To measure the characteristics of the surface coating of a moving metal strip, such as the alliation level of a coating including zinc and iron, the product is exposed to the radiation of a radiative source with a predetermined wavelength directed orthogonally to the surface of the product and the energy reflected by the surface is measured also in a direction orthogonal to the surface so as to overcome reflectivity variations due to the morphological characteristics of the surface and these operations are performed with the help of off-the-shelf optical fibres previously stripped at their free ends of their normal optical focusing accessories so that they can be brought as near to each other as possible and placed parallel to each other.