A simple focusing method for area array detectors using single-aperture scanning

Abstract

A simple focusing method for an array detector using a single-hole scanning method belongs to the field of detector image plane focusing. The implementation method of the present invention is as follows: a black screen with a small hole is placed in front of the objective lens, and 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 the position of the light spot center of mass is obtained by a centroid calculation algorithm; the black screen is moved in the vertical direction, and the light spot images at different positions of the small hole are collected. The position of the light spot center of mass of each light spot image is calculated respectively, and compared with the position of the light spot center of mass initially formed, it is judged whether the detector is in the front or back of the focus at this time; the defocus amount is obtained according to the defocus amount formula; if there is a deviation between the positions of the various center of mass, the installation position is adjusted; if the center of mass of the corresponding light spot is in the same position of the detector before and after the movement, the detector is exactly on the focal plane of the objective lens, indicating that the detector has been installed in place, that is, the simple focusing of the array detector is achieved. The positioning accuracy of the present invention can reach one hundredth of a 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 detector image plane focusing. 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 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 small hole aperture is moved. If the center of mass of the corresponding light spot is in the same position of the detector before and after the movement, 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 positions, 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 using a single-aperture 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 uses the center of mass calculation algorithm to obtain the position of the light spot centroid.

[0009] Step 2: Move the black screen and collect the spot images at different positions of the pinhole. Calculate the centroid of each spot and compare it with the position of the centroid of the spot formed in step 1 to determine whether the detector is in the front or back focus position.

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

[0011]

[0012] Where 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 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.

[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 center of mass of the spot in the two images are calculated. The defocus amount can be calculated according to formula (1).

[0015] If the detector needs to be focused with high precision, the pinhole position 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, which is used 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 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 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 in front of or after focus and the calculated defocus amount, or the gaskets are appropriately ground to achieve simple focusing of the detector. Because there is no need 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 changes in the center of mass position of the light spot on the detector to complete the focusing. Therefore, the use 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 objective lens aperture 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 instrument used in this method is simple and all optical laboratories have the conditions to implement it.

[0019] 3. The present invention discloses a simple focusing method for an area array detector using a single-hole scanning method. If the focusing accuracy of the detector is not required to be high, focusing can be completed by collecting images twice. Two positions far apart in the vertical direction are respectively used as the positions of the pinhole, and then spot images are collected respectively. The position of the centroid of the spot in the two images is calculated, and the defocus amount is calculated according to the defocus amount formula. If the detector requires high-precision focusing, the pinhole position is moved multiple times within the aperture and spot images are collected respectively. The defocus amount is calculated 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 these defocus amounts to obtain the defocus amount with the smallest variance, which is used as a reference for focusing the detector. Selecting the focusing operation according to 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 perforated flat plate used in Example 1 and Example 2;

[0023] Figure 4 This is a schematic diagram of the placement of the flat plate with a hole 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 with reference to the embodiments and accompanying drawings.

[0026] Example 1

[0027] 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.

[0028] A simple focusing method for an area array detector using a single-aperture 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, and move the pinhole vertically upward by 20mm. The detector collects the spot image, such as Figure 4 As shown, with the center of the image as the origin, the coordinates of the center of mass of the light spot are (0mm, -0.0399mm); move the small hole vertically downward by 40mm, as shown Figure 5 As shown, the detector collects the spot image again, and the coordinates of the spot center of mass are (0mm, 0.0399mm). By comparing the positions of the spot center of mass, it can be found that the position of the pinhole moves downward, while the position of the spot center of mass changes upward. Therefore, 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 fixed structure system, focus adjustment can be accomplished by adding a 0.5mm thick spacer at the detector mounting location.

[0033] 5) The focus operation is completed.

[0034] Example 2

[0035] In usage scenarios requiring extremely high focus 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-aperture 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 direction of incident 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 multiple times, recording the distance moved each time. Then, collect spot images at these positions, calculate the center of mass of the spot using the centroid algorithm, and record the center of mass position. The recording results are shown in Table 1. Based on the changes in the center of mass positions of the two spot images, it can be determined that the detector is in the front focus.

[0039] Table 1 Pinhole position and corresponding spot centroid position

[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 squares method is used for fitting, and the linear regression model is obtained as d = 0.1874h + 0.7885. The defocus with the smallest variance is determined to be 0.8mm, which is used as the basis for adjustment; h is the distance between the two spot centroids. The measurement principle is as follows: Figure 2 shown.

[0043] Table 2 Pinhole movement relative to zero point movement distance and defocus

[0044]

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

[0046] 5) The focus operation is completed.

[0047] 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 using a single-aperture scanning method, characterized by: 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 uses the center of mass calculation algorithm to obtain the center of mass position of the light spot. Step 2: Move the black screen vertically to collect spot images at different positions of the pinhole. 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 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 achieved by adding or removing spacers at the detector mounting 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 pinhole, and then the spot images are collected respectively. The positions of the center of mass of the spot in the two images are calculated, and the defocus amount can be calculated according to formula (1).

3. The method for simple focusing of 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 pinhole position is moved multiple times within the aperture and the spot images are collected respectively. The defocus amount is calculated according to the above formula (1) and then the least square method is used to fit the defocus amount to obtain the defocus amount with the smallest variance as a reference for focusing the detector.

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