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Section 3.2 The Physics of Color 74

1.2

1
Power 0.8

Spectral 0.6 Illuminant D65 Illuminant A

Relative 0.4

0.2

0
400 450 500 550 600 650 700
Wavelength in nm

FIGURE 3.4: There is a variety of illuminant models; the graph shows the relative spectral
power distribution of two standard CIE models, illuminant A—which models the light from
a 100W Tungsten filament light bulb, with color temperature 2800K—and illuminant D-
65—which models daylight. Figure plotted from data available at http://www.cvrl.org/.

effects, usually modeled rather roughly by the Mie scattering model, described in
Lynch and Livingston (2001) or in Minnaert (1993)) One author remembers vivid
sunsets in Johannesburg caused by dust in the air from mine dumps, and there are
records of blue and even green moons caused by volcanic dust in the air.

Artificial Illumination
Typical artificial light sources are commonly of a small number of types:
• An incandescent light contains a metal filament that is heated to a high tem-
perature. The spectrum roughly follows the black-body law (Section 3.2.1),
but the melting temperature of the element limits the color temperature of
the light source, so the light has a reddish tinge.
• A fluorescent light works by generating high-speed electrons that strike gas
within the bulb. The gas releases ultraviolet radiation, which causes phos-
phors coating the inside of the bulb to fluoresce. Typically the coating consists
of three or four phosphors, which fluoresce in quite narrow ranges of wave-
lengths. Most fluorescent bulbs generate light with a bluish tinge, but some
bulbs mimic natural daylight (Figure 3.5).
• In some bulbs, an arc is struck in an atmosphere consisting of gaseous met-
als and inert gases. Light is produced by electrons in metal atoms dropping