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Section 3.2 The Physics of Color 75

from an excited state to a lower energy state. Typical of such lamps is strong
radiation at a small number of wavelengths, which correspond to particular
state transitions. The most common cases are sodium arc lamps and mercury
arc lamps. Sodium arc lamps produce a yellow-orange light extremely effi-
ciently and are quite commonly used for freeway lighting. Mercury arc lamps
produce a blue-white light and are often used for security lighting.

Figure 3.5 shows a sample of spectra from different light bulbs.

1
Metal halide
0.9 Standard flourescent
Moon white flourescent
0.8 Daylight flourescent
0.7
energy 0.6

Relative 0.5
0.4

0.3
0.2
0.1

0
350 400 450 500 550 600 650 700 750
Wavelength in nm
FIGURE 3.5: The relative spectral power distribution of four different lamps from the Mit-
subishi Electric Corporation. Note the bright, narrow bands that come from the flourescing
phosphors in the fluorescent lamp. The figure was plotted from data made available by the
Coloring Info Pages at http://www.colorpro.com/info/data/lamps.html;the data was
measured by Hiroaki Sugiura.

Black Body Radiators
One useful abstraction is the black body, a body that reflects no light. A heated
black body emits electromagnetic radiation. It is a remarkable fact that the spectral
power distribution of this radiation depends only on the temperature of the body.
If we write T for the temperature of the body in Kelvins, h for Planck’s constant,
k for Boltzmann’s constant, c for the speed of light, and λ for the wavelength, we
have
1 1
E(λ) ∝ 5 .
λ (exp(hc/kλT) − 1)
This means that there is one parameter family of light colors corresponding to
black body radiators—the parameter being the temperature—and so we can talk
about the color temperature of a light source. This is the temperature of the black