In all quartz lamps which have a gas fill, there is a small area that looks like a nipple. It is in fact the location of the exhaust tube where the lamp was filled with its gas mixture. The position of the nipple is very important and if it’s facing the wrong way, it may show up as a shadow in the optical path. Whenever possible, the exhaust tip should be orientated straight up, or up to 45 degrees from vertical. This will keep the tip in the hottest location, and prevent the iodides and rare earth metals from collecting in it as the lamp cools. If the tip is orientated down, the rare earth metals will collect in it over time and the color of the lamp will shift as they are no longer included in the plasma arc.