Research On Laboratory Calibration Methods Of The Directional Polarimetric Camera

The directional polarimetric camera(DPC)is a space-borne polarimetric sensor with the POLDER like design,which has the characteristics of ultra-wide angle and low-distortion imaging.It can simultaneously measure atmospheric radiation with spectral,angular and polarimetric to characterize the characteristics of atmospheric aerosols,theoretically maximize the sensitivity of its observation on aerosol characteristics.To ensure the accuracy of data measured by DPC and the reliability of inversion results,laboratory calibration is an important part.For large field polarimetric imagers such as DPC,the contents of laboratory calibration include the geometric calibration,the radiometric calibration,and the polarimetric calibration.Although stray light correction does not belong to the scope of calibration,in order to improve the quality of DPC measurement data,it is necessary to carry out stray light correction research in the laboratory calibration link.In addition,it is necessary to verify the performance of DPC after calibration,especially the polarization measurement accuracy.This paper studies the four main components of DPC laboratory calibration.In terms of geometric calibration,the geometric model of DPC is rebuilt and a geometric calibration method based on two-dimensional turntable and Euler transform is proposed,which eliminates the calibration error caused by the deviation of angle information obtained by the original method.Experimental results show that the maximum residual error of the proposed method is only 2×10-1 pixel,which is better than that of the original method of 3.5×10-1 pixel.In terms of radiometric polarimetric calibration,the principle,equipment and specific calibration process of DPC radiometric calibration and polarimetric calibration are introduced systematically.As for polarimetric calibration,the principle and method of radiation calibration and polarization calibration about DPC laboratory,the equipment and the specific calibration process are systematically introduced.The radiometric calibration of DPC is carried out by a large aperture integrating sphere,a spectral radiometer and a two-dimensional turntable.The absolute radiometric calibration coefficient and the relative response coefficient of the whole field of view are obtained.The calibration methods of the diattenuation of the optics,relative transmission of polarized channel and absolute azimuth angle are improved and upgraded.The calibration results show that the measuring accuracy of the diattenuation of the optics,relative transmission of polarized channel and absolute azimuth angle obtained by the improved method improved from ±1%,± 1.5%and ±0.1° to ±0.4%,±0.4%and ±0.05°,respectively.In the aspect of stray light correction,the stray light characteristics of DPC are systematically analyzed.For the first kind of stray light,a method of oblique edge method combined with iterative optimization of mathematical model is designed to construct the stray light diffusion function corresponding to each field of view of DPC,and a deconvolution method based on the stray light diffusion function of multiple fields of view is proposed.For the second kind of stray light,the matrix method applied to linear CCD is improved to the matrix method applicable to planar CCD,and the acquisition process of stray light matrix is optimized.The experimental results show that the stray light in the selected area is reduced by at least 93%.In the aspect of polarization measurement accuracy analysis and verification,the factors causing DPC polarization measurement error are divided into instrument measurement error including noise and stray light and polarimetric parameters calibration error.In the laboratory environment,two verification experiments based on a non-polarized radiation source and a polarizing system were carried out in the laboratory respectively.The experimental results show that the DoLPs obtained by using the modified parameters inversion are closer to the set value of the polarized light source.This proves that the parameters obtained by the improved polarization calibration methods are closer to the real state of the instrument.The maximum polarization measurement deviations of three polarized band of DPC are 1.1 ×10-2,8.3×10-3 and 9×10-3 respectively,and the maximum polarization measurement deviations of three polarized band of DPC for partially polarized light are 3.99×10-3,3.51×10-3 and 4.97×10-3 respectively.