Wheat leaf bidirectional reflectance measurements: Description and quantification of the volume, specular and hot-spot scattering features

This study focuses on the directionality of wheat leaf reflectance as a function of leaf surface characteristics. Wheat leaf BRF measurements were completed under 45° zenith illumination angle in three visible broad spectral bands with a conoscope that provides very high angular resolution data over a large portion of the whole hemisphere, including around the illumination direction. The measurements show a clear anisotropy with a specular lobe in the forward scattering direction and a small but significant hotspot feature in the backward scattering direction. The BRF directional features further depend on the illumination orientation because of the leaf roughness created by longitudinal veins: the specular lobe was more pronounced when the illumination was perpendicular to the veins, while specular reflection was more spread over azimuths for longitudinal illumination. Moreover, a sharp hotspot feature was observed for transversal illumination where the apparent roughness is the largest. The scattering was tentatively decomposed into specular, hotspot and isotropic components. Results showed that the hotspot contribution to the directional hemispherical reflectance factor (DHRF) was marginal conversely to that of the specular component that ranges between 0.036 and 0.050 (absolute DHRF value). The specular component was almost the same in the three visible bands considered. The isotropic component originating from volume scattering was contributing the most to the DHRF and was depending on wavelength, ranging between 0.055 and 0.097 in absolute DHRF value. A simple model was proposed to estimate the volume scattering from the isotropic and the surface components. Consequences of these findings were drawn on the ability to estimate leaf biochemical composition independently from leaf surface scattering, as well as on the interpretation of remote sensing at the canopy level.