Page 105 - Computational Colour Science Using MATLAB

92 CHROMATIC-ADAPTATION TRANSFORMS AND COLOUR APPEARANCE

6.3 CAMs

According to Fairchild’s definition of a CAM, CIELAB can be considered to be
a CAM but it makes relatively poor predictions of colour appearance in most
cases. The main problem of using CIELAB as a CAM is demonstrated by the
simple example where a grey patch is shown surrounded by either a white
background or a black background (Figure 6.3).
The CIELAB colour coordinates for the two grey patches shown in Figure 6.3
are identical but the colour appearances of the two grey patches are quite
different. Although the normalization of the tristimulus values by the white point
in the computation of CIELAB values attempts to deal with some issues of
colour appearance (those caused by the ability of the visual system to
compensate for changes in the illumination) it does not contain any spatial
component. Yet in everyday viewing the colours that we see are almost always
related colours. That is, we see colours in relation to the surrounding colours in a
scene. A CAM should, for example, be able to predict an increase in lightness
when a grey paper is viewed against a dark background compared with when it is
viewed against a light background. For many technologies, of course, colour
appearance is not important and basic colorimetry is sufficient (Berns, 2000). So,
for example, if we wish to compare a trial fabric sample with a standard we are
often only concerned with whether the trial matches the standard rather than
with what the two samples actually look like. The importance of this distinction
(see Westland, 2002) is critical if the recent work on colour appearance is to be
understood.

Figure 6.3 The grey patch looks lighter on the dark background than it does on the light
background