H mercury vapor lamp (Hstands for
forhydrargyrum, the Greek word
for mercury and the source of its
chemical symbol Hg)
M metal halide lamp
S or LU high-pressuresodium lamp (LUis
short for the trademarked name of
several manufacturers’ HPS lamps)
SB self-ballasted mercury lamp (for
incandescent retrofit)
T self-extinguishing mercury or metal
halide lamps (these stop operating
if the outer bulb is broken to protect
people from exposure to excessive
UV radiation; the designation is
omitted with non-self-extinguishing
lamps.^1
39 with mercury lamps, two-digit num-
bers denote electrical properties
and kind of ballast required;
lamps with the same numbers are
electrically interchangeable (double
numbers such as 43/44 indicate
that the lamp will operate from
more than one ballast).
100 with metal halide and HPS lamps,
the number specifying nominal
lamp wattage.
/C phosphor-coated (metal halide)
/D diffuse-coated (HPS)
/U manufacturer-designated symbols
appear after the next slant line;
these commonly identify color or
burning position (e.g., U =univer-
sal burning position).
/BD basedown to horizontal ± 15°
/BU baseup to horizontal ± 15°
/DX deluxe white phosphor
/HOR basehorizontal ± 15°
/MED medium screw base
/T tubular bulb
/U universal burning position
For optical control, clear lamps offer a rel-
atively small “point source” of 2^18 in to 9½ in
in length. The phosphor-coated lamps enlarge
the optical size of the source to the outer bulb
wall; although the phosphor coating enhances
the lamps’ color-rendering abilities, the
increased optical size dictates the use of large
reflectors for useful optical control.Lamp Operation
As with fluorescent lamps, HID lamps require
ballasts to regulate the arc current flow and
to deliver the proper voltage to strike the arc.
Electronic ballasts are more efficient and
provide more precise control of the arc tube
voltage over the lamp’s life, resulting in more
consistent color and longer life.
Extinction of a lamp occurs in one of
three ways: (1) a power interruption of more
than half a cycle, (2) a severe voltage dip of
more than a few cycles, or (3) insufficient
voltage maintained from the ballast.
Before the lamp will relight, it must cool
sufficiently to reduce the vapor pressure to a
point where the arc will restrike. The time
required to cool depends partly on a
luminaire’s ability to dissipate heat. Typically
in a luminaire, mercury vapor lamps will
relight in three to ten minutes. Metal halide
lamps require ten to twenty minutes to
restrike; pulse-start metal halide lamps
relight in four to eight minutes. High-pres-
sure sodium lamps usually restrike in
approximately one minute. “Instant-strike”
HPS lamps have a second arc tube that pro-
duces light instantly after a momentary
power interruption.INTERIOR LIGHTING FOR DESIGNERS
(^1) Mercury and metal halide lamps will cause serious skin
burn and eye inflammation from shortwave ultraviolet
radiation after only a few minutes when the outer enve-
lope of the lamp is broken or punctured if adequate
shielding is not used.