Consider a perfect blackbody at absolute temperature T. We assume its surroundings also to act as a perfect blackbody at absolute temperature T0 , where T0 < T.
The power per unit area radiated from the surface of a blackbody at temperature T is its emissive power Rb at that temperature. According to the Stefan-Boltzmann law,
Rb = σT4
where σ is the Stefan constant. At the same time, the body absorbs radiant energy from the surroundings. The radiant energy absorbed per unit time per unit area by the black-body is σT04.
Therefore, the net rate of loss of radiant energy per unit area by the blackbody is σ(T4 - T04).
[Note : If the body at temperature T has emissivity e < 1, (i.e., it is not a perfect blackbody) the net rate of loss of radiant energy per unit area is eσ(T4 - T04).]
