Fresnel lens energy flow distribution acquisition method and system

Abstract

The invention provides a Fresnel lens energy flow distribution obtaining method and system, and belongs to the technical field of solar condensation, and the method comprises the following steps: employing a CCD camera to shoot a plurality of pictures of the same scene at different exposure times, and constructing a camera response function through the pixel gray value, the light source irradiance and the exposure time of each picture; according to the path of a Fresnel lens focusing light cone, a Lambert target is placed in any cross section of the light cone to form an elliptical light spot, and an elliptical light spot grid is constructed according to a Lambert surface coordinate and a focusing light cone coordinate; a CCD camera and the Lambert surface are oppositely arranged, the oval light spots are shot, gray level distribution of oval light spot grids is obtained, and the energy flux density of the oval light spot grids is calculated according to a camera response function; and creating a cross section circle of a Fresnel lens focusing light cone, and mapping the energy flow density of the elliptical light spot to the cross section circle to obtain energy flow distribution of the Fresnel lens. According to the invention, the measurement flexibility can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of solar concentrator technology, and particularly relates to a method and system for obtaining the energy flux distribution of a Fresnel lens. Background Art

[0002] Due to its simple structure, low cost, and high focusing efficiency, the Fresnel lens is widely used in fields such as solar energy concentration, optical imaging, and laser spot measurement. In these applications, the measurement of the energy flux intensity of the spot is a key means to evaluate the performance of the concentrator system. According to different measurement methods, the measurement of the energy flux intensity of the spot is generally divided into direct measurement method and indirect measurement method.

[0003] The direct measurement method directly measures the energy flux density on the surface of the receiver through an energy flux density meter, which has the advantages of simple operation and strong real-time performance, and can quickly reflect the energy distribution of the spot. However, due to the point-focusing characteristic of the Fresnel lens, the energy flux density from the center to the outer diameter of the spot shows a significant gradient change. Due to equipment and site limitations, the direct measurement method has a low spatial resolution and is difficult to accurately capture such small changes in the gradient. At the same time, the cost of the measurement equipment is relatively high and it usually needs to be fixed on the surface of the receiver, which limits its application in large-area measurement or complex scenarios. The indirect measurement method measures other physical quantities related to the target quantity (such as spot size, brightness, etc.), and then indirectly calculates the energy flux intensity of the spot by combining mathematical models or physical laws. Currently, the common indirect measurement methods mainly use computer simulation technology, which has high spatial resolution and low cost, and can be flexibly applied to different scenarios. However, this method is easily affected by optical system errors and model assumption conditions, which may lead to deviations in the calculation results. In contrast, the real-time measurement method based on a CCD camera can directly capture the energy distribution of the spot and has high spatial resolution, so it is widely used in actual measurement.

[0004] However, currently, the CCD camera is usually fixed at the center of the Fresnel lens, which limits the shooting angle, so that the camera can only shoot the cross-section of the spot parallel to the lens, restricting the measurement range and having low flexibility. Summary of the Invention

[0005] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a method for obtaining the energy flux distribution of a Fresnel lens, including the following steps: Use a CCD camera to take multiple pictures of the same scene at different exposure times, and construct a camera response function through the pixel gray value, light source irradiance, and exposure time at each pixel position of each picture; According to the path of the focused light cone of the Fresnel lens, place the Lambert target within any cross-section of the light cone. The Lambert surface on the Lambert target forms an elliptical light spot with the cross-section of the focused light cone. Construct an elliptical light spot grid based on the Lambert surface coordinates and the focused light cone coordinates. Set the CCD camera opposite to the Lambert surface. Use the CCD to capture the elliptical light spot to obtain the gray-scale distribution of the elliptical light spot grid. Calculate the energy flux density of the elliptical light spot according to the camera response function. Create a cross-sectional circle of the focused light cone of the Fresnel lens, and map the energy flux density of the elliptical light spot onto the cross-sectional circle to obtain the energy flux distribution of the Fresnel lens.

[0006] Preferably, calculating the energy flux density of the elliptical light spot according to the camera response function includes the following steps: Introduce a regularization term into the camera response function to obtain a non-linear mapping relationship between the pixel gray value and the light source irradiance. Combined with the non-linear mapping relationship, use a heat flux meter to calibrate the relationship between the pixel gray value and the solar light spot irradiance. Calculate the energy flux density of the elliptical light spot grid according to the relationship between the pixel gray value and the solar light spot irradiance.

[0007] Preferably, creating the cross-sectional circle of the focused light cone of the Fresnel lens and mapping the energy flux density of the elliptical light spot onto the cross-sectional circle to obtain the energy flux density of the cross-sectional circle includes the following steps: Obtain the light ray vector from the node of the elliptical light spot grid to the focus of the focused light cone of the Fresnel lens, determine the intersection position of the light ray vector and the cross-sectional circle. The intersection point is the point where the light ray vector is mapped onto the cross-sectional circle. Determine the area of the cross-sectional circle grid cell according to the intersection point. Calculate the energy flux density of the cross-sectional circle grid cell according to the area of the cross-sectional circle grid cell, the area of the elliptical light spot grid cell, and the energy flux density of the elliptical light spot grid cell. Traverse all elliptical light spot grid cells, calculate the energy flux density of all cross-sectional circle grid cells, and obtain the energy flux distribution of the Fresnel lens.

[0008] Preferably, determining the intersection position of the light ray vector and the cross-sectional circle includes the following steps: Construct the parametric equation of the light ray through the direction and starting point of the light ray. Define the plane equation through a point on the plane and the normal vector. Substitute the parametric equation of the light ray into the plane equation to obtain the intersection position of the light ray vector and the cross-sectional circle.

[0009] Preferably, the camera response function is specifically as follows: ; In the formula, is the pixel gray value, is the light source irradiance, is the exposure time, is the camera response function.

[0010] Preferably, the non-linear mapping relationship between the pixel gray value and the radiance is specifically as follows: ; wherein, ; In the formula, is the pixel gray value, is the weight function, is the camera response function, is the light source irradiance, is the exposure time, is the smoothing coefficient, is the minimum value of the pixel gray value, is the maximum value of the pixel gray value, is the L2 regularization term, is the total number of pixel points in a picture; is the total number of pictures taken.

[0011] Preferably, the calculation of the energy flux density of the cross-sectional circular grid unit according to the area of the cross-sectional circular grid unit, the area of the elliptical spot grid unit, and the energy flux density of the elliptical spot grid unit is specifically carried out through the following formula: q 椭圆 · A 椭圆 = q 圆 · A 圆 ; q 圆 = q 椭圆 · A 椭圆 / A 圆 ; In the formula, q 椭圆 is the energy flux density of the elliptical spot grid unit; A 椭圆 is the area of the elliptical spot grid unit; q 圆 is the energy flux density of the cross-sectional circular grid unit; A 圆 is the area of the cross-sectional circular grid unit.

[0012] The present invention also provides a Fresnel lens energy flux distribution acquisition system, including: A response function construction module, which is used to use a CCD camera to take multiple pictures of the same scene at different exposure times, and construct a camera response function through the pixel gray values, light source irradiance, and exposure time at each pixel position of each picture; An elliptical spot grid acquisition module, which is used to place a Lambert target in any cross-section of the light cone according to the path of the Fresnel lens focusing light cone. An elliptical spot is formed between the Lambert surface on the Lambert target and the cross-section of the focusing light cone, and an elliptical spot grid is constructed according to the Lambert surface coordinates and the focusing light cone coordinates; An energy flux distribution acquisition module, which is used to set the CCD camera opposite to the Lambert surface, use the CCD to take pictures of the elliptical spot, obtain the gray distribution of the elliptical spot grid, and calculate the energy flux density of the elliptical spot grid according to the camera response function; create a cross-sectional circle of the Fresnel lens focusing light cone, map the energy flux density of the elliptical spot to the cross-sectional circle, and obtain the energy flux distribution of the Fresnel lens.

[0013] The method for obtaining the energy flux distribution of the Fresnel lens provided by the present invention has the following beneficial effects: The present invention constructs a camera response function by using a CCD camera to take multiple pictures of the same scene at different exposure times and through the pixel gray values, light source irradiance, and exposure time at each pixel position of each picture; calculates the energy flux density of the elliptical spot by using a CCD camera to take pictures of the elliptical spot in any cross-section of the Fresnel lens focusing light cone and using the camera response function; and obtains the energy flux density of the cross-sectional circle by mapping the energy flux density of the elliptical spot to the cross-sectional circle. This method does not limit the position of the CCD camera. As long as a complete spot can be photographed, the energy flux density of different spot cross-sections can be obtained by using the camera response function, thereby expanding the measurement range and improving the flexibility of measurement. Description of the Drawings

[0014] In order to more clearly illustrate the embodiments of the present invention and its design solutions, the accompanying drawings required for this embodiment will be briefly introduced below. The accompanying drawings in the following description are only partial embodiments of the present invention. For those of ordinary skill in the art, other accompanying drawings can be obtained based on these drawings without creative efforts.

[0015] Figure 1 It is a flowchart of the method for obtaining the energy flux distribution of the Fresnel lens according to the embodiment of the present invention; Figure 2 It is a diagram of the elliptical spot grid; Figure 3 It is a diagram of the position of the cross-sectional circle; Figure 4 It is a cross-sectional circle grid of the light cone formed by the intersection points of the light ray vectors and the cross-sectional circle. Detailed Embodiments

[0016] To enable those skilled in the art to better understand the technical solution of the present invention and be able to implement it, the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The following embodiments are only used to more clearly illustrate the technical solution of the present invention and cannot be used to limit the protection scope of the present invention.

[0017] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "axial", "radial", "circumferential", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the technical solution of the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0018] In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. In the description of the present invention, it should be noted that unless otherwise clearly specified or limited, the terms "connected" and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances. In the description of the present invention, unless otherwise stated, the meaning of "plurality" is two or more, which will not be elaborated here.

[0019] Embodiment The present invention provides a method for obtaining the energy flow distribution of a Fresnel lens, specifically as Figure 1 shown, including the following steps: Step 1: Use a CCD camera to take multiple pictures of the same scene at different exposure times, and construct a camera response function through the pixel gray value, light source irradiance, and exposure time at each pixel position of each picture.

[0020] The present invention uses a CCD camera to take pictures of the same scene at different exposure times j ( j = 6) pictures, requiring that the pictures contain all values within the range of pixel gray values from 0 to 255, and the shooting time is as short as possible to ensure that the brightness of the scene remains unchanged. Each picture has i pixel gray values, and each pixel position corresponds to a constant light source irradiance , and an equation can be listed according to each pixel gray value , and a total of equations, where g(0) to g(255) are unknowns, i ones E i are unknowns. There are a total of 256 + i unknowns. Due to the large number of pixel points, it is unrealistic to directly construct all equations to solve for the unknowns. Select more than 256 + i pixel points and list the equations to construct the functional relationship between the pixel gray value and the light source irradiance and the exposure time as follows: ; In the formula, is the pixel gray value, is the light source irradiance, is the exposure time, is the camera response function.

[0021] Step 2: Introduce a regularization term into the camera response function to obtain a non - linear mapping relationship between the pixel gray value and the light source irradiance.

[0022] Since the camera response function is relatively steep when the gray level is close to the extreme value, a simple hat function is used as the weighting function to reduce the influence of the extreme value. To increase the smoothness of the fitting curve, the L2 regularization term is introduced to obtain the non - linear mapping relationship between the pixel gray value and the light source irradiance as follows: ; where, ; In the formula, is the pixel gray value, is the weight function, is the camera response function, is the light source irradiance, is the exposure time, is the smoothing coefficient, is the minimum value of the pixel gray value, is the maximum value of the pixel gray value, is the L2 regularization term, is the total number of pixel points in one picture; is the total number of pictures taken.

[0023] Through the non - linear mapping relationship, when the pixel gray value and the exposure time of the known pixel points are known, the magnitude of the light source irradiance can be directly deduced.

[0024] Step 3: Combine the non-linear mapping relationship and use the heat flow meter to calibrate the relationship between the pixel gray value and the solar spot irradiance The specific process is as follows: The solar spot irradiance and the spot brightness L have a linear mapping relationship: ; In the formula, is L and 's proportionality coefficient, b is a constant.

[0025] The spot brightness L and the light source irradiance have a linear mapping relationship: ; In the formula, is the camera response function, is the exposure time, is L and 's proportionality coefficient; Therefore, from the pixel gray value the solar spot irradiance at the corresponding position can be deduced. Specifically, through the following relationship: ; Use the heat flow meter to read at a certain position, the pixel gray value at the corresponding position in the picture and the exposure time of the picture, fit the coefficients A and B to obtain the relationship between the pixel gray value and the solar spot irradiance at the corresponding position. The solar spot irradiance at each position is directly applied to the elliptical spot surface according to its position in the picture. These values are distributed as scatter data on the elliptical spot surface and do not exactly correspond to the coordinates of the grid nodes. In order to obtain the incident energy flux density q of the grid nodes, an interpolation method (such as the scattered interpolation function scatteredInterpolant) is used to convert the scatter data into the q values on the regular grid nodes, realizing the smooth distribution of the energy flux density.

[0026] Step 4: According to the path of the focused light cone of the Fresnel lens, place the Lambert target within any cross-section of the light cone. The Lambert surface on the Lambert target forms an elliptical light spot with the cross-section of the focused light cone. Construct an elliptical light spot grid based on the Lambert surface coordinates and the focused light cone coordinates. The specific process is as follows: According to the path of the focused light cone of the Fresnel lens, place the Lambert target within any cross-section of the light cone. It is only necessary to ensure that the CCD camera lens faces the Lambert surface, and there is no need to fix it at the center of the lens, as Figure 2 shown. The focused light cone is a cone, and the cross-section of the Lambert surface and the light cone is an elliptical light spot (elliptical sunlight spot). Construct an elliptical light spot grid on the Lambert surface based on the Lambert surface coordinates and the light cone coordinates, as specifically Figure 3 shown.

[0027] Step 5: Set the CCD camera opposite to the Lambert surface. Use the CCD to capture the elliptical light spot to obtain the gray-scale distribution of the elliptical light spot grid. Calculate the energy flux density of the elliptical light spot grid according to the relationship between the pixel gray-scale value and the solar irradiance of the light spot; create a cross-sectional circle of the focused light cone of the Fresnel lens, and map the energy flux density of the elliptical light spot onto the cross-sectional circle to obtain the energy flux distribution of the Fresnel lens. Specifically, it includes the following steps: Record the connection vectors between the elliptical grid nodes and the focus of the light cone. These connection vectors are the light vectors passing through the elliptical light spot grid nodes.

[0028] Create the cross-sectional circle of the light cone to be solved, and determine the intersection points of the light vectors and the cross-sectional circle. These intersection points form the cross-sectional circle grid of the light cone, as Figure 4 shown.

[0029] Based on the geometric principle of the intersection of a light ray and a plane, use the parametric equation of the light ray to determine the intersection point position, and improve the intuitiveness and practicality of the intersection point position analysis through screening and visualization means. The specific process is as follows: The direction and starting point of a light ray can be represented by a vector. The parametric equation of the light ray is: ; where P( t ) is the coordinate of any point on the light ray, is the starting point of the light ray, is the direction vector of the light ray, is the parameter used to determine the position of any point on the light ray extending from the starting point along the direction vector .

[0030] A plane is defined by a point on the plane and a normal vector. The plane equation is: ; where, , , is the plane normal vector, is the constant of the plane equation, x , y , z are the coordinates of any point on the plane, and all points on this plane satisfy this relationship.

[0031] Substitute the parametric equation of the light ray into the plane equation to obtain the expression of the parameter : ; is the normal vector of the plane, calculate and substitute it into the parametric equation of the light ray to obtain the intersection position, which is the point where the light ray is mapped on the cross-sectional circle, thus obtaining the cross-sectional circle grid of the elliptical spot grid mapping.

[0032] The corresponding elliptical spot grid unit ( Figure 4 the elliptical grid unit in Figure 4 ) and the cross-sectional circle grid unit ( n ray ) have the same number of light rays passing through. The energy flux density of the cross-sectional circle grid unit is calculated by the ratio of their areas, as shown in the following formula: q 椭圆 · A 椭圆 = q 圆 · A 圆 ; q 圆 = q 椭圆 · A 椭圆 / A 圆 ; In the formula, q 椭圆 is the energy flux density of the elliptical spot grid unit; A 椭圆 is the area of the elliptical spot grid unit; q 圆 is the energy flux density of the cross-sectional circle grid unit; A 圆 is the area of the cross-sectional circle grid unit.

[0033] Traverse the elliptical grid unit and use the above formula to solve the energy flux distribution of any circular cross-section of the light cone.

[0034] The present invention also provides a Fresnel lens energy flux distribution acquisition system, including: A response function construction module, configured to use a CCD camera to capture multiple images of the same scene at different exposure times, and construct a camera response function based on the pixel gray values, light source irradiance, and exposure time at each pixel position of each image; An elliptical spot grid acquisition module, configured to place a Lambert target within an arbitrary cross-section of the light cone according to the path of the Fresnel lens focusing light cone, form an elliptical spot where the Lambert surface on the Lambert target and the cross-section of the focusing light cone are located, and construct an elliptical spot grid based on the Lambert surface coordinates and the focusing light cone coordinates; An energy flux distribution acquisition module, configured to relatively arrange the CCD camera and the Lambert surface, use the CCD to capture the elliptical spot, obtain the gray value distribution of the elliptical spot grid, calculate the energy flux density of the elliptical spot grid according to the camera response function; create a cross-sectional circle of the Fresnel lens focusing light cone, map the energy flux density of the elliptical spot onto the cross-sectional circle, and obtain the energy flux distribution of the Fresnel lens.

[0035] The above embodiments are only preferred specific embodiments of the present invention, and the protection scope of the present invention is not limited thereto. Any simple changes or equivalent replacements of technical solutions that can be obviously obtained by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for obtaining the energy flow distribution of a Fresnel lens, characterized in that, It includes the following steps: Use a CCD camera to take multiple pictures of the same scene at different exposure times, and construct a camera response function based on the pixel gray values, light source irradiance, and exposure time at each pixel position of each picture; According to the path of the Fresnel lens focusing light cone, place a Lambert target within an arbitrary cross-section of the light cone. The Lambert surface on the Lambert target forms an elliptical light spot with the cross-section of the focusing light cone. Construct an elliptical light spot grid based on the Lambert surface coordinates and the focusing light cone coordinates; Set the CCD camera opposite to the Lambert surface, use the CCD to capture the elliptical light spot, obtain the gray distribution of the elliptical light spot grid, and calculate the energy flux density of the elliptical light spot grid according to the camera response function; Create a cross-sectional circle of the Fresnel lens focusing light cone, and map the energy flux density of the elliptical light spot onto the cross-sectional circle to obtain the energy flux distribution of the Fresnel lens.

2. The method for obtaining the energy flow distribution of the Fresnel lens according to claim 1, wherein The calculating the energy flux density of the elliptical light spot according to the camera response function includes the following steps: Introduce a regularization term into the camera response function to obtain a non-linear mapping relationship between the pixel gray value and the light source irradiance; Combine the non-linear mapping relationship and use a heat flow meter to calibrate the relationship between the pixel gray value and the solar light spot irradiance; Calculate the energy flux density of the elliptical light spot grid according to the relationship between the pixel gray value and the solar light spot irradiance.

3. The method for obtaining the energy flow distribution of the Fresnel lens according to claim 1, wherein The creating a cross-sectional circle of the Fresnel lens focusing light cone and mapping the energy flux density of the elliptical light spot onto the cross-sectional circle to obtain the energy flux density of the cross-sectional circle includes the following steps: Obtain the light ray vectors from the nodes of the elliptical light spot grid to the focus of the Fresnel lens focusing light cone, determine the intersection positions of the light ray vectors and the cross-sectional circle. The intersection points are the points where the light ray vectors are mapped onto the cross-sectional circle, and determine the area of the cross-sectional circle grid cells according to the intersection points; Calculate the energy flux density of the cross-sectional circle grid cells according to the area of the cross-sectional circle grid cells, the area of the elliptical light spot grid cells, and the energy flux density of the elliptical light spot grid cells; Traverse all the elliptical light spot grid cells, calculate the energy flux density of all the cross-sectional circle grid cells, and obtain the energy flux distribution of the Fresnel lens.

4. The method for obtaining the energy flow distribution of the Fresnel lens according to claim 3, characterized in that, Determining the intersection positions of the light ray vectors and the cross-sectional circle includes the following steps: Construct the parametric equation of the light ray through the direction and starting point of the light ray; Define the plane equation through a point on the plane and the normal vector; Substitute the parametric equation of the light ray into the plane equation to obtain the intersection positions of the light ray vectors and the cross-sectional circle.

5. The method for obtaining the energy flow distribution of a Fresnel lens according to claim 1, wherein The camera response function is specifically as follows: ； In the formula, is the pixel gray value, is the light source irradiance, is the exposure time, is the camera response function.

6. The method for obtaining the energy flow distribution of the Fresnel lens according to claim 5, wherein, The non-linear mapping relationship between the pixel gray value and the radiance is specifically as follows: ； Among them, ; Wherein, is the pixel gray value, is the weight function, is the camera response function, is the light source irradiance, is the exposure time, is the smoothing coefficient, is the minimum value of the pixel gray value, is the maximum value of the pixel gray value, is the L2 regularization term, is the total number of pixel points in an image; is the total number of captured images.

7. The method for obtaining the energy flow distribution of the Fresnel lens according to claim 3, wherein, The calculating the energy flux density of the cross-sectional circle grid cells according to the area of the cross-sectional circle grid cells, the area of the elliptical light spot grid cells, and the energy flux density of the elliptical light spot grid cells is specifically carried out through the following formula: q 椭圆 · A 椭圆 = q 圆 · A 圆 ； q 圆 = q 椭圆 · A 椭圆 / A 圆 ； Wherein, q 椭圆 is the energy flux density of the elliptical light spot grid unit; A 椭圆 is the area of the elliptical light spot grid unit; q 圆 is the energy flux density of the cross-sectional circular grid unit; A 圆 is the area of the cross-sectional circular grid unit.

8. A Fresnel lens energy flow distribution acquisition system, characterized in that, It includes: A response function construction module for using a CCD camera to take multiple pictures of the same scene at different exposure times, and constructing a camera response function based on the pixel gray values, light source irradiance, and exposure time at each pixel position of each picture; An elliptical spot grid acquisition module, which is used to place a Lambert target within an arbitrary cross-section of the light cone according to the path of the Fresnel lens focusing light cone. The Lambert surface on the Lambert target forms an elliptical spot with the cross-section of the focusing light cone, and an elliptical spot grid is constructed based on the Lambert surface coordinates and the focusing light cone coordinates; An energy flux distribution acquisition module, which is used to set the CCD camera opposite to the Lambert surface, use the CCD to capture the elliptical spot, obtain the gray-scale distribution of the elliptical spot grid, and calculate the energy flux density of the elliptical spot grid according to the camera response function; create a cross-sectional circle of the Fresnel lens focusing light cone, map the energy flux density of the elliptical spot onto the cross-sectional circle, and obtain the energy flux distribution of the Fresnel lens.

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