Retroreflection by a rough surface is defined as a brightness increase of the surface as the source-surface- observer angle is decreased to exactly zero degrees. The retroreflectance peaking has been acknowledged to be caused in two ways: (1) geometrically, by interparticulate shadowing, or (2) by physical interference caused be weak field localization of temporally and spatially coherent incident radiation scattered by surface edges or asperities. The distinction between the two is suggested as occurring when the correlation widths of the surface edges and asperities are on the order of or less than the coherence length of the incident wavelength. Anomalous unexplained repeatable polarized multiple retro-peaking occurs for many surfaces, adjacent to the main retroreflection peak. To test the applicability of the two theories of retroreflectance, direct and cross polarization data are presented on 18 substances in various configurations at high and low spatial resolution using a HeNe laser at 0.6328 micrometers wavelength. Depolarization of the incident coherent radiation is a significant factor to characterize the retroreflection phenomenology.
Walter G. Egan,
"Polarized laser retroreflectance", Proc. SPIE 3121, Polarization: Measurement, Analysis, and Remote Sensing, (3 October 1997); doi: 10.1117/12.278962; https://doi.org/10.1117/12.278962