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HID Technology
How HID Technology Works
The technology in high intensity discharge lighting is in some
ways similar to fluorescent technology: an arc is established between
two electrodes in a gas-filled tube which causes a metallic vapor
to produce radiant energy. In this case, however, a combination
of factors shifts the wavelength of much of this energy to within
the visible range, so light is produced without any phosphors. In
addition, the electrodes are only a few inches apart (at opposite
ends of a sealed arc tube) and the gases in the tube are highly
pressurized. This allows the arc to generate extremely high temperatures,
causing metallic elements within the gas atmosphere to vaporize
and release large amounts of visible radiant energy. There are three
main types of HID lamps: mercury vapor, metal halide and sodium.
The names refer to the elements that are added to the gases in the
arc stream which cause each type to have somewhat different color
characteristics and overall lamp efficiency.
Ballasts and Warm-Up Time
Like any gaseous discharge light source, HID lamps have special
electrical requirements that must be supplied by a ballast. With
HID sources, however, the ballast must be specifically designed
for the lamp type and wattage being used. In addition, HID lamps
require a warm-up period to achieve full light output. Even a momentary
loss of power can cause the system to restrike and have to warm
up again a process that can take several minutes. In applications
where constant illumination is important for safety and security,
a backup system is often required. The LUMALUX? Standby lamp offers
instant restrike capabilities once power is restored, making it
an ideal choice for applications where safety is a concern.
Metal Halide Lamps
Metal halide lamps, such as the SYLVANIA METALARC? products, are
among the most energy efficient sources of white light available
today. These lamps feature special chemical compounds known as ??halides??
that produce light in most regions of the spectrum. They offer high
efficacy, excellent color rendition, long service life and good
lumen maintenance. Because of their numerous advantages, metal halide
lamps are used extensively in outdoor applications and in commercial
interiors. Recently, a wide range of low wattage METALARC? lamps
has been developed, offering high performance in a more compact
size and bringing HID benefits to applications such as retail and
display lighting. There is also an entire family of metal halide
lamps called METALARC? Ceramic that employs ceramic arc tube technology
to improve color rendition and consistency. METALARC PRO-TECH? lamps
represent an additional innovation, featuring a special shroud that
surrounds the arc tube, enabling use in open fixtures.
Sodium Lamps
High-pressure sodium sources, such as SYLVANIA??s LUMALUX? lamps,
were developed primarily for their energy efficiency. Mercury and
sodium vapors in the ceramic arc tube produce a yellow/orange light
with extremely high LPW performance and exceptionally long service
life (up to 40,000 hours). High-pressure sodium lamps render colors
poorly, which tends to limit their use to outdoor and industrial
applications where high efficacy and long life are priorities. Variations
within the LUMALUX family of lamps include a Standby version with
2 arc tubes for rapid re-strike after power interruption. LUMALUX
PLUS? ECO? eliminates cycling at end-of-life. Low pressure sodium
sources, are also available. Since these lamps produce light at
only one wavelength in the yellow region of the spectrum, they are
used where energy efficiency and long life are the only requirements.
Mercury Vapor Lamps
Mercury vapor lighting is the oldest HID technology. The mercury
arc produces a bluish light that renders colors poorly. Therefore,
most mercury vapor lamps have a phosphor coating that alters the
color temperature and improves color rendering to some extent. Other
HID types that offer higher LPW and better color properties have
largely superceded the use of this lamp.
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