An apparatus for acquiring a background knife-edge matched map and a method for acquiring a background streak map

Abstract

The application discloses a background knife-edge matching image acquisition device and a background schlieren image acquisition method, and belongs to the technical field of schlieren imaging. The background knife-edge matching image acquisition device uses a transparent background particle plate as a particle background of a background schlieren image, and places the transparent background particle plate in a knife-edge schlieren light path to form a fusion light path. The knife-edge schlieren image and the background schlieren image are integrated on the same fusion light path, so that the matching image between the background schlieren image and the knife-edge schlieren image under the same flow field and the same visual angle condition of a gas density disturbance flow field can be collected. Further, the image A and the image B are subjected to pixel difference value calculation and filtering to remove particle residual images, so as to obtain an image D which only carries the knife-edge schlieren image; the image A and the image D are subjected to difference value calculation and particle morphology enhancement to obtain an image E which only contains particle information, and then particle displacement calculation is performed; the operation is simple, the calculation complexity is low, and the accurate background schlieren image can be quickly obtained.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of schlieren imaging, and more particularly relates to a background knife-edge matching image acquisition device and a background schlieren image acquisition method. BACKGROUND

[0002] Transparent fluid display technology gradually developed in the process of studying fluid, which makes the fluid flow invisible to the human eye visible to the naked eye, allowing people to directly observe various flow phenomena, such as tracer technology, schlieren technology, particle image velocimetry technology, fluorescent tracer technology, etc. Schlieren technology is a frequently used flow field visualization technology, which has a relatively simple structure and high sensitivity. Schlieren technology is a non-contact optical method for fluid measurement by observing the refraction of light, which was initially used to test the uniformity of optical glass and the quality of optical elements. Currently, various schlieren technologies such as black and white schlieren, color schlieren, and interference schlieren have been developed and widely used in boundary layer, shock wave, wake, combustion, and wind tunnel observation and measurement. Schlieren technology has many advantages, such as non-contact measurement, no interference with the measured flow field, fast response, real-time measurement of flow field parameters, direct flow image, and quantitative measurement data. In traditional schlieren technology, large-aperture lenses are usually used for experiments, which are extremely costly, so they are mostly used for small-scale flow display and are limited to the laboratory, which cannot be carried out outdoors and cannot meet the demand for large field measurement.

[0003] In contrast, background schlieren imaging technology is a new emerging schlieren technology suitable for large field of view, which has been widely used in wind tunnel testing, shock wave observation, and aircraft external flow field measurement. Its principle is to collect background spot images before and after disturbance, calculate particle displacement through image registration algorithm, and combine particle image velocimetry technology to infer the density change of the flow field. Compared with traditional schlieren technology, background schlieren technology has obvious advantages. First, background schlieren no longer quantifies light deflection by light intensity changes, but by particle image processing technology to obtain background spot displacement, thereby quantitatively obtaining light deflection from a certain area, avoiding the influence of ambient light or self-luminous measurement flow field. Second, it no longer requires a large number of optical lenses, avoiding the limitation of lens size on the range of measurement field. Thus, it can achieve observation and measurement of a larger flow field, saving a large amount of experimental equipment cost. Background schlieren technology can also use natural background, which is no longer limited to the particle background in the laboratory. However, background schlieren has its own limitations due to the combination with particle image processing technology. Since particle image processing technology obtains the displacement change of the entire window by solving the iterative window, in addition to the limitation of physical resolution, the window size in the algorithm also limits its resolution, making the background schlieren imaging resolution much lower than that of traditional knife-edge schlieren.

[0004] With the development of deep learning technology, it is possible to map a low-resolution image to a high-resolution image based on matching image data, and the difficulty lies in how to obtain a matching image of a traditional knife-edge schlieren and a background schlieren with the same field of view and the same perspective. SUMMARY

[0005] In view of the above defects or improvement needs of the prior art, the present application provides a background knife-edge matching image acquisition device and a background schlieren image acquisition method, which aims to solve the technical problem that the prior art is difficult to obtain a matching image of a traditional knife-edge schlieren and a background schlieren with the same field of view and the same perspective.

[0006] To achieve the above-mentioned purpose, according to one aspect of the present application, a background knife-edge matching image acquisition device is provided, comprising:

[0007] a light source for emitting a first light signal;

[0008] a transparent background particle plate arranged on the propagation path of the first light signal, the transparent background particle plate being a transparent plate carrying particles thereon, the light-blocking property of the particles being higher than that of the transparent plate; the transparent background particle plate is used for receiving the first light signal and partially transmitting it to obtain a second light signal carrying a background schlieren; wherein the second light signal is propagated to a flow field to be measured and partially transmitted through the flow field to be measured, to obtain a third light signal carrying a background schlieren and flow field information;

[0009] a knife-edge arranged on the propagation path of the third light signal, for receiving the third light signal and blocking part of the light spot to obtain a fourth light signal carrying a background schlieren, flow field information and a knife-edge schlieren;

[0010] an image collector arranged on the propagation path of the fourth light signal, for converting the collected fourth light signal into a matching image between the background schlieren and the knife-edge schlieren.

[0011] In one embodiment, the light source, the transparent background particle plate and the knife-edge are arranged on the same main optical axis, further comprising:

[0012] a collimating lens arranged on the exit light path of the first light signal, for collimating the first light signal so that the collimated first light signal is propagated to the transparent background particle plate;

[0013] a converging lens arranged on the propagation path of the third light signal, for converging the third light signal onto the knife-edge.

[0014] In one embodiment, the light source, the collimating lens, the transparent background particle plate, the converging lens, the knife-edge and the image collector are arranged on the same main optical axis.

[0015] In one of the embodiments, the light source, the transparent background particle plate and the knife edge are not arranged on the same principal optical axis, and the transparent background particle plate is arranged on the same optical axis as the knife edge, and the apparatus further comprises:

[0016] a first concave mirror arranged on the light path of the first light signal and used for reflecting the first light signal to the transparent background particle plate;

[0017] The distance between the light source and the first concave mirror is twice the focal length of the first concave mirror.

[0018] In one of the embodiments, the light source, the transparent background particle plate and the knife edge are not arranged on the same principal optical axis, and the apparatus further comprises:

[0019] a second concave mirror arranged on the light path of the first light signal and used for reflecting the first light signal to the transparent background particle plate;

[0020] a third concave mirror arranged on the light path of the third light signal and used for reflecting the third light signal to the knife edge.

[0021] In one of the embodiments, the apparatus further comprises a condenser lens and a slit.

[0022] The condenser lens is arranged on the light path of the first light signal and used for focusing the first light signal to the slit and emitting the first light signal to the second concave mirror through the slit.

[0023] According to another aspect of the present application, a method for obtaining a background schlieren image is provided, comprising:

[0024] S1: performing pixel difference calculation and filtering to remove particle residual image between image A and image B, and obtaining an image D carrying only knife edge schlieren;

[0025] The image A is a matching image collected by an image collector of a background knife edge matching image obtaining apparatus, and the image B is an image directly collected by the image collector without a flow field to be measured.

[0026] S2: performing difference calculation and particle morphology enhancement between the image A and the image D to obtain an image E containing only particle information;

[0027] S3: performing particle displacement calculation on the image E to obtain a background schlieren image.

[0028] In one of the embodiments, the S1 comprises:

[0029] The first difference image is obtained by pixel-wise difference calculation of image A and image B, the first difference image is superimposed with image C, and filtering is performed to remove particle residual image, thereby obtaining image D.

[0030] The image C is an image directly collected by the image collector when the transparent background particle plate is removed and there is no flow field to be measured in the background knife edge matching image acquisition device.

[0031] In one embodiment, S2 includes: performing difference calculation on image A and image D to obtain a second difference image, superimposing the second difference image with image C, and performing particle morphology enhancement to obtain image E.

[0032] In one embodiment, S3 includes: performing particle displacement calculation on image E by using image B to obtain the background schlieren image.

[0033] Overall, compared with the prior art, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0034] (1) The present application provides a background knife edge matching image acquisition device, which uses a transparent background particle plate as the particle background of the background schlieren, and places the transparent background particle plate in the knife edge schlieren light path to form a fusion light path. The knife edge schlieren and the background schlieren are integrated on the same fusion light path, which realizes the collection of the matching image between the background schlieren and the knife edge schlieren under the same flow field and the same viewing angle condition of the gas density disturbance flow field.

[0035] (2) The light source, flow field, lens, knife edge and image collector in the transmission type schlieren light path are arranged on the same main optical axis. The structure is simple and the light path loss is small.

[0036] (3) The single-mirror off-axis reflection type schlieren light path places a concave mirror and a light source and an image collector at both ends. The light source and the image collector are arranged on both sides of the central axis of the mirror. The distance between the light source and the concave mirror is twice the focal length. A knife edge is placed at the focal point of the mirror on the same side as the image collector to cut the light spot. The single-mirror off-axis reflection type schlieren light path can adapt to the scene where the light source, the transparent background particle plate are not arranged on the same main optical axis, and the transparent background particle plate and the knife edge are arranged on the same optical axis.

[0037] (4) The Z-shaped light path is folded into a Z shape by two concave mirrors (second concave mirror and third concave mirror). The light emitted by the light source is focused by the lens, passes through the slit, and then is incident on the second concave mirror. After reflection by the second concave mirror, the light becomes parallel light. The Z-shaped light path can adapt to the scene where the light source, the transparent background particle plate and the knife edge are not arranged on the same main optical axis.

[0038] (5) The application also provides a background schlieren image acquisition method. When the above device is used to collect image C, the optical path remains in the initial state, the transparent background particle plate is not placed, and the flow field is not applied. Then the transparent background particle plate is placed, but the flow field disturbance is still not applied, and image B is collected. When image A is collected, the transparent background particle plate is kept in place, and the flow field disturbance is added. The operation is simple, and complex algorithms are not required. Further, image A and image B are calculated by pixel difference value and filtered to remove particle residual images to obtain image D which only carries the knife edge schlieren; image A and image D are calculated by difference value and particle morphology enhancement to obtain image E which only contains particle information; the particle vector displacement is obtained from image E by using the optical flow algorithm, cross-correlation algorithm and the like, and finally the horizontal or vertical component amplitude of the particle vector displacement is obtained to obtain the background schlieren image. The operation is simple, the calculation complexity is low, and the accurate background schlieren image can be quickly obtained. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is the transmission type schlieren and background schlieren optical path provided by the embodiment 1 of the application.

[0040] Figure 2 is the single mirror off-axis reflection type schlieren and background schlieren optical path provided by the embodiment 1 of the application.

[0041] Figure 3 is the Z-shaped schlieren and background schlieren optical path provided by the embodiment 1 of the application.

[0042] In all the above drawings, the same reference signs are used to represent the same elements or structures, wherein 1 is a light source, 2 is a collimating lens, 3 is a transparent background particle plate, 4 is a flow field to be measured, 5 is a converging lens, 6 is a knife edge, 7 is an image collector, 8 is a first concave mirror, 9 is a condenser lens, 10 is a slit, 11 is a second concave mirror, and 12 is a third concave mirror.

[0043] Figure 4 is the algorithm flowchart for acquiring the background schlieren image provided by the embodiment 1 of the application. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical scheme and advantages of the application clearer, the application is further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the application and do not limit the application. In addition, the technical features involved in each embodiment of the application described below can be combined with each other as long as they do not conflict with each other.

[0045] Embodiment 1

[0046] This embodiment provides a device for acquiring a background schlieren matching image, including: a light source 1, a transparent background particle plate 3, a schlieren 6, and an image acquisition device 7. The light source 1 emits a first light signal; the transparent background particle plate 3 is disposed in the propagation path of the first light signal, and is a transparent plate carrying particles, the particles having a higher light-blocking capacity than the transparent plate; the transparent background particle plate 3 receives the first light signal and partially transmits it to obtain a second light signal carrying background schlieren; the second light signal is propagated onto the flow field 4 to be measured and partially transmitted through the flow field 4 to obtain a third light signal carrying background schlieren and flow field information; the schlieren 6 is disposed in the propagation path of the third light signal, and receives the third light signal and partially blocks the light spot to obtain a fourth light signal carrying background schlieren, flow field information, and schlieren; the image acquisition device 7 is disposed in the propagation path of the fourth light signal, and converts the acquired fourth light signal into a matching image between the background schlieren and the schlieren.

[0047] for Figure 1 , Figure 2 , Figure 3 The key aspects of the transparent background particle board 3 lie in the selection criteria of the background substrate, the particle morphology and layout characteristics, and the particle optical properties. For the transparent background particle board 3, to minimize the attenuation of light intensity in the knife-edge lithography optical path and allow a high proportion of light to pass through, the background substrate needs good light transmittance. Transparent PVC, glass, etc., can be used as the substrate material. To ensure that the background substrate does not affect the knife-edge lithography optical path, the substrate thickness should be uniform to avoid non-uniform reflection or refraction when light passes through it. To ensure the stable morphology of the transparent background particle board 3 at different image acquisition stages, the background substrate should have physical stability, meaning its morphology and light transmittance should not be affected by vibration, temperature, airflow, etc., in the experimental environment.

[0048] Regarding the particle morphology on the transparent background particle board 3, the particles should be close to dots, that is, the particle diameter and shape should be such that they appear as dots in the image captured by the image acquisition device 7. Preferably, the particle dot size is controlled at 3 to 5 pixels.

[0049] In terms of particle layout, the particles should be as evenly and randomly distributed as possible in the area through which the light path passes, with a spacing between particle centers of at least 3 times the average diameter of the particles, so that most of the light is not blocked by the particles. Preferably, the total area of the particles can be set to cover 5% to 15% of the light path, and a density that is too high will affect the light transmittance of the transparent background particle plate 3, and a density that is too low will result in low displacement processing accuracy. The particle morphology and layout can be generated using open source tools such as Imagico Dot Pattern Generator. This tool can adjust the particle density, particle size, and randomness of the pattern, and is suitable for background patterns in background schlieren. In terms of particle optical properties, in order to observe clear particles in the image captured by the final image collector 7, the particles in the transparent background particle plate 3 should have poorer light transmittance relative to the substrate. Preferably, the light transmittance between particles and within particles tends to be consistent, but not completely blocking light transmission. The particles can be printed by inkjet printing, laser printing, manual production, etc.

[0050] In one embodiment, the light source 1, the transparent background particle plate 3, and the knife edge 6 are disposed on the same main optical axis, further comprising: a collimating lens 2 disposed on the outgoing light path of the first light signal, for collimating the first light signal, so that the collimated first light signal is propagated onto the transparent background particle plate 3; a converging lens 5 disposed on the propagation light path of the third light signal, for converging the third light signal onto the knife edge 6. Further, the light source 1, the collimating lens 2, the transparent background particle plate 3, the converging lens 5, the knife edge 6, and the image collector 7 are disposed on the same main optical axis.

[0051] As shown in Figure 1 When setting up the light path of the transmission schlieren combined with the background schlieren, first, the optical centers of the light source (such as a point light source) 1, the collimating lens 2, the converging lens 5, and the image collector 7 are placed at the same horizontal height. The light emitted by the light source 1 becomes collimated light after passing through the collimating lens 2, and is converged by the converging lens 5. The knife edge 6 placed at the focal point of the converging lens 5 blocks part of the light spot (preferably, half of the focal point light spot), and the part of the light spot not blocked by the knife edge 6 is finally captured by the image collector 7. The knife edge 6 has no light transmittance and has a thickness as thin as possible. Optionally, it can be a blade like a Swiss knife. The transparent background particle plate 3 and the flow field 4 to be observed are placed in turn between the collimating lens 2 and the converging lens 5 according to the subsequent image acquisition process.

[0052] In one embodiment, the light source 1 and the transparent background particle plate 3 are not disposed on the same main optical axis, and the transparent background particle plate 3 and the knife edge 6 are on the same optical axis, further comprising: a first concave mirror 8 disposed on the outgoing light path of the first light signal, for reflecting the first light signal onto the transparent background particle plate 3.

[0053] As shown in Figure 2As shown, the optical path for combining single-mirror off-axis reflective schlieren and background schlieren includes a light source 1, a knife edge 6, a first concave mirror 8, and an image acquisition device 7, all with their principal optical axes on the same plane. First, the light source 1 is placed near twice the focal length of the first concave mirror 8, on one side of its principal optical axis. The light emitted from the light source 1 is reflected by the first concave mirror 8 and converges on the other side of its principal optical axis. The knife edge 6 is then placed at the converged spot to partially block the light, and the unblocked portion is ultimately captured by the image acquisition device 7. Similarly, the transparent background particle plate 3 and the flow field to be observed 4 are placed sequentially between the first concave mirror 8 and the light source 1, following the image acquisition process described later, and are positioned closer to the first concave mirror 8 to obtain the largest possible measurement field of view.

[0054] In one embodiment, the light source 1, the transparent background particle plate 3, and the blade edge 6 are not positioned on the same principal optical axis. The system further includes: a second concave reflector 11, disposed in the outgoing optical path of the first light signal, for reflecting the first light signal onto the transparent background particle plate 3; and a third concave reflector 12, disposed in the propagation optical path of the third light signal, for reflecting the third light signal onto the blade edge 6. Further, the system includes: a condenser lens 9 and a slit; the condenser lens 9, in the outgoing optical path of the first light signal, focuses the first light signal onto the slit, and then outputs it through the slit to the second concave reflector 11.

[0055] like Figure 3 As shown, when constructing the optical path combining the Z-shaped reflective schlieren and the background schlieren, the light emitted by the light source 1 is focused at the slit 10 by the condenser lens 9, and then forms a collimated light measurement area between two concave mirrors 11 and 12 with identical parameters. After the light spot is focused by the third concave mirror 12, a knife edge 6 is placed at the spot to block part of it. Preferably, half of the spot is blocked. Finally, the unblocked spot is captured by the image acquisition device 7 to form an image of the measurement area. Similarly, the transparent background particle plate 3 and the flow field to be observed 4 are placed between the second concave mirror 11 and the third concave mirror 12 in sequence according to the image acquisition process proposed later.

[0056] Example 2

[0057] This embodiment provides a method for obtaining a background schlieren image, such as... Figure 4As shown, comprising: S1: calculate the difference between image A and image B according to pixels, and filter out particle residues to obtain image D carrying only knife edge moire. Wherein, image A is the matching image collected by image collector 7 in the acquisition device for providing background knife edge matching image of embodiment 1; image B is the image directly collected by image collector 7 when the acquisition device for providing background knife edge matching image of embodiment 1 has no flow field 4. S2: calculate the difference between image A and image D, and perform particle morphology enhancement to obtain image E containing only particle information. S3: perform particle displacement calculation on image E to obtain a background moire image.

[0058] The above device is used to collect image A (which can be a frame in a continuous video) with a transparent particle plate and a flow field, image B with a transparent particle plate and no flow field, and image C with no transparent particle plate and no flow field. First, the above optional fusion moire optical system is built, and the height of light source 1, lens, concave mirror, knife edge 6, transparent particle plate, flow field, and image collector 7 is adjusted so that their optical centers are at the same height. When adjusting the positions of the devices in the optical path and the focal length of image collector 7, the light emitted by light source 1 should fill the entire image field in the image of image collector 7 as much as possible, so as to eliminate the influence of ambient light. Then, the brightness of light source 1 and the exposure parameters of image collector 7 are adjusted so that the image collected by image collector 7 is evenly illuminated and not overexposed. Finally, image collection is performed in stages, and the controllability of experimental variables and the consistency of the perspective and field of view of the schlieren image are ensured by gradually introducing the transparent background particle plate and the flow field while keeping other settings and parameters unchanged.

[0059] After the above steps are completed, image collector 7 can capture the knife edge schlieren and background schlieren fusion image C without a transparent particle plate and a flow field. Then, only the transparent background particle plate 3 is placed, and image collector 7 captures image B. Finally, the flow field is further applied, and image collector 7 captures image or video A. During the capture of image or video A, image B, and image C, all system parameters remain constant except for the sequential placement of transparent background particle plate 3 and the application of the flow field.

[0060] Through the above acquisition process, the present application constructs a fusion image acquisition system that can simultaneously capture knife edge schlieren and background schlieren, and can acquire fusion images of knife edge schlieren and background schlieren with the same field of view and flow field. This avoids the mismatch between background schlieren and knife edge schlieren caused by changes in the optical path and different perspectives, and is compatible with different types of optical elements and image collection devices.

[0061] In one of the embodiments, S1 comprises: calculating a first difference image by pixel-wise difference between image A and image B, and filtering to remove particle residue, to obtain image D; wherein image C is an image directly collected by image collector 7 when transparent background particle plate 3 is removed and there is no flow field 4 to be measured in the image acquisition device for obtaining the background knife-edge matching image. Further, S2 comprises: calculating a second difference image by difference between image A and image D, and performing particle morphology enhancement, to obtain image E.

[0062] In one of the embodiments, S1 comprises: calculating a first difference image by pixel-wise difference between image A and image B, and filtering to remove particle residue, to obtain image D; wherein image C is an image directly collected by image collector 7 when transparent background particle plate 3 is removed and there is no flow field 4 to be measured in the image acquisition device for obtaining the background knife-edge matching image provided in embodiment 1. Further, S2 comprises: calculating a second difference image by difference between image A and image D, and performing particle morphology enhancement, to obtain image E.

[0063] Specifically, input image A with transparent background particle plate and flow field (if it is a video, extract each frame of the video as a series of images A), image B with transparent background particle plate and no flow field, and image C with no transparent background particle plate and no flow field. According to the aforementioned acquisition steps, images A, B and C have the same size. If the three images are obtained in other ways, they need to be pre-processed to the same size. To obtain a separate knife-edge schlieren image, preferably, the image data type is first converted to floating point type. According to step S1, the result of A-B+C is calculated by pixel-by-pixel calculation. Step S1 can also be selected to calculate the result of A-B by pixel-by-pixel calculation, but the overall image is darker. Optionally, a fixed constant is added to the subsequent full image to improve the brightness of the image. For example, for an image with pixel value boundary of 0 and 255, the fixed constant can be selected as 128. Subsequently, preferably, the data type of the calculation result is converted to unsigned integer type. To keep the image size unchanged, preferably, the image is expanded by copying the edge pixel value.

[0064] Regarding the removal of particle residue. In the filtering method, the method of two times of median filtering can be selected: in the first time of median filtering, for each pixel point, the pixel values in its 7x7 neighborhood are selected, sorted and the median value is taken to replace the original pixel value. In the second time of median filtering, a 9x9 neighborhood is selected for the same operation. After median filtering, the particles in the image are basically removed, only containing the fluid image, to obtain the knife-edge schlieren image D.

[0065] Subsequently, the data type of the calculation result is converted to unsigned integer. Next, the particle and background contrast of the A-D+C result is increased by particle morphological enhancement, and the particles are filled to be solid. The black hat algorithm is optional, and the processing is performed by a 13x13 rectangular structure element. The black hat algorithm is a morphological operation for enhancing dark details or hole structures in an image. The core step of the black hat operation is to perform the operation of first expansion and then corrosion, and then subtract the original image, which can be realized by using the function in the open source tool such as OpenCV.

[0066] Subsequently, the data type of the calculation result is converted to unsigned integer. Next, the particle and background contrast of the A-D+C result is increased by particle morphological enhancement, and the particles are filled to be solid. The black hat algorithm is optional, and the processing is performed by a 13x13 rectangular structure element. The black hat algorithm is a morphological operation for enhancing dark details or hole structures in an image. The core step of the black hat operation is to perform the operation of first expansion and then corrosion, and then subtract the original image, which can be realized by using the function in the open source tool such as OpenCV.

[0067] In one embodiment, S3 includes: performing particle displacement calculation on image E by using image B to obtain a background streak image.

[0068] Specifically, in the particle displacement calculation, two particle images are used, and the particle vector displacement can be obtained by using the optical flow algorithm, the cross-correlation algorithm, etc., and then the horizontal or vertical component size of each particle vector displacement is used as the background streak image. In the two particle images, in optional scheme one, any two images E are used, and in optional scheme two, one image E and image B are used for particle morphological enhancement.

[0069] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present application, and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A device for acquiring a background knife edge matching image, characterized in that, include: A light source, used to emit the first light signal; A transparent background particle plate is disposed in the propagation optical path of the first optical signal. The transparent background particle plate is a transparent plate carrying particles, and the light-blocking property of the particles is higher than that of the transparent plate. The transparent background particle plate is used to receive the first optical signal and partially transmit it to obtain a second optical signal carrying background schlieren. The second optical signal is propagated to the flow field to be measured and partially transmitted through the flow field to obtain a third optical signal carrying background schlieren and flow field information. The blade is positioned on the propagation optical path of the third optical signal to receive the third optical signal and block part of the light spot, thereby obtaining a fourth optical signal carrying background schlieren, flow field information and blade schlieren. An image acquisition device is set in the propagation optical path of the fourth optical signal to convert the acquired fourth optical signal into a matching map between the background schlieren and the knife edge schlieren.

2. The device for acquiring the background blade edge matching image as described in claim 1, characterized in that, The light source, the transparent background particle board, and the blade are arranged on the same main optical axis, and the system further includes: A collimating lens is disposed in the outgoing light path of the first light signal to collimate the first light signal so that the collimated first light signal is propagated onto the transparent background particle plate. A converging lens is disposed in the propagation optical path of the third optical signal to converge the third optical signal onto the knife edge.

3. The device for obtaining the background blade edge matching image as described in claim 2, characterized in that, The light source, the collimating lens, the transparent background particle plate, the converging lens, the blade, and the image acquisition device are arranged on the same main optical axis.

4. The device for acquiring a background blade edge matching image as described in claim 1, characterized in that, The light source and the transparent background particle board are not positioned on the same main optical axis, while the transparent background particle board and the blade are on the same optical axis. The system also includes: A first concave reflector is disposed in the outgoing light path of the first light signal and is used to reflect the first light signal onto the transparent background particle plate. The distance between the light source and the first concave mirror is twice the focal length of the first concave mirror.

5. The device for acquiring a background blade edge matching image as described in claim 1, characterized in that, The light source, the transparent background particle board, and the blade edge are not all on the same main optical axis, and the system further includes: The second concave reflector is disposed in the outgoing light path of the first light signal and is used to reflect the first light signal onto the transparent background particle plate. A third concave reflector is disposed in the propagation path of the third optical signal to reflect the third optical signal onto the knife edge.

6. The device for acquiring the background blade edge matching image as described in claim 5, characterized in that, Also includes: Condensing lens and slit; The focusing lens is used to focus the first light signal onto the slit in the outgoing light path of the first light signal, and then outgoing it through the slit to the second concave reflector.

7. A method for obtaining a background schlieren image, characterized in that, include: S1: Calculate the difference between image A and image B according to pixels and filter to remove particle afterimages, to obtain image D that only carries knife edge tattoos; Wherein, image A is the matching image acquired by the image acquisition device of the background knife edge matching image acquisition device according to any one of claims 1-6; image B is the image directly acquired by the image acquisition device when there is no flow field to be measured; S2: Perform difference calculation and particle morphology enhancement on image A and image D to obtain image E containing only particle information; S3: Perform particle displacement calculation on the image E to obtain a background schlieren image.

8. The method for obtaining a background schlieren image as described in claim 7, characterized in that, S1 includes: The first difference map is obtained by calculating the difference between image A and image B according to the pixel. The first difference map is superimposed on image C and filtered to remove particle afterimages to obtain image D. Wherein, image C is the image directly acquired by the image acquisition device when the transparent background particle plate is removed and there is no flow field to be measured in the background knife edge matching image acquisition device according to any one of claims 1-6.

9. The method for obtaining a background schlieren image as described in claim 8, characterized in that, S2 includes: The difference between image A and image D is calculated to obtain a second difference map. The second difference map is superimposed on image C and particle morphology enhancement is performed to obtain image E.

10. The method for obtaining a background schlieren image as described in claim 7, characterized in that, S3 includes: using image B to perform particle displacement calculation on image E to obtain the background schlieren image.

Images