Measurement Definition: The directional albedo or directional-hemispherical reflectance (also called black-sky albedo) is the integration of the bi-directional reflectance over the viewing hemisphere. It assumes all energy is coming from a direct radiation from the sun and is computed for a specific time. The hemispherical albedo or bi-hemispherical reflectance (also called white-sky albedo) is the integration of the directional albedo over the illumination hemisphere. It assumes a complete diffuse illumination.