High-intensity discharge (HID) lighting can provide high efficacy and long service life, and both HIDs and LEDs can save significant energy when they replace incandescent lighting. 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 some time to produce light, they are most suitable for applications in which they stay on for hours at a time.
HID ballast technology is very old and often highly inefficient. Interfacing HID with advanced controls is difficult and dimming is not practical. LEDs are rapidly displacing many traditional HID lighting markets, such as stadium and roadway lighting, because they turn on instantly and are easily dimmed and controlled.
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—were formerly used for street lighting, but are now only rarely used for that purpose. Nearly all new lamps sold in North America today for street lighting are either metal halide or LEDs, which have also displaced mercury vapor lamps in sports arenas and gymnasiums. Mercury vapor lamps provide about 50 lumens per watt, but ballast loss can reduce the system efficacy to about 30 lumens per watt, which is not competitive with LEDs.
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 parking 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.
High-Pressure Sodium Lamps
High-pressure sodium (HPS) lighting is a type of high-intensity discharge lighting used for street and outdoor area lighting, parking garages, and some industrial applications. Although HPS lamps can be efficient and long-lasting, they typically have poor color rendering compared to other lamp types, and are being displaced by LEDs in many applications.
