Polarimetry is a powerful observational tool for astrophysics research. It can derive crucial physical and geometric information of astronomical objects, which is unobtainable otherwise. The effect of instrumental polarization of telescopes, which modifies the incident polarization signal before it is analyzed, is the primary technical problem for high-accuracy astronomical polarimetry. The instrument polarization is mainly caused by the oblique reflection of light on the metal coating mirror. To perform high-accurate polarimetry with metal coating mirrors, it is essential to investigate the polarization properties of metal coatings comprehensively based on precise laboratory polarimetry. This paper introduces the optical properties of the metal film, introduces the reflection model of light on the metal surface and the structure of the metal film with a dielectric layer, and gives a corresponding method for solving the complex reflection coefficient. The reflection coefficient ratio is used to link the optical properties of the metal film with the polarization properties. This paper introduces the structure and measurement principle of the Mueller matrix ellipsometer and phase modulation ellipsometer. First, use the effective rank of the system matrix to judge whether the number of measurement data sets is sufficient. Then based on metrics such as condition number and modulation efficiency, an optimized configuration method suitable for the study of polarization characteristics of metal films is given. This method can direct all measurement accuracy to key elements. This paper proposes a polarization properties measurement method for the protected aluminum coating. This method can directly measure the key elements of the Mueller matrix of the protected aluminum coating and standard aluminum coating by two photoelectric modulations at a multiple-angle-of-incidence spectrum. The retrieval results of the method in this paper are consistent with those obtained by traditional methods, and the method in this paper can fully characterize the polarization properties. We find that the polarization properties of the aluminum coating are anisotropic at high angles. When a uniform and isotropic optical model is used to characterize the polarization properties of the aluminum coating, the applicable angle-of-incidence range of the model is limited. And we use the retrieved parameters to predict the polarization properties of the aluminum coating at the angle of incidence below 50°, where the accuracy of the prediction results will be higher than 5e-4.
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