Shadow-hiding effect in inhomogeneous layered particulate media

Shadow hiding is studied for particulate media consisting of opaque, spheri cal particles with Lambertian scattering phase functions. The present compu ter modeling allows geometric optics calculations of the photometric charac teristics of layered particulate media with accuracies better than 1%. For statistically homogeneous particulate media, packing density is the single parameter that characterizes the opposition effect due to shadow hiding. Th e opposition effects become sharper with decreasing packing density both fo r monodisperse and polydisperse particulate media. There are no differences between the phase curves of monodisperse and polydisperse media with equal packing densities. A monolayer of hemispherical particles placed randomly on a plane-parallel, Lambertian background medium always produces phase cur ves that are convex upward. A monolayer of spherical particles can give alm ost linear phase curves in the phase angles of 0-40 degrees. In a double-la yer model, an optically thin layer of small particles can sharpen the oppos ition effect, but that depends strongly on the method of conjugation of the two layers. A dependence of the opposition effect on the size of the upper -layer particles really exists (Hillier's effect), but it is rather small a t about 5% according to our calculations. Sparsely distributed large partic les on a plane-parallel dusty surface can also produce sharpening of the ph ase function near opposition. (C) 1999 Elsevier Science Ltd. All rights res erved.