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"Lamp" Defined
"Lamp" is this guide refers to the capsule that contains a tungsten
filament that heats to a white-hot temperature when supplied with
12v current. The temperature of the filament within the hard glass
(quartz) capsule filled with a mixture of (halogen) gases determines
the quality of the emitted light.
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Light Quality
Quartz-halogen sources are characterized by a white light that renders
objects lighted in their true, natural color. This is different
from the yellow cast of standard incandescent light. Modern automotive
headlamps (which operate at 12v) provide this type of white light,
and it is even possible to identify an older vehicle at night by
the color of the headlamps. The gases in the lamp envelope and the
high operating temperature cause the tungsten which evaporates off
the filament to re-deposit on the filament, thus extending lamp
life; eventually, the filament will break, causing the lamp to fail.
Quartz-halogen lamps designed for 12v operation are generally long-lived.
Lamp manufacturers publish lamp life data as "average lamp life".
This figure is arrived at by burning a number of lamps continuously
until half of them fail. Thus, if an array of 1000 identical lamps
are simultaneously energized, the time elapsed until 500 lamps "burn
out" is the manufacturer's "average lamp life". Note that the lamp
manufacturers do not guarantee lamp life, they merely provide estimates
of lamp longevity.
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Proper Voltage
Since a well-designed 12v lighting system delivers a maximum of
12v and a minimum of 10v to the lamps, one would expect that the
quartz-halogen lamps will meet or exceed the manufacturer"s
"average lamp life" criteria. This ranges from 2,000 to 10,000 operating
hours in theory; in practice, the life of a quartz-halogen lamp
depends on operating voltage, manufacturing tolerances and cost:
expensive lamps last longer. Cost-benefit factors yield an actual
average lamp life of 2,000 to 5,000 hours. Since an outdoor lighting
system switched on from dusk to midnight accumulates a total of
about 2,000 operating hours annually, lamp life can be accurately
assessed. Some lamps will fail "early"; in the example above, 500
lamps failed during the test period to yield "average lamp life"
for 1,000 lamps. Lamps operating at less than 12v but more than
10v last longer. Thus, if the voltage at each lamp is known, the
failure rate (barring defective lamps) can be predicted.
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Lamp Life Warranty
It is reasonable to expect that your outdoor lighting company warrant
lamp life for at least three years as part of the original equipment
warranty. Since the company that installed the system should know
the operating voltage of each lamp, they should bear the risk of
premature failure. It is important to know that lamps operating
outside the 12v to 10v range will actually have shorter life. A
visible variation in lamp-to-lamp brightness is a good indication
that a system has not been well designed or properly engineered.
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