DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

The term sharpening is used in the
color science community to
describe certain 3×3 transforms
of cone fundamentals. This terminology
is unfortunate, because in
image science, sharpening refers
to spatial phenomena.
X, Y, and Z are pronounced big-X,
big-Y, and big-Z, or cap-X, cap-Y,
and cap-Z, to distinguish them
from little x and little y, to be
described in a moment.
The CIE 1931 functions are appropriate to estimate the
visual response to stimuli subtending angles of about 2°
at the eye. In 1964, the CIE standardized a set of CMFs
suitable for stimuli subtending about 10°; this set is
generally unsuitable for image reproduction.
The functions of the CIE Standard Observer were standardized
based upon experiments with visual color
matching. Research since then revealed the spectral
absorbance of the three types of cone cells – the cone
fundamentals. We would expect the CIE CMFs to be
intimately related to the properties of the retinal photoreceptors;
many experimenters have related the cone
fundamentals to CIE tristimulus values through 3×3
linear matrix transforms. None of the proposed
mappings is very accurate, apparently owing to the
intervention of high-level visual processing. For engineering
purposes, the CIE functions suffice.
The Y(λ) and Z(λ) CMFs each have one peak – they are
“unimodal.” However, the X(λ) CMF has a secondary
peak, between 400 nm and 500 nm. This does not
directly reflect any property of the retinal response;
instead, it is a consequence of the mathematical
process by which the X(λ), Y(λ), and Z(λ) curves are
constructed.
CIE XYZ tristimulus
Weighting an SPD under the Y(λ) color-matching function
yields luminance (symbol Y), as I described on
page 205. When luminance is augmented with two
other values, computed in the same manner as luminance
but using the X(λ) and Z(λ) color-matching functions,
the resulting values are known as XYZ tristimulus
values (denoted X, Y, and Z ). XYZ values correlate to
the spectral sensitivity of human vision. Their amplitudes
– always positive – are proportional to intensity.
Tristimulus values are computed from a continuous SPD
by integrating the SPD under the X(λ), Y(λ), and Z(λ)
color-matching functions. In discrete form, tristimulus
values are computed by a matrix multiplication, as illustrated
in Figure 21.5 overleaf.
CHAPTER 21 THE CIE SYSTEM OF COLORIMETRY 217