  Non-equilibrium radiation is addressed theoretically by means of a stochastic
lattice-gas model. We consider a resonating transmission line composed of a
chain of radiation resonators, each at a local equilibrium, whose boundaries
are in thermal contact with two blackbody reservoirs at different temperatures.
In the long chain limit, the stationary state of the non-equilibrium radiation
is obtained in a closed form. The corresponding spectral energy density departs
from the Planck expression, yet it obeys a useful scaling form. A macroscopic
fluctuating hydrodynamic limit is obtained leading to a Langevin equation whose
transport parameters are calculated. In this macroscopic limit, we identify a
local temperature which characterises the spectral energy density. The
generality of our approach is discussed and applications for the interaction of
non-equilibrium radiation with matter are suggested.
