Metal-halide gas-discharge lamp

The metal-halide lamp is a high-pressure mercury vapor lamp for lighting purposes. It contains a small amount of rare metals such as thallium, indium, dysprosium or scandium. Pure metal would require very high temperatures to vaporize, so the metals are inserted in a form of halogenides - compounds with e. g. iodine. The light color may be white with relatively good color rendering, or also green, blue or violet. Some types were used as high-intensity ultraviolet sources. Metal-halide lamps are still used in stadiums, shopping malls or historic monuments. The most powerful type made by Tesla Holešovice had 3500 W power. A similar type is the ordinary high-pressure mercury vapor lamp with no rare metals. It produces sharp bluish-greenish light, but it is usually used with phosphore coating, which produces a pale white light. These lamps were formerly used for street lighting. They were replaced with sodium lamps that have much better efficiency, but provide only orange light. Today, gas-discharge lamps are gradually replaced with LED technology.