Editing Color rendering index (section)

==History==

Researchers use daylight as the benchmark to which to compare color rendering of electric lights.  In 1948, daylight was described as the ideal source of [[Illumination (image)|illumination]] for good color rendering because "it (daylight) displays (1) a great variety of colors, (2) makes it easy to distinguish slight shades of color, and (3) the colors of objects around us obviously look natural".<ref>{{cite book |author=P. J. Bouma |year=1948 |title=''Physical aspects of colour; an introduction to the scientific study of colour stimuli and colour sensations'' |publisher=(Eindhoven: Philips Gloeilampenfabrieken (Philips Industries) Technical and Scientific Literature Dept.)}}</ref>

Around the middle of the 20th century, color scientists took an interest in assessing the ability of [[artificial light]]s to accurately reproduce colors. European researchers attempted to describe illuminants by measuring the [[spectral power distribution]] (SPD) in "representative" spectral bands, whereas their North American counterparts studied the [[colorimetric]] effect of the illuminants on reference objects.<ref>American approach is expounded in {{harvtxt|Nickerson|1960}}, and the European approach in {{harvtxt|Barnes|1957}}, and {{harvtxt|Crawford|1959}}. See {{harvtxt|Schanda|Sándor|2003}} for a historical overview.</ref>

The [[International Commission on Illumination|CIE]] assembled a committee to study the matter and accepted the proposal to use the latter approach, which has the virtue of not needing [[spectrophotometry]], with a set of [[Munsell color system|Munsell]] samples. Eight samples of varying hue would be alternately lit with two illuminants, and the color appearance compared. Since no color appearance model existed at the time, it was decided to base the evaluation on color differences in a suitable color space, [[CIE 1964 color space|CIEUVW]]. In 1931, the CIE adopted the first formal system of [[colorimetry]], which is based on the trichromatic nature of the [[human visual system]].<ref name="Color rendering: Beyond pride and prejudice - Rea - 2010 - Color Research &amp; Application - Wiley Online Library" >{{cite journal |last1=Rea |first1=M. S. |last2=Freyssinier |first2=J. P. |title=Color rendering: Beyond pride and prejudice |journal=Color Research and Application |year=2010 |volume=35 |issue=6 |pages=401–409 |doi=10.1002/col.20562}}</ref><ref>{{cite magazine |title=Background |magazine=Guide to Light and Color in Retail Merchandising |volume=8 |issue=1 |date=March 2010 |page=5 |publisher=Alliance for Solid-State Illumination Systems and Technologies |url= http://www.lrc.rpi.edu/programs/solidstate/assist/pdf/AR-ColorGuideforRetailLighting-March2010.pdf}}</ref> CRI is based upon this system of colorimetry.<ref>{{cite web |last1=Rea |first1=M. |last2=Deng |first2=L. |last3=Wolsey |first3=R. |date=2004 |work=NLPIP Lighting Answers |title=Light Sources and Color |location=Troy, NY |publisher=Rensselaer Polytechnic Institute |url=http://www.lrc.rpi.edu/nlpip/publicationDetails.asp?id=901&type=2 |access-date=June 17, 2010 |archive-url= https://web.archive.org/web/20100611200908/http://www.lrc.rpi.edu/nlpip/publicationDetails.asp?id=901&type=2 |archive-date=June 11, 2010 }}</ref>

To deal with the problem of having to compare light sources of different correlated color temperatures (CCT), the CIE settled on using a reference [[black body]] with the same color temperature for lamps with a CCT of under 5000&nbsp;K, or a phase of CIE [[standard illuminant]] D (daylight) otherwise. This presented a continuous range of color temperatures to choose a reference from. Any chromaticity difference between the source and reference illuminants were to be abridged with a von Kries-type [[chromatic adaptation transform]]. There are two extent versions of CRI: the more commonly used R<sub>a</sub> of {{harvtxt|CIE|1995}} (actually from 1974) and R96<sub>a</sub> of {{harvtxt|CIE|1999}}.