Mercury Vapor Lighting Systems


Mercury vapor lamps, recognizable by the bluish cast to their light output, are the least efficient member of the High Intensity Discharge (HID) family. They were developed to overcome problems with fluorescent lamps for outdoor use and are actually less efficient than fluorescents. Existing mercury lamp systems should always be considered for retrofit with other high efficiency alternatives, unless they operate very few hours a year.

The mercury vapor lamp produces light when the electrical current passes through a small amount of mercury vapor. The lamp consists of two glass envelopes: an inner envelope in which the arc is struck, and an outer protective envelope. The mercury vapor lamp, like the fluorescent lamp, requires a ballast designed for its specific use. Special ballasts are required for dimming.

The 1992 National Energy Policy Act severely restricts the production of Mercury Vapor lamps. It's just a matter of time before they will become obsolete and all production totally stopped.



  • Three times as efficient as incandescent.
  • Available in a wide variety of ratings, colors, sizes and shapes.
  • Relatively low unit cost and a high average rated life (about 24,000 hours when operated on a 10 hour cycle). This is 24 times longer life than incandescent.



  • Except for incandescents, they are the most inefficient source of light, with an efficacy in the 25 to 55 lumens per watt range.
  • Mercury lamps may be greatly affected by lamp lumen depreciation and should be replaced after 24,000 hours.
  • Poor lumen maintenance compared to other HID sources.
  • Require a maximum warm-up period of five to seven minutes before giving full light output, depending on lamp type, ballast and ambient temperature. They can be used only where this delay is acceptable.
  • Four to five minute cooling and restart time.
  • New outdoor installations are illegal under some state laws.
  • Lamps are quite voltage sensitive. Unless special ballasts are used, care must be taken to keep the supply voltage within plus or minus 5% of the rated voltage.
  • Mercury lamp ballasts are noisy. When this presents a problem, remote mounted ballasts may be an alternative.
  • A special dimming ballast is required to dim mercury lamps.



The wattage at which a lamp operates will vary from its nominal rating depending on the ballast, the supply voltage and the lamp voltage. When designing a Mercury Vapor installation, it's important to take these variables into account as the lumen output of the lamp varies with the wattage at which the lamp operates.

Unless a specified operating position is indicated, Mercury Vapor lamps are designed to be operated in any position.


Best Applications

Mercury vapor lamps have found greatest use in industrial applications and outdoor lighting, because of their low cost and long life (16,000 to 24,000 hours). Lamp sizes range from 40 to 1,000 watts.


Technology Types (Resource)

The color rendering qualities of the mercury vapor lamp are not as good as those of incandescent and fluorescent lamps. A significant portion of the energy radiated is in the ultraviolet region. Through use of phosphor coatings on the inside of the outer envelope, some of this energy is converted to visible light. As a result, the color rendition and lamp efficiency of phosphor-coated mercury vapor lamps is better than that of their clear (no phosphor coating) counterparts. The development of phosphor-coated mercury vapor lamps has enabled lighting designers to use this type of HID lighting for many indoor applications, in lobbies and hallways, retail display areas, and many others.

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