CIE Color-Rendering Index
- Color rendering (of a light source)
- Color rendering index
measure of the degree to which the psychophysical color of an object illuminated by the test illuminant conforms to that of the same object illuminated by the reference illuminant, suitable allowance having been made for the state of chromatic adaptation
- CIE 1974 special color rendering index [Ri]
measure of the degree to which the psychophysical color of a CIE test color sample illuminated by the test illuminant conforms to that of the same sample illuminated by the reference illuminant, suitable allowance having been made for the state of chromatic adaptation
- CIE 1974 general color rendering index [Ra]
mean of the CIE 1974 special color rendering indices for a specified set of eight test color samples
The word rendering is used in different meanings in computer graphics, illuminating engineering, color science, etc. In illuminating engineering and colorimetry, it describes how a scene will look under a specified illumination, compared to some sort of reference. The term color rendering is used in a very restricted form; in the CIE definition, it describes what we nowadays call color fidelity of a light source. Light source color rendering encompasses also color preference and color discrimination.
CIE Color Rendering Index
The first CIE color rendering index was based on the dissimilarity of the spectrum of the test and a reference light source , performing the comparison in a number of spectral bands. It was soon realized that it is more important to describe the color rendering by the description the test source has on different colored samples, and CIE decided to base the new index on the color difference of the color of test samples illuminated with the test source and a reference illuminant of equal correlated color temperature . CIE published an updated, revised edition of this publication in 1974  and republished it later with minor editorial changes .
Colorimetry used in the calculation of above CRI was the best correlate for color appearance at the time of its elaboration. Since the 1960s, the description of color appearance progressed considerably; new color appearance models have been developed . During the past 30 years, a large number of papers were published that partly criticized the CIE Test Sample Method and showed some evidence where the method breaks down and how a new method could be developed, but they were not conclusive enough to be able to come up with a better method (see e.g., [8, 9, 10, 11, 12, 13, 14, 15]).
CIE dealt with the question of updating the color rendering index in several technical committees and submitted several internal recommendations [16, 17] but was unable to come up with a recommendation that would have suited all participants. The latest CIE technical committee (TC 1–69) faces similar fate.
Color Fidelity, Preference, and Discrimination
Parallel to the work to update the color rendering test method, several attempts were made to add further color quality descriptors of light sources, such as flattery/color preference index and color discrimination index. Judd coined the term flattery index already in 1967 . The flattery index was intended to describe whether a light source renders colors in a more pleasant (flattery) way than a reference illuminant. Jerome discussed the differences between flattery and rendition in detail . Later, the word preference was used instead of flattery . Thornton’s calculation showed that color rendering and color preference indices do not have their optimum value at the same spectral distribution and discussed the question of color discrimination as well , see also .
Recently, much interest was raised by increasing the brightness appearance of the illuminated scene, also called apparent or spatial brightness, and investigating how this might correlate with some further descriptors of light source color quality [23, 24, 25, 26, 27].
Recent investigations show that instead of the classical CRI, one would need in the future several indices.
The color fidelity index could be a replacement of the current CIE test method . This new metric  tries to update every aspect of the CIE metric: The CIE-UCS is used only to find the corresponding reference illuminant with equal correlated color temperature; in the other colorimetric calculations, the CIE 10° observer is used, as in color rendering one usually sees larger surfaces and the 10° observer is not flawed by the erroneous V(λ) function. It uses two sets of test samples: one artificial set is constructed to prohibit a visually not supported optimization of the test light source spectrum, and a further large set uses both color constant and color inconstant samples  to find out which colors will be less reliably rendered. Colorimetric calculations are performed in CIECAM02 space with CAM02-UCS extension that provides an advanced chromatic adaptation transform and good uniformity. Square root averaging gives higher weight to larger color differences in calculating the general color fidelity index, and a sigmoid function between ΔE and R avoids negative indices and adjusts the scale to human perception.
A color preference index might be based on the proposal by Davis and Ohno , who recommended in their CIELAB-based formula not to punish sources if they render test samples providing higher chroma and favored hue shift, also allowing for lower numbers if the light source color is extremely reddish or bluish (very low or very high CCT); see also .
Further Color Quality Proposals
There are number of further proposals in the literature that emphasize one aspect of color preference or another. A few titles that might lead the reader to further readings are the following:
Rea and Freyssinier argued that a proper description of light source color quality can be achieved by the help of the CRI and gamut area descriptors [31, 32]. Hashimoto and coworkers described preference based on the feeling of contrast . Smet and coworkers based their metric on memory colors . Szabó and coworkers discussed in their paper the question how light source color quality can be described by evaluating the color harmony in the investigated scene .
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