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Depending on the choice of primaries, many different sets
of color-matching functions are possible, all of which de-
scribe the same color-matching behavior. Figure 1 shows
experimental data for the primaries 435.8, 546.1, andFIGURE 2 Estimates of human cone action spectra (Konig fun- ¨
damentals) derived by V. Smith and J. Pokorny. [From Wyszecki,
G., and Stiles, W. S. (1982). “Color Science: Concepts and Meth-
ods, Quantitative Data and Formulate,” 2nd ed. Copyright ©1982
John Wiley & Sons, Inc. Reprinted by permission of John Wiley &
Sons, Inc.]
700.0 nm. Depicted in Fig. 2 are current estimates of
the spectral sensitivities of the three types of cone pho-
toreceptors. These functions, which have been inferred
from the data of psychophysical experiments of various
kinds, agree reasonably well with direct microspectropho-
tometric measurements of the absorption spectra of outer
segments of human cone photoreceptors containing the
photopigments that are the principal determinants of the
spectral sensitivity of the cones.
The cone spectral sensitivities may be regarded as color-
matching functions based on primaries that are said to be
imaginary in the sense that, although calculations of color
matches based on them are possible, they are not phys-
ically realizable. To exist physically, each such primary
would uniquely excite only one type of cone, whereas
real primaries always excite at least two types.
Another set of all-positive color-matching functions,
based on a different set of imaginary primaries, is given
in Fig. 3. This set, which makes very similar predictions
about color matches as the cone sensitivity curves, was
adopted as a standard by the International Commission on
Illumination (CIE) in 1931.
By simulating any of these sets of sensitivity func-
tions in three optically filtered photocells, it is possible
to remove the human observer from the system of color
measurement (colorimetry) and develop a purely physical
(though necessarily very limited) description of color, one
that can be implemented in automated colorimeters.