Estimating the Complex Index of Refraction and View Angle of an Object using Multiple Polarization Measurements

A passive polarization based imaging system records the polarization state of light reflected by objects that are illuminated with an unpolarized and generally uncontrolled source. Such systems can be useful in many remote sensing applications including target detection, object segmentation and material classification. In this paper we present a method to jointly estimate the complex index of refraction and the view angle of a target from multiple measurements collected by a passive polarimeter. This generalizes our previous work which was applicable only to dielectric targets. An expression for the degree of polarization is derived from the microfacet polarimetric bidirectional reflectance model for the case of scattering in the place of incidence. Using this expression, we develop nonlinear least squares estimation algorithms for extracting the complex index of refraction and view angle from multiple polarization measurements. The effectiveness of the proposed method is validated with data collected in laboratory conditions. Experimental results indicate that the proposed method is effective for recovering the parameters of interest for real world data and that the complex index of refraction thus computed provides a feature vector that is robust to the view angle.