Page 96 - Computational Colour Science Using MATLAB

CATS 83

Figure 6.1 Schematic diagram to show the chromaticities of a test stimulus (+) under
illuminant A, the stimulus under illuminant D65 ( ) and the hypothetical psychophysically
measured corresponding colour (*)

illuminations. Therefore the observer would select the same grey chip under
the reference illumination as was viewed under the test illumination. Usually,
in such experiments the corresponding colours are not those predicted by a
process that discounts the illumination change perfectly. In Figure 6.1 the
corresponding colour (*) determined under the reference illuminant (D65) to
match a neutral chip viewed under illuminant A (+) is shown for a hypothetical
experiment.
Fairchild (1998) has classified chromatic-adaptation mechanisms into two
groups: sensory and cognitive. Sensory chromatic-adaptation mechanisms refer
to those that respond automatically to the stimulus and are thought to relate to
control mechanisms in the sensitivities of the long-, medium- and short-
wavelength-sensitive cone classes. Cognitive chromatic-adaptation mechanisms
refer to higher level cognitive processes that may relate to our understanding of
scene content. Research has shown that chromatic-adaptation mechanisms are
quite rapid, being 50% complete after 4 s, 90% complete after 70 s, and 99%
complete after 110 s (Fairchild and Lennie, 1992; Fairchild and Reniff, 1995).
A CAT is a method for computing the corresponding colour under a reference
illuminant for a stimulus defined under a test illuminant. Most modern CATs are