Abstract: In a pulsed laser which is periodically energized so that it emits a pulse of radiation following each periodic energization, a feedback loop control system is provided for controlling energization of the laser so that the laser output radiation pulses are substantially constant over a relatively long period of operation, the feedback loop including a detector for detecting the laser pulses and producing an electrical signal representative of each pulse and a comparing circuit responsive to those electrical signals and a reference signal that represents the constant laser output level desired, for comparing the electric signals and the reference signals producing a control signal for controlling the energy of the periodic energization.