A mercury vapor lamp is a type of high intensity discharge (HID) lamp that is predominantly used for commercial and outdoor lighting. It was the first metal vapor style of light fixture to be mass produced for general lighting. The mercury vapor lamp produces a bluish white light by introducing an electric current to mercury vapor within a sealed glass tube. The bulb is comprised of an inner tube, called the arc tube, and an outer tube.
The arc tube typically is made of quartz and has an electrode at each end. Inside the arc tube is a small amount of mercury and a buffer gas, usually argon. The buffer gas helps to carry the discharge while the lamp warms up, which can take up to 10 minutes. It also produces the heat required to vaporize the mercury.
The outer tube of a mercury vapor lamp is generally filled with nitrogen, or a mixture of nitrogen and argon. Its job is to provide thermal insulation and protect users from the ultraviolet (UV) radiation that is common with mercury lighting. This protective shell can be made of clear, borosilicate glass, but it is often coated with a phosphorous finish to correct color and add further UV protection.
As with fluorescent lights, mercury vapor lamps require a ballast to supply the proper voltage and regulate the current to the electrodes. The ballast is designed to be used with a specific size, or wattage, of bulb. Using a mercury vapor lamp with a ballast that was made for a higher wattage bulb might damage it or cause it to explode. Using too small of a ballast will result in lower light output and can shorten the life of the bulb.
When used with the correct ballast and properly maintained, a mercury vapor lamp has a long service life. The average life expectancy is more than 24,000 hours, or nearly three years of continuous use. Unless they are damaged, these lamps typically do not burn out completely. Over time, deposits build on the inside of the arc tube wall, causing a phenomenon called lumen depreciation. As a general rule, lumen depreciation will cause a mercury vapor lamp to produce 50 percent less light every five years.
As more efficient types of HID lighting fixtures have entered the market, mercury lamps have become less popular. Although the bulbs are still widely available, the United States banned the sale of mercury vapor lamp ballasts in 2008. According to the ban, existing fixtures can remain in service but, as they burn out, they must be replaced with newer, more efficient lighting alternatives. Similar laws have been passed in the European Union, which set 2015 as the year when mercury lamps will no longer be approved for lighting purposes.