directed rib or flange 152b the screen 200 has a doubly folded edge 201 the assembly process is as follows the folded edge 201 is fitted into the groove 113a and then covered with a compressible annular seal 210 when the lens 150 is fitted the rib 152a compresses the seal 210 on to the edge 201 in the groove 113a a number of u shaped clips 220 are then fitted to hold the components together by clipping on the flange 152b of the lens 150 and on the flange containing the groove 113a this manner of assembly gives a hermetically sealed assembly which can be dismantled the screen 200 is preferably made of a transparent thin plastics material such as a methacrylate or a polycarbonate it is preferably rigid but may alternatively be a resiliently deformable film it is transparent or at least translucent and can be coloured although its optical function is not a basic feature of the present invention the basic function of the screen 200 is as follows if the lens 150 should break for example as a result of a blow from a stone or some other impact the screen 200 should nearly always remain intact because of its distance from the central part 151 of the lens 150 the screen 200 prevents any broken glass dust or moisture from entering the space 120 and there is no risk of the optical properties of the reflector 130 being impaired in the absence of a lens the damaged headlamp can be used until the vehicle reaches a repair garage whereupon the clips 220 can be released and the remains of the lens 150 replaced by a new lens without removing the screen 200 so that space 120 is protected throughout against.