Light Output
Depreciation in light output during life occurs
mainly because of the escape of electron-
emissive material and tungsten from the
cathodes to the walls of the arc tube. This
depends in part on the frequency of starting;
therefore, long burning cycles increase lamp
life and lumen maintenance. Other factors
affecting lumen maintenance are operating
current and the current wave form produced
by the ballast design.
The light output of metal halide lamps
declines more rapidly than either mercury
vapor or HPS lamps. With ceramic metal
halide lamps, lumen maintenance is
improved approximately 30 percent. Fre-
quent starting is most harmful to metal
halide, less harmful to HPS, and least harm-
ful to mercury vapor lamps.
Lamp Life
Lamp life varies considerably depending on
the kind of HID lamp and its burning orienta-
tion. The rated average life of HID lamps is
the point at which approximately 50 percent
of the lamps in a large group have burned
out and 50 percent remain burning. Pub-
lished lamp-life ratings are based on ten
hours per start.
The normal mode of failure of a mercury
vapor lamp is its inability to light. Almost all
mercury vapor lamps have rated average
lives of 24,000+ hrs (the plus sign indicates
that in 50 percent of such lamps “burnout”
occurs in excess of 24,000 hrs). It is wise to
relamp before reaching the point of failure,
however, because the lamps continue to
operate long after they are a useful light
source.
Metal halide lamps have rated average
lives of 7,500 to 20,000 hrs, depending on
lamp wattage. Lives of metal halide lamps
are shorter than those of other HID lamps
because of inferior lumen maintenance and
the presence of iodides in the arc tubes. The
normal mode of failure is the inability to light
because of an increased starting voltage
requirement. Metal halide lamps are particu-
larly sensitive to frequency of starting. As
with all lamps, over-wattage operation also
shortens life.
Almost all HPS lamps have rated aver-
age lives of 24,000 hrs. Normal end of life
occurs when the lamp begins to cycle on and
off, the result of lamp voltage having
increased to the point where the ballast volt-
age is insufficient to keep the lamp lighted.
Over-wattage operation causes voltage to
rise faster; slight under-wattage does not
affect lamp life.
As with fluorescent lamps, the initial
lumen rating for HID lamps is measured after
the first one hundred hours of operation.
This “seasoning” is necessary because the
lamps depreciate rapidly during these first
hundred hours, when cleanup of impurities
takes place.Dimming
It is possible to dim some HID lamps using
special ballasts, but operating HID lamps at
less than full output will produce color shifts
and reduced lamp efficacy. As wattage
decreases, the color-rendering properties of
metal halide lamps approach the color of
mercury vapor; HPS lamps approach the
yellow-amber color of low-pressure sodium.
mercury vapor lamps will retain their already
inferior color properties reasonably well, but
lumen maintenance and length of life are
reduced.Low-Pressure Sodium (LPS) Lamps
Low-pressure sodium (LPS)lamps, although
technically not high-intensity discharge
sources, are used in limited applications.
They have high efficacies—up to 200DISCHARGE LAMPS