  Using a quantum-kinetic many-body approach, exact results for the interacting
system of field and matter in a specified geometry are presented. It is shown
that both the spectral function of photons and the field fluctuations split up
into vacuum- and medium-induced contributions, for which explicit expressions
are derived. Using Poynting's theorem, the incoherent emission is analyzed and
related to the coherent absorption as may be measured in a linear
transmission-reflection experiment. Their ratio defines the medium-induced
population of the modes of the transverse electromagnetic field and so
generalizes Planck's law to an arbitrarily absorbing and dispersing medium in a
nonequilibrium steady state. For quasi-equilibrium, this population develops
into a Bose distribution whose chemical potential marks the crossover from
absorption to gain and, also, characterizes the degree of excitation.
Macroscopic quantum phenomena such as lasing and quantum condensation are
discussed on this footing.
