  Short pulses of a probe laser have been used in the past to measure whether a
two-level atom is in its ground or excited state. The probe pulse couples the
ground state to a third, auxiliary, level of the atom. Occurrence or absence of
resonance fluorescence were taken to mean that the atom was found in its ground
or excited state, respectively. In this paper we investigate to what extent
this procedure results in an effective measurement to which the projection
postulate can be applied, at least approximately. We discuss in detail the
complications arising from an additional time development of the two-level
system proper during a probe pulse. We extend our previous results for weak
probe pulses to the general case and show that one can model an ideal
(projection-postulate) measurement much better with a strong than a weak probe
pulse. In an application to the quantum Zeno effect we calculate the slow-down
of the atomic time development under n repeated probe pulse measurements and
determine the corrections compared to the case of n ideal measurements.
