Abstract: An apparatus for automatically controlling the rate of vacuum deposition of thin films on a substrate, consisting of an optical diaphragm or shutter in the form of two overlapping plates which together define a shutter opening between an evaporation crucible and a substrate. The overlapping plates are attached to a pair of rack gears to be moved in opposite directions by a pinion gear to vary the size of the shutter opening through which the evaporated material must pass to be deposited on the substrate. The rate of evaporation is sensed by a quartz crystal monitor and a signal therefrom is compared to a reference signal. The difference between the two signals is used to control the movement of the apertured plates thereby correcting the rate of deposition of the thin film.