for such lamps is in photographic lighting and stage lighting fixtures, where they are commonly generically known as MSD or HMI lamps and are generally used in 150, 250, 400, 575 and 1,200 watt ratings, especially intelligent lighting. Because of their wide spectrum and good efficiency they are used for indoor growing applications, specifically cannabis and are quite popular with reef aquarists who need a high intensity light source for their corals. Operation Like other gas-discharge lamps such as the very-similar mercury-vapor lamps, metal-halide lamps produce light by ionizing a mixture of gases in an electric arc. In a metal-halide lamp, the compact arc tube contains a mixture of argon or xenon, mercury, and a variety of metal halides, such as sodium iodide and scandium iodide. The particular mixture of metal halides influences the correlated color temperature and intensity (making the light more blue or red, for example). When started, the argon gas in the lamp is ionized first, which helps to maintain the arc across the two electrodes with the applied starting voltage. The heat generated by the arc and electrodes then ionizes the mercury and metal halides into a plasma, which produces an increasingly brighter white light as the temperature and pressure increases to operating conditions. The arc-tube operates at anywhere from 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 have negative resistance (with the rare exception of self-ballasted lamps with a filament), and so require a ballast to provide proper starting and operating voltages while regulating the current flow through 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%. Components Metal-halide lamps consist of an arc tube with electrodes, an outer bulb, and a base. Arc tube Inside the fused quartz arc tube, two tungsten electrodes doped with thorium are sealed into each end and an AC voltage is applied to them through molybdenum foil seals fused in silica. It is the arc between the two electrodes where the light is actually created. Besides mercury vapor, the lamp contains iodides or bromides of different metals. Iodine and bromine are of the halogen group of the periodic table, and so are termed "halides" when ionized. Scandium and sodium are also used in