Page 23 - Computational Colour Science Using MATLAB

10 INTRODUCTION
stimulus by dimensions of lightness, chroma and hue and it is therefore
reasonable to describe CIELAB as a colour-appearance space, whereas this label
is not appropriate for tristimulus space which is strictly only for colour
specification. However, if predictions of colour constancy using CIELAB are
compared with empirical measurements of colour constancy, then it is found that
the predictions are quite poor in general. The field of colour appearance has been
actively researched over the last decade in particular and several advanced colour
spaces (e.g. CIECAT94 and CIECAM97s) are now available for predicting
colour appearance.
Thirdly, the non-linear transform of tristimulus values in the CIELAB
equations allows the Euclidean distance between two points in the new space to
better predict the visual colour difference between the colour stimuli represented
by those two points. Consequently, the colour difference metric known as DE ab *
and computed by the formula
2 2 2 1=2
DE ab * ¼½ðDL*Þ þðDa*Þ þðDb*Þ Š , ð1:6Þ
where DL*, for example, denotes the difference in L* between the two samples,
has been used effectively to quantify colour difference in a wide range of
industries. The values of DL*, Da* and Db* are given by

* *
DL* ¼ L 1 L 2
* *
Da* ¼ a 1 a 2
and

* *
Db* ¼ b 1 b 2
where the subscripts refer to the two stimuli concerned.
Unfortunately, although CIELAB is more perceptually uniform than XYZ
space it is still a long way from being perceptually uniform. Industrial
practitioners of colour science would like to be able to apply a single tolerance
on the value of DE* that defines the perceptibility or acceptability boundaries
ab
throughout colour space, but this is not possible. The last two decades of the
twentieth century saw a great deal of research into the development of effective
colour-difference formulae. The CMC formula (named after the Colour
Measurement Committee of the Society of Dyers and Colourists) was introduced
in 1983 and has been widely used in industry (Clarke et al., 1984). However, a
new recommendation for colour difference was recently introduced by the CIE
and is known as CIEDE2000 (Luo et al., 2001). CIEDE2000, like its predecessor
CMC, is not in itself a colour space (it computes colour difference starting from
differences in CIELAB space) but rather describes a method for combining and
weighting the differences that is more complex, and certainly more effective, than
simply measuring the Euclidean distance.