Speckle interferometry system and method

Abstract

The invention provides a speckle interference measurement system and method, and relates to an optical measurement system, and the system comprises a parallel light generation device which is used for generating parallel light; part of the parallel light is reflected by the reflector I, then irradiates the beam splitter prism I, and is divided into reflected light I and transmission light by the beam splitter prism I; part of the reflected light I irradiates a measured surface to generate diffuse reflection, so that reflected light II is formed; part of the reflected light II sequentially passes through the beam splitter prism I, the Fourier lens I, the diaphragm, the Fourier lens II and the beam splitter prism II and then is irradiated on the imaging surface of the camera as object light; part of the transmission light is reflected by the reflector II, the reflector III and the beam splitter prism II in sequence and then is irradiated on the imaging surface of the camera as reference light; the system provided by the invention is helpful to solve the problem of decorrelation effect caused by large in-plane deformation in the prior art.

Description

Technical Field

[0001] The present invention relates to an optical measurement system, and in particular to a speckle interferometry measurement system and method. Background Art

[0002] Laser irradiation causes diffuse reflection on the surface of a rough object, and random interference on the image plane produces speckle. Speckle interferometry measures the superposition of the complex amplitudes of the speckle pattern to obtain the fringe pattern corresponding to the deformation information of the object. The contrast of the fringes depends on the correlation of the speckle field. When the environment is disturbed and the object vibrates, resulting in out-of-plane and in-plane displacements, the speckle field will change in relative position. When the relative position change exceeds the correlation distance of the speckle field, it will cause decorrelation. The decorrelation caused by out-of-plane displacement can be suppressed by reducing the aperture stop and increasing the axial correlation distance of the speckle. The in-plane correlation distance of the speckle is relatively short. At the same time, since the in-plane deformation often has a large amplitude, it is easy to exceed the lateral correlation distance of the speckle, making the speckle field before and after deformation completely uncorrelated, making speckle interferometry measurement impossible.

[0003] Therefore, how to solve the de-correlation effect caused by large in-plane deformation is an urgent problem to be solved. Summary of the Invention

[0004] (1) Technical problems solved

[0005] In view of the shortcomings of the existing technology, the present invention provides a speckle interferometry measurement system and method to solve the technical problem of the decorrelation effect caused by large in-plane deformation in the existing technology.

[0006] (2) Technical solution

[0007] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0008] The present invention provides a speckle interferometry measurement system, comprising a parallel light generating device for generating parallel light;

[0009] Part of the parallel light is reflected by the reflector 1 and then irradiated on the beam splitter prism 1, and is split into the reflected light 1 and the transmitted light by the beam splitter prism 1;

[0010] Part of the reflected light 1 is diffusely reflected on the measured surface to form reflected light 2;

[0011] Partially reflected light 2 passes through beam splitter prism 1, Fourier lens 1, aperture, Fourier lens 2, and beam splitter prism 2 in sequence, and then illuminates the camera imaging surface as object light;

[0012] Part of the transmitted light is reflected by the second reflector, the third reflector and the second beam splitter prism in sequence, and then irradiates the camera imaging surface as reference light;

[0013] Among them, Fourier lens 1 and Fourier lens 2 constitute a 4f system, the measured surface is on the front focal plane of Fourier lens 1, the camera imaging surface is on the back focal plane of Fourier lens 2, and the aperture is on the spectrum plane of the 4f system.

[0014] Furthermore, the diaphragm is an aperture diaphragm with adjustable aperture.

[0015] Furthermore, the second reflector is connected to a driving structure 1 for adjusting its reflection angle.

[0016] Furthermore, the reflector 3 is connected to a driving structure 2 for adjusting its reflection angle.

[0017] Furthermore, the parallel light generating device includes a laser and a collimating beam expander, and the laser light generated by the laser is expanded by the collimating beam expander and then irradiated on the first beam splitter prism as parallel light.

[0018] A second aspect of the present invention further provides a method for performing speckle interferometry using the above system, the method comprising:

[0019] Obtain two pairs of speckle interferograms before and after deformation of the measured surface in the same coordinate system;

[0020] Get the amplitude correlation coefficient μ corresponding to each element in the search set and the selected block amc ,and <> is the statistical average, * is the conjugate operation;

[0021] (x, y) are the coordinates of a point in a two-dimensional coordinate system on the camera imaging surface 8, and the origin of the two-dimensional coordinate system is the intersection of the optical axis and the camera imaging surface 8 in the 4f system;

[0022] O(x,y) is the complex amplitude of the object light in the selected block, O shift (x, y) is the complex amplitude of the object light within the traversal block; the selected block is obtained by selecting the search box in the speckle interferogram obtained before the measured surface is deformed, and the traversal block is obtained by selecting the search box in the speckle interferogram obtained after the measured surface is deformed according to the set traversal rules. All traversal blocks selected by the search box in the speckle interferogram obtained after the measured surface is deformed according to the set traversal rules constitute the search set;

[0023] Then obtain the traversal block corresponding to the maximum value of all amplitude correlation coefficients and use it as the offset block;

[0024] Obtaining a corrected interferogram obtained after the interferogram corresponding to the traversed block is offset, and the center coordinates of the offset block in the corrected interferogram are the same as the center coordinates of the selected block;

[0025] The out-of-plane deformation of the measured surface is calculated using the corrected interference pattern and the corresponding interference pattern of the selected block.

[0026] Furthermore, the center coordinates of the selected block coincide with the origin of the two-dimensional coordinate system.

[0027] Furthermore, the speckle interferogram corresponding to the selected block is a speckle interferogram obtained before the measured surface is deformed.

[0028] (3) Beneficial effects

[0029] The present invention provides a speckle interferometry system and method. Compared with the prior art, the system has the following advantages:

[0030] 1. The illumination light is parallel and the imaging light path is telecentric in the object space to avoid the internal changes of the speckle field caused by the change of the illumination light angle when the object vibrates in the plane;

[0031] 2. Use spatial carrier technology to extract the complex amplitude of the light field from a single speckle interferogram to meet the needs of dynamic measurement scenarios;

[0032] 3. The amplitude correlation coefficient is proposed to effectively calculate the correlation of the speckle field before and after deformation, so as to determine the distance of in-plane displacement caused by vibration;

[0033] 4. The offset is calculated using the amplitude correlation coefficient of the object light field before and after deformation. After correction, the phase before and after deformation is subtracted to obtain a high-quality fringe pattern, thereby improving the deformation measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0035] Figure 1 A schematic diagram of the optical path structure of a speckle interferometry measurement system provided in Example 1 is shown;

[0036] In the picture:

[0037] 1. Parallel light generating device; 2. Reflector 1; 3. Beam splitter prism 1; 4. Fourier lens 1; 5. Aperture; 6. Fourier lens 2; 7. Beam splitter prism 2; 8. Camera imaging surface; 9. Reflector 2; 10. Reflector 3. DETAILED DESCRIPTION

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0039] Example 1

[0040] Combined with attachment Figure 1 This embodiment provides a speckle interferometry measurement system, including a parallel light generating device 1 for generating parallel light. In some embodiments, the parallel light generating device 1 includes a laser and a collimating beam expander, and the laser light generated by the laser is expanded by the collimating beam expander and then irradiated onto a beam splitter prism 3 as parallel light to obtain the parallel light.

[0041] The parallel light generated by the parallel light generating device 1 is first irradiated on the reflector 2, wherein part (or all) of the parallel light is reflected by the reflector 2 and then irradiated on the dichroic prism 3, and is divided into reflected light 1 and transmitted light by the dichroic prism 3; wherein, the reflector 2 is used to reflect the parallel light on the dichroic prism 3, and the reflection angle of the reflector 2 is adjustable. In some embodiments, it is connected to the driving structure 3 to adjust the reflection angle of the parallel light as needed.

[0042] A portion (or all) of the reflected light 1 reflected by the dichroic prism 13 is irradiated on the measured surface and is diffusely reflected to form reflected light 2.

[0043] Part (or all) of the reflected light 2 passes through the beam splitter prism 1 3, the Fourier lens 1 4, the aperture 5, the Fourier lens 2 6 and the beam splitter prism 2 7 in sequence, and then illuminates the camera imaging surface 8 as object light;

[0044] Part (or all) of the transmitted light from the parallel light passing through the beam splitter prism 1 3 is reflected in sequence by the reflector 2 9, the reflector 3 10 and the beam splitter prism 2 7, and then irradiates the camera imaging surface 8 as reference light.

[0045] Among them, Fourier lens 1 4 and Fourier lens 2 6 form a 4f system, and the measured surface is on the front focal plane of Fourier lens 1 4, the camera imaging surface 8 is on the back focal plane of Fourier lens 2 6, and the aperture 5 is on the spectrum plane of the 4f system.

[0046] In order to facilitate the adjustment of the size of the speckle particles, in this embodiment, the diaphragm 5 is an aperture diaphragm with an adjustable aperture.

[0047] In order to facilitate the adjustment of the respective reflection angles of the second reflector 9 and the third reflector 10, in this embodiment, the second reflector 9 is connected to the driving structure 1 for adjusting its reflection angle, and the third reflector 10 is connected to the driving structure 2 for adjusting its reflection angle.

[0048] That is, the reflector 2 9 and the reflector 3 10 can be adjusted separately through the driving structure 1 and the driving structure 2.

[0049] During the actual adjustment process, in this embodiment, the second reflector 9 is used to adjust the incident angle of the reference light, while the third reflector 10 is used to adjust the spot position of the reference light to ensure that the reference light and the object light spots coincide with each other, thereby maximizing the interference area and adjusting the carrier frequency introduced by the reference light.

[0050] refer to Figure 1 The speckle interferometry measurement system provided in this embodiment constitutes a measurement system of parallel light illumination and telecentric imaging light path, wherein, before the measured surface is deformed, the object light wave U irradiated on the camera imaging surface 8 o (x,y) and the reference light wave U r They are:

[0051]

[0052] Among them, A o(x,y) 、 A r 、 are the object light amplitude, object light phase, reference light amplitude, and reference light phase in the speckle interferogram obtained on the camera imaging surface 8 before the measured surface is deformed, i is an imaginary unit, 2πf x x is the carrier introduced in the x direction by the reference light in the speckle interferogram obtained on the camera imaging surface 8, f x is the frequency of the introduced carrier.

[0053] In this embodiment, since the reference light is set to be a plane wave, its amplitude and phase are constant (ie, A r 、 and 2πf x It is constant before and after the deformation of the measured surface) and is not affected by the deformation of the measured surface. Therefore, in order to simplify the derivation process, in this embodiment, it is set that the reference light only introduces the carrier in the x direction.

[0054] Among them, before the measured surface is deformed and the reference light is not introduced into the carrier, the speckle interference pattern I(x,y) obtained on the camera imaging surface 8 is:

[0055]

[0056] Then it is obvious that after the reference light is introduced into the carrier, the above formula becomes:

[0057] where k c =2πf x .

[0058] Then when the measured surface undergoes out-of-plane deformation, the speckle interference pattern I obtained on the camera imaging surface 8 is ′ (x,y) has:

[0059]

[0060] φ out (x, y) is the phase introduced when the measured surface undergoes out-of-plane deformation. When the measured surface undergoes out-of-plane deformation and causes in-plane deformation at the same time, the corresponding speckle interference pattern I shift (x,y) has:

[0061]

[0062] u and v are the deformation of the measured surface in the x-axis and y-axis directions respectively.

[0063] The speckle interferograms obtained before and after deformation are Fourier transformed, and a bandpass filter is designed according to the diameter of the system aperture and the size of the introduced carrier frequency. The high-frequency part containing the object light information is extracted through the bandpass filter and the inverse Fourier transform is performed to obtain the complex amplitude of the object light O(x, y) before the deformation of the measured surface and the complex amplitude of the object light O(x, y) after the deformation of the measured surface. shift (x,y).

[0064] O(x,y)=IFFT(FFT(I)*BandpassFilter);

[0065] O shift (x,y)=IFFT(FFT(I shift )*BandpassFilter);

[0066] Where BandpassFilter is a bandpass filter manually selected according to the carrier frequency.

[0067] Because in-plane vibration can cause an overall misalignment of the complex amplitude of the object light, the speckle field in the speckle interferogram obtained before and after deformation is no longer consistent, resulting in de-correlation. Quantifying the degree of de-correlation becomes the key to correction. To this end, an evaluation index such as the correlation coefficient is introduced to measure the consistency between the complex amplitude field before and after deformation, thereby evaluating the image misalignment caused by in-plane deformation and providing a quantitative basis for subsequent image matching and displacement compensation. In general, the correlation of the complex amplitude is measured by calculating the complex correlation coefficient, which is defined as follows:

[0068]

[0069] Where <> is the statistical average and * is the conjugate operation.

[0070] It can be found that if the complex correlation coefficient is calculated directly, it will be disturbed by the existence of phase difference. The phase difference comes from the translation of the initial random speckle field before and after deformation and the out-of-plane deformation of the object. However, the amplitude of the complex amplitude of the object light is not affected by the phase, but is modulated by the in-plane translation of the object light. Therefore, for the conjugate multiplication of the numerator, the modulus is first calculated and then averaged, that is, the amplitude correlation of the complex amplitude of the object light before and after deformation is calculated. This can avoid the influence of phase and obtain an accurate correlation coefficient. Therefore, this embodiment proposes the amplitude correlation coefficient μ amc ,make:

[0071]

[0072] After extracting the complex amplitudes of the object light from the speckle patterns before and after deformation, the present invention introduces the amplitude correlation coefficient as a matching criterion in the search algorithm. This coefficient measures the correspondence by comparing the similarity between the corresponding complex amplitude moduli (i.e., amplitudes) of two selected regions of equal size in the speckle interferograms obtained before and after deformation. Essentially, it serves as the "objective function" or "similarity evaluation index" in the search process.

[0073] Compared to the traditional complex correlation coefficient, the amplitude correlation coefficient is unaffected by phase differences and can more stably reflect the speckle translation caused by in-plane deformation. When traversing blocks at different offsets within the search area, it serves as an optimization criterion to evaluate the degree of match between each pair of regions. Ultimately, the offset that maximizes the amplitude correlation coefficient is selected as the optimal correspondence between the images, thereby effectively correcting speckle decorrelation.

[0074] That is, when performing speckle interferometry using the above-mentioned speckle interferometry system, the following method is used. Specifically, the method includes:

[0075] Obtain two pairs of speckle interferograms before and after deformation of the measured surface in the same coordinate system;

[0076] Get the amplitude correlation coefficient μ corresponding to each element in the search set and the selected block amc ,and <> is the statistical average, * is the conjugate operation;

[0077] (x, y) are the coordinates of a point in a two-dimensional coordinate system on the camera imaging surface 8, and the origin of the two-dimensional coordinate system is the intersection of the optical axis and the camera imaging surface 8 in the 4f system;

[0078] O(x,y) is the complex amplitude of the object light in the selected block, O shift(x, y) is the complex amplitude of the object light in the traversal block;

[0079] The selected block is obtained by selecting a speckle interferogram with a search box, and the traversed block is obtained by selecting a speckle interferogram with a search box according to the set traversal rules. All traversed blocks selected by the search box in the corresponding speckle interferogram according to the set traversal rules constitute the search set.

[0080] Then obtain the traversal block corresponding to the maximum value of all amplitude correlation coefficients and use it as the offset block;

[0081] Obtain a corrected interferogram obtained after the interferogram corresponding to the traversed block is offset, and the center coordinates of the offset block in the corrected interferogram are the same as the center coordinates of the selected block;

[0082] The out-of-plane deformation of the measured surface is calculated using the corrected interference pattern and the corresponding interference pattern of the selected block.

[0083] That is, by obtaining the center coordinates of the offset block and comparing them with the center coordinates of the selected block, the offset of the speckle interferogram corresponding to the offset block relative to the speckle interferogram corresponding to the selected block can be obtained (the offset is the difference between the center coordinates of the offset block and the center coordinates of the selected block). Based on the offset, the speckle interferogram corresponding to the offset block can be shifted by the offset so that the center coordinates of the offset block in the corrected interferogram obtained after the shift are the same as the center coordinates of the selected block in the speckle interferogram corresponding to the selected block (without offset). Then, the corrected interferogram and the interferogram corresponding to the selected block can be used to calculate the out-of-plane deformation of the measured surface.

[0084] Among them, the aforementioned setting traversal rule in this embodiment is specifically to set the search box by the obtained speckle interference Figure 1 The search is started from the corner (such as the lower left corner) of the speckle interferogram. For example, in this embodiment, the search box is a rectangular box. Initially, it is located at a corner of the speckle interferogram and one corner of the two overlaps. At the same time, the search box is located in the speckle interferogram. Then, the block selected by the box is used as the traversal block, and the amplitude correlation coefficient is calculated with the selected block. At the same time, the selected block is used as an element in the search set. When traversing, the search box is stepped to one side of the speckle interferogram with a step distance of 1 pixel, and a new traversal block is obtained after the step. When the search box steps to one corner and overlaps with a corner of the speckle interferogram ( For example, the lower right corner of the search box, the search box steps upward by 1 pixel to obtain a new traversal block, and then gradually traverses to the left with a step distance of 1 pixel to obtain a new traversal block. After the left side of the search box coincides with the left side of the speckle interferogram, the search box steps upward by 1 pixel to obtain a new traversal block, and then gradually traverses to the right with a step distance of 1 pixel to obtain a new traversal block. The above steps are repeated until the upper right corner of the search box coincides with the upper right corner of the speckle interferogram, completing the traversal. The step distance can be set as needed, for example, 2 pixels, 3 pixels, etc., and will not be described in detail in this embodiment.

[0085] The speckle interferogram corresponding to the selected block can be the speckle interferogram obtained before the object is deformed or after the object is deformed, which does not affect the speckle interferometry measurement, and the center coordinates of the selected block coincide with the origin of the two-dimensional coordinate system.

[0086] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or apparatus comprising the element.

[0087] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A speckle interferometry system, characterized in that: It includes a parallel light generating device for generating parallel light; Part of the parallel light is reflected by the reflector 1 and then irradiated on the beam splitter prism 1, and is split into the reflected light 1 and the transmitted light by the beam splitter prism 1; Part of the reflected light 1 is diffusely reflected on the measured surface to form reflected light 2; Partially reflected light 2 passes through beam splitter prism 1, Fourier lens 1, aperture, Fourier lens 2, and beam splitter prism 2 in sequence, and then illuminates the camera imaging surface as object light; Part of the transmitted light is reflected by the second reflector, the third reflector and the second beam splitter prism in sequence, and then irradiates the camera imaging surface as reference light; Among them, Fourier lens 1 and Fourier lens 2 constitute a 4f system, the measured surface is on the front focal plane of Fourier lens 1, the camera imaging surface is on the back focal plane of Fourier lens 2, and the aperture is on the spectrum plane of the 4f system.

2. The speckle interferometry system according to claim 1, characterized in that: The diaphragm is an aperture diaphragm with adjustable aperture.

3. The speckle interferometry system according to claim 1, characterized in that: The second reflector is connected to a driving structure 1 for adjusting its reflection angle.

4. The speckle interferometry system according to claim 1, characterized in that: The reflector 3 is connected to a driving structure 2 for adjusting its reflection angle.

5. The speckle interferometry system according to claim 1, characterized in that: The parallel light generating device includes a laser and a collimating beam expander, and the laser light generated by the laser is expanded by the collimating beam expander and then irradiated on the first beam splitter prism as parallel light.

6. A method for speckle interferometry using the system according to any one of claims 1 to 5, characterized in that: The method comprises: Obtain two pairs of speckle interferograms before and after deformation of the measured surface in the same coordinate system; Get the amplitude correlation coefficient μ corresponding to each element in the search set and the selected block amc ,and <> is the statistical average, * is the conjugate operation; (x, y) are the coordinates of a point in a two-dimensional coordinate system on the camera imaging surface 8, and the origin of the two-dimensional coordinate system is the intersection of the optical axis and the camera imaging surface 8 in the 4f system; O(x,y) is the complex amplitude of the object light in the selected block, O shift (x, y) is the complex amplitude of the object light within the traversal block; the selected block is obtained by selecting the search box in the speckle interferogram obtained before the measured surface is deformed, and the traversal block is obtained by selecting the search box in the speckle interferogram obtained after the measured surface is deformed according to the set traversal rules. All traversal blocks selected by the search box in the speckle interferogram obtained after the measured surface is deformed according to the set traversal rules constitute the search set; Then obtain the traversal block corresponding to the maximum value of all amplitude correlation coefficients and use it as the offset block; Obtaining a corrected interferogram obtained after the interferogram corresponding to the traversed block is offset, and the center coordinates of the offset block in the corrected interferogram are the same as the center coordinates of the selected block; The out-of-plane deformation of the measured surface is calculated using the corrected interference pattern and the corresponding interference pattern of the selected block.

7. The method according to claim 6, characterized in that The center coordinates of the selected block coincide with the origin of the two-dimensional coordinate system.

8. The method according to claim 6, characterized in that The speckle interferogram corresponding to the selected block is the speckle interferogram obtained before the measured surface is deformed.

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