radiation is volumetric phenomenon, and solids, liquids, and gases emit, absorb,or transfer radiation to varying degrees. however,radiation is usually considered to be a surface phenomenon for solids that opaque to thermal radiation such material,wood,and rocks since the radiation emitted by the interior regions of such material can never reach the surface, and that radiation incident on bodie usually absorbed within a few microns from the surface.
the maximum rate radiation that can be emited from a surface at a thermodynamic temperature T ( in K or R) is given by the stefan-boltzmann law as