A NEW METHOD FOR INVESTIGATING COLOUR HARMONY DISTORTION OF LIGHT SOURCES
Szabó, F.1 Sárvári, G.1 Tóth, K.2 Balázs, L.2 Schanda J.1
1
Virtual Environment and Imaging Technologies Laboratory, University of Pannonia, Veszprém,
Hungary, 2 GE Hungary Ltd., Budapest, Hungary
szabof@szafeonline.hu
Abstract
The failure of the current CIE Colour Rendering Index1 (CRI) for modern (especially LED based) light
sources is anecdotal. CIE TC1-62 recommended the development of a new colour rendering index or
more indices to make an improved description of light source colour quality2. One of the authors
previously developed new colour harmony models based on visual experiments 3. Based on these new
models of colour harmony, a Colour Harmony Rendering Index (Rhr) was introduced4 to describe the
colour harmony rendering property of light sources as a new colour quality metric. Until now, this
metric is the only one, which deals with the colour harmony rendering property of light sources. A need
of the demonstration of colour harmony rendering phenomenon was raised by lamp designers and
manufacturers as well as end-users, to get familiar with this new quantity. During this workshop
presentation, a newly developed computer software will be introduced.
Keywords: Colour Harmony Rendering, Colour Harmony, Colour Rendering, Colour Quality
1 Introduction
Colour theory has long had the goal of predicting or specifying those colour combinations that
would “work well together” or appear harmonious. Scientists and artists of the last centuries (Itten5,
Munsell6, Ostwald7) and nowadays (Nemcsics8) developed colour order systems where they defined
rules to establish harmonic sets of colours. Other authors (Judd and Wyszecki9) define colour
harmony as a more universal concept: “when two or more colours seen in neighbouring areas produce
a pleasing effect, they are said to produce colour harmony”. Also, there is no consistency among the
principles and the keywords of colour harmony: It is completeness according to Goethe10, order
according to Nemcsics8 and Chevreul11, and balance according to Munsell6. A quantitative model for
two-colour combinations based on the CIELAB colour space was developed by Ou et al12. One of the
authors previously developed new colour harmony models based on CIECAM02 13 colour appearance
model3.
According to the opinion of the authors, it is possible that it is not the colour differences of the test
colour samples under the test light source and the reference light source 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 sample pairs or among three or more colour samples – this is
the issue of the so-called “colour harmony rendering”.
2 Software demonstration of Colour Harmony Rendering
In the following subsections, a computer software will be introduced to demonstrate the
phenomenon of colour harmony rendering. Main features of the software will be introduced as
subsections. Caleidoscope images will be used for visual demonstration.
2.1 Application of Display Calibration Models
In order to achieve colour accurate appearance of coloured figures, display calibration models can be
applied in the program. CRT and LCD calibration models14 are implemented. User has to choose
display type and fill in the parameters of the display (inverse tri-stimulus matrix, gain and gamma
descriptors of the channel characteristic). The Display Calibration sheet of the software can be seen in
Figure 1.
Figure 1. Display calibration sheet.
2.2 Caleidoscope Images
In Figures sheet of the software the visual appearance of coloured patches can be seen in case of two
loaded SPDs. The two loaded SPDs are not necessarily in the reference and test illuminant relation in
this case, they can be two SPDs to be compared according to the users wish. The reflection spectra of
each coloured patch could be changed separately, to test the colour fidelity and colour harmony
rendering capability of light source on different colour sample sets (CIE 13.3 test samples, MBCCC,
FM100 Hue test, etc).
Figure 2. Figures sheet.
2.3 CIECAM02 correlates
CIECAM02 sheet of the software can be divided into two parts: one is related to the light source and
viewing condition section, another is related to the appearance of the different shapes of the
caleidoscope. The parameters of viewing conditions can be set in the “Select Surround” section. In
case of “average”, “dim” and “dark” viewing condition, the parameters F, c and Nc are changed
according to the definitions of viewing conditions in the CIECAM02 colour appearance model.
Parameters can be seen in Table 1. Light source tri-stimulus values (X,Y,Z at Reference White
section) are calculated from the loaded SPDs.
Table 1. Parameters of viewing condition
F
c
Nc
Average
1
0.69
1
Dim
0.9
0.59
0.95
Dark
0.8
0.525
0.8
CIECAM02 data and tri-stimulus values (XYZ section at the bottom of this sheet) are calculated in
case of reference and test light source for each shape (each symbolizing one reflecting sample)
separately. Actual shape can be selected in the “Select shape” section at the top of the sheet.
Figure 3. CIECAM02 sheet.
2.4 Data Table
Table sheet is for the numerical comparison of CIECAM02 data of reflectance shapes. All important
CIECAM02 correlates (Lightness, Brightness, Colourfulness, Chroma, Saturation, Hue composition,
Hue angle) of all shapes can be seen together.
Figure 4. Table sheet.
2.5
Colour Coordinate Vector
Vectors sheet demonstrates the moving of colour co-ordinates of each shape loaded as test colour
samples (symbolizing reflecting samples) in the CIELAB a*-b* plane. One arrow demonstrates the
moving of the colour co-ordinates of one reflectance surface. The origin of one arrow is the a*-b* coordinate of one particular reflecting sample under the illuminant, which have been loaded to the left
side of the display. The destination of the same arrow is the a*-b* co-ordinate of the same reflecting
sample under the illuminant, which have been loaded to the right side of the display. On the right side
of the user interface, the CIE Colour Rendering index Ra1, the value of CRI-CAM02UCS15 (based on
colour differences in the CIECAM02 UCS space) and the Harmony Rendering Index (Rhr)4 can be
seen. CCT of both illuminants are shown.
Figure 5. Vectors sheet.
2.6
Spectral Data of Test and HRI reference light sources
On HRI references sheet the relative spectral power distribution of the 2 lamps (loaded to the right and
left side of the screen) can be seen in the first raw. In the second raw, the SPD of reference
illuminants can be seen (those are selected according to the method of Harmony Rendering Index).
Reference illuminant is selected from the illuminants in Table 2.
Table 2. Reference light sources used by calculating Harmony Rendering Index (Rhr)
CCT[K]
Light source
x
y
Rhr
CIE 13.3 Ra
2686
Incandescent lamp
0.4631
0.4134
100
99.6
3000
Halogen lamp
0.4471
0.4077
99.9
99.5
3834
Halogen lamp
0.3942
0.3960
100
91.8
5000
Daylight simulator D50
0.3459
0.3525
99.1
100
6504
Daylight simulator D65
0.3133
0.3221
99.2
100
Figure 6. HRI references sheet.
2.7
HRI vectors
One of the authors proved, that the distortion of observer’s colour harmony impression depends on the
moving of colour coordinates of colour samples composing the observed colour combination. The socalled “shift invariance” property of colour harmony was proved in a former paper16. In case of
“equivalent-shifts” each of the three perceived attributes (hue, chroma and lightness) changes to the
same extent, this being valid for all colours of the triad. In case of equivalent-shifts, the distortion of
visual colour harmony caused by the illuminant is not significant. In case of “non-equivalent shifts”,
each of the three attributes (hue, chroma and lightness) changes in different extent for the colours
composing the colour combination (that practically means vectors with different directions and
magnitude on the CIELAB a*-b* plane) were also shown to examine the distortion of perceived colour
harmony. HRI vectors sheet shows the moving of colour co-ordinates of the colour samples, which are
composing the two-colour combinations used by calculating Harmony Rendering Index (Rhr). Each
arrow starts from the colour co-ordinate of the test colour sample illuminated by the reference light
source. This reference light source is selected from the light sources listed in Table 2, to be the closest
to the test light source CCT.
Figure 7. HRI vectors of test colour combinations used by calculating HRI.
On the right hand side of Figure 7 Harmony Rendering Vectors of light source with relatively high Rhr
value can be seen. Main measures of light source colour quality (CIE 13.3 Ra, CRI CAM02-UCS, Rhr)
are shown below each test light source diagram separately. On the left hand side of the same Figure
Harmony Rendering Vectors of light source with relatively low Rhr value can be seen. As can be seen,
moving of colour co-ordinates can be said to be much more equivalent, than in case of light source
with low Rhr value.
3 Conclusion
In this paper, a demonstration method of colour harmony rendering property of light sources have
been described. Among light source colour quality measures, Harmony Rendering Index is the only
one, which is based on observers’ colour harmony impression. With the help of this computer
software, phenomenon of colour harmony rendering can be understood and two light sources can be
compared according to this method with each other at the same time.
References
[1] CIE, 1995 Publication 13.3-1995: Method of Measuring and Specifying Colour Rendering Properties of Light Sources.
Vienna: CIE Central Bureau; 1995.
[2]
Commission Internationale de l’Éclairage, CIE TC 1-62, Colour Rendering of White LED Light Sources, 2007, Technical
Report, Publication CIE 177:2007.
[3] SZABÓ F., BODROGI P., SCHANDA J. 2010, Experimental Modelling of Colour Harmony, Color Research and Application
35:1, 34-49
[4] SZABÓ F., BODROGI P., SCHANDA J. 2009, A Colour Harmony Rendering Index Based on Predictions of Colour Harmony
Impression, Lighting Research and Technology 41: 165-182.
[5] ITTEN, J.: Kunst der Farbe, Maier-Verlag, Ravensburg, 1961
[6] MUNSELL, A. H.: A Grammar of Colour. Stratmore Paper Co., Mittineague, Mass., 1921.
[7] OSTWALD, W.: Farbatlas als Ausgabe: Colour Harmony Manual Beziehl. Durch. Musterschmidt-Verlag, Göttingen, 1956.
[8] NEMCSICS A., Das Koloroid, ein farbdynamisches Farbensystem. Period. Polytechn. Arch. (Budapest) 16, 37 (1972a)
[9] JUDD, D. B. – WYSECKI, G.: Color in Business, Science and Industry, Wiley, New York – London, 1963
[10] GOETHE, J. W., 1840, Theory of Colours. Translation by C. L. Eastlake (1840) from the German “Farbenlehre” of 1810.
Reprinted in 1970, Massachusetts: The MIT Press.
[11] CHEVREUL, M. E.,1854, The Principles of Harmony and Contrast of Colors. Translation by C. Martel (1854) from the
French edition of 1839. New York: Van Nostrand Reinhold; 1981.
[12] Li-Chen OU, Ming Ronnier LUO. A Study of Colour Harmony for Two-colour Combinations. Color Research and
Application 2006;31: 191-204
[13] Commission Internationale de l’Éclairage (CIE), Publication 159-2004: A Colour Appearance Model for Colour Management
Systems: CIECAM02, Vienna: CIE Central Bureau; 2004
[14] BERNS R. S, KATOH N 2002, Methods of characterizing displays. Color Engineering: Achieving Device Independent
Colour, 127-164. P. Green and L. W. MacDonald, John Wiley & Sons, England,
[15] LI C., LUO M. R., LI C. J, 2011, The CRI-CAM02UCS Colour Rendering Index, Col. Res Appl., 36(2011) in press.
[16] SZABÓ F., SCHANDA J., BODROGI P., 2007, Experimental Investigation of the Distortion of Colour Harmony, AIC
International Colour Harmony Conference, Budapest, Hungary