Operation
Like other gas-discharge lamps such as the very-similar mercury-vapor lamps, metal-halide lamps produce light by making an electric arc in a mixture of gases. In a metal-halide lamp, the compact arc tube contains a high-pressure mixture of argon or xenon, mercury, and a variety of metal halides, such as sodium iodide and scandium iodide,. The particular mixture of halides influences the correlated color temperature and intensity (making the light bluer, or redder, for example). The argon gas in the lamp is easily ionized, which facilitates striking the arc across the two electrodes when voltage is first applied to the lamp. The heat generated by the arc then vaporizes the mercury and metal halides, which produce light as the temperature and pressure increases.
Common operating conditions inside the arc tube are 5-50 atm or more (70–700 psi or 500-5000 kPa) and 1000-3000 °C. Like all other gas-discharge lamps, metal-halide lamps, with the rare exception of self-ballasted lamps with a filament, require auxiliary equipment to provide proper starting and operating voltages and regulate the current flow in the lamp. About 24% of the energy used by metal-halide lamps produces light (an efficacy of 65–115 lm/W), making them substantially more efficient than incandescent bulbs, which typically have efficiencies in the range 2-4%.
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