The CIE 1976 L*a*b* color model
is based on the CIE 1964 color model which was further enhanced because
of two problems with the specification of colors in terms of tristimulus
values and chromaticity
space. Firstly, this specification is not easily interpreted in terms
of the psychophysical dimensions of color perception namely, brightness,
hue, and colourfulness. Secondly, the XYZ system and the associated chromaticity
diagrams are not perceptually uniform. The second of these points is a
problem if we wish to estimate the magnitude of the difference between
two color stimuli. The need for a uniform colour space led to a number
of non-linear transformations of the CIE 1931 XYZ space and finally resulted
in the specification of one of these transformations as the CIE 1976 L*
a* b* color space.
In fact in 1976 the CIE specified two color spaces; one of these was
intended for use with self-luminous colors and the other was intended
for use with surface colors. The other model was the CIE
1976 L*u*v* color model being described in more detail below. These
notes are principally concerned with the latter known as CIE 1976 L* a*
b* color space or CIELAB.
CIELAB allows the specification of color perceptions in terms of a three-dimensional
space.
The L*-axis is known as the lightness and extends from 0 (black) to
100 (white). The other two coordinates a* and b* represent redness-greeness
and yellowness-blueness respectively. Samples for which a* = b* = 0 are
achromatic and thus the L*-axis represents the achromatic scale of greys
from black to white.
The quantities L*, a*, and b* are obtained from the tristimulus values
according to the following transformations:
for Y/Yn > 0.008856
for Y/Yn <= 0.008856
for X/Xn > 0.008856)
for X/Xn <= 0.008856
for Z/Zn > 0.008856
for Z/Zn <= 0.008856
where Xn,
Yn,
and Zn
are the tristimulus values of the illuminant
(perfect reflecting diffuser) that was used for the calculation of X, Y,
and Z of the sample.
Color differences will be measured as total difference of the L*, a*
and b* values of a sample and the standard using the following equations:
∆L* = L*sample
- L*standard
positive ∆L* means, the sample is lighter than the standard
negative ∆L* means, the sample is darker than the standard
∆a* = a*sample
- a*standard
positive ∆a* means, the sample is redder than the standard
negative ∆a* means, the sample is greener than the standard
∆b* = b*sample
- b*standard
positive ∆b* means, the sample is yellower than the standard
negative ∆b* means, the sample is bluer than the standard
In addition, there are two other delta values
that are related to this scale, ∆C* and ∆H*. The ∆C* is
the difference in chroma between the sample and standard as described
in a polar coordinate system. The ∆H* is the difference in hue angle between
the sample and standard as described in a polar coordinate system.
(this is called meric chroma)
