Modularized coaxial schlieren imaging system and use method thereof

Abstract

The invention relates to the field of optical imaging, in particular to a modular coaxial schlieren imaging system and a use method thereof. A modularized coaxial schlieren imaging system comprises a light source module, a spectroscope module, a knife edge module, a camera module, a test area module and a reflector module. According to the system, high flexibility and expandability are realized through modular design, and free adjustment and configuration can be carried out according to different experiment requirements. Due to the design of the coaxial optical path, the optical path error is greatly reduced, the imaging precision is improved, and a high-resolution schlieren image is ensured. Meanwhile, the system is compact and portable in overall design, and can be conveniently used in a scene with limited space or needing frequent movement. Besides, through the application of a high-precision optical element and a sensitivity-adjustable knife edge module, tiny fluid density changes can be accurately captured, the device is suitable for scientific research, engineering application and high-precision industrial detection, and the experiment and detection efficiency is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of optical imaging, and in particular to a modular coaxial schlieren imaging system and a method of using the same. Background Art

[0002] Schlieren imaging is an optical technique used to visualize changes in fluid density, especially showing small differences in refractive index in air or other gaseous media. The core principle of Schlieren imaging is that when light propagates in different media, it is deflected due to small changes in refractive index, resulting in brightness differences in the imaging system. This difference can reflect the density gradient in the fluid, making the invisible gas flow phenomenon visible.

[0003] Traditional Schlieren imaging systems usually include a point light source, collimating lens, test area, knife edge, imaging lens, camera, etc. With the advancement of technology and the diversification of application requirements, traditional Schlieren imaging systems have certain limitations in modularity, flexibility and coaxial design.

[0004] 1. Large size: Due to its complex optical path and large-scale optical components, it is difficult to use in environments with limited space or high portability requirements.

[0005] 2. Lack of flexibility: Traditional systems cannot adjust components such as light sources and imaging devices according to specific experimental requirements, lack modular design, and have limited application scope.

[0006] 3. Complex optical coaxial design: Since the optical axes of the optical components are not collinear, the optical path is prone to deviation, resulting in reduced imaging accuracy and difficulty in maintaining consistency.

[0007] It is difficult for existing technologies to achieve coaxial and modular systems while maintaining high-precision imaging. This has led to an urgent need for a more advanced, modular, and coaxial Schlieren imaging system. Summary of the invention

[0008] In order to solve the above problems, the present invention proposes a modular coaxial schlieren imaging system to achieve high-precision and flexibly configured schlieren imaging.

[0009] A modular coaxial Schlieren imaging system comprises a light source module, a beam splitter module, a knife-edge module, a camera module, a test area module and a reflector module; the light source module, the beam splitter module, the knife-edge module and the camera module are assembled by a detachable cage bracket, the knife-edge module is located between the camera module and the beam splitter module, the light source module is fixed on one side of the beam splitter module, and the test area module and the reflector module are placed on the other side of the beam splitter module and in the vertical direction of the line connecting the beam splitter module and the camera module;

[0010] The light source module includes an energy optical fiber and an LED light source. The light source module outputs a point light source by coupling the LED light source with the energy optical fiber. The core diameter of the energy optical fiber is adapted to the brightness of the light source. The numerical aperture of the energy optical fiber is adapted to the diameter and focal length of the concave reflector in the reflector module. The point light source of the light source module is located at the curvature center of the concave reflector of the reflector module, so that the light is reflected along the incident path and accurately returns to the position of the point light source.

[0011] Furthermore, the intensity, color temperature and spot size of the light source of the light source module are adjustable; the spectroscope module, knife-edge module, test area module and reflector module are all provided with adjustment devices, which are adjusted according to the different objects to be tested or the environments to be tested, so that the optical axes of the light source, concave reflector, knife-edge and camera module are strictly aligned.

[0012] Furthermore, the cage bracket includes an upper bottom, a lower bottom, an upper partition, a lower partition and a plurality of pillars, the pillars and the upper and lower bottoms are installed by screws, the upper and lower partitions installed on the pillars have through holes, the spectrometer module is installed below the upper bottom, and the camera module is installed on the pillars through the lower partition; the knife edge module is installed on the upper partition between the camera module and the spectrometer module, and the light source module is installed on one side of the upper partition so that the emission point of the point light source is located directly behind the spectrometer; the cage bracket is surrounded by partitions with uniform thickness, and the partitions are provided with an opening in front of the spectrometer for the light beam to pass through.

[0013] Furthermore, the beam splitter of the beam splitter module is mounted on the upper end of the cage bracket, the beam splitter is fixed on the mounting bracket, the mounting bracket is mounted on the upper bottom of the cage bracket through precision fine-thread adjustment screws, and the precision fine-thread adjustment screws fine-tune the reflection direction of the beam splitter.

[0014] Furthermore, the blade module includes a blade, a fixing plate with a groove and a circular ring-shaped lever with a protrusion. The blade is pasted on the fixing plate. One end of the fixing plate is fixed to the circular ring of the lever by a pin, and the protrusion at the other end corresponds to the limiting hole in the groove of the fixing plate. Driven by the lever, the blade swings slightly in the groove of the fixing plate with the protrusion of the fixing plate as a fulcrum. The blade position is accurately adjusted by the lever according to the sensitivity requirement of the refractive index change in the experiment to increase or decrease the sensitivity of the system to tiny light deflection.

[0015] Furthermore, the test area module includes a box with optical windows at both ends and a clamping device installed on the bottom of the box. The clamping device can adjust the height and rotation angle to fix experimental objects of different sizes and shapes, flexibly adjust the position and angle of the object to be tested, and ensure that the light beam can fully pass through the area to be tested; the reserved optical window ensures that the light beam can pass through the test area without interference.

[0016] Furthermore, the camera module includes a high-resolution imaging sensor, and the exposure time, frame rate or resolution parameters are adjusted according to different experimental environments to ensure that the clarity and dynamic range of the Schlieren image are optimal.

[0017] Furthermore, the reflector module includes a bracket and a concave reflector mounted on the bracket by snap-fitting, and three adjustment screws are arranged around the concave reflector.

[0018] The modular coaxial Schlieren imaging system of this application has the following optical path design:

[0019] ①The light source module generates light and outputs a point light source through optical fiber coupling;

[0020] ②The light beam passes through the beam splitter and is divided into two perpendicular paths; one path is used as the illumination beam of the system's illumination structure light source, and the other path is shielded;

[0021] ③ After the illumination beam passes through the test area and is incident on the concave reflector, the light is reflected along the incident path, and the reflected beam is split into two perpendicular paths again by the beam splitter; one path returns to the location of the light source, and the other path converges to the knife edge;

[0022] ④ In the test area, the light beam is deflected due to a slight change in the refractive index. The deflected light beam passes through the knife-edge module, and the knife-edge intercepts part of the deflected light, forming a light-dark contrast;

[0023] ⑤ The remaining light enters the camera module, and the lens focuses the light onto the sensor of the camera module; the image captured by the camera shows the brightness difference after the refractive index change, forming a visual Schlieren image.

[0024] In addition, the present application also provides a method for using the modular coaxial schlieren imaging system.

[0025] The method for using the modular coaxial Schlieren imaging system comprises the following steps:

[0026] (1) First, according to different objects or environments to be tested, concave reflectors with different diameters and focal lengths are selected, and then light sources and energy fibers with different core diameters and numerical apertures are selected;

[0027] (2) Assemble the modules according to the structural design, and then test and adjust the modules in turn so that the light emitted from the light source propagates along a coaxial path throughout the system;

[0028] (3) Select the camera based on the light source, frame rate, resolution and other parameters;

[0029] (4) Implant the object to be tested or the environment to be tested into the test area and conduct the experiment.

[0030] The method of adjusting different modules in step (2) comprises the following steps:

[0031] a. Observe the optical fiber light-emitting hole and the image of the knife edge reflected by the beam splitter through the window directly in front of the point light source, then adjust the lever of the knife edge to ensure that the light-emitting hole and the knife edge can be clearly seen at the same time, and then adjust the three precision fine-thread adjustment screws of the beam splitter module to make the depth of field of the light-emitting hole and the knife edge on the same imaging plane, and finally fine-tune the knife edge lever to make the knife edge cut half of the light-emitting hole;

[0032] b. Place the assembled module in the correct position according to the optical path design, then turn on the light source, take a piece of white paper and place it above the beam splitter module close to the two adjustment screws on the rear side, and finally adjust the lowest adjustment screw of the concave reflector counterclockwise until a reflected light spot of the point light source appears on the white paper;

[0033] c. Adjust the two adjustment screws on the concave reflector to move the reflected light spot horizontally so that the reflected light spot, the point light source and the center of the concave reflector are in the same vertical plane. Then move the assembly where the beam splitter is located slightly forward and backward to focus the reflected light spot. Finally, adjust the lowest adjustment screw on the concave reflector clockwise to move the reflected light spot to a position near the point light source.

[0034] d. Open the control software of the industrial camera on the computer, play the pictures taken by the camera, and adjust the lever of the blade. When the reflected light spot is completely unobstructed, a bright circular field of view will be seen in the center of the picture. When the reflected light spot is completely blocked, the field of view will be completely black. When the reflected light spot is half blocked, there will be three situations in the field of view observed in the center of the picture: one is that the upper half of the circular field of view is bright and the lower half is black, one is that the upper half of the circular field of view is black and the lower half is bright, and the third is that the circular field of view is dark as a whole and the upper and lower parts are equally bright;

[0035] e. If the first two situations occur in the previous step, it is necessary to fine-tune the position of the assembly where the beam splitter is located forward and backward until the third situation occurs.

[0036] The technical implementation of the present invention is described in detail below.

[0037] (1) System composition:

[0038] The modular coaxial schlieren imaging system of the present invention is mainly composed of the following parts:

[0039] Light source module: used to provide the lighting source for the system. The light source can be LED light source, laser, etc. The specific selection such as color temperature and spot size depends on the experimental requirements. The light source module is equipped with an adjustment device to control the intensity of the light source. In the light source module, the coupling design of the optical fiber and the LED light is very critical. The optical fiber can effectively limit the spot diameter transmitted from the LED light source, making the output of the optical fiber an approximately ideal point light source.

[0040] Beam splitter module: located after the light source module, responsible for splitting the light beam into two perpendicular beams. The reflection direction of the beam splitter can be fine-tuned.

[0041] Test area module: The core area of ​​the system, used to place the object to be tested or the environment to be tested (such as airflow, heat flow, etc.). The module is equipped with an adjustable clamping device to fix experimental objects of different sizes and shapes. Optical windows are reserved on both sides of the test area to ensure that the light beam can pass through the test area without interference.

[0042] Reflector module: Concave reflector, located behind the experimental area. The point light source is located at the center of curvature of the concave mirror, and the light is reflected along the incident path and returns to the location of the light source accurately.

[0043] Knife-edge module: Installed behind the test area, it is used to cut the light that is deflected after passing through the test area. The knife-edge module can precisely adjust the position of the knife-edge up and down to adjust the sensitivity of the imaging. The precise adjustment of the knife-edge is the key to the imaging effect of the system.

[0044] Camera module: used to collect optical images after passing through the imaging lens. The camera module contains a high-resolution imaging sensor, which can adjust parameters such as exposure time and sampling frequency according to different experimental requirements to ensure the best imaging quality.

[0045] (2) Modular adjustment function:

[0046] The modular design of the invention allows the individual components to be adjusted according to experimental requirements:

[0047] Light source adjustment: The brightness, spot size and color temperature of the light source module can be adjusted according to experimental requirements.

[0048] Knife-edge sensitivity adjustment: The fine-tuning device of the knife-edge allows the user to precisely adjust its position according to the sensitivity requirements of the refractive index change in the experiment. By adjusting the position of the knife-edge, the sensitivity of the system to small light deflections can be increased or decreased. Test area adjustment: The device in this area can flexibly adjust the position and angle of the object to be tested to ensure that the light beam can fully pass through the test area.

[0049] Camera parameter setting: The camera module allows users to adjust parameters such as exposure time, frame rate, resolution, etc. according to different experimental environments to ensure the best clarity and dynamic range of the Schlieren image.

[0050] The present invention provides a modular coaxial Schlieren imaging system, which achieves high flexibility and scalability through modular design and can be freely adjusted and configured according to different experimental requirements. The design of the coaxial optical path greatly reduces the optical path error, improves the imaging accuracy, and ensures high-resolution Schlieren images. At the same time, the overall design of the system is compact and portable, which is convenient for use in scenes with limited space or frequent movement. In addition, through the application of high-precision optical elements and a knife-edge module with adjustable sensitivity, the present invention can accurately capture tiny changes in fluid density, which is suitable for scientific research, engineering applications and high-precision industrial detection, greatly improving the efficiency of experiments and detection.

[0051] It has the following beneficial effects:

[0052] (1) All components of the system (such as light source, spectroscope, knife edge, camera, test area, and reflector) adopt modular design and can be flexibly adjusted and combined according to different experimental requirements. The characteristics of modular design include:

[0053] Components are removable and replaceable: Light sources, cameras, reflectors, etc. can all be replaced or adjusted according to experimental requirements, improving the versatility of the system.

[0054] Flexible configuration: Different modules can be customized as needed to adapt to different scenarios and conditions of the experiment.

[0055] · Scalability: The modular design allows for the addition of new features or upgrading of existing components in the future to adapt to changing research needs.

[0056] (2) Coaxial optical design

[0057] The system uses a precise cage coaxial optical design to ensure that the propagation path of light in the system always remains coaxial. The key design points include:

[0058] Coaxial arrangement of optical system: The optical axes of light source, knife edge, camera and reflector are kept consistent, which reduces the optical path error and ensures the clarity of the image.

[0059] Reduce errors and optical losses: The coaxial design reduces the complexity of optical path adjustment in traditional schlieren systems, reduces errors and optical losses, and improves imaging accuracy.

[0060] System simplification: The coaxial design reduces the overall complexity of the system, making the equipment more compact and easier to deploy.

[0061] (3) High-precision imaging

[0062] The system ensures the accuracy and clarity of imaging through high-precision optical components and design solutions:

[0063] High-resolution imaging: The system is equipped with a high-resolution camera that can clearly capture tiny changes in fluid density gradients, improving the accuracy of experimental data.

[0064] Sensitivity adjustment: The knife edge position in the system can be flexibly adjusted to enhance the sensitivity to refractive index changes and adapt to different experimental environments.

[0065] Minimized optical distortion: Use high-quality optical reflectors and precision alignment technology to ensure that light will not be distorted after passing through the test area, ensuring image accuracy.

[0066] (4) Portability and compact design

[0067] The system pays special attention to compact design, which enables the system to be used in space-constrained environments and has good portability:

[0068] Small size: The components are 3D printed, which makes the whole device small and compact, easy to transport and quickly deploy in different experimental scenarios.

[0069] Rapid assembly: The modular design allows the system to be quickly assembled and disassembled, which is suitable for experimental scenarios that require frequent movement and setup.

[0070] High integration: While maintaining high performance, the system design is simple, reducing unnecessary components, making system installation and debugging more convenient.

[0071] (5) Adapt to different experimental conditions

[0072] Adjustable light source: According to the experimental requirements, the brightness, color temperature and other parameters of the light source can be adjusted to ensure the best imaging effect under different conditions.

[0073] Multi-scenario application: The system design can adapt to a variety of experimental scenarios, including laboratory fixed test environments and outdoor portable applications, and has strong versatility.

[0074] The modular coaxial schlieren imaging system of this application is mainly used to visualize the refractive index changes in gas and is widely used in the following fields:

[0075] (1) Fluid mechanics: used to study density gradients and eddy currents in airflow, such as pressure waves in air flow.

[0076] (2) Aerodynamics: Used for testing the aerodynamic performance of aircraft, rockets and other aircraft, and observing the airflow distribution and aerodynamic effects in the air.

[0077] (3) Combustion research: used to monitor the hot air flow generated during the combustion process and its changes to help optimize the combustion process.

[0078] (4) Thermophysics research: used to study the density gradient of the fluid during heat conduction and understand the changes in heat transfer.

[0079] (5) Industrial applications: It can be used to detect air leakage, gas mixing, spray cooling and other phenomena, and is suitable for non-contact measurement in engineering and industrial scenarios.

[0080] This application provides a flexible, accurate and efficient schlieren imaging solution that can capture tiny refractive index changes in transparent gases in real time and non-contact, and is suitable for scientific research experiments, engineering tests and high-precision industrial detection occasions. (4) Specific embodiments

[0082] In aerodynamic experiments, the system is used to observe the flow field changes when high-speed air flows over the surface of an object. By placing the object in the test area, the system successfully captures the density gradient changes in the air flow and generates clear schlieren images, showing the flow field structure and turbulence phenomena when the air flows around the object. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Figure 1 This is the overall system diagram after assembly of the system for this application.

[0084] Figure 2 This is a diagram of the assembly of the beamsplitter module and the upper and lower parts of the cage bracket.

[0085] Figure 3 The diagram shows the assembly of the knife-edge module structure and the upper partition of the cage bracket, a is the schematic diagram after assembly, and b is the exploded diagram.

[0086] Figure 4 Picture of the camera module and the lower spacer assembly of the cage bracket.

[0087] Figure 5 It is a schematic diagram of the assembly of the test area module and the reflector module, a is the assembly diagram, and b is the perspective diagram.

[0088] Figure 6 Schematic diagram of the optical path when the system of this application is used.

[0089] Figure 7 Example of Schlieren Imaging - Thermal Convection.

[0090] Light source module 1, LED light source 1-1, energy optical fiber 1-2 and point light source (emitting port) 1-3; spectrometer module 2, spectrometer 2-1, fine-tooth adjustment screw 2-2, blade module 3, blade 3-1, paddle 3-2, fixing part 3-3, fixing plate 3-4, positioning hole 3-5, camera module 4, test area module 5, optical window 5-1, box 5-2, clamping device 5-3, reflector module 6, reflector 6-1, bracket 6-2, adjustment screw 6-3, buckle 6-4, cage bracket 7, upper bottom 7-1, upper partition 7-2, support 7-3, lower partition 7-4.

[0091] A modular coaxial schlieren imaging system is provided, wherein a light source module 1, a spectroscope module 2, a knife-edge module 3 and a camera module 4 are assembled onto a cage bracket 7, wherein the cage bracket 7 comprises an upper bottom 7-1, a lower bottom, an upper partition 7-2, a lower partition 7-3 and a plurality of pillars 7-4, wherein the pillars 7-4 are mounted to the upper and lower bottoms by screws, and the upper and lower partitions 7-3 mounted on the pillars 7-4 have through holes, the spectroscope module 2 is mounted below the upper bottom 7-1, and the camera module 4 is mounted on the pillars 7-4 through the lower partition 7-3; the knife-edge module 3 is mounted on the upper partition 7-2 between the camera module 4 and the spectroscope module 2, and the light source module 1 is mounted on one side of the upper partition 7-2 so that the emission point of the point light source is located directly behind the spectroscope; partitions with uniform thickness are arranged around the cage bracket 7, and the partitions are provided with openings in front of the spectroscope for light beams to pass through. The knife-edge module 3 is located between the camera module 4 and the spectroscope module 2, the light source module 11 is fixed on one side of the spectroscope module 2, the test area module 5 and the reflector module 6 are placed on the other side of the spectroscope module 2 and in the vertical direction of the line connecting the spectroscope module 2 and the camera module 4.

[0092] The beam splitter 2-1 of the beam splitter module 2 is mounted on the upper end of the cage bracket 7, and the beam splitter 2-1 is fixed on the mounting bracket 2-3. The mounting bracket 2-3 is mounted on the upper bottom 7-1 of the cage bracket 7 through the precision fine-thread adjustment screw 2-2, and the precision fine-thread adjustment screw 2-2 finely adjusts the reflection direction of the beam splitter 2-1. The knife edge module 3 includes a knife edge 3-1, a fixing plate 3-4 with a groove, and a circular ring-shaped lever 3-2 with a protrusion. The knife edge 3-1 is pasted on the fixing plate 3-3. One end of the fixing plate 3-3 is fixed to the ring of the lever 3-2 through a pin, and the protrusion of the other end corresponds to the limit hole 3-5 in the groove of the fixing plate 3-4. Driven by the lever 3-2, the knife edge 3-1 swings slightly in the groove of the fixing plate 3-4 with the protrusion of the fixing plate 3-3 as the fulcrum; according to the sensitivity requirements of the refractive index change in the experiment, the position of the knife edge is accurately adjusted through the lever 3-2 to increase or decrease the sensitivity of the system to the deflection of tiny light. The test area module 5 includes a box 5-2 with optical windows 5-1 at both ends and a clamping device 5-3 fixed on the bottom surface of the box. The clamping device 5-3 can adjust the height and rotation angle, and is used to fix experimental objects of different sizes and shapes, flexibly adjust the position and angle of the object to be tested, and ensure that the light beam can fully pass through the test area; the reserved optical window 5-1 ensures that the light beam can pass through the test area without interference. The reflector module 6 includes a bracket 6-2 and a concave reflector 6-1 installed on the bracket 6-2 through a buckle 6-4, and three adjustment screws 6-3 are arranged around the concave reflector 6-1.

[0093] According to the test object: the hot updraft formed by the candle flame and the cold downdraft formed by the popsicle, the spectroscope module 2, the knife edge module 3, the test area module 5 and the reflector module 6 are adjusted to ensure that the optical axes of the light source, the concave reflector, the knife edge and the camera module 4 are strictly aligned. The specific operation is:

[0094] (1) First, select concave reflectors with different diameters and focal lengths according to different objects or environments to be tested, and then select light sources and energy fibers with different core diameters and numerical apertures; the brightness of the light source is 1000LX (1000 lumens per square meter).

[0095] (2) Assemble the modules according to the structural design, and then test and adjust the modules in turn according to the following steps, so that the light emitted from the light source propagates along a coaxial path throughout the system;

[0096] a. Observe the optical fiber light-emitting hole and the image of the knife edge reflected by the beam splitter through the window directly in front of the point light source, then adjust the lever of the knife edge to ensure that the light-emitting hole and the knife edge can be clearly seen at the same time, and then adjust the three precision fine-thread adjustment screws of the beam splitter module to make the depth of field of the light-emitting hole and the knife edge on the same imaging plane, and finally fine-tune the knife edge lever to make the knife edge cut half of the light-emitting hole;

[0097] b. Place the assembled module in the correct position according to the optical path design, then turn on the light source, take a piece of white paper and place it above the beam splitter module close to the two adjustment screws on the back, and finally adjust the bottom adjustment screw of the concave reflector counterclockwise.

[0098] Screw until a reflected light spot of the point light source appears on the white paper;

[0099] c. Adjust the two adjustment screws on the concave reflector to move the reflected light spot horizontally so that the reflected light spot, the point light source and the center of the concave reflector are in the same vertical plane. Then move the assembly where the beam splitter is located slightly forward and backward to focus the reflected light spot. Finally, adjust the bottom adjustment screw of the concave reflector clockwise to move the reflected light spot to the point.

[0100] Location near the light source;

[0101] d. Open the control software of the industrial camera on the computer, play the picture taken by the camera, and adjust the lever of the blade. When the reflected light spot is completely unobstructed, a bright circular field of view will be seen in the center of the picture. When the reflected light spot is completely blocked, the field of view is completely black. When the reflected light spot is half blocked, there will be three situations in the field of view observed in the center of the picture. One is that the upper half of the circular field of view is bright and the lower half is black, one is that the upper half of the circular field of view is black and the lower half is bright, and the third is that the entire circular field of view is dark.

[0102] Darken the upper and lower parts to be equally bright;

[0103] e. If the first two situations occur in the previous step, it is necessary to fine-tune the position of the assembly where the beam splitter is located forward and backward until the third situation occurs.

[0104] (3) Camera acquisition parameters: 3072*2048 (pixels), 30FPS (frame rate), exposure time 1000us (microseconds); (4) Implant the object to be tested or the environment to be tested into the test area and conduct the experiment.

[0105] Experimental Schlieren imaging of the hot updraft formed by a candle flame and the cold downdraft formed by a popsicle. Figure 7 shown.

Claims

1. A modular coaxial Schlieren imaging system, comprising a light source module, a beam splitter module, a knife-edge module, a camera module, a test area module and a reflector module; the light source module, the beam splitter module, the knife-edge module and the camera module are assembled by a detachable cage bracket, the knife-edge module is located between the camera module and the beam splitter module, the light source module is fixed on one side of the beam splitter module, and the test area module and the reflector module are placed on the other side of the beam splitter module and in the vertical direction of the line connecting the beam splitter module and the camera module; The light source module includes an energy optical fiber and an LED light source. The light source module outputs a point light source by coupling the LED light source with the energy optical fiber. The core diameter of the energy optical fiber is adapted to the brightness of the light source. The numerical aperture of the energy optical fiber is adapted to the diameter and focal length of the concave reflector in the reflector module. The point light source of the light source module is located at the curvature center of the concave reflector of the reflector module, so that the light is reflected along the incident path and accurately returns to the position of the point light source.

2. The modular coaxial Schlieren imaging system according to claim 1, characterized in that: The light source of the light source module is an LED light source or a laser, and the intensity, color temperature and spot size of the light source are adjustable; the spectroscope module, the knife-edge module, the test area module and the reflector module are all provided with adjustment devices, which are adjusted according to the different objects to be tested or the environments to be tested, so that the optical axes of the light source, the concave reflector, the knife-edge and the camera module are strictly aligned.

3. The modular coaxial Schlieren imaging system according to claim 1, characterized in that: The cage bracket includes an upper bottom, a lower bottom, an upper partition, a lower partition and a plurality of pillars. The pillars are installed with the upper and lower bottoms by screws. The upper and lower partitions installed on the pillars have through holes. The spectrometer module is installed below the upper bottom. The camera module is fixed to the pillars through the partition. The knife edge module is installed on the partition between the camera module and the spectrometer module. The light source module is installed on one side of the partition where the knife edge is located. The emission point of the point light source is located directly behind the spectrometer. The cage bracket is surrounded by partitions with uniform thickness. The partitions are provided with openings in front of the spectrometer for light beams to pass through.

4. The modular coaxial Schlieren imaging system according to claim 1, characterized in that: The beam splitter of the beam splitter module is installed on the upper end of the cage bracket, the beam splitter is fixed on the mounting bracket, the mounting bracket is installed on the upper bottom of the cage bracket through a precision fine-thread adjustment screw, and the precision fine-thread adjustment screw fine-tunes the reflection direction of the beam splitter.

5. The modular coaxial Schlieren imaging system according to claim 1, characterized in that: The knife edge module includes a knife edge, a fixing plate with a groove and a circular ring-shaped lever with a protrusion. The knife edge is pasted on the fixing plate. One end of the fixing plate is fixed to the circular ring of the lever by a pin, and the protrusion at the other end corresponds to the limiting hole in the groove of the fixing plate. Driven by the lever, the knife edge swings slightly in the groove of the fixing plate with the protrusion of the fixing plate as a fulcrum. The position of the knife edge is accurately adjusted by the lever according to the sensitivity requirement of the refractive index change in the experiment, so as to increase or decrease the sensitivity of the system to tiny light deflection.

6. The modular coaxial Schlieren imaging system according to claim 1, characterized in that: The test area module includes a box with optical windows at both ends and a clamping device fixed on the bottom of the box. The clamping device can adjust the height and rotation angle and is used to fix experimental objects of different sizes and shapes, flexibly adjust the position and angle of the object to be tested, and ensure that the light beam can fully pass through the area to be tested; the reserved optical window ensures that the light beam can pass through the test area without interference.

7. The modular coaxial Schlieren imaging system according to claim 1, characterized in that: The camera module includes a high-resolution imaging sensor, and the exposure time, frame rate or resolution parameters are adjusted according to different experimental environments to ensure that the clarity and dynamic range of the Schlieren image are optimal; the reflector module includes a bracket and a concave reflector mounted on the bracket by a buckle, and 3 adjustment screws are provided around the concave reflector.

8. The modular coaxial Schlieren imaging system according to claim 1, characterized in that: The optical path design is as follows: ①The light source module generates light and outputs a point light source through optical fiber coupling; ②The light beam passes through the beam splitter and is divided into two perpendicular paths; one path is used as the illumination beam of the system's illumination structure light source, and the other path is shielded; ③ After the illumination beam passes through the test area and is incident on the concave reflector, the light is reflected along the incident path, and the reflected beam is split into two perpendicular paths again by the beam splitter; one path returns to the location of the light source, and the other path converges to the knife edge; ④ In the test area, the light beam is deflected due to a slight change in the refractive index. The deflected light beam passes through the knife-edge module, and the knife-edge intercepts part of the deflected light, forming a light-dark contrast; ⑤ The remaining light enters the camera module, and the lens focuses the light onto the sensor of the camera module; the image captured by the camera shows the brightness difference after the refractive index change, forming a visual Schlieren image.

9. The method for using the modular coaxial schlieren imaging system according to any one of claims 1 to 8, comprising the following steps: (1) First, according to different objects or environments to be tested, concave reflectors with different diameters and focal lengths are selected, and then light sources and energy fibers with different core diameters and numerical apertures are selected; (2) Assemble the modules according to the structural design, and then test and adjust the modules in turn so that the light emitted from the light source propagates along a coaxial path throughout the system; (3) Select the camera based on the light source, frame rate, resolution and other parameters; (4) Implant the object to be tested or the environment to be tested into the test area and conduct the experiment.

10. The method of use according to claim 9, characterized in that: The method of adjusting different modules in step (2) comprises the following steps: a. Observe the optical fiber light-emitting hole and the image of the knife edge reflected by the beam splitter through the window directly in front of the point light source, then adjust the lever of the knife edge to ensure that the light-emitting hole and the knife edge can be clearly seen at the same time, and then adjust the three precision fine-thread adjustment screws of the beam splitter module to make the depth of field of the light-emitting hole and the knife edge on the same imaging plane, and finally fine-tune the knife edge lever to make the knife edge cut half of the light-emitting hole; b. Place the assembled module in the correct position according to the optical path design, then turn on the light source, take a piece of white paper and place it above the beam splitter module close to the two adjustment screws on the rear side, and finally adjust the lowest adjustment screw of the concave reflector counterclockwise until a reflected light spot of the point light source appears on the white paper; c. Adjust the two adjustment screws on the concave reflector to move the reflected light spot horizontally so that the reflected light spot, the point light source and the center of the concave reflector are in the same vertical plane. Then move the assembly where the beam splitter is located slightly forward and backward to focus the reflected light spot. Finally, adjust the lowest adjustment screw on the concave reflector clockwise to move the reflected light spot to a position near the point light source. d. Open the control software of the industrial camera on the computer, play the pictures taken by the camera, and adjust the lever of the blade. When the reflected light spot is completely unobstructed, a bright circular field of view will be seen in the center of the picture. When the reflected light spot is completely blocked, the field of view will be completely black. When the reflected light spot is half blocked, there will be three situations in the field of view observed in the center of the picture: one is that the upper half of the circular field of view is bright and the lower half is black, one is that the upper half of the circular field of view is black and the lower half is bright, and the third is that the circular field of view is dark as a whole and the upper and lower parts are equally bright; e. If the first two situations occur in the previous step, it is necessary to fine-tune the position of the assembly where the beam splitter is located forward and backward until the third situation occurs.

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