A colored or filtered incandescent lamp
produces colored light by starting with
“white” light and filtering out the undesired
portions of the spectrum. Yet most colored
light sources, even those that appear highly
saturated, are not truly monochromatic.
They emit a fairly wide band of wavelengths,
often including small amounts of energy in
other hue regions. The less saturated the
color, the greater the content of other hues.
Color Filters
The predominant method of producing col-
ored light is the use of colorfilterswith a
“white” light source. The white source con-
tains all of the colors of the spectrum; the
filter absorbs the unwanted parts of the
spectrum and transmits the wavelengths
that make up the desired color.
Color filters are usually designed for
incandescent lamps. Other types of light
sources, lacking a truly continuous spec-
trum, are seldom used with color filters. The
greatest use of colored light is in retail store
windows and in theatre, television, and pho-
tographic lighting.
Gelatin filters (“gels”) are thin, colored,
transparent plastic sheets available in a wide
variety of colors as well as multicolored and
diffusing sheets. Deeper saturations are
obtained by using more than one thickness.
Gels have a short service life because their
color fades rapidly when they are transmit-
ting intense light and heat.
Colored plastic panels are available for
use with fluorescent lamps but are unsatis-
factory for use with hot incandescent fila-
ments. Colored glass filters, which can
withstand the heat of incandescent lamps,
come smooth, stippled, prismatic, or split;
they are highly stable.
Interference filters consist of one or
more layers of ultrathin film coating on clear
glass that reflect rather than absorb the
unwanted wavelengths. The number and
thickness of the film coatings determine the
transmission (hue and saturation). Because
unwanted wavelengths are not absorbed,
interference filters remain cool.
Some interference filters are designed
to reflect or transmit a portion of the spec-
trum: infrared or ultraviolet or both. Broad-
band interference filters are often called
dichroic (“two-colored”) because they trans-
mit one color and reflect the complimentary
color (figure 6.14).
It is advisable to determine the approxi-
mate spectral composition of the “white”
light source before selecting a filter. If the
desired wavelengths are not present in the
original source, the filter will be ineffective.
An extreme example is a red lamp with a
green filter, which will transmit no visible
light.Colored Lamps
Incandescent colored sign and decorative
lamps have outside ceramic enamels,
sprayed finishes, or dip coatings applied to
clear bulbs to obtain colored light by the
subtractive method: by absorbing the light of
those colors that are undesirable.
Transparent ceramic enamels are used
to coat clear glass bulbs; the finely ground
colored glass is fired into the bulb to fuse the
coating into a hard, permanent finish. The
coating is applied before the bulbs are made
into lamps. This makes these lamps resis-
tant to scuffing, chipping, and weather, but
they are less transparent than lamps with
lacquers or plastic coatings.
Sprayed finishes, usually shellacs or sili-
cones, are applied to the completed lamp.
Although these sprayed coatings have good
adhesion, they lack the hardness of the
ceramic enamels and have less resistance
to scratches or scuffing. Sprayed lacquers
are highly transparent and therefore often
used when the sparkle of a visible filament is
desired.INTERIOR LIGHTING FOR DESIGNERS