The Hunter Lab color scale evolved during the 1950s and 1960s. At that time, many of the scientists involved with color measurement were working on uniform color scales. The XYZ system was being used, but it did not give a good indication of sample color based solely on the numbers. The uniform color scales being investigated gave better indications of the color of a sample based solely on the numbers. There were several permutations of the Hunter Lab color scale before the current formulas were released in 1966.
The Hunter Lab color scale is more visually uniform than the XYZ 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 Hunter Lab color space is organized in a cube form. The L axis runs from top to bottom. The maximum for L is 100, which would be a perfect reflecting diffuser. The minimum for L would be zero, which would be 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 of the Hunter Lab color space.
There are delta values (∆L, ∆a, and ∆b) associated with this color scale. These values indicate how much a standard and sample differ from one another in L, a, and b. The ∆L, ∆a, and ∆b values are often used for quality control or formula adjustment. Tolerances may be set for the delta values. Delta values that are out of tolerance 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. ∆E is a single value that takes into account the differences between the L, a, and b of the sample and standard. It does not indicate which parameter is out of tolerance 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 the tolerance.
The Hunter Lab color scale may be used on any object whose color may be measured. It is not used as frequently today as it was in the past because the CIE L*a*b* scale, which was released in 1976, has gained popularity.
Conditions for Measurement
Instrumental: Any HunterLab color measurement instrument
Standard Observer Function: Either 2 or 10 degree
Transmission and/or Reflectance: Either
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 tables, formulas, and illustrations)