HID's or High Intensity Discharge lamps are a general group of lamps consisting of mercury, metal
halide and high pressure sodium. Compared to fluorescent and incandescent lamps, high-intensity
discharge (HID) lamps produce a large quantity of light in a small package. They are commonly used for
warehouses and outdoor applications. Mercury vapor lamps, which originally produced a bluish-green
light, were the first commercially available HID lamps. Today, they are also available in a color corrected,
whiter light, but they are still often being replaced by the newer, more efficient high-pressure sodium and
metal halide lamps. Standard high-pressure sodium lamps have the highest efficacy of all HID lamps, but
they produce a yellowish light. High-pressure sodium lamps that produce a whiter light are now available,
but efficiency is somewhat sacrificed. Metal halide lamps are less efficient but produce an even whiter,
more natural light.
HID lamps are typically used when high levels of light are required over large areas and when energy
efficiency and/or long life are desired. These areas include gymnasiums, large public areas, warehouses,
outdoor activity areas, roadways, parking lots, and pathways. More recently, however, HID sources,
especially metal halide, have been used in small retail and residential environments.