星载多角度偏振成像仪非偏通道全视场偏振效应测量及误差分析

星载多角度偏振成像仪自身的光学系统有一定的偏振效应,会影响非偏通道的辐射测量精度.斜入射到光学元件上的光透射率是偏振敏感的,导致了光学系统的线偏振效应.为精确反演解析出观测目标光的辐射强度,需对成型仪器本身的偏振效应进行准确的测量、定标、校正.通过分析仪器原理和光路结构,详细推导出仪器非偏通道含线偏振效应的辐射测量模型,并根据实际镜头特点合理简化了模型.提出了基于不同偏振角的完全线偏光在仪器全视场内稀疏入射并最小二乘拟合响应值的方法,对非偏通道全视场线偏振效应进行测量和定标,同时对此方法的定标过程进行了仿真.另外,分析了仪器主要物理参数有偏差时对不同偏振态入射光的反演误差,如仪器单像元方位角、显式起偏效应、低频透过率.对仪器开展了实验室定标实验,得到了仪器主要物理参数范围及其拟合偏差量,进一步算出显式起偏效应参数偏差引起的辐射定标强度相对误差最大为0.4%,满足仪器辐射精度5%的要求并留足余量.该研究为仪器非偏通道全视场的高精度辐射测量、定标及后期数据处理提供了理论依据及实验指导.

Full field of view polarization effect measurement and error analysis of non-polarized channels of spaceborne directional polarimetric camera

The optical system of spaceborne directional polarimetric camera that employs an ultra wide-angle lens for its multiangle, a filter wheel for its multispectral and also its multipolarization capability, a CCD itself for its imaging has a certain polarization effect, which can affect the radiometric accuracy of the non-polarized channels of the instrument. The transmittance of the oblique light rays that are incident on the optical element surfaces is sensitive to the orientation of the electric field, contributing to the linear polarization effect of optical system. The precise polarization measurement and calibration of the passive imaging polarimetry are in urgent need to eliminate the instrumental polarization effect and to improve its radiometric accuracy for observation scenes. The non-polarized channel radiometric model considering the linear polarization effect is deduced in detail by analyzing the instrumental principle and optical structures. Moreover, the reasonably simplified model is established based on the actual lens characteristics. A calibration method in which completely linearly polarized light with different kinds of polarization angles irradiates sparsely the instrument within full field of view and subsequently fits the response in the least square sense, is proposed and simulated. In addition, the measured relative errors of the intensity of incident light with different kinds of polarization states caused by the calibration deviations of instrumental principal physical parameters are analyzed and calculated, such as the azimuthal angle of single pixels, explicit optic polarization rate and low frequency spatial relative transmittance. The actual instrumental parameter values and their calibration deviation amounts are acquired by carrying out the laboratory calibration experiment for instrument and combining the least square fitting. Furthermore, the maximum radiometric calibration relative error caused by the deviation of the physical parameter called explicit optic polarization rate is calculated to be 0.4%, fulfilling completely the requirement of radiometric relative accuracy 5% and retaining abundant tolerance. The study provides a theoretical basis and an experimental guidance in high accurately measuring radiation, calibrating and processing data for the instrumental non-polarized channels with full field of view.