The 1976 CIELAB color space is 3-dimensional with the dimensions labeled as L*, a*, and b*. Unfortunately, some implementations of the CIELAB color space in software applications have not followed the proper notation and have simply labeled the three dimensions as L, a, and b. To people outside of the color science community, this may appear to be a trivial difference that does not alter the meaning of the color space. But to people within the color science community, the difference is not trivial.
Here is a brief history of two of the many steps that led to the introduction of the 1976 CIELAB color space and the CIE L*, a*, and b* coordinates.
In 1948, Richard S. Hunter published a paper in which he described a photoelectric color-difference meter that would measure color and deliver 3 values to quantify the color. The axes in this three-dimensional, Cartesian coordinate system where labeled L, a, and b. This color space is known as the Hunter 1948 Lab color space, and the calculations for L, a, and b are based on measurement of the CIE 1931 XYZ tristimulus values with the photoelectric color-difference meter.
The Hunter 1948 Lab color space incorporates the opponent-colors theory—proposed by Ewald Hering in 1878—with the a-axis representing the redness or greenness of a color and the b-axis representing the yellowness or blueness of a color. The L-axis represents the perceived lightness of the color.
Building upon the Lab Cartesian coordinate system for a color space, Glasser, McKinney, Reilly, and Schnelle published a paper in 1958 in which they described a visually uniform color coordinate system and the use of cube-root functions to calculate the values for the three dimensions of the color space. They used L*, a*, and b* to denote the three dimensions. Although the notations and goals were similar, the equations for L*, a*, and b* had very little in common with the Hunter 1948 Lab equations.
There were several other sets of equations that were proposed for the determination of color differences by other scientists, but for the sake of brevity I will not go into those details. However, the existence and use of so many sets of equations prompted the CIE to develop and recommend one set of equations for the calculation of color differences. The result of this effort was the CIE 1976 L*a*b* color space in which the CIE accepted the opponent-colors theory, adopted the Lab approach to the notation for the three axes, and incorporated the cube-root approach proposed by Glasser et al. for the nonlinearity between physical energy measurements and perceptual responses. Recognizing the previous use of Lab for the Hunter 1948 Lab color space and the use of L*, a*, and b* by Glasser et al., the CIE denoted their uniform color space with their initials, the year, and the three axes: CIE 1976 L*a*b*. Some publications added another formality and enclosed the axes in parentheses: CIE 1976 (L*a*b*).
I hope from this explanation that you can see that the Lab notation refers to the Hunter 1948 Lab color space, and the L*a*b* notation refers to the equations proposed by Glasser et al. To avoid confusion, and disdain from color scientists, please use CIE 1976 L*a*b*, or the alternative CIELAB notation, when referring to the CIE 1976 (L*a*b*) uniform color space.
Post written by Parker Plaisted
References:
G. Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, John Wiley & Sons, New York, N.Y. (1986).
Colorimetry, second edition. CIE Publication 15.2 (1986).
Hunter, R. S., Photoelectric Color-Difference Meter, J. Opt. Soc. Am. 38, 661 (1948).
Glasser, L. G., McKinney, A. H., Reilly, C. D., and Schnelle, P. D., Cube-Root Color Coordinate System, J. Opt. Soc. Am. 48, 736 (1958).
Hering, E., Zur Lehre vom Lichtsinne, Carl Gerold’s Sohn, Wien (1878).