Universidade Federal de Minas Gerais, C.P. 702, 30161-970 Belo Horizonte, M.G., BrazilAbstractThe exact reduced dynamics for the independent oscillator model in the RWA approximation atzero and finite temperatures is derived. It is shown that the information about the interaction and theenvironment is encapsulated into three time dependent coefficients of the master equation, one of whichvanishes in the zero temperature case. In currently used optical cavities all the information about thefield dynamics is contained into two (or three) experimentally accesible and physically meaningful realfunctions of time. From the phenomenological point of view it suffices then to carefully measure two(three) adequate observables in order to map the evolution of any initial condition, as shown with severalexamples: (generalized) coherent states, Fock states, Schr¨odinger cat states, and squeezed states.