Metal Halide Bulbs

A metal-halide lamp is an electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides(compounds of metals with bromine or iodine). It is a type of high-intensity discharge (HID) gas discharge lamp. Developed in the 1960s, they are similar to mercury vapor lamps but contain additional metal halide compounds in the quartz arc tube, which improve the efficiency and color rendition of the light. The most common metal halide compound used is sodium iodide. Once the arc tube reaches its running temperature, the sodium dissociates from the iodine, adding orange and reds to the lamp's spectrum from the sodium D line as the metal ionizes. As a result, metal-halide lamps have a high luminous efficacy of around 75–100 lumens per watt, which is about twice that of mercury vapor lights and 3 to 5 times that of incandescent lights, and produce intense white light. Lamp life is 6,000 to 15,000 hours. As one of the most efficient sources of high CRI white light, metal halides as of 2005 were the fastest-growing segment of the lighting industry. They are used for wide area overhead lighting of commercial, industrial, and public places, such as parking lots, sports arenas, factories, and retail stores, as well as residential security lighting and automotive headlamps (xenon headlights).

The lamps consist of small fused quartz or ceramic arc tube which contains the gases and the arc, enclosed inside a larger glass bulb that has a coating to filter out the ultraviolet light produced. They operate at a pressure between 4 and 20 atmospheres and require special fixtures to operate safely, as well as an electrical ballast. Metal atoms produce most of the light output. They require a warm-up period of several minutes to reach the full light output.