1. The nCRI Colour Rendering Index
János Schanda
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 colour fidelity index proposal
Summary

3. Brief History of CIE colour rendering
1948: 8 band Spectral Band Method (SBM), deviation from full radiator
1955: established WC 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 colour rendering
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 colour rendering
1980s: new TC worked on subject, but closed without recommendations due to member disagreements
1995: Publication 13 (3rd edition), no basic changes 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-CAM02UCS nCRI
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. 3-LED model gamut expanded
CIE Test Samples
3-LED-1
3-LED-2
3-LED-3
3-LED model gamut expanded
4-LED-1 max Ra
4-LED-2 max R(9-12)
4-LED no yellow
4-LED with yellow
Ph-LED YAG
DE*ab
CAM02-Ri
CIE1
93,5
77,9
90,5
79,4
97,6
89,7
88,6
98,7
83,2 5
4,93
68,0
95,1
92,1
89,0
88,5
82,0
92,8
87,9
76,5 19
3,54
91,0
80,7
89,8
78,7
97,5
91,4
88,3
96,3 31
1,07
61,5
75,5
66,5
97,9
70,9
94,7
95,7
83,3
CIE2
89,3
82,5
98,0
88,8
93,7
90,3
89,5
88,7
97,3 2
5,57
91,3
74,3
71,1
90,9
96,1
92,4
90,7
91,7 4
2,20
78,5
71,2
77,8
78,0
85,9
84,9
70,6
91,6 14
9,32
71,8
78,3
86,3
94,8
80,5
84,2 18
4,13
88,2
72,1
84,6
97,4
87,7
85,6 20
5,77
86,8
85,2
67,8
51,9
92,5
84,4 24
6,38
74,2
89,1
83,9
82,8
89,4
76,7
98,1 25
8,87
86,4
75,6
48,4
91,1
84,1

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 should be 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
Light source
CIE 13.2
CIELAB
CIECAM02
CoolWhite CFL(940)
-0,39
-0,42
-0,29
CoolWhite CFL(840)
0,67
0,57
0,64
CoolWhite Fl., tradit.
0,69
0,50
0,75
CoolWhite Tri-band
0,49
0,47
0,60
P-LED
-0,62
-0,37
0,04
RGB-LED 1
0,74
0,79
RGB-LED 2
0,82
0,89

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
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 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.

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,2012 and 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/ project.