Abstract: A method and apparatus for quickly and easily calibrating a relatively unstable light detector, such as a photomultiplier tube. The system is particularly designed to effect large scale semi-automatic calibration of many photomultiplier tube-preamplifier combinations that may exist in, for example, a laser scanner system. Each preamplifier (10) includes an automatic gain control circuit (20) connected in a loop which is momentarily closed during a calibrate mode when a predetermined level of a calibrated light source (15) is directed upon the associated tube (12). An error amplifier (34) develops a voltage in accordance with the deviation of the video output of the tube from the expected level during calibration, and the error voltage is preferably stored in digital form (38) so that the gain of the preamplifier (10) may be maintained at the calibrated level between successive calibrations.