Simple focus fixing method for area array detector adopting single-hole scanning mode

Abstract

The invention discloses a simple area array detector focus fixing method adopting a single-hole scanning mode, and belongs to the field of detector image plane focus fixing. The implementation method comprises the steps that a blank screen with a small hole is placed in front of an objective lens, incident light passes through the small hole and then passes through the objective lens to form a light spot, a detector collects a light spot image, and the light spot mass center position is obtained through a mass center calculation algorithm; moving the blank screen in the vertical direction, collecting light spot images at different positions of the small hole, respectively calculating to obtain the light spot mass center position of each light spot image, comparing the light spot mass center position with the initially formed light spot mass center position, 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 a defocusing amount formula; if deviation exists between the positions of the centroids, the installation position is adjusted; if the mass centers of the corresponding light spots are located at the same position of the detector before and after movement, the detector is just located on the focal plane of the objective lens, the detector is installed in place, and simple focus fixing of the area array detector is achieved. According to the invention, the positioning precision can reach one hundredth pixel.

Description

Technical Field

[0001] The invention relates to a simple focusing method for an area array detector adopting a single-hole scanning mode, and belongs to the field of focusing of a detector image plane. 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 an array detector using a single-hole scanning method. This method adds a black screen with a small hole in front of the objective lens. The light beam passing through the small hole 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 is obtained by the centroid calculation algorithm, and then the position of the pinhole aperture is moved. If the center of mass of the corresponding light spot before and after the movement is in the same position of the detector, 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 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 using a single-hole scanning method comprises the following steps:

[0008] Step 1: Place a black screen with a small hole in front of the objective lens. The incident light passes through the small hole 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 center of mass of the light spot is obtained by the center of mass calculation algorithm.

[0009] Step 2: Move the black screen, collect the spot images at different positions of the pinhole, 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] In the formula, L is the distance between the two pinholes, f is the focal length of the objective lens, and h is the distance between the centroids of the two light spots;

[0013] 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.

[0014] If the focus accuracy of the detector is not high, it is only necessary to collect two images to complete the focus. Two positions far apart in the vertical direction are taken as the positions of the pinhole, and then the spot images are collected respectively. The positions of the centroids of the spots in the two images are calculated. The defocus value can be calculated according to formula (1).

[0015] If the detector needs to be focused with high precision, the position of the pinhole is moved multiple times within the aperture and the spot images are collected respectively. The defocus amounts are calculated respectively according to the method of calculating the defocus amount according to the above formula (1). The defocus amounts are affected by various aberrations and are not consistent. The least squares method is used to fit these defocus amounts to obtain the defocus amount with the smallest variance as a reference for focusing the detector.

[0016] Beneficial effects:

[0017] 1. The present invention discloses a simple focusing method for an area array detector using a single-hole scanning method. A black screen with a small hole is added in front of an objective lens. The light beam passing through the small hole 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 is obtained by a center of mass calculation algorithm. Then, the position of the small hole diaphragm is moved. Accurate focusing adjustment is achieved by adding or removing gaskets at the detector installation location. The addition or removal of gaskets is determined based on whether the detector is before or after focus and the calculated defocus amount, or the gaskets are appropriately ground, so that the simple focusing of the detector is completed. Because it is not necessary to move the detector back and forth or replace the gaskets multiple times during the focusing process, it is only necessary to change the position of the small hole diaphragm 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.

[0018] 2. The present invention discloses a simple focusing method for an area array detector using a single-hole scanning method. This method places a black screen with a pinhole in front of the objective lens, and realizes light beam scanning within the aperture of the objective lens by moving the position of the pinhole diaphragm. The focusing process 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.

[0019] 3. The present invention discloses a simple focusing method for an array detector using a single-hole scanning method. 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. Two positions far apart in the vertical direction are taken as the positions of the pinholes, and then the spot images are collected respectively. The positions of the centroids of the spots in the two images are calculated, and the defocus is calculated according to the defocus formula. If the detector needs high-precision focusing, the pinhole position is moved multiple times within the aperture and the spot images are collected respectively. The defocus is calculated according to the above-mentioned method for calculating the defocus. The defocus is not consistent due to the influence of various aberrations. The least squares method is used to fit these defocus to obtain the defocus 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

[0020] Figure 1 This is the principle diagram of the optical path of the photoelectric autocollimator;

[0021] Figure 2 This is a schematic diagram of the principle of calculating the detector defocus amount according to the present invention;

[0022] Figure 3 The plate with holes used in Example 1 and Example 2;

[0023] Figure 4 This is a schematic diagram of the placement of the flat plate with holes when collecting the spot image for the first time in Example 1;

[0024] Figure 5 This is a schematic diagram of the placement of the holed flat plate in Example 1 when collecting the spot image for the second time. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with the embodiments and drawings.

[0026] Example 1

[0027] 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.

[0028] A simple focusing method for an area array detector using a single-hole scanning method is described. The specific implementation steps are as follows:

[0029] 1) The focal length of the objective lens is 250mm, the effective aperture is Φ50mm, prepare a black screen with a small hole, the diameter of the small hole is Φ2mm, and the size of the black screen is 100mm×100mm, such as Figure 3 As shown, the fixture holding the black screen can move in the vertical direction, and the black screen is placed in front of the objective lens, that is, in the incident direction of parallel light;

[0030] 2) Adjust the fixture holding the black screen, align the pinhole with the center of the objective lens, move the pinhole vertically upward by 20 mm, and collect the spot image with the detector. Figure 4 As shown, take the center of the image as the origin, and the coordinates of the centroid of the light spot are (0mm, -0.0399mm); move the small hole vertically downward by 40mm, as shown in Figure 5 As shown, the detector collects the spot image again, and the coordinates of the spot mass center are (0mm, 0.0399mm); by comparing the position of the spot mass center, it can be found that the pinhole position moves downward, while the position of the spot mass center changes upward, so it is judged that the detector is in the post-focus state at this time;

[0031] 3) According to formula (1), the defocus of the detector can be calculated to be 0.4989 mm;

[0032] 4) In a system with a fixed structure, the focus adjustment can be completed by adding a 0.5mm thick gasket at the detector installation;

[0033] 5) The focus setting operation is completed.

[0034] Example 2

[0035] 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.

[0036] A simple focusing method for an area array detector using a single-hole scanning method is described. The specific implementation steps are as follows:

[0037] 1) The focal length of the objective lens is 250mm, the effective aperture is Φ50mm, prepare a black screen with a small hole, the diameter of the small hole is Φ2mm, and the size of the black screen is 100mm×100mm, such as Figure 3 As shown, the fixture holding the black screen can move in the vertical and horizontal directions, and the black screen is placed in front of the objective lens, that is, in the incident direction of parallel light;

[0038] 2) Adjust the fixture holding the black screen, adjust the pinhole to align with the center of the objective lens, and use this as the origin to move the pinhole position multiple times, record the distance moved each time, and then collect spot images at these positions respectively, 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 changes in the center of mass position of the two spot images, it can be determined that the detector is in the front focus;

[0039] Table 1. The position of the pinhole and the corresponding centroid position of the light spot

[0040]

[0041]

[0042] 3) According to formula (1), the defocus of the detector can be calculated from the two sets of data in step 2), as shown in Table 2. Then, the least square method is used for fitting, and the linear regression model is d = 0.1874h + 0.7885. The defocus with the smallest variance is 0.8mm, which is used as the basis for adjustment. h is the distance between the centroids of the two light spots. The measurement principle is as follows: Figure 2 shown.

[0043] Table 2 The relative distance of the pinhole movement and the defocus amount

[0044]

[0045] 4) In a system with a fixed structure, precise focus adjustment can be achieved by reducing the 0.8mm spacer on the detector;

[0046] 5) The focus setting operation is completed.

[0047] 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 using a single-hole scanning method, characterized in that: The following steps are included: Step 1: Place a black screen with a small hole in front of the objective lens. The incident light passes through the small hole and then passes through the objective lens to form a light spot. The detector collects the light spot image, and then the center of mass position of the light spot is obtained through the center of mass calculation algorithm. Step 2: Move the black screen vertically to collect the spot images at different positions of the pinhole, calculate the spot centroid position of each spot image, compare it with the spot centroid position formed in step 1, and judge 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, L is the distance between the pinholes before and after the two movements, f is the focal length of the objective lens, and h is the distance between the centroids of the two light spots; 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. A simple focusing method for an area array detector using a single-aperture scanning method as claimed in claim 1, characterized in that: If the focus accuracy of the detector is not high, two positions far apart in the vertical direction are selected as the positions of the pinholes, and then the spot images are collected respectively. The positions of the centroids of the spots in the two images are calculated, and the defocus value can be calculated according to formula (1).

3. A simple focusing method for an area array detector using a single-aperture scanning method as claimed in claim 1, characterized in that: If the detector needs to be focused with high precision, the position of the pinhole is moved multiple times within the aperture and the spot images are collected respectively. The defocus values ​​are calculated respectively according to the above formula (1), and then the defocus values ​​are fitted using the least squares method to obtain the defocus value with the smallest variance as a reference for focusing the detector.

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