Abstract: A mechanism for positioning one of a plurality of lamps, each carried by a lamp socket, at a desired focal point of an optical system is comprised of a first lamp socket positioned in an operative position such that a lamp carried by that socket will be at a desired focal point of the optical system. At least one other lamp socket is positioned in an inoperative position. A mechanism for enabling movement of the first lamp socket from the operative position to an inoperative position while simultaneously moving the other lamp socket from the inoperative position to the operative position is provided. The present invention enables a failed lamp to be replaced by a backup lamp in such a manner that no degradation or change in optical performance occurs.