Particle image velocimetry system and method based on superlens

Abstract

The application provides a kind of particle image velocimetry system and method based on super lens, comprising: super lens, digital camera, laser light source and computer;The light intensity and light source thickness of the laser light source can be adjusted, and the whole test area is uniformly illuminated;The super lens contains a plurality of super lens units, and the light scattered or emitted by the tracer particles in the test area is imaged, and the image is located on the plane where the photosensitive chip of the digital camera is located;The super lens unit is composed of a substrate and a nanocolumn array covered on the substrate, and the phase of incident light is controlled to realize the imaging function, and different phase compensation is designed in practice for different needs;The camera parameters are calculated by calibrating the image, and the particle displacement and three-dimensional flow field data of the test area are obtained by three-dimensional cross-correlation or particle matching tracking analysis calculation of the experimental image;The application benefits from the optical structure of the new system, which can realize the convenient adjustment of system parameters like traditional binocular 3D-PIV.

Description

Technical Field

[0001] This invention relates to the field of flow field measurement technology, and more specifically, to a particle image velocimetry system and method based on metalenses. Background Technology

[0002] Flow field measurement is an important component of fluid mechanics research, and the main measurement methods can be divided into contact measurement and non-contact measurement. Among them, non-contact measurement methods are favored due to their advantages such as no interference with the flow field, wide applicability, convenient operation, and high measurement accuracy. Non-contact flow field measurement mainly uses optical measurement techniques, including particle image velocimetry (PIV), laser speckle velocimetry (LSP), and particle tracking velocimetry (PTV).

[0003] Particle image velocimetry (PIV) is a non-contact measurement method widely used in fluid mechanics research. PIV technology has attracted widespread attention due to its advantages such as no interference to the test area, ability to obtain instantaneous and full-field flow velocity information, high resolution, and wide applicability. Stereo PIV technology uses multiple cameras to image the same area within the flow field, thereby resolving the velocity field within that test area. However, the use of multiple cameras increases the overall system complexity, and multi-camera imaging requires calibration and synchronization of each camera. Compared to traditional PIV, this method is more complex and difficult to use. Therefore, researchers have begun to explore different methods to simplify the overall system complexity.

[0004] Philippe M. Bardet proposed a single-camera stereo PIV technique that uses a prism for beam splitting, allowing the left and right sides of the image sensor to image the test area separately. Shi et al. proposed a 3D-PIV technique based on a light field camera, which collects information about the test area from multiple perspectives by adding a microlens array between the main lens and the image sensor, thus capturing the entire flow field. These methods, to some extent, address the need for multiple cameras and the difficulty of synchronous control in stereo PIV systems. However, all of these methods have their drawbacks: using beam splitting devices increases the number of optical components, raising the overall system complexity; and using a light field camera sacrifices some resolution. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the purpose of this invention is to provide a particle image velocimetry system and method based on metalenses.

[0006] A particle image velocimetry system based on metalens provided by the present invention includes:

[0007] The system includes a meta-lens, a digital camera, a laser light source, and a computer, wherein the computer is electrically connected to the digital camera and the laser light source.

[0008] The laser source can uniformly illuminate the entire test area according to instructions, and tracer particles are set in the test area.

[0009] The metalens comprises two or more metalens units, and the light scattered or emitted by the tracer particles in the test area enters the digital camera through the metalens units;

[0010] The digital camera can acquire images of tracer particles in the test area through a meta-lens according to instructions;

[0011] The computer controls the digital camera to acquire experimental images, performs three-dimensional cross-correlation or particle matching tracking analysis on the experimental images, and obtains particle displacement and three-dimensional flow field data of the test area.

[0012] Preferably, the meta-lens comprises two meta-lens units located on the same plane with the same focal length, and the imaging area of ​​each of the two meta-lens units covers the entire test area.

[0013] Alternatively, the meta-lens may comprise three or more meta-lens units located on the same plane with the same focal length. Each pair of meta-lens units forms a lens group, and the imaging areas of the two meta-lens units in each lens group cover the same part of the test area. The sum of the imaging areas of all lens groups covers the entire test area.

[0014] Preferably, the metalens employs a spherical aberration-free phase modulation, using a single micrometer-thick two-dimensional planar metalens to refract and focus light rays incident from different directions onto the focal plane, with the difference between the focal spot size and the diffraction limit of the metalens being less than a preset standard.

[0015] And / or, the metalens is a single-wavelength metalens with a narrow transmission band.

[0016] Preferably, the laser is directly facing the direction of motion of the vortex ring of the tracer particle, and the laser plane coincides with the direction of motion of the center of the vortex ring;

[0017] The meta-lens and digital camera are located on one side of the normal vector of the laser plane and are parallel to the laser plane.

[0018] Preferably, the computer controls the digital camera to acquire calibration images and experimental images, calculates camera parameters through the calibration images, determines the mapping relationship between the actual physical distance and the pixel width in the image based on the camera parameters, performs three-dimensional cross-correlation or particle matching tracking analysis on the experimental images to obtain the number of particle displacement pixels in the test area, converts it into the physical distance in the real world according to the mapping relationship, and calculates the three-dimensional flow field data corresponding to the physical scale of the real world.

[0019] Preferably, in the meta-lens unit, different meta-lens units can use different operating wavelengths, or use the same operating wavelength, with the same focal length at different operating wavelengths.

[0020] A particle image velocimetry method based on metalensing provided by the present invention includes:

[0021] Step S1: Distribute tracer particles in the test area to uniformly illuminate the entire test area with a laser light source;

[0022] Step S2: The digital camera acquires images of the tracer particles in the test area through the metalens;

[0023] Step S3: Perform three-dimensional cross-correlation or particle matching tracking analysis on the image to obtain particle displacement and three-dimensional flow field data of the test area.

[0024] Preferably, the meta-lens comprises two meta-lens units located on the same plane with the same focal length, and the imaging area of ​​each of the two meta-lens units covers the entire test area.

[0025] Alternatively, the meta-lens may comprise three or more meta-lens units located on the same plane with the same focal length. Each pair of meta-lens units forms a lens group, and the imaging areas of the two meta-lens units in each lens group cover the same part of the test area. The sum of the imaging areas of all lens groups covers the entire test area.

[0026] Preferably, the metalens employs a spherical aberration-free phase modulation, using a single micrometer-thick two-dimensional planar metalens to refract and focus light rays incident from different directions onto the focal plane, with the difference between the focal spot size and the diffraction limit of the metalens being less than a preset standard.

[0027] And / or, the metalens is a single-wavelength metalens with a narrow transmission band;

[0028] The laser is directly facing the direction of the vortex ring motion of the tracer particle, and the laser plane coincides with the direction of motion of the center of the vortex ring;

[0029] The meta-lens and digital camera are located on one side of the normal vector of the laser plane and are parallel to the laser plane;

[0030] The computer controls the digital camera to acquire calibration images and experimental images. It calculates camera parameters through the calibration images, determines the mapping relationship between the actual physical distance and the pixel width in the image based on the camera parameters, performs three-dimensional cross-correlation or particle matching tracking analysis on the experimental images, obtains the number of particle displacement pixels in the test area, converts it into the physical distance in the real world according to the mapping relationship, and calculates the three-dimensional flow field data corresponding to the physical scale of the real world.

[0031] In the aforementioned meta-lens unit, different meta-lens units can use different operating wavelengths, or use the same operating wavelength, with the same focal length at different operating wavelengths.

[0032] Preferably, in step S5:

[0033] Regions of Interest (ROIs) are defined, and corresponding regions containing the flow structure are identified. Two-dimensional cross-correlation is performed on the left and right images to obtain a 2D-2C velocity field, generating two velocity fields in the x and y directions for each view. The velocity vector is solved by matching particle clusters within windows in the two frames using the two-dimensional cross-correlation. The window used to calculate the velocity vector is a reference window with a size of M×N pixels, selected from the first frame image. From the second frame image, a reference window of size M is selected. g ×N g Given a matching window for pixels, labeling the first and second frame images as f1 and f2, the cross-correlation Φ(m,n)| between the two windows is... (i,j) Represented as:

[0034]

[0035] Where m is the horizontal length of the reference window, n is the vertical length of the reference window, i is the number of pixels the matching window is horizontally shifted in the second frame image, and j is the number of pixels the matching window is vertically shifted in the second frame image.

[0036] The two windows with the highest matching degree represent the positions of the same particle cluster in the two frames, and the displacement vector d(i,j) is represented as:

[0037]

[0038] The velocity v(i,j) is obtained by the time interval and displacement:

[0039]

[0040] Where k is a coefficient that converts pixel size into actual distance, and Δt is the time interval between two frames;

[0041] Based on the corresponding 2D-2C velocity field and the mapping matrix of the left and right images, the 2D-3C velocity field of the region is reconstructed using the Soloff method. The two-dimensional velocity vectors obtained by the left and right eyes are the result of projecting the real velocity vectors onto the imaging plane. Using the mapping matrix obtained during the calibration process, the component in the z-depth direction is calculated to generate the final three-directional component velocity field. In post-processing, erroneous velocity vectors are eliminated by global and local verification and filled by linear interpolation.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. This invention achieves 3D-PIV measurement using only a single camera shot. It employs a metalens instead of a traditional lens, resulting in a simpler and more compact optical structure while maintaining image quality. This structure is highly efficient and does not require camera synchronization. Furthermore, the metalens can be designed for aberration-free phase modulation, using only a single micrometer-thick two-dimensional planar metalens to accurately refract and focus light rays incident from different directions onto the focal plane. Additionally, the focal spot size of the metalens is close to the diffraction limit, resulting in better imaging resolution for the same numerical aperture.

[0044] 2. This invention achieves single-camera 3D imaging through binocular / multi-lens metamorphic lenses, possessing full-field velocity field measurement capability. Furthermore, DIC measurements can be optimized by designing different metamorphic surfaces. By designing a single-wavelength metamorphic lens with a narrow transmission band, images illuminated by monochromatic lasers from PIVs can be accurately imaged, without chromatic aberration and with reduced influence from other ambient light sources.

[0045] 3. This invention can also be effectively extended to special measurement fields such as high speed and microscopy, thereby effectively reducing hardware costs and testing space requirements;

[0046] 4. Thanks to the optical structure of the new system, the system parameters of this invention can be easily adjusted, just like traditional binocular 3D-PIV. Attached Figure Description

[0047] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0048] Figure 1 This is a schematic diagram of the structure of Embodiment 3 of the present invention;

[0049] Figure 2 This is a schematic diagram of the experimental apparatus in Embodiment 3 of the present invention;

[0050] Figure 3 The image shows the tracer particles collected in Embodiment 3 of the present invention.

[0051] Figure 4 This is a three-dimensional velocity field image of Embodiment 3 of the present invention;

[0052] Figure 5 This is a schematic diagram of the binocular meta-lens of Embodiment 3 of the present invention;

[0053] Figure 6 This is a schematic diagram of the multi-lens meta-lens of Embodiment 4 of the present invention; Detailed Implementation

[0054] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0055] Example 1:

[0056] This invention discloses a particle image velocimetry system and method based on a metalens 2 and a single digital camera 4. The method includes the following steps: First, using the metalens 2 as the main optical element, it forms an image capturing system with the digital camera 4; then, it acquires an image of the flow region to be measured, which is illuminated by a laser and uniformly sprinkled with tracer particles; finally, it uses a traditional three-dimensional PIV algorithm based on the acquired image to obtain the flow field of the test region. This invention uses a metalens 2 to replace the traditional optical lens, resulting in a compact structure, easily adjustable system parameters, and more convenient and effective high-precision three-dimensional measurement in special fields such as microscopy and high speed. It solves the core problem of single-camera flow field measurement and promotes the development of particle image velocimetry flow field measurement technology.

[0057] According to the particle image velocimetry method based on metalens 2 provided by the present invention, the particle image velocimetry system based on metalens 2 is used, such as... Figures 1-2 As shown, the execution includes:

[0058] Step S1: Distribute tracer particles evenly in the test area, ensuring that the tracer particles follow the measurement requirements in the test area.

[0059] Step S2: Adjust the meta-lens 2 and the digital camera 4 so that each meta-lens unit can clearly image the tracer particles in the area and meet the preset standard;

[0060] Step S3: Adjust the camera and light source parameters for the required flow field measurement, and synchronously control the light source and camera so that the laser illuminates the entire test area. Use computer 3 to control the camera to acquire images.

[0061] Step S4: Capture calibration images and obtain calibration parameters;

[0062] Step S5: Separate the acquired images and perform 3D cross-correlation or 3D particle tracking analysis to obtain the 3D velocity field. The 3D cross-correlation algorithm is a type of particle matching tracking analysis, which includes 3D cross-correlation.

[0063] Specifically, the metalens 2 comprises two metalens units located on the same plane and having the same focal length; within the designed working distance, the pair of metalens units can simultaneously image the test area; and the imaging areas of each of the pair of metalens units cover the entire test area.

[0064] The meta-lens 2 comprises two meta-lens units, which image the light scattered or emitted by the tracer particles in the test area, and the resulting image is located on the plane of the image sensor chip of the digital camera 4.

[0065] Specifically, the meta-lens unit consists of a substrate and a nanopillar array covering the substrate, which enables the modulation of the phase of the incident light to achieve imaging function. In practice, different phase compensations are designed for different needs.

[0066] The meta-lens units are located on the same plane and have the same focal length; within the designed working distance, there are multiple pairs of meta-lens units that can simultaneously image all or part of the object or region under test; in the multiple pairs of meta-lens units, the imaging areas of the two meta-lens units in each pair contain the same test area; the sum of the imaging areas of the multiple pairs of meta-lens units covers the entire test area.

[0067] The digital camera 4 can be a monochrome camera or a color camera;

[0068] In the aforementioned meta-lens unit, different meta-lens units use different operating wavelengths, but have the same focal length at different operating wavelengths;

[0069] The laser light source 1 used has the ability to emit light at the aforementioned working wavelength.

[0070] Specifically, in step S5:

[0071] Regions of Interest (ROIs) are defined, and corresponding regions containing the flow structure are identified. Two-dimensional cross-correlation is performed on the left and right images to obtain a 2D-2C velocity field, generating two velocity fields in the x and y directions for each view. The velocity vector is solved by matching particle clusters within windows in the two frames using the two-dimensional cross-correlation. The window used to calculate the velocity vector is a reference window with a size of M×N pixels, selected from the first frame image. From the second frame image, a reference window of size M is selected. g ×N gGiven a matching window for pixels, labeling the first and second frame images as f1 and f2, the cross-correlation Φ(m,n)| between the two windows is... (i,j) Represented as:

[0072]

[0073] The two windows with the highest matching degree represent the positions of the same particle cluster in the two frames, and the displacement vector d(i,j) is represented as:

[0074]

[0075] The velocity v(i,j) is obtained by the time interval and displacement:

[0076]

[0077] Where k is a coefficient that converts pixel size into actual distance, and Δt is the time interval between two frames;

[0078] Based on the corresponding 2D-2C velocity field and the mapping matrix of the left and right images, the 2D-3C velocity field of the region is reconstructed using the Soloff method. The two-dimensional velocity vectors obtained by the left and right eyes are the result of projecting the real velocity vectors onto the imaging plane. Using the mapping matrix obtained during the calibration process, the component in the z-depth direction is calculated to generate the final three-directional component velocity field. In post-processing, erroneous velocity vectors are eliminated by global and local verification and filled by linear interpolation.

[0079] Example 2:

[0080] Example 2 is a preferred embodiment of Example 1, and is used to illustrate the present invention in more detail.

[0081] The present invention also provides a particle image velocimetry system based on metalens. The particle image velocimetry system based on metalens can be implemented by executing the process steps of the particle image velocimetry method based on metalens. That is, those skilled in the art can understand the particle image velocimetry method based on metalens as a preferred embodiment of the particle image velocimetry system based on metalens.

[0082] A particle image velocimetry system based on metalens provided by the present invention includes:

[0083] 2. Super lens; 4. Digital camera; 1. Laser light source; and 3. Computer.

[0084] The intensity and thickness of the laser source 1 can be adjusted to uniformly illuminate the entire test area.

[0085] The meta-lens 2 contains multiple meta-lens units, which image the light scattered or emitted by the tracer particles in the test area, and the image formed is located on the plane of the image sensor chip of the digital camera 4.

[0086] The meta-lens unit consists of a substrate and a nanopillar array covering the substrate, which enables the modulation of the phase of the incident light to achieve imaging function. In practice, different phase compensations are designed for different needs.

[0087] The superlens 2 is designed to eliminate spherical aberration and phase modulation. It uses a single micrometer-thick two-dimensional planar superlens to refract and focus light rays incident from different directions to the focal plane. The difference between the focal spot size and the diffraction limit of the superlens is less than the preset standard.

[0088] The digital camera 4 is used to acquire images of tracer particles in the test area formed by the metalens 2;

[0089] The computer 3 is used to process calibration and experimental image data acquired by the digital camera 4, calculate camera parameters through calibration images, and perform three-dimensional cross-correlation or particle matching tracking analysis on experimental images to obtain particle displacement and three-dimensional flow field data of the test area.

[0090] The light emitted by the laser source 1 has the operating wavelength of the superlens 2 as designed.

[0091] Specifically, the meta-lens 2 comprises two meta-lens units located on the same plane with the same focal length; within the designed working distance, the pair of meta-lens units can simultaneously image the test area; and the imaging areas of each of the two meta-lens units cover the entire test area.

[0092] Specifically, the meta-lens 2 includes two meta-lens units to image the light scattered or emitted by the tracer particles in the test area, and the image formed is located on the plane of the photosensitive chip of the digital camera 4.

[0093] The meta-lens unit consists of a substrate and a nanopillar array covering the substrate, which enables the modulation of the phase of the incident light to achieve imaging function. In practice, different phase compensations are designed for different needs.

[0094] Specifically, the meta-lens units are located on the same plane and have the same focal length; within the designed working distance, there are multiple pairs of meta-lens units that can simultaneously image all or part of the object or region under test; in the multiple pairs of meta-lens units, the imaging areas of the two meta-lens units in each pair contain the same test area; the sum of the imaging areas of the multiple pairs of meta-lens units covers the entire test area.

[0095] Specifically, the digital camera 4 is either a monochrome camera or a color camera.

[0096] Specifically, in the meta-lens unit, different meta-lens units use different operating wavelengths, but have the same focal length at different operating wavelengths;

[0097] The laser light source 1 used has the ability to emit light at the aforementioned working wavelength.

[0098] Example 3:

[0099] Example 3 is a preferred example of Example 1, and is used to illustrate the present invention in more detail.

[0100] like Figures 3-4 As shown, this invention proposes a three-dimensional particle image velocimetry system and method based on metalenses. Considering the imaging characteristics of metalenses, a three-dimensional particle image velocimetry method based on metalenses is proposed. A metalens 2 is used as the main optical element to form an image capturing system with a digital camera 4. Images are acquired of the flow region to be measured, which is illuminated by a laser and uniformly sprinkled with tracer particles. Based on the acquired images, a traditional three-dimensional PIV algorithm is used to obtain the flow field of the test region. This invention uses a metalens 2 to replace the traditional optical lens, resulting in a compact structure, easily adjustable system parameters, and more convenient and effective high-precision three-dimensional measurement in special fields such as microscopy and high speed.

[0101] A measurement method for a three-dimensional particle image velocimetry system using a metalens 2 and a single digital camera 4 includes the following steps:

[0102] 1) The tracer particles are evenly distributed in the test area, and the tracking performance of the tracer particles in the test area meets the measurement requirements;

[0103] 2) Place the three-dimensional particle image velocimetry system in front of the test area, and adjust the meta-lens 2 and digital camera 4 so that each meta-lens unit can clearly image the tracer particles in the area.

[0104] 3) Adjust the camera and light source parameters for the required flow field measurement, and synchronously control the light source and camera so that the laser illuminates the entire test area. Use computer 3 to control the camera to acquire images.

[0105] 4) Capture calibration images and obtain calibration parameters;

[0106] 5) The acquired images are separated and subjected to three-dimensional cross-correlation or three-dimensional particle tracking analysis to obtain the three-dimensional velocity field.

[0107] A three-dimensional particle image velocimetry system based on a metalens 2 and a single digital camera 4 includes a metalens 2, a digital camera 4, a laser light source 1, and a computer 3; the laser light source 1 is used to uniformly illuminate the entire test area, enabling the camera to easily acquire images with suitable grayscale values, and the light intensity and light source thickness are adjustable; the metalens 2 includes two metalens units ( Figure 5 The superlens unit is used to image the light scattered or emitted by the tracer particles in the test area onto the photosensitive chip; the superlens unit consists of a substrate and a nanopillar array covering the substrate, which realizes the control of the phase of the incident light to achieve the imaging function; the digital camera 4 is used to acquire the image of the tracer particles in the test area formed by the superlens 2; the computer 3 is used to process the calibration and experimental image data acquired by the digital camera 4, calculate the camera parameters through the calibration image, and then perform three-dimensional cross-correlation or particle matching tracking analysis on the experimental image to obtain the particle displacement and three-dimensional flow field data of the test area.

[0108] In operation, first fill the water tank with purified water, then pour the tracer particles into the water tank and mix them evenly. Fix the laser to one side of the water tank using optical elements, move the bottom displacement stage of the laser to face the direction of the vortex ring's movement, and adjust the bottom rotation stage of the laser to align the laser plane with the direction of the vortex ring's center movement. Set the camera and metalens 2 at the same height using optical elements, and adjust the elevation mechanism to ensure the metalens 2 is parallel to the camera. Then move the metalens 2 imaging system to the normal vector side of the laser plane, and adjust the rotation mechanism to make the metalens 2 imaging system parallel to the laser plane. Adjust the image distance and object distance by adjusting the bottom displacement mechanism of the metalens 2 to ensure clear imaging of the tracer particles on the camera's image sensor. Adjust the laser intensity to allow the camera to easily acquire tracer particle images with suitable grayscale values. A vortex ring is ejected through the vortex ring generator, and the computer 3 controls the camera to acquire images. Perform PIV analysis on the acquired images to calculate the corresponding three-dimensional velocity field.

[0109] Example 4:

[0110] Example 4 is a preferred example of Example 1, which is used to illustrate the present invention in more detail.

[0111] A three-dimensional particle image velocimetry system based on a metalens 2 and a single digital camera 4 includes a metalens 2, a digital camera 4, a laser light source 1, and a computer 3; the laser light source 1 is used to uniformly illuminate the entire test area, enabling the camera to easily acquire images with suitable grayscale values, and the light intensity and light source thickness are adjustable; the metalens 2 includes four metalens units (…). Figure 6The superlens unit is used to image the light scattered or emitted by the tracer particles in the test area onto the photosensitive chip; the superlens unit consists of a substrate and a nanopillar array covering the substrate, which realizes the control of the phase of the incident light to achieve the imaging function; the digital camera 4 is used to acquire the image of the tracer particles in the test area formed by the superlens 2; the computer 3 is used to process the calibration and experimental image data acquired by the digital camera 4, calculate the camera parameters through the calibration image, and then perform three-dimensional cross-correlation or particle matching tracking analysis on the experimental image to obtain the particle displacement and three-dimensional flow field data of the test area.

[0112] Those skilled in the art will understand that, besides implementing the system and its various devices, modules, and units provided by this invention in the form of purely computer-readable program code, the same functions can be achieved entirely through logical programming of the method steps, making the system and its various devices, modules, and units of this invention function in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers. Therefore, the system and its various devices, modules, and units provided by this invention can be considered as a hardware component, and the devices, modules, and units included therein for implementing various functions can also be considered as structures within the hardware component; alternatively, the devices, modules, and units for implementing various functions can be considered as both software modules implementing the method and structures within the hardware component.

[0113] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A particle image velocimetry method based on metalensing, characterized in that, include: Step S1: Distribute tracer particles in the test area to uniformly illuminate the entire test area with a laser light source; Step S2: The digital camera acquires images of the tracer particles in the test area through the metalens; Step S3: Perform three-dimensional cross-correlation or particle matching tracking analysis on the image to obtain particle displacement and three-dimensional flow field data of the test area; When separating the acquired images and performing 3D cross-correlation or particle matching tracking analysis, a region of interest (ROI) is defined, and a corresponding region containing the flow structure is defined. Two-dimensional cross-correlation is performed on the left and right images to obtain a 2D-2C velocity field, generating two velocity fields in the x and y directions for each image. The 2D cross-correlation solves for the velocity vector by matching particle clusters within windows in the two frames. The window used to calculate the velocity vector is a reference window with a size of M×N pixels, and the first frame image is selected. From the second frame image, a reference window of size M is selected. g ×N g Given a matching window for pixels, labeling the first and second frame images as f1 and f2, the cross-correlation Φ(m,n)| between the two windows is... (i,j) Represented as: Where m is the horizontal length of the reference window, n is the vertical length of the reference window, i is the number of pixels the matching window is horizontally shifted in the second frame image, and j is the number of pixels the matching window is vertically shifted in the second frame image. The two windows with the highest matching degree represent the positions of the same particle cluster in the two frames, and the displacement vector d(i,j) is represented as: The velocity v(i,j) is obtained by the time interval and displacement: Where k is a coefficient that converts pixel size into actual distance, and Δt is the time interval between two frames; Based on the corresponding 2D-2C velocity field and the mapping matrix of the left and right images, the 2D-3C velocity field of the region is reconstructed using the Soloff method. The two-dimensional velocity vectors obtained by the left and right eyes are the result of projecting the real velocity vectors onto the imaging plane. Using the mapping matrix obtained during the calibration process, the component in the z-depth direction is calculated to generate the final three-directional component velocity field. In post-processing, erroneous velocity vectors are eliminated by global and local verification and filled by linear interpolation.

2. The particle image velocimetry method based on metalens according to claim 1, characterized in that: The meta-lens comprises two meta-lens units located on the same plane with the same focal length. The imaging areas of each of the two meta-lens units cover the entire test area. Alternatively, the meta-lens may comprise three or more meta-lens units located on the same plane with the same focal length. Each pair of meta-lens units forms a lens group, and the imaging areas of the two meta-lens units in each lens group cover the same part of the test area. The sum of the imaging areas of all lens groups covers the entire test area.

3. The particle image velocimetry method based on metalens according to claim 1, characterized in that: The superlens employs phase modulation to eliminate spherical aberration. It uses a single two-dimensional planar superlens with a thickness of micrometers to refract and focus light rays incident from different directions onto the focal plane. The difference between the focal spot size and the diffraction limit of the superlens is less than the preset standard. And / or, the metalens is a single-wavelength metalens with a narrow transmission band; The laser is directly facing the direction of the vortex ring motion of the tracer particle, and the laser plane coincides with the direction of motion of the center of the vortex ring; The meta-lens and digital camera are located on one side of the normal vector of the laser plane and are parallel to the laser plane; The computer controls the digital camera to acquire calibration images and experimental images. The camera parameters are calculated through the calibration images. Based on the camera parameters, the mapping relationship between the actual physical distance and the pixel width in the image is determined. The experimental images are subjected to three-dimensional cross-correlation or particle matching tracking analysis to obtain the number of particle displacement pixels in the test area. After converting the mapping relationship into the physical distance in the real world, the three-dimensional flow field data corresponding to the physical scale of the real world is calculated. In the aforementioned meta-lens unit, different meta-lens units can use different operating wavelengths, or use the same operating wavelength, with the same focal length at different operating wavelengths.

4. A system for implementing the particle image velocimetry method based on metalens as described in any one of claims 1-3, characterized in that, include: The system includes a meta-lens, a digital camera, a laser light source, and a computer, wherein the computer is electrically connected to the digital camera and the laser light source. The laser source can uniformly illuminate the entire test area according to instructions, and tracer particles are set in the test area. The metalens comprises two or more metalens units, and the light scattered or emitted by the tracer particles in the test area enters the digital camera through the metalens units; The digital camera can acquire images of tracer particles in the test area through a meta-lens according to instructions; The computer controls the digital camera to acquire experimental images, performs three-dimensional cross-correlation or particle matching tracking analysis on the experimental images, and obtains particle displacement and three-dimensional flow field data of the test area.

5. The particle image velocimetry system based on metalens according to claim 4, characterized in that: The meta-lens comprises two meta-lens units located on the same plane with the same focal length. The imaging areas of each of the two meta-lens units cover the entire test area. Alternatively, the meta-lens may comprise three or more meta-lens units located on the same plane with the same focal length. Each pair of meta-lens units forms a lens group, and the imaging areas of the two meta-lens units in each lens group cover the same part of the test area. The sum of the imaging areas of all lens groups covers the entire test area.

6. The particle image velocimetry system based on metalens according to claim 4, characterized in that: The superlens employs phase modulation to eliminate spherical aberration. It uses a single two-dimensional planar superlens with a thickness of micrometers to refract and focus light rays incident from different directions onto the focal plane. The difference between the focal spot size and the diffraction limit of the superlens is less than the preset standard. And / or, the metalens is a single-wavelength metalens with a narrow transmission band.

7. The particle image velocimetry system based on metalens according to claim 4, characterized in that: The laser is directly facing the direction of the vortex ring motion of the tracer particle, and the laser plane coincides with the direction of motion of the center of the vortex ring; The meta-lens and digital camera are located on one side of the normal vector of the laser plane and are parallel to the laser plane.

8. The particle image velocimetry system based on metalens according to claim 4, characterized in that: The computer controls the digital camera to acquire calibration images and experimental images. It calculates camera parameters through the calibration images, determines the mapping relationship between the actual physical distance and the pixel width in the image based on the camera parameters, performs three-dimensional cross-correlation or particle matching tracking analysis on the experimental images, obtains the number of particle displacement pixels in the test area, converts it into the physical distance in the real world according to the mapping relationship, and calculates the three-dimensional flow field data corresponding to the physical scale of the real world.

9. The particle image velocimetry system based on metalens according to claim 4, characterized in that: In the aforementioned meta-lens unit, different meta-lens units can use different operating wavelengths, or use the same operating wavelength, with the same focal length at different operating wavelengths.

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