{{Short description|Ability of a light source to reproduce colors}}

The '''color rendering''' of a [[light source]] refers to its ability to reveal the [[color]]s of various objects faithfully (i.e. to produce [[Metamerism (color)|illuminant metamerism]]) in comparison with an ideal or natural light source. Light sources with good color rendering are desirable in color-critical applications such as [[neonatal care]]<ref>{{Cite web |title=Neonatal intensive care unit lighting: Update and Recommendations |url=https://www.researchgate.net/publication/305334032}}</ref> and [[art restoration]]. It is defined by the [[International Commission on Illumination]] (CIE) as follows:<ref>{{cite web |url=http://www.cie.co.at/publ/abst/17-4-89.html |title=CIE 17.4-1987 International Lighting Vocabulary |access-date=2008-02-19 |archive-url=https://web.archive.org/web/20100227034508/http://www.cie.co.at/publ/abst/17-4-89.html |archive-date=2010-02-27 |url-status=dead}}</ref>

<blockquote> Effect of an illuminant on the [[color appearance]] of objects by conscious or subconscious comparison with their color appearance under a reference illuminant. </blockquote>

== Quantitative measures == A wide variety of quantitative measures have been devised to measure the color rendering of a light source, to the human eye or to the camera. Notable ones include:

* [[Color rendering index]] (CRI), CIE 1974. Currently acknowledged as flawed, but still widely used for consumer lighting. Updated 1999, but rarely followed. * [[#Television lighting consistency index|Television lighting consistency index]] (TLCI), [[European Broadcasting Union|EBU]] 2012. Address the spectral response of cameras and screens, which can have significantly different results with [[high-CRI LED lighting]].<ref name=TLCI>{{cite web |title=Television Lighting Consistency Index 2012 |url=https://tech.ebu.ch/tlci-2012 |website=tech.ebu.ch |language=en |date=31 May 2016}}</ref> * [[#Spectral similarity index|Spectral similarity index]] (SSI), [[Academy of Motion Picture Arts and Sciences|AMPAS]] 2016 (revised 2020). Ditches the concept of color samples in CRI and TLCI to directly address the shape of the spectrum. As there are no color samples, SSI doesn't measure color rendering, but is included here because it is often used to indicate the potential color rendering quality of a test light source as compared to a reference source. Published as [[SMPTE]] 2122.<ref name=ssi>{{cite web |title=Spectral Similarity Index (SSI) |url=https://www.oscars.org/science-technology/projects/spectral-similarity-index-ssi |website=Oscars.org {{!}} Academy of Motion Picture Arts and Sciences |language=en |date=21 April 2017}}</ref> * [[Illuminating Engineering Society|IES]] [[#TM-30|TM-30]], 2015 (revised 2020). A metric related to CRI with updated color transformation, more (99) color samples, and scoring for additional "pleasantness" factors such as gamut size and hue shift. Still uses human participants. Endorsed by CIE in 2015 to replace CRI.<ref name=ES_TM_30>{{cite web |title=Evaluating Color Rendering With TM-30 |author=Michael Royer |date=March 31, 2016 |website=ENERGY STAR Webinar (US DOE) |url=https://www.energystar.gov/sites/default/files/asset/document/TM-30%20ES%20%28Final%29_0.pdf}}</ref>

== Background == Researchers used daylight as the benchmark to which to compare color rendering of electric lights. In 1948, daylight was described as the ideal source of illumination for good color rendering because "it (daylight) displays (1) a great variety of colours, (2) makes it easy to distinguish slight shades of colour, and (3) the colours 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>

== Scales == === Color rendering index === {{main|Color rendering index}}

The color rendering index (CRI) of 1974 is the product of a CIE committee's study on the topic of color rendering. It uses the American colorimetric approach with a panel of human subjects instead of requiring [[spectrophotometry]]. 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]]. The residual difference in chromaticity is resolved with a [[chromatic adaptation transform]] before comparing to the reference illuminant. Each color difference was translated to a sub-score, eight of which are averaged to produce the final score of R<sub>a</sub>.<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=2010-06-17 |archive-url= https://web.archive.org/web/20100611200908/http://www.lrc.rpi.edu/nlpip/publicationDetails.asp?id=901&type=2 |archive-date=2010-06-11 |url-status=dead}}</ref>

=== Television lighting consistency index === As early as 1971, an analogue of CRI for televisions have been devised by workers at the [[BBC]].<ref>Sproson, W. N. & Taylor, E. W. (1971). A colour television illumination consistency index. BBC Research Department Report 1971-45</ref> At that time, the relatively broad-band nature of light sources meant that the CRI still approximated the color rendering for television cameras, an assumption quickly broken by the advent of [[LED lighting]]. As a result, the [[European Broadcasting Union]] re-introduced the concept of a television lighting consistency index (TLCI) in 2012, followed by a television luminaire matching factor (TLMF) in 2013 for mixed lights.<ref name=TLCI/>

To calculate a TLCI, a full measure of the spectral power distribution (SPD) of the light source is first taken. From this SPD a [[correlated color temperature]] (CCT) is found, which provides the reference illuminant. Under the test and reference illuminant, an image of the [[ColorChecker]] is simulated using known reflectivities and the color curves of an average HDTV camera and display. The differences are calculated in CIEDE2000. With the TLMF, the reference is not specified by a CCT, but by a user directly.<ref>{{cite web |author=European Broadcasting Union |title=[Tech 3355] Method for the Assessment of the colorimetric properties of luminaires: The Television Lighting Consistency Index (TLCI-2012) &Television Luminaire Matching Factor (TLMF-2013) |url=https://tech.ebu.ch/docs/tech/tech3355.pdf |access-date=16 November 2021}}</ref>

=== Spectral similarity index === The spectral similarity index (SSI) of 2016 is a scale that completely forgoes the comparison of color samples, instead directly comparing the SPDs of one light source to the reference.<ref name=ssi/> Its developers argue that difference among cameras mean that TLCI can only describe three-chip television cameras, not the more-varied spectral sensitivities of single-chip digital cinema, still cameras, or film.<ref name=ssi-wp>{{cite web |last1=Academy of Motion Picture Arts and Sciences |title=Academy Spectral Similarity Index (SSI): Overview |url=https://www.oscars.org/sites/oscars/files/ssi_overview_2020-09-16.pdf |date=2020-09-16}}</ref> (In theory, [[color gel]]s also introduce variations that are hard to be captured by TLCI.)

The SSI is calculated by taking two integrated, normalized SPDs in the 5-nm intervals from 375 to 675&nbsp;nm and finding a weighted relative difference between them. This weighted relative difference is convolved, and the magnitude of the result is translated into a 100-point value. A low SSI only warns of potential color-rendering issues, but neither confirms the presence of one nor indicates what errors are likely to occur.<ref name=ssi-wp/>

=== TM-30 === <!-- Need a CVG example to show what the Rg "gamut area" is about. --> TM-30 is the current (as of 2021) CIE recommended measure for color rendering as perceived by humans. It generates a large set of outputs, including an overall fidelity index (R<sub>f</sub>), an overall [[gamut]] index (R<sub>g</sub>) for changes in [[colorfulness|chroma]], a gamut shape graph, and detailed values for chroma, hue, and color fidelity for each of the 16 hue ranges, plus color fidelity scores for each of the 99 sample colors. It uses the [[CIECAM02|CAM02-UCS]] color space. The R<sub>f</sub> has been adopted by the CIE as CIE 224:2017 "color fidelity index" (CFI).<ref name="tm30-tuto">{{cite journal |last1=Royer |first1=Michael P. |title=Tutorial: Background and Guidance for Using the ANSI/IES TM-30 Method for Evaluating Light Source Color Rendition |journal=LEUKOS |date=16 March 2021 |volume=18 |issue=2 |pages=191–231 |doi=10.1080/15502724.2020.1860771 |s2cid=233697915 |url=https://www.researchgate.net/publication/350116795}}</ref>

As with other newer scales, TM-30 is calculated from a SPD with reference to a SPD of the same CCT.<ref name="tm30-tuto"/> The uniqueness of TM-30 is that it goes beyond ''fidelity'' (accuracy of color reproduction) to describe other aspects of color rendering. This extra information allows for, e.g. fidelity to be sacrificed for vividness of skin tones under a certain design criterion. Three reference design intents and priority levels are defined in TM-30 Annex E.<ref>{{cite web |title=Using TM-30 to Improve Your Lighting Design – Illuminating Engineering Society |url=https://www.ies.org/fires/using-tm-30-to-improve-your-lighting-design/}}</ref>

=== Other scales === Before the aforementioned scales are devised to replace CRI, a number of other measures have been proposed. None of them have seen wide use, however:

; [[Color rendering index#R96a method|R96<sub>a</sub>]], 1999 : A revision of the CRI to account for the obsolescence/loss of original color samples and some improvements in colorimetry. Uses ColorChecker samples, CIELAB, and CIECAT94.<ref name=CRI99>{{citation|author=CIE|series=Publication 135/2|year=1999|url=http://cie.kee.hu/newcie/publ/abst/135-99.html|title=Colour rendering (TC 1–33 closing remarks)|isbn=3-900734-97-6|publisher=CIE Central Bureau |location=Vienna|access-date=2008-07-16|archive-url=https://web.archive.org/web/20080922013149/http://cie.kee.hu/newcie/publ/abst/135-99.html|archive-date=2008-09-22|url-status=dead}}</ref> ; [[Color quality scale]], 2005 : [[NIST]]-proposed replacement for CRI R<sub>a</sub>. Uses more saturated samples, CIELAB, and [[CIECAM02|CMCCAT2002]]. Modifications to scoring.<ref>{{cite journal |url=http://proceedings.spiedigitallibrary.orgproceeding.aspx?articleid=873209 |title=Toward an improved color rendering metric |author1=Davis, W. |author2=Ohno, Y. |editor-first1=Ian T. |editor-first2=John C. |editor-first3=Tsunemasa |editor-first4=Ian E. |editor-last1=Ferguson |editor-last2=Carrano |editor-last3=Taguchi |editor-last4=Ashdown |journal=Proc. SPIE |volume=5941 |pages=59411G–1 |doi=10.1117/12.615388|series=Fifth International Conference on Solid State Lighting |year=2005 |bibcode=2005SPIE.5941..283D |citeseerx=10.1.1.470.2414 |s2cid=121431482 }}</ref> ; Gamut area index, 2010 : A measure of [[gamut]] area. Use in conjunction with a fidelity measure (such as CRI) predicts preference better than using either alone.<ref>{{cite magazine |title=Color Rendering |publisher=Alliance for Solid-State Illumination Systems and Technologies |magazine=Recommendations for Specifying Color Properties of Light Sources for Retail Merchandising |volume=8 |issue=2 |date=March 2010 |page=6 |access-date=2020-09-14 |url= http://www.lrc.rpi.edu/programs/solidstate/assist/pdf/AR-SpecifyColorRec-March2010.pdf}}</ref> <!-- These are the ones that are mentioned in the CRI article. For more we can pick from: Royer M, Houser KW, David A. 2018b. Chroma Shift and Gamut Shape: Going Beyond Average Color Fidelity and Gamut Area. Leukos. 14(3):149–165. doi:10.1080/15502724.2017.1372203 -->

== Typical values == <!-- Hopefully we can have a table similar to the CRI one, so we can show some sense of comparison between the scales -->

== References == {{reflist}}

===Works cited=== {{Refbegin|2}} * {{cite journal |last=Barnes |first=Bentley T. |title=Band Systems for Appraisal of Color Rendition |journal=[[JOSA]] |volume=47 |issue=12 |date=December 1957 |pages=1124–1129 |doi=10.1364/JOSA.47.001124 |bibcode=1957JOSA...47.1124B}} * {{cite journal |last=Crawford |first=Brian Hewson |title=Measurement of color rendering tolerances |journal=[[JOSA]] |date=December 1959 |volume=49 |issue=12 |pages=1147–1156 |doi=10.1364/JOSA.49.001147 |bibcode=1959JOSA...49.1147C}} * {{cite journal |last=Nickerson |first=Dorothy |title=Light sources and color rendering |journal=[[JOSA]] |date=January 1960 |volume=50 |issue=1 |pages=57–69 |doi=10.1364/JOSA.50.000057 |bibcode=1960JOSA...50...57N}} * {{cite conference |first1=János |last1=Schanda |first2=Norbert |last2=Sándor |year=2003 |title=Colour rendering, past – present – future |pages=76–85 |conference=International Lighting and Colour Conference, Cape Town, South Africa, Nov 2–5, 2003}} {{refend}}

[[Category:Color|rendering]] [[Category:Lighting]]