ABSTRACT
In the previous chapters, we have discussed radiative exchange between objects by implicitly assuming that they are far from each other. The “closeness” or “proximity” between structures for radiative transfer is a relative term as it should be quantified in relation to the wavelengths of thermal emission. We can therefore express the last statement in a different way by saying that radiant energy exchanges between bodies separated by distances much longer than the dominant emitted wavelength can be described by the radiative transfer equation (RTE) and the Planck blackbody distribution. In that case, transport is assumed to be incoherent as thermal radiation is conceptualized as a particle (photon). This regime is considered as the “far-field regime.” The approximation that radiation is incoherent in the far-field regime is acceptable since the coherence length of a blackbody is of the same order of magnitude as the dominant wavelength of thermal emission [Chen (2005)].
