An Experimental Study of Light Scattering by Large, Irregular Particles

Abstract

The intensity and polarization of light scattered by a variety of types of artificial particles large compared to the wavelength were measured as a function of phase angle. Shape, surface roughness, absorption coefficient, and internal scattering coefficient were varied systematically and their effects studied. Scattering by clear, smooth-surfaced spheres is in quantitative agreement with the predictions of the geometrical optics (ray theory) approximation to physical optics (Mie theory). However, almost any change from these ideal characteristics causes major departures from Mie theory. Hence, Mie theory is a poor predictor of the scattering properties of most large particles encountered in nature. An ideal sphere is strongly forward scattering, but irregularities redirect rays from the forward peak into smaller phase angles. This has the effect of decreasing the internal path length followed by an average ray as it traverses an absorbing dielectric particle, which increases the scattering efficiency. Internal scatterers can cause a particle to be back scattering. The phase functions of almost all of the particles measured have both forward and backward scattering lobes. Neither a first-order Legendre polynomial series nor a single Henyey-Greenstein function adequately describes the scattering functions of such particles. A two-parameter, double Henyey-Greenstein function generally provides reasonably good descriptions of the data, while keeping the number of free parameters to the minimum necessary. On a double Henyey-Greenstein parameter plot all of the particles fall into an L-shaped area of restricted size in which the location is characteristic of the particle type. Hence, this type of plot should be useful for estimating certain properties of a particle from its angular scattering function and vice versa. Formalisms based on the equivalent slab model are also given for estimating the scattering efficiency of a large, irregular particle. For most dielectric particles the transmitted, forward scattered light is partially negatively polarized. It is this component, and not the Brewster angle, that is responsible for the well-known maximum in the polarization curves of planetary regoliths at phase angles around 100°. For phase angles between about 30° and 70° the internally scattered light is found to be randomly polarized in the particles studied here, so that the only contribution to the second component of the Stokes vector is by Fresnel reflection from the particle surface. If this empirical result is general, measurement of the second Stokes vector of the light scattered from a regolith at these angles may provide a method of remotely measuring the mean refractive index.