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精密测角算法直观简洁,易于实现且精度较高,是实验室内标定测绘相机内方位元素采用的一种经典算法,其实质就是根据像点坐标数据和对应的入射光线角度数据,寻找一种主点、主距以及畸变系数的计算方法,使得相机的光学畸变符合一定的约束条件。
基于测角法的内方位元素和畸变测试原理如图1所示。图1中,
$ {H}{{'}} $ 为被测相机物镜的后节点,O为像面中心,P为像面主点位置, f为被测相机主距,$ {S}_{i} $ 为被测点,$ {S}_{i}{{'}} $ 为被测点的理想位置,$ {L}_{i} $ 为$ {S}_{i} $ 距像面中心O点的距离,$ {W}_{i} $ 为对应被测点的偏角,角度$ \Delta W $ 是主点和像面中心偏差所成的角度[11-12]。图 1 一维内方位元素和畸变测量原理
Figure 1. Measurement principle of one dimensional interior orientation element and distortion
采用精密测角法测试时将光学镜头置于精密转台上,镜头焦面处放置模拟靶标,测量时将标定过的网格板刻划面精确地安置在镜头焦面上,并且使其刻线中心与镜头光轴重合。测试时,转动高精度二维转台,获取模拟靶标上不同刻线位置,记录转台角度值,根据约束条件即可求解镜头畸变。常用的约束条件包括平均值法、谢尔兴算法、畸变平方和最小算法等[13],其中全视场畸变平方和最小约束方法较为常用,采用该约束条件时,可得畸变、主距的计算公式如下:
$$ {D}_{i}={L}_{i}-f\;{\tan}\;{\omega }_{i}+p\;{{\tan}}^{2}\;{\omega }_{i} $$ (1) $$ f=\frac{\left(\displaystyle\sum {L}_{i}{\;{\tan}}^{2}{\omega }_{i}\cdot \displaystyle\sum {\;{\tan}}^{3}{\omega }_{i}\right)-\left(\displaystyle\sum {L}_{i}\;{\tan}{\omega }_{i}\cdot \displaystyle\sum {\;{\tan}}^{4}{\omega }_{i}\right)}{\left(\displaystyle\sum {\;{\tan}}^{3}{\omega }_{i}\right)^{2}-\left(\displaystyle\sum {\;{\tan}}^{2}{\omega }_{i}\cdot \displaystyle\sum {\;{\tan}}^{4}{\omega }_{i}\right)} $$ (2) 根据精确标定的网格板不同刻线对应的像高Li和相机旋转的角度Wi,利用上述公式即可求出对应相机的主距以及对应各点的畸变值Di。
精密测角算法的标定精度与转台精度密切相关。国内外各个研究机构进行遥感相机畸变测试的基本原理均基于精密测角法,目前国际主流的测量水平可满足1∶10000比例尺航天测绘相机的研制需求。
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用上述的测试方法对被测相机进行了实际测试。被测相机焦距为8 m,视场角
$ 2\omega \geqslant 2.{2}^{\circ } $ ,主镜通光口径为800 mm。测试时全视场共取21个测试点,进行了三次有效测试,经过数据处理得到线阵方向畸变如表2所示,其中最大相对畸变值为:表 1 两种方法标定结果比对
Table 1. Comparison of calibration results of two methods
Traditional measurement Improvement measurement Image height/μm Distortion/μm Image height/μm Distortion/μm 776 −717.6 808 −723.9 7456 −418.4 7430 −424.6 14624 −299.1 14608 −301.1 21870 −153.2 21860 −161.3 29208 −76.4 29258 −79.9 36180 −53.3 36166 −45.6 43366 −13.5 43390 −9.9 49226 0 49280 0 55988 −21.1 56022 −31.2 63148 −56.2 63146 −54.2 70278 −87.0 70336 −84 77206 −171.8 77216 −180.6 84314 −320.9 84358 −311.2 91284 −426.3 91248 −433.1 101426 −727.5 101470 −731.4 $$ {D_{{\rm{max}}}= \frac{1\;809.2}{255\;889.2-129\;037.3}}=1.43 \text{%}$$ 如图7所示,通过对该相机光学系统模型的畸变网格数据可知,该相机线阵方向最大相对畸变设计值为1.5%。经过对比可以看出,文中提出的测试方法取得的测试结果与理论设计值吻合较好。三次重复测试,各采样点畸变测试的均方根小于3 μm,说明测试过程中随机误差较小,测试精度高。
表 2 三次畸变标定结果
Table 2. Cubic distortion calibration results
The first time The second time The third time RMS DIST/μm Image height/μm Distortion/μm Image height/μm Distortion/μm Image height/μm Distortion/μm 2158.1 1812.8 2154.6 1817.8 2156.8 1817.3 2.8 14887.6 1338.5 14885.5 1335.5 14886.2 1335.3 1.8 27508.6 937.6 27509.3 936.6 27508.9 937.5 0.6 40187.0 643.8 40189.1 644.8 48189.1 643.1 0.9 52900.4 372.3 52903.2 374.3 52901.1 376.2 2.1 65581.6 232.8 65578.1 227.8 65578.8 230.4 2.5 78271.2 85.9 78273.3 90.9 78269.3 89.2 2.5 90940.5 66.4 90937.7 65.4 90946.6 66.7 0.7 103671.4 30.8 103670.0 31.8 103684.7 32.7 1.0 116334.4 19.0 116335.1 14.0 116332.8 19.2 2.9 129037.3 0.0 129038.7 −1.0 129034.5 0 0.6 141724.1 4.3 141725.5 9.3 141724.9 8.3 2.6 154356.3 36.6 154356.3 32.6 154353.5 35.2 2.0 167048.7 68.1 167051.5 64.1 167052.0 65.0 2.1 179721.5 87.9 179721.5 87.9 179721.2 86.5 0.8 192423.0 258.8 192419.5 257.8 192426.3 258.2 0.5 205118.9 419.2 205119.6 416.2 205119.0 419.5 1.8 217805.0 681.9 217807.8 686.9 217819.0 683.9 2.5 230508.6 956.4 230505.8 952.4 230506.5 955.1 2.0 243236.7 1350.7 243233.9 1355.7 243210.1 1354.9 2.7 255889.2 1809.2 255892.0 1806.2 255903.2 1810.8 2.3 -
(1)主点测试误差
主点的测试精度主要取决于靶球的中心位置,靶球中心位置的获取受以下因素影响:干涉仪测试误差,干涉仪的测试精度优于0.1λ,因此可以忽略不计;激光跟踪仪测距精度16 μm;经纬仪自准直测角误差0.5″,测试中两次使用经纬仪,因此测角误差为0.7″。综上所述,主点测试误差为17 μm。
(2)主距测试误差
1) 主距的测量精度与畸变算法精度有关,基于测角法的主距误差计算公式为:
$$ {\delta }_{f}=\sqrt{\left(\frac{1}{\displaystyle\sum {\;{\tan}}^{2}{\omega }_{i}}{\delta }_{L}^{2}\right)^{2}+\frac{\displaystyle\sum {L}_{i}^{2}{{{\rm{sec}}}}^{4}{\omega }_{i}}{(\displaystyle\sum {\;{\tan}}^{2}{\omega }_{i}{)}^{2}}{\delta }_{\omega }^{2}} $$ (3) 式中:
$ {\delta }_{L} $ 为$ {L}_{i} $ 的测试误差,取决于靶标像位移计算精度;$ {\delta }_{\omega } $ 为角度$ {\omega }_{i} $ 的测试误差。2) 靶标像位移靠激光跟踪仪测量获取,激光跟踪仪测距精度为16 μm,即
$ {\delta }_{L} $ 精度为16 μm;3) 测角误差:测角误差包括光电自准直仪测角误差和多齿分度台测角误差。光电自准直仪测角误差为0.02″,多齿分度台经过高精度标定和校准测角误差可以达到0.1″,因此测角误差为0.11″。
综上所述,主距测试时质心计算误差优于16 μm,测角误差为0.11″,间隔0.5°进行测试采样,则相机主距f(8000 mm)的单轮测试误差
$ {\delta }_{f}=494.59 \;\text{μ} {\rm{m}} $ 。(3)畸变测试误差
基于测角法的畸变测试误差
$ {\delta }_{D} $ 主要包括[8-9]:1) 角度测试误差
$ {\delta }_{\omega } $ ;2) 像高测量误差
$ {\delta }_{L} $ ;3) 主点位置测量误差
$ {\delta }_{P} $ ;4) 主距测量误差
$ {\delta }_{f} $ ;5) 测试环境的影响:在测试中,要对温度、震动、气流等环境因素进行控制,尽量选择隔振地基、并且对测试环境严格控温、控气流扰动等,尽量减小或避免环境因素的影响;
6)测量间隔:在等精度条件下,测试间隔即镜头有效视场的测试点数对结果也有影响,测试间隔越小测试精度越高,测试间隔过大会对测试误差有较大影响。
畸变测试误差的表达式为:
$$ {\delta }_{D}^{2} = {\sum \left(\frac{\partial D}{\partial L}\right)}^{2}{\delta }_{L}^{2} + {\sum \left(\frac{\partial D}{\partial \omega }\right)}^{2}{\delta }_{\omega }^{2}+{\sum \left(\frac{\partial D}{\partial f}\right)}^{2}{\delta }_{f}^{2} + {\sum \left(\frac{\partial D}{\partial p}\right)}^{2}{\delta }_{p}^{2} $$ (4) 式中:
${\delta }_{f}=\sqrt{\left(\dfrac{1}{\sum {\;{\tan}}^{2}{\omega }_{i}}{\delta }_{L}^{2}\right)^{2}+\dfrac{\displaystyle\sum {L}_{i}^{2}{{{\rm{sec}}}}^{4}{\omega }_{i}}{({\;{\tan}}^{2}{\omega }_{i}{)}^{2}}{\delta }_{\omega }^{2}}$ ;$\left(\dfrac{\partial D}{\partial \omega }\right)^{2}= (-f{{{\rm{sec}}}}^{2}{\omega }_{i}-2 p{\tan}{\omega }_{i}{{{\rm{sec}}}}^{2}{\omega }_{i}{)}^{2}$ ;$\left(\dfrac{\partial D}{\partial p}\right)^{2}={{\tan}}^{4}{\omega }_{i}$ ;$\left(\dfrac{\partial D}{\partial L}\right)^{2}=1$ ;$\left(\dfrac{\partial D}{\partial f}\right)^{2}={\mathit{{\rm{tan}}}}^{2}{\omega }_{i}$ 。文中方法中高精度测角系统的测角误差
$ {\delta }_{\omega } $ 为±0.1″,像高$ {L}_{i} $ 由激光跟踪仪测试得到,激光跟踪仪测距误差为16 μm,代入公式(4),再根据相机的参数和测试间隔,经过计算可得,该方法对相机畸变的测试精度小于20 μm。
Calibration technique of geometric distortion for space high resolution optical system
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摘要: 遥感相机光学系统畸变系数作为影响相机在轨成像质量的关键因素,其检测精度一直以来都是遥感相机研制过程中的核心环节。传统的测角法主要依靠高精度二维转台,实现了光学系统视场角与像高之间的精准对应,该方法对测试设备和测试环境要求苛刻。随着相机焦距、口径和体积的增大,对于转台设备的尺寸与测量精度也日渐提升,单纯依靠提升测试设备性能无法满足后续各类高性能遥感相机的研制需求,尤其对于垂直装调类超大口径空间高分辨率光学系统,该方法不可行。在传统精密测角法的基础上,提出一种基于干涉原理的空间高分辨率光学系统几何畸变标定技术,相比于传统的精密测角法,该方法在同等测试精度的基础上,具备更广泛的适用性,其不再受限于测试设备的尺寸与精度限制,可同时满足各种类型遥感相机的畸变测试需求。文中详细介绍了该畸变测试方的基本原理、测试方法与误差链路,并对该畸变测试方法进行了应用验证,将结果与传统畸变测试方法进行对照,表明该方法的测试精度满足遥感相机的研制要求且适用范围更广,对航天长焦距大口径遥感相机研制及畸变测试有参考借鉴意义。Abstract:
As an important internal parameter of the camera, the measurement accuracy of distortion directly affects the image processing accuracy and the geometric positioning accuracy of the camera on orbit. The traditional high-precision laboratory calibration method relying on three-axis turntable has strict requirements for test equipment and test environment. With the increase of camera focus, aperture and volume, this method has increasingly high requirements for equipment and site. The idea of achieving high-precision geometric distortion calibration simply by improving the volume and accuracy of equipment is not applicable. On the basis of the traditional precision angle measurement method, this paper proposes a geometric distortion calibration technology of large aperture and long focal length optical system based on the interference principle. Compared with the traditional precision angle measurement method, this method does not require a high-precision experimental turntable, has good robustness and high accuracy. This paper introduces the basic principle, test method and error link of the distortion test method. The test results of this method are compared with the traditional distortion test method, which shows that the test accuracy of this method meets the requirements of remote sensing camera development and has a wider application range. It can be used for reference for the distortion test of aerospace long focal length and large aperture remote sensing camera. Objective As an important internal parameter of the camera, the measurement accuracy of distortion directly affects the image processing accuracy and the geometric positioning accuracy of the camera on orbit. At present, the distortion test methods for optical cameras are generally divided into three categories, the high-resolution spaceflight remote sensing camera is more suitable for the test method based on the precision angle measurement theory. However, the precision angle measurement method has strict requirements for the test equipment and the test environment. With the increase of the focal length, aperture and volume of the spatial high-resolution optical system, higher requirements are also put forward for the size and test accuracy of the turntable. It is difficult to realize in engineering and cannot meet the development requirements of various types of space high-resolution remote sensing cameras. Meanwhile, for the space high-resolution optical systems with ultra-large aperture and ultra-long focal length, in order to reduce the influence of gravity in the process of alignment, the vertical method is usually used for alignment. The visual axis of the lens is always perpendicular to the earth in this case, and it is impossible to use the traditional precision angle measurement method to calibrate the distortion of the optical lens in the laboratory. In order to solve this problem, a geometric distortion calibration technology of large aperture and long focal length optical system based on the interference principle is proposed. Methods The whole test system includes laser tracker, special target ball, 4D interferometer, a measured lens, high-precision angle measuring system and plane reflector (Fig.2, Fig.5). In the measurement space coordinate system, the target ball of the laser tracker is placed at the image point (Fig.3). When the center of the target ball coincides with the focus position of the interferometer, the interference self-collimation fringe can be formed (Fig.4). When the target ball is precisely positioned at the image point, the laser tracker can be used to test the image point coordinates to obtain the image height data. The field angle corresponding to the image height of the lens can be obtained by using the high-precision angle measurement system (Fig.6), and the lens distortion value can be calculated by the image height and its corresponding field angle. Results and Discussions Comparative experiments were carried out on optical lens with a focal length of 2 000 mm and a field angle of 2.8° using the traditional angle measurement method and the distortion measurement method based on the interference principle. The calibration results show that the results of the two test methods are highly consistent, and the maximum relative distortion of the two methods are 1.48% and 1.49% (Tab.1). This shows that the method based on interference principle can meet the development requirements of remote sensing camera. A long focal length and large aperture optical lens is tested with the new method. During the test, a total of 21 test points were taken from the full field of view, and three effective tests were conducted (Tab.1). The root-mean-square distortion of the three tests is less than 3 microns, and the maximum relative distortion value is 1.43%. The maximum relative distortion design value of the lens linear array direction is 1.5% (Fig.7), and the test results are in good agreement with the theoretical design value. Conclusions Based on the traditional angle measurement method, a distortion measurement method for aerospace large aperture and long focal length optical system is proposed. This method can meet the distortion test requirements of various types of optical systems. It is used to calibrate the distortion of the traditional optical lens and the large-aperture high-resolution optical lens with vertical adjustment in the laboratory. The results are consistent with the design values, which provides a reference for the development and test of space high-resolution optical remote sensor. -
表 1 两种方法标定结果比对
Table 1. Comparison of calibration results of two methods
Traditional measurement Improvement measurement Image height/μm Distortion/μm Image height/μm Distortion/μm 776 −717.6 808 −723.9 7456 −418.4 7430 −424.6 14624 −299.1 14608 −301.1 21870 −153.2 21860 −161.3 29208 −76.4 29258 −79.9 36180 −53.3 36166 −45.6 43366 −13.5 43390 −9.9 49226 0 49280 0 55988 −21.1 56022 −31.2 63148 −56.2 63146 −54.2 70278 −87.0 70336 −84 77206 −171.8 77216 −180.6 84314 −320.9 84358 −311.2 91284 −426.3 91248 −433.1 101426 −727.5 101470 −731.4 表 2 三次畸变标定结果
Table 2. Cubic distortion calibration results
The first time The second time The third time RMS DIST/μm Image height/μm Distortion/μm Image height/μm Distortion/μm Image height/μm Distortion/μm 2158.1 1812.8 2154.6 1817.8 2156.8 1817.3 2.8 14887.6 1338.5 14885.5 1335.5 14886.2 1335.3 1.8 27508.6 937.6 27509.3 936.6 27508.9 937.5 0.6 40187.0 643.8 40189.1 644.8 48189.1 643.1 0.9 52900.4 372.3 52903.2 374.3 52901.1 376.2 2.1 65581.6 232.8 65578.1 227.8 65578.8 230.4 2.5 78271.2 85.9 78273.3 90.9 78269.3 89.2 2.5 90940.5 66.4 90937.7 65.4 90946.6 66.7 0.7 103671.4 30.8 103670.0 31.8 103684.7 32.7 1.0 116334.4 19.0 116335.1 14.0 116332.8 19.2 2.9 129037.3 0.0 129038.7 −1.0 129034.5 0 0.6 141724.1 4.3 141725.5 9.3 141724.9 8.3 2.6 154356.3 36.6 154356.3 32.6 154353.5 35.2 2.0 167048.7 68.1 167051.5 64.1 167052.0 65.0 2.1 179721.5 87.9 179721.5 87.9 179721.2 86.5 0.8 192423.0 258.8 192419.5 257.8 192426.3 258.2 0.5 205118.9 419.2 205119.6 416.2 205119.0 419.5 1.8 217805.0 681.9 217807.8 686.9 217819.0 683.9 2.5 230508.6 956.4 230505.8 952.4 230506.5 955.1 2.0 243236.7 1350.7 243233.9 1355.7 243210.1 1354.9 2.7 255889.2 1809.2 255892.0 1806.2 255903.2 1810.8 2.3 -
[1] Wang Qiaoping, Qi Wenwen, Tan Wei. Maneuvering imaging quality of space TDI push scan optical remote sensor [J]. Infrared and Laser Engineering, 2022, 51(10): 20220094. (in Chinese) [2] Qin Zichang, Ren Chengming, Qi Yunsheng, et al. Low error-sensitive design of small-sized high-resolution space camera optical system [J]. Infrared and Laser Engineering, 2022, 51(10): 20220365. (in Chinese) [3] Jiang Xiwen, Zhao Jinyu, Lu Tianyu, et al. Design and alignment of large-aperture prime focus optical system [J]. Optics and Precision Engineering, 2022, 30(23): 2987-2994. (in Chinese) doi: 10.37188/OPE.20223023.2987 [4] Lu Bo, Feng Rui, Kou Wei, et al. Optical system design and stray light suppression of catadioptric space camera [J]. Chinese Optics, 2020, 13(4): 822-831. (in Chinese) doi: 10.37188/CO.2019-0036 [5] Fan Wenqiang, Wang Zhichen, Chen Baogang, et al. Review of the active control technology of large aperture ground telescopes with segmented mirrors [J]. Chinese Optics, 2020, 13(6): 1194-1208. (in Chinese) doi: 10.37188/CO.2020-0032 [6] Li Chongyang, Zhang Zhifei, Lv Chong, et al. System integration and test of GF-7 bi-linear array stereo mapping sensing camera [J]. Infrared and Laser Engineering, 2021, 50(1): 20200143. (in Chinese) doi: 10.3788/IRLA20200143 [7] Zhang Jiyou, Wang Dongjie, Ma Lina. The self-calibration technology of camera intrinsic parameters calibration methods [J]. Imaging Science and Photochemistry, 2016, 34(1): 15-22. (in Chinese) [8] Cheng Qiang, Hu Haixiang, Li Longxiang, et al. Distortion analysis and focal length testing of off-axis optical system [J]. Optics and Precision Engineering, 2022, 30(22): 2839-2846. (in Chinese) doi: 10.37188/OPE.20223022.2839 [9] Li Chongyang, Dong Xin, Yue Liqing, et al. Testing method of distortion for space remote sensing camera with large field of view [J]. Infrared and Laser Engineering, 2018, 47(11): 0117003. (in Chinese) [10] Yang Shourui, Duan Wanying, Ai Wenyu, et al. Light field camera modeling and distortion correction improvement method [J]. Infrared and Laser Engineering, 2023, 52(1): 20220326. (in Chinese) doi: 10.3788/IRLA20220326 [11] Fu Ruimin, Zhang Yuanming, Zhang Jiyou. Study on geometric measurement methods for line-array stereo mapping camera [J]. Spacecraft Recovery & Remote Sensing, 2011, 32(6): 62-67. (in Chinese) [12] Liu Weiyi, Jia Jiqiang, Ding Yalin, et al. Measurement error impact on intrinsic parameters calibration in precise angle measurement method [J]. Infrared and Laser Engineering, 2009, 38(4): 705-706. (in Chinese) [13] Zhang Jiyou. Simulation of geometric measurement method for stereo mapping camera. [J]. Spacecraft Recovery & Remote Sensing, 2012, 33(3): 48-53. (in Chinese)