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Section 3.2 The Physics of Color 76

body that looks most similar. At relatively low temperatures, black bodies are red,
passing through orange to a pale yellow-white to white as the temperature increases
(Figure 3.12 shows this locus). When hc kλT , wehave1/(exp(hc/kλT) − 1) ≈
exp(−hc/kλT ), so
exp(−hc/kλT )
E(λ; T )= C
λ 5
where C is the constant of proportionality; this model is somewhat easier to use
than the exact model (Section 3.5.2).

3.2.2 The Color of Surfaces

The color of surfaces is a result of a large variety of mechanisms, including dif-
ferential absorbtion at different wavelengths, refraction, diffraction, and bulk scat-
tering (for more details, see, for example Lamb and Bourriau (1995), Lynch and
Livingston (2001), Minnaert (1993), or Williamson and Cummins (1983)). If we ig-
nore the physical effects that give rise to the color, we can model surfaces as having
a diffuse and a specular component. Each component has a wavelength-dependent
albedo. The wavelength-dependent diffuse albedo is sometimes referred to as the
spectral reflectance (sometimes abbreviated to reflectance or, less commonly, spec-
tral albedo). Figures 3.6 and 3.7 show examples of spectral reflectances for a number
of different natural objects.

1
0.9
0.8
yellow
0.7
flower orange flower
white
Reflectance 0.5 petal white flower
0.6

0.4
0.3 violet orange
flower
0.2 berry
0.1
blue flower
0
400 450 500 550 600 650 700
Wavelength in nm

FIGURE 3.6: Spectral albedoes for a variety of natural surfaces measured by Esa
Koivisto, Department of Physics, University of Kuopio, Finland, plotted against
wavelength in nanometers. These figures were plotted from data available at
http://www.it.lut.fi/ip/research/color/database/database.html.

There are two color regimes for specular reflection. If the surface is dielectric