Abstract: In order to track a distinctive line on a workpiece area to be followed by an operating implement, specifically a junction line between two workpieces to be welded by a torch advancing along that line, a narrow beam is trained generally perpendicularly from a pulsating source of luminous radiation--e. g. a laser--upon that area and is shifted parallel to itself around the implement so as to intersect the workpiece area along a closed trace substantially bisected by the line being tracked. The source of radiation is part of a photosensor including an objective with an optical axis inclined to the beam at a small angle to generate an image of the latter on a slanting straight line parallel to a central ray which passes through the point of intersection of the beam axis with the front focal plane of the objective, this image line starting at the intersection of the rear focal plane with a central ray parallel to the beam.