much as 20%. Most modern ballasts are now also equipped with a dimmer, which uses pulse-width modulation to dim the lamp up to 50%, or as much as one stop of light. Unlike a tungsten-based light, which has a negative color temperature shift with a drop in power, the mercury emission spectra takes over with a drop in power (approximately 200 K bluer at 50% output). Safety HMI lamps are approximately the same color temperature as the sun at noon (6000 K), and like other high intensity discharge lamps which contain mercury, generate ultra-violet light. Each HMI fixture has a UV safety glass cover that should be used to protect people who may be in front of the light. Exposure to an unprotected lamp can cause retinal damage and severe skin burns. HMI lamps can reach ignition voltages of up to 70,000 V when striking hot, and are considered very dangerous if miswired. It is good practice to strike the light from the ballast and not the head, in the event that there is a short circuit in the lamp head. Proper striking procedures should be followed as well, such as calling out a vocal warning whenever a light is turned on to warn persons in the area. Also, the header cable should be properly and securely connected. Nearly all present-day header cables are equipped with 'twist-lock' fittings at either end to ensure a safe and precise connection, both at the ballast and the lamp housing. 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 oriented 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 oriented 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. In addition to these concerns, HMI lamps have been known to shatter at the