A Laboratory Geometric Distortion Calibration Method for Large-Aperture Cameras

Abstract

This invention discloses a laboratory geometric distortion calibration method for large-aperture cameras, belonging to the field of aerospace optical remote sensor technology. The method first establishes a demonstration and verification environment for precise calibration of camera geometric distortion, with the target sphere of the laser tracker located at the infinity focal plane of the camera's main lens. The planar reflector is rotated multiple times within the full field of view of the camera's main lens. After each rotation, the target sphere is positioned at the infinity focal plane of the camera's main lens, and the current rotation angle of the planar reflector and the position of the target sphere are tested. Based on each rotation angle of the planar reflector and the position of the target sphere, the principal point, principal distance, and distortion of the camera's main lens are calculated. Through the application of this invention, high-precision calibration of the geometric distortion of large-aperture cameras is achieved.

Description

Technical Field

[0001] This invention relates to a laboratory geometric distortion calibration method for large-aperture cameras, belonging to the field of aerospace optical remote sensor technology. Background Technology

[0002] Earth observation satellites are entrusted with the important mission of "understanding the Earth, accurately measuring targets, and providing real-time navigation." However, "seeing clearly" does not mean "being able to pinpoint accurately." Due to the complexity of the observation environment and the limitations of technology, the quality of current satellite data, especially geometric quality, often fails to meet the accuracy requirements of mapping in practical applications, which greatly restricts its practical application and service capabilities. Specifically, this manifests as: (1) large-aperture optical imaging systems have large lens distortion; (2) laboratory geometric distortion calibration methods have stringent requirements for testing equipment and environment, and the testing accuracy is difficult to meet mapping needs; (3) there is a lack of high-precision on-orbit geometric calibration methods. Existing camera distortion calibration methods using camera rotation are not suitable for large-aperture camera lenses, resulting in unstable imaging spots after camera rotation. Summary of the Invention

[0003] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose a laboratory geometric distortion calibration method for large-aperture cameras, so as to achieve high-precision calibration of camera geometric distortion and solve the problem that the ground calibration of large optical cameras relies too much on turntables and is difficult to implement in engineering.

[0004] The technical solution of this invention is:

[0005] A method for calibrating the geometric distortion of a large-aperture camera in the laboratory, comprising:

[0006] A demonstration and verification environment for precise calibration of camera geometric distortion was built based on a 4D interferometer, a camera main lens, a plane mirror, and a laser tracker. The target sphere of the laser tracker was located at the infinity focal plane of the camera main lens.

[0007] The current position of the test target ball is the initial principal point position for the distortion test; the current angle of the test plane mirror is the initial angle position for the distortion test.

[0008] The plane mirror is rotated multiple times within the full field of view of the camera's main lens. After each rotation, the interferometer and target ball are adjusted so that the target ball is located at the infinity focal plane of the camera's main lens. The current rotation angle of the plane mirror and the position of the target ball are then tested.

[0009] Based on the obtained rotation angle of the plane mirror and the position of the target ball for each time, calculate the rotation of the plane mirror relative to the initial position of the angle and the displacement deviation of the target ball relative to the initial principal point position for each time, and solve the principal point, principal distance and distortion of the camera's main lens.

[0010] Preferably, a demonstration and verification environment for precise calibration of camera geometric distortion is built based on a 4D interferometer, camera main lens, plane mirror, and laser tracker, including:

[0011] The main lens of the camera is placed on a three-dimensional adjustment stage, and the plane mirror is placed on a two-dimensional turntable. The center heights of the camera optical axis and the plane mirror are the same, and the camera optical axis is parallel to the normal direction of the plane mirror.

[0012] A 4D interferometer is set up, and the laser emitted by the 4D interferometer returns to the interference fringes through the main lens of the camera and a plane mirror, with zero power.

[0013] A three-dimensional adjustment stage is set up between the 4D interferometer and the main lens of the camera. A laser tracker is set up on the three-dimensional adjustment stage, and the target ball of the laser tracker is located at the infinity focal plane of the main lens of the camera.

[0014] Preferably, the method for determining the position of the target ball at the infinity focal plane of the camera's main lens is as follows:

[0015] If the laser emitted by the interferometer returns to the interference fringes after passing through the target sphere and the power is zero, the current position of the target sphere is the position of the infinity focal plane of the camera's main lens.

[0016] Preferably, a 4D interferometer is installed, and the laser emitted by the interferometer returns to the interference fringes through the camera lens and a plane mirror, resulting in zero power, including:

[0017] Adjust the angle of the 4D interferometer so that the laser emitted by the interferometer enters the camera lens through the camera's infinity focal plane, is emitted through the camera lens to the plane mirror, and is then reflected back into the interferometer in the opposite direction by the plane mirror.

[0018] Adjust the position of the interferometer so that the laser emitted by the interferometer returns to the interference fringes through the camera lens and the plane mirror, and the power is zero.

[0019] Preferably, the horizontal angle measurement accuracy and the pitch angle measurement accuracy of the two-dimensional turntable are both better than 2″.

[0020] Preferably, the camera's main lens optical path is adjusted by using a theodolite to aim at the angle, so that the camera's optical axis is parallel to the normal direction of the plane mirror.

[0021] Preferably, after each rotation angle, the 4D interferometer and the target sphere are adjusted so that the target sphere is positioned at the infinity focal plane of the camera's main lens. The current rotation angle of the plane mirror and the position offset of the target sphere are then tested, including:

[0022] Remove the target ball;

[0023] Adjust the position of the 4D interferometer so that the laser emitted by the 4D interferometer returns to the interference fringes through the camera lens and the plane mirror, and the power is zero.

[0024] Place the 3D adjustment platform and target ball again, with the target ball located at the infinity focal plane of the main lens of the camera, and test the position of the target ball;

[0025] Test the angle of the plane mirror.

[0026] Preferably, the angle of the plane mirror is tested using the following method:

[0027] A multi-tooth indexing stage and an opto-autocollimator are placed between the plane mirror and the camera body. The opto-autocollimator is in an autocollimated state with the plane mirror. The multi-tooth indexing stage is rotated in the opposite direction to the plane mirror and at the same angle. The rotation angle of the plane mirror is tested using the opto-autocollimator.

[0028] Preferably, the diameter of the camera's main lens and the diameter of the plane mirror are both over 1 meter.

[0029] The advantages of this invention compared to the prior art are:

[0030] (1) This invention breaks through the laboratory geometric distortion calibration technology of ultra-large aperture camera based on the principle of interference tracking, constructs a demonstration and verification environment for precise calibration of optical camera geometric distortion, realizes high-precision calibration of camera geometric distortion and perception and correction of on-orbit changes, solves the problem that the ground calibration of large optical cameras relies too much on turntables and is difficult to implement in engineering, and fully explores the mapping application capabilities of current on-orbit optical cameras.

[0031] (2) Based on the original research on the internal orientation element testing method using a three-dimensional turntable rotating camera, this invention establishes an internal orientation element testing method that does not require rotating the camera but only rotating the plane mirror, which solves the problem of unstable imaging spot after the large aperture camera is rotated and greatly improves the testing accuracy of internal orientation camera elements of large aperture cameras.

[0032] (3) The present invention uses a combination of laser tracker and 4D interferometer to accurately calibrate the camera's infinity focal plane. The infinity focal plane determination accuracy is better than 1 micrometer by using interferometric measurement, which greatly improves the accuracy of the camera's infinity focal plane determination.

[0033] (4) The present invention utilizes the laser emitted by the interferometer to emit parallel light through the camera lens as the light source for testing the orientation elements inside the camera lens, and then reflects it back to the camera lens through a plane mirror for testing, thereby realizing the self-calibration of the camera and solving the problem that the light source of a large-aperture camera is too small to test the full aperture. Attached Figure Description

[0034] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0035] Figure 1 This is a flowchart of a laboratory geometric distortion calibration method for a large-aperture camera according to an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of the orientation element test within the camera lens according to an embodiment of the present invention. Detailed Implementation

[0037] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0038] A laboratory geometric distortion calibration method for large-aperture cameras is proposed. The method uses parallel light emitted by the camera lens itself to perform self-calibration of the camera lens, realizing full-aperture testing of large-aperture cameras with an aperture of more than 1 meter. The method uses a laser tracker to test the position of the target ball in real time, achieving high-precision positioning of the camera at infinity.

[0039] like Figure 1 As shown, the method includes:

[0040] (1) The main lens of the camera is placed on a three-dimensional adjustment stage, and the large-aperture plane mirror is placed on a high-precision two-dimensional turntable. The horizontal angle measurement accuracy and the pitch angle measurement accuracy of the high-precision two-dimensional turntable are both better than 2″.

[0041] (2) Adjust the three-dimensional adjustment stage and the two-dimensional turntable so that the center height of the camera optical axis and the large-aperture plane mirror are the same;

[0042] (3) Use the theodolite to aim at the angle and adjust the camera lens optical path so that the camera optical axis is parallel to the normal direction of the large-aperture plane mirror;

[0043] (4) Place a 720-tooth multi-tooth indexing stage and an opto-autocollimator between the plane mirror and the camera body, and adjust the position and angle of the opto-autocollimator so that the opto-autocollimator and the plane mirror are in an autocollimated state.

[0044] (5) Set up a 4D interferometer near the focal plane of the camera to be tested, adjust the angle of the interferometer so that the laser emitted by the interferometer enters the camera lens through the infinity focal plane of the camera, is emitted through the camera lens to the plane mirror, and then is reflected back to the 4D interferometer in the opposite direction by the plane mirror. Adjust the position of the 4D interferometer so that the laser emitted by the 4D interferometer returns to the interference fringe with zero power through the camera lens and the plane mirror. At this time, determine the position of the 4D interferometer during the test.

[0045] (6) Set up a three-dimensional adjustment stage between the 4D interferometer and the lens, and place the target ball of the laser tracker on the three-dimensional adjustment stage. Adjust the position of the target ball, such as... Figure 2 As shown, the laser emitted by the 4D interferometer returns to the interference fringes after passing through the target sphere and the power is zero. At this time, the position of the target sphere is the position of the infinity focal plane of the camera lens.

[0046] (7) Use a laser tracker to test the position of the target ball, which is the initial principal point position for the distortion test;

[0047] (8) Use an opto-autocollimator to test the angle of the plane mirror at this time, which is the initial position of the distortion test angle;

[0048] (9) Remove the three-dimensional adjustment stage and target ball set up between the camera lens and the 4D interferometer;

[0049] (10) Rotate the angle of the plane mirror according to the test requirements and adjust the position of the 4D interferometer so that the laser emitted by the 4D interferometer returns to the interference fringes through the camera lens and the plane mirror with zero power.

[0050] (11) Adjust the position of the target ball according to step (6) so that the laser emitted by the 4D interferometer returns to the interference fringe with zero power after passing through the target ball. At this time, the target ball is located at the infinity focal plane of the camera lens.

[0051] (12) Test the position of the target ball using a laser tracker according to step (7);

[0052] (13) Rotate the multi-tooth indexing table in the opposite direction to the rotation direction of the plane mirror, and use the photoelectric autocollimator to measure the rotation angle of the plane mirror with high precision according to step (8).

[0053] (14) Repeat steps (9) to (13) to rotate the plane mirror within the full field of view of the camera lens and test the rotation angle of the plane mirror and the position of the target ball at different rotation angles; the rotation angle of the plane mirror is determined according to the performance parameters of the camera lens.

[0054] (15) Using the rotation angle of the plane mirror and the position of the target ball, calculate the rotation of the plane mirror relative to the initial angular position and the displacement deviation of the target ball relative to the initial principal point position for each time, and solve for the principal point, principal distance and distortion of the camera lens. Among them, the distortion value is the camera geometric distortion calibration value, and the principal point value and principal distance value are the camera's interior orientation element values.

[0055] The distortion calibration method for large-aperture cameras proposed in this invention was used for the first time during distortion testing of a certain type of camera in my country. The test used a 1m diameter plane mirror, a laser tracker and its target sphere, a theodolite, and a 4D interferometer. Tests showed that after applying this invention, the parallel light during camera lens distortion testing was consistent with the camera aperture, achieving full-aperture testing. Different fields of view of the camera were tested by rotating the plane mirror, and the target sphere position was stable using the laser tracker, resulting in high testing accuracy and providing effective assurance for high-precision distortion testing of large-aperture cameras.

[0056] This invention can be applied to various fields such as interior orientation element testing of high-precision stereo mapping cameras and accurate positioning testing of camera focal planes, greatly improving testing accuracy.

[0057] The embodiments described above are merely preferred embodiments of the present invention. Ordinary variations and substitutions made by those skilled in the art within the scope of the technical solution of the present invention should be included within the protection scope of the present invention.

Claims

1. A large-aperture camera laboratory geometric distortion calibration method, characterized in that, The application relates to a camera geometric distortion accurate calibration demonstration verification environment based on a 4D interferometer, a camera main lens, a plane mirror and a laser tracker, wherein the target ball of the laser tracker is located at an infinite focus surface of the camera main lens. The current position of the target ball is the initial principal point position of the distortion test; and the current angle of the plane mirror is the initial angle position of the distortion test. The angle of the plane mirror is rotated multiple times within the full field of view of the camera main lens; after each rotation, the current rotation angle of the plane mirror and the position of the target ball are tested by adjusting the interferometer and the target ball so that the target ball is located at the infinite focus surface of the camera main lens. According to the obtained rotation angle of the plane mirror and the position of the target ball at each time, the rotation amount of the plane mirror relative to the initial angle position and the displacement deviation amount of the target ball relative to the initial principal point position are calculated, and the principal point, the principal distance and the distortion of the camera main lens are solved. The camera geometric distortion accurate calibration demonstration verification environment based on the 4D interferometer, the camera main lens, the plane mirror and the laser tracker comprises the following steps: the camera main lens is placed on a three-dimensional adjustment table, the plane mirror is placed on a two-dimensional rotary table, the height of the camera optical axis is the same as that of the center of the plane mirror, and the camera optical axis is parallel to the normal direction of the plane mirror; the 4D interferometer is erected, the laser emitted by the 4D interferometer returns to the interference fringes with a power of zero through the camera main lens and the plane mirror; a three-dimensional adjustment table is erected between the 4D interferometer and the camera main lens, and a laser tracker is erected on the three-dimensional adjustment table, wherein the target ball of the laser tracker is located at the position of the infinite focus surface of the camera main lens. After each rotation, the current rotation angle of the plane mirror and the position deviation of the target ball are tested by adjusting the 4D interferometer and the target ball so that the target ball is located at the infinite focus surface of the camera main lens, which comprises the following steps: the target ball is removed; the position of the 4D interferometer is adjusted so that the laser emitted by the 4D interferometer returns to the interference fringes with a power of zero through the camera main lens and the plane mirror; the three-dimensional adjustment table and the target ball are placed again, the target ball is located at the infinite focus surface of the camera main lens, and the position of the target ball is tested; and the angle of the plane mirror is tested. The method for determining the position of the target ball at the infinite focus surface of the camera main lens comprises the following steps:

2. The method of claim 1, wherein, If the laser emitted by the interferometer returns to the interference fringes with a power of zero after passing through the target ball, the current position of the target ball is the position of the infinite focus surface of the camera main lens. The 4D interferometer is erected, and the laser emitted by the interferometer returns to the interference fringes with a power of zero through the camera main lens and the plane mirror, which comprises the following steps:

3. The method of claim 1, wherein, The angle of the 4D interferometer is adjusted so that the laser emitted by the interferometer is incident on the camera lens through the infinite focus surface of the camera, is emitted to the plane mirror through the camera lens, and is reflected back to the interferometer through the plane mirror in the extension direction; The position of the interferometer is adjusted so that the laser emitted by the interferometer returns to the interference fringes with a power of zero through the camera lens and the plane mirror. The horizontal angle measurement accuracy and the pitch angle measurement accuracy of the two-dimensional rotary table are both better than 2".

4. The method of claim 1, wherein, The camera main lens optical path is adjusted by using the theodolite aiming angle so that the camera optical axis is parallel to the normal direction of the plane mirror.

5. The method of claim 1, wherein, ​ 6. The method of claim 1, wherein, The angle of the plane mirror is tested by the following method: A multi-tooth indexing table and a photoelectric autocollimator are arranged between the plane mirror and the camera body, the photoelectric autocollimator is in an autocollimation state with the plane mirror, the multi-tooth indexing table is rotated in the opposite direction of the rotation direction of the plane mirror at the same angle, and the rotation angle of the plane mirror is tested by using the photoelectric autocollimator.

7. The method for calibrating geometric distortion of a large-format camera laboratory according to claim 1, wherein, The camera body lens aperture and the plane mirror aperture are both more than 1 meter.

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