A method and system for detecting thickness of waterborne polyurethane film

Abstract

The present application relates to the technical field of film thickness measurement, and specifically to a method and system for detecting the thickness of a waterborne polyurethane film, the method comprising: collecting interference images of each film sample on a waterborne polyurethane film in each phase shift state; obtaining the interference fringe intensity of the target point by comparing the grayscale distribution difference between the target point on the film sample and the pixels in its neighborhood on all interference images; performing phase measurement on the interference images in all phase shift states to obtain the phase value of the target point; obtaining the interference phase change value according to the phase change degree and phase difference of the target point in the local area; comprehensively analyzing the clustering of the interference phase change values ​​and interference fringe intensity of all position points, extracting the zero-order fringe area of ​​the upper and lower surfaces, and calculating the thickness of the film sample; comprehensively analyzing the numerical distribution of the thickness of all film samples to obtain the thickness of the waterborne polyurethane film. The present application can improve the measurement accuracy of the thickness of waterborne polyurethane films.

Description

Technical Field

[0001] The present application relates to the technical field of film thickness measurement, and in particular to a method and system for detecting the thickness of a waterborne polyurethane film. Background Art

[0002] Waterborne polyurethane refers to a material prepared by a specific synthesis process using water as a dispersion medium. Compared with traditional solvent-based polyurethane, it has good flexibility, wear resistance and chemical corrosion resistance. Waterborne polyurethane films of different thicknesses have great differences in performance. Thicker films have better wear resistance and corrosion resistance, while thinner films have better flexibility and air permeability. Therefore, it is necessary to test the thickness of waterborne polyurethane films, which is conducive to improving the synthesis process and producing products with a higher pass rate.

[0003] At present, the interference method is usually used to optically measure the thickness of waterborne polyurethane film. The traditional white light phase shift interferometry method uses the principle of white light interference. When white light is irradiated on the waterborne polyurethane film, interference will occur due to the reflected light from the upper and lower surfaces of the film. The thickness of the film is calculated by measuring the zero-order fringe position of the interference fringe and combining the phase information. However, due to the large scattering of white light by waterborne polyurethane film, phase discontinuity may occur, causing deviation in the recognition of the zero-order fringe position, thereby causing errors in the calculation of film thickness. Summary of the invention

[0004] In order to solve the above technical problems, the purpose of this application is to provide a method and system for detecting the thickness of a waterborne polyurethane film. The technical solutions adopted are as follows:

[0005] In a first aspect, an embodiment of the present application provides a method for detecting the thickness of a waterborne polyurethane film, the method comprising the following steps:

[0006] S1, collecting interference images of each film sample on the waterborne polyurethane film under a preset light source in each phase shift state;

[0007] S2, record any position point on the film sample as the target point; obtain the interference fringe intensity of the target point based on the grayscale distribution difference between the target point and the pixels in its neighborhood on all interference images;

[0008] S3, performing phase measurement on the interference images in all phase shift states to obtain the phase value of the target point; obtaining the phase gradient change value of the target point according to the phase change degree of the target point in the local area; obtaining the phase continuity of the target point according to the phase difference between the target point and the position point in its local area; and obtaining the interference phase change value by combining the phase gradient change value and phase continuity of the target point;

[0009] S4, comprehensively analyzing the interference phase change values ​​and interference fringe intensity clustering of all positions on the film sample, extracting the zero-order fringe area on the upper surface and the zero-order fringe area on the lower surface of the film sample; obtaining the thickness of the film sample based on the distance between the zero-order fringe area on the upper surface and the zero-order fringe area on the lower surface; and obtaining the thickness of the waterborne polyurethane film based on the numerical distribution of the thickness of all film samples.

[0010] Furthermore, the calculation formula for the interference fringe intensity of the target point is: ; In the formula, represents the interference fringe intensity of the target point; M represents the number of phase shift states; P represents the number of pixels in the eight-neighborhood of the pixel at the same position of the target point on the interference image; Represents the gray value of the pixel at the same position on the interference image of the target point in the i-th phase shift state; Represents the gray value of the jth pixel in the eight-neighborhood of the pixel at the same position on the interference image of the target point in the i-th phase shift state; It represents the mean gray value of the pixels at the same position on the interference image of the target point in all phase shift states.

[0011] Further, the phase measurement is performed on the interference images in all phase shift states to obtain the phase value of the target point, including:

[0012] The four-step phase shift method is used to obtain the preliminary phase image for the interference images in all phase shift states; the preliminary phase image is phase unwrapped to obtain the phase distribution image; the phase of the target point at the pixel point at the same position in the phase distribution image is taken as the phase value of the target point.

[0013] Furthermore, the method for obtaining the phase gradient change value of the target point includes:

[0014] Perform gradient detection on the phase distribution image to obtain the gradient amplitude of each pixel point; the gradient amplitude of each pixel point is the phase gradient of the same position point on the thin film sample; obtain the phase standard deviation of the target point and all target points in its eight neighborhoods; and take the product of the standard deviation and the phase gradient of the target point as the phase gradient change value of the target point.

[0015] Furthermore, the method for calculating the phase continuity of the target point includes:

[0016] The mean phase difference between the target point and all the points in its eight neighborhoods is calculated as the phase continuity of the target point.

[0017] Further, the phase gradient change value and phase continuity of the integrated target point are used to obtain the interference phase change value, including:

[0018] The product of the normalized value of the phase continuity of the target point and the normalized value of the phase gradient change value is taken as the interference phase change value of the target point.

[0019] Further, the method for extracting the zero-order fringe area on the upper surface and the zero-order fringe area on the lower surface of the film sample comprises:

[0020] Calculate the product of the interference fringe intensity and the interference phase change value of each position point as the interference value of each position point; cluster all the position points with the minimum value, the second minimum value and the maximum value of the interference value of all the position points as cluster centers to obtain clusters; record the area composed of all the position points in the cluster with the minimum value as the cluster center as the zero-order fringe area of ​​the upper surface; record the area composed of all the position points in the cluster with the second minimum value as the cluster center as the zero-order fringe area of ​​the lower surface.

[0021] Furthermore, the method for obtaining the thickness of the film sample specifically includes:

[0022] The absolute values ​​of the differences in the Euclidean distances from the center of gravity of the zero-order fringe area on the upper surface and the center of gravity of the zero-order fringe area on the lower surface to the upper edge of the film sample are calculated respectively; the refractive index of the waterborne polyurethane film under a preset light source is obtained, and the ratio of the absolute value of the difference to the refractive index of the waterborne polyurethane film is used as the thickness of the film sample.

[0023] Further, the method for obtaining the thickness of the waterborne polyurethane film comprises:

[0024] Calculate the standard deviation and average value of the thickness of all film samples; calculate the absolute value of the difference between the thickness of each film sample and the average value, and record it as the thickness difference; record the film samples with a thickness difference greater than the standard deviation as error samples; calculate the average thickness of the other film samples except all error samples as the thickness of the waterborne polyurethane film.

[0025] In a second aspect, an embodiment of the present application further provides a waterborne polyurethane film thickness detection system, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, wherein when the processor executes the computer program, the steps of any one of the above-mentioned waterborne polyurethane film thickness detection methods are implemented.

[0026] This application has at least the following beneficial effects:

[0027] The present application analyzes the phase discontinuity problem in the interference image caused by the scattering of white light by the waterborne polyurethane film, first analyzes the phase change of the local area where the zero-order fringe appears, and obtains the interference fringe intensity according to the change of the pixel gray value in the interference image, and more accurately judges the interference fringe intensity in the local area; further, by analyzing the influence of the noise of the interference fringe and the defects on the surface of the film on the phase stability of the local area, the interference phase change value is obtained by calculating the degree and continuity of the phase change according to the phase gradient, and combined with the characteristic that the phase usually changes continuously near the zero-order fringe, the interference phase change value of all positions on the film sample and the clustering of the interference fringe intensity are comprehensively analyzed to extract the zero-order fringe area, thereby improving the recognition accuracy of the zero-order fringe. This embodiment obtains a more accurate zero-order fringe position, improves the calculation accuracy of the film sample thickness, solves the thickness measurement error caused by the film scattering white light, and improves the measurement accuracy of the waterborne polyurethane film thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions and advantages in the embodiments of the present application or the prior art, the drawings required for use in the embodiments or the prior art descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 A flow chart of the steps of a method for detecting the thickness of a waterborne polyurethane film provided in one embodiment of the present application;

[0030] Figure 2 A schematic diagram of the thickness distribution of a thin film sample provided for one embodiment of the present application. DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. In the absence of conflict, the embodiments of the present application and the technical features in the embodiments can be combined with each other. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0032] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0033] The specific scheme of a waterborne polyurethane film thickness detection method and system provided by the present application is described in detail below with reference to the accompanying drawings.

[0034] See also Figure 1 , which shows a flow chart of a method for detecting the thickness of a waterborne polyurethane film provided by an embodiment of the present application, the method comprising the following steps:

[0035] S1, collecting interference images of each film sample on the waterborne polyurethane film under a preset light source in each phase shift state.

[0036] A specific implementation scenario of the embodiment of the present application is the thickness measurement scenario of the waterborne polyurethane film; since temperature changes will cause the waterborne polyurethane film to expand or contract, and humidity will also cause the optical properties of the film to change, this embodiment controls the temperature of the detection environment at 20°C ± 2°C and the humidity at 40%RH~60%RH to avoid introducing more errors. Among them, the interference objective lens selects the Mirau interference objective lens, the positioner selects the high-precision piezoelectric ceramic (PZT) positioner, and the light source selects the white light LED light source with an average wavelength of 600nm, and measures the refractive index n of the waterborne polyurethane film under this light source. A CCD digital camera and a high-resolution image acquisition card are used to collect images, and the detection system is built on an air-floating vibration isolation platform to avoid vibration interference.

[0037] N samples are randomly selected from the waterborne polyurethane film. In this embodiment, N is 10, the sample size is 1cm×1cm, the scanning step is 20nm, the scanning range is the entire film sample, the phase shift method is mechanical phase shift through a PZT positioner, the phase shift amount is π / 4, and the number of phase shifts is 4.

[0038] Adjust the optical path, turn on the white light LED light source, and divide it into two beams through the beam splitter, one as the reference light and the other as the measurement light. Make the reference light and the measurement light meet at the detector, and make the measurement light evenly irradiate the film. After being reflected by the upper and lower surfaces of the film, use the interference objective lens to focus the measurement light onto the film surface, and collect the light reflected by the film to interfere with the reference light to obtain relevant optical information of the film.

[0039] Taking one of the thin film samples as an example, the image acquisition process is as follows: the PZT positioner carries the objective lens to vertically scan the film according to the set scanning step and scanning range, and the scanning mode is line-by-line scanning. The phase shift is achieved by changing the optical path difference between the measurement light and the reference light through the PZT positioner, and the phase shift amount is changed each time, and a CCD digital camera is used to synchronously capture an interference image. Phase shift and image acquisition are performed in sequence according to the set phase shift amount and phase shift number, that is, four interference images under different phase shift states are collected. Median filtering is used for denoising of all the interference images collected. The median filtering denoising process belongs to the well-known technology, and the specific content will not be repeated.

[0040] White light is composed of light of multiple wavelengths and has a certain coherence length. When white light is irradiated onto the film, interference occurs in the light reflected from the upper and lower surfaces of the film. Since light of different wavelengths propagates at different optical paths in the film, the intensity and position of the interference fringes change with the optical path difference. At the position where the optical path difference is zero, the interference fringes will have a maximum value, namely the zero-order fringes, and this position is related to the height information of the film surface. By scanning the surface of the film and recording the interference fringes information at different positions, the thickness of the film can be obtained. The phase discontinuity in the interference image caused by the scattering of white light by the waterborne polyurethane film causes deviations in the thickness calculation. Therefore, this embodiment extracts the zero-order fringes by analyzing the phase changes in the local area where the zero-order fringes appear in the interference image.

[0041] S2, any position point on the film sample is recorded as the target point; according to the difference in grayscale distribution between the target point and the pixels in its neighborhood on all interference images, the interference fringe intensity of the target point is obtained.

[0042] When calculating the film thickness, the four-step phase shift method is usually used to establish a quantitative relationship between phase and light intensity through the inverse tangent function; the phase information is further obtained through the unwrapping algorithm, and the zero-order fringe position is obtained to calculate the film thickness. Since the waterborne polyurethane film scatters white light more, the phase is discontinuous, which causes the zero-order fringe position recognition deviation, resulting in errors in the film thickness calculation.

[0043] The zero-order fringe usually corresponds to the maximum intensity of the interference fringe. Near the zero-order fringe, since the optical path difference is zero, the interference is constructive, the intensity is large and the relative change is small. At a position far away from the zero-order fringe, the intensity of the interference fringe will change with the change of the optical path difference, and the relative change is also large. By analyzing the relative change of the intensity of the interference fringe, the area with relatively stable intensity can be found, thereby determining the position of the zero-order fringe.

[0044] Therefore, in this embodiment, the interference fringe intensity at each position can be calculated from the grayscale change of the collected interference image, and any position point on the film sample is recorded as a target point. The calculation formula for the interference fringe intensity of the target point is:

[0045]

[0046] In the above formula, represents the interference fringe intensity of the target point; M represents the number of phase shift states, which is 4 in this embodiment; P represents the number of pixels in the eight neighborhoods of the pixel at the same position of the target point on the interference image; Represents the gray value of the pixel at the same position on the interference image of the target point in the i-th phase shift state; Represents the gray value of the jth pixel in the eight-neighborhood of the pixel at the same position on the interference image of the target point in the i-th phase shift state; It represents the mean gray value of the pixels at the same position on the interference image of the target point in all phase shift states.

[0047] Among them, the smaller the intensity of the interference fringes, the larger the grayscale value of the pixel, and the smaller the change in the grayscale value in the neighborhood of the pixel, that is, the more stable the intensity of the area where the pixel is located, the more likely the pixel is to belong to the position of the zero-order fringe. On the contrary, it means that the optical path difference in the area where the pixel is located changes greatly, the intensity of the interference fringes is unstable, and it is less likely to be the position of the zero-order fringe.

[0048] S3, perform phase measurement on the interference images in all phase shift states to obtain the phase value of the target point; obtain the phase gradient change value of the target point according to the phase change degree of the target point in the local area; obtain the phase continuity of the target point according to the phase difference between the target point and the position point in its local area; and obtain the interference phase change value by combining the phase gradient change value and phase continuity of the target point.

[0049] The zero-order fringe can be calculated by combining the change in interference intensity with the preliminary phase image, but the position of the zero-order fringe may deviate due to the phase discontinuity problem. In the four-step phase shift method, the interference images in all phase shift states are used as input, and the preliminary phase value of each pixel on the interference image is calculated by the four-step phase shift method to obtain a preliminary phase image. The four-step phase shift method is a well-known technology, and the specific steps will not be repeated. It should be noted that the phase value at this time is constrained to between.

[0050] In order to obtain a more accurate phase image, an unwrapping algorithm is used to unwrap the wrapped phase. In this embodiment, the branch cutting method is selected. The branch cutting method is a commonly used phase unwrapping algorithm. It first identifies phase discontinuities, which are usually caused by noise from interference fringes, defects on the surface of the film, or other factors. Then, these discontinuities are connected by constructing branch cuts to form a branch cut network. During the unwrapping process, the phase is increased or decreased along the direction of the branch cut to ensure that the phase after unwrapping is continuous, which can solve the problem of phase discontinuity.

[0051] Specifically, the preliminary phase image is used as input, and the branch cutting method is used for phase unwrapping to obtain the phase distribution image. At this time, the phase at each pixel point on the phase distribution image is a true continuous phase value, and the phase of the target point at the same position pixel point in the phase distribution image is used as the phase value of the target point. The specific implementation process of the branch cutting method is a well-known technology and will not be described in detail.

[0052] Near the zero-order fringe, the phase usually changes relatively continuously; while at a position far from the zero-order fringe, the phase changes greatly. Therefore, the interference phase change is calculated based on the phase relationship of each pixel in the phase distribution image to calculate the position of the zero-order fringe; specifically, by calculating the phase gradient change of the pixel and analyzing its change, the area with a small change rate can be found, thereby determining the possible position of the zero-order fringe.

[0053] In this embodiment, the interference phase change can be obtained from the phase gradient change and phase continuity in the phase distribution image. Near the position of the zero-order fringe, the phase usually changes relatively continuously, and its phase gradient changes slightly. At a position far away from the zero-order fringe, the phase change is large, and the phase gradient change rate is correspondingly large. Specifically, by calculating the phase gradient change of the pixel and analyzing its change, an area with a small change rate can be found, thereby determining the possible position of the zero-order fringe. The phase distribution image is gradient detected to obtain the phase gradient of each pixel point; the method for gradient detection of the phase distribution image in this embodiment is to use the sobel operator to obtain the gradient amplitude of each pixel point in the phase distribution image; the pixel value of each pixel point in the phase distribution image is the phase value of the pixel point at the same position on the thin film sample, and the gradient amplitude of each pixel point is the phase gradient of the same position on the thin film sample; the phase standard deviation of the target point and all target points in its eight neighborhoods is obtained; the product of the standard deviation and the phase gradient of the target point is used as the phase gradient change value of the target point.

[0054] When the phase gradient is small, it means that the phase change in the area is relatively gentle, and it is likely to be near the zero-order fringe. If the phase standard deviation is small, it means that the phase change is relatively gentle and concentrated, which can measure the discreteness of the phase in the area, so as to determine whether it is close to the zero-order fringe position; the smaller the final phase gradient change value, the smaller the pixel phase standard deviation of the pixel and its neighborhood, and the smaller the gradient change rate, that is, the smaller the phase change in the area where the pixel is located, the more likely it is to be at the location of the zero-order fringe, otherwise it is less likely to be at the location of the zero-order fringe.

[0055] Near the zero-order fringe, the phase usually changes continuously, and the phase difference with the adjacent position point is small. Further analysis of the phase continuity in the local area can measure the degree of phase continuity in the entire image area. A higher degree of phase continuity means that the phase changes of most pixels in the image are relatively gentle and are more likely to be near the zero-order fringe. Specifically, the phase continuity of the target point is obtained based on the phase difference between the target point and the position point in its local area.

[0056] In this embodiment, the calculation method of the phase continuity of the target point is:

[0057]

[0058] In the above formula, represents the phase continuity of the target point, Q represents the number of position points in the eight-neighborhood of the target point, Represents the phase value of the target point, Indicates the phase value of the bth position point in the eight-neighborhood of the target point.

[0059] The smaller the value of the phase continuity is, the more continuous the phase change near the pixel in the image is, that is, the pixel is more likely to be in the area where the zero-order fringe is located. On the contrary, it means that the phase change near the pixel is discontinuous, that is, the phase change is more obvious, and it is less likely to be located at the position where the zero-order fringe is located.

[0060] In summary, the phase gradient change value of a pixel in the phase distribution diagram indicates the phase change degree of the region where the pixel is located, and the phase continuity indicates the phase change continuity degree near the region where the pixel is located. The phase change degree and the phase change continuity degree of the region where the pixel is located both reflect the degree of membership of the corresponding position point to the zero-order fringe. Therefore, this embodiment combines the phase gradient change value and phase continuity of each position point to obtain the interference phase change value, and further measures the degree of membership of the position point to the zero-order fringe.

[0061] In this embodiment, the product of the normalized value of the phase continuity of the target point and the normalized value of the phase gradient change value is used as the interference phase change value of the target point. The normalized value is calculated by using a linear normalization function. The smaller the phase continuity, the greater the phase change degree of the target point; the smaller the phase gradient change value, the smaller the phase change degree of the pixel area. At this time, the smaller the interference phase change value of the position point, the greater the degree of membership to the zero-order fringe, and the more likely it is to belong to the area where the zero-order fringe is located.

[0062] S4, comprehensively analyzing the interference phase change values ​​and interference fringe intensity clustering of all positions on the film sample, extracting the zero-order fringe area on the upper surface and the zero-order fringe area on the lower surface of the film sample; obtaining the thickness of the film sample based on the distance between the zero-order fringe area on the upper surface and the zero-order fringe area on the lower surface; and obtaining the thickness of the waterborne polyurethane film based on the numerical distribution of the thickness of all film samples.

[0063] A total of four interference images are obtained from the above steps, and then a phase distribution image is obtained from the four interference images. Zero-order fringes are obtained by clustering the phase distribution in the phase distribution image. The interference fringe intensity and interference phase change value at the zero-order fringes on the upper and lower surfaces of the film sample are relatively small. Therefore, this embodiment calculates the product of the interference fringe intensity and the interference phase change value of each position point as the interference value of each position point; clusters all position points with the minimum value, the second minimum value and the maximum value of the interference value of all position points as cluster centers to obtain each cluster cluster; the area composed of all position points in the cluster cluster with the minimum value as the cluster center is recorded as the upper surface zero-order fringe area; the area composed of all position points in the cluster cluster with the second minimum value as the cluster center is recorded as the lower surface zero-order fringe area. Then the area composed of all position points in the cluster cluster with the maximum value as the cluster center is the non-zero-order fringe area.

[0064] The optical path difference is related to the thickness of the object. When light passes through the waterborne polyurethane film, its propagation speed and wavelength change, thus affecting the optical path. The specific methods for calculating the film thickness based on the optical path difference when light passes through the waterborne polyurethane film include:

[0065] The absolute values ​​of the differences in the Euclidean distances from the center of gravity of the zero-order fringe area on the upper surface and the center of gravity of the zero-order fringe area on the lower surface to the upper edge of the film sample are calculated respectively; the refractive index of the waterborne polyurethane film under the current light source is obtained, and the ratio of the absolute value of the difference to the refractive index of the waterborne polyurethane film is used as the thickness of the film sample.

[0066] Furthermore, in order to eliminate the influence of interference factors such as image noise and ambient light on thickness measurement in the above calculation process, by analyzing the numerical distribution of the thickness of all film samples and eliminating the thickness calculation results with large errors, a more accurate thickness calculation value is obtained.

[0067] Specifically, the thickness of each film sample after scanning is calculated, and the scanning positions of the film samples are numbered in sequence, such as Figure 2 The figure shows the thickness distribution of the film samples, where the horizontal axis is the scanning position number of the film samples and the vertical axis is the thickness of the film samples. The standard deviation and average value of the thickness of all film samples are calculated; the absolute value of the difference between the thickness of each film sample and the average value is calculated and recorded as the thickness difference; the film samples with a thickness difference greater than the standard deviation are recorded as error samples; the average thickness of the other film samples except all error samples is calculated as the thickness of the waterborne polyurethane film.

[0068] Based on the same inventive concept as the above method, an embodiment of the present application also provides a waterborne polyurethane film thickness detection system, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, wherein when the processor executes the computer program, the steps of any one of the above-mentioned waterborne polyurethane film thickness detection methods are implemented.

[0069] Through the above description of the implementation method in combination with the accompanying drawings, technical personnel in the relevant field can understand that for the convenience and simplicity of description, only the division of the above-mentioned functional modules is used as an example. In actual applications, the above-mentioned functions can be assigned to different functional modules as needed, that is, the internal structure of the device can be divided into different functional modules to complete all or part of the functions described above.

[0070] The above contents are only specific implementation methods of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be covered by the protection scope of the present application.

Claims

1. A method for detecting the thickness of a waterborne polyurethane film, characterized in that: The method comprises the following steps: S1, collecting interference images of each film sample on the waterborne polyurethane film under a preset light source in each phase shift state; S2, record any position point on the film sample as the target point; obtain the interference fringe intensity of the target point based on the grayscale distribution difference between the target point and the pixels in its neighborhood on all interference images; S3, performing phase measurement on the interference images in all phase shift states to obtain the phase value of the target point; obtaining the phase gradient change value of the target point according to the phase change degree of the target point in the local area; obtaining the phase continuity of the target point according to the phase difference between the target point and the position point in its local area; and obtaining the interference phase change value by combining the phase gradient change value and phase continuity of the target point; S4, comprehensively analyzing the interference phase change values ​​and interference fringe intensity clustering of all positions on the film sample, extracting the zero-order fringe area on the upper surface and the zero-order fringe area on the lower surface of the film sample; obtaining the thickness of the film sample based on the distance between the zero-order fringe area on the upper surface and the zero-order fringe area on the lower surface; and obtaining the thickness of the waterborne polyurethane film based on the numerical distribution of the thickness of all film samples.

2. A method for detecting the thickness of a waterborne polyurethane film according to claim 1, characterized in that: The calculation formula of the interference fringe intensity of the target point is: ; In the formula, represents the interference fringe intensity of the target point; M represents the number of phase shift states; P represents the number of pixels in the eight-neighborhood of the pixel at the same position of the target point on the interference image; Represents the gray value of the pixel at the same position on the interference image of the target point in the i-th phase shift state; Represents the gray value of the jth pixel in the eight-neighborhood of the pixel at the same position on the interference image of the target point in the i-th phase shift state; It represents the mean gray value of the pixels at the same position on the interference image of the target point in all phase shift states.

3. A method for detecting the thickness of a waterborne polyurethane film according to claim 1, characterized in that: The phase measurement of the interference images in all phase shift states to obtain the phase value of the target point includes: The four-step phase shift method is used to obtain the preliminary phase image for the interference images in all phase shift states; the preliminary phase image is phase unwrapped to obtain the phase distribution image; the phase of the target point at the pixel point at the same position in the phase distribution image is taken as the phase value of the target point.

4. A method for detecting the thickness of a waterborne polyurethane film as claimed in claim 3, characterized in that: The method for obtaining the phase gradient change value of the target point includes: Perform gradient detection on the phase distribution image to obtain the gradient amplitude of each pixel point; the gradient amplitude of each pixel point is the phase gradient of the same position point on the thin film sample; obtain the phase standard deviation of the target point and all target points in its eight neighborhoods; and take the product of the standard deviation and the phase gradient of the target point as the phase gradient change value of the target point.

5. A method for detecting the thickness of a waterborne polyurethane film according to claim 1, characterized in that: The method for calculating the phase continuity of the target point includes: The mean phase difference between the target point and all the points in its eight neighborhoods is calculated as the phase continuity of the target point.

6. A method for detecting the thickness of a waterborne polyurethane film according to claim 1, characterized in that: The phase gradient change value and phase continuity of the integrated target point are used to obtain the interference phase change value, including: The product of the normalized value of the phase continuity of the target point and the normalized value of the phase gradient change value is taken as the interference phase change value of the target point.

7. A method for detecting the thickness of a waterborne polyurethane film according to claim 1, characterized in that: The method for extracting the zero-order fringe area on the upper surface and the zero-order fringe area on the lower surface of the film sample comprises: Calculate the product of the interference fringe intensity and the interference phase change value of each position point as the interference value of each position point; cluster all the position points with the minimum value, the second minimum value and the maximum value of the interference value of all the position points as cluster centers to obtain clusters; record the area composed of all the position points in the cluster with the minimum value as the cluster center as the zero-order fringe area of ​​the upper surface; record the area composed of all the position points in the cluster with the second minimum value as the cluster center as the zero-order fringe area of ​​the lower surface.

8. A method for detecting the thickness of a waterborne polyurethane film according to claim 1, characterized in that: The method for obtaining the thickness of the film sample specifically comprises: The absolute values ​​of the differences in the Euclidean distances from the center of gravity of the zero-order fringe area on the upper surface and the center of gravity of the zero-order fringe area on the lower surface to the upper edge of the film sample are calculated respectively; the refractive index of the waterborne polyurethane film under a preset light source is obtained, and the ratio of the absolute value of the difference to the refractive index of the waterborne polyurethane film is used as the thickness of the film sample.

9. A method for detecting thickness of a waterborne polyurethane film according to claim 1, characterized in that: The method for obtaining the thickness of the waterborne polyurethane film comprises: Calculate the standard deviation and average value of the thickness of all film samples; calculate the absolute value of the difference between the thickness of each film sample and the average value, and record it as the thickness difference; record the film samples with a thickness difference greater than the standard deviation as error samples; calculate the average thickness of the other film samples except all error samples as the thickness of the waterborne polyurethane film.

10. A waterborne polyurethane film thickness detection system, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that: When the processor executes the computer program, the steps of a method for detecting the thickness of a waterborne polyurethane film as described in any one of claims 1 to 9 are implemented.

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