The present invention relates to an automated in-line method for measuring the surface cleanness of a continuously running metal sheet or strip, characterized by the following steps: a beam of radiation or a particle beam or a spark is focused onto the surface of the running strip, the transmitted power and the focal diameter being chosen so as to obtain a power density sufficient to create a plasma or hot spot which locally etches the metal in the form of a central zone surrounded by a peripheral oxidation ring; the characteristics of a zone encompassing said oxidized ring and possibly said central zone are analyzed by means of an optical image acquisition device and image processing; and an objective value indicative of the surface cleanness is deduced therefrom.