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

Field goniometer measurements were obtained to examine the angular variation 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) SpectralonTM exhibit different non-Lambertian properties with respect to direct beam irradiance. The angular variation in the reflectance of diffuse radiation by the panel appears independent of the panel type but varies with the diffuse to global (D : G) irradiance ratio, especially at large solar zenith angles. The combined effect of the angular response to direct beam and diffuse radiation is that panel reflectance of the global flux shows only slight variation with angle for solar zenith angles up to 55 for optical grade Spectralon^TM. For larger solar zenith angles panel reflectance increases markedly with angle.