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A metal halide lamp is a very efficient type of lamp which operates by creating an electrical discharge through a mixture of gasses such as mercury vapour and sodium iodide in order to give the full spectrum of white light. They are not used domestically except as the light source in a data projector as they can take several minutes to reach full brightness, run very hot, and can explode if abused, but are frequently used in shops and as floodlighting.