  We study thermal radiation outside equilibrium. The situation considered
consists of two bodies emitting photons at two different temperatures. We show
that the system evolves to a stationary state characterized by an energy
current which satisfies a Stefan-Boltzmann-like law expressing it as the
difference of the temperatures to the fourth power of the emitters . The
results obtained show how the classical laws governing the thermal radiation at
equlibrium can be generalized away from equilibrium situations.
