Opposition-effect experimentation

Abstract

Observations of the retroreflective or opposition effect in the backscatter of visible radiation from common dielectric surfaces at small phase or observation angles have been made with a beam-splitting technique at zero phase angle with phase-angle resolution just under 0.2°, the smallest known resolution reported to date. The observations on many surfaces are still obviously instrument-resolution limited. On the many surfaces that were examined with linearly polarized radiation incident upon them, no opposition effect was observed in the linearly polarized backscattered radiation component that was polarized orthogonally to the incident-polarization direction. The opposition effect was found to be present on single surfaces of vegetation, so that the opposition effect observed from vegetation on a gross scale (e.g., the bright spot about the shadow of an airplane at sufficient altitude) may consist of both vegetation structure and single-surface effects.