The emitting capability of a body surface is described by the spectral emissivity, ε(λ), a dimensionless value ranging between 0 and 1 and varying on the basis of the wavelength (λ) and the geometric configuration of the surface. Formally, spectral 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).
A blackbody is an ideal radiator that totally absorbs and then reemits all energy incident upon it. By definition the spectral emissivity of a blackbody is equal to one (the maximum) at whatever wavelength and temperature. A blackbody radiates a continuous spectrum. Real materials do not behave like a blackbody. Natural matter could radiates more in selected spectral region (like in the case of atomic or molecular gases) more frequently with a continuous spectrum (like in the case of solids) always with spectral emissivity minor or equal to 1.
Another important concept is the one related to the graybody. For gray bodies, the spectral emissivity value is constant for each wavelength value, as for black bodies, but is always less than 1. Therefore, for any given wavelength the emitted energy of a graybody is a fraction of that of a blackbody. This behavior could be quite important even for limited spectral ranges. For instance the spectral emissivity of the sea in the TIR (Thermal InfraRed) spectral range 8-14 microns (TIR atmospheric window) can be assumed constant (about 0,98) with significant simplifications in the determination of SST (Sea Surface Temperature) from satellite sensors operating in that spectral region.
As said above, the emissivity of the most of the bodies present in nature varies depending on the wavelength. These objects are referred to as selective radiators or as being selectively radiant. This means that some materials may behave as black bodies at certain wavelengths (ε close to 1) and may have reduced emissivity at other wavelengths.