Such a light source, such as a lamp, is generally associated with a parabolic reflector, the focus of which is located between the two filaments of the lamp.  The headlight further includes a cover lens which has sets of prisms and/or striations such as to spread the beam generally horizontally and along the inclined part of the cut-off of the dipped beam, so as to give comfortable lighting and also to satisfy the photometric requirements laid down in regulations. 
In addition, Valeo Vision S.A. has for a number of years been developing reflectors which, associated with filaments that have no masking screen, generates beams such as a European type dipped beam, which not only provide a precise V-shaped cut-off, but which also give good lateral spread of the light.  In that case, the cover lens may be smooth or nearly smooth, which is of advantage, not only as regards its selling costs, but also from the aesthetic point of view.  One headlight of that type is described in French patent specification No.  FR 2 664 677A in particular. 
Given the foregoing, the purpose of this type of reflective surface is, as has been indicated above, to cooperate with a filament having no masking screen, and has been considered a priori as being of no interest where the filament does have a screen for making the cut-off.  Now, twin filament lamps, in which one filament is associated with a screen, and more particularly the so-called "H4" normalised lamps, continue to be widely used, mainly in headlights which have both a main beam and a dipped beam function. 
It is also known, from French patent specification No.  FR 2 720 476A in the name of Valeo Vision S.A., to use mathematically defined surfaces such as are cited above, with a lamp of the H4 type or equivalent, to obtain better quality beams.  The headlights described in that document do however have certain limitations.  In particular, if it is required to make, in accordance with the descriptions in that patent, headlights which are adapted for use under different regulations, and in particular a headlight with main and dipped beam functions in which the dipped beam is a normalised European beam for driving on the right, or a headlight with main and dipped beam functions in which the dipped beam is a normalised European beam for driving on the left, or again, a headlight with main and dipped beam functions in which the dipped beam will satisfy regulations in the United States of America, then all the various reflectors have to be made each time not only with different formers in the mould (i.e. the core, or male member, which cooperates with the mould cavity to produce the moulded reflector), but the mould cavities themselves also have to be different. 
This is explained by the fact that the base surfaces from which the various reflectors are made differ quite radically from each other, and the use of a common mould cavity would lead to major variations in thickness of the reflector, leading to high consumption of moulding material and to risks of mechanical instability at high temperatures. 
It is true that European patent specification No.  EP 0 736 726A proposes a headlight capable of generating a chopped beam, in which the same mould cavity can be used whether the reflector is to be used for driving on the left or driving on the right.  However, the disclosures in that European patent document do not enable a headlight with both main beam and dipped beam facilities to be made with the same advantages.  In particular, nothing is said about the positioning of the masking screen.