  A generalized probe sequence typical of trapped ion experiments using
shelving is studied. Detection efficiency is analyzed for finite shelved state
lifetimes and using multi-modal count distributions. Multi-modal distributions
are more appropriate for measurements that use a small number of ions than the
simple Poisson counting statistics usually considered and have a larger
variance that may be significant in determining uncertainties and in making
weighted fits. Optimal probe times and the resulting state detection efficiency
and sensitivity are determined for arbitrary cooling rates, initial states and
shelved state lifetimes, in terms of a probe coherence time {\tau}p. A
universal optimal probe time of tp ~ 0.43{\tau}p is shown to give an almost
optimal probe sensitivity for most systems.
