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The nCRI Colour Rendering Index

The nCRI Colour Rendering Index. János Schanda University of Pannonia Virtual Environments and Imaging Technologies Laboratory. Overview. The CIE system of colour rendring evaluation Colour fidelity and colour preference Why updating the CIE CRI calculation method?

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The nCRI Colour Rendering Index

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  1. The nCRI Colour RenderingIndex JánosSchanda University of Pannonia Virtual Environments and Imaging Technologies Laboratory

  2. Overview • The CIE system of colour rendring evaluation • Colour fidelity and colour preference • Why updating the CIE CRI calculation method? • The colourfidelity index proposal • Summary

  3. Brief History of CIE colourrendering 1948: 8 band Spectral Band Method (SBM), deviation from full radiator 1955: established WC 1.3.2 to address terminology and compare SBM with test sample method 1961: agreed on test color method, with 8 test samples 1964: Publication 13 (1st edition), published test sample method

  4. Brief History of CIE colourrendering 1974: Publication 13 (2nd edition) Defined reference illuminants Test samples: 8 + 6 Von Kries chromatic adaptation transform Use the CIE 1964 UCS Scaling: Warm white halophosphate lamp = Ra=51 This is the CRI as we know it today

  5. CIE 1974 Test method • Select a reference illuminant • Planckian if TCC<5000 K • Phase of Daylight if TCC>5000K • Determine Test Sample Tristimulus Values • Transform into U*V*W* space • Apply von Kries chromatic adaptation transform • Calculate colour differences • Calculate special and general colour rendering indices

  6. Brief History of CIE colourrendering 1980s: new TC worked on subject, but closed without recommendations due to member disagreements 1995: Publication 13 (3rd edition), no basicchanges from 2nd edition, just fixed some errors 1999: Another TC (TC 1-33) closed without reaching consensus. Industry opposed proposed new procedures due to lack of visual experiments

  7. Recent CIE Activity 2007: TC 1-62 published CIE 177 Studied color rendering of white LED sources Recommended the development of a new color rendering metric (or set of metrics) Recommended concurrent use of CRI and new metric, at least at first The new metric should be applicable to all types of light sources

  8. Current Standardization Work TC 1-69: Colour Rendition by White Light Sources Started in late 2006 Chair: Wendy Davis, US Terms 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.

  9. Proposed metrics • Rank-order based color rendering index (RCRI) • Feel of contrast index (FCI) • CRI-CAM02UCSnCRI • Color quality scale (CQS) • Harmony rendering index (HRI) • Categorical color rendering index (CCRI) • Memory CRI (MCRI)

  10. Questions raised with current CIE Test method • Reference illuminant: • Use single reference? • CIE D65? • What about visual scene in the evening?

  11. Questions raised with current CIE Test method • Colorimetry • U*,V*,W* space outdated • Von Kries chromatic adaptation transform outdated • Use CIECAM02, with • built in chromatic adaptation transform • and its extendsion: UCS colour space

  12. Questions raised with current CIE test method • Test samples • CIE 8 unsaturated plus • 4 basic colours+ complexion & foliage • Problem: • Samples from few pigments • Not representative real world samples

  13. CIE Test Samples

  14. Colour 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 shouldbe high enough to avoid scaling differences using different sample sets. • The method should replace the current colour rendering test method

  15. Visual observations in double booth with Macbeth Color Checker Chart samples

  16. Fidelity experiment Correlation between visual ranking and measured colour differences

  17. Recommended new colour fidelity metric: nCRI • Flowchart of the new metric: • Determine. Ref. Ill. SPD • Use new test samples • Calc. Tristim. Values • Transform to CIECAM-UCS • Calc. Col. Diff. • Average • Calculate CRI2012

  18. Reference illuminant in nCRI • Reference illuminant: • Stay with CIE 13.3 ref. illuminants: • Below 5000 K: Planck distribution • Above 5000 K: Phases of daylight • Determination 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.

  19. Test samples in nCRI • New test samples to avoid lamp spectrum optimisation to maximize CRI • 3 sets of samples: • HL17, artificial set of 17 colours • sampling in the colour solid • 90 samples of high colour constancy • 90 samples of low colour constancy • Artist colours & skin tones

  20. Tristimulus and CIECAM02 calculations • USE CIE 10° observer • CIECAM 02 parameters: • Background Yb=20 • Average surround condition with F, Nc, c parameters of 1, 1, 0.69, respectively. • Luminance of test adapting field: LA=100 cd/m2 • Adaptation factor: D=1.

  21. Colour difference and averaging calculation • Colour difference in CIECAM 02-UCS space • Root-mean-square averaging: • Avoid negative Rivalues: • 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.

  22. Practical examples

  23. nCRI output • The Excel program provides • spectrum of the lamp • the CCT and (uv) of the test lamp • the following general colour rendering indices: • Ra, 2012 • CIE Ra • CQS Qa

  24. nCRI output • Pictorial information: • Gamut of the 17 samples under test lamp and reference illuminant • Visualisation of the 17 samples’ colours • Information of the worst colour from the 210 sample set

  25. Auxiliary information • If requested the program can output also such informations as • vectors 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

  26. Correlation between Ra,2012and CIE Ra, CQS Qa Above Ra,2012=80 correlation high, For Warm White CFLs Ra,2012 is lower than other metrics provide The program is available for trial.

  27. Thanks for your kind attention! This publication/research has been supported by the TÁMOP-4.2.2/B-10/1-2010-0025 project.

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