Polarimetric BRDF in the Microfacet Model: Theory and Measurements

A key to modeling polarimetric signatures of painted objects is the polarimetric bidirectional reflectance distribution function (BRDF). In particular, it is important to correctly capture the out-of-plane behavior of the BRDF. From the theoretical point of view, the BRDF is a complex object – a 4 x 4 matrix of functions, each depending on three angles. Brute force tabulation of such an object is unlikely to ever be a feasible modeling approach. For this reason, model forms of the BRDF, rooted in physics, are very valuable for extending measured data and for use in modeling codes. Only a limited number of such models have been mentioned in the literature. Perhaps the one most relevant to paints, such as CARC (Chemical Agent Resistive Coating) coatings, is the microfacet model. Unpolarized versions of this model have been used in various forms for years. This paper presents a full polarimetric version of this model and discusses its properties. Recently acquired polarimetric data is compared with the model. It is found that the model is successful in capturing qualitative aspects of this data.