|In a high-intensity discharge lamp, electricity arcs
between two electrodes, creating an intensely
bright light. Mercury, sodium, or metal halide gas
actS as the conductor.
HID lamps use an electric arc to produce intense light. Like fluorescent lamps, they require ballasts. They also take up to 10 minutes to produce light when first turned on because the ballast needs time to establish the electric arc. Because of the intense light they produce at a high efficacy, HID lamps are commonly used for outdoor lighting and in large indoor arenas. Because the lamps take awhile to establish, they are most suitable for applications in which they stay on for hours at a time. They are not suitable for use with motion detectors.
The three most common types of high-intensity discharge lamps are:
Mercury Vapor Lamps
Mercury vapor lamps—the oldest types of high-intensity discharge lighting—are used primarily for street lighting.
Mercury vapor lamps provide about 50 lumens per watt. They cast a very cool blue/green white light. Most indoor mercury vapor lamps in arenas and gymnasiums have been replaced by metal halide lamps. Metal halide lamps have better color rendering and a higher efficacy. However, like high-pressure sodium lamps, mercury vapor lamps have longer lifetimes (16,000-24,000 hours) than metal halide lamps.
Significant energy savings are also possible by replacing old mercury vapor lamps with newer high-pressure sodium lamps.
Metal Halide Lamps
Metal halide lamps produce a bright, white light with the best color rendition among high-intensity lighting types. They are used to light large indoor areas, such as gymnasiums and sports arenas, and outdoor areas, such as car lots.
Metal halide lamps are similar in construction and appearance to mercury vapor lamps. The addition of metal halide gases to mercury gas within the lamp results in higher light output, more lumens per watt, and better color rendition than from mercury gas alone.
Metal halide lamps have shorter lifetimes (5,000-20,000 hours) than mercury vapor and high-pressure sodium lamps.
High-Pressure Sodium Lamps
High-pressure sodium lighting—a type of high-intensity discharge lighting—is becoming the most common type of outdoor lighting.
High-pressure sodium lamps have an lumens per watt—an efficiency exceeded only by low-pressure sodium lamps. They produce a warm white light.
Like mercury vapor lamps, high-pressure sodium lamps have poorer color rendition than metal halide lamps but longer lifetimes (16,000-24,000 hours).