Abstract: Certain non-optical characteristics and, in particular, mechanical characteristics are not measurable in situ in the industrial production of layers using plasma-assisted reactive electron beam vaporization, particularly if high demands are made on the hardness, wear resistance and barrier action, so as to be able to reproducibly apply the layers. The values of the optical layer characteristics are to be used as a control signal. In this method, immediately after the substrate has passed through the vaporizing zone, the reflection and/or transmission and absorption capacity are measured in the wavelength range .DELTA..sub.k =150 to 800 nm and from this are determined the refractive index and optical absorption coefficient. These determined values are compared with an experimentally determined desired value. A control signal obtained therefrom, in the case of a constant reactive gas partial pressure, controls the plasma and maintains constant the optical characteristics of the layer.