This paper addresses the problem of the sensor engineer to determine what actual performance he might expect from a real (non-ideal) photodetector and how his sensor design may affect that performance. The paper shows the relationship between the external electrical parameters of the detector and its internal parameters and how these internal parameters may be deduced from the electrical measurements. The paper shows how geometrical and spectral shielding of the detector affects the sensor performance and how the detector performance when mounted in the sensor may differ from that measured on the detector test bench. The conditions where the detectivity may exceed the "classical" theoretical limit for photoconductors are derived. Finally, the theoretical ideas developed are applied to actual HgCdTe detectors in a practical illustration.