Simple focus fixing method of area array detector for carrying out light beam scanning based on glass plate

Abstract

The invention discloses a simple area array detector focus fixing method for light beam scanning based on a glass parallel plate, and belongs to the field of optical measuring instruments. The implementation method comprises the steps that a glass parallel plate is placed in front of an objective lens, the parallel plate is perpendicular to a light beam, thin parallel incident light passes through the glass parallel plate and then is focused on a detector through the objective lens to form a light spot, the detector collects a light spot image, and then the position of the light spot mass center is obtained through a mass center calculation algorithm; rotating the glass parallel plate, acquiring light spot images of the glass parallel plate at different rotation angles, and judging whether the detector is in a pre-focus state or a post-focus state at the moment; obtaining a defocusing amount according to the position of the mass center of the light spot; precise focus fixing adjustment is completed by increasing or decreasing gaskets at the installation position of the detector, the increase or decrease of the gaskets is determined or the gaskets are properly ground according to the fact that the detector is judged to be in a pre-focus state or a post-focus state and the calculated defocusing amount, and simple and convenient focus fixing of the detector is completed. The centroid positioning precision of the invention can reach the precision of one percent pixel.

Description

Technical Field

[0001] The invention relates to a simple focusing method for a surface array detector based on a glass parallel flat plate for light beam scanning, and belongs to the field of optical measuring instruments. Background Art

[0002] In the photoelectric autocollimator, the light emitted by the point light source is collimated into parallel light after passing through the collimating objective lens. The parallel light is reflected to the objective lens by the plane reflector and converged to the focal point after being focused by the objective lens. The detector (CCD or CMOS) is placed on the focal plane of the objective lens, so that the detector can collect a spot image. The principle diagram of the optical path of the photoelectric autocollimator is shown as follows: Figure 1 As shown. When the angle of the reflector deflects, the position of the light spot on the detector will also change accordingly. The center of mass calculation algorithm can be used to obtain the center of mass of each light spot. By comparing the changes in the center of mass positions before and after, a displacement can be obtained. Then, the angle change of the reflector can be calculated through geometric relationships. In the photoelectric autocollimator, whether the detector is accurately installed on the focal plane of the objective lens has a great influence on the angle measurement accuracy. However, due to the existence of interference factors such as temperature changes and environmental vibrations during the use of this type of system, the system that originally completed the focus will be offset. Therefore, before use, it is necessary to check whether the system is in a defocus state, and only after re-focusing can subsequent measurements be performed. The photoelectric autocollimator measures the angle change by calculating the change in the center of mass of the front and rear light spots. Therefore, during the installation and adjustment of the photoelectric autocollimator, whether the detector is accurately installed on the focal plane of the objective lens directly affects the accuracy and reliability of the angle measurement. When the object to be measured has a large deflection angle, only part of the reflected light from the measurement beam emitted by the autocollimator can return to the autocollimator system. Theoretically, if the detector is exactly on the focal plane, all parts of the light are focused on the same point, so no matter which part of the light is used for angle measurement, the data obtained is the same. However, if the detector is in a defocused state, the center of mass position of the light spot focused on the detector by light from different positions will change. Therefore, it is necessary to accurately focus the installation position of the detector to ensure that the center of mass of the light spot after the light beam at any position within the aperture is focused is at the same position, so as to ensure the accuracy of angle measurement. In satellite cameras, if the detector cannot be accurately installed on the focal plane, the collected image will be blurred. Therefore, a simple, reliable and high-precision detector focusing method is crucial in such systems where the detector needs to be accurately focused.

[0003] Among the traditional focusing methods, the solution most similar to the present invention is the spot diameter method. According to the principle of geometric optics, parallel light is focused on the focal plane after passing through the objective lens and converges into a light spot. The diameter of the spot is the smallest on the focal plane. Keeping the rest of the optical path unchanged, move the detector back and forth along the optical axis, collect the spot image at different positions, and then perform image processing operations such as denoising and edge extraction on the spot image to obtain the size of the spot diameter. Then compare the spot diameters at different axial positions. When the spot diameter is the smallest, it is considered that the detector is in the focal plane position.

[0004] This method requires moving the detector back and forth during the focusing process, which is cumbersome and cannot be used in systems with fixed structures. Moreover, when the detector is in a defocused state, it is impossible to determine whether the detector is in front of or behind the focal plane based on the spot image collected by the detector. This method determines whether the detector is installed in place by the change in the diameter of the spot. In theory, the highest spot positioning accuracy is sub-pixel accuracy, so in a high-precision photoelectric autocollimation system, the focal plane positioning accuracy of the detector will become a key factor restricting the further improvement of angle measurement accuracy. Summary of the invention

[0005] In order to solve the problem of low precision and cumbersome focusing of the existing autocollimator focusing method, the purpose of the present invention is to provide a simple focusing method for a planar array detector based on a glass parallel plate for light beam scanning. This method places a glass parallel plate in front of the objective lens. The fine light beam passing through the glass parallel plate is focused by the objective lens and falls on the detector to form a light spot. The position of the center of mass of this light spot can be obtained by the center of mass calculation algorithm. Then the parallel plate is rotated. If the center of mass of the corresponding light spot is in the same position of the detector before and after the rotation, the detector is just on the focal plane of the objective lens, indicating that the detector has been installed in place. If there is a deviation between the positions of the various center of mass, it means that the detector is in a defocused state at this time and the installation position needs to be adjusted. The center of mass positioning accuracy of the present invention can reach an accuracy of one hundredth of a pixel.

[0006] The purpose of the present invention is achieved through the following technical steps.

[0007] A simple focusing method for an area array detector based on a glass parallel plate for beam scanning includes the following steps:

[0008] Step 1: Place a glass parallel plate in front of the objective lens. The parallel plate is perpendicular to the light beam. After the incident light passes through the glass parallel plate, it is focused by the objective lens onto the detector to form a light spot. The detector collects the light spot image, and then the position of the center of mass of the light spot is obtained by the center of mass calculation algorithm.

[0009] Step 2: rotate the glass parallel plate, collect the spot images of the glass parallel plate at different rotation angles, calculate the centroid of each spot, compare it with the position of the centroid of the spot formed in step 1, and judge whether the detector is in front of or behind the focus at this time;

[0010] Step 3: Obtain the defocus value △d through the centroid position of the light spot in steps 1 and 2:

[0011]

[0012] Where f is the focal length of the objective lens, h is the distance between the centroids of the two light spots, and L is the center distance of the light beams before and after the rotating parallel plate, which is calculated according to Snell's law. The calculation process is shown in formula (2):

[0013]

[0014] θ3=θ1-θ2 (3)

[0015]

[0016] Where n1 is the refractive index of air, n2 is the refractive index of the glass parallel plate, d is the thickness of the glass parallel plate, θ1 is the rotation angle of the glass parallel plate, θ2 is the angle between the light in the glass and the normal, and θ3 is the angle between the light in the glass and the incident light.

[0017] Step 4: In a system with a fixed structure, precise focusing adjustment can be achieved by adding or removing shims at the detector installation location. The addition or removal of shims or appropriate grinding of the shims is determined based on whether the detector is in front of or behind the focus and the calculated defocus amount, thus completing simple focusing of the detector.

[0018] If the focusing accuracy of the detector is not high, it is only necessary to collect two images to complete the focusing. The glass parallel plate is rotated to generate two parallel light beams with a large deviation in the vertical direction. Then, the spot images are collected separately, and the positions of the centroids of the spots in the two images are calculated. The defocus value is calculated according to formula (1).

[0019] If the detector needs high-precision focusing, the glass parallel plate is rotated multiple times within the aperture and the spot images are collected respectively. The defocus amounts are calculated respectively according to the above-mentioned method for calculating the defocus amount. The defocus amounts are not uniformly affected by various aberrations. The defocus amounts are fitted with the least squares method to obtain the defocus amount with the minimum variance. The detector is focused with reference to the defocus amount with the minimum variance.

[0020] Beneficial effects:

[0021] 1. The present invention discloses a simple focusing method for an area array detector based on a glass parallel plate for light beam scanning. A glass parallel plate is placed in front of an objective lens. A fine light beam passing through the glass parallel plate is focused by the objective lens and falls on the detector to form a light spot. The position of the center of mass of the light spot can be obtained by a center of mass calculation algorithm. Then the parallel plate is rotated. If the center of mass of the corresponding light spot is at the same position of the detector before and after the rotation, the detector is just on the focal plane of the objective lens. Because the detector does not need to be moved back and forth or the gasket is replaced multiple times during the focusing process, it is only necessary to change the rotation angle of the parallel plate and then compare the change in the center of mass position of the light spot on the detector to complete the focusing. Therefore, the use of the method in a structural fixed system is not restricted, and the focusing method is simple and the focusing accuracy is high.

[0022] 2. The present invention discloses a simple focusing method for an area array detector based on a glass parallel plate for light beam scanning. The method places a glass parallel plate in front of an objective lens and realizes light beam scanning within the aperture of the objective lens through the angle of the glass parallel plate. The focusing method is based on the original optical path of the optical system and does not require additional system construction. Moreover, the method uses simple instruments and is available in all optical laboratories.

[0023] 3. The present invention discloses a simple focusing method for an array detector based on a glass parallel plate for light beam scanning. If the focusing accuracy of the detector is not required to be high, it is only necessary to collect two images to complete the focusing. The glass parallel plate is rotated to generate two parallel light beams with a large deviation in the vertical direction. Then, the spot images are collected respectively, and the positions of the spot centroids in the two images are calculated. The defocus amount is calculated according to the defocus amount formula. If the detector needs to be focused with high precision, the glass parallel plate is rotated multiple times within the aperture and the spot images are collected respectively. The defocus amount is calculated respectively according to the above-mentioned method for calculating the defocus amount. The defocus amount is not uniformly affected by various aberrations. The least squares method is used to fit the defocus amount to obtain the defocus amount with the smallest variance as a reference for focusing the detector. Selecting the focusing operation according to the actual use requirements can achieve both simple and fast focusing and high-precision focusing. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the optical path of the photoelectric autocollimator;

[0025] Figure 2 Schematic diagram of beam scanning using glass parallel plates;

[0026] Figure 3 The size of the parallel plates of glass and the height difference of the center of the light rays;

[0027] Figure 4 This is a schematic diagram of the placement of the glass parallel plate when collecting the spot image for the first time in Example 1;

[0028] Figure 5 This is a schematic diagram of the placement of the glass parallel plate in Example 1 when collecting the spot image for the second time. DETAILED DESCRIPTION

[0029] In order to better illustrate the purpose and advantages of the present invention, the invention is further described below with reference to the accompanying drawings and examples.

[0030] Example 1

[0031] In a usage scenario where there is no extremely high focus accuracy requirement, the present invention can quickly complete the focus operation of the detector while ensuring a certain accuracy.

[0032] A simple focusing method for an area array detector based on a glass parallel plate for beam scanning is provided. The specific implementation steps are as follows:

[0033] 1) Prepare a glass parallel plate with a size of 60mm×60mm and a thickness of 20mm. Select a monochromatic parallel light with a central wavelength of 540nm and a beam diameter of 2mm as the light source. Place the glass parallel plate in front of the objective lens, i.e., in the incident direction of the beam.

[0034] 2) Take the position of the glass parallel plate perpendicular to the optical axis as the zero point, rotate the glass parallel plate, the clockwise rotation angle is recorded as positive, the counterclockwise rotation angle is recorded as negative, rotate the glass parallel plate 30°, such as Figure 4 As shown, the spot image is collected, and the center of the image is taken as the origin, and the centroid position of the spot is obtained as (0mm, 0.0285mm). Then the glass parallel plate is rotated to -30°, as shown in FIG. Figure 5 As shown, the spot image is collected, and the spot centroid position is (0mm, -0.0285mm). When the glass plate is rotated counterclockwise, the thin parallel light beam produces a vertical upward offset, and the spot centroid position changes downward, indicating that the detector is in the post-focus state at this time;

[0035] 3) According to formulas (1) to (4), the defocus of the detector is calculated to be 1.2 mm;

[0036] 4) In a system with a fixed structure, the focus adjustment can be completed by adding a 1.2mm thick gasket at the detector installation;

[0037] 5) The focus setting operation is completed.

[0038] Example 2

[0039] In usage scenarios that require extremely high focusing accuracy, the present invention can simply achieve a centroid positioning accuracy of one hundredth of a pixel.

[0040] A simple focusing method for an area array detector based on a glass parallel plate for beam scanning is provided. The specific implementation steps are as follows:

[0041] 1) Prepare a glass parallel plate with a size of 60mm×60mm and a thickness of 20mm. Select a monochromatic parallel light with a central wavelength of 540nm and a beam diameter of 2mm as the light source. Place the glass parallel plate in front of the objective lens, i.e., in the incident direction of the beam.

[0042] 2) Taking the position of the glass parallel plate perpendicular to the optical axis as the zero point, rotate the glass parallel plate, record the clockwise rotation angle as positive, and the counterclockwise rotation angle as negative, rotate the glass parallel plate multiple times, and record the rotation angle each time, then collect spot images at these positions, calculate the center of mass of the spot by the center of mass algorithm, and record the center of mass position. The recording results are shown in Table 1, and according to the position changes of the two spot images, it can be determined that the detector is after focus;

[0043] Table 1 Glass parallel plate rotation angle and corresponding center of mass position

[0044]

[0045] 3) According to formulas (1) to (4), the defocus of the detector can be calculated from the data in step 2), as shown in Table 2. Then, the least square method is used for fitting to determine that the defocus with the smallest variance is 2.1 mm, which is used as the basis for adjustment.

[0046] Table 2 Parallel beam deviation caused by the rotation of the glass parallel plate and the corresponding defocus

[0047]

[0048] 4) In a system with a fixed structure, the focus adjustment can be completed by adding a 2.1mm thick gasket at the detector installation;

[0049] 5) The focus setting operation is completed.

[0050] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A simple focusing method for an area array detector based on a glass flat plate for beam scanning, characterized in that: The following steps are included: Step 1: Place a glass plate in front of the objective lens. The incident light passes through the glass plate and then passes through the objective lens to form a light spot. The detector collects the light spot image, and then obtains the center of mass position of the light spot through the center of mass calculation algorithm. Step 2: Rotate the glass plate to collect the spot images at different rotation angles of the glass plate, calculate the centroid position of each spot image, and compare it with the position of the spot centroid formed in step 1 to determine whether the detector is in the front or back focus at this time; Step 3: Obtain the defocus value △d through the centroid position of the light spot in steps 1 and 2: In the formula, f is the focal length of the objective lens, h is the distance between the centroids of the two light spots before and after the rotation, and L is the center distance of the light beams before and after the rotation of the parallel plate. Through formulas (2) to (4), we can get: θ3=θ1-θ2 (3) Where n1 is the refractive index of air, n2 is the refractive index of the glass plate, d is the thickness of the glass plate, θ1 is the rotation angle of the flat glass, θ2 is the angle between the light in the glass and the normal, and θ3 is the angle between the light in the glass and the incident light. Step 4: In a system with a fixed structure, based on whether the detector is in the front or back focus position determined in step 2 and the defocus value obtained in step 3, precise focus adjustment can be completed by adding or removing shims at the detector installation location.

2. The method for focusing an area array detector based on a glass flat plate for beam scanning as claimed in claim 1, characterized in that: If the detector needs high-precision focusing, the glass plate is rotated multiple times within the aperture and the spot images are collected respectively, the defocus amounts are calculated respectively, and the defocus amounts are fitted using the least squares method to obtain the defocus amount with the minimum variance. The detector is focused with reference to the defocus amount with the minimum variance.

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