CN115265399A - Dynamic speckle interferometry system and method - Google Patents

Dynamic speckle interferometry system and method Download PDF

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CN115265399A
CN115265399A CN202210910096.XA CN202210910096A CN115265399A CN 115265399 A CN115265399 A CN 115265399A CN 202210910096 A CN202210910096 A CN 202210910096A CN 115265399 A CN115265399 A CN 115265399A
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phase
phase shift
speckle
light
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CN115265399B (en
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赵宏
杜怡君
赵自新
李均祥
樊晨
周孟航
张旭超
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Xian Jiaotong University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
    • G01B11/161—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by interferometric means
    • G01B11/162—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by interferometric means by speckle- or shearing interferometry
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors

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Abstract

The invention discloses a dynamic speckle interferometry system and a method, wherein the speckle interferometry system based on an SLM is built, and the positions of double CCDs are adjusted to make the shooting areas of the double CCDs consistent; fine-tuning the axial position of the second camera CCD2 by using a micro-motion displacement platform, and determining the optimal axial position of the second camera CCD2 by using a zero-mean normalized cross-correlation coefficient; loading a phase diagram containing characteristic information on the SLM, and establishing projection mapping between double CCD imaging surfaces to obtain a homography matrix; loading a phase shift phase diagram on the SLM, synchronously shooting and recording speckle interference fields of the detected surface in various deformation states by the double CCDs, converting pictures shot by the second camera CCD2 into a pixel coordinate system of the first camera CCD1 according to a homography matrix and pixel coordinate conversion, and obtaining information containing deformation phase information by utilizing a multi-frame phase shift demodulation algorithm
Figure DDA0003773781030000011
The wrapped phase diagram is subjected to phase unwrapping to calculate the deformation, and the dynamic speckle interferometry is completed. The requirements of actual engineering measurement occasions on real-time performance and accuracy can be met.

Description

Dynamic speckle interferometry system and method
Technical Field
The invention belongs to the technical field of rough surface dynamic deformation high-precision measurement, and particularly relates to a dynamic speckle interferometry system and method.
Background
In the fields of mechanical engineering, aerospace, biomedicine and the like, real-time monitoring and analysis are carried out aiming at micro deformation, stress strain and surface defect evolution of core parts, and the method has important significance for ensuring system safety and reliability and guiding aspects such as model design and production process control. Speckle interferometry is widely used as a non-contact optical measurement method capable of performing high-precision full-field displacement and deformation evaluation tracking on rough surfaces.
The speckle interferometry is a technology capable of realizing full-field and nondestructive measurement on an optically rough surface, and the object plane change measurement is realized by analyzing the phase change of a speckle interferogram. The basic idea is as follows: the wavelets in all directions, which are reflected by the rough surface of the object, of the laser are mutually interfered to form original speckles, the original speckles and the reference light are interfered to form speckle interference fields, the original speckle interference fields before and after the object is deformed or speckle interference fringes obtained through relevant calculation are subjected to phase analysis, and state change information of the measured object can be obtained. Different from the traditional interference measurement process, speckle in the speckle interference measurement is used as a carrier of surface information of a measured object and is coherent noise which can seriously influence phase solving precision, so that the difference exists between actual measurement precision and theoretical precision.
The multiframe phase shift demodulation method is a mainstream method for eliminating speckle noise and improving speckle interference phase solving precision, generates phase shift based on the change of an optical path difference between a test light path and a reference light path, and is widely applied to the field of high-precision measurement. The phase shift method can be divided into a temporal phase shift method and a spatial phase shift method according to the time-space domain relationship of the phase shift fringe image. The time phase shift method is used for acquiring a multi-frame phase shift interferogram on a time sequence, sacrifices the measurement efficiency and is suitable for static measurement occasions; the spatial phase shift method can generate pixel-level spatial phase shift by introducing carrier waves or acquire phase shift interferograms at different spatial positions by using a light splitting mode, and the mode is suitable for dynamic measurement occasions, but the precision and the spatial resolution are easily influenced.
In speckle interferometry, a time phase shift method needs to collect multiple frames of phase shift images before and after a measured object is deformed, obtain speckle phases before and after deformation respectively by using a multi-step phase shift formula, and obtain deformation information after subtraction. Based on the thought, the phase shift process of modes such as 'N + N', 'N + 1', 'N + 2' and the like is generated, but is limited by the phase shift process, is only suitable for static deformation measurement, and cannot meet the dynamic measurement requirement. The spatial phase shift method can be subdivided into a spatial carrier phase shift method and a method for instantaneously acquiring multi-frame phase shifts by using a spatial optical path. The spatial carrier phase shift is to decompose a single-frame carrier fringe pattern into multiple-frame phase shift subgraphs and then use a multiple-frame phase shift algorithm to calculate the phase. In order to meet the assumption of consistency of background terms and phase terms, a high-frequency carrier wave must be generated in a single speckle, namely, the size and the carrier frequency of the speckle need to be strictly controlled and coordinated, so that the system is high in construction difficulty, and the light energy utilization rate and the spatial resolution are poor. The phase shift technology based on the spatial optical path difference is to distribute a plurality of phase shift interference fields at different spatial positions, thereby realizing the synchronous acquisition of multi-frame phase shift. The phase-shift interferograms can be recorded by a plurality of cameras respectively, or can be recorded by only using a single camera, a Michelson optical path is used as an interference field replication structure in the prior art, a linear polarization array is used for collecting multi-frame speckle interferograms on a single CCD based on a polarization phase-shift principle, and dynamic measurement of deformation is realized. However, the single CCD acquisition may cause the reduction of spatial resolution and measurement field range, and limit the spatial frequency of the deformation to be measured, and the depolarization phenomenon caused by the scattering property of the measured surface and the adjustment error of the polarizing element may affect the phase shift consistency and accuracy, resulting in a very small adjustable range of the phase shift amount and reducing the accuracy of the phase shift demodulation. Therefore, in summary, the use of multiple cameras to record multiple frames of speckle interference phase shift maps is a better way to take account of spatial resolution, measurement accuracy and speed, but the premise of achieving high-accuracy measurement lies in establishing a more practical and accurate phase shift mechanism, ensuring that the spatial positions of the multiple cameras are consistent, and establishing a conversion relation between pixel point coordinates, so that the spatial correlation between multiple phase shift speckle fields can be effectively ensured, which is the basis for performing multi-frame phase shift demodulation calculation, but the problem in the relevant research of speckle interference measurement is not elaborated in detail.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a dynamic speckle interferometry system and method aiming at the deficiencies in the prior art, which realizes Spatial phase-shifting speckle interferometry based on polarization multiplexing by means of the electric control birefringence effect of a phase Liquid Crystal Spatial Light Modulator (LC-SLM), and solves the contradiction between the measurement accuracy and the measurement speed in the traditional speckle interferometry phase-shifting technology; by optimizing a polarization state modulation strategy, the limitation of the depolarization problem of the scattering surface on the phase shift amount is overcome, so that accurate 90-degree phase shift can be realized; meanwhile, a collection mode of recording a phase shift interference pattern by using double CCDs is adopted, so that the problem of spatial resolution loss in the existing spatial phase shift speckle interference measurement method is solved; in addition, by means of speckle correlation, characteristic point matching and the like, the problem of accurate alignment of the space positions of the double CCDs in speckle interference is solved; the whole method can completely meet the requirements of real-time performance and accuracy of actual engineering measurement occasions.
The invention adopts the following technical scheme:
a system of a dynamic speckle interferometry method comprises a first depolarization beam splitter prism, wherein laser is divided into reference light and object light by the first depolarization beam splitter prism;
the reference light forms plane waves after being expanded and collimated, the plane waves are irradiated on the target surface of the spatial light modulator through the second depolarizing beam splitter prism and reflected after the polarization direction of the reference light is adjusted through the first polaroid, the target surface of the spatial light modulator sequentially passes through the third lens, the third depolarizing beam splitter prism, the second lens and the polarizing beam splitter prism to reach the image surfaces of the first camera and the second camera, and meanwhile, the target surface of the spatial light modulator is imaged to the image surfaces of the first camera and the second camera;
the object light irradiates to the surface to be measured to form speckles through the divergent lens after being refracted by the first reflecting mirror and the second reflecting mirror, the polarization state of a speckle field is adjusted through the second polaroid, the light intensity after passing through the polarization beam splitting prism is kept consistent, the back focal length of the imaging lens is prolonged through the first lens and the second lens, the image is formed to the image surfaces of the first camera and the second camera, the interference is generated with the reference light, and the speckle interference field is formed.
Another technical solution of the present invention is a dynamic speckle interferometry method, comprising the steps of:
s1, building a two-channel spatial phase shift speckle interferometry system based on a spatial light modulator, and adjusting the shooting areas of a first camera and a second camera in the two-channel spatial phase shift speckle interferometry system to be consistent;
s2, determining the optimal axial position of the second camera by utilizing a zero-mean normalized cross-correlation coefficient based on the two-channel space phase shift speckle interferometry system constructed in the step S1;
s3, adjusting the position of the second camera according to the optimal axial position obtained in the step S2, loading a phase diagram containing characteristic information on a spatial light modulator, and establishing projection mapping between the first camera and the imaging surface of the second camera to obtain a homography matrix HM;
s4, loading a phase shift phase diagram on the spatial light modulator, adjusting and compensating the phase shift amount delta by changing the gray value gv of the phase diagram to realize 90-degree phase shift, synchronously shooting and recording speckle interference fields of the measured surface in various deformation states by the first camera and the second camera, converting the picture shot by the second camera into the pixel coordinate system of the first camera according to the homography matrix HM obtained in the step S3 and the pixel coordinate conversion, and obtaining information containing deformation phase by using a multi-frame phase shift demodulation algorithm
Figure BDA0003773781010000041
The wrapped phase diagram is subjected to phase unwrapping to calculate the deformation, and the dynamic speckle interference is completedTo measurements.
Specifically, in step S1, the position of the first camera is adjusted to make the first camera and the measured surface conform to the object-image conjugate relationship and fixed, the reference light path is shielded, and a speckle pattern Sp0 is recorded; and adjusting the position of the second camera to make the shooting area of the second camera consistent with that of the first camera.
Specifically, in step S2, the axial position of the second camera is finely adjusted, and a series of speckle patterns Sp1 and Sp2 … Spn are recorded respectively; based on speckle correlation, a zero-mean normalized cross-correlation coefficient is used for searching a corresponding point P1 … Pn of an Sp0 center pixel point P0 in Sp1 and Sp2 … Spn, a correlation value is recorded, when the value of the zero-mean normalized cross-correlation coefficient reaches the maximum value, the axial position difference between a second camera and a first camera is minimum, and the position of the second camera is fixed to serve as the optimal axial position of the second camera.
Further, the zero-mean normalized cross-correlation coefficient C is:
Figure BDA0003773781010000042
wherein R is0Representing a reference image, R1Which represents the image to be evaluated and is,
Figure BDA0003773781010000043
and
Figure BDA0003773781010000044
the average value of each image is represented by M, N, which is the number of pixels.
Specifically, in step S3, the establishing of the homography matrix HM in the image registration process based on the feature points specifically includes:
adding a third polaroid into the reference light path which is built in the step S1 and is based on the speckle interferometry system of the spatial light modulator, wherein the included angle between the polarization direction of the third polaroid and the polarization direction of the e light is 45 degrees, extracting the component of the e light in the polarization direction of the third polaroid, and enabling the two beams of light which pass through the polarization beam splitter prism to contain e light information, so that the first camera and the second camera respectively record characteristic phase diagrams T1 and T2; and searching a series of corresponding homonymous points in the characteristic phase diagrams T1 and T2 based on a characteristic point detection algorithm, and calculating to obtain a homography matrix HM.
Further, the homography matrix HM is:
Figure BDA0003773781010000051
wherein, a11、a12、a21、a22For relative rotation and skew between the imaging planes of the two cameras, tx、tyA single offset in the x, y directions.
Specifically, in step S4, the corresponding relationship between the phase shift δ and the phase diagram gray level gv is as follows:
Figure BDA0003773781010000052
ne(gv) denotes the e-light refractive index, n, of the SLM at different input grayscalesoThe refractive index of o light, the thickness of a liquid crystal molecular layer, and the wavelength of laser light are shown in the specification; synchronously shooting and recording speckle interference fields of the detected surface in various deformation states at intervals by using a first camera and a second camera; at any two sampling stages, with I1、I2、I3、I4Representing the intensity distribution of the interference field recorded by the first camera and the second camera; converting the picture shot by the second camera into a pixel coordinate system of the first camera according to the homography matrix HM and the pixel coordinate conversion formula obtained in the step S3 to obtain I3 H、I4 H(ii) a Subtracting every two of the four speckle interference patterns to obtain a three-frame phase-shift speckle interference fringe pattern sub1、sub2、sub3(ii) a Obtaining information containing deformation phase by using multi-frame phase shift demodulation algorithm
Figure BDA0003773781010000053
Wrapped phase map of (a); and performing unwrapping operation to obtain the real phase distribution condition.
Further, by using a multi-frame phase shift demodulation algorithm, taking a phase shift fringe pattern sequence as an original variable, and rapidly realizing phase demodulation through matrix decomposition, wherein a phase shift fringe pattern matrix is as follows:
Figure BDA0003773781010000054
wherein a is a background light intensity vector, c and s are respectively used for describing sine and cosine vector components of a modulation phase term, u and v are respectively used for describing sine and cosine vectors of a phase shift term, and T represents transposition operation.
Further, the true phase Φ (x, y) obtained by phase unwrapping is:
φ(x,y)＝ψ(x,y)+2k(x,y)π
where (x, y) is the pixel position coordinate, ψ (x, y) is the wrapping phase, and k (x, y) represents the wrapping order.
Compared with the prior art, the invention at least has the following beneficial effects:
the dynamic speckle interferometry system adopts the depolarization dispersion prisms BS1, BS2 and BS3 to split and convert light and avoid the change of polarization state; the polaroid P1 is used for adjusting the polarization state of the reference light to enable the reference light to simultaneously comprise two polarization components of o light and e light, wherein only the e light can be modulated by the SLM to generate phase change, and at the moment, the reference light comprises two components which are vertical to the two polarization states and have phase delay, and the two components are respectively recorded by the two CCDs after being split by the PBS, so that spatial phase shift is generated, and the influence of a time phase shift process on the measurement speed is avoided; the polaroid P2 is used for adjusting the polarization state of the speckle field, and aims to adjust the light intensity of the object light field on different CCDs to be matched with the reference light intensity so as to obtain high-contrast speckle interference fringes; the imaging system composed of the lenses L3 and L2 aims at imaging the SLM target surface to the CCD, so that a characteristic phase diagram can be effectively recorded, and the registration process of pixel coordinates of the double CCD is facilitated; the imaging system composed of the lenses L1 and L2 aims to prolong the back focal length of the imaging lens, so that more optical elements are conveniently arranged between the imaging lens and the CCD.
A dynamic speckle interferometry method realizes space phase shift speckle interferometry based on polarization multiplexing by means of an electric control birefringence effect of a phase liquid crystal spatial light modulator, and solves the contradiction between measurement precision and measurement speed in the traditional speckle interferometry phase shift technology; by optimizing the polarization state modulation strategy, the limitation of the depolarization problem of the scattering surface on the phase shift amount is overcome, so that accurate 90-degree phase shift can be realized; meanwhile, the acquisition mode of recording the phase shift interferogram by the double CCD is adopted, so that the problems of spatial resolution and measurement field range loss in the existing spatial phase shift speckle interferometry are solved; in addition, by means of speckle correlation, characteristic point matching and the like, the problem of accurate alignment of the space positions of the double CCDs in speckle interference is solved; and on the basis of synchronously acquiring the phase shift interference field by using the double channels, a three-step phase shift fringe pattern is obtained, so that a multi-frame random phase shift demodulation algorithm can be combined to realize phase recovery with higher precision. The whole method can completely meet the requirements of real-time performance and accuracy of actual engineering measurement occasions.
Further, the speckle pattern Sp0 recorded in the step S1 is used as an evaluation reference of the optimal axial position in the step S2, and the shot areas of the second camera CCD and the first camera CCD are roughly adjusted manually to be consistent, so that the complexity of subsequent registration work can be reduced, and the registration efficiency can be improved.
Further, in the step S2, the axial position of the CCD is determined by evaluating the correlation near the central pixel point between the speckle fields recorded by the second camera CCD at different positions and the reference speckle field, so that the longitudinal correlation between the speckle fields collected by the second camera and the first camera can be recovered, which is the basis for performing the deformation phase recovery based on the multi-camera collected phase-shift speckle interference field.
Furthermore, the zero-mean normalized cross-correlation coefficient is adopted to calculate the speckle correlation, so that the method has the advantages of simple and quick calculation, low possibility of being influenced by background light intensity change and environmental noise, and better robustness and reliability.
Furthermore, the purpose of establishing the homography conversion matrix of the first camera CCD and the second camera CCD based on the image feature point registration process is to realize the alignment of the pixel coordinate positions of the two cameras and ensure the complete consistency of the shooting areas, thereby recovering the transverse correlation between the speckle fields collected by the second camera CCD and the first camera CCD and being the basis for carrying out deformation phase recovery based on the phase-shift speckle interference field collected by the multiple cameras.
Further, the homography matrix HM clearly describes the relative pose relationship between the two cameras, and can be used for converting between the pixel coordinates of the two cameras to realize high-precision alignment.
Further, the phase shift amount can be set according to actual needs by loading phase shift phase diagrams with different gray scales to the SLM, and the closer the phase shift amount is to 90 degrees theoretically, the higher the phase solving accuracy is. Moreover, the object of transforming the pixel coordinates of the interferogram recorded by the second camera CCD on the basis of a homography matrix is to obtain I3 H、I4 HAnd obtaining three-frame phase-shift speckle interference fringe pattern sub by subtraction operation1、sub2、sub3Therefore, a three-step phase shift process can be realized based on a two-channel acquisition mode, and compared with a two-channel two-step phase shift process in the existing method, the demodulation precision is further improved.
Furthermore, the influence of inconsistent actual phase shift amount on demodulation precision caused by environmental disturbance can be avoided by utilizing a multi-frame random phase shift demodulation algorithm. The multi-frame demodulation algorithm based on the matrix decomposition principle has the advantages of high efficiency and high precision, and can effectively give consideration to the rapidity and the accuracy of measurement, but is not limited to the algorithm.
In conclusion, the invention solves the problem that the measurement speed and the measurement precision are difficult to be considered in the rough surface deformation measurement, realizes the spatial phase shift speckle interference measurement based on polarization multiplexing by utilizing the SLM electric control birefringence effect, has adjustable phase shift amount, and overcomes the problem that the phase shift amount is easily influenced by the depolarization of the scattering surface and the adjustment error of the polaroid in the polarization phase shift technology; the double CCD is adopted to record the phase shift interference pattern, so that the spatial resolution and the measurement field range are not lost; the requirements of actual engineering measurement occasions on real-time performance and accuracy can be completely met.
The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.
Drawings
FIG. 1 is a schematic diagram of the optical path of the present invention;
FIG. 2 is a schematic diagram of an optical path for completing calibration of a spatial position relationship of a dual CCD and a characteristic phase diagram loaded on an SLM;
FIG. 3 is a measurement flow chart of the method of the present invention;
FIG. 4 is a speckle interference phase shift fringe pattern of a measured object in a certain deformation state;
fig. 5 shows the measurement results for different deformation phases.
Wherein: BS. depolarizing beam splitter prism, p polarizer, m reflector, l lens, pbs polarizing beam splitter prism, slm, spatial light modulator, ccd, camera.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "one side", "one end", "one side", and the like indicate orientations or positional relationships based on those shown in the drawings, merely for convenience of description and simplification of description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed in a particular orientation, and be operated, and therefore, are not to be construed as limiting the present invention. In addition, in the description of the present invention, "a plurality" means two or more unless otherwise specified.
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
It will be understood that the terms "comprises" and/or "comprising," when used in this specification and the appended claims, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
It is also to be understood that the terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in this specification and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
It should be further understood that the term "and/or" as used in this specification and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.
Various structural schematics according to the disclosed embodiments of the invention are shown in the drawings. The figures are not drawn to scale, wherein certain details are exaggerated and possibly omitted for clarity of presentation. The shapes of various regions, layers and their relative sizes and positional relationships shown in the drawings are merely exemplary, and deviations may occur in practice due to manufacturing tolerances or technical limitations, and a person skilled in the art may additionally design regions/layers having different shapes, sizes, relative positions, according to actual needs.
The invention provides a dynamic speckle interferometry system and a method, which realize space phase shift based on polarization multiplexing by means of an electric control birefringence effect of an SLM (spatial light modulator), wherein the phase shift amount can be controlled by loading different phase images to the SLM, and the phase shift interferograms are synchronously acquired by two cameras, so that the real-time measurement requirement is met and the space resolution is not lost; on the basis of finishing the alignment of the space position of the double cameras and the establishment of the pixel coordinate conversion relation, the deformation is accurately demodulated by combining a multi-frame phase shift algorithm, and the dynamic and high-precision detection of the deformation is realized.
Referring to fig. 1, the dynamic speckle interferometry system of the present invention includes a first depolarizing beam splitter BS1, a second depolarizing beam splitter BS2, a third depolarizing beam splitter BS3, a first polarizer P1, a second polarizer P2, a spatial light modulator SLM, a first reflector M1, a second reflector M2, a first lens L1, a second lens L2, a third lens L3, a polarizing beam splitter PBS, a first camera CCD1, and a second camera CCD2.
Laser emitted by the laser is divided into reference light and object light through a first depolarization beam splitter prism BS 1; the reference light forms plane waves after being expanded and collimated, the plane waves are irradiated on the target surface of the spatial light modulator SLM through the second depolarization beam splitter BS2 after the polarization direction of the reference light is adjusted through the first polaroid P1, the second depolarization beam splitter BS2 images the target surface of the spatial light modulator SLM to the image surfaces of the first camera CCD1 and the second camera CCD2 through the third lens L3, the third depolarization beam splitter BS3 and the second lens L2 in sequence, and the matching of pixel coordinates of the first camera CCD1 and the second camera CCD2 is completed by loading a characteristic phase diagram to the spatial light modulator SLM.
After the object light is refracted by the first reflecting mirror M1 and the second reflecting mirror M2, the object light irradiates to the surface to be measured through the diverging lens to form speckles, the imaging lens adjusts the polarization state of a speckle field through the second polaroid P2, the object reference light intensity is matched, the back focal length of the imaging lens is prolonged through the first lens L1, the third depolarizing beam splitter BS3 and the second lens L2, so that more optical elements are arranged, the coupling of the imaging lens and a subsequent light path is reduced through the arrangement, and the imaging lens is convenient to replace according to different measured objects or measurement requirements.
The spatial light modulator SLM is located in the reference light path for achieving spatial phase shift by adjusting the reference light phase. The electrically controlled birefringence effect of the SLM means that the liquid crystal molecules have different refractive indexes for ordinary light (o) and extraordinary light (e), and as the input voltage changes (i.e. changes in the loaded phase pattern gray scale), the refractive index of e light changes and the refractive index of o light keeps consistent, i.e. the SLM only has a phase modulation effect on e light.
Therefore, a first polarizer P1 is arranged in front of the spatial light modulator SLM, the included angle between the polarization angle of the first polarizer P1 and the polarization direction of the e light is 45 degrees, at this time, the light beam incident to the spatial light modulator SLM is linearly polarized light simultaneously containing two polarization components of the o light and the e light, a phase shift phase diagram loaded on the spatial light modulator SLM only influences the phase of the e light component, the phase difference between the e light component and the o light component can be changed by loading phase diagrams corresponding to different phase shift components,
the correspondence between the phase shift magnitude δ and the phase map gray scale (gray value, gv) is:
Figure BDA0003773781010000111
wherein n ise(gv) denotes the e-light refractive index, n, of the SLM at different input grey levelsoDenotes the refractive index of o light (which does not vary with the input signal), d denotes the thickness of the liquid crystal molecular layer, and λ denotes the wavelength of the laser light.
And finally, separating the two components by using a Polarization Beam Splitter (PBS), respectively interfering with different polarization components of the same speckle field, and synchronously recording two phase-shift speckle interference fields by using a first camera CCD1 and a second camera CCD2 so as to meet the dynamic measurement requirement.
Before and after the measured surface is deformed, the speckle interference field intensity distribution recorded by the first camera CCD1 and the second camera CCD2 is respectively expressed as:
CCD1
Figure BDA0003773781010000112
CCD2
Figure BDA0003773781010000113
wherein I represents the speckle interference field intensity, A, B are respectively a background term and a modulation term,
Figure BDA0003773781010000114
is the speckle phase, delta is the phase difference, i.e., phase shift component, between the e and o light components,
Figure BDA0003773781010000115
is the deformation phase.
Referring to fig. 3, the dynamic speckle interferometry system and method of the present invention includes the following steps:
s1, building a speckle interferometry system based on an SLM, adjusting the position of a first camera CCD1 to enable the first camera CCD1 and a measured surface to be in accordance with an object-image conjugate relation and to be fixed, shielding a reference light path, and recording a speckle pattern Sp0; adjusting the position of the second camera CCD2 to make the shooting area of the second camera CCD2 consistent with that of the first camera CCD 1;
s2, fine-tuning the axial position of the second camera CCD2 by using a micro-motion displacement platform, and determining the optimal axial position of the second camera CCD2 by using a zero-mean normalized cross-correlation coefficient;
finely adjusting the axial position of the second camera CCD2 and respectively recording a series of speckle patterns Sp1 and Sp2 … Spn; based on speckle correlation, a Zero-mean normalized cross correlation coefficient (ZNCC) is used for searching a corresponding point P1 … Pn of an Sp0 center pixel point P0 in Sp1 and Sp2 … Spn, and recording a correlation value, when the ZNCC value is maximum, the axial position difference between a second camera CCD2 and a first camera CCD1 is minimum, and the position of the second camera CCD2 is fixed;
the formula for carrying out correlation evaluation according to ZNCC is as follows:
Figure BDA0003773781010000121
wherein C represents the correlation calculation result, R0Represents a radicalQuasi image, R1Which represents the image to be evaluated and is,
Figure BDA0003773781010000122
and with
Figure BDA0003773781010000123
The average value of each image is represented by M, N, which is the number of pixels.
R0In the process of searching a corresponding point P1 … Pn for a small correlation evaluation area which is selected from a speckle pattern SP0 and has the width of B and the center pixel point P0 as the center, a search range which has the width of B and the center pixel point of the speckle pattern Sp1 … Spn as the center is set, and each pixel point in the search range has a neighborhood which has the width of B, namely corresponds to R1Calculating R point by point1And R0Correlation of (2), when the correlation is maximum R1The central point of (1) is the corresponding point P1 … Pn of P0 in the speckle pattern Sp1 … Spn, and the correlation value at this time is recorded. And finally, comparing the n correlation calculation results, and taking the position corresponding to the maximum value as the optimal axial position of the second camera CCD2.
S3, loading the phase diagram containing rich characteristic information on an SLM, determining a double-CCD pixel coordinate conversion relation, namely establishing projection mapping between imaging surfaces of a first camera CCD1 and a second camera CCD2 to obtain a homography matrix HM;
referring to fig. 2, the image only includes two gray-scale values 0 and 255, the resolution is consistent with the SLM resolution, and after the image is loaded on the SLM image plane, the image is respectively imaged on the image planes of the two CCDs through the 4F system composed of L2 and L3, the edge shape of the characteristic phase diagram can be clearly observed on the CCDs, and the registration of the pixel coordinates of the dual CCDs can be performed by using the edge and corner information. The characteristic phase map is not limited to the pattern given in fig. 2, but only needs to contain sufficiently rich edges and corners.
The image of the first camera CCD1 is selected as a reference image, and the process of projecting the image shot by the second camera CCD2 to the image coordinates of the first camera CCD1 is expressed as follows:
Figure BDA0003773781010000131
Figure BDA0003773781010000132
where HM is a homography matrix of 3 × 3, (u)1,v1,1)、(u2,v2And 1) denote homogeneous pixel coordinates of the first camera CCD1 and the second camera CCD2, respectively.
And establishing the homography matrix by using an image registration process based on the characteristic points.
Firstly, because the SLM has no modulation effect on o light components, a third polarizing film P3 needs to be added into a reference light path, referring to fig. 2, an included angle between the polarization direction of the third polarizing film P3 and the polarization direction of e light is 45 °, which is equivalent to extracting the component of e light on the polarization direction of the third polarizing film P3, so that two beams of light passing through the polarization splitting prism PBS can be ensured to contain e light information, and the purpose is to enable two CCDs to record characteristic phase diagrams, which are respectively represented as T1 and T2;
secondly, searching corresponding homonymous points in the T1 and the T2 based on a characteristic point detection algorithm, and substituting coordinates of the series of homonymous points into the formula to obtain a homography matrix HM through calculation.
And S4, loading a phase shift phase diagram on the SLM, namely, introducing a phase shift delta between speckle interference fields shot by the first camera CCD1 and the second camera CCD2, wherein the double CCDs can simultaneously record the phase shift interference fields without carrying out an additional phase shift process under a certain state corresponding to the surface to be measured, namely, a phase shift diagram sequence can be obtained by single-trigger shooting, and the dynamic measurement speed is not limited by the phase shift process and only depends on the camera acquisition time.
The measurement procedure is described as follows:
the state of the surface to be measured continuously changes, and the two CCDs synchronously shoot and record speckle interference fields of the surface to be measured in various deformation states at certain sampling intervals. In any two sampling stages, the intensity distribution of the interference field recorded by the first camera CCD1 and the second camera CCD2 can utilize I in S11、I2、I3、I4And (4) showing.
Converting the picture shot by the second camera CCD2 into a pixel coordinate system of the first camera CCD1 according to the homography matrix HM and the pixel coordinate conversion formula obtained in the step S3 to obtain I3 H、I4 H。
Subtracting every two of the four speckle interference patterns to obtain three frames of phase-shift speckle interference fringe patterns, which are respectively expressed as:
Figure BDA0003773781010000141
Figure BDA0003773781010000142
Figure BDA0003773781010000143
it can be seen that the three-frame phase-shifted fringe pattern sub1、sub2、sub3The phase shift amount between the two is delta and 2 delta, as shown in fig. 4 and 5, the multi-frame phase shift demodulation algorithm can be used to obtain the phase information containing deformation according to the three-frame phase shift fringe pattern
Figure BDA0003773781010000144
Wrapped phase diagram of (1).
The multi-frame phase shift demodulation method based on matrix decomposition is taken as an example for introduction, and the demodulation method has the advantages of high efficiency and high precision and can further ensure the rapidity and the accuracy of an operation process and a result. The method is based on a matrix decomposition principle, a fringe pattern sequence is regarded as a product form of a phase matrix component V and a phase shift matrix component U, the phase shift fringe pattern sequence is used as an original variable, and phase demodulation is rapidly realized through matrix decomposition.
The phase shift fringe pattern sequence can be represented by the following light intensity distribution expression after being rearranged:
Figure BDA0003773781010000145
where the subscript n denotes the number of frames of the phase shift map, AsAnd BsRepresenting the background term and the modulation term.
The above formula is equivalent to:
Figure BDA0003773781010000146
the pixel position in each sub-interferogram is denoted by M, which denotes the total number of pixels.
Column vectors are represented using lower case letters in bold and matrices are represented using upper case letters in bold.
Figure BDA0003773781010000147
And
Figure BDA00037737810100001413
respectively for describing the sine and cosine vector components of the modulation phase term,
Figure BDA00037737810100001414
and
Figure BDA00037737810100001410
respectively, for describing the sine and cosine vectors of the phase shift terms. The elements of the vectors correspond to each other
Figure BDA00037737810100001411
un＝cosδnAnd vn＝sinδn。
Finally, the process is carried out in a batch,
Figure BDA00037737810100001415
for representing the background light intensity vector, having the element amUsing the above symbols, a phase-shifted fringe pattern matrix form is constructed, expressed as:
Figure BDA0003773781010000151
where T denotes a transposition operation.
The first step of the matrix VU decomposition algorithm is to perform initial estimation on the phase shift quantity of the fringe pattern sequence:
Figure BDA0003773781010000152
wherein u is0Cos delta term, v, corresponding to the initial guess0The initial estimate may be assigned randomly corresponding to the initial guess sin δ term.
Using u0 TComputing an intermediate term matrix B0Comprises the following steps:
B0＝U0 TU0
calculating a phase term matrix V0Comprises the following steps:
V0＝IU0B0 -1
the nonlinear disturbance is realized by the following formula to make V0Is updated to V1：
Figure BDA0003773781010000153
V (: 1) represents the first column of data of the V matrix, and so on, and
Φ0＝arctan(-V0(:,3),V0(:,2))
by means of V1Computing the intermediate term matrix A1Comprises the following steps:
A1＝V1 TV1
further calculate the phase shift term matrix U1Comprises the following steps:
U1 T＝A1 -1A1I
at this point, the first iteration loop of the VU decomposition is complete.
Repeating the above process, in the k-th iteration, VkThe updating is as follows:
Figure BDA0003773781010000165
the convergence policy is set to:
Figure BDA0003773781010000161
wherein the content of the first and second substances,
Figure BDA0003773781010000162
and
Figure BDA0003773781010000163
the deformation phases obtained in the k iteration loops and the k-1 iteration loops, respectively. ξ is a preset convergence condition, e.g., ξ =10-5When the condition is met, iteration is terminated, and the distribution of the deformation phase to be measured is obtained
Figure BDA0003773781010000164
As most phase demodulation methods work through an arctan function to obtain corresponding phase distribution results, the demodulated phase distribution value range is wrapped between (-pi, pi). Therefore, unwrapping operation is also needed to obtain the true phase distribution, and the basic principle of phase unwrapping is represented as follows:
φ(x,y)＝ψ(x,y)+2k(x,y)π
where (x, y) is the pixel position coordinate, phi (x, y) and psi (x, y) correspond to the true phase and wrapped phase, respectively, and k (x, y) represents the wrapped order, which is an integer.
The process of unpacking is the process of solving the k value of each pixel point.
The wrapped phase obtained based on multi-frame phase shift demodulation has higher precision and less speckle noise, so that the deformation information can be recovered by most interference phase unwrapping methods, and the details are not repeated herein.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be obtained by a person skilled in the art without inventive step based on the embodiments of the present invention, are within the scope of protection of the present invention.
In order to show the actual measurement effect of the method, dynamic deformation measurement is carried out on an aluminum disc which is fixed at the periphery and loaded at the center on the basis of completing adjustment and calculation of the axial position of the CCD and the pixel coordinate homography transformation matrix according to the description in the steps S2 and S3. Fig. 4 is a three-frame phase-shift speckle interference fringe image obtained by subtracting two speckle interference patterns from four speckle interference patterns acquired based on the CCD1 and the CCD2 according to the description in step S4 at a certain deformation stage. It can be seen that2、sub3And sub1Slight difference in fringe contrast is caused mainly because it is impossible to ensure completely the background light intensity of interference field recorded by two CCDs under experimental conditions to be identical, but the difference can be eliminated by recording one reference light intensity image R in advance by two CCDsf1And Rf2In calculating sub2、sub3While subtracting Rf1And Rf2The fringe contrast can be improved, i.e.:
sub2＝[(I4 H-Rf2)-(I1-Rf1)]2
sub3＝[(I2-Rf1)-(I3 H-Rf2)]2
fig. 5 shows speckle interference fringe sequences, wrapped phase diagrams and true phase distributions obtained at three different deformation stages in the process of deformation of the aluminum disc, and further verifies the feasibility and the practicability of the method applied to the field of dynamic high-precision measurement.
In summary, the dynamic speckle interferometry system and method provided by the invention realize the spatial phase shift speckle interferometry process based on polarization multiplexing by means of the SLM liquid crystal molecule electric control birefringence effect, and overcome the limitation of the depolarization problem of the scattering surface on the phase shift amount by optimizing the polarization state modulation strategy, so that the accurate 90-degree phase shift can be realized, and the contradiction between the measurement precision and the measurement speed in the traditional phase shift technology is solved; meanwhile, the acquisition mode of recording the phase shift interferogram by the double CCD is adopted, so that the problems of spatial resolution and measurement field range loss in the existing spatial phase shift speckle interferometry are solved; in addition, the problem of accurate matching of the space positions of the double CCDs in speckle interference is solved by means of speckle correlation, characteristic point matching and the like; and on the basis of synchronously acquiring the phase shift interference field by using the double channels, a three-step phase shift fringe pattern is obtained, so that a multi-frame random phase shift demodulation algorithm can be combined to realize phase recovery with higher precision. The requirements of actual engineering measurement occasions on real-time performance and accuracy can be completely met.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.

Claims (10)

1. A dynamic speckle interferometry system is characterized by comprising a first depolarization beam splitter (BS 1), wherein laser is divided into reference light and object light by the first depolarization beam splitter (BS 1);
the reference light forms plane waves after being expanded and collimated, the plane waves are irradiated on the target surface of the spatial light modulator through a second depolarizing beam splitter prism (BS 2) after the polarization direction of the reference light is adjusted through a first polaroid (P1) and then reflected, the reference light sequentially reaches the image surfaces of a first camera (CCD 1) and a second camera (CCD 2) through a third lens (L3), a third depolarizing beam splitter prism (BS 3), a second lens (L2) and a polarizing beam splitter Prism (PBS), and the target surface of the spatial light modulator is imaged to the image surfaces of the first camera (CCD 1) and the second camera (CCD 2);
the object light irradiates to the surface to be measured to form speckles after being refracted by the first reflecting mirror (M1) and the second reflecting mirror (M2) through the divergent lens, the polarization state of a speckle field is adjusted through the second polaroid (P2), the light intensity after passing through the polarization beam splitter Prism (PBS) is kept consistent, the back focal length of the imaging lens is prolonged through the first lens (L1) and the second lens (L2), and the image is imaged to the image surfaces of the first camera (CCD 1) and the second camera (CCD 2) and interferes with reference light to form a speckle interference field.
2. A dynamic speckle interferometry method, using the dynamic speckle interferometry system of claim 1, comprising:
s1, building a two-channel spatial phase shift speckle interferometry system based on a spatial light modulator, and adjusting the shooting areas of a first camera and a second camera in the two-channel spatial phase shift speckle interferometry system to be consistent;
s2, determining the optimal axial position of a second camera by using a zero-mean normalized cross-correlation coefficient based on the two-channel spatial phase shift speckle interferometry system established in the step S1;
s3, adjusting the position of the second camera according to the optimal axial position obtained in the step S2, loading a phase diagram containing characteristic information on a spatial light modulator, and establishing projection mapping between the first camera and the imaging surface of the second camera to obtain a homography matrix HM;
s4, loading a phase shift phase diagram on the spatial light modulator, adjusting and compensating the phase shift amount delta by changing the gray value gv of the phase diagram to realize 90-degree phase shift, synchronously shooting and recording speckle interference fields of the measured surface in various deformation states by the first camera and the second camera, converting the picture shot by the second camera into the pixel coordinate system of the first camera according to the homography matrix HM obtained in the step S3 and the pixel coordinate conversion, and obtaining information containing deformation phase by using a multi-frame phase shift demodulation algorithm
Figure FDA0003773778000000011
The wrapped phase diagram is subjected to phase unwrapping to calculate the deformation, and the dynamic speckle interferometry is completed.
3. The dynamic speckle interferometry method as claimed in claim 2, wherein in step S1, the position of the first camera is adjusted to make the first camera and the measured surface conform to the conjugate relationship of the object and image and fixed, the reference light path is shielded, and a speckle pattern Sp0 is recorded; and adjusting the position of the second camera to make the shooting area of the second camera consistent with that of the first camera.
4. The dynamic speckle interferometry method of claim 2, wherein in step S2, the axial position of the second camera is fine-tuned and a series of speckle patterns Sp1, sp2 … Spn are recorded separately; based on speckle correlation, a zero-mean normalized cross-correlation coefficient is used for searching a corresponding point P1 … Pn of an Sp0 center pixel point P0 in Sp1 and Sp2 … Spn, a correlation value is recorded, when the value of the zero-mean normalized cross-correlation coefficient reaches the maximum value, the axial position difference between the second camera and the first camera is minimum, and the position of the second camera is fixed to be the optimal axial position of the second camera.
5. The dynamic speckle interferometry method of claim 4, wherein the zero-mean normalized cross-correlation coefficient C is:
Figure FDA0003773778000000021
wherein R is0Representing a reference image, R1Which represents the image to be evaluated and is,
Figure FDA0003773778000000022
and with
Figure FDA0003773778000000023
Which represents the mean value of the respective images, M, N is the number of pixels.
6. The dynamic speckle interferometry method according to claim 2, wherein in step S3, the establishing of the homography matrix HM based on the image registration process of the feature points specifically comprises:
adding a third polaroid into the reference light path which is built in the step S1 and is based on the speckle interferometry system of the spatial light modulator, wherein the included angle between the polarization direction of the third polaroid and the polarization direction of the e light is 45 degrees, extracting the component of the e light in the polarization direction of the third polaroid, and enabling the two beams of light which pass through the polarization beam splitter prism to contain e light information, so that the first camera and the second camera respectively record characteristic phase diagrams T1 and T2; and searching a series of corresponding homonymous points in the characteristic phase diagrams T1 and T2 based on a characteristic point detection algorithm, and calculating to obtain a homography matrix HM.
7. The dynamic speckle interferometry method of claim 6, wherein the homography matrix HM is:
Figure FDA0003773778000000031
wherein, a11、a12、a21、a22For relative rotation and deflection between the imaging planes of the two cameras, tx、tyA single offset in the x, y directions.
8. The dynamic speckle interferometry method according to claim 2, wherein in step S4, the correspondence between the phase shift δ and the phase map gray level gv is:
Figure FDA0003773778000000032
ne(gv) denotes the e-light refractive index, n, of the SLM at different input grayscalesoThe refractive index of o light, the thickness of a liquid crystal molecular layer, and the wavelength of laser light are represented by d and lambda respectively; synchronously shooting and recording speckle interference fields of the detected surface in each deformation state at intervals by using a first camera and a second camera; is arbitrarily twiceSampling phase with I1、I2、I3、I4Representing the intensity distribution of the interference field recorded by the first camera and the second camera; converting the picture shot by the second camera into the pixel coordinate system of the first camera according to the homography matrix HM and the pixel coordinate conversion formula obtained in the step S3 to obtain I3 H、I4 H(ii) a Subtracting every two of the four speckle interference patterns to obtain three-frame phase-shift speckle interference fringe pattern sub1、sub2、sub3(ii) a Obtaining information containing deformation phase by using multi-frame phase shift demodulation algorithm
Figure FDA0003773778000000033
The wrapped phase map of (a); and performing unwrapping operation to obtain a real phase distribution condition.
9. The dynamic speckle interferometry method of claim 8, wherein phase demodulation is rapidly implemented by matrix decomposition using a multi-frame phase shift demodulation algorithm with a phase shift fringe pattern sequence as an original variable, and the phase shift fringe pattern matrix is:
Figure FDA0003773778000000034
wherein a is a background light intensity vector, c and s are respectively used for describing sine and cosine vector components of a modulation phase term, u and v are respectively used for describing sine and cosine vectors of a phase shift term, and T represents transposition operation.
10. The dynamic speckle interferometry method of claim 8, wherein the true phase phi (x, y) obtained by phase unwrapping is:
φ(x,y)＝ψ(x,y)+2k(x,y)π
where (x, y) is the pixel position coordinate, ψ (x, y) is the wrapping phase, and k (x, y) represents the wrapping order.
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