HID lamps or High-intensity discharge lamps produce light by striking an electrical arc across tungsten (Wolfram) electrodes housed inside a specially designed inner fused quartz or fused alumina tube. This tube is filled with both gas & metals. The gas aids in the starting of the lamps. Then, the metals produce the light once they are heated to a point of evaporation, forming a plasma (a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule).

High-intensity discharge (HID) lamps include these types of electrical lamps:

• mercury vapor,
• metal halide (also HQI),
• high-pressure sodium (Son),
• low-pressure sodium (Sox)
• less common, xenon short-arc lamps.

The light-producing element of these lamp types is a well-stabilized arc discharge contained within a refractory envelope (in arc tube) with wall loading in excess of 3 W/cm

Compared with fluorescent & incandescent lamps, HID lamps produce a far higher quantity of light per unit area of lamp package.