Light field particle image velocimetry system and method based on meta-lens array

Abstract

The application provides a kind of light field particle image velocimetry system and method based on super lens array, comprising: super lens array, digital camera, main imaging lens, laser light source and computer;The super lens array contains a series of super lenses, constitutes a regular array, the light scattered or emitted by the tracer particle in the region to be measured is imaged, and the image formed is located in the plane of the digital camera photosensitive chip;The digital camera is used to collect the image of tracer particle in the flow field to be measured formed by the super lens array;The computer processes the calibration and experimental image data collected by the digital camera, calculates the camera parameters through the calibration image, and performs three-dimensional cross-correlation or particle matching tracking analysis on the experimental image to obtain the particle displacement of the region to be measured and three-dimensional flow field data;The application can also be effectively expanded to high-speed, microscopic and other special measurement fields, thereby effectively reducing the hardware cost and test space requirement.

Description

Technical Field

[0001] The present invention relates to the field of flow field measurement technology, and in particular, to a light field particle image velocimetry system and method based on a metalens array, and more particularly, to a particle image velocimetry system and method based on a metalens array and a single digital camera. Background Art

[0002] Flow field measurement is an important component of fluid dynamics research. The main measurement methods can be divided into contact and non-contact measurement. Non-contact measurement methods are highly favored due to their non-interference to the flow field, wide applicability, ease of operation, and high measurement accuracy. Non-contact flow field measurement primarily relies on 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 with the flow field to be measured, ability to obtain instantaneous and full-field flow velocity information, high resolution, and wide range of applications. Three-dimensional PIV technology uses multiple cameras to shoot the same area in the flow field, thereby analyzing the velocity field in the area to be measured. However, the use of multiple cameras makes the entire system more complicated. At the same time, multi-camera shooting requires calibration and synchronization of each camera. Compared with traditional PIV, the system complexity and difficulty of use of this method are relatively higher. Therefore, researchers have begun to study different ways to simplify the overall system complexity. For example, the use of microlens arrays to replace traditional lenses to form light field cameras can effectively simplify the system.

[0004] Mei et al. proposed a dual-light-field camera PIV technique, which uses two light-field cameras to measure tracer particles within the target area, effectively improving the resolution and accuracy of the light-field cameras. Tadd T. Truscott et al. proposed a three-dimensional microscopic light-field particle image velocimetry technique, combining a light-field camera with a microscope to achieve flow field measurement within a tiny area. However, all of these methods have a drawback: while the use of microlenses can effectively simplify the overall system complexity, the manufacturing errors of traditional microlens arrays can lead to unavoidable aberrations. Summary of the Invention

[0005] In view of the defects in the prior art, the purpose of the present invention is to provide a light field particle image velocimetry system and method based on a meta-lens array.

[0006] According to the present invention, a light field particle image velocimetry system based on a metalens array is provided, comprising:

[0007] Metalens array, digital camera, primary imaging lens, laser light source, and computer;

[0008] The intensity and thickness of the laser light source can be adjusted to evenly illuminate the entire test area.

[0009] The meta-lens array comprises a plurality of meta-lenses forming a regular array, which images the light scattered or emitted by the tracer particles in the area to be measured, and the image is located on the plane where the digital camera photosensitive chip is located;

[0010] The meta-lens includes a substrate and a nano-pillar array covering the substrate, which can control the phase of the incident light and realize the imaging function; different phase compensations are designed according to different needs;

[0011] The meta-lens is a phase modulation lens with a spherical aberration. It uses a single micron-thick two-dimensional meta-lens to refract and focus light incident from different directions onto the focal plane. The focal spot size of the meta-lens is close to the diffraction limit.

[0012] The digital camera is used to collect images of tracer particles in the flow field to be measured formed by the meta-lens array;

[0013] The computer processes calibration and experimental image data collected by the digital camera, calculates camera parameters based on the calibration images, and performs three-dimensional cross-correlation or particle matching pursuit analysis and calculation on the experimental images to obtain particle displacement and three-dimensional flow field data of the area to be measured;

[0014] The light emitted by the laser light source has an operating wavelength when the meta-lens is used.

[0015] Preferably, in the metalens array:

[0016] The metalens array comprises a series of metalenses, whose periodic arrangement includes a regular array of tetragonal and hexagonal lattices. The metalenses are arranged in a regular pattern, tangential to each other or with a preset gap between them. The metalenses are located in the same plane and have the same focal length. Within a designed working distance, the metalens array can simultaneously image the area to be measured.

[0017] The particle imaging velocimetry measurements are optimized by designing different metasurfaces. By designing a single-wavelength metalens with a narrow transmission band, the samples illuminated by the monochromatic laser of the particle imaging velocimetry can be accurately imaged.

[0018] Preferably, in the digital camera:

[0019] Use a black and white camera or a color camera.

[0020] Preferably, different meta-lenses use different operating wavelengths and have the same focal length at different operating wavelengths; the laser light source used has the ability to emit light at different operating wavelengths.

[0021] Preferably, the digital camera and the metalens array are combined to form a light field camera. The plane where the metalens array is located is parallel to the plane where the camera photosensitive chip is located. There are two options for the distance between the two planes:

[0022] The distance between the two planes is the same as the focal length of the metalens;

[0023] Or, if the Gaussian imaging formula is satisfied, when the distance between the two planes is a, the focal length of the metalens is f, and the distance between the main lens image plane and the metalens array plane is b, then 1 / a+1 / b=1 / f.

[0024] Preferably, the geometric shapes of the metalenses in the metalens array include circular, elliptical and polygonal.

[0025] According to a light field particle image velocimetry method based on a metalens array provided by the present invention, the light field particle image velocimetry system based on a metalens array is used to perform the following steps:

[0026] Step S1: evenly spread tracer particles in the area to be measured, and the tracking performance of the tracer particles in the flow field to be measured meets the measurement requirements;

[0027] Step S2: adjusting the main lens, meta-lens array, and digital camera so that the imaging system images the tracer particles in the area;

[0028] Step S3: Adjust the camera and light source parameters according to the flow field to be measured, synchronously control the light source and camera so that the laser illuminates the entire area to be measured, and use a computer to control the camera to capture images;

[0029] Step S4: Capture the calibration image and obtain calibration parameters;

[0030] Step S5: Separate the collected images and perform three-dimensional cross-correlation or three-dimensional particle tracking analysis and calculation to obtain a three-dimensional velocity field.

[0031] Preferably, in the metalens array:

[0032] The metalens array comprises a series of metalenses, whose periodic arrangement includes a regular array of tetragonal and hexagonal lattices. The metalenses are arranged in a regular pattern, tangential to each other or with a preset gap between them. The metalenses are located in the same plane and have the same focal length. Within a designed working distance, the metalens array can simultaneously image the area to be measured.

[0033] Different meta-lenses use different working wavelengths, and have the same focal length at different working wavelengths; the laser light source used has the ability to emit light at different working wavelengths.

[0034] Preferably, in the digital camera:

[0035] Use a black and white camera or a color camera;

[0036] The digital camera and the meta-lens array are combined to form a light field camera. The plane where the meta-lens array is located is parallel to the plane where the camera's photosensitive chip is located. There are two options for the distance between the two planes:

[0037] The distance between the two planes is the same as the focal length of the metalens;

[0038] Or if the Gaussian imaging formula is satisfied, when the distance between the two planes is a, the focal length of the metalens is f, and the distance between the primary lens image plane and the metalens array plane is b, then 1 / a+1 / b=1 / f;

[0039] The geometric shapes of the metalenses in the metalens array include circular, elliptical and polygonal;

[0040] The particle imaging velocimetry measurements are optimized by designing different metasurfaces. By designing a single-wavelength metalens with a narrow transmission band, the samples illuminated by the monochromatic laser of the particle imaging velocimetry can be accurately imaged.

[0041] Preferably, in step S5:

[0042] The reconstruction and cross-correlation algorithm uses a light field particle reconstruction algorithm based on the product algebraic reconstruction technology MART and the weight coefficient calculation method of dense ray tracing. The three-dimensional particle spatial distribution map is reconstructed from the light field particle image;

[0043] The steps are as follows: using dense ray tracing method to determine the weight coefficient of each pixel affected by non-zero voxel Voxel, according to the recorded pixel value and weight coefficient, using MART algorithm represented by the following formula to iteratively calculate the particle voxel value;

[0044]

[0045] Where, E(X j ,Y j ,Z j ) is the intensity of the j-th voxel, I(x i ,y i ) is the intensity of the i-th pixel, obtained from the captured light field image, w i,j is a weighting coefficient that quantifies the contribution of the light of the jth voxel to the intensity of the i-th pixel;

[0046] According to the dual-plane model, the main lens is discretized according to the number of pixels under the microlens. For a specific pixel, its weight coefficient is obtained by multiplying two parts: the ratio of the beam area of ​​all beams emitted by the point light source that hit a microlens to the total beam area is the weight coefficient w1; the ratio of the beam area on the CCD pixel below the hit microlens to the beam area entering this microlens is w2;

[0047] After obtaining the particle intensity field, a three-dimensional transient velocity field is obtained using a three-dimensional cross-correlation algorithm based on FFT, and the calculated velocity vector field is subjected to median filtering and linear interpolation.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] 1. This invention can achieve 3D-PIV measurement with only a single camera shot. It uses a meta-lens to replace the traditional micro-lens array, making the system structure more compact and the parameters easier to adjust while maintaining imaging quality. The meta-lens is designed as a phase modulation with a spherical aberration. Using only a single micron-thick two-dimensional meta-lens, light incident from different directions can be accurately refracted and focused onto the focal plane. Furthermore, the focal spot size of the meta-lens approaches the diffraction limit, resulting in better imaging resolution at the same numerical aperture.

[0050] 2. This invention uses a metalens array to achieve single-camera 3D imaging, capable of full-field velocity field measurement. Furthermore, PIV measurements can be optimized by designing different metasurfaces. By designing a single-wavelength metalens with a narrow transmission band, the sample illuminated by the monochromatic laser in PIV can be accurately imaged without chromatic aberration and with minimal influence from other ambient light sources.

[0051] 3. The present invention can also be effectively expanded to special measurement fields such as high-speed and microscopic measurements, thereby effectively reducing hardware costs and test space requirements;

[0052] 4. Thanks to the new optical structure of the system, the present invention can achieve convenient adjustment of system parameters like traditional Tomo-PIV. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0054] Figure 1 This is a schematic structural diagram of Example 4 of the present invention;

[0055] Among them, 1 is the laser light source, 2 is the main imaging lens, 3 is the computer, 4 is the meta-lens array, and 5 is the digital camera. DETAILED DESCRIPTION

[0056] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0057] Example 1:

[0058] The present invention discloses a particle image velocimetry system and method based on a metalens array and a single digital camera. The method comprises the following steps: first, a light field camera is formed using a metalens array 4 in combination with a single digital camera 5, with the light field camera being the main optical element and forming an image capture system with a traditional lens; then, an image of the flow area to be measured, which is illuminated by a laser and evenly distributed with tracer particles, is captured; finally, a traditional three-dimensional light field PIV algorithm is used based on the captured image to obtain the flow field of the area to be measured. The present invention replaces the traditional microlens array with a metalens, which has a compact structure, easy adjustment of system parameters, high optical design freedom, and elimination of the inevitable aberrations caused by the processing errors of the traditional microlens array. When conducting three-dimensional flow velocity field measurement experiments in special fields such as microscopy and high speed, it can more conveniently and effectively complete high-precision three-dimensional measurements. It solves the core problem of single-camera flow fields and promotes the development of particle image velocimetry flow field measurement technology.

[0059] This patent utilizes the imaging characteristics of meta-lens and proposes a three-dimensional particle image velocimetry method based on a meta-lens array. The meta-lens array is used to replace the traditional microlens array, and the image of the tracer particles in the test area is collected through the meta-lens array single camera system to calculate the three-dimensional velocity field. The number of cameras is reduced without adding unnecessary optical elements, and the time for camera calibration and synchronization is correspondingly reduced, which greatly increases the shooting efficiency and can more conveniently and effectively complete high-precision three-dimensional measurement.

[0060] According to a light field particle image velocimetry method based on a metalens array provided by the present invention, the light field particle image velocimetry system based on a metalens array is used to perform the following steps:

[0061] Step S1: evenly spread tracer particles in the area to be measured, and the tracking performance of the tracer particles in the flow field to be measured meets the measurement requirements;

[0062] Step S2: adjusting the main lens, metalens array 4 and digital camera 5 so that the imaging system images the tracer particles in the area;

[0063] Step S3: adjust the flow field camera and light source parameters for the required measurement, synchronously control the light source and the camera, so that the laser illuminates the entire area to be measured, and the camera is controlled by the computer 3 to collect images;

[0064] Step S4: take a calibration image to obtain calibration parameters;

[0065] Step S5: separate the collected images and perform three-dimensional cross-correlation or three-dimensional particle tracking analysis calculation to obtain a three-dimensional velocity field.

[0066] Preferably, in the superlens array 4:

[0067] The superlens array 4 includes a series of superlenses, and the periodic arrangement mode includes: a regular array composed of a square lattice arrangement and a hexagonal lattice arrangement; the superlenses satisfy the regular arrangement, are tangent to each other, or have a predetermined gap; the superlenses are located in the same plane and have the same focal length; within the designed working distance, the superlens array 4 can simultaneously image the measured area;

[0068] Different superlenses use different working wavelengths, and the focal lengths under different working wavelengths are the same; the laser light source 1 used has the ability to emit light of different working wavelengths.

[0069] Preferably, in the digital camera 5:

[0070] A black and white camera or a color camera is used;

[0071] The digital camera 5 and the superlens array 4 form a light field camera, the plane where the superlens array 4 is located is parallel to the plane where the camera photosensitive chip is located, and the distance between the two planes has two options:

[0072] The distance between the two planes is the same as the focal length of the superlens;

[0073] Or it satisfies the Gaussian imaging formula, that is, when the distance between the two planes is a, the focal length of the superlens is f, and the distance between the main lens image plane and the superlens array 4 plane is b, there is 1 / a+1 / b=1 / f;

[0074] The geometric shape of the superlens in the superlens array 4 includes a circle, an ellipse, and a polygon;

[0075] The particle imaging velocimetry technology measurement is optimized by designing different superlens surfaces; by designing a single-wavelength superlens with a transmission narrow band, the single-color laser illumination example of the particle imaging velocimetry technology can be accurately imaged.

[0076] Preferably, in the step S5:

[0077] The reconstruction and cross-correlation algorithm uses a light field particle reconstruction algorithm based on the product algebraic reconstruction technology MART and the weight coefficient calculation method of dense ray tracing. The three-dimensional particle spatial distribution map is reconstructed from the light field particle image;

[0078] The steps are as follows: using dense ray tracing method to determine the weight coefficient of each pixel affected by non-zero voxel Voxel, according to the recorded pixel value and weight coefficient, using MART algorithm represented by the following formula to iteratively calculate the particle voxel value;

[0079]

[0080] Where, E(X j ,Y j ,Z j ) is the intensity of the j-th voxel, I(x i ,y i ) is the intensity of the i-th pixel, obtained from the captured light field image, w i,j is a weighting coefficient that quantifies the contribution of the light of the jth voxel to the intensity of the i-th pixel;

[0081] According to the dual-plane model, the main lens is discretized according to the number of pixels under the microlens. For a specific pixel, its weight coefficient is obtained by multiplying two parts: the ratio of the beam area of ​​all beams emitted by the point light source that hit a microlens to the total beam area is the weight coefficient w1; the ratio of the beam area on the CCD pixel below the hit microlens to the beam area entering this microlens is w2;

[0082] After obtaining the particle intensity field, a three-dimensional transient velocity field is obtained using a three-dimensional cross-correlation algorithm based on FFT, and the calculated velocity vector field is subjected to median filtering and linear interpolation.

[0083] Example 2:

[0084] Example 2 is a preferred example of Example 1 and is used to illustrate the present invention in more detail.

[0085] The present invention also provides a light field particle image velocimetry system based on a metalens array. The light field particle image velocimetry system based on a metalens array can be implemented by executing the process steps of the light field particle image velocimetry method based on a metalens array. That is, those skilled in the art can understand the light field particle image velocimetry method based on a metalens array as a preferred embodiment of the light field particle image velocimetry system based on a metalens array.

[0086] According to the present invention, a light field particle image velocimetry system based on a meta-lens array is provided. Figure 1 Shown, including:

[0087] Metalens array 4, digital camera 5, main imaging lens 2, laser light source 1 and computer 3;

[0088] The light intensity and thickness of the laser light source 1 can be adjusted to evenly illuminate the entire test area;

[0089] The meta-lens array 4 comprises a plurality of meta-lenses forming a regular array, which images the light scattered or emitted by the tracer particles in the area to be measured, and the image is located on the plane where the photosensitive chip of the digital camera 5 is located;

[0090] The meta-lens includes a substrate and a nano-pillar array covering the substrate, which can control the phase of the incident light and realize the imaging function; different phase compensations are designed according to different needs;

[0091] The meta-lens is a phase modulation lens with a spherical aberration. It uses a single micron-thick two-dimensional meta-lens to refract and focus light incident from different directions onto the focal plane. The focal spot size of the meta-lens is close to the diffraction limit.

[0092] The digital camera 5 is used to collect images of tracer particles in the flow field to be measured formed by the meta-lens array 4;

[0093] The computer 3 processes the calibration and experimental image data collected by the digital camera 5, calculates the camera parameters through the calibration image, and performs three-dimensional cross-correlation or particle matching pursuit analysis and calculation on the experimental image to obtain the particle displacement and three-dimensional flow field data of the test area;

[0094] The light emitted by the laser light source 1 has the working wavelength of the meta-lens.

[0095] Specifically, in the metalens array 4:

[0096] The metalens array 4 comprises a series of metalenses, whose periodic arrangement includes a regular array of tetragonal and hexagonal lattices. The metalenses are arranged in a regular pattern, tangential to each other or with predetermined gaps between them. The metalenses are located in the same plane and have the same focal length. Within a designed working distance, the metalens array 4 can simultaneously image the area to be measured.

[0097] The particle imaging velocimetry measurements are optimized by designing different metasurfaces. By designing a single-wavelength metalens with a narrow transmission band, the samples illuminated by the monochromatic laser of the particle imaging velocimetry can be accurately imaged.

[0098] Specifically, in the digital camera 5:

[0099] Use a black and white camera or a color camera.

[0100] Specifically, different meta-lenses use different working wavelengths, and have the same focal length at different working wavelengths; the laser light source 1 used has the ability to emit light at different working wavelengths.

[0101] Specifically, the digital camera 5 is combined with the metalens array 4 to form a light field camera. The plane where the metalens array 4 is located is parallel to the plane where the camera photosensitive chip is located. There are two options for the distance between the two planes:

[0102] The distance between the two planes is the same as the focal length of the metalens;

[0103] Or, if the Gaussian imaging formula is satisfied, when the distance between the two planes is a, the focal length of the metalens is f, and the distance between the main lens image plane and the metalens array 4 plane is b, then 1 / a+1 / b=1 / f.

[0104] Specifically, the geometric shapes of the metalenses in the metalens array 4 include circular, elliptical and polygonal.

[0105] Example 3:

[0106] Example 3 is a preferred example of Example 1 and is used to illustrate the present invention in more detail.

[0107] This paper proposes a three-dimensional particle image velocimetry system and method using a metalens array. Taking into account the imaging characteristics of metalenses, a three-dimensional particle image velocimetry method based on a metalens array is proposed. Images are captured of the flow region to be measured, which is illuminated by a laser and evenly distributed with tracer particles. Based on the captured images, a traditional three-dimensional PIV algorithm is used to determine the flow field in the region to be measured.

[0108] The technical solution adopted by the present invention to solve the technical problem is:

[0109] A light field three-dimensional particle image velocimetry system based on a metalens array and a single digital camera, the system comprising a metalens array 4, a digital camera 5, a main imaging lens 2, a laser light source 1 and a computer 3;

[0110] The intensity and thickness of the laser light source 1 are adjustable, so that the entire test area can be evenly illuminated, and the camera can easily capture images with appropriate values;

[0111] The meta-lens array 4 comprises a series of small meta-lenses arranged in a regular array to image the light scattered or emitted by the tracer particles in the region to be measured. The image formed is located on the plane where the photosensitive chip of the digital camera 5 is located.

[0112] The metalens, composed of a substrate and an array of nanopillars covering it, modulates the phase of incident light to achieve imaging. In practice, different phase compensations can be designed for different operating wavelengths, focal length requirements, and locations on the metasurface.

[0113] The digital camera 5 is used to collect images of tracer particles in the flow field to be measured formed by the meta-lens array 4;

[0114] The computer 3 is used to process the calibration and experimental image data collected by the digital camera 5. First, the camera parameters are calculated using the calibration image. Then, three-dimensional cross-correlation or particle matching pursuit analysis and calculation are performed on the experimental image to obtain the particle displacement and three-dimensional flow field data of the test area.

[0115] Furthermore, the light emitted by the laser light source 1 has the working wavelength when designing the above-mentioned meta-lens.

[0116] Furthermore, the metalens array 4 includes a series of small metalenses, whose periodic arrangements include: a regular array composed of a tetragonal lattice arrangement and a hexagonal lattice arrangement; the small metalenses satisfy the regular arrangement and can be closely tangent to each other or have a certain gap between them; the metalenses are located in the same plane and have the same focal length; within the designed working distance, the metalens array 4 can simultaneously image the area to be measured.

[0117] Furthermore, the digital camera 5 is a black and white camera or a color camera;

[0118] Furthermore, in the metalens array 4, different metalenses can use different working wavelengths, but have the same focal length at different working wavelengths; correspondingly, the laser light source 1 used also has the ability to emit light at the above working wavelength.

[0119] Furthermore, the digital camera 5 is combined with the above-mentioned metalens array 4 to form a light field camera. The plane where the metalens array 4 is located is parallel to the plane where the camera photosensitive chip is located. There are two options for the distance between the two: 1. The distance between the two is the same as the focal length of the metalens; 2. The Gaussian imaging formula is satisfied, that is, when the distance between the two is a, the focal length of the metalens is f, and the distance between the main lens image plane and the metalens array 4 plane is b, 1 / a+1 / b=1 / f.

[0120] Furthermore, the geometric shapes of the metalenses in the metalens array 4 include circular, elliptical and polygonal.

[0121] A measurement method for a three-dimensional particle image velocimetry system using a metalens array 4 and a single digital camera 5 comprises the following steps:

[0122] 1) Uniformly distribute tracer particles in the region to be measured, the following ability of the tracer particles in the flow field to be measured meets the measurement requirements;

[0123] 2) Place the three-dimensional particle image velocimetry system in front of the region to be measured, adjust the main lens, the superlens array 4 and the digital camera 5 so that the imaging system can clearly image the tracer particles in the region;

[0124] 3) Adjust the camera and light source parameters of the flow field to be measured, synchronously control the light source and the camera, so that the laser illuminates the entire region to be measured, and use the computer 3 to control the camera to collect images;

[0125] 4) Take calibration images to obtain calibration parameters;

[0126] 5) Separate the collected images and perform three-dimensional cross-correlation or three-dimensional particle tracking analysis and calculation to obtain a three-dimensional velocity field.

[0127] A light field three-dimensional particle image velocimetry system based on a superlens array 4 and a single digital camera 5 includes a superlens array 4, a digital camera 5, a main imaging lens 2, a laser light source 1 and a computer 3; the laser light source 1 is used to uniformly illuminate the entire test region, so that the camera can conveniently collect images with suitable gray scale values, and the light intensity and the thickness of the light source are adjustable; the superlens array 4 includes a series of small superlenses, which are used to image the light scattered or emitted by the tracer particles in the region to be measured on a photosensitive chip; the superlens is composed of a substrate and a nanocolumn array covering the substrate, which realizes the regulation of the phase of incident light to realize the imaging function; the digital camera 5 is used to collect the images of the tracer particles in the flow field to be measured formed by the superlens array 4; the computer 3 is used to process the calibration and experimental image data collected by the digital camera 5, calculate the camera parameters through the calibration images, and then perform three-dimensional cross-correlation or particle matching tracking analysis and calculation on the experimental images to obtain the particle displacement of the region to be measured and the three-dimensional flow field data.

[0128] During use, the water tank is first filled with pure water, then the tracer particles are poured into the water tank and mixed evenly. Optical components are used to secure the laser to one side of the water tank. The laser's bottom translation stage is moved so that it faces the vortex ring's motion direction, and the laser's bottom rotation stage is adjusted so that the laser plane coincides with the vortex ring's center motion direction. Optical components are used to set the camera, metalens array 4, and main imaging lens 2 at the same height. The metalens array 4, camera, and main imaging lens 2 are aligned using a pitch mechanism to ensure parallelism. The metalens array 4 imaging system is then moved to the side of the laser plane's normal vector, and the rotation mechanism is adjusted to parallelize the metalens imaging system with the laser plane. The image distance and object distance are adjusted by adjusting the translation mechanism and main imaging lens 2 to ensure a clear image of the tracer particles on the camera's photosensitive chip. The laser intensity is adjusted so that the camera can easily capture images of the tracer particles with appropriate grayscale values. A vortex ring is ejected from the vortex ring generator, and a computer 3 is used to control the camera to capture images. PIV analysis is performed on the captured images to calculate the corresponding three-dimensional velocity field.

[0129] Those skilled in the art will appreciate that, in addition to implementing the system and its various devices, modules, and units provided by the present invention in purely computer-readable program code, it is entirely possible to implement the same functions of the system and its various devices, modules, and units provided by the present invention in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered 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; the devices, modules, and units for implementing various functions can also be considered as both software modules implementing the method and structures within the hardware component.

[0130] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. A light field particle image velocimetry system based on a metalens array, characterized in that: include: Metalens array, digital camera, primary imaging lens, laser light source, and computer; The intensity and thickness of the laser light source can be adjusted to evenly illuminate the entire area to be measured; The meta-lens array comprises a plurality of meta-lenses forming a regular array, which images the light scattered or emitted by the tracer particles in the area to be measured, and the image is located on the plane where the digital camera photosensitive chip is located; The meta-lens includes a substrate and a nano-pillar array covering the substrate, which can control the phase of the incident light and realize the imaging function; different phase compensations are designed according to different needs; The meta-lens is a phase modulation lens with a spherical aberration. It uses a single micron-thick two-dimensional meta-lens to refract and focus light incident from different directions onto the focal plane. The focal spot size of the meta-lens is close to the diffraction limit. The digital camera is used to collect images of tracer particles in the area to be measured formed by the meta-lens array; The computer processes calibration and experimental image data collected by the digital camera, calculates digital camera parameters based on the calibration images, and performs three-dimensional cross-correlation or particle matching pursuit analysis and calculation on the experimental images to obtain particle displacement and three-dimensional flow field data of the area to be measured; The wavelength range of the light emitted by the laser light source includes the operating wavelength of the metalens.

2. The light field particle image velocimetry system based on a metalens array according to claim 1, characterized in that: In the metalens array: The metalens array comprises a series of metalenses, whose periodic arrangement includes a regular array of tetragonal and hexagonal lattices. The metalenses are arranged in a regular pattern, tangential to each other or with a preset gap between them. The metalenses are located in the same plane and have the same focal length. Within a designed working distance, the metalens array can simultaneously image the area to be measured. The particle imaging velocimetry measurement is optimized by designing different metasurfaces; and the particles illuminated by the monochromatic laser of the particle imaging velocimetry technology can be accurately imaged by designing a single-wavelength metalens with a narrow transmission band.

3. The light field particle image velocimetry system based on a metalens array according to claim 1, characterized in that: In the digital camera: Use a black and white camera or a color camera.

4. The light field particle image velocimetry system based on a metalens array according to claim 1, characterized in that: Different meta-lenses use different working wavelengths, and have the same focal length at different working wavelengths; the laser light source used has the ability to emit light at different working wavelengths.

5. The light field particle image velocimetry system based on a metalens array according to claim 1, characterized in that: The digital camera and the meta-lens array are combined to form a light field camera. The plane where the meta-lens array is located is parallel to the plane where the digital camera's photosensitive chip is located. There are two options for the distance between the two planes: The distance between the two planes is the same as the focal length of the metalens; Or, if the Gaussian imaging formula is satisfied, when the distance between the two planes is a, the focal length of the metalens is f, and the distance between the main imaging lens plane and the metalens array plane is b, then 1 / a+1 / b=1 / f.

6. The light field particle image velocimetry system based on a metalens array according to claim 1, characterized in that: The geometric shapes of the metalenses in the metalens array include circular, elliptical and polygonal.

7. A light field particle image velocimetry method based on a meta-lens array, characterized in that: The light field particle image velocimetry system based on a metalens array according to any one of claims 1 to 6 is used to perform the following steps: Step S1: evenly spread tracer particles in the area to be measured, and the tracking performance of the tracer particles in the area to be measured meets the measurement requirements; Step S2: adjusting the main imaging lens, meta-lens array, and digital camera so that the imaging system images the tracer particles in the area; Step S3: Adjust the parameters of the digital camera and laser light source according to the flow field to be measured, synchronously control the laser light source and the digital camera so that the laser illuminates the entire area to be measured, and use a computer to control the digital camera to capture images; Step S4: Capture the calibration image and obtain calibration parameters; Step S5: Separate the collected images and perform three-dimensional cross-correlation or three-dimensional particle tracking analysis and calculation to obtain a three-dimensional velocity field.

8. The light field particle image velocimetry method based on a metalens array according to claim 7, characterized in that: In the metalens array: The metalens array comprises a series of metalenses, whose periodic arrangement includes a regular array of tetragonal and hexagonal lattices. The metalenses are arranged in a regular pattern, tangential to each other or with a preset gap between them. The metalenses are located in the same plane and have the same focal length. Within a designed working distance, the metalens array can simultaneously image the area to be measured. Different meta-lenses use different working wavelengths, and have the same focal length at different working wavelengths; the laser light source used has the ability to emit light at different working wavelengths.

9. The light field particle image velocimetry method based on a metalens array according to claim 7, characterized in that: In the digital camera: Use a black and white camera or a color camera; The digital camera and the meta-lens array are combined to form a light field camera. The plane where the meta-lens array is located is parallel to the plane where the digital camera's photosensitive chip is located. There are two options for the distance between the two planes: The distance between the two planes is the same as the focal length of the metalens; Or if the Gaussian imaging formula is satisfied, when the distance between the two planes is a, the focal length of the metalens is f, and the distance between the main imaging lens plane and the metalens array plane is b, then 1 / a+1 / b=1 / f; The geometric shapes of the metalenses in the metalens array include circular, elliptical and polygonal; The particle imaging velocimetry measurement is optimized by designing different metasurfaces; and the particles illuminated by the monochromatic laser of the particle imaging velocimetry technology can be accurately imaged by designing a single-wavelength metalens with a narrow transmission band.

10. The light field particle image velocimetry method based on a metalens array according to claim 7, characterized in that: In step S5: The reconstruction and cross-correlation algorithm realizes the light field particle reconstruction algorithm by using the weight coefficient calculation method based on the product algebraic reconstruction technology MART and dense ray tracing. The three-dimensional particle spatial distribution map is reconstructed from the light field particle image. The steps are as follows: using dense ray tracing method to determine the weight coefficient of each pixel affected by non-zero voxel Voxel, according to the recorded pixel value and weight coefficient, using MART algorithm represented by the following formula to iteratively calculate the particle voxel value; Where, E(X j ,Y j ,Z j ) is the intensity of the j-th voxel, I(x i ,y i ) is the intensity of the i-th pixel, obtained from the captured light field image, w i,j is a weighting coefficient that quantifies the contribution of the light of the jth voxel to the intensity of the i-th pixel; According to the dual-plane model, the main imaging lens is discretized according to the number of pixels below the metalens. For a specific pixel, its weight coefficient is obtained by multiplying two parts: the ratio of the beam area of ​​all beams emitted by the point light source that hits a metalens to the total beam area is the weight coefficient w1; and the ratio of the beam area on the CCD pixel below the hit metalens to the beam area entering this metalens is w2. After obtaining the particle intensity field, a three-dimensional transient velocity field is obtained using a three-dimensional cross-correlation algorithm based on FFT, and the calculated velocity vector field is subjected to median filtering and linear interpolation.

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