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. This figure shows typical mercury lamp designs:
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.
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.
Strengths:
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.
Weaknesses:
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.
Have a distinctive bluish cast light output.
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.
