tems of color notation and color rendition
have been developed.
Color Temperature
Color temperaturedescribes how a lamp
appears when lighted. Color temperature is
measured in kelvin (K), a scale that starts at
absolute zero (–273°C).
At room temperature, an object such as
a bar of steel does not emit light, but if it is
heated to a certain point it glows dull red.
Instead of a bar of steel, physicists use an
imaginary object called ablackbody radiator.
Similar to a steel bar, the blackbody radiator
emits red light when heated to 800 K; a
warm, yellowish “white” at 2800 K; a day-
light-like white at 5000 K; a bluish, daylight
white at 8000 K; and a brilliant blue at
60,000 K. The theoretical blackbody is nec-
essary because the bar of steel would melt
at these higher temperatures.
Color temperature is not a measure of
the surface temperature of an actual lamp or
any of its components. Color temperature
refers to the absolute temperature of the
laboratory blackbody radiator when its visible
radiation matches the color of the light
source.
Incandescent lamps closely resemble
blackbody radiators in that they emit a contin-
uous spectrum of all of the visible colors of
light (color plate 13). Consequently, the
incandescent spectrum is accurately speci-
fied by color temperature in kelvin. Fluores-
cent andhigh-intensity discharge(HID) lamps
produce a discontinuous spectrum with blank
areas punctuated by bands at specific fre-
quencies (color plates 14–17 and 19–31).
These bands combine to give the impression
of “white” light; fluorescent and HID lamp
color appearance is specified by itsapparent
orcorrelated color temperature(CCT).
Incandescent lamps used in architec-
tural lighting have color temperatures in the
2600 K to 3100 K range; fluorescent lamps
are available with apparent color tempera-
tures from 2700 K to 7500 K; north skylight
is arbitrarily called 10,400 K.
Unfortunately, the apparent color tem-
perature of discontinuous spectrum light
sources fails to provide information about its
spectral energy distribution. For example,
warm white and RE-930 fluorescent lamps
have the same apparent color temperature,
yet their spectral distribution curves and
their color rendition of objects and materials
are vastly different. This same limitation
applies when using color temperature nota-
tions for high-intensity discharge sources,
includingmercury vapor, metal halide, and
high-pressure sodium lamps.Color Rendering
To remedy this limitation, color rendering
expresses how colors appear under a given
light source. For example, a shade of red will
be rendered lighter or darker, more crimson
or more orange, depending on the spectral-
distribution properties of the light falling on it.
The most accepted method to deter-
mine the color-rendering ability of a light
source is a rating system called theColor
Rendering Index(CRI).
The CRI first establishes the real or
apparent color temperature of a given light
source. Then, it establishes a comparison
between the color rendition of the given light
source and of a reference light source. If the
color temperature of a given source is 5000 K
or less, the reference source is the blackbody
radiator at the nearest color temperature. If
the given color temperature is above 5000 K,
the reference source is the nearest simulated
daylight source.
The comparison is expressed as an Ra
factor, on a scale of 1 to 100, which indi-
cates how closely the given light source
matches the color-rendering ability of the
reference light source. Since the reference
for CRI changes with color temperature, theCOLOR