Abstract: A laser rangefinder comprises a transmitter (19) controlled by sequence-controlled pulses emitted by a timer (16) following actuation of a switch (17) by an operator. The receiver of the rangefinder comprises an avalanche photodiode (APD) (10) the sensitivity of which is controlled in a time-programmed manner by a bias voltage controller (11) which receives timing pulses from the timer (16). The time-programmed control operates to render the APD 10 insensitive to protect the receiver against optical element backscatter and atmospheric backscatter prior to rendering the APD 10 sensitive to monitor return pulses from remote targets.