Method and device for detecting interference pigments in coatings

Abstract

The invention relates to a method for identifying interference pigments (103) in a coating (101), comprising at least the following steps: inc1 ) irradiates the coating (101) applied to the surface with light at a relative angle to the first irradiation angle (θ inc1 ) of the Bragg angle (θ spec1 ) measured by the first non-specular measurement angle (θ as1 ) recording a first image of the coating (101) applied to the surface at at least one second illumination angle (θ) different from the first illumination angle relative to the surface normal inc2 ) irradiates the coating (101) applied to the surface with respect to the at least one second irradiation angle (θ inc2 ) of the Bragg angle (θ spec2 ) measured at the first non-specular measurement angle; - spatially and spectrally analyzing and evaluating the first image and the at least one second image, - based on the first image and the at least one second image, the first and the at least one second illumination angle (θ inc1 ,θ inc2 ) and the first non-specular measurement angle (θ as1 ), identifying the interference pigment (103). A corresponding device and a computer-readable medium are also disclosed.

Description

Technical Field

[0001] The present invention relates to a method and a device for identifying interference pigments in a coating applied to a surface. Background Art

[0002] Coatings, especially color coatings, are found everywhere: in automobile construction, in architecture, in entertainment electronics and in everyday objects. Such coatings can protect the respective surface to which they are applied and enhance the design through various color, gloss and glitter effects and thus can increase the attractiveness of the product. Using effect pigments, unique, unmistakable color effects can be produced. In addition, resistance to weathering effects and UV rays is often achieved thereby.

[0003] The precise color coordination of paints for such coatings requires the precise identification of the individual pigment types to be used in each case and their amounts. The white, black or color pigments present in finely distributed manner in paints and coatings produced based on paints interact almost isotropically with the incident light. This means that their respective reflection spectra have only a low angular dependence and the color impression produced by the color pigments is almost independent of the illumination angle at which the surface with the coating is illuminated or the viewing angle at which the surface with the coating is viewed or measured.

[0004] In contrast, the reflection spectra of interference pigments present in a finely distributed manner in paints or paint-based coatings generally have a dependence on the illumination angle at which light is incident on the surface of the coating to which the interference pigments have been applied. In this case, the paints that usually contain black, white or colored pigments influence the reflection spectrum of the interference pigments. In addition, such interference pigments can only be detected or observed within a small range of angular divergence around the specular angle. These characteristics inherent in such interference pigments prevent the interference pigments from being clearly identified by individual, spectrally resolved pictures or images of the corresponding coatings containing such interference pigments.

[0005] US2017 / 0200288A describes a method in which an image of the target coating is generated by means of a processor, the image generated thereby is subjected to image analysis, the sparkle points are identified and their color is determined by means of a hue analysis, and the sparkle color distribution is statistically calculated. However, it is not possible to locally identify individual interference pigments in this way. Summary of the invention

[0006] It was therefore an object of the present invention to provide a possible manner in which even individual interference pigments within a coating can be determined as precisely and easily as possible.

[0007] This object is achieved by a method and a device having the features of the independent claims. Developments of the invention can be gathered from the dependent claims and the description.

[0008] A method for identifying interference pigments in a coating is provided, comprising at least the following steps:

[0009] - irradiating the coating applied to the surface (of the sample platelet) with light at a first illumination angle relative to the surface normal,

[0010] - recording a first image of the coating applied to the surface at a first aspecular measurement angle or observation angle measured relative to the Bragg angle or specular angle assigned to a reference surface normal,

[0011] - irradiating the coating applied to the surface with light at at least one second illumination angle relative to the surface normal that is different from the first illumination angle,

[0012] - recording at least one second image of the coating applied to the surface at a first non-specular measurement angle or a corrected first non-specular measurement angle, wherein the first non-specular measurement angle is measured relative to a Bragg angle or a specular angle assigned to at least one second illumination angle with reference to the surface normal, and the corrected first non-specular measurement angle is measured relative to a Bragg angle assigned to at least one second illumination angle, wherein the corrected first non-specular measurement angle is obtained from the first non-specular measurement angle by correction according to an optical refraction of light that changes at at least one second illumination angle compared to the first illumination angle,

[0013] - spatially analyzing and evaluating the first image and the at least one second image,

[0014] - spectrally analyzing and evaluating the first image and the at least one second image,

[0015] - Based on the spatial and spectral analysis and evaluation of the first image and the at least one second image, the first and the at least one second illumination angle and the first non-specular measurement angle or the corrected first non-specular measurement angle, the interference pigment is identified.

[0016] The term "measurement arrangement" is understood in the context of the present disclosure to mean all of those components of a color measuring instrument that are used to illuminate a measurement point on the surface of an object to be measured with light and to capture the light radiated back from the measurement point or to capture the first and at least one second image. "Surface normal" is understood to mean a (imaginary) line fixed relative to the device, which (ideally) is perpendicular to the surface of the object to be measured and defines the center point of the measurement point during actual use of the color measuring instrument. Irradiation angle or illumination direction is understood to mean the angle or direction at which the measurement point is illuminated with light. By analogy, observation or measurement direction or observation or measurement angle is understood to mean the direction or angle at which the light radiated back from the measurement point is recorded and measured. "Mirror direction" or "Bragg angle" is understood to mean the nominal observation direction or observation angle or measurement angle reflected at the surface of the (plane) object to be measured. When there are multiple possible illumination directions of the measuring instrument, the corresponding mirror directions are also multiple. The non-specular observation or measurement angle at which the first image and the at least one second image, respectively, are recorded is measured in each case with respect to the corresponding specular direction obtained according to the corresponding illumination direction. The "measuring plane" is understood to mean a plane passing through the normal (normal to the instrument) and all illumination directions and observation directions as well as specular directions. All angular indications relate to directions lying in the measuring plane. The object to be measured is a sample platelet having a coating applied to the surface, wherein the coating comprises a pigment (finely) distributed in a carrier material.

[0017] According to the method of the present invention, the illumination angle at which a surface or a coating applied to the surface is illuminated, i.e. the angle of incidence (AOI), is varied accordingly, while the coating is spectrally and spatially evaluated based on the recorded images (i.e. the first image and at least one second image) at a non-specular observation angle that remains the same or is corrected for the varied light refraction.

[0018] When changing the illumination angle, the spectral variation of the reflection (spectral resolution analysis) of the independent interference pigment (spatial resolution analysis) is evaluated in each case with the non-specular observation angle that remains unchanged or is corrected according to the light refraction of the variation, but by association, is evaluated again with the observation or measurement angle that has changed relative to the surface normal. As a result, the corresponding interference pigment type can be clearly identified. Due to the light wavelength of the variation in the coating comprising the interference pigment, the change of the spectral reflection or color that follows the illumination angle variation represents a new measured variable for identifying the corresponding interference pigment, because the corresponding interference pigment type shows the characteristic change of the spectral reflection that depends on the illumination angle. The corresponding characteristic change of the spectral reflection that depends on the illumination angle assigned to the corresponding interference pigment type is stored in the database. In addition, the corresponding non-specular observation angle as the corresponding interference pigment type feature may be stored in the database retrievably together with the angular divergence range or tolerance of the corresponding non-specular observation angle that still shows reflection in each case, and is assigned to the corresponding interference pigment type.

[0019] As defined from US 2017 / 0200288 A1, in the case of the method according to the invention, the statistical distribution of the colors of the pigment flakes (pigment platelets) distributed in the paint is not determined in order to determine the recipe of the paint, but instead the spectral changes of the individual effect pigments are evaluated in order to identify the individual pigment types.

[0020] Interference pigments are effect pigments whose effect is mainly or completely based on the interference of light on a thin, highly refractive layer. In this case, the incident light is partially reflected at the interface between the interference pigment and the carrier material as another component of the coating. Another part is refracted into the interior of the coating. Finally, the remaining or additional light is reflected at the interface between the coating and the surface of the substrate or sample platelet or, in the case of a multilayer system, a further thin layer and then reflected again at its surface before leaving the coating. The light rays leaving the surface of the coating at the same point must cover different wavelengths, so that certain wavelengths are eliminated and other wavelengths are enhanced by superposition. As a result, colors are produced in the visible spectral range, precisely because the difference in wavelength depends on the angle of incidence of the light impinging in the angle-dependent color impression (angle-dependent color). In the invisible spectral range, the effect is usually manifested by the detectable angle-dependent wavelength of the reflected light.

[0021] Generally, in the context of the present disclosure, "light" is understood to mean electromagnetic radiation in the visible spectral range and in the invisible spectral range, for example in the infrared range or in the UV range. This means that the method according to the invention and the system according to the invention can be applied or used in the visible spectral range and in the invisible spectral range. Interference pigments that show their effect particularly or especially in the infrared range may be relevant to a variety of sensor systems, for example in the case of a lidar for measuring, for example, the range or speed of a vehicle coated with a coating.

[0022] Interference pigments usually take the form of platelets (flakes) which are oriented in the coating or paint in a respective direction which generally does not extend parallel to the coating surface. In order to reflect incident light in the intended viewing direction at a given illumination angle relative to the surface normal of the object to be measured, the platelets in the coating or paint must be oriented in a specific direction, precisely so that the incident light is reflected exactly specularly in the intended viewing direction when it strikes the respective platelet. There are some interference pigment types which align themselves very well, i.e. parallel or almost parallel to the surface of the object to be measured, and other types which align themselves relatively poorly, i.e. at a clearly measurable angle greater than 0° to the surface of the object to be measured.

[0023] According to the invention, it is envisaged that light (especially white light) is irradiated at a certain angle relative to the surface normal, or at an angle of 45° (θ inc , see Figure 1) illuminates the surface of a sample platelet, which is coated with a coating or paint of unknown formulation. A known conventional transparent coating can be applied over the coating of unknown formulation. Then, with the help of a sensor or camera, the non-specular measurement angle (θ as , see FIG. 1 ), such as, for example, as15°, i.e., relative to the Bragg angle (θ spec , see FIG. 1 ), captures the spectral radiation reflected from the surface of the object to be measured in a spatially resolved manner, in particular in the form of an image, where “as” stands for “non-specular”.

[0024] If the coating of the unknown formulation includes interference pigments, those interference pigments, i.e. their interference pigment platelets aligned with respect to the test piece surface in such a way that they reflect specularly in the direction of the non-specular measurement angle (see Figure 1), are visible in the captured image. surrounding = 1) and the coating around the interference pigment (usually, n coating=1.5, see FIG1 ), the paired combination of irradiation direction (irradiation angle) and observation direction (measurement angle) can be assigned a direction α (see FIG1 ) in which the interference pigment flakes (platelets) in the coating or paint must be oriented in order to reflect the incident light specularly in the observation direction. For the non-specular angle θ of specular reflection of the interference pigment oriented in α relative to the surface as , according to Figure 1, depends on the incident angle θ of the light on the surface of the test piece inc (measured relative to the normal), the following equation applies:

[0025] θ as = arcsin[n coating / n surrounding * sin(arcsin(n surroundings / n coating * sin(θ inc )) + α)] - θ inc (1)

[0026] For the changing incident angle θ of light inc2 , according to the above equation, for the specular reflection of an interference pigment oriented α relative to the surface, the viewing direction also changes (see Figure 1b ). However, in the first approximation (small angles θ and α), θ as1 ≈θ as2 , so the interference pigment continues to be specularly reflected at the same non-specular viewing angle (see Figure 1c ). In addition, the optics of the camera used have a certain acceptance angle (numerical aperture), so that θ as of slight deviations.

[0027] Therefore, individual interference pigments can be measured for various measurement geometries (e.g., 45as15 and 55as15) while maintaining the non-specular observation angle. If necessary, the above equation can be used to correct the non-specular angle to obtain the aforementioned corrected first non-specular angle. Since the non-specular observation angle is (almost) retained when recording the first and at least one second image, in at least two measurements, that is, when recording the first image and at least one second image, the same interference pigment shows a specular reflection in the viewing direction and is therefore visible in both the first image and the at least one second image. In this case, the specular reflection of the same interference pigment at different illumination angles or incident angles is evaluated. It is also possible to use two or more different incident angles in sequence and evaluate the spectral reflection individually and as a whole. As described above, the corresponding interference pigment type shows that its specular reflection depends on the changing characteristics of the illumination angle.

[0028] In a further step, an illumination angle is selected that is different from the first angle of incidence or illumination angle, but the recording of another image is performed at the (almost) same non-specular angle relative to the Bragg angle generated by the illumination angle. This means that the real observation angle relative to the surface normal of the object to be measured must be determined and changed accordingly.

[0029] In a further development of the method according to the invention, the step of spectral analysis and evaluation comprises deriving corresponding reflection spectra for the first and the at least one second image.

[0030] In a further development of the method according to the invention, the derivation of the respective reflection spectra for the first and the at least one second image is carried out with the aid of a simulation algorithm.

[0031] In a further development of the method according to the invention, at least one camera, in particular a multi-angle color camera or a hyperspectral camera, is used to record the first and the at least one second image.

[0032] As already explained above, it is conceivable to record a plurality of second images of the coating applied to the surface at a respective plurality of second illumination angles at a first non-specular measurement angle measured relative to the Bragg angle assigned to the respective second illumination angle, or at a first non-specular measurement angle corrected according to equation (1).

[0033] Furthermore, in order to identify a plurality of different interference pigments, the method can be carried out accordingly a plurality of times, while the non-specular measuring angle can likewise be varied a plurality of times.

[0034] The present disclosure also relates to a computer-implemented method for identifying interference pigments or interference pigment types in a coating, comprising at least the following steps:

[0035] - receiving a first image, wherein the first image of the coating applied to the surface (of the sample platelet) is recorded by an image capture unit under illumination of the coating applied to the surface with light at a first illumination angle relative to the surface normal at a first non-specular measurement angle measured relative to a Bragg angle assigned to the first illumination angle with reference to the surface normal,

[0036] - receiving at least one second image, wherein the at least one second image of the coating applied to the surface is recorded by an image capture unit under illumination of the coating applied to the surface with light at a second illumination angle different from the first illumination angle relative to the surface normal, at a first non-specular measurement angle measured relative to a Bragg angle assigned to at least one second illumination angle referenced to the surface normal, or at a corrected first non-specular measurement angle measured relative to a Bragg angle assigned to at least one second illumination angle, wherein the corrected first non-specular measurement angle is obtained from the first non-specular measurement angle by correction (according to equation (1)) according to an optical refraction of light that changes at at least one second illumination angle compared to the first illumination angle,

[0037] - performing an image analysis of the first image and the at least one second image by using at least one processor,

[0038] - detecting interference pigments in the first image and in the at least one second image by spatially resolved analysis of the images,

[0039] - evaluating, calculating and / or simulating corresponding reflection spectra for the first image and at least one second image,

[0040] - identifying the interference pigment or the interference pigment type based on the spatial and spectral analysis and evaluation of the first image and the at least one second image, the first and the at least one second illumination angle and the first non-specular measurement angle or the corrected first non-specular measurement angle.

[0041] Furthermore, the subject of the present invention is also a device, comprising at least:

[0042] -database,

[0043] - a processor communicatively connected to a database and configured to communicate or exchange data with the database, wherein the processor is programmed to perform the following steps:

[0044] - receiving a first image, wherein a first image of a coating applied to a surface (of a sample platelet) is recorded by an image capture unit under illumination of the coating applied to the surface with light at a first illumination angle relative to a surface normal at a first non-specular measurement angle, the first non-specular measurement angle being measured relative to a Bragg angle assigned to the first illumination angle with reference to a surface normal,

[0045] - receiving at least one second image, wherein the at least one second image of the coating applied to the surface is recorded by an image capture unit at a first non-specular measurement angle or a corrected (based on equation (1)) first non-specular measurement angle under illumination of the coating applied to the surface with light at a second illumination angle relative to the surface normal that is different from the first illumination angle, wherein the first non-specular measurement angle is measured relative to the Bragg angle assigned to the at least one second illumination angle with reference to the surface normal, and the corrected first non-specular measurement angle is measured relative to the Bragg angle assigned to the at least one second illumination angle, wherein the corrected first non-specular measurement angle is obtained from the first non-specular measurement angle by correction according to an optical refraction of light that changes at the at least one second illumination angle compared to the first illumination angle,

[0046] - performing an image analysis on the first image and at least one second image,

[0047] - detecting interference pigments in the first image and in the at least one second image by spatially resolved analysis of the images,

[0048] - evaluating, calculating and / or simulating corresponding reflection spectra for the first image and at least one second image,

[0049] - identifying the interference pigment or the interference pigment type based on the spatial and spectral analysis and evaluation of the first image and the at least one second image, the first and the at least one second illumination angle and the first non-specular measurement angle or the corrected first non-specular measurement angle, and by comparison with corresponding characteristic changes in the specular reflection at different illumination angles (i.e. depending on the illumination angle) stored in a database and assigned to the corresponding interference pigment (or interference pigment type).

[0050] The image capture unit can be a camera, in particular a color camera or a hyperspectral camera, preferably a multi-angle color camera. In the case of a hyperspectral camera, the corresponding reflection spectrum is directly output and therefore only needs to be evaluated. The image capture unit can also be a system that (also) operates in the non-visible spectral range, such as a multispectral camera (from UV to NIR).

[0051] In a development, the device according to the invention further comprises an image capture unit which is communicatively connected to the processor and is configured to record the first image and at least one second image.

[0052] The device according to the invention is particularly configured to carry out the method having the above-mentioned features.

[0053] The present disclosure also relates to a non-volatile computer-readable medium comprising a computer program having program code means designed to perform the following steps when the computer program is run on a computing unit, in particular on said processor of the apparatus as described above:

[0054] - receiving a first image, wherein the first image of a coating applied to a surface is recorded by an image capture unit under illumination of the coating applied to the surface with light at a first illumination angle relative to a surface normal at a first non-specular measurement angle measured relative to a Bragg angle assigned to the first illumination angle with reference to the surface normal,

[0055] - receiving at least one second image, wherein the at least one second image of the coating applied to the surface is recorded by an image capture unit at a first non-specular measurement angle or a corrected first non-specular measurement angle under illumination of the coating applied to the surface with light at a second illumination angle relative to the surface normal that is different from the first illumination angle, wherein the first non-specular measurement angle is measured relative to a Bragg angle assigned to at least one second illumination angle referenced to the surface normal, and the corrected first non-specular measurement angle is measured relative to a Bragg angle assigned to at least one second illumination angle, wherein the corrected first non-specular measurement angle is obtained from the first non-specular measurement angle by correcting (according to equation (1)) for an optical refraction of light that changes at at least one second illumination angle compared to the first illumination angle,

[0056] - performing an image analysis on the first image and at least one second image,

[0057] - detecting interference pigments in the first image and in the at least one second image by spatially resolved analysis of the images,

[0058] - evaluating, calculating and / or simulating corresponding reflection spectra for the first image and at least one second image,

[0059] - identifying the interference pigment or the interference pigment type based on the spatial and spectral analysis and evaluation of the first image and the at least one second image, the first and the at least one second illumination angle and the first non-specular measurement angle or the corrected first non-specular measurement angle, and by comparison with corresponding characteristic changes in the specular reflection stored in a database and assigned to the corresponding interference pigment (or interference pigment type).

[0060] The non-transitory computer-readable medium is designed to perform the method as described above, when the computer program runs on a computing unit, in particular on a processor of an apparatus as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The invention is schematically represented by exemplary embodiments in the drawings and is further described with reference to the drawings.

[0062] Figure 1 Figure 1a A possible measurement geometry is shown in the schematic diagram of for capturing an image of a coating applied to the surface of a sample platelet, wherein the sample platelet comprises a pigment platelet; Figure 1b A possible measurement geometry is shown in FIG. 1 for capturing a first and at least one second image of a coating applied to a surface of a sample platelet, wherein the sample platelet comprises a pigment platelet; and Figure 1c The dependence of the non-specular viewing angle on the alignment of the interference pigment relative to the coating surface is shown in the diagram of;

[0063] Figure 2 shows an image of the coating recorded by a camera, which is recorded at a non-specular measurement angle measured relative to the Bragg angle assigned to the illumination angle selected here;

[0064] Figure 3 Shown are simulated and normalized reflection spectra of a TiO2 coating for different illumination angles and a measurement geometry with a (nearly) fixed non-specular measurement angle relative to the corresponding illumination angle. DETAILED DESCRIPTION

[0065] The method according to the invention, the device according to the invention and the computer program product according to the invention can be used both for automotive refinishing paints of cars or car bodies or body molding parts and also for other types of coatings, including stains and industrial finishes. The embodiments of the invention described below are not limiting.

[0066] Embodiments of the method according to the invention, of the device according to the invention and of the computer-readable medium according to the invention can be used in many fields, such as for example for comparing and / or coordinating designs and / or fashion products.

[0067] Embodiments of the method according to the invention may be at least partially executed or implemented in a computer system, which may be a standalone unit or comprise one or more external terminals or equipment communicating with a central computer via a network, such as for example the Internet or an Intranet.

[0068] The computers or processors described in this disclosure and the components coupled thereto or integrated therein may thus be part of a local computer system or a remote computer or online system or a combination thereof.

[0069] The databases described in the context of the present disclosure and the computer programs described herein may be stored or retrievably stored in internal computer memory or in non-volatile computer readable media.

[0070] Embodiments of the method according to the invention and / or of the device according to the invention use an image capture unit, which may for example be a multi-angle color camera or a multi-angle multispectral camera, possibly in combination with a spectrophotometer, thereby making it possible to produce improved and simplified results of pigment characteristics and sample properties.

[0071] The following statements apply to the description of the following figures: If no individual reference numerals are entered in a figure, reference is made in this respect to the other figures and the relevant part of the description.

[0072] "Specular direction" is understood to mean the nominal viewing direction reflected at the surface of the planar object to be measured, taken from the illumination direction. Multi-angle color measuring instruments or generally multi-angle measuring instruments for measuring specular changes of coatings depending on the illumination and / or observation (measurement) direction have a plurality of illumination directions and a plurality of observation directions. The measuring plane is understood to mean the plane passing through the instrument normal and all illumination and observation directions as well as the specular direction. All angular indications relate to directions lying in the measuring plane.

[0073] Figure 1a The schematic diagram shows a beam path 102 of a light beam which is directed at an angle θ inc Interference pigments 103 that enter the coating 101 and are reflected by the coating 103 present therein and aligned non-parallel to the coating surface are reflected back in the direction of the surface of the object to be measured. In this case, the object to be measured corresponds to a sample platelet having a coating applied to the surface of the sample platelet.

[0074] According to the invention, it is envisaged that light (especially white light) is irradiated at a specific angle relative to the surface normal, for example at an angle θ of 45° relative to the surface normal. inc The surface of the sample platelet is illuminated, and the surface of the sample platelet is coated with a coating 101 or paint of unknown formulation. A known conventional transparent coating can be applied on the coating 101 of unknown formulation. Then, with the help of a sensor or camera, the angle θ is measured at a specific non-specular surface. as , such as for example as15°, i.e. relative to the Bragg angle θ spec The 15° sensor captures the spectral radiation reflected from the surface of the object to be measured in a spatially resolved manner, in particular in the form of an image, where "as" stands for "as-specular".

[0075] If the coating of unknown formulation includes interference pigments 103, then those interference pigments 103, i.e., the interference pigments 103, are measured at their non-specular angle θ as Their interference pigment platelets, aligned in a specular manner with respect to the surface of the object to be measured, are visible in the captured image. surroundings = 1) and the coating around the interference pigment (usually n coating =1.5), the irradiation direction θ can be single-shot. inc (illumination angle) and non-specular measurement angle θ as The pairwise combination of is assigned a direction α in which the interference pigment flakes (platelets) in the coating or in particular in the substrate must be oriented in order to be visible in the viewing direction θ. spec +θ asUpper mirror surface (θ pig ) reflects the irradiated or incident light 102. For the interference pigment 103 oriented α relative to the surface, it is mirrored (θ pig ) The non-specular angle of reflection θ as ,according to Figure 1a , depends on the incident angle θ of the light on the surface of the object to be measured inc (about the normal being measured), the following equation applies:

[0076] θ as = arcsin[n coating / n surroundings * sin(arcsin(n surroundings / n coating * sin(θ inc )) + α)] - θ inc (1)

[0077] Figure 1b Two possible measurement geometries are shown for capturing first and second images of a coating 101 applied to a surface of an object to be measured comprising a pigment platelet 103, of which only one is shown here for simplicity. The first measurement geometry comprises illumination at position 111 with a first incident angle θ and a camera at position 121. inc1 The coating 101 is illuminated, and the camera is directed relative to the first incident angle θ assigned to inc1 The Bragg angle θ spec1 The first non-specular angle θ is measured as1 Or in the first viewing direction θ as1 +θ spec1 The second measurement geometry includes illumination at position 112 and a camera at position 122, wherein the illumination uses light at a second incident angle θ. inc2 The coating is illuminated, and the camera is in a second viewing direction θ as2 +θ spec2 The second image is recorded on the inc2 The change of Bragg angle θ spec2 Also changes accordingly, but the non-mirror angle θ as2 ≈θ as1 Remains almost unchanged.

[0078] For the first incident angle θ inc1 Varying light incident angle θ inc2 , based on the above equation (1), which is used with respect to the first incident angle θ assigned to inc1 The first viewing direction θ as1 +θ spec1Specular reflection of the interference pigment 103 oriented in α relative to the surface, observation direction θ as2 +θ spec2 Therefore, it will also change. However, in the first approximation (small angles θ and α), θ as1 ≈θ as2 , and therefore the interference pigment 103 continues to be specularly reflected at the same non-specular measuring angle (see Figure 1c ). In addition, the optics of the camera used have a certain acceptance angle (numerical aperture), so that θ as of slight deviations.

[0079] Figure 1c shows the inc The non-specular observation angle θ of the specular reflection of the interference pigment located in the coating as Dependence on the alignment α of the interference pigment relative to the coating surface or the surface of the object to be measured. The alignment α of the interference pigment is expressed as an angle [°] on the x-axis. The Bragg angle of the interference pigment, i.e. the aspecular angle θ as , expressed as an angle [°] on the y-axis. Curve 151 shows the irradiation angle θ inc The curve 152 shows the correlation when the illumination angle θ is 5°. inc The curve 153 shows the correlation when the illumination angle θ is 15°. inc The curve 154 shows the correlation when the illumination angle θ is 25°. inc The curve 155 shows the correlation when the illumination angle θ is 35°. inc The curve 156 shows the correlation when the illumination angle θ is 45°. inc The curve 157 shows the correlation when the illumination angle θ is 55°. inc Correlation at 65°.

[0080] Figure 2An image recorded by a camera as part of an embodiment of an apparatus according to the invention is shown. In this case, a surface of an object to be measured, comprising a sample platelet, on the surface of which a coating comprising interference pigments has been applied, is illuminated with white collimated light at a predetermined angle of incidence. In this case, the camera recording the image is arranged at a specified non-specular angle relative to the Bragg angle of the reference surface normal given by the angle of incidence. The flashing points visible here represent interference pigments contained by the coating, which usually take the form of interference pigment flakes or platelets finely distributed in the coating. In a next step, another further image, i.e. a second image, is recorded, wherein, in this case, the surface of the sample platelet coated with the coating is illuminated at another second angle of incidence, however, the recording of the image is recorded at the same first non-specular angle or a first non-specular measurement angle corrected for optical refraction at the boundary surface relative to the Bragg angle given by the second angle of incidence. Since the non-specular angle is (almost) retained when recording the second image, the same interference pigment within the coating shows a specular reflection in the direction of the camera and is therefore visible in both images. Based on the (at least almost) preserved non-specular angle relative to the Bragg angle resulting from the respective angle of incidence, the orientation of the interference pigment flakes visible in the respective image, i.e. in the first and at least one second image, can be deduced. After the respective images have been analyzed, it is thus possible to clearly infer the interference pigment type based on the characteristic change in the spectral reflection that can be derived from the image and is dependent on the angle of incidence with the aid of a database.

[0081] Figure 3The simulated and normalized reflection spectra of a single TiO2-coated mica platelet embedded in a standard matrix (constant refractive index n=1.5) are shown for measuring geometric configurations with various angles of incidence. In this case, the wavelength of the reflected light is plotted in nanometers on the x-axis 201. The normalized reflection is plotted on the y-axis 202. The reflection spectrum 211 shows the light reflected from the coated mica platelet (= object to be measured) at an angle of incidence of 25° and a non-specular measurement angle of 15° relative to the Bragg angle generated by the angle of incidence. In this case, a strong reflection can be seen in the range of 460nm, and a smaller local maximum can be seen in the range of 575nm. The reflection spectrum 212 shows the light reflected from the coated mica platelet or its surface at an angle of incidence of 45° and a retained non-specular angle of 15° measured relative to the Bragg angle generated by the angle of incidence. Here, the reflection maximum is in the range of 450nm, while the smaller local maximum is in the range of 560nm. The reflection spectrum 213 was recorded for an angle of incidence of 65°, wherein the measuring instrument provided for recording the reflection spectrum was arranged at a non-specular angle of 15°. Here, the reflection maximum is obtained in the range of 440 nm, while a smaller local maximum is obtained in the range of 540 nm. When evaluating the resulting reflection spectrum, in particular when taking into account the angle of incidence-dependent changes / variations of the spectral reflection or spectral reflection profile that can be derived from the reflection spectrum, it is possible to clearly infer the interference pigment that exhibits precisely this reflection profile or precisely this change, which change / variation is in each case characteristic of the corresponding interference pigment type.

Claims

1. A method for identifying interference pigments (103) in a coating (101), comprising at least the following steps: - Use light at a first illumination angle (θ) relative to the surface normal inc1 ) irradiating said coating (101) applied to said surface, - using light at an angle relative to the first illumination angle (θ inc1 ) of the Bragg angle (θ spec1 ) measured by the first non-specular measurement angle (θ as1 ) recording a first image of said coating (101) applied to said surface, - at least one second illumination angle (θ) relative to the surface normal that is different from the first illumination angle inc2 ) irradiating said coating (101) applied to said surface, - using the first non-mirror measurement angle or the corrected first non-mirror measurement angle (θ as2 ) records at least one second image of the coating (101) applied to the surface, wherein the first non-specular measurement angle is relative to a wavelength assigned to the at least one second illumination angle (θ inc2 ) of the Bragg angle (θ spec2 ) is measured, the corrected first non-specular measurement angle (θ as2 ) relative to the at least one second illumination angle (θ inc2 ) of the Bragg angle (θ spec2 ) is measured, wherein the corrected first non-specular measurement angle (θ as2 ) by the first irradiation angle (θ inc1 ) compared to the at least one second illumination angle (θ inc2 ) is corrected by the optical refraction of the light that changes under the first non-mirror surface and the angle (θ as1 )get, - spatially analyzing and evaluating said first image and said at least one second image, - spectrally analyzing and evaluating said first image and said at least one second image, and - based on spatial and spectral analysis and evaluation of the first image and the at least one second image, the first illumination angle (θ inc1 ) and the at least one second illumination angle (θ inc2 ) and the first non-specular measurement angle (θ as1 ) or the corrected first non-specular measurement angle (θ as2 ), identifying the interference pigment (103); Wherein, the first non-mirror measurement angle (θ as1 ) is calculated according to the following formula: i as1 =arcsin[n coating / n surroundings *sin(arcsin(n surroundings / n coating *sin(θ inc1 ))+α)]-θ inc1 Among them, n coating represents the refractive index of the coating (101), n surroundings represents the refractive index of the surrounding environment of the coating (101), θ inc1 represents the first illumination angle, and α represents the orientation of the interference pigment (103) relative to the surface of the coating (101).

2. The method of claim 1, wherein: The step of spectral analysis and evaluation comprises deriving respective reflectance spectra for the first image and the at least one second image.

3. The method of claim 2, wherein: The derivation of the respective reflection spectra for the first image and the at least one second image is performed with the aid of a simulation algorithm.

4. The method of claim 1, wherein: At least one camera is used to record the first image and the at least one second image.

5. The method of claim 4, wherein: The at least one camera is a multi-angle color camera and / or a hyperspectral camera and / or a camera system equivalent to a multi-angle color camera and / or a hyperspectral camera.

6. The method of claim 4, wherein: The at least one camera is a system that also operates in the invisible spectral range.

7. The method of claim 1, wherein: A plurality of second images of the coating applied to the surface is recorded at a corresponding plurality of second illumination angles at the first non-specular measurement angle or the corrected first non-specular measurement angle, the first non-specular measurement angle or the corrected first non-specular measurement angle being measured in each case relative to a Bragg angle assigned to the second illumination angle.

8. The method as claimed in claim 1 , wherein the method is performed a plurality of times for the identification of a plurality of pigments, the non-specular measuring angle being likewise changed a plurality of times, and wherein the non-specular measuring angle is kept constant or corrected only as a function of the optical refraction of the light which changes at the at least one second illumination angle compared to the first illumination angle for the identification of an interference pigment in each case.

9. A computer-implemented method for identifying interference pigments (103) in a coating (101), comprising at least the following steps: - receiving a first image, wherein the first image of the coating (101) applied to the surface is obtained by irradiating light at a first illumination angle (θ) relative to the normal of the surface inc1 ) of the coating (101) applied to the surface, by an image capture unit at a Bragg angle (θ) relative to the first illumination angle assigned to the coating (101) spec1 ) measured by the first non-specular measurement angle (θ as1 )Record, - receiving at least one second image, wherein the at least one second image of the coating (101) applied to the surface is obtained by irradiating light at a second illumination angle (θ) relative to the surface normal that is different from the first illumination angle. inc2 ) is applied to the coating (101) on the surface, and the image capture unit is used to capture the first non-mirror measurement angle or the first corrected non-mirror measurement angle (θ as2 ) record, wherein the first non-specular measurement angle is relative to the at least one second illumination angle (θ inc2 ) of the Bragg angle (θ spec2 ) is measured, the corrected first non-specular measurement angle (θ as2 ) is measured relative to the Bragg angle assigned to the at least one second illumination angle, wherein the corrected first non-specular measurement angle (θ as2 ) by the first irradiation angle (θ as1 ) compared to the at least one second illumination angle (θ as2 ) is corrected by the optical refraction of the light that changes under the first non-mirror surface and the angle (θ as1 )get, - performing an image analysis of said first image and said at least one second image by using at least one processor, - detecting said interference pigment (103) in said first image and in said at least one second image by spatially resolved analysis of said images, - evaluating, calculating and / or simulating corresponding reflection spectra for said first image and said at least one second image, and - based on spatial and spectral analysis and evaluation of the first image and the at least one second image, the first illumination angle (θ inc1 ) and the at least one second illumination angle (θ inc2 ) and the first non-specular measurement angle (θ as1 ) or the corrected first non-specular measurement angle (θ as2 ), identifying the interference pigment (103); Wherein, the first non-mirror measurement angle (θ as1 ) is calculated according to the following formula: i as1 =arcsin[n coating / n surroundings *sin(arcsin(n surroundings / n coating *sin(θ inc1 ))+α)]-θ inc1 Among them, n coating represents the refractive index of the coating (101), n surroundings represents the refractive index of the surrounding environment of the coating (101), θ inc1 represents the first illumination angle, and α represents the orientation of the interference pigment (103) relative to the surface of the coating (101).

10. A device comprising at least: - databases, and - a processor, which is communicatively connected to the database and configured to exchange data with the database, wherein the processor is programmed to perform the following steps: - receiving a first image, wherein the first image of the coating (101) applied to the surface is obtained by irradiating light at a first illumination angle (θ) relative to the normal of the surface inc1 ) for the coating (101) applied to the surface, by an image capture unit with respect to the first illumination angle (θ inc1 ) of the Bragg angle (θ spec1 ) measured by the first non-specular measurement angle (θ as1 )Record, - receiving at least one second image, wherein the at least one second image of the coating (101) applied to the surface is obtained by irradiating light at a second illumination angle (θ) relative to the surface normal that is different from the first illumination angle. inc2 ) is applied to the coating (101) on the surface, and the image capture unit is used to capture the first non-mirror measurement angle or the first corrected non-mirror measurement angle (θ as2 ) record, wherein the first non-specular measurement angle is relative to the at least one second illumination angle (θ inc2 ) of the Bragg angle (θ spec2 ) is measured, the corrected first non-specular measurement angle (θ as2 ) relative to the at least one second illumination angle (θ inc2 ) of the Bragg angle (θ spec2 ) is measured, wherein the corrected first non-specular measurement angle (θ as2 ) by the first irradiation angle (θ inc1 ) compared to the at least one second illumination angle (θ inc2 ) is corrected by the optical refraction of the light that changes under the first non-mirror surface and the angle (θ as1 ) obtained, - performing an image analysis on said first image and said at least one second image, - detecting interference pigments (103) in said first image and in said at least one second image by spatially resolved analysis of said images, - evaluating, calculating and / or simulating corresponding reflection spectra for said first image and said at least one second image, and - based on spatial and spectral analysis and evaluation of the first image and the at least one second image, the first illumination angle (θ inc1 ) and the at least one second illumination angle (θ inc2 ) and the first non-specular measurement angle (θ as1 ) or the corrected first non-specular measurement angle (θ as2 ), identifying the interference pigment (103); Wherein, the first non-mirror measurement angle (θ as1 ) is calculated according to the following formula: i as1 =arcsin[n coating / n surroundings *sin(arcsin(n surroundings / n coating *sin(θ inc1 ))+α)]-θ inc1 Among them, n coating represents the refractive index of the coating (101), n surroundings represents the refractive index of the surrounding environment of the coating (101), θ inc1 represents the first illumination angle, and α represents the orientation of the interference pigment (103) relative to the surface of the coating (101). 11 . The apparatus of claim 10 , further comprising the image capture unit, the image capture unit being communicatively connected to the processor and configured to record the first image and the at least one second image.

12. The apparatus of claim 10, configured to perform the method as claimed in claim 1.

13. The apparatus of claim 10, configured to perform the method as claimed in claim 9.

14. A non-transitory computer-readable medium comprising a computer program having program code means, said program code means being designed to perform the following steps when said computer program is run on said processor of the apparatus as claimed in claim 10: - receiving a first image, wherein The first image of the coating (101) applied to the surface is illuminated by light at a first illumination angle (θ inc1 ) for the coating (101) applied to the surface, by an image capture unit with respect to the first illumination angle (θ inc1 ) of the Bragg angle (θ spec1 ) measured by the first non-specular measurement angle (θ as1 )Record, - receiving at least one second image, wherein the at least one second image of the coating (101) applied to the surface is obtained by irradiating light at a second illumination angle (θ) relative to the surface normal that is different from the first illumination angle. inc2 ) is applied to the coating (101) on the surface, and the image capture unit is used to capture the first non-mirror measurement angle or the first corrected non-mirror measurement angle (θ as2 ) is recorded, wherein the first non-specular measurement angle is relative to the at least one second illumination angle (θ inc2 ) of the Bragg angle (θ spec2 ) is measured, the corrected first non-specular measurement angle (θ as2 ) relative to the at least one second illumination angle (θ inc2 ) of the Bragg angle (θ spec2 ) is measured, wherein the corrected first non-specular measurement angle (θ as2 ) by the first irradiation angle (θ inc1 ) compared to the at least one second illumination angle (θ inc2 ) is corrected by the optical refraction of the light that changes under the first non-mirror surface and the angle (θ as1 )get, - performing an image analysis on said first image and said at least one second image, - detecting the interference pigment (103) in the first image and in the at least one second image by spatially resolved analysis of the images, - evaluating, calculating and / or simulating corresponding reflection spectra for said first image and said at least one second image, - based on spatial and spectral analysis and evaluation of the first image and the at least one second image, the first illumination angle (θ inc1 ) and the at least one second illumination angle (θ inc2 ) and the first non-specular measurement angle (θ as1 ) or the corrected first non-specular measurement angle (θ as2 ), identifying the interference pigment (103).

15. A non-transitory computer-readable medium comprising a computer program with program code means designed for performing the method as claimed in claim 1 when the computer program is run on a computing unit.

16. A non-transitory computer-readable medium comprising a computer program having program code means designed to perform the method as claimed in claim 9 when the computer program is run on a computing unit.

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