Laser altimeter for Apollo lunar orbit.
The function of the altimeter is to provide precise measurements of Apollo vehicle height above known positions on the lunar surface. A short duration light pulse from the laser is directed through a telescope in a narrow beam to the lunar surface. A portion of the light is reflected by the moon back to the altimeter where it is intercepted by a receiving telescope and relayed to a detector. The time from the moment when the light pulse leaves the laser until its return, which is measured with the aid of a quartz crystal controlled clock oscillator, is a measure for the altitude.
Woodward, J. H.↗