Liquid phase applicable schlieren measurement device and method based on background light intensity field gradient

Abstract

The invention relates to a liquid phase applicable schlieren measuring device and method based on background light intensity field gradient, the device comprises an LED dot matrix lamp box and a high-speed camera which are respectively located at two sides of a to-be-measured area, the outside of the LED dot matrix lamp box is covered with a layer of parchment paper, the included angle between the LED dot matrix lamp box and the imaging plane of the high-speed camera can be adjusted, and the included angle between the LED dot matrix lamp box and the imaging plane of the high-speed camera can be adjusted. The directional background light intensity field gradient is used for generating a directional background light intensity field gradient, and the direction of the background light intensity field gradient is consistent with the refractive index gradient direction of a medium in a to-be-detected area. Compared with the prior art, the LED dot matrix lamp box is adopted as an area light source, the background light intensity gradient field with directivity is formed in the imaging area through adjustment, and under the action of the gradient field, deflected light rays penetrating through the refractive index change area are highlighted, so that a schlieren image with good contrast is formed; the high-resolution schlieren imaging can be simply obtained in real time, imaging blurring is avoided, and meanwhile the method is suitable for measurement scenes sensitive to refractive index changes.

Description

Technical Field

[0001] This invention relates to the field of schlieren imaging technology, and in particular to a liquid-phase applicable schlieren measurement device and method based on the background light intensity field gradient. Background Technology

[0002] Schlieren imaging is a method that makes invisible flowing elements (such as gases, air, and other transparent media) visible. It is also a fundamental method for observing minute refractive index changes in flow fields. Its image contrast is more acute than shadow imaging, and it is simpler to set up than an interferometer. Schlieren measurement is the technical means to achieve schlieren imaging. Specifically, it uses optical principles to observe changes in flow field density. Based on the fact that the refractive index gradient of light in a flow field is proportional to the airflow density, knife-edge modulation converts the density gradient into a difference in light intensity, thereby visualizing flow field characteristics such as shock waves and compression waves.

[0003] Currently, schlieren measurement techniques can be broadly categorized into two types. One type involves traditional optical path setup: a specific optical path is established along the light propagation path using various optical lenses (convex lenses, concave mirrors, Fresnel lenses, etc.). Then, a knife edge or knife edge grating is used to filter the refracted light after passing through the medium. Depending on the optical path design and knife edge selection, this can be further subdivided into transmission schlieren, Z-shaped reflection schlieren, color schlieren, and focused schlieren methods. Each of these methods has specific requirements for the placement of lenses and knife edges, and generally requires a large space to build the optical path. As the area to be measured increases, the cost and difficulty of building the optical path also increase. Due to the presence of the knife edge, this type of schlieren is extremely sensitive to changes in refractive index, making it unsuitable for measurements in liquid phases with large refractive index variations. The other type of schlieren is background schlieren, also known as composite schlieren. The principle of this type of schlieren technique is very similar to that of digital image correlation (speckle measurement) in solid mechanics experiments. It does not require any light source or optical lenses; it only needs to place a speckle background image behind the area to be measured. The difference in refractive index will cause these speckles to produce tiny displacements on the imaging surface. By recording these displacements with a camera, the refractive index difference can be calculated. The matching calculation method used for the speckle displacement can directly apply existing algorithms from digital image correlation techniques. Background schlieren cleverly utilizes the displacement of speckles to reconstruct the refractive index distribution. Although it is inexpensive and can easily achieve large-scale measurements, the post-processing requirements prevent it from achieving real-time measurements. Furthermore, its resolution is directly limited by the accuracy of the speckle matching algorithm. In addition, it has a focusing problem: if the focus is on the surface of the object being measured, defocusing on the background speckles may reduce accuracy; if the focus is on the background speckles, it may be impossible to clearly delineate the specific area of ​​the object being measured. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art by providing a liquid-phase applicable schlieren measurement device and method based on the background light intensity field gradient, which can obtain high-resolution schlieren imaging in a simple and real-time manner, avoid imaging blur, and is suitable for measurement scenarios that are sensitive to changes in refractive index.

[0005] The objective of this invention can be achieved through the following technical solution: a liquid-phase applicable schlieren measurement device based on background light intensity field gradient, comprising an LED dot matrix light box and a high-speed camera, wherein the LED dot matrix light box is covered with a layer of sulfuric acid paper, the LED dot matrix light box and the high-speed camera are respectively located on both sides of the area to be measured, and the angle between the LED dot matrix light box and the imaging plane of the high-speed camera can be adjusted to generate a directional background light intensity field gradient, wherein the direction of the background light intensity field gradient is consistent with the direction of the refractive index gradient of the medium in the area to be measured.

[0006] Furthermore, the LED dot matrix light box includes multiple LED units with adjustable luminous power.

[0007] Furthermore, the LED dot matrix light box is rotatably mounted on one side of the area to be measured, and the high-speed camera is fixedly mounted on the other side of the area to be measured.

[0008] Furthermore, the distance between the high-speed camera and the area to be measured is less than or equal to a preset spacing.

[0009] Furthermore, the sulfuric acid paper is selected from sulfuric acid paper with a gradual change in light transmittance.

[0010] A liquid-phase applicable schlieren measurement method based on background light intensity field gradient includes the following steps: S1. Cover and fix the outside of the LED dot matrix light box with tracing paper, place the LED dot matrix light box on one side of the area to be measured, and place the high-speed camera on the other side of the area to be measured. S2. Turn on the LED dot matrix light box and construct the background light intensity field gradient by adjusting the working parameters of the high-speed camera and the working status of the LED dot matrix light box. S3. Induce flow field changes in the area to be measured, and record the corresponding schlieren image sequence by a high-speed camera.

[0011] Further, step S2 includes the following steps: S21. Open the LED dot matrix light box so that the light generated from the LED dot matrix light box, after being softened by tracing paper, passes through the area to be tested and forms refraction; S22. Adjust the working parameters of the high-speed camera and adjust the working state of the LED dot matrix light box to construct a background light intensity field gradient, the direction of which is parallel to the refractive index gradient direction.

[0012] Furthermore, the operating parameters of the high-speed camera include the shooting distance and exposure time.

[0013] Furthermore, step S22 specifically involves adjusting the shooting distance of the high-speed camera to be less than or equal to a preset distance from the area to be measured, in order to reduce the diffusion of refracted light. The exposure time of the high-speed camera is adjusted to the microsecond level, so that the imaging area shows a light intensity gradient distribution with one side bright and the other side dark.

[0014] Furthermore, in step S22, adjusting the working state of the LED dot matrix light box specifically involves rotating the LED dot matrix light box to form a specific angle between it and the imaging plane of the high-speed camera, or adjusting the luminous power of each LED unit within the LED dot matrix light box.

[0015] Compared with the prior art, the present invention has the following advantages: This invention employs an LED dot matrix light box covered with a layer of tracing paper to transform it into a uniform diffuse reflection light source. The LED dot matrix light box and a high-speed camera are positioned on opposite sides of the area to be measured. The angle between the LED dot matrix light box and the imaging plane of the high-speed camera is adjustable to generate a directional background light intensity gradient. This gradient aligns with the refractive index gradient of the medium within the measured area. Under the influence of this gradient, light rays deflected after passing through the refractive index change region are highlighted, resulting in a schlieren image with good contrast. Compared to traditional schlieren systems built using optical paths, this invention eliminates the need for optical lenses or complex optical path designs, significantly reducing system costs and simplifying experimental setup. Furthermore, this invention can obtain high-resolution schlieren images in real time without post-processing, resulting in higher imaging efficiency and suitability for dynamic process observation. Simultaneously, the high-speed camera in this invention only needs to focus on the measured area, eliminating the need to balance focus position between the background and the object, thus avoiding image blurring.

[0016] In this invention, the LED dot matrix light box is designed to include multiple LED units with adjustable luminous power. By independently adjusting the luminous power of each LED unit, the effect of constructing a directional background light intensity field gradient can also be achieved.

[0017] This invention sets the distance between the high-speed camera and the area to be measured to be less than or equal to a preset distance, so that the high-speed camera and the area to be measured are close enough to reduce the diffusion of refracted light and avoid a decrease in image contrast; this invention also adjusts the exposure time of the high-speed camera to the microsecond level so that the imaging area presents a light intensity gradient distribution with one side bright and the other side dark.

[0018] In this invention, the directionality, sensitivity, and observation range of the constructed light intensity gradient can be flexibly achieved by adjusting the rotation angle and size of the LED dot matrix light box. This makes it well-suited for scenarios with large refractive index gradients in liquid phases and high sensitivity to imaging, thus making it more suitable for liquid phase flow field measurements. Furthermore, this invention is also applicable to qualitative and visual measurements of gas phase flow fields with significant refractive index changes, demonstrating good applicability and scalability. Attached Figure Description

[0019] Figure 1 This is a top view of the device of the present invention; Figure 2 This is a schematic diagram of the method flow of the present invention; Figure 3 This is a schematic diagram of the cold water jet in the test area in Example 2; Figure 4 This is a schematic diagram of the schlieren results of the cold water jet in Example 2; The markings in the diagram are as follows: 1. LED dot matrix light box, 2. Area to be tested, 3. High-speed camera, 201. Glass dropper, 202. Glass tank hot water pool, 203. Thermocouple. Detailed Implementation

[0020] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0021] Example 1 like Figure 1 As shown, a liquid-phase schlieren measurement device based on background light intensity field gradient includes an LED dot matrix light box 1 (as a surface light source) and a high-speed camera 3 (for recording schlieren images). The LED dot matrix light box 1 is covered with a layer of tracing paper (to transform the LED dot matrix light box 1 into a uniform diffuse light source). The LED dot matrix light box 3 and the high-speed camera 3 are located on opposite sides of the area to be measured 2. The angle between the imaging plane of the LED dot matrix light box 1 and the high-speed camera 3 is adjustable to generate a directional background light intensity field gradient. The direction of this background light intensity field gradient is consistent with the direction of the refractive index gradient of the medium in the area to be measured 2. Light is generated from the LED dot matrix light box 1, softened by the tracing paper, passes through the area to be measured 2, refracts, and is subsequently captured and recorded by the high-speed camera 3 to form a schlieren image.

[0022] In this embodiment, the LED dot matrix light box 1 is rotatably installed on one side of the area to be tested 2, and the high-speed camera 3 is fixedly installed on the other side of the area to be tested 2. The distance between the high-speed camera 3 and the area to be tested 2 is less than or equal to a preset distance, so as to reduce the diffusion of refracted light and avoid the decrease in image contrast.

[0023] In practical applications, the LED dot matrix light box 1 can be configured to include multiple LED units with adjustable luminous power. The directional background light intensity field gradient can be generated by independently adjusting the luminous power of each LED unit. In addition, tracing paper with a light transmittance gradient characteristic can also be used to generate the directional background light intensity field gradient.

[0024] Based on the above device, a liquid-phase applicable schlieren measurement method based on the background light intensity field gradient is realized, such as... Figure 2 As shown, it includes the following steps: S1. Cover and fix the outside of the LED dot matrix light box with tracing paper, place the LED dot matrix light box on one side of the area to be measured, and place the high-speed camera on the other side of the area to be measured. S2. Turn on the LED dot matrix light box and construct the background light intensity field gradient by adjusting the working parameters of the high-speed camera and the working status of the LED dot matrix light box. S3. Induce flow field changes in the area to be measured, and record the corresponding schlieren image sequence by a high-speed camera.

[0025] Step S2 includes the following steps: S21. Open the LED dot matrix light box so that the light generated from the LED dot matrix light box, after being softened by tracing paper, passes through the area to be tested and forms refraction; S22. Adjust the operating parameters of the high-speed camera (including the shooting distance and exposure time of the high-speed camera), and adjust the working state of the LED dot matrix light box to construct a background light intensity field gradient, the direction of which is parallel to the refractive index gradient direction.

[0026] In step S22, the shooting distance of the high-speed camera is adjusted to be less than or equal to a preset distance from the area to be measured, so as to reduce the diffusion of refracted light. The exposure time of the high-speed camera is adjusted to the microsecond level, so that the imaging area shows a light intensity gradient distribution with one side bright and the other side dark.

[0027] In addition, adjusting the working state of the LED dot matrix light box in step S22 specifically involves rotating the LED dot matrix light box to form a specific angle between it and the imaging plane of the high-speed camera, or adjusting the luminous power of each LED unit inside the LED dot matrix light box.

[0028] In summary, the core of this scheme lies in constructing a background light intensity field with a gradient distribution, ensuring that the direction of this gradient aligns with the refractive index gradient direction in the medium under test. Based on this principle, this scheme designs a directional background light intensity field gradient by rotating the LED dot matrix light box, which serves as the background light source. Alternatively, this gradient can be achieved through other methods, such as independently controlling the luminous power of each LED unit in the LED dot matrix light box, or constructing it using tracing paper with gradually changing transmittance.

[0029] This solution uses an LED dot matrix lightbox as a surface light source. By adjusting the angle between the lightbox and the imaging plane of the high-speed camera, a directional background light intensity gradient field is formed within the imaging area. Under the influence of this gradient field, light rays that are deflected after passing through the region of refractive index change are highlighted, thus forming a schlieren image with good contrast. The basic principle can be understood as: deflecting light rays originally in high-intensity areas to low-intensity areas, or vice versa, thereby enhancing image contrast through background brightness differences and visualizing refractive index perturbations.

[0030] Compared to traditional schlieren systems built using optical paths, this approach eliminates the need for optical lenses or complex optical path designs, significantly reducing system costs and simplifying experimental setup. Compared to existing background schlieren methods, although both are based on background structures, this approach obtains high-resolution schlieren images in real time without post-processing. The schlieren images are directly recorded by a high-speed camera, resulting in higher imaging efficiency and suitability for observing dynamic flow fields. Furthermore, the high-speed camera only needs to focus on the area under test, eliminating the need to balance focus position between the background and the object, thus avoiding image blurring.

[0031] The directionality, sensitivity, and observation range of the light intensity gradient generated by this scheme can be flexibly adjusted by changing the rotation angle and size of the LED dot matrix lightbox. In terms of sensitivity, this scheme is suitable for scenarios with large refractive index gradients in liquid phases and high sensitivity to imaging, making it more suitable for liquid phase flow field measurements. Furthermore, this scheme is also suitable for qualitative visualization measurements of gas phase flow fields with significant refractive index changes, exhibiting good applicability and scalability. The sensitivity of this scheme can be increased by increasing the background light intensity gradient. Depending on the gradient construction method, this operation can be further subdivided into: increasing the lightbox rotation angle; setting a larger spatial power gradient for the LED units; using tracing paper with a larger gradient transmittance, etc. The size of the measurement area can be directly adjusted by replacing LED dot matrix lightboxes of different sizes, exhibiting high scalability and theoretically having no limitation on the observation range.

[0032] Example 2 To verify the effectiveness of this solution, this embodiment applies the solution described in Embodiment 1 to perform schlieren measurement on a cold water jet in hot water, thereby demonstrating the imaging effect of this solution. Figure 3 As shown, a glass tank hot water pool 202 is set in the area to be tested. Its length, width, and height are 10cm × 10cm × 8cm. The temperature of the hot water inside is 80℃, and the temperature of the pool is measured by thermocouple 203. The temperature of the cold water is room temperature (20℃). After being drawn by glass dropper 201, it is injected into the glass tank hot water pool 202 to form convection.

[0033] In this embodiment, the LED dot matrix light box, tracing paper, glass container, glass dropper, and thermocouple are all commercially available models, and the high-speed camera used is the Thousand-Eyed Wolf high-speed camera.

[0034] The specific application process of this embodiment includes: Step 101: Cover and fix the tracing paper onto the LED dot matrix light box 1, then turn on the LED dot matrix light box 1 and adjust the power to maximum. Place the LED dot matrix light box 1 upright in the area to be tested 2. After placing the glass tank hot water pool 202, adjust the shooting distance of the high-speed camera 3 so that the high-speed camera 3 is close enough to the object to be tested (in this embodiment, the shooting distance is less than 80cm) to reduce the diffusion of refracted light and avoid a decrease in image contrast. Focus the lens of the high-speed camera 3 onto the thermocouple 203 in the glass tank hot water pool 202, using this as a reference to determine the subsequent working plane.

[0035] Step 102: Rotate the LED dot matrix light box 1 appropriately to form an angle between it and the imaging plane of the high-speed camera 3 (i.e., the plane where the lens is located) to construct a sufficient background light intensity gradient. In this embodiment, this angle should be greater than 30°. Simultaneously, adjust the exposure time of the high-speed camera 3 to the microsecond level so that the imaging area exhibits a light intensity gradient distribution with one side bright and the other side dark. When generating the gradient light intensity field, it is necessary to ensure that the gradient direction remains parallel to the expected refractive index gradient direction.

[0036] Step 103: Using a glass dropper 201, draw room temperature (approximately 20°C) cold water. Based on the position of thermocouple 203, inject the cold water into the glass tank's hot water pool 202 within the focused area of ​​the high-speed camera 3, inducing a heat convection process. Simultaneously, the high-speed camera 3 is activated to record the schlieren image sequence formed during the convection process (e.g., ...). Figure 4 As shown in the diagram, the black cylinder is a thermocouple, and the arrow points to the inlet of the cold water jet.

Claims

1. A liquid phase applicable stria shadowgraph measuring device based on background light intensity field gradient, characterized in that, The application relates to a device for measuring the refractive index gradient of a medium in a measuring area (2), which comprises an LED dot matrix lamp box (1) and a high-speed camera (3), the LED dot matrix lamp box (1) is externally covered with a layer of sulfuric acid paper, the LED dot matrix lamp box (1) and the high-speed camera (3) are respectively arranged on the two sides of the measuring area (2), and the included angle between the LED dot matrix lamp box (1) and the imaging plane of the high-speed camera (3) can be adjusted and set, so as to generate a directional background light intensity field gradient, and the direction of the background light intensity field gradient is consistent with the refractive index gradient direction of the medium in the measuring area (2).

2. The liquid phase applicable stria shadowgraph device based on background light intensity field gradient according to claim 1, characterized in that, The LED dot matrix lamp box (1) comprises a plurality of LED units with adjustable light emitting power.

3. The liquid phase applicable stria shadowgraph device based on background light intensity field gradient according to claim 1, characterized in that, The LED dot matrix lamp box (1) is rotatably arranged on one side of the measuring area (2), and the high-speed camera (3) is fixedly arranged on the other side of the measuring area (2).

4. The liquid phase applicable stria shadowgraph device based on background light intensity field gradient according to claim 1, characterized in that, The distance between the high-speed camera (3) and the measuring area (2) is less than or equal to a preset interval.

5. The liquid phase applicable stria shadowgraph device based on background light intensity field gradient according to claim 1, characterized in that, The sulfuric acid paper is selected from sulfuric acid paper with light transmittance gradient characteristics.

6. A liquid-phase applicable stria shadowgraph measurement method based on a background light intensity field gradient, applied to the liquid-phase applicable stria shadowgraph measurement device based on the background light intensity field gradient according to claim 1, characterized in that, The application further discloses a method for measuring the refractive index gradient of a medium in a measuring area (2), which comprises the following steps: S1, covering and fixing the sulfuric acid paper on the outside of the LED dot matrix lamp box (1), arranging the LED dot matrix lamp box (1) on one side of the measuring area (2), and arranging the high-speed camera (3) on the other side of the measuring area (2); S2, opening the LED dot matrix lamp box (1), adjusting the working parameters of the high-speed camera (3) and the working state of the LED dot matrix lamp box (1), and constructing a background light intensity field gradient; S3, inducing flow field changes in the measuring area (2) and recording corresponding schlieren image sequences by the high-speed camera (3).

7. The liquid phase applicable striae measurement method based on background light intensity field gradient according to claim 6, characterized in that, The step S2 comprises the following steps: S21, opening the LED dot matrix lamp box (1) to make light generated from the LED dot matrix lamp box (1), the light is softened by the sulfuric acid paper, and then passes through the measuring area (2) to form refraction; S22, adjusting the working parameters of the high-speed camera (3) and the working state of the LED dot matrix lamp box (1) to construct a background light intensity field gradient, and the direction of the background light intensity field gradient is parallel to the refractive index gradient direction.

8. The liquid phase applicable striae measurement method based on background light intensity field gradient according to claim 7, characterized in that, The working parameters of the high-speed camera (3) comprise the shooting distance and the exposure time of the high-speed camera (3).

9. The liquid phase applicable striae measurement method based on background light intensity field gradient according to claim 8, characterized in that, In the step S22, the shooting distance of the high-speed camera (3) is adjusted to be less than or equal to the preset interval from the measuring area (2), so as to reduce the diffusion of refracted light; The exposure time of the high-speed camera (3) is adjusted to the microsecond level, so that the imaging area presents a light intensity gradient distribution of one side being bright and the other side being dark.

10. The liquid phase applicable striae measurement method based on background light intensity field gradient according to claim 7, characterized in that, In the step S22, the working state of the LED dot matrix lamp box (1) is adjusted, specifically, rotating the LED dot matrix lamp box (1) to form a specific included angle with the imaging plane of the high-speed camera (3), or adjusting the light emitting power of each LED unit in the LED dot matrix lamp box (1).

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