An adaptive wavelength phase-shift sampling algorithm for harmonic anti-aliasing

Abstract

The present invention discloses an adaptive harmonic anti-aliasing wavelength phase-shift sampling algorithm, which includes a spectrum analysis target selection method, a sub-algorithm, and three spectrum criteria. The sampling result is judged according to the designed sub-algorithm and spectrum criteria. If the criteria are not met, the phase shift value and the number of sampling frames are changed and the cyclic sampling judgment is performed again; the three spectrum criteria include: criterion 1 is the judgment of the valley value between frequency peaks and the height of the tail spectrum, criterion 2 is the judgment of the head peak interval, and criterion 3 is the judgment of the relative height of the peak. The algorithm is easy to implement, has low technical difficulty, and is novel in method.

Description

Technical Field

[0001] The present invention relates to an adaptive harmonic anti-aliasing wavelength phase-shifting sampling algorithm, in particular to a sampling method in multi-surface wavelength phase-shifting interferometry that determines and reselects the current sampling frequency through sampling performance analysis, and is applied in the field of high-precision optical measurement. Background Art

[0002] Optically transparent parallel plates (hereinafter referred to as optical plates) have high-quality surface morphology distribution and high parallelism, so they are widely used in the design and construction of optical systems. If there are surface defects on the surface of the optical plate, it will cause plasma blockage and damage subsequent optical components. Therefore, high-precision surface measurement of parallel plates has important practical significance and research value for efficient and high-quality optical component processing and surface quality assessment.

[0003] In the surface detection technology of optical flat plates, the main measurement method currently adopted is optical detection. Optical detection is an important part of modern precision detection technology. It can demodulate the initial phase distribution of the measured object with high precision, thereby realizing wavefront reconstruction and accurately obtaining the morphological distribution of each surface. The characteristics of this detection method are: high precision, fast speed, modularization, and non-contact non-destructive surface detection. Therefore, it is widely used in industrial component detection, medical detection, aerospace, astronomical observation and other fields. When interferometric measurement is performed on a multi-surface optical flat plate, the collected interference pattern is the result of the superposition of harmonics of the interference between each surface of the measured object and the reference surface (these harmonics include: interference signals between the measured front surface and the reference mirror, the measured rear surface and the reference mirror, and the measured front and rear surfaces). Therefore, it is impossible to directly use the traditional single-surface method to reconstruct the wavefront.

[0004] In the past, when measuring multiple surfaces, it is important to note that since it is impossible to eliminate the influence of the corresponding harmonics of each DUT on the interference pattern, this information can only be suppressed by applying matting materials such as Vaseline on the non-measured surfaces. However, in the process of applying and cleaning the matting material, it is very easy to damage the high-precision surface of the DUT, and when measuring transparent DUTs with multiple surfaces in this way, it is impossible to obtain the morphological distribution of multiple surfaces at one time.

[0005] In optical detection technology, the wavelength phase shifting method can avoid the inevitable multi-harmonic frequency aliasing problem in traditional hardware phase shifting. The current multi-surface interferometry measurement method has strict restrictions on the interference cavity length of each harmonic, and has poor robustness to measurement errors such as phase shift errors, and cannot meet the requirements of high-precision multi-surface measurement under free cavity length. The traditional method can estimate the relative frequency of each harmonic based on the measurement distance between the reference mirror inside the interferometer and the measured object and the optical thickness of the measured object itself, but this method has a high estimation error.

[0006] The present invention proposes an adaptive harmonic anti-aliasing wavelength phase-shift sampling algorithm, which analyzes the sampling performance and reselects the sampling frequency by judging the spectrum of the interference intensity signal of multi-harmonic superposition obtained at the current sampling frequency. The advantages are as follows:

[0007] (1) It can realize the sampling performance determination under free cavity length and adaptive change of sampling frequency, and is robust to measurement errors such as phase shift error;

[0008] (2) It does not require accurate measurement of prior information such as the measurement distance, thus avoiding the estimation error of the relative frequency of harmonics;

[0009] (3) The purpose of multi-surface measurement of the measured object can be achieved through one or more sets of interference patterns, and there is no need to rotate the measured object during the measurement process. Summary of the invention

[0010] In order to solve the problem of high-precision multi-surface measurement under free cavity length without relying on prior information, the present invention proposes an adaptive harmonic anti-aliasing wavelength phase-shifting sampling algorithm, which analyzes the sampling performance and reselects the sampling frequency by judging the spectrum of the interference intensity signal of multi-harmonic superposition obtained at the current sampling frequency. This method can overcome the problem that the traditional multi-surface measurement method cannot be applied to the free cavity length, and does not require measurement of prior information, and can adaptively judge the optimal sampling frequency. The method is easy to implement, has low technical difficulty, and is novel. The steps of the wavelength phase-shifting interferometry method can be briefly described as: changing the wavelength of the light source according to a certain wavelength tuning amount, and collecting interference patterns to obtain interference patterns of a certain number of frames (such as 30 frames, that is, 30 interference patterns); processing the collected interference patterns to obtain the phase of the measured surface; post-processing the phase to obtain surface information.

[0011] According to the above inventive concept, the present invention adopts the following technical solutions:

[0012] As a basis, the important parameters in multi-surface phase-shifting interferometry are first given:

[0013] With n1 as the refractive index of the measured object, T as the average thickness of the measured object, under the long cavity long interference condition (meaning that the optical thickness n1T of the measured object is less than the distance H between the reference mirror and the front surface of the measured object), for the thickness change signal, the front surface signal and the back surface signal, the harmonic relative frequency F=1, M, M+1, where M is the ratio of the distance between the reference mirror and the front surface of the measured object to the optical thickness of the measured object, that is, H / (n1T). The phase shift reference coefficient N can determine the single-step phase shift value of the fundamental frequency signal (the signal with the minimum harmonic frequency) to be 2π / N. Therefore, for the thickness change signal, the front surface signal and the back surface signal, the phase shift values ​​are 2π / N, M(2π / N), (M+1)(2π / N) respectively. The sampling sequence length set in this application is XN, N is greater than or equal to 8, where X can be selected according to demand, X is greater than or equal to 1, and XN should be an integer. The frequency spectrum of the interference signal can be obtained by using the Fast Fourier Transform (FFT) algorithm on the interference signal. Since the spectrum is symmetrical, the right half of the spectrum is taken for analysis, and the frequency of each harmonic can be obtained by searching the amplitude peak (peak value for short) within this half of the spectrum.

[0014] Under the condition of long cavity long interference, considering the low-order terms after Taylor expansion in the wavelength change, and ignoring the influence of high-order nonlinearity on the phase shift process, the frequency of each harmonic can be expressed as:

[0015]

[0016] In the above formula, the harmonic frequency vm subscript m = 1, 2, 3 corresponds to the thickness change signal, the front surface signal and the back surface signal respectively, (x, y) is the coordinate on the interference pattern. Since the front and back surfaces of the measured object have a high degree of parallelism, and each measured surface is also parallel to the reference surface in the reference mirror, the harmonic frequency of each coordinate position is the same, and (x, y) can be omitted. In the above formula, the single-step wavelength tuning amount is Δλ, and the starting wavelength of the laser is λ0. The harmonic frequency vm multiplied by 2π is the phase shift value of the harmonic.

[0017] When the harmonic cavity length (including the distance from the measured object to the reference mirror and the optical thickness of the measured object itself) is difficult to measure accurately, the method of estimating the relative frequency of harmonics based on the advance measurement of prior information has poor robustness to the phase shift error, and requires accurate measurement of the prior information, which will undoubtedly increase the measurement cost. In order to overcome this problem, from the perspective of spectrum analysis, an adaptive harmonic anti-aliasing wavelength phase shift sampling algorithm is proposed. The algorithm includes a spectrum analysis target selection method, a sub-algorithm, and three spectrum criteria. The sampling results are judged according to the designed sub-algorithms and spectrum criteria. If the criteria are not met, the phase shift value and the number of sampling frames are changed and the cyclic sampling judgment is performed again. By combining with the FFT frequency solution algorithm, the method proposed in this application can achieve high-precision solution of the harmonic signal frequency, automatic determination of sampling performance, and selection of the optimal sampling frequency.

[0018] First, select the area and object to be analyzed.

[0019] Preferably, the spectrum analysis target selection method can be described as: selecting a target area in the interference pattern, adding the interference light intensity of the center coordinates of the target area and 8 points immediately surrounding the center (9 points in total) under different interference pattern frame numbers to obtain intensity data of the 9 points, then subtracting the average value of this group of intensity data from the intensity data of the 9 points to obtain the intensity data of the 9 points after de-averaging, and removing the intensity data of the first and last positions (denoted as I0) from the intensity data of the 9 points after de-averaging, and taking I0 as the object for FFT.

[0020] After performing FFT on I0, its spectrum can be obtained. In the spectrum of I0, the horizontal axis is frequency and the vertical axis is amplitude. A certain point on the horizontal axis (frequency axis) is used as the starting position, and a point with a certain amplitude at this frequency is used as the ending position. The line connecting the above starting position and the ending position is called the spectrum line in the spectrum. Using FFT on I0 can obtain the spectrum of the interference signal. Since the spectrum is symmetrical, the right half of the spectrum is taken for analysis. The frequency of each harmonic can be obtained by searching based on the peak value of the amplitude in this half of the spectrum (referred to as the peak value). In the case of good sampling performance, the distance between the harmonic frequencies in the obtained spectrum should be far enough, and there should be no adhesion between the spectrum lines and the distribution should be clear. In this case, spectrum aliasing will not occur.

[0021] There are three main factors that affect the spectrum distribution of multi-surface superposition interference signals, namely sampling frequency and sequence length (determined by the N value), interference cavity length frequency doubling (determined by M) and measurement error (determined by whether the measurement conditions are stable). In order to achieve multi-surface interferometry without harmonic frequency aliasing under free cavity length, this application will start from the perspective of spectrum analysis, mainly by changing the N value to change the frequency distribution of each harmonic, and design the corresponding spectrum criterion and variable sampling algorithm.

[0022] First, based on the ideal spectrum distribution characteristics of multi-surface interferometry, when the sampling performance is good, there should be three frequency peaks in the spectrum in addition to the DC component, and the horizontal coordinate values ​​corresponding to these three frequency peaks have a certain difference. Considering that the spectrum amplitude of the harmonic sub-signal is related to the corresponding harmonic contrast, and the contrast is related to the number of reflections and transmissions of the laser beam on the surfaces and inside of the measured object, it can be used to assist in determining the spectrum distribution type. In order to avoid harmonic aliasing, a sub-algorithm of the variable sampling method is first designed based on FFT.

[0023] Preferably, the sub-algorithm can be described as: after solving the spectrum of I0 by FFT algorithm, the bottom layer is set to zero and the number of peaks is judged in the spectrum. The general process of the sub-algorithm can be summarized as follows: in the obtained spectrum, the frequencies corresponding to the three highest amplitude peaks are first searched, and the heights of the remaining frequency spectrum lines that are less than 1 / 3 of the amplitude of the third peak are set to 0. Specifically, with the current phase shift reference coefficient N=N C After phase shifting and interferogram acquisition, the interference data after averaging the central 9-point pixels is subjected to FFT calculation, and the spectrum obtained is expressed as F NC The spectrum peak search operation can be expressed as: Search the spectrum F NC The first derivative F' NC The zero-crossing point with positive on the left and negative on the right (the horizontal coordinate of the zero-crossing point is n ZR ), which shall meet the following conditions:

[0024] [F′ NC (n ZR -1)>0]∧[F′ NC (n ZR +1)<0]

[0025] In the formula, ^ represents the judgment condition of "and".

[0026] After the bottom layer is set to zero, the peak value of the processed spectrum is judged. If the number of peak values ​​P=3 at this time, the condition is met.

[0027] However, merely judging the number of peaks cannot satisfy the frequency anti-aliasing under the free cavity length. To this end, the present application proposes three criteria to process and identify the aliasing of the harmonic sub-signal spectrum.

[0028] Preferably, criterion-1 of the three spectrum criteria can be described as:

[0029] Criteria-1: Determination of the peak-to-peak valley value and the height of the tail spectrum

[0030] When the phase shift value of each harmonic is close to π, if the environmental disturbance is large or the sampling is insufficient, there will be a higher spectral line distribution at the end of the spectrum. And between the other two peaks, there will be a spectrum adhesion or leakage where the minimum spectral line height is still high. In order to deal with such problems, criterion -1 is set as follows: if the spectral line height of the three adjacent points at the end of the spectrum is greater than 1 / 5 of the minimum peak amplitude in the spectrum, or the frequency amplitude distributed between every two peak amplitudes is also greater than 1 / 5 of the minimum frequency peak amplitude, then it is determined that the corresponding sampling frequency does not meet the current criterion.

[0031] Preferably, criterion-2 of the three spectrum criteria can be described as:

[0032] Criterion-2: Head peak interval determination

[0033] At the same time, when the frequencies of two harmonics are too close, it will also have an adverse effect on wavefront reconstruction. In order to deal with this problem, set criterion-2 as follows: take the corresponding coordinates of the peak with the smaller horizontal coordinate in the spectrum as the calculation basis, sort from left to right along the frequency axis, and the rightmost side is the end of the spectrum. If the difference between the horizontal coordinates of the second peak and the first peak from left to right along the frequency axis is at least 0.8 times the horizontal coordinate value of the first peak, and the difference between the horizontal coordinates of the third peak and the second peak is at least 1 times the horizontal coordinate value of the first peak, then it is determined that the current sampling performance meets the requirements.

[0034] Preferably, criterion-3 of the three spectrum criteria can be described as:

[0035] Criterion-3: Peak relative height determination

[0036] In order to avoid the influence of serious spectrum leakage on the measurement results, criterion-3 is set as follows: the peak heights of the three frequency peaks in the spectrum are judged. If the maximum difference between the three frequency peaks does not exceed half of the minimum peak amplitude, the current sampling performance is judged to meet criterion-3.

[0037] Preferably, the cyclic sampling determination can be described as follows: first, N=N S As the phase shift reference coefficient, the interference pattern is sampled, and the sampling frame number is set to XN. If the spectrum distribution of the interference intensity signal I0 obtained by the current sampling cannot meet the condition of the number of peaks P=3 after being processed by the sub-algorithm, it is necessary to increase the phase shift reference coefficient N=N+N D , N DThe sampling amount is increased, and the interference pattern is sampled again, and the judgment after the next sampling is entered.

[0038] If criterion-1 and criterion-2 cannot be met, it is also necessary to increase the phase shift reference coefficient N = N + N D , and sample again. When criterion-3 cannot be met, the measurement distance from the reference surface to the front surface of the measured object is changed, and the change in the measurement distance should be no less than 30% of the last measurement distance.

[0039] In addition, when the sampling frequency is changed, the tuning performance of the current hardware and the stability of the measurement environment need to be considered comprehensively. If the random disturbance is too large, the above criteria cannot be met. Therefore, the limit threshold N is set here. LIM is a positive number greater than or equal to 10 and less than or equal to 100. If N is less than or equal to the limit threshold N LIM , one sub-algorithm and three spectrum criteria are still executed, and whether to increase the sampling of the phase shift reference coefficient is determined according to the judgment result. If N is greater than the limit threshold N LIM The loop is jumped out. This measure is to avoid the situation where the algorithm fails and the algorithm is invalidly looped due to too large random disturbance in the measurement. If all the above criteria are met, the harmonic frequency solution and phase demodulation can be performed. S 、N D and N LIM It needs to be set according to different measurement requirements.

[0040] At this point, the multi-harmonic adaptive frequency anti-mixing algorithm using the variable sampling method has been designed. This algorithm only needs to roughly estimate the optical thickness of the device under test and calculate the wavelength tuning amount, and can realize the multi-surface wavelength phase-shifting interferometry measurement of frequency anti-mixing under free cavity length through automatic judgment and iteration, and has good robustness to common errors such as phase shift error.

[0041] Compared with the prior art, the present invention has the following obvious outstanding substantial features and significant advantages:

[0042] 1. The adaptive harmonic anti-aliasing wavelength phase-shift sampling algorithm designed in the present invention can adaptively determine the sampling performance;

[0043] 2. The adaptive harmonic anti-aliasing wavelength phase-shifting sampling algorithm designed in the present invention can avoid the problem of failure of multi-surface wavelength interferometry measurement caused by frequency aliasing and other problems;

[0044] 3. The adaptive harmonic anti-aliasing wavelength phase-shifting sampling algorithm designed in the present invention provides an excellent data basis for high-precision multi-surface measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention is further described below with reference to the accompanying drawings and example processes.

[0046] Figure 1 The light intensity and spectrum under ideal sampling conditions;

[0047] Figure 2 Flowchart of the designed sampling judgment method. DETAILED DESCRIPTION

[0048] In order to avoid the problem of harmonic frequency aliasing in multi-surface interferometry, from the perspective of spectrum analysis, an adaptive harmonic anti-aliasing wavelength phase shift sampling algorithm is proposed. The algorithm includes a spectrum analysis target selection method, a sub-algorithm, and three spectrum criteria. The phase shift value and the number of sampling frames are changed according to the judgment result, and the cyclic sampling judgment is performed again. By combining with the FFT frequency solution algorithm, the algorithm proposed in this application can achieve high-precision solution of harmonic sub-signal frequencies, automatic determination of sampling performance, and selection of the optimal sampling frequency. The preferred embodiments of the present invention are described in detail as follows in conjunction with the accompanying drawings:

[0049] Embodiment 1:

[0050] First, the area to be analyzed and the object to be analyzed are selected. The target area is selected in the interference pattern, and the interference light intensity of the center coordinates of the target area and the 8 points immediately surrounding the center (a total of 9 points) under different interference pattern frame numbers is added to obtain the intensity data of the 9 points. Then, the average value of this group of intensity data is subtracted from the intensity data of the 9 points to obtain the intensity data of the 9 points after the mean is removed, and the intensity data of the first and last positions (recorded as I0) are removed from the intensity data of the 9 points after the mean is removed, and I0 is used as the object for FFT.

[0051] After performing FFT on I0, its spectrum can be obtained. In the spectrum of I0, the horizontal axis is frequency, and the vertical axis is amplitude (or height). A certain point on the horizontal axis (frequency axis) is used as the starting position, and a point with a certain amplitude at this frequency is used as the ending position. The line connecting the above starting position and the ending position is called the spectrum line in the spectrum. Using FFT on I0 can obtain the spectrum of the interference signal. Since the spectrum is symmetrical, the right half of the spectrum is taken for analysis. The frequency of each harmonic can be obtained by searching based on the peak value of the amplitude in this half of the spectrum (referred to as the peak value). In the case of good sampling performance, the distance between each harmonic frequency in the obtained spectrum should be far enough, and there should be no adhesion between the spectrum lines and the distribution should be clear. In this case, spectrum aliasing will not occur, as shown in the attached figure. Figure 1 As shown ( Figure 1 The left picture is the light intensity distribution diagram, and the right picture is the spectrum diagram).

[0052] Based on the ideal spectrum distribution characteristics of multi-surface interferometry, when the sampling performance is good, there should be three frequency peaks in the spectrum in addition to the DC component, and the horizontal coordinate values ​​corresponding to these three frequency peaks have a certain difference. Considering that the spectrum amplitude of the harmonic sub-signal is related to the corresponding harmonic contrast, and the contrast is related to the number of reflections and transmissions of the laser beam on the surfaces and inside of the measured object, it can be used to assist in determining the spectrum distribution type.

[0053] Embodiment 2:

[0054] In this embodiment, after the spectrum of I0 is solved by FFT algorithm, the bottom layer is set to zero and the number of peaks is determined in the spectrum. The general process of this sub-algorithm can be summarized as follows: in the obtained spectrum, the frequencies corresponding to the three highest amplitude peaks are first searched, and the heights of the remaining frequency spectrum lines that are less than 1 / 3 of the amplitude of the third peak are set to 0. Specifically, with the current phase shift reference coefficient N=N C After phase shifting and interferogram acquisition, the interference data after averaging the central 9-point pixels is subjected to FFT calculation, and the spectrum obtained is expressed as F NC The spectrum peak search operation can be expressed as: Search the spectrum F NC The first derivative F' NC The zero-crossing point with positive on the left and negative on the right (the horizontal coordinate of the zero-crossing point is n ZR ), which shall meet the following conditions:

[0055] [F′ NC (n ZR -1)>0]∧[F′ NC (n ZR +1)<0]

[0056] In the formula, ^ represents the judgment condition of "and".

[0057] After the bottom layer is set to zero, the peak value of the processed spectrum is judged. If the number of peak values ​​P=3 at this time, the condition is met.

[0058] However, merely judging the number of peaks cannot satisfy the frequency anti-aliasing under the free cavity length. To this end, the present application proposes three criteria to process and identify the aliasing of the harmonic sub-signal spectrum.

[0059] Criteria-1: Determination of the peak-to-peak valley value and the height of the tail spectrum

[0060] When the phase shift value of each harmonic is close to π, if the environmental disturbance is large or the sampling is insufficient, there will be a higher spectral line distribution at the end of the spectrum. And between the other two peaks, there will be a spectrum adhesion or leakage where the minimum spectral line height is still high. In order to deal with such problems, criterion -1 is set as follows: if the spectral line height of the three adjacent points at the end of the spectrum is greater than 1 / 5 of the minimum peak amplitude in the spectrum, or the frequency amplitude distributed between every two peak amplitudes is also greater than 1 / 5 of the minimum frequency peak amplitude, then it is determined that the corresponding sampling frequency does not meet the current criterion.

[0061] Criterion-2: Head peak interval determination

[0062] At the same time, when the frequencies of two harmonics are too close, it will also have an adverse effect on wavefront reconstruction. In order to deal with this problem, set criterion-2 as follows: take the corresponding coordinates of the peak with the smaller horizontal coordinate in the spectrum as the calculation basis, sort from left to right along the frequency axis, and the rightmost side is the end of the spectrum. If the difference between the horizontal coordinates of the second peak and the first peak from left to right along the frequency axis is at least 0.8 times the horizontal coordinate value of the first peak, and the difference between the horizontal coordinates of the third peak and the second peak is at least 1 times the horizontal coordinate value of the first peak, then it is determined that the current sampling performance meets the requirements.

[0063] Criterion-3: Peak relative height determination

[0064] In order to avoid the influence of serious spectrum leakage on the measurement results, criterion-3 is set as follows: the peak heights of the three frequency peaks in the spectrum are judged. If the maximum difference between the three frequency peaks does not exceed half of the minimum peak amplitude, the current sampling performance is judged to meet criterion-3.

[0065] Embodiment three:

[0066] In this embodiment, first, N=N S As the phase shift reference coefficient, the interference pattern is sampled, and the sampling frame number is set to XN. If the spectrum distribution of the interference intensity signal I0 obtained by the current sampling cannot meet the condition of the number of peaks P=3 after being processed by the sub-algorithm, it is necessary to increase the phase shift reference coefficient N=N+N D , N D The sampling amount is increased, and the interference pattern is sampled again, and the judgment after the next sampling is entered.

[0067] If criterion-1 and criterion-2 cannot be met, it is also necessary to increase the phase shift reference coefficient N = N + N D , and sample again. When criterion-3 cannot be met, the measurement distance from the reference surface to the front surface of the measured object is changed, and the change in the measurement distance should be no less than 30% of the last measurement distance.

[0068] In addition, when the sampling frequency is changed, it is necessary to comprehensively consider the tuning performance of the current hardware and the stability of the measurement environment. If the random disturbance is too large, the above criteria cannot be met. Therefore, it is set here that if N is less than or equal to the limit threshold N LIM , one sub-algorithm and three spectrum criteria are still executed, and whether to increase the sampling of the phase shift reference coefficient is determined according to the judgment result. If N is greater than the limit threshold N LIM The loop is jumped out. This measure is to avoid the situation where the algorithm fails and the algorithm is invalidly looped due to too large random disturbance in the measurement. If all the above criteria are met, the harmonic frequency solution and phase demodulation can be performed. S 、N D and N LIM It needs to be set according to different measurement requirements.

[0069] The above process can be shown in the form of a flow chart, as shown in the accompanying figure. Figure 2 As shown. So far, the multi-harmonic adaptive frequency anti-mixing algorithm using the variable sampling method has been designed. This algorithm only needs to roughly estimate the optical thickness of the device under test and calculate the wavelength tuning amount. It can realize the multi-surface wavelength phase-shifting interferometry measurement of frequency anti-mixing under free cavity length through automatic judgment and iteration, and has good robustness to common errors such as phase shift error.

[0070] The above describes the embodiments of the present invention in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Various changes can be made according to the purpose of the invention. Any changes, modifications, substitutions, combinations or simplifications made according to the spirit and principle of the technical solution of the present invention should be equivalent replacement methods. As long as they meet the purpose of the invention and do not deviate from the technical principles and inventive concepts of the present invention, they belong to the protection scope of the present invention.

Claims

1. A wavelength phase-shift sampling algorithm for adaptive harmonic anti-aliasing, characterized by: The algorithm includes a spectrum analysis target selection method, a sub-algorithm, and three spectrum criteria. The sampling result is judged according to the designed sub-algorithm and spectrum criteria. If the criteria are not met, the phase shift value and the number of sampling frames are changed and the cyclic sampling judgment is performed again; the three spectrum criteria include: criterion-1 is the judgment of the valley value between frequency peaks and the height of the tail spectrum, criterion-2 is the judgment of the head peak interval, and criterion-3 is the judgment of the relative height of the peak; The spectrum analysis target selection method is described as follows: select a target area in the interference pattern, add the interference light intensity of the center coordinates of the target area and the 8 points immediately surrounding the center under different interference pattern frame numbers to obtain 9-point added intensity data, then subtract the average value of this group of intensity data from the 9-point added intensity data to obtain the 9-point added intensity data after removing the average value, and remove the intensity data of the first and last positions from the 9-point added intensity data after removing the average value, record it as I0, and use I0 as the object for FFT; The sub-algorithm is described as follows: after solving the spectrum of I0 by FFT algorithm, the bottom layer is set to zero and the number of peaks is judged in the spectrum; the general process of the sub-algorithm can be summarized as follows: in the obtained spectrum, the frequencies corresponding to the three highest amplitude peaks are first searched, and the heights of the remaining frequency spectrum lines that are less than 1 / 3 of the amplitude of the third peak are set to 0, which is the bottom layer zeroing; with the current phase shift reference coefficient N=N C After phase shifting and interferogram acquisition, the interference data after averaging the central 9-point pixels is subjected to FFT calculation, and the spectrum obtained is expressed as F NC The spectrum peak search operation is expressed as: search for F in the spectrum NC The first derivative F' NC The zero-crossing point is positive on the left and negative on the right. The horizontal coordinate of the zero-crossing point is n ZR , which should meet the following conditions: [F′ NC (n ZR -1)>0]∧[F′ NC (n ZR +1)<0] In the formula, ^ represents the judgment condition of "and"; After the bottom layer is set to zero, the peak value of the processed spectrum is judged. If the number of peaks P = 3 at this time, the condition is met; The criterion-1 of the three spectral criteria is described as: Criterion-1: The peak-to-peak valley value and the tail spectrum height judgment. If the spectral line height of the three adjacent points at the tail of the spectrum is greater than 1 / 5 of the minimum peak amplitude in the spectrum, or the frequency amplitude distributed between every two peak amplitudes is also greater than 1 / 5 of the minimum frequency peak amplitude, then it is determined that the corresponding sampling frequency does not meet the current criterion; Criterion-2 of the three spectrum criteria can be described as: Criterion-2: head peak interval judgment, taking the corresponding coordinates of the peak with the smaller horizontal coordinate in the spectrum as the calculation basis, sorting from left to right along the frequency axis, and the rightmost side is the tail of the spectrum. If the difference between the horizontal coordinates of the second peak and the first peak from left to right along the frequency axis is at least 0.8 times the horizontal coordinate value of the first peak, and the difference between the horizontal coordinates of the third peak and the second peak is at least 1 times the horizontal coordinate value of the first peak, then it is determined that the current sampling performance meets the requirements; Criterion-3 among the three spectrum criteria can be described as: Criterion-3: peak relative height judgment, the peak heights of the three frequency peaks in the spectrum are judged. If the maximum difference between the three frequency peaks does not exceed half of the minimum peak amplitude, it is judged that the current sampling performance meets Criterion-3.

2. The wavelength phase-shift sampling algorithm for adaptive harmonic anti-aliasing according to claim 1, characterized in that: The cyclic sampling determination is described as follows: first, N=N S As the phase shift reference coefficient, the interference pattern sampling is performed, and the sampling frame number is set to XN, N is greater than or equal to 8, X is greater than or equal to 1, and XN should be an integer. If the spectrum distribution of the interference intensity signal I0 obtained by the current sampling cannot meet the condition of the peak number P=3 after being processed by the sub-algorithm, it is necessary to increase the phase shift reference coefficient N=N+N D , N D The sampling amount is increased, and the interference pattern is sampled again, and the judgment after the next sampling is entered.

3. The wavelength phase-shift sampling algorithm for adaptive harmonic anti-aliasing according to claim 1, characterized in that: If criterion-1 and criterion-2 cannot be met, it is necessary to increase the phase shift reference coefficient N = N + N D , and sample again; When criterion 3 cannot be met, the measurement distance from the reference surface to the front surface of the measured object shall be changed, and the change in the measurement distance shall be no less than 30% of the last measurement distance.

4. The wavelength phase-shift sampling algorithm for adaptive harmonic anti-aliasing according to claim 1, characterized in that: Limit threshold N LIM is a positive number greater than or equal to 10 and less than or equal to 100. Set if N is less than or equal to the limit threshold N LIM , one sub-algorithm and three spectrum criteria are still executed, and whether to increase the sampling of the phase shift reference coefficient is determined according to the judgment result. If N is greater than the limit threshold N LIM Then exit the loop.

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