High Intensity Discharge (HID) is a type of electrical lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. This tube is filled with both gas and metal salts. The gas facilitates the arc’s initial strike. Once the arc is started, it heats and evaporates the metal salts forming a plasma, which greatly increases the intensity of light.
Large amounts of light are released as an electric arc is passed between two cathodes, vaporizing gas or metallic salts in a pressurized tube causing a plasma which greatly intesifies the amount of light.
Do to using no filament the rated life is greatly increased up to 24,000 hours. The Lumens per watt is also greatly increased.
A variety of gasses and metallic additives may be used in this process. Generally they are Mercury Vapar, Metal Halide, and High Pressure Sodium.
Murcury Vapor lamps are the oldest member of the HID family and are recognized by their blue-green light. They product the lowest lumbens per watt (50-60) and a CRI (Color Rendering Index) of only 50. Mercury Vapor lamps are still in use in landscape, sign ligting, street lighting, etc.
Metal Halide lamps produce a brilliant white light, and much better lumens per watt (80-115) with a respectable CRI up to 80. Lamp life is a little shorter than that of the Mercuy Vapor, but most find the light quality and color vastly superior and worth the small sacrifice.
High Pressure Sodium
High Pressure Sodium (HPS) lamps produce a golden color, even more yellow than the standard incandescent color. HPS lamps also product a very high lumens per watt ratio (90-140), but the yellow light reduces the appearance of light to the human eye. In addition to not appearing to be as bright as their Metal Halide counter part, they produce a very low CRI (22) which can produce some security issues.