A general class of lamps called high-intensity discharge (HID) includes mercury-vapor, metal halide, and high-pressure sodium lamps. All electric discharge lamps include cylindrical transparent or translucent arc tubes that enclose the mercury, various gases, and metal salts and confine the electric discharge. The arc tubes are enclosed in a glass bulb which performs three functions:

• Excludes air to prevent oxidation of metal

• Stabilizes operating temperature of the lamp

• Reduces ultraviolet radiation emitted by the excitation of the vapors

HID lamps are packaged in a wide range of bulb styles, including BT, E, ET, PAR, R, and T. There are also specialized double-ended and single-ended styles. The bases include a range of screw-type, bayonet, and ceramic styles. Some HID lamps are designed for base-up, base-down, or horizontal operating positions, while others can be operated in any position. Like fluorescent lamps, all HID lamps require ballasts.

These lamps operate at high pressures and very high temperatures. They can be ruptured by mishandling, misapplication, or ballast failure. An arc-tube rupture can burst and shatter the outer glass bulb, resulting in an explosion that scatters glass fragments at temperatures as high as 1832 F (1000 C). To reduce the risk of personal injury, property damage, or fire, the lamp must be enclosed in a rigid housing with a combination lens and diffuser that can contain these hot fragments. HID lamps give full light output over a wide range of ambient temperatures. This makes them suitable for street, stadium, and...