A photosensor gain detection apparatus that includes a detector including a photosensor configured to output a signal. Also included in the apparatus is an after-pulse/dark-pulse detector device that detects an after-pulse or a dark-pulse in the signal output by the photosensor, and outputs an indication signal when the after-pulse or the dark-pulse is detected, the after-pulse and the dark-pulse representing after-events in the photosensor triggered from a previous photon generating event. The apparatus additionally includes an integrator device that integrates the signal output by the photosensor and to output an integrated signal, a histogram device connected to the integrator and the after-pulse/dark-pulse detector device, and that generates a histogram from the integrated signal and the indication signal, a gain determination device that determines a gain of the photosensor based on the generated histogram, and a memory configured to store the determined gain.