New Metric on Light Source Colour Quality:
Colour Harmony Rendering Index
Ferenc
SzabÓ
New metric on light
source
colour quality:
colourUniversity
harmony rendering
index
of Pannonia
Ferenc Szabó
University of Pannonia
ABSTRACT: This study describes the development of a
new metric intended to improve the description of light
source colour quality. Previously, the author has developed
several colour harmony formulae to predict observer’s
impressions of colour harmony for two- and three-colour
combinations seen under well-defined viewing conditions.
One of the applications of these formulae is to quantify the
distortion of colour harmony that can occur on changing
from one light source to another. Based on the colour
harmony predictions of these formulae for a sample set of
colours seen under reference and test light sources a new
light source colour quality metric, the Colour Harmony
Rendering Index is defined. Visual experiment was carried
out to investigate the correlation between visual
observations and this new colour quality metric.
1. INTRODUCTION:
The failure of the current CIE Colour Rendering Index
(CRI)1 for modern (especially LED based) light sources has
been demonstrated. Experience has shown that the current
CRI based ranking of a set of light sources containing white
LED light sources contradicts the visual ranking2-6. Various
attempts have been made to clarify the possible reasons3,4
and to improve7,8 the CRI, but no internationally adoptable
solution has been found. One group of authors identified
the reason for the failure as the use of the outdated CIE
U*V*W* colour space and its colour difference metrics9.
Several attempts have been made to evaluate the visual
colour differences with new colour difference formulae3,5.
Other authors believed that the small number of test
samples may be the problem, and have tried to develop a
sample-independent CRI10 by calculating the maximum
colour solids under reference and test illuminants. Others
believed the choice of the reference illuminant to be the
main problem11. CIE Technical Committee TC 1-69,
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“Colour rendition by white light sources” decided to
abandon colour rendering and to use colour quality as the
generic term to describe the ability of a light source to
render object colours, and to distinguish between colour
fidelity, colour discrimination12, colour preference13,
flattery14 and visual clarity15. A colour quality scale16 has
already been recommended. All these factors influence the
final acceptance of a light source for indoor lighting. The
author would like to extend the above list with a further
aspect of colour appearance. The pleasantness of an interior
depends to a large degree on the harmonious appearance of
the colours seen side by side in the interior. Thus it seemed
to be obvious to suggest as a further light source quality
index a descriptor as to how strongly a light source distorts
the harmony of colours seen in the environment. In the
authors opinion, it is possible that it is not the colour
differences of the colour samples under the test light source
and the reference light source (colour fidelity) which is
visually relevant but the general appearance of all colours
in the field of view under the test and reference light
source, especially the relation between colour samples –
this is the issue of “colour harmony rendering”7.
2. DISTORTION OF COLOR HARMONY UNDER
DIFFERENT LIGHT SOURCES
2.1 Experimental Method
My first hypothesis was that the impression of colour
harmony might change if we changed the reference
illuminant to a test light source. To investigate this
phenomenon an experiment with two- and three-colour
combinations was conducted with the help of a double
viewing booth. The experiment was carried out in a
completely dark room with black walls. The only light
source in this room was the test light source in the case of
1
the visual estimations (full adaptation to the light source
colour). Observers spent 10 minutes before starting the
experiment to adapt to the lighting conditions. The
difference of CCTs of test light sources was within 200 K.
Various sets of light sources, including incandescent lamps,
compact fluorescent lamps, halogen lamps, high pressure
gas discharge lamps, white phosphor LEDs, RGB LEDs
were tested. Light sources were chosen to form three
groups according to their correlated colour temperature
(2700 K, 4000 K and 6500 K). The luminance of the
viewing plane was standardized at 170 cd/m2 ±5% for all
light sources. The uniformity of luminance was within
±10% in the viewing plane. The colorimetric properties of
reference and test light sources are summarized in Table 1.
completely harmonious) scale, according to their
impression of colour harmony. Observers did not have any
time limit, but it took about one minute to make their
judgements.
Table 1b.: Colorimetric data of test illuminants.
CCT[K]
2700
Table 1a.: Colorimetric data of reference illuminants.
Light source
x
y
Rhr
Ra
Nd glass incandescent lamp
0.4571
0.3986
79.9
76
White phosphor LED #1 – High
CRI
0.4616
0.4234
102.3
94
RGB LED
0.4648
0.4121
51.2
32
0.4624
0.4222
116.4
67
0.4609
0.4208
112.2
80
0.4589
0.4162
105.6
94
0.3896
0.3774
98.3
82
White phosphor LED #2 – Low
CRI
White phosphor LED #3 – Mid
CRI
White phosphor LED #4 – High
CRI
CWR LED lamp (containing
6500 K „CoolWhite” and RED
LEDs)
White phosphor LED #5 – Low
CRI
White phosphor LED #6 – High
CRI
CCT[K]
Light source
x
y
Rhr
Ra
2686
Incandescent lamp
0.4631
0.4134
100
100
3000
Halogen lamp
0.4471
0.4077
100
100
3834
Halogen lamp
0.3942
0.3960
100
92
0.3459
0.3525
100
100
Compact Fluorescent Lamp
0.3133
0.3221
100
100
RGB LED
5000
6504
Daylight simulator
D50
Daylight simulator
D65
Among these 5 reference light sources, the incandescent
lamp was used as the reference for the 2700 K light source
group, the halogen lamp was used for the 4100 K light
source group and the daylight simulator D65 was used for
the 6500 K group. Three series of observations were carried
out within 4 months with nine observers. They were all
young adults, university students familiar with colours. All
observers had normal colour vision according to MunsellFarnsworth 100 Hue Test. The inside of the lighting booth
was painted grey to provide a middle grey background
(L*=50) for the observations.
In the experiment observers were presented with 17
different two- and 5 different three-colour combinations
(one combination at one time) made up from Munsell matte
paper samples. Observers were asked to rate the colour
combination seen under the test light source on a -5
(meaning completely disharmonious) to +5 (meaning
4000
6500
White phosphor LED #7 – High
CRI
White phosphor LED #8 – High
CRI
0.3803
0.3800
117.2
76
0.3906
0.3890
110.0
88
0.3929
0.3952
100.1
82
0.3937
0.3745
51.1
34
0.3151
0.3352
102.6
93
0.3147
0.3303
99.3
98
RGB LED
0.3091
0.3213
52.7
47
Compact Fluorescent Lamp
0.3197
0.3527
96.9
74
The spectral power distribution of the reflected light from
the single samples illuminated by the reference and test
light sources was measured with a Photo Research PR-705
spectro-radiometer. The appearance of all test stimuli was
modelled with the help of the CIECAM02 appearance
model19 (using the measured in-situ values (background
luminance and white point) as parameters and average
viewing condition (c=0.69, Nc=1.0, F=1.0)). Colour
harmony predictions were made by using recently
developed colour harmony formulae18 for all two- and
three-colour combinations in all lighting situations.
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2.2 Results
Observer responses were analysed statistically. In Figure 1,
the experimental results can be seen.
Figure 1. Experimental results. Each column represents the mean
visual colour harmony rating from 9 observers making 3 repeated
observations of 17 two-colour combinations and 5 three-colour
combinations. Results are shown with 95% confidence intervals.
As can be seen from Figure 1 colour harmony can change if
the illuminant is changed. In our experiments the RGB
LED test lamp turned out to show the worst colour
harmony rendering. For certain kinds of the white phosphor
LEDs no colour harmony distortion was observed. In some
other cases large harmony changes were found. One
possible reason of the changes in the observer’s visual
colour harmony impression is suggested by the data shown
in Figure 2. Figure 2 shows the CIECAM02 coordinates19
for the specific case of a three-colour combination under
D65 reference, white phosphor LED and RGB LED
illumination. The CCT of the three illuminants are within a
tolerance limit of 20 K. Figure 2 shows that for the RGB
LED test lamp, non-systematic colour shifts are apparent:
The shifts of colour co-ordinates are very different not only
in size but also in direction. In case of the white phosphor
LED, the colour co-ordinates of the sample illuminated by
the reference light source and the test light source
practically coincide.
3. QUANTIFYING THE DISTORTION OF COLOUR
HARMONY
After predicting the colour harmony score for all the test
stimuli for all light sources, the differences between the
colour harmony predictions computed under the reference
54
and the test light sources can be an expressive way to
describe the distortion of colour harmony.
Figure 2. The shift in colour appearance from the D65 reference
for two kinds of test light sources on the CIECAM02 ac-bc plane19
for a specific three-colour combination.
In Figure 3 the colour harmony scores of the test stimuli
resulting from the present visual estimation experiment can
be seen as a function of the predicted colour harmony
scores (CH) for several light sources. Visual estimation
scores on Figure 3 were calculated by averaging the [5...0...+5] judgements of 9 observers’ 3 repetitions for 17
two-colour combinations and 5 three-colour combinations.
Figure 3. The relationship between the colour harmony (CH)
predictions19 and the visual estimation measurements. Each point
denotes mean visual estimation scores (along the abscissa) and
mathematical predictions (along the ordinate) for all test
combinations under one particular light source.
3
As can be seen from Figure 3 there is a significant
correlation between CH and the visual results. In the next
section, the concept of a new light source colour quality
metric, using colour harmony predictions, the Harmony
Rendering Index (HRI) will be described.
+5 are plotted against the Harmony Rendering Index (Rhr)
of the test light sources.
4. COLOUR HARMONY RENDERING INDEX
Perceived colour harmony is subject to change when the
light source illuminating the colour samples changes from a
reference light source to a test light source. Distortion of
colour harmony can be observed if there are very different
and non-systematic colour shifts - concerning both
magnitudes and directions20 - among the samples in the
CIECAM02 colour space19. Similar to the concept of colour
rendering, we suggest calculating the differences in colour
harmony predictions (using our recently developed models)
for a set of test colour sample combinations between the
appearance of test samples under the test light source and
the reference light source. These differences will
characterize the "colour harmony rendering" property of the
test light source by quantifying the extent of distortion of
the perceived colour harmony of a set of test colour
combinations that are harmonious under the reference
illuminant. A "harmony rendering index" (HRI) can be
calculated e.g. by the following formula:
n
R hr = 100 + k * ∑ CHF i , ref − CHF i ,test
(1)
i =1
where
CHFi ,ref is calculated under the reference light
source, CHFi ,test under the test light source, n indicates the
number of test colour sample combinations (harmonious
under the reference light source) used to compute Rhr, and k
is a constant, which was optimised to achieve reasonable
Rhr values. The optimised value of k is 5. This value was
chosen in order to scale Rhr values at a similar numerical
range as Ra. CHF2M, CHF2D, CHF3T and CHF3T can be
substituted in equation (1) depending on the set of 2 or 3
test colour combinations used.
In Figure 4, the results of the visual estimation experiment
mean colour harmony scores measured on a scale of -5 to
Figure 4. The relationship between the mean colour harmony
scores of the visual estimation experiment and the value of Rhr of
the test light sources
As can be seen from Figure 4, Rhr is a good descriptor of
the harmony rendering property of the test light sources. A
correlation of r2=0.783 was found between the colour
harmony ratings and the Rhr value of the test light sources.
5. DISCUSSION AND CONCLUSIONS
In this paper, colour harmony has been investigated to
develop a new Harmony Rendering Index (Rhr) that can be
used to describe light source colour quality. This Rhr metric
is based on the colour harmony predictions, provided by
four recently developed colour harmony formulae. These
formulae predict colour harmony impression from the
CIECAM02 correlates of the member colours of two- or
three-colour combinations. A new colour harmony dataset
was collected in laboratory experiments using 15 light
sources and 17 colour combinations.
From the experimental results, one possible reason for the
change in colour harmony under different light sources was
discovered: Non-systematic shifts of colour appearance
under the reference light source produced by the application
of the test light sources. The occurrence of this
phenomenon is closely related to the distortion of colour
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harmony. Some test light sources produced large nonsystematic shifts while others did not.
After the statistical analysis of results, colour harmony
predictions were made for all the colour combinations used
for all light sources. These predictions correlated well with
the results of the experiment.
A new Harmony Rendering Index (Rhr) was also defined
from the sum of the differences of colour harmony
predictions for a set of colour combinations for the
reference and test light sources. Rhr was calculated for a set
of test light sources including incandescent and halogen
lamps, compact fluorescent lamps, white phosphor LEDs,
RGB LEDs at various CCTs as well as for theoretical
spectral power distributions. This harmony rendering index
(Rhr) based on the recently developed colour harmony
formulae, showed good correlations with the results of the
experiments. The authors suggest using Rhr to supplement
the current Colour Rendering Index of the CIE to describe
the colour quality of light sources.
REFERENCES:
[1]
[2]
[3]
[4]
[5]
[6]
Commission Internationale de l’Éclairage (CIE), Publication
13.3-1995: Method of Measuring and Specifying Colour
Rendering Properties of Light Sources. Vienna: CIE Central
Bureau; 1995.
Schanda J., Madár G., Sándor N., Szabó F., Colour
rendering – colour acceptability: Proceedings of the 6th
International Lighting Research Symposium on Light and
Colour, Orlando, FL, 2006
Sándor N., Schanda J. Visual colour-rendering based on
colour difference evaluations. Lighting Research and
Technology 2006; 38: 225-239.
Bodrogi P., Szabó F., Csuti P., Schanda J. Why does the CIE
colour rendering index fail for white RGB LED light
sources?: Proceedings of CIE Expert Symposium on LED
light sources, Tokyo, 2004
Sándor N., Bodrogi P., Csuti P., Kránicz B., Schanda J.
Direct visual assessment of colour rendering: Proceedings
of the 25th CIE Session, San Diego 2003, CIE Publ. CIE
152:2003 D1-42-45
Guo X, Houser KW. A review of colour rendering indices
and their application to commercial light sources. Lighting
Research and Technology 2004; 36: 183-199.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
Commission Internationale de l’Éclairage, CIE TC 1-62,
Colour Rendering of White LED Light Sources, Technical
Report, Publication CIE 177:2007.
van Kemenade JTC, van der Burgt PJM, Towards a user
oriented description of colour rendition of light sources,
CIE 119-1995-23rd Session, New Delhi, pp. 43-46, 1995.
Commission Internationale de l’Eclairage: Colour rendering,
TC 1-33 closing remarks. Publ. CIE 135/2 1999.
H. Xu. Sample-Independent Color Rendering Index, Color
Research and Application, 1995; 20:251-254
Rea MS, Freyssinier-Nova JP, A Tale of Two Metrics,
Colour Research and Application, 2008; 33:193.
Thornton WA. Colour-discrimination index. Journal of the
Optical Society of America 1972; 62: 191-194.
Einhorn HD. Colour preference index (Principles and
formulation for Warm White lighting), CIE Compte Rendu,
London, 1975; pp. 297-304.
Jerome CW. Flattery vs colour rendition. Journal of
Illuminating Engineering Society 1972; April, p. 208-211.
Bellchambers HE, Godby AC. Illumination, colour
rendering and visual clarity. Lighting Research and
Technology, 1972; 4: 104-106.
Davis W, Ohno Y. Toward an improved color rendering
metric. Fifth International Conference on Solid State
Lighting, Proc. SPIE, 5941, 59411G, 2005.
Ou LC, Luo, MR. A Study of Colour Harmony for Twocolour Combinations. Color Research and Application
2006;31: 191-204
Szabó F., Bodrogi P., Schanda J.: Experimental Modelling
of Colour Harmony. Color Research and Application 2010;
35: 34-49
Commission Internationale de l’Éclairage (CIE), Publication
159-2004: A Colour Appearance Model for Colour
Management Systems: CIECAM02, Vienna: CIE Central
Bureau; 2004
Szabó F., Schanda J., Bodrogi P. Experimental Investigation
of the Distortion of Colour Harmony: Proceedings of the
AIC International Conference on Colour Harmony;
Budapest, 2007
Ohno, Y., Color Rendering and Luminous Efficacy of White
LED Spectra: Proceedings of the SPIE 4th International
Conference on Solid State Lighting (Denver, CO, August
2004), 5530, p. 88-98.
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