3.14 non uniformity of cie system color differences
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  • 1. In many ways the chief limitation of the CIE system is its non- uniformity.
  • 2. 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 result is a colour-difference formula which, for a pair of samples, gives a number that is intended to be proportional to the difference seen.
  • 3. CIE LAB, CMC two RECOMMENDED Formula The CIE L*a*b* (CIELAB) formula was one of two recommended for general use 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.
  • 4. Associated Color SpaceThis is obtained by usingL*, a* and b*as mutually perpendicularaxes(Figure 4.5) where L*, a* and b* are defined (except for very low tristimulus values) by Eqn 3.21:
  • 5. 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).
  • 6. and DE is intended to be proportional to the difference seen.An idea of the size of DE units can be gained byconsidering that the difference between black and whitecorresponds to 100 DE units,while commercial tolerances are often between 1 and 2 DE units.