The CIE defined two sets of color-matching functions for use as standard observers. The first was the CIE 1931 Standard Colorimetric Observer. The second was the CIE 1964 Supplementary Standard Colorimetric Observer. The difference is based on the field of view used in the collection of the experimental data.
The CIE 1931 Standard Colorimetric Observer is based on experiments with matching fields of two degrees of angular subtense. The upper limit of 2 degrees was imposed to constrain the image to the fovea within each eye of each observer. The fovea contains a dense concentration of cones and no rods, and the cones are our color receptors. Therefore, the color-matching functions for the CIE 1931 Standard Colorimetric Observer describe cone vision within the fovea.
The CIE 1964 Supplementary Standard Colorimetric Observer is based on experiments with matching fields of ten degrees of angular subtense. This larger field extended the image beyond the fovea and gathered information about cone vision outside of the fovea. One of the goals was to determine if cone vision outside the fovea differed from cone vision within the fovea, so efforts were made to discount the central 2-degree field of view in the color-matching experiments of Stiles and Burch (1959) and Speranskaya (1959) which formed the basis for the CIE 1964 Supplementary Standard Colorimetric Observer. Since the larger field also included rods and the scientists sought to exclude rod vision from the experimental results, efforts were made to suppress the influence of the rods. To quote Wyszecki and Stiles, “Thus, the color-matching functions embodied in the CIE 1964 supplementary standard colorimetric observer aim to define the matching properties of the rod-suppressed retina for a large visual field.” In CIE Publication 15.2 (1986), the CIE recommends the use of the color-matching functions of the CIE 1964 Supplementary Standard Colorimetric Observer “whenever correlation with visual colour matching of fields of angular subtense greater than about 4 degrees at the eye of the observer is desired.”
Which set of color-matching functions is relevant for monitor calibration? From one perspective, the act of calibrating a color monitor involves measuring large color patches that are displayed on the screen of the monitor. The patches are clearly larger than a 4-degree field of view. In addition, we usually judge the whiteness (or coolness or warmness of white) of the monitor by looking at a relatively large white patch that is larger than a 4-degree field of view. However, we must keep in mind that monitor calibration is implemented to facilitate the evaluation and editing of photographic images, not large uniform patches of color, and the CIE 1931 Standard Colorimetric Observer is preferred for colorimetry applied to photographic images with detail that is small enough to fit within the 2-degree subtense. Furthermore, the CIE daylight locus is mapped on the CIE 1931 (x,y)-chromaticity diagram with chromaticity coordinates based on the CIE 1931 Standard Colorimetric Observer. Thus, the color-matching functions for the CIE 1931 Standard Colorimetric Observer (2-degree observer) are more appropriate for determining the CIE tristimulus values and chromaticity coordinates that are used for monitor calibration for a digital photography workflow.
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).
R. S. Berns, Billmeyer and Saltzman’s Principles of Color Technology, 3rd Edition, John Wiley & Sons, New York, N.Y. (2000).