  In this paper, using the viewpoint that quantum mechanics can be constructed
as a classical field theory without any quantization I build a fully classical
theory of thermal radiation. Planck's law for the spectral energy density of
thermal radiation and the Einstein A-coefficient for spontaneous emission are
derived in the framework of classical field theory without using the concept of
"photon". It is shown that the spectral energy density of thermal radiation is
apparently not a universal function of frequency, as follows from the Planck's
law, but depends weakly on the nature of atoms, while Planck's law is valid
only as an approximation in the limit of weak excitation of atoms.
