  In this paper a general and unified treatment of the DQE is exposed, both in
the space-domain and in the frequency-domain. The meaning of the DQE as quantum
efficiency appears to be of paramount importance for a correct interpretation
of the physical parameters involved in its formulation and measurement. The
treatment in the frequency-domain turns out to be a direct extension of that in
the space-domain. An operational procedure is introduced to consider the effect
of the filtering of the detector in the space-domain.
