Particle size effect on the opposition spike and negative polarization

We report results of experiments designed to increase our understanding of the influence of particle size on the photometric opposition spike and negative polarization observed in the reflectance and polarization phase curves of particulate surfaces. We concentrate our studies on particle-size separates of alumina (bright powders) and boron carbide (absorbing powders). We use two photopolarimeters that span small (0.2-17°) and large (2-160°) phase-angle ranges. The results suggest that the negative polarization has two dominant mechanisms: (1) the coherent-backscatter enhancement and (2) single-particle scatter, and that the contributions of the mechanisms are a function of particle size. The measured photometric and polarimetric phase functions can be applied to evaluate models used to calculate scattering properties of particulate surfaces.