The nCRI Colour Rendering Index
János SchandaUniversity 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? The colour fidelity index proposal Summary
Brief History of CIE colour rendering
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
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
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
R Ri a
E i
U*,V*,W*
X,Y,Z
Adapt.Chrom.
Source
SamplesTest
TestCCT
U*,V*,W*
X,Y,Z
Illum.
Ref.
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
Recent CIE Activity2007: 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
Current Standardization WorkTC 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.
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 method
Reference illuminant: Use single reference?
CIE D65? What about visual
scene in the evening?
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
Questions raised with current CIE test method
Test samples CIE 8 unsaturatedplus 4 basic colours
+ complexion & foliage
Problem: Samples from few
pigments Not representative real world
samples
CIE Test Samples3-
LED-1
3-LED-
2
3-LED-
3
3-LED mod
el gamut
expanded
4-LED-
1max Ra
4-LED-
2max R(9-12)
4-LED no
yellow
4-LEDwith yello
w
Ph-LED YAG
E*ab CAM02-Ri
CIE1 93,5 77,9 90,5 79,4 97,6 89,7 88,6 98,7 83,25 4,93 68,0 95,1 92,1 89,0 88,5 82,0 92,8 87,9 76,519 3,54 91,0 80,7 89,8 78,7 97,5 91,4 88,3 96,3 88,531 1,07 77,9 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 97,32 5,57 70,9 91,3 74,3 71,1 90,9 96,1 92,4 90,7 91,74 2,20 78,5 71,2 77,8 78,0 85,9 84,9 70,6 91,6 91,314 9,32 71,8 78,3 88,7 86,3 94,8 88,6 80,5 84,2 94,818 4,13 88,2 72,1 92,8 84,6 97,4 87,7 90,5 84,2 85,620 5,77 86,8 85,2 67,8 51,9 92,5 88,2 84,4 88,3 93,524 6,38 74,2 89,1 91,0 83,9 82,8 82,5 89,4 76,7 98,125 8,87 86,4 85,2 75,6 48,4 89,5 91,1 78,7 84,1 90,7
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
Visual observations in double booth with Macbeth Color Checker
Chart samples
Fidelity experiment
Light source CIE 13.2 CIELAB CIECAM02CoolWhite 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 0,04RGB-LED 1 0,75 0,74 0,79RGB-LED 2 0,82 0,82 0,89
Correlation between visual ranking and measured colour differences
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
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.
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
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.
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.
2
1rms
ΔΔ
N
ii
EE
N
1,5rms
2
,2012 Δ 1
2100a k ER
e
Practical examples
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
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
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
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.
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.00.010.020.030.040.050.060.070.080.090.0
100.0
CIE RaCQS Qa
Ra,2012
Ra, Q
a
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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