Calibration of polarization errors introduced by folding mirror in imaging spectropolarimeters

Abstract

The folding mirror has been always avoided in Imaging Spectropolarimeters (ISPs) because of its polarization characteristics which can introduce errors to the polarization detecting system. However, it is a useful component in optic systems to fold the light path and save the volume of optic systems, especially in space remote-sensing field. In this paper, we proposed a calibration method based on Mueller matrix calculation to correct the polarization after a folding mirror for an ISP with a wide wavelength range. In the Mueller matrix of the folding mirror, the elements are expressed by reflection coefficients and change along with the incident angle and wavelength. Those two factors are considered in the calibration method. The first one is the incident angle. To a fixed folding mirror the incident angle changes very little because it depends on the little change of the field of view (FOV) angle of the ISPs. As a result, the influence of the change of incident angle is ignored. In our experiment, the reflect angle of the mirror was set to 45°, and the influence of the FOV angle was simulated. The second one is the wavelength, which should be considered because of its wide range. Therefore, in our experiment, a series of polarization before and after the reflecting mirror were acquired and the elements of the Mueller matrix of the folding mirror was calculated by solving the overdetermined equations. A 3D correction matrix was obtained, in which every page is a Mueller matrix for one wavelength. And the polarization of a wide wavelength range can be corrected by multiplying by the inverse matrix of it. The effectiveness of the method was proved by the experimental results. And the method offers a new way to save the volume of the IPSs and makes the optical system design of the IPSs more flexible.