CIE L*a*b* Color Scale - an07_96a


In 1976, the CIE recommended the CIE L*a*b*, or CIELAB, color scale for use. CIE Publication 15.2 (1986), Section 4.2, contains details on this color scale. It was intended to provide a standard, approximately uniform color scale which could be used by everyone so that color values could be easily compared.

The CIELAB color scale is an approximately uniform color scale. In a uniform color scale, the differences between points plotted in the color space correspond to visual differences between the colors plotted. The CIELAB color space is organized in a cube form. The L* axis runs from top to bottom. The maximum for L* is 100, which represents a perfect reflecting diffuser. The minimum for L* is zero, which represents black. The a* and b* axes have no specific numerical limits. Positive a* is red. Negative a* is green. Positive b* is yellow. Negative b* is blue. Below is a diagram representing the CIELAB color space.

There are delta values associated with this color scale.  ∆L*, ∆a*, and ∆b* indicate how much a standard and sample differ from one another in L*, a*, and b*.  These delta values are often used for quality control or formula adjustment. Tolerances may be set for the delta values. Delta values that are out of the tolerances indicate that there is too much difference between the standard and the sample. The type of correction needed may be determined by which delta value is out of tolerance. For example, if ∆a* is out of tolerance, the redness/greenness needs to be adjusted. Whether the sample is redder or greener than the standard is indicated by the sign of the delta value. For example, if ∆a* is positive, the sample is redder than the standard.
The total color difference, ∆E*, may also be calculated. The ∆E* is a single value which takes into account the differences between the L*, a*, and b* of the sample and standard. It does not indicate which parameter(s) (L*, a*, and/or b*) are out of tolerances if ∆E* is out of tolerance. It may also be misleading in some cases where ∆L*, ∆a*, or ∆b* is out of tolerance, but ∆E* is still within tolerance.
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.
The CIELAB color scale may be used on any object whose color may be measured. It is used extensively in many industries. As was intended, it provides a standard scale for comparison of color values.

Conditions for Measurement
Instrumental: Any HunterLab color measurement instrument
Illuminant: Any
Standard Observer Function: 2 or 10 degree
Transmittance and/or Reflectance: Either. 

Typical Applications This color scale may be used for measurement of the color of any object whose color can be measured. 

(See attached pdf file for the complete article with diagrams, tables, and Formulas)

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