Reference panel anisotropy and diffuse radiation - some implications for field spectroscopy

Field goniometer measurements were obtained to examine the angular variatio n in the reflectance of direct beam, diffuse and global radiation from two types of Spectralon(TM) panel. The results indicate that optical grade (99% reflective) and grey (20% reflective) Spectralon(TM) exhibit different non -Lambertian properties with respect to direct beam irradiance. The angular variation in the reflectance of diffuse radiation by the panel appears inde pendent of the panel type but varies with the diffuse to global (D:G) irrad iance ratio, especially at large solar zenith angles. The combined effect o f the angular response to direct beam and diffuse radiation is that panel r eflectance of the global flux shows only slight variation with angle for so lar zenith angles up to 55 degrees for optical grade Spectralon(TM). For la rger solar zenith angles panel reflectance increases markedly with angle.