Mercury vapor lamps are HID lamps used for commercial and outdoor lighting. They produce bluish white light by introducing a mercury vapor electric current inside a sealed glass tube. They require a ballast and have an average life expectancy of over 24,000 hours. However, they have become less popular due to more efficient alternatives, and the US and EU have banned their sale.
A mercury vapor lamp is a type of high intensity discharge (HID) lamp used primarily for commercial and outdoor lighting. It was the first type of metal vapor lamp to be mass-produced for general lighting. The mercury vapor lamp produces a bluish white light by introducing a mercury vapor electric current inside a sealed glass tube. The bulb consists of an inner tube, called an arc tube, and an outer tube.
The arc tube is typically made from 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 carry the discharge while the lamp warms up, which can take up to 10 minutes. It also produces the heat needed to vaporize mercury.
The outer tube of a mercury vapor lamp is usually filled with nitrogen or a mixture of nitrogen and argon. Its job is to provide thermal insulation and protect users from ultraviolet (UV) radiation which is common with mercury lighting. This protective shell can be made from clear borosilicate glass, but is often coated with a phosphor finish to correct color and add extra UV protection.
As with fluorescent lights, mercury vapor lamps require a ballast to supply the correct voltage and regulate current to the electrodes. The ballast is designed for use with a specific size, or wattage, of bulb. Using a mercury vapor lamp with a ballast made for a higher wattage bulb could damage it or cause it to explode. Using a ballast that is too small will result in lower light output and can shorten bulb life.
When used with the correct ballast and properly maintained, a mercury vapor lamp has a long life. The average life expectancy is over 24,000 hours, which is almost three years of continuous use. Unless damaged, these lamps typically don’t burn out completely. Over time, deposits build up within 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% less light every five years.
As more efficient types of HID luminaires 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. Under the ban, existing fixtures can remain in service but, when they burn out, must be replaced with one-light fixtures. newer and more efficient alternatives. Similar laws have been passed in the European Union, which set 2015 as the year in which mercury lamps will no longer be approved for lighting.
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