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An HQI is a type of light source where the light is produced using a very high voltage arc through a gas.

The 'bulb' consists of an outer glass sheild surrounding two voltage rails which attach to each end of a smaller bulb containing the gas and with one rod inserted from each end connected to each respective voltage rail.

The lamp is powered by a 'ballast' which consits of a large capacitor and transformer to enable a constant very high voltage to create the arc in the smaller bulb.

These lamps can produce many colours using different metal halides and the most common form are 'Sodium Floods' which produce a slightly orange light and are found around many buildings.