  The exact reduced dynamics for the independent oscillator model in the RWA
approximation at zero and finite temperatures is derived. It is shown that the
information about the interaction and the environment is encapsulated into
three time dependent coefficients of the master equation, one of which vanishes
in the zero temperature case. In currently used optical cavities all the
information about the field dynamics is contained into {\it two} (or three)
experimentally accesible and physically meaningful real functions of time. From
the phenomenological point of view it suffices then to carefully measure two
({\it three}) adequate observables in order to map the evolution of any initial
condition, as shown with several examples: (generalized) coherent states, Fock
states, Schr\"odinger cat states, and squeezed states.
