The fundamental and intrinsic property governing the complete reflectance behavior of a target. Earth surface irradiance is made up of a direct and diffuse component. This energy again may be absorbed, transmitted, or reflected by a vegetated canopy layer. Energy that passes through this layer may again be absorbed or reflected by the soil background layer. There is both a direct and a diffuse component.
E varies with time of day, sky conditions, clouds, season, etc.
A simpler approach, however, involves the use of a standard reference plate, a surface target of known reflectance properties.