DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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CIE No 15.2, Colorimetry, Second
Edition (Vienna, Austria: Commission
Internationale de L’Éclairage,
1986); reprinted with corrections
in 1996.
In CIE No 15.2, color matching
functions are denoted x _ (λ), y _ (λ),
and z _ (λ) [pronounced ECKS-bar,
WYE-bar, ZEE-bar]. CIE No 15.3 is
in draft status, and I have
adopted its new notation X(λ),
Y(λ), and Z(λ).
Some authors refer to CMFs
as color mixture curves, or CMCs.
That usage is best avoided,
because CMC denotes a particular
color difference formula defined
in British Standard BS:6923.
Z(λ)
Y(λ)
X(λ)
400 500 600 700
Wavelength, λ, nm
Figure 21.4 CIE 1931, 2° color-matching functions. A camera with 3 sensors must have these
spectral response curves, or linear combinations of them, in order to capture all colors. However,
practical considerations make this difficult. These analysis functions are not comparable to spectral
power distributions!
different colors. For a scanner or a camera to see color
as the eye does, the filter sensitivity curves must be
intimately related to the response of human vision.
The famous “color-matching experiment” was devised
during the 1920s to characterize the relationship
between physical spectra and perceived color. The experiment
measures mixtures of different spectral distributions
that are required for human observers to match
colors. From statistics obtained from experiments
involving observers participating in these experiments, in
1931 the CIE standardized a set of spectral weighting
functions that models the perception of color.
These curves are called the X(λ), Y(λ), and Z(λ) colormatching
functions (CMFs) for the CIE Standard
Observer. They are illustrated at the bottom of
Figure 21.3, and are graphed at a larger scale in
Figure 21.4 above. They are defined numerically; they
are everywhere nonnegative.
216 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES