  The attenuation of fringe contrast in a Ramsey interferometer induced by the
atom-field interaction is analyzed. We show that short-time power series
expansion is not the proper tool to find the relevant time scale for such
process. Analytical expressions to quantify the relevant time scale for the
fringe contrast decay and to characterize the long-term effectiveness of this
process are proposed. For Pegg-Barnett phase state initial conditions, these
expressions suggest that: the increase of the energy of the field leads to
slower vanishing of fringe contrast; the increase of the field photon number
variance leads to more effective attenuation in the final regime. Numerical
simulations with coherent and thermal initial states are in qualitative
agreement with such results.
