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[[File:CIE illuminants A,B,C.svg|thumb|300px|Relative [[spectral power distribution]]s (SPDs) of CIE illuminants A, B, and C from 380nm to 780nm.]]
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A '''standard illuminant''' is a theoretical source of [[visible light]] with a profile (its [[spectral power distribution]]) which is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting.
 
==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:<ref name=schanda/>
{{quote|At present no artificial source is recommended to realize CIE standard illuminant D65 or any other illuminant D of different CCT. It is hoped that new developments in light sources and filters will eventually offer sufficient basis for a CIE recommendation.|CIE|Technical Report (2004) Colorimetry, 3rd ed., Publication 15:2004, CIE Central Bureau, Vienna}}
 
Nevertheless, they do provide a measure, called the [[Metamerism Index]], to assess the quality of daylight simulators.<ref>{{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.|publisher=Bureau central de la CIE|location=Paris}}</ref><ref>{{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)].</ref> The [[Metamerism (color)|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.<ref>{{cite journal|title=Evaluation of the quality of different D65 simulators for visual assessment|first=Yuk-Ming|last=Lam|coauthors=Xin, John H.|date=August 2002|journal=Color Research & Application|volume=27|issue=4|pages=243–251|doi=10.1002/col.10061}}</ref>
 
===Illuminant A===
 
The CIE defines illuminant A in these terms:
 
{{quote|CIE standard illuminant A is intended to represent typical, domestic, tungsten-filament lighting. Its relative spectral power distribution is that of a Planckian radiator at a temperature of approximately 2856&nbsp;K. CIE standard illuminant A should be used in all applications of colorimetry involving the use of incandescent lighting, unless there are specific reasons for using a different illuminant.|CIE|[http://www.cie.co.at/publ/abst/s005.html CIE Standard Illuminants for Colorimetry]}}
 
The [[spectral radiance]] of a [[black body]] follows [[Planck's law]]:
 
<math>M_{e,\lambda}(\lambda,T)=\frac{c_1 \lambda^{-5}}{\exp\left(\frac{c_2}{\lambda T}\right)-1}</math>
 
At the time of standardizing illuminant A, both <math>c_1=2\pi \cdot h \cdot c^2</math> (which does not affect the relative SPD) and <math>c_2=h \cdot c/k</math> were different. In 1968, the estimate of c<sub>2</sub> was revised from 0.01438&nbsp;m·K to 0.014388&nbsp;m·K (and before that, it was 0.01435 m·K when illuminant A was standardized). This difference shifted the [[Planckian locus]], changing the color temperature of the illuminant from its nominal 2848&nbsp;K to 2856&nbsp;K:
 
<math>T_{new}=T_{old} \times \frac{1.4388}{1.435} = 2848\ \text{K} \times 1.002648 = 2855.54\  \text{K}</math>
 
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 c<sub>2</sub>:<ref name=schanda/>
 
<math>S_{A}(\lambda)=100 \left(\frac{560}{\lambda}\right)^5 \frac{\exp \frac{1.435 \times 10^7}{2848 \times 560}-1}{\exp\frac{1.435 \times 10^7}{2848 \lambda}-1}</math>
 
The coefficients have been selected to achieve a peak SPD of 100 at 560&nbsp;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&nbsp;K, while C represented average day light with a CCT of 6774&nbsp;K. They are poor approximations of any common light source and deprecated in favor of the D series:<ref name=schanda>{{cite book|title=Colorimetry: Understanding the CIE System|first=János|last=Schanda|publisher=[[Wiley Interscience]]|year=2007|chapter=3: CIE Colorimetry|isbn=978-0-470-04904-4|pages=37–46}}</ref>
 
{{quote|Illuminant C does not have the status of a CIE standard but its relative spectral power distribution, tristimulus values and chromaticity coordinates are given in Table T.1 and Table T.3, as many practical measurement instruments and calculations still use this illuminant.|CIE|Publication 15:2004<ref name=cie2004/>}}
 
The liquid filters, designed by [[Raymond Davis, Jr.]] and Kasson S. Gibson in 1931,<ref>{{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}}</ref> 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===
 
{{see also|CIE Standard Illuminant D65}}
 
[[File:CIE illuminants D and blackbody small.gif|right|frame|Relative spectral power distribution of illuminant D and a black body of the same correlated color temperature (in red), normalized about 560nm.]]
 
Derived by Judd, MacAdam, and Wyszecki,<ref name=jmw>{{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]]|date=August 1964|volume=54|issue=8|pages=1031–1040|doi=10.1364/JOSA.54.001031}}</ref> 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 [[Ottawa]], H. R. Condit and F. Grum of the [[Eastman Kodak Company]] in [[Rochester, New York]],<ref name=Condit1964>{{cite journal
| author = Condit, Harold R.
| coauthors = Grum, Frank
|date=July 1964
| 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
| doi = 10.1364/JOSA.54.000937
}}</ref> and S. T. Henderson and D. Hodgkiss of [[Thorn Electrical Industries]] in [[Enfield Town|Enfield]]<ref>{{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
}}<br>
{{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
}}</ref> had independently measured the spectral power distribution (SPD) of daylight from 330 to 700&nbsp;nm, totaling among them 622 samples. Judd ''et al.'' analyzed these samples and found that the (x,y) chromaticity coordinates had a simple, [[quadratic function|quadratic]] relation:
 
:<math>y=2.870x - 3.000x^2 - 0.275</math>.
 
Simonds supervised the [[characteristic vector analysis]] of the SPDs.<ref>{{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|date=August 1963| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-53-8-968|doi=10.1364/JOSA.53.000968}}</ref><ref>{{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|date=April 2005|doi=10.1002/col.20086}}</ref> Application of his method revealed that the SPDs could be satisfactorily approximated by using the mean (S<sub>0</sub>) and first two characteristic vectors (S<sub>1</sub> and S<sub>2</sub>):
 
:<math>S(\lambda) = S_0(\lambda) + M_1 S_1(\lambda) + M_2 S_2(\lambda)</math>
 
In simpler terms, the SPD of the studied daylight samples can be expressed as the [[linear combination]] of three, fixed SPDs. The first vector (S<sub>0</sub>) 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 (S<sub>1</sub>) corresponds to yellow–blue variation, accounting for changes in the correlated color temperature due to presence or absence of clouds or direct sunlight.<ref name=jmw/> The third vector (S<sub>2</sub>) corresponds to pink–green variation caused by the presence of water in the form of vapor and haze.<ref name=jmw/>
 
To construct a daylight simulator of a particular correlated color temperature one merely needs to know the coefficients M<sub>1</sub> and M<sub>2</sub> of the characteristic vectors S<sub>1</sub> and S<sub>2</sub>.
 
[[File:CIE illuminants D components.svg|right|thumb|250px|Characteristic vectors of illuminant D; component SPDs S0 (blue), S1 (green), S2 (red).]]
 
Expressing the chromaticities x and y as:
 
<math>x=\frac{X_0+M_1 X_1+M_2 X_2}{S_0+M_1 S_1 + M_2 S_2}</math>
 
<math>y=\frac{Y_0+M_1 Y_1+M_2 Y_2}{S_0+M_1 S_1 + M_2 S_2}</math>
 
and making use of known tristimulus values for the mean vectors, they were able to express M<sub>1</sub> and M<sub>2</sub> as follows:
[[File:Planckian-locus.png|thumb|Kelly's figures depicted the lines of constant correlated color temperature on the CIE 1960 UCS, as shown here, as well as the familiar xy diagram.]]
 
<math>M_1=\frac{-1.3515-1.7703x+5.9114y}{0.0241+0.2562x-0.7341y}</math>
 
<math>M_2=\frac{0.0300-31.4424x+30.0717y}{0.0241+0.2562x-0.7341y}</math>
 
The only problem is that this left unsolved the computation of the coordinate <math>(x,y)</math> 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&nbsp;K, 6500&nbsp;K, and 7500&nbsp;K. For other color temperatures, one could consult figures made by Kelly.<ref name=kelly>{{cite journal|last=Kelly|first=Kenneth L.|date=August 1963|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|doi=10.1364/JOSA.53.000999}}</ref> 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&nbsp;K to 25,000&nbsp;K.<ref>{{cite conference|title=Proceedings of the 15th Session, Vienna|author=Commission Internationale de l'Eclairage|year=1964}}</ref> The y coordinate trivially followed from Judd's quadratic relation.
 
Judd ''et al.'' then extended the reconstituted SPDs to 300–330&nbsp;nm and 700–830&nbsp;nm by using Moon's spectral absorbance data of the Earth's atmosphere.<ref>{{cite journal|title=Proposed standard solar-radiation curves for engineering use| first=Parry|last=Moon|date=November 1940|volume=230|issue=5|pages=583–617|journal=Journal of the Franklin Institute|doi=10.1016/S0016-0032(40)90364-7}}</ref>
 
The tabulated SPDs presented by the CIE today are derived by [[linear interpolation]] of the 10&nbsp;nm data set down to 5&nbsp;nm. 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 function]]s are only tabulated from 380 to 780&nbsp;nm in increments of 5&nbsp;nm.<ref>[http://www.cie.co.at/main/freepubs.html CIE 1931 and 1964 Standard Colorimetric Observers] from 380nm to 780nm in increments of 5nm.</ref>
 
Similar studies have been undertaken in other parts of the world, or repeating Judd ''et al.'''s analysis with modern computational methods.  In several of these studies, the daylight locus is notably closer to the Planckian locus than in Judd ''et al.''<ref>Studies from the 1960s and 1970s include:<br>{{cite journal|journal=[[JOSA]]|volume=56|issue=4|date=April 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.|doi=10.1364/JOSA.56.000456}}<br>{{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|date=March 1965|pages=319–323| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-55-3-319|doi=10.1364/JOSA.55.000319}}<br>{{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|date=March 1968| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-58-3-391|doi=10.1364/JOSA.58.000391}}<br>{{cite journal|title=Locus of daylight chromaticities in relation to atmospheric conditions|last=Sastri|first=V.D.P.|volume=9|issue=1|date=January 11, 1976|pages=L1–L3|journal=[[Journal of Physics D: Applied Physics]]|doi=10.1088/0022-3727/9/1/001}}<br>{{cite journal|title=Spectral distribution of Australian daylight|last=Dixon|first=E.R.|journal=[[JOSA]]|volume=68|issue=4|date=April 1978|pages=437–450| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-68-4-437|doi=10.1364/JOSA.68.000437}}<br>Analyses using the faster computation of the 1990s and 2000s include:<br>{{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|date=February 20, 1998|url=http://www.opticsinfobase.org/abstract.cfm?URI=ao-37-6-971|doi=10.1364/AO.37.000971|pmid=18268673}}<br>{{cite journal|title=Color and spectral analysis of daylight in southern Europe|volume=18|issue=6|date=June 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}}<br>{{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}}
</ref>
 
; Computation:
 
The relative [[spectral power distribution]] (SPD) <math>S_D (\lambda)</math> of a D series illuminant can be derived from its [[chromaticity coordinate]]s in the [[CIE 1931 color space]], <math>(x_D,y_D)</math>:<ref name=bruce>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.</ref>
 
<math>x_D = \begin{cases}
0.244063 + 0.09911 \frac{10^3}{T} + 2.9678 \frac{10^6}{T^2} - 4.6070 \frac{10^9}{T^3} & 4000K \leq T \leq 7000K \\
0.237040 + 0.24748 \frac{10^3}{T} + 1.9018 \frac{10^6}{T^2} - 2.0064 \frac{10^9}{T^3} & 7000K < T \leq 25000K
\end{cases}</math>
 
<math>y_D = -3.000 x_D^2 + 2.870 x_D - 0.275</math>
 
[[File:Daylight-locus-in-CIE-1960-UCS.png|right|thumb|250px|Daylight locus in the CIE 1960 UCS. The isotherms are perpendicular to the Planckian locus. The two sections of the daylight locus, from 4000–7000&nbsp;K and 7000–25000&nbsp;K, are color-coded. Note that the two loci are separated by a fairly even distance, of around <math>\Delta_{uv}=0.003</math>.]]
 
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:
 
<math>S_D(\lambda)=S_0(\lambda)+M_1 S_1(\lambda)+M_2 S_2(\lambda)</math>
 
<math>M_1=(-1.3515-1.7703x_D+5.9114y_D)/M</math>
 
<math>M_2=(0.03000-31.4424x_D+30.0717y_D)/M</math>
 
<math>M=0.0241+0.2562x_D-0.7341y_D</math>
 
where <math>S_0(\lambda), S_1(\lambda), S_2(\lambda)</math> are the mean and first two [[eigenvector]] SPDs, depicted above.<ref name=bruce/> The characteristic vectors both have a zero at 560&nbsp;nm, since all the relative SPDs have been normalized about this point.
 
The CCTs of the canonical illuminants, D<sub>50</sub>, D<sub>55</sub>, D<sub>65</sub>, and D<sub>75</sub>, differ slightly from what their names suggest. For example, D50 has a CCT of 5003&nbsp;K ("horizon" light), while [[CIE Standard Illuminant D65|D65]] has a CCT of 6504&nbsp;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. In order to match all significant digits of the published data of the canonical illuminants the values of M<sub>1</sub> and M<sub>2</sub> have to be rounded to three decimal places before calculation of ''S<sub>D</sub>.''<ref name="schanda" />
 
===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.<ref name=schanda/>
 
[[File:Planckian-locus.png|thumb|right|Illuminant E is beneath the Planckian locus, and roughly isothermal with D<sub>55</sub>.]]
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&nbsp;K. (Of the canonical illuminants, D<sub>55</sub> is the closest.) Manufacturers sometimes compare light sources against Illuminant E to calculate the [[colorfulness#Excitation purity|excitation purity]].<ref>{{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}}</ref>
 
===Illuminant series F===
 
The '''F''' series of illuminants represent various types of [[Fluorescent lamp|fluorescent lighting]].
 
F1–F6 "standard" fluorescent lamps consist of two semi-broadband emissions of [[antimony]] and [[manganese]] activations in calcium halophosphate [[phosphor]].<ref>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}}</ref> 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.<ref name=cie2004>{{cite book|author=CIE Technical Report|year=2004|title=Colorimetry|edition=3rd|series=Publication 15:2004|publisher=CIE Central Bureau, Vienna|isbn=3-901906-33-9|url=http://www.cie.co.at/publ/abst/15-2004.html}}</ref><ref>[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&nbsp;nm increments from 380 to 780&nbsp;nm.</ref>
 
<gallery widths=200px heights=175px>
Image:CIE illuminants F 1 to 6.svg|'''FL 1–6''': Standard
Image:CIE illuminants F 7-9.svg|'''FL 7–9''': Broadband
Image:CIE illuminants F 10-12.svg|'''FL 10–12''': Narrowband
</gallery>
 
==White point==
 
{{main|White point}}
 
The spectrum of a standard illuminant, like any other profile of light, can be converted into [[tristimulus value]]s.  The set of three tristimulus coordinates of an illuminant is called a ''white point''.  If the profile is [[Normalizing constant|normalized]], then the white point can equivalently be expressed as a pair of [[chromaticity coordinate]]s.
 
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 temperature]]s (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 color space|sRGB]] display calibration.
 
{| cellspacing="0" align="center" class="wikitable" style="width:60em"
|+'''White points'''<ref name=babel>{{cite web | author = Danny Pascale | publisher = Babel Color  | url = http://www.babelcolor.com/download/A%20review%20of%20RGB%20color%20spaces.pdf | title = A Review of RGB color spaces}}</ref><ref>[http://www.hunterlab.com/appnotes/an05_05.pdf Equivalent White Light Sources, and CIE Illuminants]</ref><ref>[http://www.colour.org/tc8-04/Data/F.txt CIE F-series Spectral Data], CIE 15.2:1986</ref>
|-
! rowspan="2" |Name
! colspan="2" |CIE 1931 2°
! colspan="2" |CIE 1964 10°
! rowspan="2" |[[Correlated color temperature|CCT]] ([[Kelvin|K]])
! rowspan="2" |[[Hue]]
! rowspan="2" style="width:25em; text-align:left; padding-left:1em;" |Note
|-
!align="center" |''x''<sub>2</sub>
!align="center" |''y''<sub>2</sub>
!align="center" |''x''<sub>10</sub>
!align="center" |''y''<sub>10</sub>
|-
|align="center" |A
|align="center" |0.44757
|align="center" |0.40745
|align="center" |0.45117
|align="center" |0.40594
|align="center" |2856
|bgcolor="#ffb263"|
|style="padding-left:1em;"|Incandescent / Tungsten
|-
|align="center" |B
|align="center" |0.34842
|align="center" |0.35161
|align="center" |0.34980
|align="center" |0.35270
|align="center" |4874
|bgcolor="#d6beab"|
|style="padding-left:1em;"|{obsolete} Direct sunlight at noon
|-
|align="center" |C
|align="center" |0.31006
|align="center" |0.31616
|align="center" |0.31039
|align="center" |0.31905
|align="center" |6774
|bgcolor="#c6c0ca"|
|style="padding-left:1em;"|{obsolete} Average / North sky Daylight
|-
|align="center" |D50
|align="center" |0.34567
|align="center" |0.35850
|align="center" |0.34773
|align="center" |0.35952
|align="center" |5003<!-- D50 was not redefined when Planck's constant was redefined, so its CCT changed -->
|bgcolor="#d1c0a8"|
|style="padding-left:1em;"|Horizon Light. [[ICC profile]] [[Color management#Profile connection space|PCS]]
|-
|align="center" |D55
|align="center" |0.33242
|align="center" |0.34743
|align="center" |0.33411
|align="center" |0.34877
|align="center" |5503<!-- D55 was not redefined when Planck's constant was redefined, so its CCT changed -->
|bgcolor="#cbc1b2"|
|style="padding-left:1em;"|Mid-morning / Mid-afternoon Daylight
|-
|align="center" |[[CIE Standard Illuminant D65|D65]]
|align="center" |0.31271
|align="center" |0.32902
|align="center" |0.31382
|align="center" |0.33100
|align="center" |6504<!-- D65 was not redefined when Planck's constant was redefined, so its CCT changed -->
|bgcolor="#c2c2c2"|
|style="padding-left:1em;"|Noon Daylight: [[Television]], [[sRGB color space]]
|-
|align="center" |D75
|align="center" |0.29902
|align="center" |0.31485
|align="center" |0.29968
|align="center" |0.31740
|align="center" |7504<!-- D75 was not redefined when Planck's constant was redefined, so its CCT changed -->
|bgcolor="#bcc3ce"|
|style="padding-left:1em;"|North sky Daylight
<!-- Commented just because it is not a standard illuminant. Otherwise we should mention 6500K etc. |-
|align="center" |9300
|align="center" |0.28315
|align="center" |0.29711
|align="center" |0.28327
|align="center" |0.30043
|align="center" |9300
|bgcolor="#b8c2df"|
|style="padding-left:1em;"|High-efficiency blue phosphor monitors -->
|-
|align="center" |E
|align="center" |1/3
|align="center" |1/3
|align="center" |1/3
|align="center" |1/3
|align="center" |5454
|bgcolor="#d3beba"|
|style="padding-left:1em;"|Equal energy
|-
|align="center" |F1
|align="center" |0.31310
|align="center" |0.33727
|align="center" |0.31811
|align="center" |0.33559
|align="center" |6430
|bgcolor="#bec4bd"|
|style="padding-left:1em;"|Daylight Fluorescent
|-
|align="center" |F2
|align="center" |0.37208
|align="center" |0.37529
|align="center" |0.37925
|align="center" |0.36733
|align="center" |4230
|bgcolor="#ddbd96"|
|style="padding-left:1em;"|Cool White Fluorescent
|-
|align="center" |F3
|align="center" |0.40910
|align="center" |0.39430
|align="center" |0.41761
|align="center" |0.38324
|align="center" |3450
|bgcolor="#eeb97d"|
|style="padding-left:1em;"|White Fluorescent
|-
|align="center" |F4
|align="center" |0.44018
|align="center" |0.40329
|align="center" |0.44920
|align="center" |0.39074
|align="center" |2940
|bgcolor="#fdb36a"|
|style="padding-left:1em;"|Warm White Fluorescent
|-
|align="center" |F5
|align="center" |0.31379
|align="center" |0.34531
|align="center" |0.31975
|align="center" |0.34246
|align="center" |6350
|bgcolor="#bbc5b8"|
|style="padding-left:1em;"|Daylight Fluorescent
|-
|align="center" |F6
|align="center" |0.37790
|align="center" |0.38835
|align="center" |0.38660
|align="center" |0.37847
|align="center" |4150
|bgcolor="#dcbf8c"|
|style="padding-left:1em;"|Lite White Fluorescent
|-
|align="center" |F7
|align="center" |0.31292
|align="center" |0.32933
|align="center" |0.31569
|align="center" |0.32960
|align="center" |6500
|bgcolor="#c2c2c2"|
|style="padding-left:1em;"|[[CIE Standard Illuminant D65|D65]] simulator, Daylight simulator
|-
|align="center" |F8
|align="center" |0.34588
|align="center" |0.35875
|align="center" |0.34902
|align="center" |0.35939
|align="center" |5000
|bgcolor="#d1c0a8"|
|style="padding-left:1em;"|D50 simulator, Sylvania F40 Design 50
|-
|align="center" |F9
|align="center" |0.37417
|align="center" |0.37281
|align="center" |0.37829
|align="center" |0.37045
|align="center" |4150
|bgcolor="#e0bc97"|
|style="padding-left:1em;"|Cool White Deluxe Fluorescent
|-
|align="center" |F10
|align="center" |0.34609
|align="center" |0.35986
|align="center" |0.35090
|align="center" |0.35444
|align="center" |5000
|bgcolor="#d1c0a7"|
|style="padding-left:1em;"|Philips TL85, Ultralume 50
|-
|align="center" |F11
|align="center" |0.38052
|align="center" |0.37713
|align="center" |0.38541
|align="center" |0.37123
|align="center" |4000
|bgcolor="#e3bc92"|
|style="padding-left:1em;"|Philips TL84, Ultralume 40
|-
|align="center" |F12
|align="center" |0.43695
|align="center" |0.40441
|align="center" |0.44256
|align="center" |0.39717
|align="center" |3000
|bgcolor="#fab46b"|
|style="padding-left:1em;"|Philips TL83, Ultralume 30
|}
 
==References==
 
{{reflist|2}}
 
==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]
 
* Konica Minolta Sensing: [http://www.konicaminolta.eu/en/measuring-instruments/learning-centre/light-measurement/the-language-of-light.html Light sources & Illuminants]
 
[[Category:Light]]

Latest revision as of 20:27, 21 August 2014

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