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Section 3.1 Human Color Perception 69

2.5

slightly cloudy, sun behind a cloud
2

1.5

cloudy, gray sky
1
slightly cloudy, sun visible
cloudless sky
cloudless sky, bright snow
0.5

cloudless sky, sunset cloudless sky, just before sunset

0
400 450 500 550 600 650 700

FIGURE 3.1: Daylight has different amounts of power at different wavelengths. These
plots show the spectral energy density of daylight measured at different times of day
and under different conditions. The figure plots relative power against wavelength for
wavelengths from 400 nm to 700 nm for a series of seven different daylight measurements,
made by Jussi Parkkinen and Pertti Silfsten, of daylight illuminating a sample of barium
sulphate (which gives a high reflectance white surface). At the foot of the plot, we show
the names used for spectral colors of the relevant wavelengths. Plot from data obtainable
at http://www.it.lut.fi/ip/research/color/database/database.html.

form, a large number of lights may be required to obtain a match, but many different
adjustments may yield a match.
Write T for the test light, an equals sign for a match, the weights—which are
non-negative—as w i , and the primaries P i . A matchcan then be writteninan
algebraic form as
T = w 1 P 1 + w 2 P 2 + ...,
meaning that test light T matches the particular mixture of primaries given by
(w 1 ,w 2 ,...). The situation is simplified if subtractive matching is allowed. In
subtractive matching, the viewer can add some amount of some primaries to the
test light instead of to the match. This can be written in algebraic form by allowing
the weights in the expression above to be negative.
Under these conditions, most observers require only three primaries to match
a test light. This phenomenon is known as the principle of trichromacy. However,