Accounting For Diffuse Irradiance On Reference Reflectance Panels

Measurements of surface reflectance factors in the field are usually made under conditions of total (direct and diffuse) irradiance. However, the reference panel reflectance factor R(r/e) used to convert the target measurement to reflectance is frequently determined using only direct irradiance. A method for determining the diffuse-irradiance reflectance factor, 11,1.4(0°,/e), of a calibrated field-reference panel from a knowledge of the direct-irrA.ance reflectance factor, Rd.,(0°,/0), is described. The magnitude of the error involved if only the direct irradiance reflectance factor is known, usually from laboratory calibration, was found to vary from -2% to +5%, depending on zenith angle, atmospheric conditions, and the particular reflectance panel used.