Civil Engineering Reference
In-Depth Information
a ballast designed for its specific use. Electronic dimming is also available. Mercury
vapor lamps are most commonly used in industrial applications and outdoor lighting
because of their low cost and long life (approximately 24,000 hours). The color-
rendering qualities of a mercury vapor lamp are not as good as those of incandescent
and fluorescent lamps. Because the color rendition and lamp efficacy 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
the application of such lamps indoors. Mercury vapor lamp sizes range from 40 to
1,250 W.
A metal halide lamp is similar in construction to a mercury vapor lamp, the major
difference being that the metal halide lamp contains various metal halide additives in
addition to mercury vapor. The efficacy of metal halide lamps is much higher than
that of mercury vapor lamps. Some of the newer metal halide lamps provide color
similar to that of incandescent lamps; others emulate daylight. Metal halide lamp sizes
range from 32 to 2,000 W. Ballasts designed specifically for metal halide lamps must
be used.
A high-pressure sodium (HPS) lamp has the highest lamp efficacy of all lamps
commonly used indoors. It produces light when electricity passes through a sodium
vapor. The lamp has two envelopes. The inner envelope is made of polycrystalline
alumina, in which the light-producing arc is struck. The outer envelope is protective
and is clear or coated. The sodium in the lamp is pressurized; hence, the light produced
is not the characteristic bright yellow associated with sodium, but rather a ''golden
white'' light. Although the HPS lamp first found its principal use in outdoor lighting,
it now is a readily accepted light source in industrial plants. HPS lamp sizes range
from 35 to 1,000 W. Ballasts designed specifically for high-pressure sodium lamps are
required.
A low-pressure sodium (LPS) lamp has the highest efficacy, providing up to 175
lm / W. It is used where color is not important, because it has a monochromatic light
output—that is, reds, blues, and other colors illuminated by an LPS light source all
appear as tones of gray or yellow. Low-pressure sodium lamps range in size from 18
to 180 W. Ballasts designed specifically for LPS lamps are required. These units are
best used for outdoor applications; indoor applications are practical only where color
is not important.
Lamp Substitution
One type of lamp substitution involves a lamp retrofit, where
one lamp is removed from a fixture and another is installed to increase efficacy. An-
other type of substitution involves replacement of fixtures themselves. Lamp efficacy
is not the only criterion to be used for selection. Other important factors include color
rendition, useful life, light distribution, restrike time, lumen depreciation rate, and
disposability.
Fixture Efficiency
Manufacturers of lighting fixtures provide product information regarding fixture effi-
ciency in terms of the coefficient of utilization (CU). This value is the percentage of
light emitted by the lamps that is delivered to the work plane. The luminaire direct
depreciation (LDD) is the amount of light lost by dirt buildup on the fixtures or lens.
This reduction of light with time requires the designer to install more fixtures than
would be required if light output did not deteriorate. When luminaires become outdated
