trodes, which lessens the darkening of the
arc tube. As a result, pulse-start lamps pro-
vide better lumen maintenance, longer lamp
life, and improved color stability compared
with standard metal halide lamps.
With high-pressure sodium (HPS)
lamps, the electric discharge is through
combined vapors of mercury and sodium,
with the latter dominating. This produces the
orange-tinted color familiar to us in street
lighting: 21 to 22 CRI (color plate 24). In
HPS lamps, the inner arc tube is constructed
of polycrystalline alumina (PCA), a ceramic
material that is resistant to sodium attack at
high temperatures and has a high melting
point. By further increasing the gas pressure
inside the lamp,white high-pressure sodium
lamps produce incandescent-like color at
2700 K with good color-rendering properties
and a CRI of 85 (color plate 25).
Ceramic metal halide lamps combine
the ceramic arc tube technology of high-
pressure sodium lamps with existing metal
halide chemistry. Instead of the quartz arc
tube used in conventional metal halide
lamps, ceramic metal halide lamps have an
arc tube made of polycrystalline alumina
(PCA). PCA permits the lamp to operate at
higher internal temperatures, increasing
color rendering, output, and efficacy. PCA
resists interaction with the chemicals inside
the tube, which stabilizes the chemical mix
over the life of the lamp, improving color
consistency and lumen maintenance. The
PCA tube is smaller than quartz tubes,
which prevents the chemical mix from dis-
persing and further improves color consis-
tency.
Many high-color-rendering HID lamps
have shorter lives and produce lower light
output than standard HID lamps, but their
superior color makes them the best choice for
areas inhabited by people. White high-pres-
sure sodium lamps offer 10,000-hr lives and a
CRI of 85, and some have prefocus bases to
provide precise location of the source in opti-
cal systems. Ceramic metal halide lamps offer
6,000- to 15,000-hr lives and CRIs of 81 to
96; some have bipin, recessed single-contact,
or mogul bipost bases to ensure accurate
alignment of the light source with the optical
system of the luminaire.Bulb shapes
HID bulbs are produced in several incandes-
cent bulb shapes. In addition, four shapes
have been specially designed for HID ser-
vice: B, BT, E, and ED. Bulb shapes (figure
7.14) include
A Arbitrary
B Bulged
BT Bulged-tubular
E Elliptical
ED Elliptical-dimpled
PAR Parabolicaluminizedreflector
R Reflector
T Tubular
The descriptive abbreviation of an HID
lamp includes a multi-letter code that identi-
fies the lamp kind or trade name and a
number that identifies the lamp’s wattage,
followed by suffixes that may include lamp
shape, a number that represents the maxi-
mum diameter of the bulb in eighths of an
inch, outer bulb finish, operating position,
base, and color. The American National Stan-
dards Institute (ANSI) code for description of
HID lamps provides suggested standardized
nomenclature among manufacturers.Example: CMH100/C/U/MED/830
CMH ceramicmetalhalide lamp, also
CDM (forceramicdischargemetal
halide) and MHC formetalhalide
ceramic)INTERIOR LIGHTING FOR DESIGNERS