Standard illuminant

A standard illuminant is a profile or spectrum of visible light which is published in order to allow images or colors recorded under different lighting to be compared.

CIE illuminants

The International Commission on Illumination (usually abbreviated CIE for its French name) is the body responsible for publishing all of the well-known standard illuminants. Each of these is known by a letter or by a letter-number combination.

Illuminants A, B, and C were introduced in 1931, with the intention of respectively representing average incandescent light, direct sunlight, and average daylight. Illuminants D represent phases of daylight, Illuminant E is the equal-energy illuminant, while Illuminants F represent fluorescent lamps of various composition.

There are instructions on how to experimentally produce light sources ("standard sources") corresponding to the older illuminants. For the relatively newer ones (such as series D), experimenters are left to measure to profiles of their sources and compare them to the published spectra:

Nevertheless, they do provide a measure, called the Metamerism Index, to assess the quality of daylight simulators. [cite book|author=CIE Technical Report|year=1999|isbn=92 9034 051 7|title=A Method for Assessing the Quality of Daylight Simulators for Colorimetry|url=http://www.cie.co.at/publ/abst/51-2-99.html|series=51.2-1999 (including Supplement 1-1999)|quote=A method is provided for evaluating the suitability of a test source as a simulator of CIE Standard Illuminants D55, D65, or D75. The Supplement, prepared in 1999, adds the CIE Illuminant D50 to the line of illuminants where the method can be applied to. For each of these standard illuminants, spectral radiance factor data are supplied for five pairs of nonfluorescent samples that are metameric matches. The colorimetric differences of the five pairs are computed for the test illuminant; the average of these differences is taken as the visible range metamerism index and is used as a measure of the quality of the test illuminant as a simulator for nonfluorescent samples. For fluorescent samples, the quality is further assessed in terms of an ultraviolet range metamerism index, defined as the average of the colorimetric differences computed with the test illuminant for three further pairs of samples, each pair consisting of a fluorescent and a nonfluorescent sample which are metameric under the standard illuminant.] [cite book|author=CIE Standard|title=Standard Method of Assessing the Spectral Quality of Daylight Simulators for Visual Appraisal and Measurement of Colour|series=S012/E:2004|year=2004|url=http://www.cie.co.at/publ/abst/s012.html Prepared by TC 1-53 "A Standard Method for Assessing the Quality of Daylight Simulators". [http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=41694 ISO Standard 23603:2005(E)] .] The Metamerism Index tests how well five sets of metameric samples match under the test and reference illuminant. In a manner similar to the Color Rendering Index, the average difference between the metamers is calculated. [cite journal|title=Evaluation of the quality of different D65 simulators for visual assessment|first=Yuk-Ming|last=Lam|coauthors=Xin, John H.|year=2002|month=August|journal=Color Research & Application|volume=27|issue=4|pages=243–251|doi=10.1002/col.10061]

Illuminant A

The CIE defines illuminant A in these terms:

The spectral radiance of a black body follows Planck's law:

$M\_\{e,lambda\}(lambda,T)=frac\{c\_1\; lambda^\{-5\{expleft(frac\{c\_2\}\{lambda\; T\}\; ight)-1\}$

At the time of standardizing illuminant A, both $c\_1=2pi\; cdot\; h\; cdot\; c^2$ (which does not affect the relative SPD) and $c\_2=h\; cdot\; c/k$ were different. In 1968, the estimate of c₂ was revised from 0.001438 m·K to 0.014388 m·K (and before that, it was 0.001435 m·K when illuminant A was standardized). This difference shifted the Planckian locus, changing the color temperature of the illuminant from its nominal 2848 K to 2856 K:

$T\_\{new\}=T\_\{old\}\; imes\; frac\{1.4388\}\{1.435\}\; =\; 2848\; ext\{K\}\; imes\; 1.002648\; =\; 2855.54\; ext\{K\}$

In order to avoid further possible changes in the color temperature, the CIE now specifies the SPD directly, based on the original (1931) value of c₂:

$S\_\{A\}(lambda)=100\; left(frac\{560\}\{lambda\}\; ight)^5\; frac\{exp\; frac\{1.435\; imes\; 10^7\}\{2848\; imes\; 560\}-1\}\{expfrac\{1.435\; imes\; 10^7\}\{2848\; lambda\}-1\}$

The coefficients have been selected to achieve a peak SPD of 100 at 560 nm. The tristimulus values are (X,Y,Z) = (109.85,100.00,35.58), and the chromaticity coordinates using the standard observer are (x,y)=(0.44758, 0.40745).

Illuminants B and C

Illuminants B and C are daylight simulators. They are derived from Illuminant A by using a liquid filters. B served as a representative of noon sunlight, with a correlated color temperature (CCT) of 4874 K, while C represented average day light with a CCT of 6774 K. They are poor approximations of any common light source and deprecated in favor of the D series:cite book|title=Colorimetry: Understanding the CIE System|first=János|last=Schanda|publisher=Wiley Interscience|year=2007|chapter=3: CIE Colorimetry|page=37-46|isbn=978-0-470-04904-4]

The liquid filters, designed by Raymond Davis, Jr. and Kasson S. Gibson in 1931, [cite journal|first=Raymond|last=Davis|coauthors=Gibson, Kasson S.|title=Filters for the reproduction of sunlight and daylight and the determination of color temperature|publisher=National Bureau of Standards|year=1931|journal=Precision Measurement and Calibration|volume=10|pages=641–805|month=January 21] have a relatively high absorbance at the red end of the spectrum, effectively increasing the CCT of the gas lamp to daylight levels. This is similar in function to a CTO color gel that photographers and cinematographers use today, albeit much less convenient.

Each filter uses a pair of solutions, comprising specific amounts of distilled water, copper sulfate, mannite, pyridine, sulfuric acid, cobalt and ammonium sulfate. The solutions are separated by a sheet of uncolored glass. The amounts of the ingredients are carefully chosen so that their combination yields a color temperature conversion filter; that is, the filtered light is still white.

Illuminant series D

Derived by Judd, MacAdam, and Wyszecki,cite journal|url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-54-8-1031| title=Spectral Distribution of Typical Daylight as a Function of Correlated Color Temperature|first=Deane B.|last=Judd|coauthors=MacAdam, David L.; Wyszecki, Günter|journal=JOSA|year=1964|month=August|volume=54|issue=8|pages=1031–1040] the D series of illuminants are constructed to represent natural daylight. They are difficult to produce artificially, but are easy to characterize mathematically.

H. W. Budde of the National Research Council of Canada in Ottowa, H. R. Condit and F. Grum of the Eastman Kodak Company in Rochester, New York,cite journal
author = Condit, Harold R.
coauthors = Grum, Frank
year = 1964
month=July
title = Spectral energy distribution of daylight
journal = JOSA
volume = 54
issue=7
pages = 937–944
url = http://www.opticsinfobase.org/abstract.cfm?URI=josa-54-7-937
accessdate = 2008-05-13] and S. T. Henderson and D. Hodgkiss of Thorn Electrical Industries in Enfield [cite journal
author = Henderson, Stanley Thomas
coauthors = Hodgkiss, D.
year = 1963
title = The spectral energy distribution of daylight
journal = British Journal of Applied Physics
volume = 14
issue = 3
pages = 125–131
doi=10.1088/0508-3443/14/3/307
accessdate = 2008-05-13] [cite journal
author = Henderson, Stanley Thomas
coauthors = Hodgkiss, D.
year = 1964
volume=15
issue=8
journal=British Journal of Applied Physics
title = The spectral energy distribution of daylight
doi=10.1088/0508-3443/15/8/310
pages=947–952
accessdate = 2008-05-13] had independently measured the SPD of daylight from 330 to 700 nm, totaling among them 622 samples. Judd "et al" analyzed these samples and found that the (x,y) chromaticity coordinates had a simple, quadratic relation:

:$y=2.870x\; -\; 3.000x^2\; -\; 0.275$.

Simonds supervised the characteristic vector analysis of the SPDs. [cite journal|journal=JOSA|first=John L.|last=Simonds|title=Application of Characteristic Vector Analysis to Photographic and Optical Response Data|volume=53|issue=8|pages=968–974|year=1963|month=August| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-53-8-968] [cite journal|title=A review of principal component analysis and its applications to color technology|journal=Color Research & Application|first=Di-Yuan|last=Tzeng|coauthors=Berns, Roy S.|volume=30|issue=2|pages=84–98|year=2005|month=April|doi=10.1002/col.20086] Application of his method revealed that the SPDs could be satisfactorily approximated by using the mean (S₀) and first two characteristic vectors (S₁ and S₂):

:$S(lambda)\; =\; S\_0(lambda)\; +\; M\_1\; S\_1(lambda)\; +\; M\_2\; S\_2(lambda)$

In simpler terms, the SPD of the studied daylight samples can be expressed as the linear combination of three, fixed SPDs. The first vector (S₀) is the mean of all the SPD samples, which is the best reconstituted SPD that can be formed with only a fixed vector. The second vector (S₁) corresponds to yellow–blue variation, accounting for changes in the correlated color temperature due to presence or absence of clouds or direct sunlight. The third vector (S₂) corresponds to pink–green variation caused by the presence of water in the form of vapor and haze.

To construct a daylight simulator of a particular correlated color temperature one merely needs to know the coefficients M₁ and M₂ of the characteristic vectors S₁ and S₂.

Expressing the chromaticities x and y as:

$x=frac\{X\_0+M\_1\; X\_1+M\_2\; X\_2\}\{S\_0+M\_1\; S\_1\; +\; M\_2\; S\_2\}$

$y=frac\{Y\_0+M\_1\; Y\_1+M\_2\; Y\_2\}\{S\_0+M\_1\; S\_1\; +\; M\_2\; S\_2\}$

and making use of known tristimulus values for the mean vectors, they were able to express M₁ and M₂ as follows:

$M\_1=frac\{-1.3515-1.7703x+5.9114y\}\{0.0241+0.2562x-0.7341y\}$

$M\_2=frac\{0.0300-31.4424x+30.0717y\}\{0.0241+0.2562x-0.7341y\}$

The only problem is that this left unsolved the computation of the coordinate $(x,y)$ for a particular phase of daylight. Judd "et al" simply tabulated the values of certain chromaticity coordinates, corresponding to commonly-used correlated color temperatures, such as 5500 K, 6500 K, and 7500 K. For other color temperatures, one could consult figures made by Kelly.cite journal|last=Kelly|first=Kenneth L.|year=1963|month=August|title=Lines of Constant Correlated Color Temperature Based on MacAdam’s (u,v) Uniform Chromaticity Transformation of the CIE Diagram|journal=JOSA|volume=53|issue=8|pages=999–1002| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-53-8-999] This problem was addressed in the CIE report that formalized illuminant D, with an approximation of the x coordinate in terms of the reciprocal color temperature, valid from 4000 K to 25,000 K. [cite conference|title=Proceedings of the 15th Session, Vienna|author=Commission Internationale de l'Eclairage|year=1964] The y coordinate trivially followed from Judd's quadratic relation.

Judd "et al" then extended the reconstituted SPDs to 300–330 nm and 700–830 nm by using Moon's spectral absorbance data of the earth's atmosphere. [cite journal|title=Proposed standard solar-radiation curves for engineering use| first=Parry|last=Moon|year=1940|month=November|volume=230|issue=5|pages=583–617|journal=Journal of the Franklin Institute|doi=10.1016/S0016-0032(40)90364-7]

The tabulated SPDs presented by the CIE today are derived by linear interpolation of the 10nm data set down to 5nm. The limited nature of the photometric data is not an impediment to the calculation of the CIEXYZ tristimulus values since the CIE standard colorimetric observer's color matching functions are only tabulated from 380 to 780 nm in increments of 5 nm. [ [http://www.cie.co.at/main/freepubs.html CIE 1931 and 1964 Standard Colorimetric Observers] from 380nm to 780nm in increments of 5nm.]

Similar studies have been undertaken in other parts of the world, [cite journal|journal=JOSA|volume=56|issue=4|month=April|year=1966|title=Spectroradiometric and Colorimetric Characteristics of Daylight in the Southern Hemisphere: Pretoria, South Africa|author=G. T. Winch, M. C. Boshoff, C. J. Kok, and A. G. du Toit|pages=456–464| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-56-4-456|quote=The derived chromaticities were found to be much closer to the full radiator locus than those previously published, which had been obtained in the northern hemisphere.] [cite journal|title=Spectral Distribution and Color of Tropical Daylight|journal=JOSA|first=S.R.|last=Das|coauthors=Sastri, V.D.P.|volume=55|issue=3|month=March|year=1965|pages=319–323| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-55-3-319] [cite journal|title=Typical Spectral Distributions and Color for Tropical Daylight|journal=JOSA|first=V.D.P.|last=Sastri|coauthors=Das, S.R.|pages=391–398|volume=58|issue=3|month=March|year=1968| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-58-3-391] [cite journal|title=Locus of daylight chromaticities in relation to atmospheric conditions|last=Sastri|first=V.D.P.|volume=9|issue=1|year=1976|month=January 11|pages=L1–L3|journal=|doi=10.1088/0022-3727/9/1/001] [cite journal|title=Spectral distribution of Australian daylight|last=Dixon|first=E.R.|journal=JOSA|volume=68|issue=4|month=April|year=1978|pages=437–450| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-68-4-437] repeating Judd "et al"'s analysis with modern computational methods. [cite journal|title=Testing Linear Models on Spectral Daylight Measurements|first=Javier|last=Hernández-Andrés|coauthors=Javier Romero, Antonio García-Beltrán, and Juan L. Nieves|journal=Applied Optics|volume=37|issue=6|pages=971–977|year=1998|month=February 20|url=http://www.opticsinfobase.org/abstract.cfm?URI=ao-37-6-971|doi=10.1364/AO.37.000971] [cite journal|title=Color and spectral analysis of daylight in southern Europe|volume=18|issue=6|month=June|year=2001|journal=JOSA A|pages=1325–1335|first=Javier|last=Hernández-Andrés|coauthors=Javier Romero, Juan L. Nieves, and Raymond L. Lee, Jr.|doi=10.1364/JOSAA.18.001325] [cite conference
title=Group theoretical investigations of daylight spectra
author=Thanh Hai Bui, Reiner Lenz, Tomas Landelius
conference=CGIV (European Conference on Colour Graphics, Imaging and Vision)
year=2004
pages=437-442
accessdate = 2008-05-13
url=http://staffwww.itn.liu.se/~reile/csp-pages/publications/reprints/cgiv04-grouptheory.pdf] In several of these studies, the daylight locus is notably closer to the Planckian locus than in Judd "et al".

; Computation :The relative spectral power distribution (SPD) $S\_D\; (lambda)$ of a D series illuminant can be derived from its chromaticity coordinates in the CIE 1931 color space, $(x\_D,y\_D)$:The coefficients differ from those in the original paper due to the change in the constants in Planck's law. See [http://www.brucelindbloom.com/index.html?Eqn_DIlluminant.html Lindbloom] for the current version, and Planckian locus for details.]

$y\_D\; =\; -3.000\; x\_D^2\; +\; 2.870\; x\_D\; -\; 0.275$

where T is the illuminant's CCT. The chromaticity coordinates of the Illuminants D are said to form the "CIE Daylight Locus". The relative SPD is given by:

$S\_D(lambda)=S\_0(lambda)+M\_1\; S\_1(lambda)+M\_2\; S\_2(lambda)$

$M\_1=(-1.3515-1.7703x\_D+5.9114y\_D)/M$

$M\_2=(0.03000-31.4424x\_D+30.0717y\_D)/M$

$M=0.0241+0.2562x\_D-0.7341y\_D$

where $S\_0(lambda),\; S\_1(lambda),\; S\_2(lambda)$ are the mean and first two eigenvector SPDs, depicted above. The characteristic vectors both have a zero at 560 nm, since all the relative SPDs have been normalized about this point.

The CCTs of the canonical illuminants, D₅₀, D₅₅, D₆₅, and D₇₅, differ slightly from what their names suggest. For example, D50 has a CCT of 5003 K ("horizon" light), while D65 has a CCT of 6504 K (noon light). As explained in a previous section, this is because the value of the constants in Planck's law have been slightly changed since the definition of these canonical illuminants, whose SPDs are based on the original values in Planck's law.

Illuminant E

Illuminant E is an equal-energy radiator; it has a constant SPD inside the visible spectrum. It is useful as a theoretical reference; an illuminant that gives equal weight to all wavelengths, presenting an even color. It also has equal CIE XYZ tristimulus values, thus its chromaticity coordinates are (x,y)=(1/3,1/3). This is by design; the XYZ color matching functions are normalized such that their integrals over the visible spectrum are the same.

Illuminant E is not a black body, so it does not have a color temperature, but it can be approximated by a D series illuminant with a CCT of 5455 K. (Of the canonical illuminants, D₅₅ is the closest.) Manufacturers sometimes compare light sources against Illuminant E to calculate the excitation purity. [cite web|author=Philips|title=Optical Testing for SuperFlux, SnapLED and LUXEON Emitters|url=http://www.philipslumileds.com/pdfs/AB08.pdf|quote=CIE has defined the color coordinates of several different white Illuminants, but within Lumileds, CIE Illuminant E is used for all color calculations]

Illuminant series F

The F series of illuminants represent various types of fluorescent lighting.

F1–F6 "standard" fluorescent lamps consist of two semi-broadband emissions of antimony and manganese activations in calcium halophosphate phosphor. [For commercial examples of calcium halophosphate fluorescents, see for example patent|US|5447660|Method for making a calcium halophosphate phosphor or patent|US|6666993|Single component calcium halophosphate phosphor] F4 is of particular interest since it was used for calibrating the CIE Color Rendering Index (the CRI formula was chosen such that F4 would have a CRI of 51). F7–F9 are "broadband" (full-spectrum light) fluorescent lamps with multiple phosphors, and higher CRIs. Finally, F10–F12 are narrow triband illuminants consisting of three "narrowband" emissions (caused by ternary compositions of rare-earth phosphors) in the R,G,B regions of the visible spectrum. The phosphor weights can be tuned to achieve the desired CCT.

The spectra of these illuminants are published in Publication 15:2004.cite book|author=CIE Technical Report|year=2004|title=Colorimetry|edition=3rd ed.|series=Publication 15:2004|publisher=CIE Central Bureau, Vienna|isbn=3 901 906 33 9|url=http://www.cie.co.at/publ/abst/15-2004.html] [ [http://www.cis.rit.edu/mcsl/online/CIE/Fluorescents.htm Spectral power distribution of Illuminants Series F] ( [http://www.cis.rit.edu/mcsl/online/CIE/Fluorescents.xls Excel] ), in 5 nm increments from 380 to 780 nm.]

FL 1–6: Standard
FL 7–9: Broadband
FL 10–12: Narrowband

White point

The spectrum of a standard illuminant, like any other profile of light, can be converted into tristimulus values. The set of three tristimulus coordinates of an illuminant is called a "white point". If the profile is normalised, then the white point can equivalently be expressed as a pair of chromaticity coordinates.

If an image is recorded in tristimulus coordinates (or in values which can be converted to and from them), then the white point of the illuminant used gives the maximum value of the tristimulus coordinates that will be recorded at any point in the image, in the absence of fluorescence. It is called the white point of the image.

The process of calculating the white point discards a great deal of information about the profile of the illuminant, and so although it is true that for every illuminant the exact white point can be calculated, it is not the case that knowing the white point of an image alone tells you a great deal about the illuminant that was used to record it.

White points of standard illuminants

A list of standardized illuminants, their CIE chromaticity coordinates (x,y) of a perfect reflecting (or transmitting) diffuser, and their correlated color temperatures (CCTs) are given below. The CIE chromaticity coordinates are given for both the 2 degree field of view (1931) and the 10 degree field of view (1964). The color swatches represent the hue of each white point, calculated with luminance Y=0.54 and the standard observer, assuming correct sRGB display calibration.

References

External links

* [http://www.cie.co.at/publ/abst/datatables15_2004/CIE_sel_colorimetric_tables.xls Selected colorimetric tables in Excel] , as published in [http://www.cie.co.at/publ/abst/15-2004.html CIE 15:2004]