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

Samples

Test

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 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

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 Samples

3-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

DE*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 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 0,75 0,74 0,79

RGB-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 E

Re

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 90.0 100.00.0

10.020.030.040.050.060.070.080.090.0

100.0

CIE RaCQS Qa

Ra,2012

Ra,

Qa

Thanks for your kind attention!

This publication/research has been supported by the TÁMOP-4.2.2/B-10/1-2010-0025 project.