a compact fused silica , electroded hid lamp for automotive forward lighting , which contains no mercury . the lamps voltage , approximately 40 volts , is developed in this lamp by vaporizing zinc iodide instead of mercury . a compromise between voltage and luminous flux is achieved through the choice of the sodium scandium molar ratio , between 4 . 5 : 1 and 6 : 1 and a zinc iodide dose of 2 to 6 micrograms per cubic millimeter that permits the lamp to operate within the north america , european and japanese automotive color specifications for white light . the voltage in the lamp can be controlled according to the zinc iodide doping level without seriously impacting the visible spectrum otherwise provided by the other known dopants in the lamp .