The invention concerns an optical monitoring system for the measurement of layer thicknesses of thin coatings applied in a vacuum, particularly on moving substrates, during the coating process, in which the light intensity of the light of a light source injected into a reference light guide and released by a first piezoelectric or electrostrictive or magnetostrictive light chopper is registered by a light detector unit in a reference phase, the light of the light source in a measuring phase is injected into a first measuring light guide and the light released by a second piezoelectric or electrostrictive or magnetostrictive light chopper is directed to the substrate, and the light intensity of the light reflected or transmitted from the substrate is registered by the light detector unit through a second measuring light guide, and a remaining light intensity is registered by the light detector unit in at least one dark phase, wherein the reference phase, the measuring phase, and the dark phase are shifted in time by the light chopper and are digitally adjusted depending on the position of the substrate.