  Some interactions between classical or quantum fields and matter are known to
be irreversible processes. Here we associate an entropy to the electromagnetic
field from well-known notions of statistical quantum mechanics, in particular
the notion of diagonal entropy. We base our work on the study of spontaneous
emission and light diffusion. We obtain a quantity which allows to quantify
irreversibility for a quantum and classical description of the electromagnetic
field, that we can study and interpret from a thermodynamical point of view.
