Effects of Surface Roughness and Albedo on Depolarization in Mueller Matrices

The Mueller matrix dependence on an objects' albedo and surface texture are measured and these effects are used to inform modifications to a microfacet model for polarized light scattering. Four different textures are imparted on red LEGO bricks which are illuminated at 451 nm and 662 nm. These wavelengths yield measurements for both low and high albedo conditions respectively. Analysis of polarizance, depolarization index, polarization entropy, and matrix roots demonstrate that texture and albedo have distinct polarization and depolarization signatures. The root mean square deviations (RMSD) of the unmodified microfacet model from the measurements are about three times greater for high albedo compared to low albedo measurements. The surface texture trend is more subtle, but in all cases the RMSD decreases as roughness increases. A major contribution of this work is an adjustment to the microfacet model so that the polarized term is wavelength dependent. This adjusted model improves the RMSD more for the low albedo compared to the high albedo measurements. To improve model fits to high albedo measurements, a modified depolarization structure is introduced to reduce the RMSD of high albedo measurements by about a factor of two.