Abstract: An illumination beam is sent onto a localized region of the surface layer of a thin-film structure which is being etched in a vacuum processing chamber. The reflective light beam is sent through a filter to the matrix sensor of a video camera, and to an optical disperser or an interference filter, a selection diaphragm, a fiber-optic cable, and an analysis slit at the input of the optical disperser or interference filter. A spectral analysis of the reflective light beam is performed to detect, by reflectometry, the transition of the following layer in a specific analysis region of the localized region.