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Section 3.2 The Physics of Color 73

fovea isn’t working).
The sensitivities of the three different kinds of receptor to different wave-
lengths can be obtained by comparing color matching data for normal observers
with color matching data for observers lacking one type of cone. Sensitivities ob-
tained in this fashion are shown in Figure 3.3. The three types of cone are properly
called Scones, Mcones,and Lcones (for their peak sensitivity being to short-,
medium-, and long-wavelength light, respectively). They are occasionally called
blue, green, and red cones; however, this is bad practice, because the sensation of
red is definitely not caused by the stimulation of red cones, and so on.

3.2 THE PHYSICS OF COLOR
Several different mechanisms result in colored light. First, light sources can produce
different amounts of light at different wavelengths. This is what makes incandescent
lights look orange or yellow, and fluorescent lights look bluish. Second, for most
diffuse surfaces, albedo depends on wavelength, so that some wavelengths may be
largely absorbed and others largely reflected. This means that most surfaces will
look colored when lit by a white light. The light reflected from a colored surface
is affected by both the color of the light falling on the surface, and by the surface,
and so is profoundly ambiguous. For example, a white surface lit by red light will
reflect red light, and a red surface lit by white light will also reflect red light.

3.2.1 The Color of Light Sources
The most important natural light source is the sun. The sun is usually modeled as
a distant, bright point. Light from the sun is scattered by the air. In particular,
light can leave the sun, be scattered by the air, strike a surface, and be reflected
into the camera or the eye. This means the sky is an important natural light
source. A crude geometrical model of the sky has it as a source consisting of a
hemisphere with constant exitance. The assumption that exitance is constant is
poor, however, because the sky is substantially brighter at the horizon than at the
zenith. A natural model of the sky is to assume that air emits a constant amount
of light per unit volume; this means that the sky is brighter on the horizon than at
the zenith because a viewing ray along the horizon passes through more sky.
A patch of surface outdoors during the day is illuminated both by light that
comes directly from the sun—usually called daylight—and by light from the sun that
has been scattered by the air (sometimes called skylight or airlight; the presence of
clouds or snow can add other, important, phenomena). The color of daylight varies
with time of day (Figure 3.1) and time of year.
For clear air, the intensity of radiation scattered by a unit volume depends on
the fourth power of the frequency; this means that light of a long wavelength can
travel much farther before being scattered than light of a short wavelength (this is
known as Rayleigh scattering). This means that, when the sun is high in the sky,
blue light is scattered out of the ray from the sun to the earth—meaning that the
sun looks yellow—and can scatter from the sky into the eye—meaning that the sky
looks blue. There are standard models of the spectral energy density of the sky at
different times of day and latitude, too. Surprising effects occur when there are fine
particles of dust in the sky (the larger particles cause much more complex scattering