A simple focusing method for area array detectors based on glass flat plate beam scanning

Abstract

A simple focusing method for a planar array detector based on a glass parallel plate for light beam scanning belongs to the field of optical measuring instruments. The implementation method of the present invention is as follows: a glass parallel plate is placed in front of an objective lens, the parallel plate is perpendicular to the light beam, and a thin parallel incident light passes through the glass parallel plate and 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 light spot center of mass is obtained by a centroid calculation algorithm; the glass parallel plate is rotated, and the light spot images of the glass parallel plate at different rotation angles are collected to determine whether the detector is in the front or back focus at this time; the defocus amount is obtained according to the position of the light spot center of mass; precise focusing adjustment is completed by adding or removing gaskets at the detector installation location, and the addition or removal of gaskets or appropriate grinding of the gaskets is determined based on the previous judgment that the detector is in the front or back focus and the calculated defocus amount, thus completing the simple focusing of the detector. The center of mass positioning accuracy of the present invention can reach an accuracy of one hundredth of a pixel.

Description

Technical Field

[0001] The invention relates to a simple focusing method for an area 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 onto 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, and then the detector can collect a light spot image. The principle diagram of the optical path of the photoelectric autocollimator is shown as follows: Figure 1 As shown in the figure, when the reflector angle deflects, the position of the light spot on the detector also changes. A centroid calculation algorithm can be used to determine the center of mass of each light spot. Comparing the changes in the front and rear center of mass positions yields a displacement, which can then be used to calculate the reflector's angular change using geometric relationships. In a photoelectric autocollimator, the accuracy of the detector's placement on the objective lens' focal plane significantly impacts angular measurement accuracy. However, during use, interference factors such as temperature fluctuations and environmental vibrations can cause the previously focused system to shift. Therefore, prior to use, the system must be checked for defocus and refocused before subsequent measurements can be performed. Photoelectric autocollimators measure angular changes by calculating the change in the center of mass of the front and rear light spots. Therefore, during the installation and calibration process, the accuracy of the detector's placement on the objective lens' focal plane directly impacts the accuracy and reliability of angular measurement. When the object under test experiences significant deflection, only a portion of the reflected light from the autocollimator's measurement beam returns to the autocollimator system. Theoretically, if the detector is exactly on the focal plane, all light rays from different parts are focused on the same point, so no matter which part of the light is used for angle measurement, the data obtained will be the same. However, if the detector is out of focus, the center of mass of the light spot focused on the detector by light rays from different positions will change. Therefore, it is necessary to accurately focus the detector installation position to ensure that the center of mass of the light spot after the light beam is focused at any position within the aperture is located 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 systems where such detectors need to be precisely focused.

[0003] Among traditional focusing methods, the one most similar to the present invention is the spot diameter method. According to the principles of geometric optics, parallel light passes through the objective lens and is focused on the focal plane, converging into a single point of light. The spot diameter is minimized on the focal plane. By keeping the rest of the optical path unchanged and moving the detector back and forth along the optical axis, the spot image is captured at different positions. Image processing operations such as denoising and edge extraction are then performed on the spot image to determine the spot diameter. The spot diameters at different axial positions are then compared. When the spot diameter is minimized, the detector is considered to be in the focal plane.

[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 out of focus, 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 changing the diameter of the spot. In theory, the highest spot positioning accuracy is sub-pixel accuracy. Therefore, in a high-precision photoelectric autocollimation system, the focal plane positioning accuracy of the detector will become a key factor restricting further improvement of angle measurement accuracy. Summary of the Invention

[0005] In order to solve the problem of low precision and cumbersome focusing of existing autocollimator focusing methods, the purpose of the present invention is to provide a simple focusing method for an 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 exactly 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 centers of mass, it means that the detector is in a defocused state 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 light spot centroid is obtained by the centroid calculation algorithm.

[0009] Step 2: Rotate the glass parallel plate to collect light spot images at different rotation angles of the glass parallel plate, calculate the centroid of each light spot, and compare it with the position of the light spot centroid formed in step 1 to determine whether the detector is in the front or back focus position 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 between 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 focus adjustment is 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 focus adjustment of the detector.

[0018] If the focus accuracy of the detector is not high, it is only necessary to collect two images to complete the focus. 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 center of mass of the spot in the two images are calculated. The defocus amount is calculated according to formula (1).

[0019] If the detector requires high-precision focusing, 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 affected by various aberrations and is not uniform. The least squares method is used to fit the defocus amount 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 light beam scanning using a glass parallel plate. 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 this light spot can be obtained using a center of mass calculation algorithm. The parallel plate is then rotated. If the center of mass of the corresponding light spot is at the same position on the detector before and after the rotation, the detector is exactly on the focal plane of the objective lens. Because the focusing process does not require moving the detector back and forth or replacing gaskets multiple times, focusing can be completed by simply changing the rotation angle of the parallel plate and then comparing the change in the center of mass position of the light spot on the detector. Therefore, the method is not restricted in use in structurally fixed systems, and the focusing method is simple and has high focusing accuracy.

[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. This method places a glass parallel plate in front of an objective lens and realizes light beam scanning within the objective lens aperture 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 instrument used in the method is simple and all optical laboratories have the conditions to implement it.

[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 separately, the positions of the centroids of the spots in the two images are calculated, and the defocus amount is calculated according to the defocus amount formula. If the detector requires high-precision focusing, the glass parallel plate is rotated multiple times within the aperture and the spot images are collected separately. The defocus amount is calculated separately according to the above-mentioned method for calculating the defocus amount. The defocus amount is affected by various aberrations and is not consistent. The least squares method is used to fit the defocus amount to obtain the defocus amount with the smallest variance, which is used as a reference for focusing the detector. The focusing operation is selected according to the actual use requirements, which 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 the principle of beam scanning using glass parallel plates;

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

[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 when collecting the spot image for the second time in Example 1. 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 requirement for extremely high focusing accuracy, the present invention can quickly complete the focusing 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 described. The specific implementation steps are as follows:

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

[0034] 2) With 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, and 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 to obtain the center of mass position of the spot (0mm, 0.0285mm). Then the glass parallel plate is rotated to -30°, as shown in FIG. Figure 5 As shown in the figure, the spot image is collected, and the center of mass of the spot is (0mm, -0.0285mm). When the glass plate is rotated counterclockwise, the thin parallel light beam produces a vertical upward offset, and the center of mass of the spot 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 fixed structure system, focus adjustment can be accomplished by adding a 1.2mm thick spacer at the detector mounting location.

[0037] 5) The focus operation is completed.

[0038] Example 2

[0039] In usage scenarios requiring extremely high focus 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 described. The specific implementation steps are as follows:

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

[0042] 2) With 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, and the counterclockwise rotation angle is recorded as negative. Rotate the glass parallel plate multiple times, and record the rotation angle for each rotation. Then, collect light spot images at these positions, calculate the center of mass of the light spot using the center of mass algorithm, and record the center of mass position. The recording results are shown in Table 1. Based on the position changes of the two light spot images, it can be determined that the detector is in the post-focus state.

[0043] Table 1 Rotation angle of glass parallel plate 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 squares 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 fixed structure system, focus adjustment can be accomplished by adding a 2.1mm thick spacer at the detector mounting location.

[0049] 5) The focus operation is completed.

[0050] The above specific description further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above description 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 by: 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 uses the center of mass calculation algorithm to obtain the center of mass position of the light spot. Step 2: Rotate the glass plate to collect spot images at different rotation angles of the glass plate. Calculate the centroid position of each spot image and compare it with the centroid position of the spot formed in step 1 to determine whether the detector is in the front or back focus position. Step 3: Obtain the defocus value △d through the centroid position of the light spot in steps 1 and 2: Where 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 between the light beams before and after the rotation of the parallel plate. The following equations (2) to (4) are used to obtain: 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 inside the glass and the normal, and θ3 is the angle between the light inside 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 achieved by adding or removing spacers at the detector mounting location.

2. The simplified focusing method for an area array detector using a glass flat plate for beam scanning as claimed in claim 1, characterized in that: If the detector requires 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 least squares method is used to fit the defocus amounts 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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