CIE Standards for assessing quality of light sourc

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CIE Standards for assessing quality of light sources
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CIE Standards for assessing quality of light sources
J Schanda
University Veszprém, Department for Image Processing and Neurocomputing, Hungary
1.
Introduction
CIE publishes Standards and Technical Reports that can be Recommendations, Guides or state
of the art reports. In the field of colorimetry there are two Standards which are joint
publications of ISO and CIE:
• CIE colorimetric illuminants (1)
• CIE colorimetric observers (2)
The International Lighting Vocabulary (3), a joint publication with IEC * contains the basic
terms and definitions used in colorimetry. CIE is currently updating this fundamental
publication.
Colorimetry, 2nd edition (4) contains all the definitions of colorimetric terms, discusses CIE
colorimetric practice and provides many useful tables to perform colorimetric calculations. CIE
Technical Committee TC 1-48 deals at the present moment with updating this publication. The
update will not change anything basic in CIE colorimetry. It will mainly get the
recommendations in line with the recommendations passed since 1986.
One such item is colour difference calculation, where a new colour difference equation was
introduced in 1994 (CIE94) (5). Another item is the assessment of the quality of daylight
simulators (6), a question we would like to discuss in more detail in this paper.
Colour rendering is a fundamental issue in characterising light-sources, and it becomes more
and more important also in imaging applications. Thus it will be another item of this paper. The
CIE publication on colour rendering (7) also needs some updating, and TC 1-33 is currently
working on the subject. The paper will discuss items related to this procedure as well.
There are a number of further publications relevant to colour and imaging enumerated in
Appendix A.
In imaging applications the original is seen either as a scene in real life, or as a paper copy,
eventually projected on screen or reproduced by a self-luminous display. The reproduction can
be again a paper copy, a projection or a soft-copy on the display. The colours seen on such
originals or reproductions depend on the illumination due to two reasons:
• The reflected/transmitted light of the object seen depends on its illumination; this can be
calculated by knowing the spectral power distribution of the source.
• The illuminant influences also the state of adaptation of our visual system and this controls
colour perception.
*
IEC: International Electrotechnical Commission, Geneva; the ILV is part of the IEC Electrotechnical Vocabulary.
Schanda: CIE Standards for assessing quality of light sources
In the present paper we will not deal with the chromatic adaptation and colour appearance
aspects of the illumination, as other papers of the meeting are discussing this effect in detail.
We will concentrate on the more direct effect the illumination has on the colours we perceive.
2.
Characterisation of the light source
In case of a light source it is usual to distinguish between the colour of the source itself, i.e. the
colour we perceive if looking at the source or at an achromatic surface illuminated by the
source. In the following this will be termed lamp colour. One has to distinguish between lamp
colour and the colour the lamp light has on coloured surfaces. This phenomenon is described by
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.”(8)
The primary physical quantity to determine the colorimetric properties of a light source is its
spectral power distribution. For constructing a description of colour rendering one needs –
according to above definition – also a reference illuminant and some objects whose colour is
evaluated.
Figure 1 shows the spectral power distribution of Standard Illuminant D65 and the necessary
amount of three spectral lights (450 nm, 550 nm and 650 nm) to get the same chromaticity coordinates as produced by D65.
Spectral power distribution
180
160
140
rel. power
120
100
80
60
40
20
0
350
400
450
500
550
600
650
700
750
800
850
900
wavelength, nm
Figure 1: Spectral power distribution of D65 and of three monochromatic radiations with the
same chromaticity co-ordinates, i.e. metameric to D65.
In Figure 2 we show a part of the x,y chromaticity diagram, with the chromaticity of D65, the
three line source, and a sample illuminated by these two sources. It is evident from Figure 1 and
2
Schanda: CIE Standards for assessing quality of light sources
Figure 2 that the chromaticity co-ordinates of the lamp colour are not enough to describe the
effect the illuminant will have on surface colours (or in case of transparencies the projector
lamp will have on the projected colours). The colour rendering index is the measure for
describing the effect lamp-light has on the appearance of object-colours.
0.342
0.340
D65
0.338
3-line
y
0.336
0.334
0.332
0.330
0.328
0.300
0.350
0.400
x
Figure 2: Chromaticity co-ordinates of the two sources (two coinciding signs on the left) whose
spectrum is shown in figure 1 and the chromaticity of a reflection sample illuminated by these
two sources (two different locations on the right).
3.
Colour rendering
As already stated in the previous section, colour rendering serves to describe the effect of an
illuminant on the colour appearance of objects. It is based on the assumption that some sources
“render” the colours of our environment in a natural form, i.e. the colour of objects looks under
these sources “natural”, while others “distort” the rendition. The most fundamental natural
source is obviously daylight, thus the primary reference for comparing the rendering of a lamp
should be the Standard Illuminant D65. Colour rendering was not an issue in the days before
electrical lighting, not even in the early days of incandescent lamp lighting, although it is quite
obvious that a low temperature incandescent lamp, as used at the beginning of our century,
rendered colours quite differently from daylight. The question of colour rendering became
apparent only when gas discharge lamps became available, and lamps with the same lamp
colour but different spectra got onto the market. These lamps rendered the colours of objects
differently.
Due to the fact that incandescent lamps were regarded to render the colours correctly, a
description of colour rendering was looked for, where the test lamp could be compared either
with a daylight chromaticity or an incandescent lamp chromaticity. It was arbitrarily decided to
take as reference illuminant a Planckian-radiator (black-body) if the test lamp had a correlated
colour temperature (CCT) below 5000 K, and a phase of daylight if the CCT of the test lamp
was higher. In both cases the reference illuminant had to have the same, or nearly the same
CCT as the test lamp.
The definition of colour rendering stated further on that the colour of objects had to be
compared. To do that in a reproducible form, some colour samples (test colour samples, TCS)
had to be selected for which the effect of illumination with the reference and test illuminant
could be compared.
3
Schanda: CIE Standards for assessing quality of light sources
3.1
CIE colour rendering index calculation
The CIE published its first recommendation on colour rendering index (CRI) calculation in
1965. The method now in use was accepted in 1974 (7). This method defines eight basic test
colour samples and five supplementary ones, selected from the Munsell book of colour. Table 1
shows the approximate Munsell notation and the colour appearance under daylight. The
spectral radiance factors can be found in the CIE recommendation (7) and as a file in (9).
k=TCS no. Approximate Munsell notation Colour appearance under daylight (10)
1
7,5 R 6 / 4
Light greyish red
2
5 Y 6/4
Dark greyish yellow
3
5 GY 6/8
Strong yellow green
4
2,5 G 6/6
Moderate yellowish green
5
10 BG 6/8
Light bluish green
6
5 PB 6/8
Light blue
7
2,5 P 6/8
Light violet
8
10 P 6/8
Light reddish purple
9
4,5 R 4/13
Strong red
10
5 Y 8/10
Strong yellow
11
4,5 G 5/8
Strong green
12
3 PB 3/11
Strong blue
13
5 YR 8/4
Light yellowish pink (human complexion)
14
5 GY 4/4
Moderate olive green (leaf green)
Table 1: CIE test colour samples used for colour rendering index calculation.
3.1.1
Reference illuminant, determination of the correlated colour temperature of the test lamp
The first step to calculate a colour rendering index is to determine the CCT of the test lamp.
This is done using the CIE 1960 Uniform Chromaticity Scale (u,v) diagram**. In this diagram
one plots the locus of the Planckian radiators of different temperatures, and determines for
which Planckian radiator the chromaticity difference between the test lamp chromaticity and the
Planckian locus is a minimum. Stated differently, one draws a perpendicular line from the test
lamp chromaticity onto the Planckian locus. This line is called iso-temperature line, as all
sources lying on this line have the same CCT. Where this line intersects the Planckian locus,
there is the Planckian radiator with the temperature one is looking for. Figure 3 shows in the
CIE x,y diagram the Planckian locus and several iso-temperature lines.
3.1.2 Colour differences between TCS colours illuminated by the two illuminants, effect of
chromatic adaptation
Once the reference illuminant has been determined, the task is to compare the colour of the
TCSs illuminated by the test lamp and the reference illuminant.
To do that, one has to determine the tristimulus values of the TCSs, once illuminated by the test
lamp and then by the reference illuminant. In the following the symbol Ti will be used for the
** The u,v chromaticity co-ordinates are calculated by the following equations:
u = 4x /( − 2 x + 12 y + 3), v = 6 y /( − 2 x + 12y + 3)
4
Schanda: CIE Standards for assessing quality of light sources
tristimulus values, with i=1 for X, 2 for Y and 3 for Z. Index t stands for the test lamp, index r
for the reference illuminant. The TCS number is shown by the index k (k =0, 1 … 14, with 0 for
the test lamp/reference illuminant). Thus the two sets of tristimulus values are:
Ti,k,r and Ti,k,t
The chromaticity difference between Ti,k=0,r and Ti,k=0,t ,
i.e. between the two source
chromaticities (called in the CIE document “adaptive colour shift”) is bridged by a von Kries
chromatic adaptation transformation (11).
The colour difference between Ti,k,r and Ti,k,t for k = 1, … 14 is then calculated using the CIE
1964 Uniform Colour Space: ∆Ei,k,r-t.
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
0.45
0.50
0.55
x
Figure 3: Planckian locus and some iso-temperature lines calculated using the u,v diagram.
3.1.3
Calculation of the special and general colour rendering indices
The ∆Ei,k,r-t values are obviously zero if the test lamp spectrum equals the spectrum of the
reference illuminant. Differences between the two spectra will cause differences between the
tristimulus values of the TCSs illuminated by the test lamp and the reference illuminant, and
thus will produce ∆Ei,k,r-t > 0 values; i.e. with increasing difference between the colour
rendering properties of the two sources, increasing ∆Ei,k,r-t values will be observed. The TC
responsible for the development of the colour rendering test method thought that this is less
convenient for general application. They suggested a further transformation, so that lamps
whose spectrum equals that of the corresponding reference illuminant should have an index of
100, and lamps whose colour rendering deviates from the colour rendering of the reference
illuminant should have an index smaller than 100. It was arbitrarily decided that the so called
Warm White fluorescent lamp should have a general colour rendering index (see later) of 50.
With this in mind, the special colour rendering indices are calculated from the colour
differences as:
5
Schanda: CIE Standards for assessing quality of light sources
Rk,t = 100 – 4,6 ∆ Ei,k,r-t.
To facilitate the evaluation of the colour rendering of a lamp, a further simplification has been
introduced: The general colour rendering index (Ra) is the average of the first eight special
colour rendering indices:
Ra =
1 8
∑ Ri
8 i =1
It has to be emphasised, however, that such an averaging necessarily decreases the information
contained in the index. If two lamps have the same general colour rendering index, it does not
mean that when illuminating a particular test colour sample with the two sources the colour
appearance of the sample will be the same. The comparison of the special colour rendering
indices will provide better insight into the similarity of the two lamps, but even in this case the
same Ris do not mean that in a complex scene colours will look the same if illuminated with one
or the other source.
This is even more so if colours produced in different media are compared: e.g. an original
interior scene illuminated with a tri-band fluorescent lamp and its reproduction on a VDU
screen. There is no standardised method to compare such situations. It is also extremely
difficult, as differences between the viewing situations (adaptation, size, etc.) have a complex
influence on the colour sensation.
3.2
Possibilities of improving the CRI calculation method
As seen in the above section, the currently used CRI method is based at several points on
outdated calculation methods. The use of the u,v-diagram to determine iso-temperature lines
was a natural choice in the 1960’s, but it can be questioned since the introduction of the
CIELUV and CIELAB spaces (4) in 1976. We will discuss this issue at the CIM’98 conference
(12).
The 1964 UCS has also been superseded by the CIELUV and CIELAB spaces. CIE has
tentatively adopted a chromatic adaptation transformation (13) and is at present in the course of
providing a further recommendation as part of its colour appearance model. We have analysed
all these questions in CIE TC 1-33, together with the possibility of using test colour samples
readily available, and other methods of selecting reference illuminants (14). This method has
not been accepted by the CIE yet, there are some questions still under consideration, but it is
hoped that in a not too distant future the colour rendering calculation can be updated and
brought in line with present day colorimetric practice.
4.
Assessing daylight simulators
Besides CRI determination, another method is also in use for evaluating daylight simulators. As
well-known with the standardisation of D55, D65 and D75 illuminants, no corresponding
sources were standardised. The CIE Technical Report 51 (6) was designed to evaluate how well
a simulator (a properly filtered real source) approximates the standard illuminant. Daylight
illuminants are used in textile, paper, etc. colorimetry to evaluate fluorescent materials, too, thus
this method had to test also the ultraviolet part of the spectral power distribution. It is based on
determining metamerism indices.
6
Schanda: CIE Standards for assessing quality of light sources
Five sets of metameric data are recommended for testing the simulator. Each set includes the
spectral reflection radiance factor of a standard and three metamers (for D55, D65 and D75,
resp.) to the standard. The CIE 1964 Supplementary Colorimetric Observer (10° Observer) is
used in the calculations.
The visible range evaluation is done by calculating tristimulus values for the five metameric
pairs (the standard and the metameric pair appropriate for the standard illuminant in question).
CIELUV or CIELAB colour difference formulae are used to calculate ∆Ei values (i stands for
describing the five sets). The visible range metamerism index is calculated from the individual
colour differences by:
MI vis =
5
∑
i =1
∆Ei
5
For the ultraviolet range the calculation is a little bit more complicated as the spectral total
radiance factor for the fluorescent sample part of each metameric pair is calculated by adding
the reflected and emitted parts of the radiation. For that the report provides data of the spectral
external radiant efficiency, the spectral reflection radiance factor and the relative spectral
distribution of radiance emitted by fluorescence for the three samples. The CIE provides a disk
that contains a program to calculate the MIvis and MI uv of a D65 simulator (15). A CIE
Technical Committee is discussing the updating of the report that it should include a method
and samples to determine a metamerism index also for a D50 simulator which is important in
imaging technologies.
Table 2 shows the five categories into which sources may be categorised. The MI values are
presented both for CIELAB and CIELUV colour difference calculations, as the average length
of colour difference vectors in the two colour spaces differ.
Category
MI (CIELAB)
MI (CIELUV)
A
<0,25
<0,32
B
0,25 < MI <0,5
0,32 < MI <0,65
C
0,5 < MI <1,0
0,65 < MI < 1,3
D
1,0 < MI < 2,0
1,3 < MI < 2,6
E
MI > 2,0
MI > 2,6
Table 2: Categories of sources based on metamterism indices.
In stating the quality of a daylight simulator by this method, first the category designator for the
visible range metamerism index has to be given, followed by the ultraviolet range category
descriptor, and the colour space used in brackets. Thus e.g. a daylight simulator of category BC
(CIELAB) is one with visible range metamerism index between 0,25 and 0,5 and ultraviolet
range metamerism index between 0,5 and 1,0. Such a simulator is found to be useful for many
applications.
It is expected that when the method becomes updated for D50 simulators, it will provide a
useful amendment to the colour rendering index in evaluating e.g. colour matching booth
lighting. The CRI evaluates only the visible part of the spectrum, the MI has two parts
evaluating both the visible and the ultraviolet range. The MI is also an average value, similarly
to the CRI, thus the colour impression a scene provides may be different if illuminated by two
simulators whose visible range metamerism index falls into the same category.
7
Schanda: CIE Standards for assessing quality of light sources
5.
CIE Technical Committees working on the above questions
During the discussion of light source colorimetric quality evaluation, we have already
mentioned that some of the standards and reports are under updating. There are a number of
CIE Technical Committees (TCs) working on such updates. Appendix B shows the list of those
TCs whose work is most relevant to the questions discussed at this meeting.
From this list we would like to draw the attention to the following:
TC 1-27 and 1-34 deal with different aspects of colour appearance. At the time of writing this
report, the simplified model of colour appearance description, CIECAMs, is in the balloting
phase, and it will certainly be published in the very near future.
TC 1-21, 1-26, 1-30, 1-42 are extending our knowledge on the brightness sensation. One can
hope that based on their work the discrepancy between results predicted by the photometric
system and some brightness perceptions will be dissolved.
TC 1-36 has the ambitious task to re-evaluate the colour matching functions and provide a more
detailed set of such functions to be used also at smaller than 2° angular subtense. This is very
important in VDU applications, as colours seen on the screen can be narrow lines. For such
colours the 2° Observer is not valid any more.
TC 1-38 is looking into the problem of getting colorimetric data into a compatible format, i.e.
using them in different applications the same wavelength ranges and increments should apply.
The development of practical daylight sources (TC 1-44) and the extension of the index of
metamerism to D50 simulators (TC 1-45) progresses as well, although the design of a practical
daylight source seems to be a difficult question.
The geometric tolerances for colorimetry (TC 2-39) was a subject requested by the imaging
technologies, but it turned out that it involves both instrumental and visual investigations and
needs detailed review.
6.
Summary
Light source colorimetry is a subject that is as old as CIE colorimetry. The first CIE
recommendations on colorimetry (1931) were passed to help describe light sources and signal
lights more completely. During the past 67 years many new recommendations were added to
the frame-work of colorimetry, which have profound influence also on light source colorimetry.
Most important among them are the introduction of uniform colour scales and colour spaces,
especially that of the CIELUV and CIELAB colour spaces, the establishment of new
descriptions of colour differences (CIE94 colour difference equation), and most recently the
introduction of the CIECAMs colour appearance model.
All these new achievements have the consequence that recommendations on light source colour
rendering, metamerism indices and similar descriptors of sources have to be reconsidered and
brought in line with findings of modern colorimetry.
The ever increasing importance of imaging technology in our everyday lives challenges also
CIE activity. The establishment of a new CIE Division, to serve the needs of the imaging
8
Schanda: CIE Standards for assessing quality of light sources
technologies, and especially the use of colour in imaging, is under way. It will be the task of
this new CIE Division to co-ordinate research among the different schools working in the field;
to organise meetings, seminars and workshops to foster progress; to develop recommendations
based on the new research results; and to prepare harmonised standards taking the needs of the
different media and industries into account.
In summary one can state that CIE Technical Committees and Divisions, working towards the
understanding and standardisation of light source and multimedia colorimetry can expect bright
and colourful future.
References
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CIE Standard; CIE Colorimetric illuminants, ISO/CIE 10526 – 1991 (CIE S001-1986)
CIE Standard; CIE colorimetric observers, ISO/CIE 10527 – 1991 (CIE S002- 1986)
CIE International Lighting Vocabulary, CIE 17.4 - 1987
CIE Technical Report; Colorimetry, 2nd Edition, CIE 15.2 - 1986
CIE Technical Report; Industrial colour-difference evaluation, CIE 116-1995
CIE Technical Report; A method for assessing the quality of daylight simulators for
colorimetry, CIE 51 - 1981
CIE Technical Report; Method of measuring and specifying colour rendering properties
of light sources, CIE 13.2-1974
CIE International Lighting Vocabulary, item 845-02-59, CIE 17.4 - 1987
CIE disk, CIE colorimetry and colour rendering tables, CIE D002
National Bureau of Standards; The ISCC-NBS method of designating colours and a
dictionary of colour names, Washington, NBS Circular No. 553, 1955
Münch W, Schultz U; Zur Berücksichtigung der Farbumstimmung in der Bewertung der
Farbwiedergabe, Farbe, vol. 19, pp130-134 1970
Schanda J and co-workers, Effects of white point definition on computer screens, Colour
Imaging in Multimedia – CIM’98 Conference 1998
CIE Technical Report; A method of predicting corresponding colours under different
chromatic and illuminance adaptations, CIE 109 - 1995
Schanda J; The new CIE colour rendering formula, Proc AIC COLOR 97, Vol. 1 pp371374 1997
CIE disk, A method for assessing the quality of D65 daylight simulators for colorimetry
(based on Publ. 51), CIE D005
Appendix A
CIE Publication relevant to colorimetry and imaging
Publ. Nr. Full title
Standards
ISO/CIE International Standard ISO/CIE 10526: CIE standard colorimetric illuminants
100526E
ISO/CIE International Standard ISO/CIE 10527: CIE standard colorimetric observers
100527E
9
Schanda: CIE Standards for assessing quality of light sources
Techn.
Reports
013.3
015.2
018.2
038
041
044
046
051
053
054
059
060
063
064
065
069
075
076
078
080
081
085
086
087
095
101
103/1
105
109
114/1
114/4
114/5
116
118
122
124
Method of measuring and specifying colour rendering properties of light
sources
Colorimetry, 2nd ed.
The basis of physical photometry, 2nd ed.
Radiometric and photometric characteristics of materials and their measurement
Light as a true visual quantity: Principles of measurement
Absolute methods for reflection measurements
A review of publications on properties and reflection values of material
reflection standards
A method for assessing the quality of daylight simulators for colorimetry
Methods of characterizing the performance of radiometers and photometers
Retroreflection: Definition and measurement
Polarization: Definitions and nomenclature, instrument polarization
Vision and the visual display unit work station
The spectroradiometric measurement of light sources
Determination of the spectral responsivity of optical radiation detectors
Electrically calibrated thermal detectors of optical radiation (absolute
radiometers)
Methods of characterizing illuminance meters and luminance meters:
Performance, characteristics and specifications
Spectral luminous efficiency functions based upon brightness matching for
monochromatic point sources, 2° and 10° fields
Intercomparison on measurement of (total) spectral radiance factor of
luminescent specimens
Brightness-luminance relations: Classified bibliography
Special metamerism index: Change in observer
Mesopic photometry: History, special problems and practical solutions
Solar spectral irradiance
CIE 1988 2° spectral luminous efficiency function for photopic vision
Colorimetry of self-luminous displays - A bibliography
Contrast and visibility
Parametric effects in colour-difference evaluation
Colour appearance analysis
Spectroradiometry of pulsed optical radiation sources
A method of predicting corresponding colours under different chromatic and
illuminance adaptations
Survey of reference materials for testing the performance of spectrophotometers
and colorimeters (reprint)
Distribution temperature and ratio temperature
Terminology relating to non-selective detectors
Industrial colour-difference evaluation
CIE Collection in Colour and Vision
The Relationship between Digital and Colorimetric Data for ComputerControlled CRT Displays
CIE Collection in colour and vision 1997
10
Schanda: CIE Standards for assessing quality of light sources
CIE Disks
D001
Disc version of CIE photometric and colorimetric data (Publ. 18.2, 86, S001 and
S002 tables)
D002
CIE colorimetry and colour rendering tables
D005
A method for assessing the quality of D65 daylight simulators for colorimetry
(based on Publ. 51)
D007
A method of predicting corresponding colours under different chromatic and
illuminance adaptations (Computer program to Publ. CIE 109)
Appnedix B
CIE TCs working on items related to colour and imaging
No.
1-21
1-26
1-27
1-30
1-33
1-34
1-36
1-38
1-42
1-43
1-44
1-45
1-46
1-47
1-48
2-25
2-33
2-39
2-42
TITLE
Testing of Supplementary Systems of Photometry
Individual Variation of Heterochromatic Brightness Matching
Specification of Colour Appearance for Reflective Media and Self-Luminous
Display Comparisons
Luminous Efficiency Functions
Colour Rendering
Testing of Colour Appearance Models
Fundamental Chromaticity Diagram with Physiologically Significant Axes
Compatibility of Tabular Spectral Data for Computational Purposes
Colour Appearance in Peripheral Vision
Rod Intrusion in Metameric Colour Matches
Practical Daylight Sources for Colorimetry
Revision of CIE Publication 51 to Include D50 Simulators
Concept and Application of Equivalent Luminance
Hue and lightness dependent correction to industrial colour difference evaluation
Revision of CIE Document 15.2 Colorimetry
Calibration Methods and Photoluminescent Standard for Total Radiance
Factor Measurement
Re-formulation of CIE Standard Illuminants A and D65 (Revision of CIE/ISO
10526)
Geometric Tolerances for Colorimetry
The Colorimetry of Visual Displays
11
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