the external focus fe a shield m in the active position is essentially orthogonal to the optical axis x x the top edge mh of the shield constitutes a cut off edge and is in the vicinity of the focus of the lens lo and of the external focus fe the last useful ray ru for the cut off beam falls at a point 2 on the reflector 1 such that the reflected ray ru 1 arrives on the peripheral edge of the lens lo thus the light rays which form in the area 3 situated in front of the point 2 will give reflected rays which will not be recovered by the lens lo the area 3 of the reflector 1 is therefore an area which is not useful optically for the production of the cut off beam according to the invention as clearly visible in fig2 the reflector 1 comprises a scalloped part 4 in the area 3 that is not useful optically for the cut off beam this scalloped part 4 is situated according to the example in fig2 at the bottom part of the reflector and has the shape of a scallop open towards the front that is to say in the direction of propagation of the light beam a secondary elliptical reflector 5 fig2 and 3 is added in the scalloped area 4 the secondary reflector 5 is formed by a ellipsoid part whose concavity is turned towards the inside of the reflector 1 the contour of the secondary reflector 5 is connected to.