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Most newer generation electronic ballasts can operate both high pressure sodium (HPS) lamps as well as metal-halide lamps. The ballast initially works as a starter for the arc by its internal ignitor, supplying a high-voltage impulse and, later, it works as a limiter/regulator of the electric flow inside the circuit.
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. [7]
Unlike regular incandescent halogen lamps where a halide gas is used to regenerate the filament and keep the evaporated tungsten from darkening the glass, the mercury vapour and the metal halides in HMI lamps are what emit the light. The high color rendering index (CRI) and color temperature are due to the specific lamp chemistry.
The lamp is powered by an electrical ballast, which regulates the current flow through the arc in the arctube, and an ignitor, to make the high voltage pulse that is necessary to start the arc in the arctube. Like all HID lamps, HQI lamps operate under high pressure and temperature, and require special light fixtures for safe use.
Like fluorescent lamps, HID lamps require a ballast to start and maintain their arcs. The method used to initially strike the arc varies: mercury-vapor lamps and some metal-halide lamps are usually started using a third electrode near one of the main electrodes, while other lamp styles are usually started using pulses of high voltage.
For example, a high pressure sodium lamp has an arc tube under 100 to 200 torr pressure, about 14% to 28% of atmospheric pressure; some automotive HID headlamps have up to 50 bar or fifty times atmospheric pressure. Metal halide lamps produce almost white light, and attain 100 lumen per watt light output. Applications include indoor lighting of ...