Method and apparatus for operating a discharge lamp

During the lamp (12) lighting phase the lamps current and/or power intensity is controlled as a function of the future evolution of luminosity exterior to the vehicle. in order to optimize the lamps life duration. The electric supply current and/or power intensity is limited to a predetermined value which is a function of the luminosity exterior to the vehicle. The electric supply control unit (30) includes light measuring unit (32) which detects exterior and interior light levels. The unit (32) is fitted to the interior of windscreen or headlamp glass and it includes a cartographic system which provides parameters representing the vehicles environment as a function of vehicle location with respect to predetermined places such as a tunnel.