The basic modes of electrical operation of photoconductive optical radiation detectors are analyzed. The nonlinearity inherent in "voltage mode" measurements can be eliminated by using "current mode" measurements. A HgCdTe photoconductive radiometer has been designed, based on this analysis, which measures the current through a biased detector. A built-in calibrating capability in the circuits of the radiometer makes it possible to eliminate the effects of a long-term drift in the bias voltage, thereby achieving a higher precision.