Presentation on theme: "The nCRI Colour Rendering Index"— Presentation transcript:
1The nCRI Colour Rendering Index János SchandaUniversity of PannoniaVirtual Environments and Imaging Technologies Laboratory
2Overview The CIE system of colour rendring evaluation Colour fidelity and colour preferenceWhy updating the CIE CRI calculation method?The colour fidelity index proposalSummary
3Brief History of CIE colour rendering 1948: 8 band Spectral Band Method (SBM), deviation from full radiator1955: established WC 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 method
4Brief History of CIE colour rendering 1974: 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 today
5CIE 1974 Test method Select a reference illuminant Planckian if TCC<5000 KPhase of Daylight if TCC>5000KDetermine Test Sample Tristimulus ValuesTransform into U*V*W* spaceApply von Kries chromatic adaptation transformCalculate colour differencesCalculate special and general colour rendering indices
6Brief History of CIE colour rendering 1980s: 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 experiments
7Recent CIE Activity 2007: TC 1-62 published CIE 177 Studied 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 sources
8Current Standardization Work TC 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.
9Proposed metrics Rank-order based color rendering index (RCRI) Feel of contrast index (FCI)CRI-CAM02UCS nCRIColor quality scale (CQS)Harmony rendering index (HRI)Categorical color rendering index (CCRI)Memory CRI (MCRI)
10Questions raised with current CIE Test method Reference illuminant:Use single reference?CIE D65?What about visual scene in the evening?
11Questions raised with current CIE Test method ColorimetryU*,V*,W* space outdatedVon Kries chromatic adaptation transform outdatedUse CIECAM02, withbuilt in chromatic adaptation transformand its extendsion: UCS colour space
12Questions raised with current CIE test method Test samplesCIE 8 unsaturatedplus4 basic colours + complexion & foliageProblem:Samples from few pigmentsNot representative real world samples
133-LED model gamut expanded CIE Test Samples3-LED-13-LED-23-LED-33-LED model gamut expanded4-LED-1 max Ra4-LED-2 max R(9-12)4-LED no yellow4-LED with yellowPh-LED YAGDE*abCAM02-RiCIE193,577,990,579,497,689,788,698,783,254,9368,095,192,189,088,582,092,887,976,5193,5491,080,789,878,797,591,488,396,3311,0761,575,566,597,970,994,795,783,3CIE289,382,598,088,893,790,389,588,797,325,5791,374,371,190,996,192,490,791,742,2078,571,277,878,085,984,970,691,6149,3271,878,386,394,880,584,2184,1388,272,184,697,487,785,6205,7786,885,267,851,992,584,4246,3874,289,183,982,889,476,798,1258,8786,475,648,491,184,1
14Colour fidelity metric Colour 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 method
15Visual observations in double booth with Macbeth Color Checker Chart samples
16Fidelity experimentCorrelation between visual ranking and measured colour differencesLight 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 10,740,79RGB-LED 20,820,89
17Recommended new colour fidelity metric: nCRI Flowchart of the new metric:Determine. Ref. Ill. SPDUse new test samplesCalc. Tristim. ValuesTransform to CIECAM-UCSCalc. Col. Diff.AverageCalculate CRI2012
18Reference illuminant in nCRI Stay with CIE 13.3 ref. illuminants:Below 5000 K: Planck distributionAbove 5000 K: Phases of daylightDetermination of equal CCT:u,v – diagram, 2° observerOne could consider to move the crossing temperature further down, as at sunset the daylight CCT might be below 5000 K.
19Test 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
20Tristimulus and CIECAM02 calculations USE 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.
21Colour difference and averaging calculation Colour difference in CIECAM 02-UCS spaceRoot-mean-square averaging:Avoid negative Ri values:Sigmoid type functions:General colour rendering index:Similar equations for the special colour rendering indicesConstant 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.
23nCRI output The Excel program provides spectrum of the lamp the CCT and (uv) of the test lampthe following general colour rendering indices:Ra, 2012CIE RaCQS Qa
24nCRI output Pictorial 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
25Auxiliary information If 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
26Correlation between Ra,2012 and CIE Ra, CQS Qa Above Ra,2012=80 correlation high,For Warm White CFLs Ra,2012 is lower than other metrics provideThe program is available for trial.
27Thanks for your kind attention! This publication/research has beensupported by theTÁMOP-4.2.2/B-10/ project.