| 摘要 |
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. Optimum performance of the APD 10 is achieved by provision of a monitor circuit (24) which in combination with controller (11) establishes the breakdown voltage of the APD 10 at the temperature prevailing on each occasion immediately prior to firing of the laser transmitter (19) and derives therefrom the corresponding APD operating voltage to be achieved when the APD 10 next becomes sensitive for return pulse monitoring, this operating voltage being a fixed fraction of the previously established APD breakdown voltage. In one arrangement for implementing circuit (24) the APD breakdown voltage is monitored by a first capacitor (C6) which is arranged to function as a voltage source during the time-programmed operation and during this operation a second capacitor (C5) is connected in parallel with the APD 10 and in series with capacitor (C6) whereby the APD operating voltage is determined by capacitor (C5). |
