Coating carried out in a sealed chamber evacuated by a mechanical pump in series with an oil diffusion pump to a pressure less than 10-4
mm of mercury, measured by a vacuum gauge. At such a low pressure the mean free path of the residual gas is greater than the dimensions of the chamber. Within this chamber is an electrically heated "boat'' that contains the material to be thin-layer coated. The surface (substrate) to be coated is relatively cooler, and molecular materials from the boat evaporate to the substrate. Optical means are employed to monitor the process and control the thickness of the deposited layer.