Incandescent lamps are usually sold by
wattage, but a watt is not a measure of
light—it is a measure of power consumed.
With electric light sources, it is a measure of
how much electricity the lamp uses during
operation. Lumens tell how much light a
lamp emits.
Extended-service (2,500-hr) incandes-
cent lamps achieve their longer life by a
reduction in light output andefficacy.Lumen
output is approximately 15 percent less than
standard 750-hr and 1,000-hr life lamps.
These lamps are more expensive than stan-
dard ones, but their longer life is useful in
locations that are difficult to relamp.
Many energy-saving or “watt-saving”
lamps are simply reduced-wattage lamps.
The reduced power consumption is accom-
plished by a reduction in light output. Some
energy-saving lamps have a more efficient
filament design, gas fill, or reflector bulb
shape to maintain light output.
Lamp Types
Incandescent lamps are divided into three
categories according to their ability to direct
light: (1) nondirectional sources emit light in
all directions and require additional compo-
nents to control their distribution, (2) semi-
directional sources give a direction to their
light output and require additional compo-
nents to complete a spatial distribution, and
(3) directional sources control the distribu-
tion of emitted light and require no addi-
tional components, being complete optical
systems in themselves.
Nondirectional Sources
Nondirectionallamps emit light in all direc-
tions. They include A, C, G, P, PS, S, and T
shapes and decorative lamps. These lamps
require external elements in the form of a
lens, reflector, or shield to modify their distri-
bution and to control their brightness (figure
6.5).
To reduce the glare from an exposed fil-
ament, many nondirectional lamps have a
coating applied to the inner surface of the
clear bulb. A two-bath acid etch or a light
coating of electrostatically applied white
powder absorbs an insignificant amount of
light, yielding a ball of light inside the bulb.
This kind of lamp has lower luminance and
less glare than the exposed filament. It is
called aninside-frostlamp (figure 6.6).
Still greater diffusion, with a further
reduction of glare and a sacrifice of about 2
percent of the light output, is achieved by a
double coating of white silica powder. This
gives a ball of light the size of the lamp, yield-
ing a bulb of almost uniform brightness. It is
called asoft-whitelamp.
In both treatments, the outer surface of
the bulb is left smooth, which makes it easy
to clean. Inside-frost lamps are preferred for
most luminaires to reduce the sharpness of
shadows and the possibility of striations on
nearby surfaces. Where the small point
source contributes to glitter, as in the spar-
kle of crystal chandeliers, clear lamps are
necessary.Semi-Directional Sources
The category of semi-directional sources
includes silver-bowl and white-bowl lamps.
Silver-bowllamps, usually used to direct light
upward, have an opaque silver coating
applied to the inside of the bowl (figure 6.7).
This functions as a specular reflector that
remains clean, and therefore efficient,
throughout the life of the lamp. Silver-bowl
lamps are available in both clear and inside
frost.
When used indirectly in a suspended
luminaire to light the ceiling, the upper part
of the bulb must be concealed to prevent
excessive brightness and glare. This is
accomplished by an assemblage of circular
rings around the lamps or by a shallow, dif-
fusing glass bowl. When silver-bowl lampsINCANDESCENT LAMPS