Harvey Schack theory for converging-diverging Gaussian beam

The scattering characteristics of random rough surfaces illuminated with three- dimensional converging-diverging Gaussian beam are investigated by applying the conventional Harvey-Shack theory in conjunction with two-dimensional plane-wave decomposition. The Gaussian beam is assumed to have an arbitrary angle of incidence and to be linearly s-polarized. Using data obtained from laser BRDF measurements on \rerevised{isotropic} random rough  surfaces \revised{with low surface roughness}, we demonstrate that the Gaussian beam Harvey-Shack theory is in better accordance with the experimental data than the conventional Harvey-Shack theory. The two models become identical for a large beam waist  radii, but are significantly different for smaller ones.