An in-depth study of the thermodynamics of nonequilibrium radiation is presented, The characterization of its nonequilibrium. macroscopic state is performed in the framework of Informational Statistical Thermodynamics. This is done in terms of a nonequilibrium Dirac-Landau-Wigner single particle density matrix, or, alternatively, in terms of the conjugated intensive nonequilibrium. thermodynamic variables. When a local description is not required, the global one can be done by giving the nonequilibrium populations in die different modes. Also, alternatively, we can introduce a nonequilibrium temperature (quasi-temperature) per mode, This is compared to a couple of contracted descriptions. The evolution of the resulting nonequilibrium thermodynamic state and the eventual experimental determination are given in the follow-up article. (C) 2001 Elsevier Science B.V. All rights reserved.30041732386402