  The coherence properties of the classical waves are discussed in terms of the
Cauchy problem for the wave equation, and of a discrete representation by an
ensemble of Hamiltonian systems. Wave quanta are related to specific "action
fields", and phase-space distributions of phonons and photons are obtained by
Wigner transform. For photons in a thermal environment, the proposed Wigner
function evolves towards the Planck equilibrium distribution. It is shown that
the free electromagnetic field can also be found in states of definite
helicity, described by a complex vector potential.
