偏振CCD相机的系统级偏振定标方法研究

The polarization properties are used to inverse the characteristic of the observed object in polarization remote sensing. However, the polarization state of the object is alterd due the surfaces of the optical len in the polarization CCD camera, and the variation relate to the field of view, azimuth angle and polarization states of the observed beams. The polarimetric accuracies limit the application of polarization remote sensing due the polarization phenomenon of the instrument.The inversion accuracies of polarization parameters have relations with the accuracy of the polarimetric calibraton which is generally more complex than radiance calibration. The only space-borne polarization CCD camera at the present time is the POLDER instrument, on which there are little references of polarimetric calibraton..The polarization imaging characteristics of the thin light beams will be studied based on the mechanics of polarization ray tracing and the entrance pupil parameters which have relations with the viewing angle of the polarization CCD camera. The polarization-altering factors of the polarization CCD camera will be analyzed; for example, the field of view, azimuth angle and incident polarization states. The system-level polarimetric model of the polarization CCD camera is to give a synthetic description of the physics of the instrument, including the relative and absolute orientation angles of the nonideal instrument polarizers and the spatial variation of the transmission within the field of view mainly due to the inter-pixel sensitivity variation. The method of system-level polarimetric calibration would get rid of the influence of polarization phenomena of the polarization CCD camera, and contribute to fundamental basis for the quantitative application of polarization remote sensing information for the Chinese polarized satellite sensor.

偏振遥感是通过测量来自目标偏振状态变化来反演目标特性的技术。而偏振CCD相机内部的多个光学表面会改变入射光的偏振状态，并且改变的程度与入射光的偏振态、视场角和方位角等有关，影响了仪器偏振测量精度，从而制约了偏振成像遥感技术的应用。偏振定标的精度决定了偏振遥感信息反演的精度，而偏振定标研究刚刚起步，目前唯一的光学在轨偏振相机POLDER，仅有极少数关于偏振定标研究的文献，且定标方法不能直接移植应用。.本项目拟从偏振光线追迹原理出发，根据偏振CCD相机入瞳参数与视场角的函数关系，研究细光束偏振成像的统计特性，探讨入射光束偏振改变状态与视场角、方位角和入射光自身偏振态的关系；分析非理想仪器偏振片透过轴的相对和绝对投影角度，以及CCD探测器像元响应差异对偏振定标的影响，构建考虑光学系统偏振效应的系统级偏振定标模型，消除仪器自身的偏振效应，为将来我国自主星载偏振遥感图像的定量化应用奠定基础。

光学成像系统的菲涅耳损失、以及透镜的非完美加工、装调等，通常会使成像光束的偏振度、方位角和椭率角等参数发生改变，即光学成像系统自身具有一定的偏振效应，并且偏振效应会随着视场角的增大而更加明显，严重影响了仪器的辐射/偏振测量精度，制约了定量化高精度偏振成像遥感技术的发展。系统级偏振定标是对偏振遥感器整机进行偏振定标，定标状态与仪器使用时的状态保持一致，有效保障仪器的偏振测量精度。. 偏振测量的基础是辐射测量，即对目标进行三个偏振方向的辐射强度测量，而偏振CCD相机获取的成像光束辐射强度信息受到光学镜头、偏振片、滤光片以及面阵CCD探测器等物理参数调制。从偏振光线追迹原理出发，明确光学系统偏振效应、各视场透过率差异、非理想偏振片/滤光片等多物理参数耦合的系统级偏振特性参数表达式，成功构建了偏振CCD相机的系统级偏振定标统计模型。在偏振CCD相机的基本性能（包括暗电流、稳定性、线性度以及面阵CCD探测器帧转移曝光效应）测试基础上，利于积分球辐射源、可调偏振度光源以及光谱偏振分析仪等定标设备，对偏振CCD相机的系统级偏振特性参数进行定标试验，获取系统级偏振特性参数的图像数据。并利于已知标准偏振光源验证和优化偏振CCD相机的偏振定标模型参数，实验结果显示：经过标定后的偏振CCD相机在0°、15°、25°视场角下的偏振度测量平均误差不大于1.5%，明显优于偏振CCD相机光学镜头在25°视场角下的自身起偏度2.81%，成功地实现了偏振CCD相机的系统级偏振定标。