Single-exposure measurement method of dual-wavelength holographic three-dimensional displacement based on digital image correlation

Abstract

The present invention relates to a dual-wavelength holographic three-dimensional displacement single-exposure measurement method based on digital image correlation assistance, comprising the following steps: obtaining a hologram containing dual-wavelength information shot by a dual-wavelength digital holographic system in a single shot; processing the hologram, reconstructing the spatial domain phase image of the physical wave, and calculating the intensity image; reconstructing the three-dimensional shape of the object in different states based on the synthetic wavelength of the dual wavelength and the reconstructed phase image; using a 2D-DIC algorithm to process the intensity image in different states to obtain the displacement field in the x and y directions, and by matching the reconstructed three-dimensional shape in each state, calculating the displacement field in the z direction, and completing the three-dimensional displacement measurement. Compared with the prior art, the present invention has the advantages of improving measurement accuracy and reducing system complexity.

Description

Technical Field

[0001] The invention relates to the technical field of digital holography, and in particular to a dual-wavelength holographic three-dimensional displacement single-exposure measurement method based on digital image correlation assistance. Background Art

[0002] In the field of digital holography, existing technologies face some challenges in measuring the 3D shape and displacement of curved diffuse reflective objects. As a non-contact full-field measurement technology, digital holography (DH) can measure the out-of-plane displacement of objects with nanometer-level accuracy. However, traditional DH technology has the following major problems in application:

[0003] 1. Limitations of 3D displacement measurement: A single DH setup cannot achieve complete 3D displacement measurement. Since DH can only measure displacements in the direction consistent with the sensitivity vector, displacements in the other two directions perpendicular to the sensitivity vector cannot be directly measured. In order to achieve simultaneous 3D displacement measurement, DH setups in three different directions are usually required, which results in multiple shots and a complex optical system.

[0004] 2. Limitation of the off-plane displacement measurement range: The off-plane displacement measurement range of traditional DH is usually limited to tens of microns. In rigid body motion, this range may be further reduced to one-quarter of the laser wavelength (λ / 4) due to the 2π phase ambiguity problem in interferometric measurement. This greatly limits the application of DH technology in large displacement measurement.

[0005] 3. Insufficient ability to measure the 3D shape of diffusely reflecting objects: Since the surface roughness of diffusely reflecting objects is usually greater than the laser wavelength, resulting in phase blur, it is difficult for traditional DH to reconstruct the 3D shape of these objects. However, for analyzing in-plane and out-of-plane deformation, it is crucial to understand the 3D shape of the object.

[0006] In order to solve the above problems, researchers proposed dual-wavelength digital holography (DWDH), which uses a synthetic wavelength generated by two wavelengths to greatly increase the measurement range of off-plane displacement. At the same time, since the surface roughness of diffuse reflection objects is smaller than the synthetic wavelength, DWDH can also be used to measure the three-dimensional shape of curved diffuse reflection objects. However, DWDH has not yet solved the problem of three-dimensional displacement measurement through a simple optical experimental setup. Some studies have attempted to use color cameras or spectral multiplexing technology to simultaneously record multiple sensitivity vectors to achieve three-dimensional displacement measurement, but this has led to a complex system setup and lost the ability of DWDH in three-dimensional shape measurement.

[0007] Although the existing solutions have been improved, they still have disadvantages such as complex systems and limited measurement range. Therefore, current technology needs further innovation to solve these problems. Summary of the invention

[0008] The purpose of the present invention is to provide a dual-wavelength holographic three-dimensional displacement single-exposure measurement method based on digital image correlation (DIC) to achieve simultaneous measurement of the three-dimensional shape and displacement of a curved diffuse reflection object while keeping the system simple. Through spectrum multiplexing technology, dual-wavelength holograms can be recorded in a single shot, thereby simplifying the system and expanding the displacement measurement range. This technological innovation not only increases the measurement range, but also reduces the complexity of the system, making it more suitable for practical application scenarios such as dynamic measurement and large displacement measurement.

[0009] The purpose of the present invention can be achieved by the following technical solutions:

[0010] A dual-wavelength holographic three-dimensional displacement single-exposure measurement method based on digital image correlation assistance comprises the following steps:

[0011] Hologram acquisition: Acquire a hologram containing dual-wavelength information captured by a single shot of a dual-wavelength digital holographic system;

[0012] Phase image reconstruction: Process the hologram, reconstruct the spatial domain phase image of the physical wave, and calculate the intensity image;

[0013] Reconstructing 3D shapes: Reconstructing the 3D shapes of objects in different states based on the dual-wavelength synthetic wavelength and reconstructed phase image;

[0014] Displacement measurement: The intensity images in different states are processed using the 2D-DIC algorithm to obtain the displacement fields in the x and y directions. By matching the reconstructed 3D shapes in each state, the displacement field in the z direction is calculated to complete the 3D displacement measurement.

[0015] The dual-wavelength digital holographic system adopts an off-axis spectrum multiplexing setting, including a dual-wavelength laser, a fiber beam splitter, a fiber coupler, a lens, an object to be measured, an imaging module and a camera. The dual-wavelength laser emits a dual-wavelength laser beam, which is divided into two object illumination waves and two reference waves by the fiber beam splitter. The object illumination wave is synthesized into a single object illumination wave by the fiber coupler, and irradiated onto the object to be measured after being collimated by the lens. The object wave reflected by the object to be measured is transmitted to the camera through the imaging module. The fiber beam splitter and the fiber coupler ensure that the two wavelengths of light beams propagate on the same path; the reference wave is collimated by the lens and transmitted to the camera at different angles to interfere with the object wave.

[0016] The imaging module adopts 4F imaging, and is composed of two coaxially placed lenses and an aperture, wherein the aperture is located between the two lenses.

[0017] The hologram is represented as: H =|O1+R c1 |2 +|O2+R c2 | 2 , where I H The recorded hologram contains the holographic information of the object at two wavelengths. O1 and O2 represent the object waves reflected by the object at different wavelengths. c1 , R c2 represents reference waves of different wavelengths, |*| 2 It means to find the square of the modulus.

[0018] The phase image reconstruction is specifically as follows: the recorded hologram is processed, the spectrum information of the object wave of different wavelengths is separated, and the light field distributions O1 and O2 of the object wave of two wavelengths are obtained, and then the phase image is calculated by filtering and inverse Fourier transform, wherein the phase image in the initial state is:

[0019]

[0020] Among them, Re means taking the real part of the complex number, Im means taking the imaginary part of the complex number, and the superscript * means the complex conjugate;

[0021] At the same time, the intensity image A1=|O1| of the light field distribution of the object wave of the first wavelength is calculated.

[0022] The three-dimensional shape reconstruction comprises the following steps:

[0023] calculating an initial three-dimensional shape based on a phase image in an initial state and a synthetic wavelength of the dual wavelength;

[0024] Calculate the phase difference between adjacent states, and calculate the height difference between each state based on the phase difference;

[0025] The 3D shape reconstruction in different states is achieved based on the initial 3D shape and the height difference between each state.

[0026] The calculation method of the initial three-dimensional shape is:

[0027]

[0028] Among them, h0 represents the three-dimensional height of the initial state, φ0 is the phase image in the initial state, and Λ is the synthetic wavelength of the dual wavelength:

[0029]

[0030] Wherein, λ1 and λ2 represent the first wavelength and the second wavelength respectively.

[0031] Calculate the phase difference between adjacent states, and calculate the height difference between each state based on the phase difference, specifically:

[0032] Based on the phase images in each state, the phase difference Δφ between adjacent states is calculated i :Δφ i =φ i -φ i-1 ;

[0033] Calculate the height difference Δh between each state based on the phase difference i :

[0034]

[0035] Wherein, Λ is the composite wavelength of the dual wavelengths.

[0036] The calculation method for three-dimensional shape reconstruction under different states is:

[0037]

[0038] Where i represents the number of states, h0 represents the three-dimensional height of the initial state, Δh j Represents the height difference between states, h i Represents the three-dimensional shape in the i-th state.

[0039] The displacement measurement is specifically:

[0040] The intensity image A1 in different states is processed using the 2D-DIC algorithm to calculate the displacement fields U and V in the x and y directions of each state compared to the original state;

[0041] According to the displacement fields U and V, the three-dimensional shape h in each state is obtained. i The point-to-point correspondence between the shape h0 in the initial state and the displacement fields U and V has sub-pixel accuracy. i Interpolate to obtain the height of the current position corresponding to each point on the shape h0 in the initial state;

[0042] Set the height h of the current position corresponding to each point on the shape h0 in the initial state i Subtract it from the height h0 to calculate the displacement field W in the z direction.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] (1) Simultaneous measurement of 3D shape and displacement is achieved

[0045] By using single-shot digital image correlation (DIC)-assisted dual-wavelength digital holography (DWDH), the 3D shape and displacement of a diffusely reflective object on a curved surface can be measured simultaneously in a single shot. Using spectrum multiplexing technology, dual-wavelength holograms can be recorded in a single exposure, thereby achieving simultaneous measurement of 3D shape and displacement without increasing system complexity. This design simplifies the measurement process and greatly improves measurement efficiency.

[0046] (2) Expanded measurement depth range

[0047] The synthetic phase image generated by DWDH technology can reconstruct the three-dimensional shape of diffuse reflective objects. Compared with traditional technologies, this method significantly expands the depth range of displacement measurement. Although the depth measurement range in the current work is only extended to 12.73 microns, this range can be further extended to the centimeter level by increasing the synthetic wavelength, thus meeting the needs of large displacement measurement.

[0048] (3) Suitable for dynamic measurement

[0049] The single-shot feature makes the present invention particularly suitable for dynamic measurement scenarios, such as high-speed vibration analysis. Since multiple shots are not required, this method avoids measurement errors caused by time delays and can more accurately capture fast-changing dynamic processes.

[0050] (4) Comprehensive use of DWDH information

[0051] By using synthetic phase images to reconstruct the 3D shape and combining the 2D-DIC algorithm to process the intensity images of different states, the system can make full use of all the information recorded by DWDH and realize the simultaneous measurement of the 3D shape and displacement of each displacement state in a single shot. This efficient information utilization greatly reduces the complexity of the system and expands the application range of digital holography in large displacement measurement and dynamic measurement.

[0052] In general, through the innovation of the above-mentioned structural design and technical solutions, the method of the present invention significantly improves the efficiency and accuracy of digital holography in three-dimensional shape and displacement measurement, overcomes many shortcomings of traditional methods, and provides a more flexible and reliable solution for actual production and scientific research applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 A schematic diagram of the DWDH off-axis spectrum multiplexing configuration of the present invention;

[0054] Figure 2 This is a workflow diagram of the DIC-assisted DWDH of the present invention;

[0055] Figure 3The three-dimensional shape measurement result of a curved diffuse reflection object obtained by using the present invention in one embodiment;

[0056] Figure 4 The displacement measurement result in the Z direction obtained by using the present invention in one embodiment;

[0057] Figure 5 FIG. 1 is a displacement measurement result in the X direction obtained by using the present invention in an embodiment. DETAILED DESCRIPTION

[0058] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0059] Example 1

[0060] This embodiment provides a dual-wavelength digital holographic system. Figure 1 As shown, the dual-wavelength digital holographic system adopts an off-axis spectrum multiplexing setting, including a dual-wavelength laser, a fiber beam splitter, a fiber coupler, a lens, an object to be measured, an imaging module and a camera. The dual-wavelength laser emits a dual-wavelength laser beam, which is divided into two object illumination waves and two reference waves by the fiber beam splitter. The object illumination wave is synthesized into a single object illumination wave by the fiber coupler, which is collimated by the lens and irradiated onto the object to be measured. The object wave reflected by the object to be measured is transmitted to the camera through the imaging module. The fiber beam splitter and the fiber coupler ensure that the two wavelengths of light beams propagate on the same path; the reference wave is transmitted into the camera at different angles and interferes with the object wave. Figure 1 Clearly showing the configuration of optical elements and the propagation direction of the light path is the key to understanding the system structure of the present invention.

[0061] When using the system for displacement measurement, the system is first initialized: initialize the dual-wavelength laser and select the laser beams with wavelengths λ1 and λ2. Use the fiber beam splitter to split the laser beam into the object illumination wavelength O I1 ,O I2 , and reference waves R1, R2. Then, O I1 ,O I2 The optical fiber coupler (FC) combines the two into a single object illumination wave. I , and then irradiated onto the object to be measured after being collimated by the lens. The waves reflected by the object (called object waves) O1 and O2 are transmitted to the camera through the 4F imaging system (composed of coaxially placed lenses L2, L3 and aperture AP). The reference waves R1 and R2 are collimated by the lens to obtain parallel reference waves R c1 , R c2, enter the camera at different angles, interfere with the object waves O1 and O2, and generate a hologram I containing dual-wavelength information H .

[0062] Example 2

[0063] Based on the system described in Example 1, this embodiment provides a dual-wavelength holographic three-dimensional displacement single exposure measurement method based on digital image correlation assistance, and its working process is as follows: Figure 2 As shown, the following steps are included:

[0064] S1, hologram acquisition: obtaining a hologram containing dual-wavelength information shot by a dual-wavelength digital holographic system in a single shot.

[0065] The hologram is represented as: I H =|O1+R c1 | 2 +|O2+R c2 | 2 , where I H The recorded hologram contains the holographic information of the object at two wavelengths. O1 and O2 represent the object waves reflected by the object at different wavelengths. c1 , R c2 represents reference waves of different wavelengths, |*| 2 It means to find the square of the modulus.

[0066] S2, phase image reconstruction: Process the hologram, reconstruct the spatial domain phase image of the physical wave, and calculate the intensity image.

[0067] The recorded hologram is processed to separate the spectrum information of the object waves of different wavelengths to obtain the light field distributions of the object waves of two wavelengths O1 and O2. Then, the phase image is calculated through filtering and inverse Fourier transform (IFFT). The phase image in the initial state is:

[0068]

[0069] Among them, Re means taking the real part of the complex number, Im means taking the imaginary part of the complex number, and the superscript * means the complex conjugate;

[0070] At the same time, the intensity image A1=|O1| of the light field distribution of the object wave of the first wavelength is calculated for use in three-dimensional displacement calculation.

[0071] S3, Reconstruct 3D shape: Reconstruct the 3D shape of objects in different states based on the synthetic wavelength of dual wavelength and the reconstructed phase image.

[0072] Specifically, the following steps are included:

[0073] S31, calculate the initial three-dimensional shape based on the phase image in the initial state and the synthetic wavelength of the dual wavelength:

[0074]

[0075] Among them, h0 represents the three-dimensional height of the initial state, φ0 is the phase image in the initial state, and Λ is the synthetic wavelength of the dual wavelength:

[0076]

[0077] Wherein, λ1 and λ2 represent the first wavelength and the second wavelength respectively.

[0078] S32, based on the phase images in each state, calculate the phase difference Δφ between adjacent states i :Δφ i =φ i -φ i-1 ; Calculate the height difference Δh between each state based on the phase difference i :

[0079]

[0080] S33, based on the initial three-dimensional shape and the height difference between each state, realize the three-dimensional shape reconstruction in different states:

[0081]

[0082] Where i represents the number of states, h0 represents the three-dimensional height of the initial state, Δh j Represents the height difference between states, h i Represents the three-dimensional shape in the i-th state.

[0083] S4, displacement measurement: The intensity images in different states are processed using the 2D-DIC algorithm to obtain the displacement fields in the x and y directions. The displacement field in the z direction is calculated by matching the reconstructed 3D shapes in each state to complete the 3D displacement measurement.

[0084] Specifically, the following steps are included:

[0085] S41, use the 2D-DIC algorithm to process the intensity image A1 in different states, and calculate the displacement fields U and V in the x and y directions of each state compared to the original state; the process of using the 2D-DIC algorithm to obtain the displacement fields U and V specifically belongs to the conventional settings in the field, and in order to avoid ambiguity in the purpose of this application, it will not be repeated here.

[0086] S42, obtaining the three-dimensional shape h in each state according to the displacement fields U and V iThe point-to-point correspondence between the shape h0 in the initial state and the displacement fields U and V has sub-pixel accuracy. i Interpolate to obtain the height of the current position corresponding to each point on the shape h0 in the initial state.

[0087] S43, subtracting the height of the current position corresponding to each point on the shape h0 in the initial state from the height of h0, and calculating the displacement field W in the z direction.

[0088] like Figure 3 As shown, the present invention is successfully applied to measure the three-dimensional shape of a curved diffuse reflection object (such as a rough cylinder). In the experiment, the three-dimensional shape of the object was reconstructed by the method of the present invention, and its accuracy is highly consistent with the theoretical calculation result. Figure 3 The figure in shows the reconstruction result of the three-dimensional shape and compares it with the theoretical value, verifying the measurement accuracy of the present invention in practical application. This result shows that the present invention can provide high-precision three-dimensional shape reconstruction in the measurement of complex shapes, which is an important improvement over the prior art.

[0089] Figure 4 and Figure 5 The results of displacement measurement using the present invention in the Z direction and the X direction are shown respectively. As can be seen from the figure, the measurement results of the present invention are highly consistent with the actual displacement value, indicating the accuracy and reliability of the present invention in large displacement measurement. Through these experimental results, the superior performance of the present invention in dynamic and large-range displacement measurement can be further demonstrated.

[0090] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning, or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A dual-wavelength holographic three-dimensional displacement single exposure measurement method based on digital image correlation, characterized in that: The following steps are involved: Hologram acquisition: Acquire a hologram containing dual-wavelength information captured by a single shot of a dual-wavelength digital holographic system; Phase image reconstruction: Process the hologram, reconstruct the spatial domain phase image of the physical wave, and calculate the intensity image; Reconstructing 3D shapes: Reconstructing the 3D shapes of objects in different states based on the dual-wavelength synthetic wavelength and reconstructed phase image; Displacement measurement: Use the 2D-DIC algorithm to process the intensity images in different states to obtain the displacement fields in the x and y directions. By matching the reconstructed 3D shapes in each state, the displacement field in the z direction is calculated to complete the 3D displacement measurement. The dual-wavelength digital holographic system adopts an off-axis spectrum multiplexing setting, including a dual-wavelength laser, a fiber beam splitter, a fiber coupler, a lens, an object to be measured, an imaging module and a camera. The dual-wavelength laser emits a dual-wavelength laser beam, which is divided into two object illumination waves and two reference waves by the fiber beam splitter. The object illumination wave is synthesized into a single object illumination wave by the fiber coupler, and irradiated onto the object to be measured after being collimated by the lens. The object wave reflected by the object to be measured is transmitted to the camera through the imaging module. The fiber beam splitter and the fiber coupler ensure that the two wavelengths of light beams propagate on the same path; the reference wave is collimated by the lens and transmitted to the camera at different angles to interfere with the object wave. The phase image reconstruction is specifically as follows: the recorded hologram is processed, the spectrum information of the object wave of different wavelengths is separated, and the light field distributions O1 and O2 of the object wave of two wavelengths are obtained, and then the phase image is calculated by filtering and inverse Fourier transform, wherein the phase image in the initial state is: Among them, Re means taking the real part of the complex number, Im means taking the imaginary part of the complex number, and the superscript * means the complex conjugate; At the same time, the intensity image A1=|O1| of the light field distribution of the object wave of the first wavelength is calculated.

2. The method for measuring three-dimensional displacement of a dual-wavelength holographic image using a single exposure and assisted by digital image correlation according to claim 1, characterized in that: The imaging module adopts 4F imaging, and is composed of two coaxially placed lenses and an aperture, wherein the aperture is located between the two lenses.

3. The method for measuring three-dimensional displacement of a dual-wavelength holographic image using a single exposure and assisted by digital image correlation according to claim 1, characterized in that: The hologram is represented as: H =|O1+R c1 | 2 +|O2+R c2 | 2 , where I H The recorded hologram contains the holographic information of the object at two wavelengths. O1 and O2 represent the object waves reflected by the object at different wavelengths. c1 , R c2 represents reference waves of different wavelengths, |*| 2 It means to find the square of the modulus.

4. The method for measuring three-dimensional displacement of a dual-wavelength holographic image using a single exposure and assisted by digital image correlation according to claim 1, characterized in that: The three-dimensional shape reconstruction comprises the following steps: calculating an initial three-dimensional shape based on a phase image in an initial state and a synthetic wavelength of the dual wavelength; Calculate the phase difference between adjacent states, and calculate the height difference between each state based on the phase difference; The 3D shape reconstruction in different states is achieved based on the initial 3D shape and the height difference between each state.

5. The method for measuring three-dimensional displacement of a dual-wavelength holographic image using a single exposure and assisted by digital image correlation according to claim 4, characterized in that: The calculation method of the initial three-dimensional shape is: Among them, h0 represents the three-dimensional height of the initial state, φ0 is the phase image in the initial state, and Λ is the synthetic wavelength of the dual wavelength: Wherein, λ1 and λ2 represent the first wavelength and the second wavelength respectively.

6. The method for measuring three-dimensional displacement of a dual-wavelength holographic image using a single exposure and assisted by digital image correlation according to claim 4, characterized in that: The phase difference between adjacent states is calculated, and the height difference between each state is calculated based on the phase difference, specifically: Based on the phase images in each state, the phase difference Δφ between adjacent states is calculated i :Δφ i =φ i -φ i-1 ; Calculate the height difference Δh between each state based on the phase difference i : Wherein, Λ is the composite wavelength of the dual wavelengths.

7. The method for measuring three-dimensional displacement of a dual-wavelength holographic image using a single exposure and assisted by digital image correlation according to claim 4, characterized in that: The calculation method for three-dimensional shape reconstruction under different states is: Where i represents the number of states, h0 represents the three-dimensional height of the initial state, Δh j Represents the height difference between states, h i Represents the three-dimensional shape in the i-th state.

8. The method for measuring three-dimensional displacement of a dual-wavelength holographic image using a single exposure and assisted by digital image correlation according to claim 1, characterized in that: The displacement measurement is specifically: The intensity image A1 in different states is processed using the 2D-DIC algorithm to calculate the displacement fields U and V in the x and y directions of each state compared to the original state; According to the displacement fields U and V, the three-dimensional shape h in each state is obtained. i The point-to-point correspondence between the shape h0 in the initial state and the displacement fields U and V has sub-pixel accuracy. i Interpolate to obtain the height of the current position corresponding to each point on the shape h0 in the initial state; Set the height h of the current position corresponding to each point on the shape h0 in the initial state i Subtract it from the height h0 to calculate the displacement field W in the z direction.

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