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Light Output
The lumen output values at specific hours of lamp life can be measured
and plotted. This lumen maintenance (or lumen depreciation) curve
contains important data for lighting designers. Though initial lumen
ratings at 100 hours are frequently the basis for comparing light
sources, mean lumens, determined at 40% of rated lamp life, are
the most important. Mean lumen ratings are based on the lamps operating
at 10 hours per start (except where noted). Lamp lumens are measured
on a reference ballast in the designed operating position at the
rated lamp wattage.
Lumen maintenance
curves represent the lamp manufacturer’s estimate of the best
lamp lumen output plotted over time. Typically, each group of lamps
tested will display a range or scatter of lumen maintenance values
at each interval measured. Therefore, individual lamps may vary
from published mean lumen ratings.
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Many factors
affect the performance of metal halide lamps over time. Most of
these factors (see table) are controllable in the design of the
lighting system. Incorporating as many of the optimized conditions
as possible will deliver the best performance from any given metal
halide lighting system. More light reducing conditions present in
the design of the lighting system create a gap between published
"optimized" ratings and actual lighting system performance.
For example,
Venture’s Uni-Form® pulse start lamp operated on an a low
current crest factor Single Voltage Hybrid or
HX magnetic ballast, and other optimized
conditions, can be expected to deliver mean lumens approaching 80%.
In contrast, a standard metal halide system operating under light-reducing
conditions may deliver only 50% lumen maintenance. Venture Lighting
publishes "optimized mean lumens."
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Even
within the Uni-Form pulse start system, you can expect a range of
lumen maintenance from 70% to 80%. (see chart below) Performance will
vary depending on the number of light-reducing conditions present.
By selecting a Uni-Form pulse start lamp, a low current crest factor
Single Voltage Hybrid or HX magnetic ballast, and optimizing the system conditions, significant
improvements in lighting system performance can be achieved.
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