In a method for the spectral analysis of a metal coating layer deposited on the surface of a steel strip, the strip is moved along an arc of the outer surface of a rotating roller with a cylindrical wall guiding the strip by contact. An ablation laser beam is guided into an internal cavity of the cylindrical wall to be placed in optical incidence under a normal axis on the outer surface of the roller on a targeted contact point of the strip and the roller. The beam passes through the wall via a wall opening, which is transparent to the beam. A plasma spectral emission distribution from the laser ablation to the contact point is collected by optical feedback in the direction of the normal axis on the outer surface of the roller and through the opening in order to be guided towards a spectral measurement unit. The normal axis on the outer surface for the optical incidence and feedback is placed in synchronous rotation with the roller.