  We present an alternative method for describing the evolution of a mode of
the quantized electromagnetic field in contact with a finite temperature
thermal bath. We use the expansion of the field density operator in terms of
coherent states and the related Glauber-Sudarshan $P$-function in phase space.
The method allows us to obtain analytical expressions of the system's
time-evolved $P$-function without needing to solve the corresponding master
equation.
