Bidirectional reflectance spectroscopy: 2. Experiments and observations

Certain aspects of the theoretical bidirectional reflectance expression derived by Hapke [this issue] were studied experimentally. The theoretical expression satisfactorily describes the measured angular scattering and spectral properties of a cobalt glass powder whose absorptance and reflectance vary greatly with wavelength. The theoretical expression for the spectral single‐scattering efficiency for irregular particles derived in paper I is shown to describe the measured efficiency of the cobalt glass particles. The predicted approximately linear dependence of the bidirectional espat‐function W on the effective particle absorption optical thickness α D e is shown experimentally to be valid for α D e at least as high as 3. The theoretical reflectance function also describes measured distributions of intensity across the surfaces of planets, except near the sunlit limb, where the macroscopic roughness influences the brightness.