temperature. Occasionally the wire is straight,
but usually it is coiled to pack more length
into a small envelope, concentrating light
and heat and increasing efficacy. Coiled fila-
ments are designated by the letterC. Some-
times the coil itself is coiled and designated
CCfor “coiled coil.”
Filament design is determined by strik-
ing a balance between light output and lamp
life. It is a function of filament temperature:
the higher the temperature at which the fila-
ment operates or “burns,” the more light it
emits and the shorter its life—the sooner it
fails or “burns out.” A long-life lamp of a
given wattage produces less light than a
standard-life lamp of the same wattage,
which consumes the same current but is
designed for a shorter life.
Lamp efficacy is the ratio of light pro-
duced (measured inlumens[lm]) to electric-
ity consumed (measured inwatts[W]). Lamp
life is measured in hours (hr).
Light Output
Lamp bulbs do not contain air, because the
incandescent tungsten will react with the
oxygen in the air and quickly evaporate.
Originally this was prevented by creating a
vacuum in the bulb. Today, filling the bulb
with an inert gas slows bulb blackening,
which is caused by condensation of evapo-
rated tungsten particles on the inner bulb
wall. Argon, nitrogen, and krypton gases are
used for this purpose. (Some incandescent
lamps, particularly those below 40 W, still
use a vacuum.)
Although reduced by the inert gas pres-
sure, the filament evaporation continues
throughout life; the tungsten wire becomes
thinner, consumes less power, and emits
less light. This light loss combined with bulb
blackening causes a steady decrease in light
output throughout the life of the lamp.
A reciprocal relationship exists between
light output and life. Over-voltage operation
results in higher wattage, higher efficacy,
and higher light output, but shorter lamp life.
Under-voltage burning results in lower watt-
age, lower efficacy, and lower light output,
but longer lamp life (figure 6.4). As a rule of
thumb, a given percentage reduction in
wattage is accompanied by double that per-
centage reduction in light.INTERIOR LIGHTING FOR DESIGNERS
Figure 6.4 Incandescent lamp characteristics as
affected by voltage (V). For example, operating a 120 V
lamp at 125 V means approximately 16% more light
(lm), 7% more power (W), and 42% less life (hr). Oper-
ating a 120 V lamp at 115 V means approximately 15%
less light (lm), 7% less power (W), and 72% more life
(hr).