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{{Short description|Accuracy of light source in showing color of objects}}
{{use mdy dates|date=September 2020|cs1-dates=ly}}▼
{{Use American English|date=March 2025}}
[[File:Simple spectroscope.jpg|thumb|upright=1.35
A '''color rendering index''' ('''CRI''') is a quantitative measure of the ability of a [[light source]] to reveal the [[color]]s of various objects faithfully in comparison with a natural or standard light source.
▲[[File:Simple spectroscope.jpg|thumb|upright=1.35|right|Emitted light spectrum determines the CRI of the lamp. An incandescent lamp (middle image) has a continuous spectrum and therefore a higher CRI than a fluorescent lamp (lower image). The top image shows the setup of the demonstration from above.]]
▲[[File:AmbientLED.png|thumb|upright=1.35|right|Color rendering index shown as color accuracy]]
The CRI of a light source does not indicate the apparent color of the light source; that information is given by the [[Color temperature|correlated color temperature
▲The CRI of a light source does not indicate the apparent color of the light source; that information is given by the [[correlated color temperature]] (CCT). The CRI is determined by the light source's [[spectrum]]. An [[incandescent lamp]] has a continuous spectrum, a [[fluorescent lamp]] has a discrete line spectrum; the incandescent lamp has the higher CRI.
The value often quoted as "CRI" on commercially available lighting products is properly called the CIE R<sub>a</sub> value, "CRI" being a general term and CIE R<sub>a</sub> being the international standard color rendering index.
Numerically, the highest possible CIE R<sub>a</sub> value is 100 and would only be given to a source whose [[spectrum]] is identical to
CIE R<sub>a</sub>'s ability to predict color appearance has been criticized in favor of measures based on [[color appearance model]]s, such as [[CIECAM02]] and for [[daylight]] simulators, the CIE [[metamerism index]].<ref>{{citation |first2=János |last2=Schanda |first1=Norbert |last1=Sándor |journal=Lighting Research and Technology |volume=38 |issue=3 |title=Visual colour rendering based on colour difference evaluations |date=September 1, 2006 |pages=225–239 |doi=10.1191/1365782806lrt168oa
| last1 = Guo
| first1= Xin
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| pages = 183–199
| doi = 10.1191/1365782804li112oa
}}</ref><ref name="CIE1995">{{citation
|author = CIE
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|publisher = Commission Internationale de l'Eclairage
|___location = Vienna
|access-date =
|archive-url = https://web.archive.org/web/20080103162323/http://www.cie.co.at/publ/abst/13-3-95.html
|archive-date = January 3, 2008
}}▼
(A verbatim re-publication of the 1974, second edition. Accompanying disk [http://www.cie.co.at/publ/abst/d008.html D008: Computer Program to Calculate CRIs]. {{Webarchive|url=https://web.archive.org/web/20080327023340/http://www.cie.co.at/publ/abst/d008.html |date=
▲}}
▲ (A verbatim re-publication of the 1974, second edition. Accompanying disk [http://www.cie.co.at/publ/abst/d008.html D008: Computer Program to Calculate CRIs]. {{Webarchive|url=https://web.archive.org/web/20080327023340/http://www.cie.co.at/publ/abst/d008.html |date=2008-03-27 }})</ref> New standards, such as the [[IES TM-30]], resolve these issues and have begun replacing the usage of CRI among professional lighting designers.<ref>Illuminating Engineering Society. 2018. ''IES Method for Evaluating Light Source Color Rendition, IES Technical Memorandum (TM) 30-18''.</ref> However, CRI is still common among household lighting products.
==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
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 & 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=
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 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
==Test method==
The CRI is calculated by comparing the color rendering of the test source to that of a "perfect" source, which is a [[black
{{citation
|title=Color rendering of light sources: CIE method of specification and its application
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# Illuminate the first eight standard samples, from the fifteen listed below, alternately using both sources.
# Using the 2° standard observer, find the co-ordinates of the light reflected by each sample in the [[CIE 1964 color space]].
# Chromatically adapt each sample by a [[Chromatic adaptation#
# For each sample, calculate the [[Euclidean distance]] <math>\Delta E_i</math> between the pair of co-ordinates.
# Calculate the special (i.e., particular) CRI using the formula <math>R_i = 100 - 4.6 \Delta E_i</math><ref>Per {{harvtxt|Schanda|Sándor|2003}}, {{harvtxt|Schanda|2002}} and, as demonstrated in the [[#Example|Example]] section, the coefficient was chosen as 4.6 so that the CRI of the CIE [[standard illuminant]] F4, an obsolete "warm white" calcium halophosphate [[fluorescent lamp]] would be 51. Today's fluorescent "[[full-spectrum light]]s" boast CRIs approaching 100; e.g., [http://www.truesun.com/philips_TL950.php Philips TL950] {{webarchive|url=https://web.archive.org/web/20071012085113/http://www.truesun.com/philips_TL950.php |date=
# Find the general CRI (R<sub>a</sub>) by calculating the [[arithmetic mean]] of the special CRIs.
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===Chromatic adaptation===
[[File:CIE CRI TCS under FL4.svg|thumb|Chromatic adaptation of TCSs lit by CIE FL4 (short, black vectors, to indicate before and after) to a [[black body]] of 2940 K (cyan circles)]]
{{harvtxt|CIE|1995}} uses this von Kries chromatic transform equation to find the [[corresponding color]] (''u''<sub>''c'',''i''</sub>, ''v''<sub>''c'',''i''</sub>) for each sample. The mixed subscripts (''t'', ''i'') refer to the [[inner product]] of the test illuminant spectrum and the spectral reflexivity of sample ''i'':
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| TCS01
| 7,5 R 6/4
| Light
| style="background:#e8a7b0;" |
|-
| TCS02
| 5 Y 6/4
| Dark
| style="background:#ccb184;" |
|-
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|}
As specified in {{harvtxt|CIE|1995}}, the original test color samples (TCS) are taken from an early edition of the [[Farnsworth-Munsell 100 hue test|Munsell]] Atlas. The first eight samples, a subset of the eighteen proposed in {{harvtxt|Nickerson|1960}}, are relatively low saturated colors and are evenly distributed over the complete range of hues.<ref>See the CIE 1960 UCS diagram towards the end of the [[#Example|Example]] section.</ref> These eight samples are employed to calculate the general color rendering index <math>R_a</math>. The last six samples provide supplementary information about the color rendering properties of the light source; the first four for high saturation, and the last two as representatives of well-known objects. The reflectance spectra of these samples may be found in {{harvtxt|CIE|2004}},<ref>[http://photometry.kriss.re.kr/wiki/img_auth.php/4/47/CIE_TCS.csv TCS spectra in CSV form] {{webarchive|url=https://web.archive.org/web/20090211042805/http://photometry.kriss.re.kr/wiki/img_auth.php/4/47/CIE_TCS.csv |date=
[[File:CIE CRI TCS SPDs.svg|300px|right]]
{{-}}
==R96<sub>a</sub> method==
[[File:CIE CRI TCS chromaticities.svg|300px|right]]
In the CIE's 1991 Quadrennial Meeting, Technical Committee 1-33 (Color Rendering) was assembled to work on updating the color rendering method, as a result of which the R96<sub>a</sub> method was developed. The committee was dissolved in 1999, releasing {{harvtxt|CIE|1999}}, but no firm recommendations, partly due to disagreements between researchers and manufacturers.<ref>"Authors' response to SA Fotios and JA Lynes" in {{harvtxt|Sándor|Schanda|2005}}: "The main message of our investigations is an answer to the lamp industry, who still use the colour rendering index and the lamp efficacy as parameters for optimizing their lamp spectra, and have turned down the work of CIE TC 1-33 by stating that there are not enough visual experiments showing the shortcomings of the CIE colour rendering calculation method."{{
The R96<sub>a</sub> method has a few distinguishing features:{{sfnp|Bodrogi|2004|p=11|loc=Past research to improve the CRI}}
* [[#New test color samples|A new set of test color samples]]
* Six reference illuminants: D65, D50, [[black bodies]] of 4200 K, 3450 K, 2950 K, and 2700 K.
* A new chromatic adaptation transform: CIECAT94.
* Color difference evaluation in CIELAB.
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<math display="block">v = \frac{6 \times 0.4031}{-2 \times 0.4402 + 12 \times 0.4031 + 3} = 0.3477.</math>
[[Image:CIE illuminant F4 and a blackbody of 2938K.svg|thumb|Relative SPD of FL4 and a [[black body]] of equal CCT. Not normalized.]]
Examining the CIE 1960 UCS reveals this point to be closest to 2938 K on the Planckian locus, which has a coordinate of (0.2528, 0.3484). The distance of the test point to the locus is under the limit (5.4×10<sup>−3</sup>), so we can continue the procedure, assured of a meaningful result:
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[[File:CIE CRI TCS under FL4.svg|thumb|upright=2.8|center|The cyan circles indicate the TCS under the ''reference'' illuminant. The short, black, vectors indicate the TCS under the ''test'' illuminant, before and after chromatic adaptation transformation (CAT). (The vectors are short because the white points are close.) The post-CAT end of the vector lies NW, mirroring the chromaticity vector between the reference and test illuminants.
The special CRIs are reflected in the length of the dotted lines linking the chromaticities of the samples under the reference and chromatically adapted test illuminants, respectively. Short distances, as in the case of TCS3, result in a high special CRI (87.9), whereas long distances, as in the case of TCS8, result in a low special CRI (10.4). In simpler terms, TCS3 reproduces better under FL4 than does TCS8 (relative to a [[black body]]).]]
== Typical values ==
{| class="wikitable" style="float:right; margin:5px;"
|-
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|}
A reference source, such as
== Special value: R9 ==
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R9 is one of the numbers of R<sub>i</sub> refers to test color samples (TCS), which is one score in extended CRI. It is the number rates the light source's color revealing ability towards TCS 09. And it describes the specific ability of light to accurately reproduce the red color of objects. Many lights manufacturers or retailers do not point out the score of R9, while it is a vital value to evaluate the color rendition performance for film and video lighting, as well as any applications that need high CRI value. So, generally, it is regarded as a supplement of color rendering index when evaluating a high-CRI light source.
R9 value, TCS 09, or in other words, the red color is the key color for many lighting applications, such as film and video lighting, textile printing, image printing, skin tone, medical lighting, and so on. Besides, many other objects which are not in red color, but actually consists of different colors including red color. For instance, the skin tone is impacted by the blood under the skin, which means that the skin tone also includes red color, although it looks much like close to white or light yellow. So, if the R9 value is not good enough, the skin tone under this light will be more paleness or even greenish in your eyes or cameras.<ref>{{cite web |url=https://www.mmsvideolight.com/why-r9-is-important-for-high-cri-lighting/ |title=Why R9 is important for High CRI Lighting? }}{{Dead link|date=March 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
== Criticism ==
{{missing information|section|CRI penalization of "pleasantly inaccurate" colorfulness-enhancing light – an important point in CQS and TM-30|date=November 2021}}
Ohno and others have criticized CRI for not always correlating well with subjective color rendering quality in practice, particularly for light sources with spiky emission spectra such as fluorescent lamps or white [[Light-emitting diode|LED]]s. Another problem is that the CRI is discontinuous at 5000 K,<ref>"Authors' response to SA Fotios and JA Lynes" in {{harvtxt|Sándor|Schanda|2005}}: "It is quite obvious that just at 5000 K, where the reference illuminant has to be changed, the present system shows discontinuity."{{
* The color space in which the color distance is calculated (CIEUVW) is obsolete and nonuniform. Use [[CIELAB]] or [[CIELUV]] instead.
* The chromatic adaptation transform used ([[Von Kries transform]]) is inadequate. Use [[CIECAM02#Chromatic adaptation|CMCCAT2000]] or [[CIECAM02|CIECAT02]] instead.
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=== Alternatives ===
{{Main|Color rendering#Scales}}
▲{{update|section|inaccurate=yes|date=November 2021}}
{{Update|section|inaccurate=yes|date=November 2021}}
{{
For a comprehensive review of alternative color rendering indexes see {{harvtxt|Guo|Houser|2004}}.
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{{harvtxt|Smet|2011}} reviewed several alternative quality metrics and compared their performance based on visual data obtained in nine psychophysical experiments. It was found that a geometric mean of the GAI index and the CIE Ra correlated best with naturalness (r=0.85), while a color quality metric based on memory colors (MCRI<ref>Smet K. A. G., Ryckaert W. R., Pointer M. R., Deconinck G., Hanselaer P. Colour Appearance Rating of Familiar Real Objects. Colour Research and Application 2011; 36(3):192–200.</ref>) correlated best for preference (''r'' = 0.88). The differences in performance of these metrics with the other tested metrics (CIE Ra; CRI-CAM02UCS; CQS; RCRI; GAI; geomean (GAI, CIE Ra); CSA; Judd Flattery; Thornton CPI; MCRI) were found to be statistically significant with ''p'' < 0.0001.<ref>Smet K. A. G., Ryckaert W. R., Pointer M. R., Deconinck G., Hanselaer P. [http://www.opticsinfobase.org/view_article.cfm?gotourl=http%3A%2F%2Fwww.opticsinfobase.org%2FDirectPDFAccess%2F3AAAA211-C63E-79CC-4E0A0772E17419BA_212731.pdf%3Fda%3D1%26id%3D212731%26seq%3D0%26mobile%3Dno&org= Correlation between color quality metric predictions and visual appreciation of light sources].</ref>
Dangol, et al., performed psychophysical experiments and concluded that people's judgments of naturalness and overall preference could not be predicted with a single measure, but required the joint use of a fidelity-based measure (e.g., Qp) and a gamut-based measure (e.g., Qg or GAI.).<ref>{{
Due to the criticisms of CRI many researchers have developed alternative metrics, though relatively few of them have had wide adoption.
==== Gamut area index (GAI) ====
Developed in 2010 by Rea and Freyssinier, the gamut area index (GAI) is an attempt to improve over the flaws found in the CRI.<ref>{{cite journal | last1 = Rea | first1 = M. S. | last2 = Freysinnier-Nova | first2 = J. P. | year = 2008| title = Color rendering: A tale of two metrics | journal = Color Research and Application | volume = 33 | issue = 3| pages = 192–202 | doi = 10.1002/col.20399 }}</ref> They have shown that the GAI is better than the CRI at predicting color discrimination on standardized Farnsworth-Munsell 100 Hue Tests and that GAI is predictive of color saturation.<ref name="Color rendering: Beyond pride and prejudice - Rea - 2010 - Color Research & Application - Wiley Online Library" /> Proponents of using GAI claim that, when used in conjunction with CRI, this method of evaluating color rendering is preferred by test subjects over light sources that have high values of only one measure. Researchers recommend a lower and an upper limit to GAI. Use of LED technology has called for a new way to evaluate color rendering because of the unique spectrum of light created by these technologies. Preliminary tests have shown that the combination of GAI and CRI used together is a preferred method for evaluating color rendering.<ref>{{cite magazine |title=Light Levels |publisher=Alliance for Solid-State Illumination Systems and Technologies |magazine=Guide to Light and Color in Retail Merchandising |volume=8 |issue=1 |page=12 |date=March 2010 |url= http://www.lrc.rpi.edu/programs/solidstate/assist/pdf/AR-ColorGuideforRetailLighting-March2010.pdf |access-date=
==== Color quality scale (CQS) ====
{{Further|Color quality scale}}
{{harvtxt|Pousset|Obein|Razet|2010}} developed a psychophysical experiment in order to evaluate light quality of LED lightings. It is based on colored samples used in the "color quality scale". Predictions of the CQS and results from visual measurements were compared.
== Film and video high-CRI LED lighting ==
{{Further|High-CRI LED lighting}}
Problems have been encountered attempting to use LED lighting on film and video sets. The color spectra of LED lighting primary colors does not match the expected color wavelength bandpasses of film emulsions and digital sensors. As a result, color rendition can be completely unpredictable in optical prints, transfers to digital media from film (DIs), and video camera recordings. This phenomenon with respect to motion picture film has been documented in an LED lighting evaluation series of tests produced by the [[Academy of Motion Picture Arts and Sciences]] scientific staff.<ref>
{{cite web
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| title = Solid State Lighting Report
| date = September 3, 2014
To that end, various other metrics such as the TLCI (television lighting consistency index) have been developed to replace the human observer with a camera observer.<ref>
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| title = EBU Technology & Innovation - Television Lighting Consistency Index 2012
| date = May 31, 2016
| url = http://www.gtc.org.uk/tlci-results.aspx
| title = The Guild of Television Cameramen: TLCI Results
| access-date =
| archive-url = https://web.archive.org/web/20140903072219/http://www.gtc.org.uk/tlci-results.aspx
| archive-date = September 3, 2014
}}</ref> Some manufacturers say that their products have TLCI values of up to 99.<ref>
{{cite web
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== Sources ==
<!-- Harvard citations used. Please do not correct "colour" to color in the references and quotes. -->
* {{citation
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==External links==
* [http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/lightsources/appendixb1.asp MATLAB script for calculating measures of light source color], [[Rensselaer Polytechnic Institute]], 2004.
* [http://stacks.iop.org/0026-1394/46/704 Uncertainty evaluation for measurement of LED colour<!-- Should be Colour not Colour, it's part of an article title, do not "correct" it--> , Metrologia]▼
▲* [http://stacks.iop.org/0026-1394/46/704 Uncertainty evaluation for measurement of LED colour, Metrologia]
* [http://www1.eere.energy.gov/buildings/ssl/cri_leds.html Color Rendering Index and LEDs], [http://www.eere.energy.gov/ United States Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE)][bad link]
* [http://www.edaphic.com.au/knowledge-base/articles/light-articles/ What is the difference between CRI and CQS?], Edaphic Scientific Knowledge Base
* [https://www.lumens.com/light-bulb-facts/color-rendering-index.html Understanding color rendering index for lighting], Lumens
[[Category:Color|rendering index]]▼
▲[[Category:Color]]
[[Category:Lighting]]
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