Metal halide lamps are also called Multi-Vapour lamps. Metal halide lamps are High Pressure, High intensity discharge light sources, similar to High Pressure Mercury Vapour lamps. These lamps produce light spectrum closest to sun light. These may be called man made sun light.The quartz arc tube contains mercury together with Sodium and Halides of Sodium, Thallium, Indium and Dysprosium. To obtain further improvements in colour halides of Rare Earth metals are used.
The principle of operation is that each metallic vapour participating in the electric discharge produces its own colour spectrum covering entire visible range. Hence there is necessity to add multiple halides of metal to be called as Metal Halide Lamps or Multi Vapour Lamps.
The life of Metal Halide lamps obtainable in practice will range from 10,000 Hrs to 15,000 Hrs for Tubular and 14,000 Hrs to 20,000 Hrs for Ellypsoidal lamps at the appropriate operating position at 10 hours per start with suitable control Gear. In general the H.O lamps used for street lighting,flood lighying,horiculture outdoors and heavy industries out doors to have more Lux levels.