One of the important factors in calculating color coordinates in the CIELAB color space is the reference white. The two primary inputs to the CIELAB equations are the set of CIE XYZ tristimulus values for the stimulus, or measured color, and the CIE XYZ tristimulus values for the reference white. The CIE does not identify a specific reference white for CIELAB, so any appropriate reference white may be used (e.g., D50). Therefore, it is important to state the reference white when using or reporting CIELAB color coordinates in order to avoid a misinterpretation of the CIELAB values.
The benefit gained from the reference white in the CIELAB equations is the chromatic adaptation provided by dividing each CIE XYZ tristimulus value of the stimulus by the corresponding CIE XYZ tristimulus value for the reference white (i.e., X/Xn, Y/Yn, and Z/Zn where Xn, Yn, and Zn are the CIE XYZ tristimulus values for the reference white).
This chromatic adaptation in CIELAB based on the CIE XYZ tristimulus values is an approximation to the von Kries chromatic adaptation model, which is applied to the retinal cone responses, but it is less accurate than a proper von Kries adaptation. A proper von Kries chromatic adaptation adjustment would require a transformation from CIE XYZ tristimulus values to cone responses, scaling of the cone responses, and then a transformation of the scaled cone responses back to CIE XYZ tristimulus values. Applying the adaptation scaling directly to the CIE XYZ tristimulus values is easier to implement than a proper von Kries adaptation and provides adaptation results that were deemed sufficient by the CIE in 1976 for color difference calculations with the CIELAB equations.
A chromatic adaptation model provides a means to account for the observer’s visual adaptation to the illumination of a measured color. This is most apparent in the CIELAB color coordinates when the measured color is white. For example, when the CIE XYZ tristimulus values for the stimulus and the reference white are the same, then the CIELAB values are L* = 100, a* = 0, and b* = 0. Under this unique condition, when the CIE XYZ tristimulus values for the stimulus and the reference white are the same, the observer is expected to be visually adapted to the reference white.
This chromatic adaptation attribute of the CIELAB color space is the reason why it is important to state the reference white when using or reporting CIELAB color coordinates. Interpretation of any given color coordinates in the CIELAB color space is relative to the reference white that was used in the calculations of the CIELAB color coordinates.
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).
M. Fairchild, Color Appearance Models, Addison-Wesley, Reading, Massachusetts (1998).
Colorimetry, second edition. CIE Publication 15.2 (1986).