The nCRI Colour Rendering Index
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Colour quality description of LED light sources for transport lighting
The nCRI Colour Rendering IndexJnos SchandaUniversity of PannoniaVirtual Environments and Imaging Technologies Laboratory
OverviewThe CIE system of colour rendring evaluationColour fidelity and colour preferenceWhy updating the CIE CRI calculation method?The colour fidelity index proposalSummaryBrief History of CIE colour rendering1948: 8 band Spectral Band Method (SBM), deviation from full radiator1955: established WC 1.3.2 to address terminology and compare SBM with test sample method1961: agreed on test color method, with 8 test samples1964: Publication 13 (1st edition), published test sample methodBrief History of CIE colour rendering1974: Publication 13 (2nd edition)Defined reference illuminantsTest samples: 8 + 6Von Kries chromatic adaptation transformUse the CIE 1964 UCSScaling: Warm white halophosphate lamp = Ra=51This is the CRI as we know it todayCIE 1974 Test methodSelect a reference illuminantPlanckian if TCC5000KDetermine Test Sample Tristimulus ValuesTransform into U*V*W* spaceApply von Kries chromatic adaptation transformCalculate colour differencesCalculate special and general colour rendering indices
Brief History of CIE colour rendering1980s: new TC worked on subject, but closed without recommendations due to member disagreements1995: Publication 13 (3rd edition), no basic changes from 2nd edition, just fixed some errors1999: Another TC (TC 1-33) closed without reaching consensus. Industry opposed proposed new procedures due to lack of visual experimentsRecent CIE Activity2007: TC 1-62 published CIE 177Studied color rendering of white LED sourcesRecommended the development of a new color rendering metric (or set of metrics)Recommended concurrent use of CRI and new metric, at least at firstThe new metric should be applicable to all types of light sourcesCurrent Standardization WorkTC 1-69: Colour Rendition by White Light SourcesStarted in late 2006Chair: Wendy Davis, USTerms of Reference: To investigate new methods for assessing the colour rendition properties of white-light sources used for illumination, including solid-state light sources, with the goal of recommending new assessment procedures.Closing the TC?CIE Div.1 will have to decide in 2012 on future; most probably TC will close with description of some possible colour fidelity and colour preference metrics.Proposed metrics Rank-order based color rendering index (RCRI) Feel of contrast index (FCI) CRI-CAM02UCS nCRI Color quality scale (CQS) Harmony rendering index (HRI) Categorical color rendering index (CCRI) Memory CRI (MCRI)Questions raised with current CIE Test methodReference illuminant:Use single reference? CIE D65?What about visual scene in the evening?
10Questions raised with current CIE Test methodColorimetryU*,V*,W* space outdatedVon Kries chromatic adaptation transform outdatedUse CIECAM02, withbuilt in chromatic adaptation transformand its extendsion: UCS colour space
Questions raised with current CIE test methodTest samplesCIE 8 unsaturatedplus4 basic colours + complexion & foliageProblem:Samples from few pigmentsNot representative real world samples
CIE Test Samples3-LED-13-LED-23-LED-33-LED model gamut expanded4-LED-1max Ra4-LED-2max R(9-12)4-LED no yellow4-LEDwith yellowPh-LED YAGDE*abCAM02-RiCIE193,577,990,579,497,689,788,698,783,254,9368,095,192,189,088,582,092,887,976,5193,5491,080,789,878,797,591,488,396,388,5311,0777,961,575,566,597,970,994,795,783,3CIE289,382,598,088,893,790,389,588,797,325,5770,991,374,371,190,996,192,490,791,742,2078,571,277,878,085,984,970,691,691,3149,3271,878,388,786,394,888,680,584,294,8184,1388,272,192,884,697,487,790,584,285,6205,7786,885,267,851,992,588,284,488,393,5246,3874,289,191,083,982,882,589,476,798,1258,8786,485,275,648,489,591,178,784,190,7Colour fidelity metricColour appearance should be the same under the test source and under the reference illuminant.Test samples should provide unique results, and/or number of samples should be high enough to avoid scaling differences using different sample sets.The method should replace the current colour rendering test methodVisual observations in double booth with Macbeth Color Checker Chart samples
Fidelity experimentLight sourceCIE 13.2CIELABCIECAM02CoolWhite CFL(940)-0,39-0,42-0,29CoolWhite CFL(840)0,670,570,64CoolWhite Fl., tradit.0,690,500,75CoolWhite Tri-band0,490,470,60P-LED-0,62-0,370,04RGB-LED 10,750,740,79RGB-LED 20,820,820,89Correlation between visual ranking and measured colour differences Recommended new colour fidelity metric: nCRIFlowchart of the new metric:Determine. Ref. Ill. SPDUse new test samplesCalc. Tristim. ValuesTransform to CIECAM-UCSCalc. Col. Diff.AverageCalculate CRI2012
Reference illuminant in nCRIReference illuminant:Stay with CIE 13.3 ref. illuminants:Below 5000 K: Planck distributionAbove 5000 K: Phases of daylightDetermination of equal CCT:u,v diagram, 2 observer One could consider to move the crossing temperature further down, as at sunset the daylight CCT might be below 5000 K.Test samples in nCRINew test samples to avoid lamp spectrum optimisation to maximize CRI3 sets of samples:HL17, artificial set of 17 colourssampling in the colour solid90 samples of high colour constancy90 samples of low colour constancyArtist colours & skin tones
Tristimulus and CIECAM02 calculationsUSE CIE 10 observerCIECAM 02 parameters:Background Yb=20Average surround condition with F, Nc, c parameters of 1, 1, 0.69, respectively.Luminance of test adapting field: LA=100 cd/m2Adaptation factor: D =1.
Colour difference and averaging calculationColour difference in CIECAM 02-UCS space
Root-mean-square averaging:Avoid negative Ri values:Sigmoid type functions:General colour rendering index:Similar equations for the special colour rendering indices Constant k sets the average of the Ra,2012 of the CIE F 1 to F12 lamps equal to the Ra index, preliminary value is k=1/55.
nCRI outputThe Excel program provides spectrum of the lampthe CCT and (uv) of the test lampthe following general colour rendering indices:Ra, 2012CIE RaCQS Qa
nCRI outputPictorial information:Gamut of the 17 samples under test lamp and reference illuminantVisualisation of the 17 samples coloursInformation of the worst colour from the 210 sample set
Auxiliary informationIf requested the program can output also such informations asvectors between corresponding test lamp and reference illuminant illuminated 210 samples,detailed information on the sample lowest Ri index, for the different groups of test samples
Correlation between Ra,2012 and CIE Ra, CQS QaAbove Ra,2012=80 correlation high,For Warm White CFLs Ra,2012 is lower than other metrics provideThe program is available for trial.
Thanks for your kind attention!This publication/research has been supported by the TMOP-4.2.2/B-10/1-2010-0025 project.