CIE 1931 and 1964 Standard Colorimetric Observers: History, Data, and Recent Assessments
Synonyms
Definition
 CIE colormatching functions

Functions \( \overline{x}\left(\lambda \right),\overline{y}\left(\lambda \right),\overline{z}\left(\lambda \right) \) in the CIE 1931 standard colorimetric system or \( {\overline{x}}_{10}\left(\lambda \right),{\overline{y}}_{10}\left(\lambda \right),{\overline{z}}_{10}\left(\lambda \right) \) in the CIE 1964 standard colorimetric system
 CIE 1931 standard colorimetric observer

ideal observer whose colormatching properties correspond to the CIE colormatching functions \( \overline{x}\left(\lambda \right),\overline{y}\left(\lambda \right),\overline{z}\left(\lambda \right) \) adopted by the CIE in 1931
 CIE 1964 standard colorimetric observer

ideal observer whose colormatching properties correspond to the CIE colormatching functions \( {\overline{x}}_{10}\left(\lambda \right),{\overline{y}}_{10}\left(\lambda \right),{\overline{z}}_{10}\left(\lambda \right) \) adopted by the CIE in 1964
Overview
Colors with different spectral compositions can look alike (i.e., be metameric). An important function of colorimetry is to determine which spectral compositions appear metameric. The use of visual colorimeters for this purpose is handicapped by variations in the color matches made among observers classified as having normal color vision, so that different observers make different matches. Visual colorimetry also tends to be timeconsuming. For these reasons, it has long been the practice in colorimetry to make use of sets of mean or standard colormatching functions to calculate tristimulus values for colors: Equality of tristimulus values for a pair of colors indicates that the color appearances of the two colors should match, when viewed under the same conditions by an observer for whom the colormatching functions apply. The use of standard sets of colormatching functions makes the comparison of tristimulus values obtained at different times and locations possible [1]. The standard colorimetric observers are defined by their colormatching functions.
If one uses monochromatic test stimuli of equal power, wavelength to wavelength along the visible spectrum (theoretically between 360 nm and 830 nm, practically between 380 nm and 780 nm), and matches these monochromatic radiations with the three selected matching stimuli (in different wavelength regions, one of the matching stimuli has to be added to the test stimulus to get a color match, and in that case the matching stimulus is shown in Eq. 1 as it would be subtracted from the other two matching stimuli), one can build up the colormatching functions.
In many applications, it is inconvenient to use negative lobes of the \( \overline{r}\left(\lambda \right),\overline{g}\left(\lambda \right),\overline{b}\left(\lambda \right) \) functions; therefore, the CIE decided in 1931 to transform the \( \overline{r}\left(\lambda \right),\overline{g}\left(\lambda \right),\overline{b}\left(\lambda \right) \) functions using a matrix transformation to imaginary CMFs (nonreal in the sense that they cannot be physically realized). The transformed CMFs are the \( \overline{x}\left(\lambda \right) \), \( \overline{y}\left(\lambda \right) \), and \( \overline{z}\left(\lambda \right) \) functions, and the tristimulus values determined using these functions are the X, Y, and Z tristimulus values. Their calculation is similar at those of the R,G, and B values shown in Eq. 2.
The original colormatching experiments were conducted with small, approximately 2° diameter homogeneous color patches, seen foveally. The central part of the fovea is covered by a yellow pigmentation, the yellow spot or macula lutea. If largercolored fields are viewed or slightly offaxis objects are viewed, the above CMFs do not hold anymore, as the yellow pigmentation absorbs light in a part of the visible spectrum.
Regarding the use of the tristimulus values and further colorimetric calculations, see chapters on “CIE chromaticity coordinates,” “CIE chromaticity diagram,” “CIE illuminants,” and “CIELAB” and other chapters on advanced colorimetry.
Short History of the CIE Colorimetric Observer
CIE colorimetry is based on the tristimulus theory developed by the greatest scientists of the nineteenth century, including Thomas Young, Helmholtz, and Maxwell (see [5]). Maxwell’s demonstrations and ideas, in particular, lead to the specification of the trichromatic theory; he showed, e.g., the three color mixture curves of the spectrum and plotted the spectrum locus in the color triangle.
Photometry and colorimetry were further developed in the USA that lead to the socalled OSA excitation curves.
During the second decade of the twentieth century, two groups in the UK performed detailed investigations of color matching: John Guild at the NPL and David Wright at the Imperial College, London. The two researchers used different primaries, and it was a great surprise that after their transformation into a common system, they matched reasonably well.
During those years the CIE formulated the wish to develop a colorimetric description of colored glasses used for traffic control. At the 1931 meetings of the CIE, the USA and UK groups discussed in detail the pros and cons of different systems and finally agreed that a mean of the Guild and Wright data should be adopted, but transformed to a system with nonnegative CMFs. These were the \( \overline{x}\left(\lambda \right) \), \( \overline{y}\left(\lambda \right) \), and \( \overline{z}\left(\lambda \right) \) CMFs that we still use today.
One major problem with the CIE 1931 XYZ system is that the values of the primaries were determined photometrically, so that the \( \overline{x}\left(\lambda \right) \), \( \overline{y}\left(\lambda \right) \), and \( \overline{z}\left(\lambda \right) \) functions had to be reconstructed using V(λ) function, the visibility function (now called spectral luminous efficiency for photopic vision). As it turned out later, the V(λ) dataset is in error in the blue part of the spectrum, and this error has been transferred to the colormatching functions.
Experiments carried out in the 1950s at NPL by Stiles and Burch led to a new set of 10deg CMFs; this time the primaries and test lights were radiometrically calibrated, so they were not contaminated by photometric errors. As mentioned in the Overview, the NPL data were harmonized with data measured by Speranskaya, and these were standardized as the CIE 10° observer in 1964.
CrossReferences
References
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 2.Schanda, J.: Chapter 3: CIE colorimetry. In: Schanda, J. (ed.) CIE Colorimetry – Understanding the CIE System. Wiley Interscience, Hoboken (2007)Google Scholar
 3.Stiles, W.S., Burch, J.M.: N.P.L. colourmatching investigation: Final report (1958). Opt. Acta 6, 1–26 (1959)ADSCrossRefGoogle Scholar
 4.Speranskaya, N.I.: Determination of spectral color coordinates for twentyseven normal observers. Opt. Spectr. 7, 424–428 (1959)ADSGoogle Scholar
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