The Hunter L*a*b* 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 X,Y,Z 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 L*a*b* color scale before the current formulas
were released in 1966.
The Hunter L*a*b* color scale is more visually uniform than the X,Y,Z
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 L,a,b color space is organized in a cube form
similar to the CIE 1976 L*a*b*
color model.
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.
The quantities L*, a*, and b* are obtained from the tristimulus values
according to the following transformations:
where
X,
Y, and Z
are the CIE tristimulus values.
Xn,
Yn,
and Zn
are the tristimulus values for the illuminant.
Yn
is 100.00.
Ka and Kb are chromaticity coefficients for the illuminant.
There are delta values ∆L*, ∆a*, and ∆b* associated with the Hunter
L*a*b* color model. These values indicate how much a standard and sample
differ from one another in L*, a*, and b*.
The ∆L*, ∆a*, and ∆b* are calculated from 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
