Process for measuring the optical properties of thin layers while they are being built up in vacuum coating installations. For this purpose, at least one test object is passed through a stationary measuring light beam and the transmission behavior of the test object is evaluated by measurement. A reference point for the measurements is fixed in each case by reference measurements at intervals of time. In addition, at least one opaque measurement zone and at least one measuring zone, which does not attenuate the measuring light beam, are disposed in path of motion of the test object. The ratio of the measured value of the test object, decreased by the measured value of the opaque measuring zone, to the measured value of the nonattenuating measuring zone, decreased by the measured value of the opaque measuring zone, is formed by an arithmetic logic unit.