Hydrargyrum quartz iodide

Hydrargyrum quartz iodide (HQI) is a special type of high-intensity discharge (or HID) lighting, where the light is produced using a very high voltage electrical arc through a gas. The "H" in "HQI" comes from hydrargyrum, the Latin name for the element mercury. When heated, mercury vapour is created inside the bulb, and deposited when it cools.

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An HQI lamp consists of a protective outer glass shield surrounding two heavy wires which are inserted into each end of a smaller inner bulb containing a gas. The lamp is powered by an electrical ballast, which regulates the current flow through the arc in the smaller inner bulb. Like all HID lamps, HQI lamps operate under high pressure and heat, and require special light fixtures for safe use.

HQI lamps can produce different color temperatures when manufactured with different metal halides. They are relatively efficient light sources producing a high lumen/watt ratio (approximately 6x that of incandescent bulbs).

Like HMI, HQI lamps are subsets (or types) of metal halide lamps, which in turn are subsets of high-intensity discharge (HID) lamps. They should not be confused with quartz halogen light bulbs, which are a specialized type of incandescent light bulb.