values ​​(MR, MG, MB, MUV, MIR) during the reflection measurement; the computing unit (24) is set up to compare the difference between two reflection measurements (MR, MG, MB, MUV) of two different colors (R, G, B, UV) and the difference between two reflection curves (R(d), G(d), B(d), UV(d)) of the same different colors when comparing the reflection measurements (MR, MG, MB, MUV) and the reflection curves (R(d), G(d), B(d), UV(d)).

12. Measuring arrangement according to claim 11, characterized in that the computing unit (24) is further configured to calculate, when comparing the reflection measurement values ​​(MR, MG, MB, MUV) and the reflection curves (R(d), G(d), B(d), UV(d)), at least one quotient (f1, f2) from the different differences of the reflection measurement values ​​(MR, MG, MB, MUV) of different February 9, 2026 I 48 P 146 WO to compare colors (R, G, B, UV) and at least one quotient of the different differences of the reflection curves (R(d), G(d), B(d), UV(d)) of the same different colors (R, G, B, UV).

13. Measuring arrangement according to claim 11 or 12, characterized in that sensors (5) of the camera (4) have at least one sensor element (6) which is configured to selectively measure light of one color (R, G, B, UV) and / or that the sensors (5) of the camera (4) have at least one sensor element (6) which is configured to measure light of several colors (R, G, B, UV) and the lighting device (3) is configured to generate the light of these several colors (R, G, B, UV) sequentially one after the other.

14. Measuring arrangement according to one of claims 11 to 13, characterized in that the measuring arrangement (20) has a transport device (22) with which the layer stack (1) with the at least one thin optically transparent layer (S) can be moved along a transport direction.

15. Measuring arrangement according to one of claims 11 to 14, characterized in that the computing unit (24) is further configured to generate a spatially resolved layer thickness image (25) from the determined layer thicknesses (d) of the thin optically transparent layer (S). February 9, 2026 I 48 P 146 WO

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