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| [[Image:Simple spectroscope.jpg|thumb|300px|right|Emitted light spectrum determines the color rendering index of the lamp.]]
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| [[File:AmbientLED.png|thumb|300px|right|Color rendering index shown as Color Accuracy.]]
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| The '''color rendering index''' ('''CRI'''), sometimes called '''color rendition index''', is a quantitative measure of the ability of a [[light source]] to reveal the [[color]]s of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as [[photography]] and [[cinematography]].<ref>
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| {{Cite web
| |
| |url=http://www.lightingdesignlab.com/articles/cri/cribig.htm
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| |title=Compare these images of a color chart taken under one light source with CRI 70 and another with CRI 85
| |
| |publisher=Lightingdesignlab.com
| |
| |date=
| |
| |accessdate=2009-04-16
| |
| }}</ref> It is defined by the [[International Commission on Illumination]] (CIE, in French) 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}}</ref>
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|
| |
|
| <blockquote>'''Color rendering''': Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant</blockquote>
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|
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|
| The CRI of a light source does not indicate the apparent color of the light source; that information is under the rubric of the [[correlated color temperature]] (CCT). In the pictures at right it can be noticed that the spectra have different structure: the [[incandescent lamp]] gives a continuum, and the [[fluorescent lamp]] gives separate lines from the mercury emission.
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| | |
| CRI'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|first1=János|last1=Schanda|first2=Norbert|last2=Sándor|journal=Lighting Research and Technology|volume=38|issue=3|title=Visual colour rendering based on colour difference evaluations |year=2005|pages=225–239|doi=10.1191/1365782806lrt168oa}}. A conference version of this article can be accessed for free: {{citation|first1=János|last1=Schanda|first2=Norbert|last2=Sándor|contribution=Visual colour-rendering experiments|title=AIC Colour '05: 10th Congress of the International Colour Association|year=2005|pages=511–514|url=http://www.knt.vein.hu/staff/schandaj/SJCV-Publ-2005/521.pdf}}</ref>
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| CRI is not a good indicator for use in visual assessment, especially for sources below 5000 [[kelvin]] (K).<ref>
| |
| {{citation
| |
| | last1 = Guo
| |
| | first1= Xin
| |
| | last2 = Houser
| |
| | first2= Kevin W.
| |
| | year = 2004
| |
| | title = A review of colour rendering indices and their application to commercial light sources
| |
| | journal = Lighting Research and Technology
| |
| | volume = 36
| |
| | issue = 3
| |
| | pages = 183–199
| |
| | doi = 10.1191/1365782804li112oa
| |
| }}</ref><ref name=CIE1995>
| |
| {{citation
| |
| |author=CIE
| |
| |series=Publication 13.3
| |
| |year=1995
| |
| |url=http://www.cie.co.at/publ/abst/13-3-95.html
| |
| |title=Method of Measuring and Specifying Colour Rendering Properties of Light Sources
| |
| |isbn=978-3-900734-57-2
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| |publisher=Commission Internationale de l'Eclairage
| |
| |location=Vienna}}
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| (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])
| |
| </ref>
| |
| | |
| Numerically, the highest possible CRI is 100, for a [[Black body]] (incandescent lamps are effectively [[Black body|blackbodies]]), dropping to negative values for some light sources. Low-pressure sodium lighting has negative CRI; fluorescent lights range from about 50 for the basic types, up to about 90 for the best tri-phosphor type. Typical LEDs have about 80+ CRI, while some manufacturers claim that their LEDs have achieved up to 98 CRI.<ref>http://www.yujiintl.com/high-cri-led-lighting</ref>
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| | |
| A newer version of the CRI, R96<sub>a</sub>, has been developed, but it has not replaced the better-known R<sub>a</sub> general color rendering index.{{citation needed|date=December 2010}}
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| | |
| ==History==
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| | |
| Researchers use daylight as the benchmark to which to compare color rendering of electric lights. In 1948, Bouma described daylight 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>
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| 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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| 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" >Rea, M.S., and Freyssinier, J.P. "Color rendering: Beyond pride and prejudice." Color Research and Application, (2010). Epub January 7, 2010 online at: < http://www3.interscience.wiley.com/cgi-bin/fulltext/123234510/HTMLSTART >. {{doi|10.1002/col.20562}}.</ref><ref>http://www.lrc.rpi.edu/programs/solidstate/assist/pdf/AR-ColorGuideforRetailLighting-March2010.pdf</ref> CRI is based upon this system of colorimetry.<ref>Rea M., L. Deng, and R. Wolsey. 2004. NLPIP Lighting Answers: Light Sources and Color. Troy, NY: Rensselaer Polytechnic Institute; National Lighting Product Information Program. Available online at: http://www.lrc.rpi.edu/nlpip/publicationDetails.asp?id=901&type=2</ref>
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| | |
| 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]].
| |
| | |
| ==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 body radiator for sources with correlated color temperatures under 5000 K, and a phase of daylight otherwise (e.g. [[CIE Standard Illuminant D65|D65]]). [[Chromatic adaptation]] should be performed so that like quantities are compared. The ''Test Method'' (also called ''Test Sample Method'' or ''Test Color Method'') needs only [[colorimetric]], rather than [[spectrophotometric]], information.<ref name=CIE1995/><ref>
| |
| {{citation
| |
| |title=Color rendering of light sources: CIE method of specification and its application
| |
| |first1=Dorothy
| |
| |last1=Nickerson
| |
| |first2=Charles W.
| |
| |last2=Jerome
| |
| |journal=Illuminating Engineering
| |
| |date=April 1965
| |
| |pages=262–271
| |
| |volume=60
| |
| |issue=4
| |
| |publisher=[[Illuminating Engineering Society of North America|IESNA]]
| |
| }}</ref>
| |
| | |
| [[Image:CIE 1960 UCS.png|thumb|CIE 1960 UCS. Planckian locus and co-ordinates of several illuminants shown in illustration below.]]
| |
| [[image:Planckian-locus.png|thumb|(u,v) chromaticity diagram with several CIE illuminants.]]
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| | |
| # Using the [[CIE 1931 color space#CIE standard observer|2° standard observer]], find the [[chromaticity]] co-ordinates of the test source in the [[CIE 1960 color space]].<ref>Note that when CRI was designed in 1965, the most perceptually uniform chromaticity space was the [[CIE 1960 color space|CIE 1960 UCS]], the [[CIELUV|CIE 1976 UCS]] not yet having been invented.</ref>
| |
| # Determine the [[correlated color temperature]] (CCT) of the test source by finding the closest point to the [[Planckian locus]] on the (''u'',''v'') chromaticity diagram.
| |
| # If the test source has a CCT<5000 K, use a black body for reference, otherwise use CIE [[standard illuminant]] D. Both sources should have the same CCT.
| |
| # Ensure that the chromaticity distance (DC) of the test source to the Planckian locus is under 5.4×10<sup>−3</sup> in the CIE 1960 UCS. This ensures the meaningfulness of the result, as the CRI is only defined for light sources that are approximately white.<ref>{{harv|CIE|1995}}, [http://resodance.com/ali/CRI_prob.html Section 5.3: Tolerance for reference illuminant]</ref> <math>DC={\Delta}_{uv}=\sqrt{(u_r-u_t)^2+(v_r-v_t)^2}</math>
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| # Illuminate the first eight standard samples, from the fifteen listed below, alternately using both sources.
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| # 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#von Kries transform|von Kries transform]].
| |
| # For each sample, calculate the [[Euclidean distance]] <math>\Delta E_i</math> between the pair of co-ordinates.
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| # 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] or {{patent|EP|1184893}}. {{harvtxt|Thornton|1972}} compares older products; {{harvtxt|Guo|Houser|2004}} compares newer ones.</ref><ref>It appeared that <math>R_i</math> could be negative (<math>\Delta E_i</math> ≥ 22), and this was indeed calculated for some lamp test colors, especially TCS9 (strong red).</ref>
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| # Find the general CRI (R<sub>a</sub>) by calculating the [[arithmetic mean]] of the special CRIs.
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| | |
| Note that the last three steps are equivalent to finding the mean [[color difference]], <math>\Delta \bar{E}_{UVW}</math> and using that to calculate <math>R_a</math>:
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| | |
| :<math>R_a=100-4.6 \Delta \bar{E}_{UVW}</math>
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| | |
| ===Chromatic adaptation===
| |
| [[Image: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).]]
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| | |
| {{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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| | |
| <math>u_{c,i}=\frac{10.872+0.404 (c_r/c_t) c_{t,i} - 4 (d_r/d_t) d_{t,i}}{16.518+1.481 (c_r/c_t) c_{t,i} - (d_r/d_t) d_{t,i}}</math>
| |
| | |
| <math>v_{c,i}=\frac{5.520}{16.518+1.481 (c_r/c_t) c_{t,i} - (d_r/d_t) d_{t,i}}</math>
| |
| | |
| <math>c=\left(4.0-u-10.0v \right)/v</math>
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| <math>d=\left(1.708v-1.481u+0.404\right)/v</math> | |
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| where subscripts ''r'' and ''t'' refer to reference and test light sources, respectively.
| |
| | |
| ===Test color samples===
| |
| | |
| {| class="wikitable" border="1" style="float:right; margin:5px;"
| |
| |-
| |
| ! Name
| |
| ! Appr. Munsell
| |
| ! Appearance under daylight
| |
| ! Swatch
| |
| |-
| |
| | TCS01
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| | 7,5 R 6/4
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| | Light greyish red
| |
| | style="background:#e8a7b0;" |
| |
| |-
| |
| | TCS02
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| | 5 Y 6/4
| |
| | Dark greyish yellow
| |
| | style="background:#ccb184;" |
| |
| |-
| |
| | TCS03
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| | 5 GY 6/8
| |
| | Strong yellow green
| |
| | style="background:#abc161;" |
| |
| |-
| |
| | TCS04
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| | 2,5 G 6/6
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| | Moderate yellowish green
| |
| | style="background:#6fc59f;" |
| |
| |-
| |
| | TCS05
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| | 10 BG 6/4
| |
| | Light bluish green
| |
| | style="background:#7fc2df;" |
| |
| |-
| |
| | TCS06
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| | 5 PB 6/8
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| | Light blue
| |
| | style="background:#8fb6ff;" |
| |
| |-
| |
| | TCS07
| |
| | 2,5 P 6/8
| |
| | Light violet
| |
| | style="background:#cca5ff;" |
| |
| |-
| |
| | TCS08
| |
| | 10 P 6/8
| |
| | Light reddish purple
| |
| | style="background:#eea3ef;" |
| |
| |-
| |
| | TCS09
| |
| | 4,5 R 4/13
| |
| | Strong red
| |
| | style="background:#e9214c;" |
| |
| |-
| |
| | TCS10
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| | 5 Y 8/10
| |
| | Strong yellow
| |
| | style="background:#fff456;" |
| |
| |-
| |
| | TCS11
| |
| | 4,5 G 5/8
| |
| | Strong green
| |
| | style="background:#0cac8a;" |
| |
| |-
| |
| | TCS12
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| | 3 PB 3/11
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| | Strong blue
| |
| | style="background:#005bc0;" |
| |
| |-
| |
| | TCS13
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| | 5 YR 8/4
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| | Light yellowish pink
| |
| | style="background:#ffe8da;" |
| |
| |-
| |
| | TCS14
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| | 5 GY 4/4
| |
| | Moderate olive green ([[leaf]])
| |
| | style="background:#6c7d4f;" |
| |
| |}
| |
| | |
| As specified in {{harvtxt|CIE|1995}}, the original test color samples (TCS) are taken from an early edition of the [[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], Korea Research Institute of Standards and Science.</ref> and their approximate Munsell notations are listed aside.<ref>[http://www.cis.rit.edu/mcsl/online/munsell.php Munsell Renotation Data], ''Munsell Color Science Laboratory'', [[Rochester Institute of Technology]]</ref>
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| | |
| [[Image:CIE CRI TCS SPDs.svg|300px|left]] {{-}}
| |
| | |
| ==R96<sub>a</sub> method== | |
| [[Image: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|Schanda|Sándor|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.''</ref>
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| | |
| The R96<sub>a</sub> method has a few distinguishing features:<ref>See "Past research to improve the CRI" in {{harvtxt|Bodrogi|2004}}</ref> | |
| | |
| * [[#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.
| |
| * Adaptation of all colors to [[CIE Standard Illuminant D65|D65]] (since CIELAB is well-tested under D65).
| |
| | |
| It is conventional to use the original method; R96<sub>a</sub> should be explicitly mentioned if used.
| |
| | |
| ===New test color samples===
| |
| {| class="wikitable" border="1" style="float:right; margin:5px;"
| |
| |-
| |
| !
| |
| ! TCS01<sup>*</sup>
| |
| ! TCS02<sup>*</sup>
| |
| ! TCS03<sup>*</sup>
| |
| ! TCS04<sup>*</sup>
| |
| ! TCS05<sup>*</sup>
| |
| ! TCS06<sup>*</sup>
| |
| ! TCS07<sup>*</sup>
| |
| ! TCS08<sup>*</sup>
| |
| ! TCS09<sup>*</sup>
| |
| ! TCS10<sup>*</sup>
| |
| |-
| |
| | L<sup>*</sup> || 40.9 || 61.1 || 81.6 || 72.0 || 55.7 || 51.7 || 30.0 || 51.0 || 68.7 || 63.9
| |
| |-
| |
| | a<sup>*</sup> || 51.0 || 28.8 || −4.2 || −29.4 || −43.4 || −26.4 || 23.2 || 47.3 || 14.2 || 11.7
| |
| |-
| |
| | b<sup>*</sup> || 26.3 || 57.9 || 80.3 || 58.9 || 35.6 || −24.6 || −49.6 || −13.8 || 17.4 || 17.3
| |
| |-
| |
| | style="height:25px;" |
| |
| | style="background:#ad3139;" |
| |
| | style="background:#d17e27;" |
| |
| | style="background:#e0cb08;" |
| |
| | style="background:#92be38;" |
| |
| | style="background:#369842;" |
| |
| | style="background:#0089a5;" |
| |
| | style="background:#413c95;" |
| |
| | style="background:#bb5593;" |
| |
| | style="background:#c99e89;" |
| |
| | style="background:#b7837d;" |
| |
| |}
| |
| | |
| As discussed in {{harvtxt|Schanda|Sándor|2005}}, {{harvtxt|CIE|1999}} recommends the use of a [[ColorChecker]] chart owing to the obsolescence of the original samples, of which only [[Metamerism (color)|metameric]] matches remain.<ref>[http://www.xrite.com/product_overview.aspx?ID=820&Action=Library X-Rite ColorChecker Chart].</ref> In addition to the eight ColorChart samples, two skin tone samples are defined (TCS09<sup>*</sup> and TCS10<sup>*</sup>). Accordingly, the updated general CRI is averaged over ten samples, not eight as before. Nevertheless, {{harvtxt|Hung|2002}} has determined that the patches in {{harvtxt|CIE|1995}} give better correlations for any color difference than the ColorChecker chart, whose samples are not equally distributed in a uniform color space.
| |
| | |
| == Example ==
| |
| | |
| The CRI can also be theoretically derived from the SPD of the illuminant and samples since physical copies of the original color samples are difficult to find. In this method, care should be taken to use a sampling resolution fine enough to capture spikes in the SPD. The SPDs of the standard test colors are tabulated in 5 nm increments {{harvtxt|CIE|2004}}, so it is suggested to use interpolation up to the resolution of the illuminant's spectrophotometry.
| |
| | |
| Starting with the SPD, let us verify that the CRI of reference illuminant F4 is 51. The first step is to determine the [[tristimulus value]]s using the 1931 standard observer. Calculation of the [[inner product]] of the SPD with the standard observer's color matching functions (CMFs) yields (''X'',''Y'',''Z'')=(109.2,100.0,38.9) (after normalizing for Y=100). From this follow the ''xy'' chromaticity values:
| |
| | |
| [[Image:CIE 1960 UCS, FL4.svg|thumb|right|200px|The tight isotherms are from 2935K–2945K. FL4 marked with a cross.]]
| |
| | |
| <math>x=\frac{109.2}{109.2+100.0+38.9}=0.4402</math>
| |
| | |
| <math>y=\frac{100}{109.2+100.0+38.9}=0.4031</math>
| |
| | |
| The next step is to convert these chromaticities to the [[CIE 1960 color space|CIE 1960 UCS]] in order to be able to determine the CCT:
| |
| | |
| <math>u=\frac{4 \times 0.4402}{-2 \times 0.4402 + 12 \times 0.4031 + 3}=0.2531</math>
| |
| | |
| <math>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|200px|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 co-ordinate 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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| | |
| <math>\begin{align} DC&=\sqrt{ (0.2531-0.2528)^2+(0.3477-0.3484)^2 } \\
| |
| & =8.12 \times 10^{-4} < 5.4 \times 10^{-3} \end{align} </math>
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| | |
| We can verify the CCT by using [[Color temperature#Approximation|McCamy's approximation algorithm]] to estimate the CCT from the ''xy'' chromaticities:
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| | |
| <math>CCT_{est.} = -449 n^3 + 3525 n^2 - 6823.3 n + 5520.33</math>, where <math>n=\frac{x-0.3320}{y-0.1858}</math>.
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| | |
| Substituting <math>(x,y)=(0.4402,0.4031)</math> yields ''n''=0.4979 and CCT<sub>est.</sub> = 2941 K, which is close enough. ([[Color temperature#Robertson's method|Robertson's method]] can be used for greater precision, but we will be content with 2940 K in order to replicate published results.) Since 2940 < 5000, we select a Planckian radiator of 2940 K as the reference illuminant.
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| | |
| The next step is to determine the values of the test color samples under each illuminant in the [[CIE 1964 color space|CIEUVW color space]]. This is done by integrating the product of the CMF with the SPDs of the illuminant and the sample, then converting from CIEXYZ to CIEUVW (with the u,v coordinates of the reference illuminant as white point):
| |
| | |
| {| class="wikitable" border="1"
| |
| |-
| |
| ! colspan = 2 | Illuminant || TCS1 || TCS2 || TCS3 || TCS4 || TCS5 || TCS6 || TCS7 || TCS8
| |
| |-
| |
| ! rowspan = 3 | Reference
| |
| ! U
| |
| | 39.22 || 17.06 || −13.94 || −40.83 || −35.55 || −23.37 || 16.43 || 44.64
| |
| |-
| |
| ! V
| |
| | 2.65 || 9.00 || 14.97 || 7.88 || −2.86 || −13.94 || −12.17 || −8.01
| |
| |-
| |
| ! W
| |
| | 62.84 || 61.08 || 61.10 || 58.11 || 59.16 || 58.29 || 60.47 || 63.77
| |
| |-
| |
| ! rowspan = 3 | CIE FL4
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| ! U
| |
| | 26.56 || 10.71 || −14.06 || −27.45 || −22.74 || −13.99 || 9.61 || 25.52
| |
| |-
| |
| ! V
| |
| | 3.91 || 11.14 || 17.06 || 9.42 || −3.40 || −17.40 || −15.71 || -10.23
| |
| |-
| |
| ! W
| |
| | 63.10 || 61.78 || 62.30 || 57.54 || 58.46 || 56.45 || 59.11 || 61.69
| |
| |-
| |
| ! rowspan = 3 | CIE FL4<br />(CAT)
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| ! U
| |
| | 26.34 || 10.45 || −14.36 || −27.78 || −23.10 || −14.33 || 9.37 || 25.33
| |
| |-
| |
| ! V
| |
| | 4.34 || 11.42 || 17.26 || 9.81 || −2.70 || −16.44 || −14.82 || −9.47
| |
| |-
| |
| ! W
| |
| | 63.10 || 61.78 || 62.30 || 57.54 || 58.46 || 56.45 || 59.11 || 61.69
| |
| |}
| |
| | |
| From this we can calculate the color difference between the chromatically adapted samples (labeled "CAT") and those illuminated by the reference. (The Euclidean metric is used to calculate the color difference in CIEUVW.) The special CRI is simply <math>R_i=100-4.6 \Delta E_{UVW}</math>.
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| | |
| {| class="wikitable" border="1"
| |
| |-
| |
| ! || TCS1 || TCS2 || TCS3 || TCS4 || TCS5 || TCS6 || TCS7 || TCS8
| |
| |-
| |
| ! <math>\Delta E_{UVW}</math>
| |
| | 12.99 || 7.07 || 2.63 || 13.20 || 12.47 || 9.56 || 7.66 || 19.48
| |
| |-
| |
| ! R<sub>i</sub>
| |
| | 40.2 || 67.5 || 87.9 || 39.3 || 42.6 || 56.0 || 64.8 || 10.4
| |
| |}
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| | |
| Finally, the general color rendering index is the mean of the special CRIs: 51.
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| | |
| [[Image:CIE CRI TCS under FL4.svg|thumb|600px|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.
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| <br /> | |
| 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).]]
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| | |
| == Typical values ==
| |
| {| class="wikitable" border="1" style="float:right; margin:5px;"
| |
| |-
| |
| ! Light source
| |
| ! CCT (K)
| |
| ! CRI
| |
| |-
| |
| | [[Sodium-vapor lamp|Low-pressure sodium]] (LPS/SOX)
| |
| | 1800
| |
| | -44
| |
| |-
| |
| | Clear [[Mercury-vapor lamp|mercury-vapor]]
| |
| | 6410
| |
| | 17
| |
| |-
| |
| | [[Sodium-vapor lamp|High-pressure sodium]] (HPS/SON)
| |
| | 2100
| |
| | 24
| |
| |-
| |
| | Coated mercury-vapor
| |
| | 3600
| |
| | 49
| |
| |-
| |
| | Halophosphate warm-white [[Fluorescent lamp|fluorescent]]
| |
| | 2940
| |
| | 51
| |
| |-
| |
| | Halophosphate cool-white fluorescent
| |
| | 4230
| |
| | 64
| |
| |-
| |
| | Tri-phosphor warm-white fluorescent
| |
| | 2940
| |
| | 73
| |
| |-
| |
| | Halophosphate cool-daylight fluorescent
| |
| | 6430
| |
| | 76
| |
| |-
| |
| | [[w:Sodium-vapor lamp|"White" SON]]
| |
| | 2700
| |
| | 82
| |
| |-
| |
| | Quartz [[Metal-halide lamp|metal halide]]
| |
| | 4200
| |
| | 85
| |
| |-
| |
| | Tri-phosphor cool-white [[Fluorescent lamp|fluorescent]]
| |
| | 4080
| |
| | 89
| |
| |-
| |
| | Ceramic metal halide
| |
| | 5400
| |
| | 96
| |
| |-
| |
| | [[Incandescent light bulb|Incandescent]]/[[Halogen lamp|halogen]] bulb
| |
| | 3200
| |
| | 100
| |
| |}
| |
| | |
| A reference source, such as blackbody radiation, is defined as having a CRI of 100. This is why [[incandescent lamp]]s have that rating, as they are, in effect, almost blackbody radiators. The best possible faithfulness to a reference is specified by a CRI of one hundred, while the very poorest is specified by a CRI below zero. A high CRI by itself does not imply a good rendition of color, because the reference itself may have an imbalanced SPD if it has an extreme color temperature.
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| == Criticism and resolution ==
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| | |
| {{harvtxt|Ohno|2006}} 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|Schanda|Sándor|2005}}: ''It is quite obvious that just at 5000 K, where the reference illuminant has to be changed, the present system shows discontinuity.'</ref> because the chromaticity of the reference moves from the [[Planckian locus]] to the [[Standard illuminant#Illuminant series D|CIE daylight locus]]. {{harvtxt|Davis|Ohno|2006}} identify several other issues, which they address in their [[Color Quality Scale]] (CQS):
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| | |
| * 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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| * Calculating the arithmetic mean of the errors diminishes the contribution of any single large deviation. Two light sources with similar CRI may perform significantly differently if one has a particularly low special CRI in a spectral band that is important for the application. Use the [[root mean square deviation]] instead.
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| * The metric is not perceptual; all errors are equally weighted, whereas humans favor certain errors over others. A color can be more saturated or less saturated without a change in the numerical value of ∆''E''<sub>''i''</sub>, while in general a saturated color is experienced as being more attractive.
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| * A negative CRI is difficult to interpret. Normalize the scale from 0 to 100 using the formula <math>R_{out}=10 \ln \left[\exp(R_{in}/10)+1\right]</math>
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| * The CRI can not be calculated for light sources that do not have a CCT (non-white light).
| |
| * Eight samples are not enough since manufacturers can optimize the emission spectra of their lamps to reproduce them faithfully, but otherwise perform poorly. Use more samples (they suggest fifteen for CQS).
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| * The samples are not saturated enough to pose difficulty for reproduction.
| |
| * CRI merely measures the faithfulness of any illuminant to an ideal source with the same CCT, but the ideal source itself may not render colors well if it has an extreme color temperature, due to a lack of energy at either short or long wavelengths (i.e., it may be excessively blue or red). Weight the result by the ratio of the [[gamut]] area of the polygon formed by the fifteen samples in CIELAB for 6500 K to the gamut area for the test source. 6500 K is chosen for reference since it has a relatively even distribution of energy over the visible spectrum and hence high gamut area. This normalizes the multiplication factor.
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| Rea and Freyssinier have developed another index, the Gamut Area Index (GAI), in an attempt to improve over the flaws found in the CRI.<ref>Rea, M.S. and Freysinnier-Nova, JP. "Color rendering: A tale of two metrics," Color Research and Application, 33(3), 192-202 (2008). {{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>Alliance for Solid-State Illumination Systems and Technologies. "ASSIST recommends…Guide to Light and Color in Retail Merchandising," 8(1), (2010). Online at: < http://www.lrc.rpi.edu/programs/solidstate/assist/recommends/lightcolor.asp > (28 May 2010)</ref><ref>Alliance for Solid-State Illumination Systems and Technologies. "ASSIST recommends…Recommendations for Specifying Color Properties of Light Sources for Retail Merchandising." 8(2), (2010). Online at: < http://www.lrc.rpi.edu/programs/solidstate/assist/recommends/lightcolor.asp > (28 May 2010)</ref>
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| | |
| {{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.
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| | |
| {{harvtxt|CIE|2007}} "reviews the applicability of the CIE color rendering index to
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| white LED light sources based on the results of visual experiments." Chaired by Davis, CIE TC 1-69(C) is currently investigating "new methods for assessing the color rendition properties of white-light sources used for illumination, including solid-state light sources, with the goal of recommending new assessment procedures ... by March, 2010."<ref>[http://www.cie.co.at/div1/ActReps/D1ActivityReport08.pdf CIE Activity Report. Division 1: Vision and Color], pg.21, January 2008.</ref>
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| | |
| 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 9 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 KAG, Ryckaert WR, Pointer MR, 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 KAG, Ryckaert WR, Pointer MR, Deconinck G, Hanselaer P. Correlation between color quality metric predictions and visual appreciation of light sources.[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=]</ref>
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| [http://lrt.sagepub.com/content/45/6/666 Dangol et al (2013)] 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>[http://lrt.sagepub.com/content/45/6/666 R. Dangol, M. Islam, M. Hyvarinen, P. Bhusal, M. Puolakka, and L. Halonen, Subjective preferences and colour quality metrics of LED light sources ,” Lighting Res. Techchnology, Vol. 45, nro 6, pp. 666-688, 2013]</ref> They carried out further experiments in real offices evaluating various spectra generated for combination existing and proposed colour rendering metrics ( see [http://lrt.sagepub.com/content/early/2013/12/06/1477153513514424.abstract Dangol et al. 2013],<ref>R. Dangol, M.S. Islam, M. Hyvärinen, P. Bhusal, M. Puolakka, and Liisa Halonen. User acceptance studies for LED office lighting: Preference, naturalness and colourfulness. Lighting Research and Technology, December , 2013, [http://lrt.sagepub.com/content/early/2013/12/06/1477153513514424.abstract doi:10.1177/1477153513514424]</ref>[http://lrt.sagepub.com/content/early/2013/12/16/1477153513514425.abstract Islam et al. 2013],<ref>M.S. Islam, R. Dangol, M. Hyvärinen, P. Bhusal, M. Puolakka, and L. HalonenUser acceptance studies for LED office lighting: lamp spectrum, spatial brightness and illuminance levelLighting Research and Technology, December , 2013, [http://lrt.sagepub.com/content/early/2013/12/16/1477153513514425.abstract doi:10.1177/1477153513514425]</ref>[http://lrt.sagepub.com/content/early/2013/12/09/1477153513515264.abstract Baniya et al. 2013]<ref>R.R. Baniya, R. Dangol, P. Bhusal, A. Wilm, E. Baur, M. Puolakka, and L. Halonen. User-acceptance studies for simplified light-emitting diode spectra. Lighting Research and Technology, December , 2013, [http://lrt.sagepub.com/content/early/2013/12/09/1477153513515264.abstract doi: 10.1177/1477153513515264.]</ref> for details).
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| | |
| == Film and video high-CRI LED lighting incompatibility ==
| |
| | |
| Problems have been encountered attempting to use otherwise high CRI 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 (DI's), 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
| |
| | url = http://www.oscars.org/science-technology/council/projects/ssl/index.html
| |
| | title = Solid State Lighting Project
| |
| }}</ref>
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| | |
| == References ==
| |
| | |
| {{Reflist|2}}
| |
| | |
| == Sources ==
| |
| <!-- Harvard citations used. Please do not correct "colour" to color in the references and quotes. -->
| |
| | |
| * {{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}}
| |
| * {{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}}
| |
| * {{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}}. Carried out by TC 1-69: Colour Rendering of White Light Sources. (Dead link)
| |
| | |
| * {{citation|title=Band Systems for Appraisal of Color Rendition|url=http://www.opticsinfobase.org/abstract.cfm?id=51703|journal=[[JOSA]]|volume=47|issue=12|date=December 1957|first=Bentley T.|last=Barnes|pages=1124–1129|doi=10.1364/JOSA.47.001124}}
| |
| * {{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}}
| |
| * {{citation|title=Measurement of color rendering tolerances|first=Brian Hewson|last=Crawford|journal=[[JOSA]]|date=December 1959|volume=49|issue=12|pages=1147–1156| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-49-12-1147|doi=10.1364/JOSA.49.001147}}
| |
| * {{citation|url=http://web.archive.org/web/20090825230627/http://physics.nist.gov/Divisions/Div844/facilities/vision/color.html|first1=Wendy|last1=Davis|first2=Yoshi|last2=Ohno|date=December 2006|title=Color Rendering of Light Sources|publisher=[[NIST]]}}
| |
| * {{citation|contribution=Sensitivity metamerism index digital still camera|series=[[Proceedings of SPIE]]|volume=4922|title=Color Science and Imaging Technologies|editors=Dazun Zhao, Ming R. Luo, Kiyoharu Aizawa|date=September 2002|pages=1–14|doi=10.1117/12.483116|first=Po-Chieh|last=Hung}}
| |
| * {{citation|title=Light sources and color rendering|first=Dorothy|last=Nickerson|journal=[[JOSA]]|date=January 1960|volume=50|issue=1|pages=57–69| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-50-1-57|doi=10.1364/JOSA.50.000057}}
| |
| * {{citation|contribution=Optical metrology for LEDs and solid state lighting|last=Ohno|first=Yoshi|year=2006|booktitle=Proceedings of [[SPIE]] |volume=6046|pages=604625–1–604625–8|title=Fifth Symposium "Optics in Industry"|editors=E. Rosas, R. Cardoso, J.C. Bermudez, O. Barbosa-Garcia|url=http://physics.nist.gov/Divisions/Div844/facilities/photo/Publications/OhnoOptInd2005.pdf |doi=10.1117/12.674617}}
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| | |
| * {{citation|first1=Nicolas|last1=Pousset|first2=Gael|last2=Obein|first3=Annick|last3=Razet|title=Visual experiment on LED lighting quality with color quality scale colored samples|journal=Proceedings of CIE 2010 : Lighting quality and energy efficiency, Vienna, Austria|year=2010|pages=722–729|url=http://nicolas_pousset.perso.neuf.fr/Recherche/Article/CQS.pdf}}
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| | |
| * {{citation|first=János|last=Schanda|contribution=The concept of colour rendering revisited|title=First European Conference on Color in Graphics Imaging and Vision|location=Univ. Poitiers, France|year=2002|month=April, 2-5|url=http://www.knt.vein.hu/staff/schandaj/SJCV-Publ-2005/462.pdf}}
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| | |
| * {{citation|first=William A.|last=Thornton|title=Color-Rendering Capability of Commercial Lamps|journal=[[Applied Optics]]|year=1972|volume=11|issue=5|pages=1078–1086| url=http://www.opticsinfobase.org/abstract.cfm?URI=ao-11-5-1078|doi=10.1364/AO.11.001078}}
| |
| * {{citation|first=K.A.G.|last=Smet|title=Correlation between color quality metric predictions and visual appreciation of light sources|journal=[[Optics Express]]|year=2011|volume=19|issue=9|pages=8151–8166| url=http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-9-8151}}
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| | |
| * R. Dangol, M. Islam, M. Hyvärinen, P. Bhusal, M. Puolakka, and L. Halonen. [http://lrt.sagepub.com/content/45/6/666 Subjective preferences and colour quality metrics of LED light sources. Lighting Research and Technology], Vol. 45, nro 6, pp. 666–688, 2013.
| |
| *
| |
| * M. Islam, R. Dangol, M. Hyvärinen, P. Bhusal, M. Puolakka, and L. Halonen. [http://lrt.sagepub.com/content/45/6/641.abstract Investigation of user preferences for LED lighting in terms of light spectrum. Lighting Research and Technology], Vol. 45, nro 6, pp. 641–665, 2013
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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://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/)
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| * [http://stacks.iop.org/0026-1394/46/704 Uncertainty evaluation for measurement of LED colour, Metrologia]
| |
| * [http://www.lightingever.com/kbase/learninglighting/light-terminology-color-rendering-index.html Color Rendering Index of Common Light Source]
| |
| * [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)]
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| * [http://www.lrc.rpi.edu/programs/solidstate/assist/recommends/lightcolor.asp Alliance for Solid State Illumination Systems and Technologies, Color Rendering]
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| {{DEFAULTSORT:Color Rendering Index}}
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| [[Category:Color]]
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| [[Category:Lighting]]
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