Characterizing illumination systems
Colour rendering and beyond
J Schanda
University Veszprém, Department of Image
Processing and Neurocomputing, Hungary
CIE Standards for assessing
quality of light sources
•
•
•
•
•
Introduction
Characterisation of the light source
Colour rendering
Assessing daylight simulators
CIE Technical Committees working on the
above questions
• New approaches
• Summary
CIE Standards and Guides
• CIE colorimetric illuminants
– ISO/CIE 10526, CIE S005
• CIE colorimetric observers
– ISO/CIE 10527
• International Lighting Vocabulary
– CIE 17.4
• Colorimetry: CIE 15.2, new edition coming
• Colour rendering: CIE 13.3, outdated experiments needed
• Quality of daylight simulators: CIE 51,
possible improvements
Illuminant, source, simulator
• CIE Standard Illuminant
– Defined as spectral distribution
CIE Standard Illuminants
CIE S 005/E
 / nm
Standard
CIE Standard Illuminants for Colorimetry
Illuminants colorimétriques normalisés CIE
CIE Normlichtarten für Farbmessung
CIE Standards are copyrighted and shall not be reproduced in any form, entirely or partly, without
the explicit agreement of the CIE.
CIE Central Bureau, Vienna
Kegelgasse 27, A-1030 Vienna, Austria
UDC:
535.65:006
535.643.2
S 005/E-1998
Descriptor: Standardisation of colour measurement
Standard colorimetric system
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
SA()
SD65()
0,930 483
0,967 643
1,005 97
1,045 49
1,086 23
1,128 21
1,171 47
1,216 02
1,261 88
1,309 10
1,357 69
1,407 68
1,459 10
1,511 98
1,566 33
1,622 19
1,679 59
1,738 55
1,799 10
1,861 27
1,925 08
1,990 57
2,057 76
0,0341 000
0,360 140
0,686 180
1,012 22
1,338 26
1,664 30
1,990 34
2,316 38
2,642 42
2,968 46
3,294 50
4,988 65
6,682 80
8,376 95
10,071 1
11,765 2
13,459 4
15,153 5
16,847 7
18,541 8
20,236 0
21,917 7
23,599 5
Illuminant, source, simulator
• CIE Standard Illuminant
– Defined as spectral distribution
• CIE Standard Source
– Source with specified spectral distribution
• Simulator
– Source approximating the illuminant
CIE colorimetric illuminants
• CIE Standard Illuminant A
– New definition, unchanged values
CIE Standard Illuminant A
SA
1 435 000
5
exp

1
 0,56 
159 488
( )  100 


14 350
  
exp
1
2 848 
CIE colorimetric illuminants
• CIE Standard Illuminant A
– New definition, unchanged values
• CIE Standard Illuminant D65
CIE Standard Illuminant D65
5. CIE standard illuminant D65
5.1
Definition
D65
The relative spectral power distribution S () of CIE standard illuminant D65 is defined by
the values given in table 1 which are presented at 1 nm intervals over the wavelength range
from 300 nm to 830 nm. If required, other intermediate values may be derived by linear
interpolation from the published values*.
CIE colorimetric illuminants
• CIE Standard Illuminant A
– New definition, unchanged values
• CIE Standard Illuminant D65
– Other daylight illuminants: D50
– Evaluation
CIE (Standard) Source(s)
• CIE Standard Source A
– Realization: incandescent lamp
• D65: no standard source
• Secondary sources
– fluorescent lamps
Source standards and imaging
• Original
– Real life: daylight illumination
– paper copy: D50, fluorescent or tungsten illum.
– screen representation
• Reproduction
– screen representation
– hard-copy
Source standards and imaging
• Original scene
– Daylight: D50, D65 (Jackson - MacDonald - Freeman pictures)
Source standards and imaging
• Reproduction
– screen representation
• CRT primaries
• chromatic adaptation
• colour appearance models
– hard-copy
• CMYK primaries
• base material (paper reflection properties)
• gamut mapping
Source standards: original scene
• Light source colour description
–
–
–
–
chromaticity of source
colour temperature
correlated colour temperature
metamerism index
• Colour rendering
– special indices: Ri
– general colour rendering index: Ra
Light source colour description
• Colour temperature, Tc
– The temperature of a Planckian radiator
whose radiation has the same chromaticity
as that of a given stimulus.
• Correlated colour temperature Tcp
– The temperature of the Planckian radiator
whose perceived colour most closely
resembles that of a given stimulus at the
same brightness and under specified
viewing conditions.
Light source colour description
u,v Iso-Temperature lines
0.50
0.45
y
0.40
0.35
0.30
0.25
0.20
0.20
0.25
0.30
0.35
0.40
x
0.45
0.50
0.55
Colour rendering
S p e c tra l p o we r d is trib u tio n
180
160
140
120
rel. p o w er
• Spectra
of two
sources
of
equal
chroma
ticity
100
80
60
40
20
0
350
400
450
500
550
600
650
w a v e le n g th , n m
700
750
800
850
900
Colour rendering
0 .3 4 2
0 .3 4 0
D65
0 .3 3 8
3 -lin e
0 .3 3 6
y
• Chromaticity
of the two
sources and
of a sample
illuminated
by these
sources
0 .3 3 4
0 .3 3 2
0 .3 3 0
0 .3 2 8
0 .3 0 0
0 .3 5 0
x
0 .4 0 0
Colour rendering
• Effect of an illuminant on the colour
appearance of objects by conscious or
subconscious comparison with their colour
appearance under a reference illuminant
Colour rendering index
• Measure of the degree to which the
phsychophysical colour of an object
illuminated by the test illuminant conforms
to that of the same object illuminated by the
reference illuminant, suitable allowance
having been made for the state of chromatic
adaptation.
Colour rendering index
• Comparison of illuminants
– test illuminant
– reference illuminant:
• Planckian if CCT < 5000 K
• Phase of Daylight  5000 K
• Objects illuminated (test colour samples)
• Chromatic adaptation
• Colour difference
Flow-chart of CRI calculation
T e s t sm p sl .
ilulm .
Re f.
ilulm ni a n t
re f . ilulm .
Equa l
CC T
XY Z
U *V W
* *
C o ol u r
C EI
d if f .
te s t sm p l.
C h rom .
Te s t
sou rce
T e s t sm p sl .
ilulm .
te s t s o u rc e
XY Z
U *V W
* *
a d a p t.
tra n s f .
CR I
CR A
Test colour samples
CIE 13.2 Test Samples
0.56
0.54
TS1
0.52
TS2
TS3
0.50
v'
TS4
0.48
TS5
TS6
0.46
TS7
TS8
0.44
D65
0.42
0.40
0.15
0.17
0.19
0.21
u'
0.23
0.25
CRI calculation - 1
• Selecting reference illuminant
• Selecting test colour samples
• Chromatic adaptation (von Kries)
• Colour difference calculation
– U*, V*, W* colour space
Chromatic adaptation
CRI calculation - 2
• Special colour rendering indices:
Rk,t = 100 – 4,6  Ei,k,r-t.
• General colour rendering index:
1 8
Ra   Ri
8 i 1
Assessing daylight simulators - 1
• For colorimetric practice the evaluation of
the UV content also important
• Assessment based on metamerism indices
– 5 metameric pairs
• 3 sets: D55, D65, D75,
• Set for D50 under development
Assessing daylight simulators - 2
• Visible range metamerism index
– average colour difference for the five
metameric pairs
Ei

5
i 1
5
MIvis
Assessing daylight simulators - 3
• UV range assessment
– reflected part of radiation +
– emitted part of radiation
• Final assessment: MI for UV and visible
• Assessment based on CIELAB or CIELUV
colour differences
– Five categories established
Assessing daylight simulators - 4
Category
A
B
C
D
E
MI (CIELAB)
<0,25
0,25 < MI <0,5
0,5 < MI <1,0
1,0 < MI < 2,0
MI > 2,0
MI (CIELUV)
<0,32
0,32 < MI <0,65
0,65 < MI < 1,3
1,3 < MI < 2,6
MI > 2,6
CIE TCs working on colorimetry
• CIECAM colour appearance models
• VDU - Reflective media comparison
• Chromaticity diagram with physiologically
significant axes
• Geometric tolerances in colorimetry
• Updating the colorimetry and colour
rendering documents
Summary
• Light source colorimetry
– lamp light colour
– brightness - task-performance relationship
– colour rendering
• Image reproduction colorimetry
– colour appearance
– colour management