States were T12 bulbs with less expensive
phosphors and poorer color rendering. The
combination of high efficacy and good color
rendering has made the T8 lamp the current
standard.
Colored Lamps
Colored fluorescent lamps emit only a partic-
ular portion of the spectrum; the color is
determined by the selection of the phos-
phors used. Different mixtures of phosphor
composition produce different colors of light.
In a few cases, additional filtering is
required to absorb mercury radiations that
will otherwise desaturate the color. For red
and deep blue lamps, a filter coating is
applied to the outside bulb wall.
The gold fluorescent lamp achieves its
color by subtraction, because no phosphors
emit mainly yellow light. A yellow filter coat-
ing on the inside of the tube absorbs the
unwanted wavelengths from a warm-white
phosphor. Whenever subtractive filtering is
used, luminous efficacy is reduced.
Black lightfluorescent lamps use a spe-
cial phosphor that emits primarily near-ultra-
violet energy, plus a small amount of visible
blue light.
Colored fluorescent lamps vary widely in
lumen output. For example, twenty-five red
lamps are required to equal the lumen
output of one green lamp. See table 6 in the
Appendix.
Different colors of light have different
degrees of effectiveness in attracting atten-
tion; this is independent of brightness inten-
sity. See table 8 in the Appendix.
Flicker and Stroboscopic Effect
The mercury arc in a fluorescent lamp oper-
ated on a 60 Hz alternating current goes on
and off 120 times per second. The light from
the lamp remains visible because the phos-
phors have some phosphorescent or “carry-
over” action: they emit a reduced quantity of
light for a short period of time after the arc is
extinguished.
The cyclic variation in light output is
known asflicker. With 60 Hz operation, the
flicker rate over the length of the lamp is 120
cycles per second. At the ends of the lamp
each alternate flash is relatively weak,
occurring at a rate of 60 flashes per second.
The 120-cycle flicker is too fast to be
visible. The 60-cycle flicker can be detected,
but only by the peripheral vision of the retina.
For this reason, lamp flicker is seldom
noticed except when seeing the ends of
lamps out of the corner of the eye.
When rapidly moving objects are
observed under discharge lighting systems,
blurred “ghost” images are sometimes
observed. This is known as stroboscopic
effect. Because of this phenomenon, an
object moving at a uniform speed will appear
to move in jerks. Under extreme conditions, a
rotating object will seem to be standing still or
even rotating in reverse direction depending
on its speed of rotation and its configuration.
Stroboscopic effect rarely causes diffi-
culty because modern phosphors have rela-
tively long carryover periods. If a problem
occurs, operating multiple ballasts on all
three phases of a three-phase circuit will
reduce stroboscopic effect because only
one-third of the lamps operate at reduced
output at a given time.High-Intensity Discharge (HID) Lamps
The term “high-intensity discharge” applies
to arc-discharge sources with a high power
density. In HID lamps, light is produced by
passing an electric current through a gas or
vapor under high pressure, as contrasted to
the low pressure in fluorescent or low-pres-
sure sodium lamps. HID lamps used for illu-
mination belong to three principal families:INTERIOR LIGHTING FOR DESIGNERS