Color rendering index: Difference between revisions

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Revision as of 15:05, 6 January 2022

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{{Short description|Measure of ability of a light source to reproduce colors in comparison with a standard light source}}{{American English editnotice}}

{{use mdy dates|date=September 2020|cs1-dates=ly}}

{{Other uses|CRI (disambiguation){{!}}CRI}}

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[[File:AmbientLED.png|thumb|upright=1.35|right|Color rendering index shown as color accuracy]]

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 ana idealnatural or naturalstandard light source. Light sources with a high CRI are desirable in color-critical applications such as [[Neonatal nursing|neonatal care]]{{Citation needed|date=December 2021}} and [[Conservation and restoration of cultural property|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>

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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]] (CCT)]]. The CRI is determined by the light source's [[spectrum]]. An [[incandescent lamp]] has a [[continuous spectrum]], a [[fluorescent lamp]] has a discrete [[Emission spectrum|line spectrum]]; implying that the incandescent lamp has the higher CRI.

The value often quoted as "CRI" on commercially available lighting products is properly called the CIE R_a value, "CRI" being a general term and CIE R_a being the international standard color rendering index.

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==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 for good color rendering because "it (daylight) displays (1) a great variety of colourscolors, (2) makes it easy to distinguish slight shades of colourcolor, and (3) the colourscolors 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>

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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''&nbsp;= 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''&nbsp;< 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>{{citation|last1=Dangol|first1=R.|title=Subjective preferences and colour quality metrics of LED light sources|date=December 2013|journal=Lighting Research and Technology|volume=45|issue=6|pages=666–688|doi=10.1177/1477153512471520|issn=1477-1535|last2=Islam|first2=M.|last3=Hyvärinen|first3=M.|last4=Bhusal|first4=P.|last5=Puolakka|first5=M.|last6=Halonen|first6=L.|s2cid=109981392}}</ref> They carried out further experiments in real offices evaluating various spectra generated for combination existing and proposed colourcolor rendering metrics.<ref>{{cite journal|last1=Dangol|first1=R|last2=Islam|first2=MS|last3=Hyvärinen|first3=M|last4=Bhushal|first4=P|last5=Puolakka|first5=M|last6=Halonen|first6=L|year=2015|title=User acceptance studies for LED office lighting: Preference, naturalness and colourfulness|journal=Lighting Research & Technology|volume=47|pages=36–53|doi=10.1177/1477153513514424|s2cid=110803300}}</ref><ref>{{cite journal|last1=Islam|first1=MS|last2=Dangol|first2=R|last3=Hyvärinen|first3=M|last4=Bhusal|first4=P|last5=Puolakka|first5=M|last6=Halonen|first6=L|year=2013|title=User acceptance studies for LED office lighting: Lamp spectrum, spatial brightness and illuminance|journal=Lighting Research & Technology|volume=47|pages=54–79|doi=10.1177/1477153513514425|s2cid=109592929}}</ref><ref>{{cite journal|last1=Baniya|first1=R. R.|last2=Dangol|first2=R.|last3=Bhusal|first3=P.|last4=Wilm|first4=A.|last5=Baur|first5=E.|last6=Puolakka|first6=M.|last7=Halonen|first7=L.|year=2015|title=User-acceptance studies for simplified light-emitting diode spectra|journal=Lighting Research and Technology|volume=47|issue=2|pages=177–191|doi=10.1177/1477153513515264|s2cid=112031599}}</ref>

Due to the criticisms of CRI many researchers have developed alternative metrics, though relatively few of them have had wide adoption.

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* {{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}}

* {{citation|author=CIE|title=CIE Colorimetric and Colour Rendering Tables|year=2004|series=[http://www.colour.org/tc8-04/Data/Render.txt Disk D002, Rel 1.3] |url=http://www.cie.co.at/publ/abst/d002.html |access-date=2008-02-16 |archive-url=https://web.archive.org/web/20080305120337/http://www.cie.co.at/publ/abst/d002.html|archive-date=2008-03-05|url-status=dead}}

* {{citation|author=CIE|series=Publication 177:2007|year=2007|url=http://www.slg.ch/pdf/publikation177.pdf |title=Colour rendering of white LED light sources|isbn=978-3-901906-57-2|publisher=CIE Central Bureau |___location=Vienna}}{{Dead link|date=January 2019 |bot=InternetArchiveBot |fix-attempted=yes }}. Carried out by TC 1-69: Colourcolor Rendering of White Light Sources.

* {{citation|title=Band Systems for Appraisal of Color Rendition|journal=[[JOSA]]|volume=47|issue=12|date=December 1957|first=Bentley T.|last=Barnes|pages=1124–1129|doi=10.1364/JOSA.47.001124|bibcode=1957JOSA...47.1124B}}

* {{citation|first=Peter|last=Bodrogi|contribution=Colour rendering: past, present(2004), and future |year=2004|title=CIE Expert Symposium on LED Light Sources|pages=10–12 |url=http://cie2.nist.gov/LED_Sympo_2004/CIE_LED_symp04_prog_abstr.pdf|access-date=2008-07-18|archive-url=https://web.archive.org/web/20110721055141/http://cie2.nist.gov/LED_Sympo_2004/CIE_LED_symp04_prog_abstr.pdf|archive-date=2011-07-21|url-status=dead}}

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* [http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/lightsources/appendixb1.asp MATLAB script for calculating measures of light source color], [[Rensselaer Polytechnic Institute]], 2004.

* [https://web.archive.org/web/20071109105911/http://www.lightinglab.fi/teaching/217/CRI_calculation.xls Excel spreadsheet with a cornucopia of data], Lighting Laboratory of the [[Helsinki University of Technology]] (Note: Cell contents in both sheets are password protected. It may be possible to unlock the individual worksheets using AAAAAAABABB/)

* [http://stacks.iop.org/0026-1394/46/704 Uncertainty evaluation for measurement of LED colourcolor, 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]

* [https://web.archive.org/web/20110718171338/http://www.lrc.rpi.edu/programs/solidstate/assist/recommends/lightcolor.asp Alliance for Solid State Illumination Systems and Technologies, Color Rendering]