Colour quality description of LED light sources for transport lighting

Report
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?
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




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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
T es t
S o u rce
R e f.
CC T
Ilul m .
T es t
C h rom .
A d a p t.
S am p el s
XY
, Z
,
XY
, Z
,
U *V
, *W
, *
U *V
, *W
, *
E
R
i
R
i
a
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 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.
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 unsaturated
plus
 4 basic colours
+ complexion
& foliage


Problem:
Samples from few
pigments
 Not representative real world samples

CIE Test Samples
Ph-LED YAG
4-LED
with yellow
CAM02-Ri
4,93
3,54
1,07
93,5
68,0
91,0
77,9
77,9
95,1
80,7
61,5
90,5
92,1
89,8
75,5
79,4
89,0
78,7
66,5
97,6
88,5
97,5
97,9
89,7
82,0
91,4
70,9
88,6
92,8
88,3
94,7
98,7
87,9
96,3
95,7
83,2
76,5
88,5
83,3
5,57
2,20
9,32
4,13
5,77
6,38
8,87
89,3
70,9
78,5
71,8
88,2
86,8
74,2
86,4
82,5
91,3
71,2
78,3
72,1
85,2
89,1
85,2
98,0
74,3
77,8
88,7
92,8
67,8
91,0
75,6
88,8
71,1
78,0
86,3
84,6
51,9
83,9
48,4
93,7
90,9
85,9
94,8
97,4
92,5
82,8
89,5
90,3
96,1
84,9
88,6
87,7
88,2
82,5
91,1
89,5
92,4
70,6
80,5
90,5
84,4
89,4
78,7
88,7
90,7
91,6
84,2
84,2
88,3
76,7
84,1
97,3
91,7
91,3
94,8
85,6
93,5
98,1
90,7
CIE2
2
4
14
18
20
24
25
4-LED
no yellow
5
19
31
4-LED-2
max R(9-12)
CIE1
4-LED-1
max Ra
3-LED model
gamut
expanded
3-LED-3
3-LED-2
3-LED-1
E*ab
Colour fidelity metric


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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
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
0,75
0,74
0,79
RGB-LED 2
0,82
0,82
0,89
Recommended new colour fidelity metric:
nCRI
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Flowchart of the new
metric:
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Determine. Ref. Ill. SPD
Use new test samples
Calc. Tristim. Values
Transform to CIECAMUCS
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

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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
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
New test samples to
avoid lamp spectrum
optimisation to
maximize CRI
3 sets of samples:
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HL17, artificial set of
17 colours
sampling in the colour
solid
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90 samples of high
colour constancy
90 samples of low
colour constancy
Artist colours & skin
tones
Tristimulus and CIECAM02 calculations
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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.
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Colour difference and averaging calculation
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Colour difference in CIECAM 02-UCS space
N
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Root-mean-square averaging:
Avoid negative Ri values:
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Δ E rm s 
 ΔE
2
i
i 1
N
Sigmoid type functions:
2
2


General colour rendering index: R a ,2012  100  k  Δ E   1 
e

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.
1 ,5
rm s
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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
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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
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If requested the program can
output also such informations as
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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
100.0
R a, Qa
80.0
60.0
CIE Ra
40.0
CQS Qa
20.0
0.0
0.0
20.0
40.0
60.0
80.0
100.0
Ra,2012
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Above Ra,2012=80 correlation high,
For Warm White CFLs Ra,2012 is lower than other metrics
provide
The program is available for trial.
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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