In many ways the chief limitation of the CIE system is its non-uniformity.

NON-UNIFORMITY OF THE CIE SYSTEM:

COLOUR DIFFERENCES

More uniform System

Equal changes in x, y or Y do not correspond to equal perceived differences.

Many attempts have been made to provide a more uniform system.

The basic approach has always been to start with the tristimulus values or chromaticity coordinates from the CIE system,

and to transform these in some way to give a more uniform system.

The end resultis a colour-difference formula which,

for a pair of samples, gives a number

that is intended to be proportional to the difference seen.

CIE LAB, CMC two RECOMMENDED Formula

The CIE L*a*b* (CIELAB) formula was one of two recommended for generaluse by the CIE in 1976 .

The formula was recommended by the Colour MeasurementCommittee of the Society of Dyers and Colourists in 1976 .

Although it was known to be far from ideal, it was the best available at that time.

One feature of the formula that has proved to be most useful has been the associated colour space.

Associated Color Space

This is obtained by using L*, a* and b* as mutually perpendicular axes (Figure 4.5)

where L*, a* and b* are defined (except for very low tristimulus values) by Eqn

3.21:

Associated Color Space CIE

where Xn, Yn and Zn are the tristimulus values, for a particular standard illuminant And observer, for a sample reflecting 100% of the light at all wavelengths .

The lightness of the sample is represented by L* on a scale running from zero for black to 100 for white.

The other attributes can be represented on a plot of b* against a*.Neutral colours plot close to the origin for any illuminant (a* = b* = 0).

Colour differences can be quantified by DE according

to Eqn 3.22:

and DE is intended to be proportional to the difference seen.

An idea of the size of DE units can be gained by considering that the difference between black and white corresponds to 100 DE units,

while commercial tolerances are often between 1 and 2 DE units.