  The physical parameters governing the dynamics of a light emitting quantum
system can be estimated from the photon counting signal. The information
available in the full detection record can be analysed by means of the
distribution of waiting times between detection events. Our theory allows
calculation of the asymptotic, long time behaviour of the sensitivity limit,
and it applies to emission processes with branching towards different final
states accompanied by the emission of distinguishable photons. We illustrate
the theory by application to a laser driven $\Lambda$-type atom.
