The emitting ability of a body surface is described by emissivity, ε(λ). This will vary with wavelength and viewing angle. A body is considered to be an ideal radiator when it totally absorbs and then reemits all energy incident upon it. Such a body is called black body and its emissivity is equal to one. Emissivity can be defined as the ratio of spectral exitance, M(λ,T), from an object at wavelength λ and temperature T, to that from a blackbody at the same wavelength and temperature, MBB(λ,T). The concept of graybody has also been introduced as the body having an emissivity of less than 1 and constant at all wavelengths.