Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

CIE Standard Illuminants and Sources

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_324

Synonyms

Definitions

CIE Standard Illuminants

Illuminants A and D65 defined by the CIE in terms of relative spectral power distributions [1].

Note 1: These illuminants are intended to represent:
  • A: Planckian radiation at a temperature of about 2,856 K

  • D65: The relative spectral power distribution representing a phase of daylight with a correlated color temperature of approximately 6,500 K (called also “nominal correlated color temperature of the daylight illuminant”)

Note 2: Illuminants B and C and other D illuminants, previously denoted as “standard illuminants,” should now be termed “CIE illuminants.”

CIE Standard Sources

Artificial sources specified by the CIE whose radiation approximate CIE standard illuminants.

Note: CIE sources are artificial sources that represent CIE illuminants.

Overview

As shown already in the section Definitions, one has to distinguish between illuminants and sources: The term “illuminant” refers to a defined spectral power distribution, not necessarily realizable or provided by an artificial source. Illuminants are used in colorimetry to compute the tristimulus values of reflected or transmitted object colors under specified conditions of illumination. The term “source” refers to a physical emitter of light, such as a lamp or the sky.

The CIE colorimetric system was established in 1931 [2], and CIE defined in those days three illuminants: illuminants A, B, and C. Illuminant A was chosen to resemble the spectral power distribution (SPD) of an average incandescent lamp, the SPD of illuminant B was near to that of average direct sunlight, and illuminant C represented average daylight. During the years, it turned out that illuminant B was very seldom used and was soon dropped. Illuminant C was defined only in the visible spectrum, and with the introduction of optical brighteners, colorimetry needed illuminants with defined ultraviolet radiation content. In 1964 the CIE recommended a new set of daylight illuminants [3], where the SPD was also defined in the ultraviolet (UV) part of the spectrum, and decided that one of these, with a correlated color temperature near to 6,500 K, should be used whenever possible. Thus finally, CIE selected illuminant A and D65 as its standard illuminants, CIE standard illuminant A and CIE standard illuminant D65. The SPD of all illuminants defined by the CIE is available in the Technical Report Colorimetry [4] and the standard on CIE Standard Illuminants for Colorimetry [5].

CIE Standard Illuminant A

Originally CIE standard illuminant A (CIE St. Ill. A) was intended to represent typical, domestic, tungsten-filament lighting. Despite the fact that tungsten incandescent lamp light looses on importance, CIE St. Ill. A is the primary standard for calibrating photometers and colorimeters.

The relative spectral power distribution (SPD) of CIE St. Ill. A, SA(λ), is defined by the equation
$$ {S}_{\mathrm{A}}\left(\lambda \right)=100{\left(\frac{560}{\lambda}\right)}^5\times \frac{ \exp \frac{1,435\times {10}^7}{2,848\times 560}-1}{ \exp \frac{1,435\times {10}^7}{2,848\ \lambda }-1} $$
where λ is the wavelength in nanometers. For practical applications, it is defined also in tabulated form over the wavelength range between 300 nm and 830 nm to six significant digits at 1 nm intervals [5]. The wavelength is to be taken as being in standard air (dry air at 15 °C and 101,325 Pa, containing 0.03 % by volume of carbon dioxide). The numerical values in the two exponential terms are defined constants originating from the first definition of illuminant A [5].

CIE Standard Illuminant D65

The most important light source is daylight, the SPD of which changes during the day and depends also on weather conditions, etc.. From detailed measurements [7], the SPD of an often encountered phase of daylight has been selected as primary reference spectrum. This SPD has a correlated color temperature of approximately 6,504 K and has been termed D65. CIE standardized this SPD as CIE standard illuminant D65 (CIE St. Ill. D65, or shortly D65); it should represent average daylight SPD. (For further details, see chapter on “Daylight Illuminants.”) For the time being, the relative SPD of CIE St. Ill. D65 is defined in tabulated form between 300 nm and 830 nm at 1 nm intervals. The wavelength values given apply in standard air. Intermediate values may be derived by linear interpolation.

CIE stated that CIE St. Ill. D65 should be used in all colorimetric calculations requiring representative daylight, unless there are specific reasons for using a different illuminant.

Short History

CIE Standard Illuminant A

CIE St. Ill. A was originally defined in 1931 as the relative spectral power distribution of a Planckian radiator of temperature T = 2,848 K, where Planck’s equation has the form
$$ {M}_{\mathrm{e},\lambda}\left(\lambda, T\right)={c}_1{\lambda}^{-5}{\left[ \exp \left({c}_2/\lambda T\right)-1\right]}^{-1}, $$
where M is radiant exitance (quotient of the radiant flux, dΦe, leaving an element of the surface containing the point, by the area, dA, of that element).
In this equation, c1 is not relevant, as SA(λ) is defined as a relative SPD; the c2 constant’s value was at the time of defining CIE St. Ill. A 14,350 μm∙K. The SPD of CIE St. Ill. A has not changed since this original definition, only the numerical value of c2 has been several times reassigned in the International Temperature Scale (ITS), and due to this, the color temperature associated with CIE St. Ill. A has changed. For the different ITSs, the relevant color temperature values are
$$ {T}_{27}=2,842\ \mathrm{K}, {T}_{48}=2,854\ \mathrm{K}, {T}_{68}={T}_{90}=2,856\ \mathrm{K}. $$
Thus, the value we use nowadays is 2,856 K.

Daylight Illuminants

In 1931 CIE defined three reference illuminants, besides CIE St. Ill. A, also a spectrum that is near to that of direct sunlight, termed illuminant B (but this has been dropped after a short time) and illuminant C, with an SPD in the visible part of the spectrum representing average daylight with a correlated color temperature of approximately 6,774 K [6].

With the introduction of optical brighteners, it became necessary to define the SPD also in the near ultraviolet. The CIE accepted a recommendation by Judd and coworkers [7] to describe phases of daylight [8]. These authors found that although daylight is highly variable, the chromaticities of different phases of daylight fall on a curve more or less parallel to the Planckian locus on the chromaticity diagram. It turned out that the SPDs of the different phases of daylight can be described using only three basic functions, termed S0(λ), S1(λ), and S2(λ); see Fig. 1.
CIE Standard Illuminants and Sources, Fig. 1

Characteristic vectors used to reconstitute phases of daylight

The SPD of a phase of daylight can be calculated using the following equation:
$$ S\left(\lambda \right)={S}_0\left(\lambda \right)+{M}_1{S}_1\left(\lambda \right)+{M}_2{S}_2\left(\lambda \right) $$
where the M1 and M2 factors are calculated by the help of the chromaticity coordinates xD, yD of the phase of daylight:
$$ {M}_1=\frac{-1.3515-1.7703\;{x}_{\mathrm{D}}+5.9114\;{y}_{\mathrm{D}}}{0.0241+0.2562\;{x}_{\mathrm{D}}-0.7341\;{y}_{\mathrm{D}}},{M}_2=\frac{0.0300-31.4424\;{x}_{\mathrm{D}}+30.0717\;{y}_{\mathrm{D}}}{0.0241+0.2562\;{x}_{\mathrm{D}}-0.7341\;{y}_{\mathrm{D}}}. $$
xD, yD are defined by the help of the correlated color temperature (Tcp) of the phase of daylight. The xD coordinate has been defined in two parts for temperatures between 4,000 K and 7,000 K as
$$ {x}_{\mathrm{D}}=\frac{-4.6070\times {10}^9}{{\left({T}_{\mathrm{cp}}\right)}^3}+\frac{2.9678\times {10}^6}{{\left({T}_{\mathrm{cp}}\right)}^2}+\frac{0.09911\times {10}^3}{\left({T}_{\mathrm{cp}}\right)}+0.244063 $$
and between 7,000 K and 25,000 K as
$$ {x}_{\mathrm{D}}=\frac{-2.0064\times {10}^9}{{\left({T}_{\mathrm{cp}}\right)}^3}+\frac{1.9018\times {10}^6}{{\left({T}_{\mathrm{cp}}\right)}^2}+\frac{0.24748\times {10}^3}{\left({T}_{\mathrm{cp}}\right)}+0.237040. $$
With the help of xD, the corresponding yD can be calculated as
$$ {y}_{\mathrm{D}}=-3.000\ {x}_{\mathrm{D}}^2+2.870\ {x}_{\mathrm{D}}-0.275. $$
Using above equations, SPDs for any phase of daylight that has a Tcp between 4,000 K and 25,000 K can be calculated. From these spectra that corresponding to a Tcp of 6,500 was selected as primary standard: D65. As discussed already in connection with CIE St. Ill. A, the International Temperature Scale was based on a different value of the c2 constant as used today. Thus, to keep the SPD of the D65 unchanged, the correlated color temperature had to be changed. To get to the correct spectrum, one has to insert in above equations the value of 6,504 instead of 6,500. (To be quite precise, the value should be 6503,616134, but for all practical purposes, 6,504 is accurate enough.) Equations have been elaborated that will produce the correct spectra for the rounded value of 6,500 K, and for that, the term “nominal correlated color temperature” is used.

Often SPDs are needed with slightly lower or higher Tcp s, thus, e.g., in graphic arts to get daylight spectra nearer to spectra used in indoor applications, a phase of daylight of \( {T}_{\mathrm{cp}}=5,003 \) K is used, termed D5000. Besides this, CIE publications often refer to D55 and D75 illuminants as well with Tcp of 5,503 and 7,504, respectively.

As can be seen from Fig. 1, the S functions are not smooth functions. Originally their values have been determined at 10 nm intervals and linear interpolation was suggested between these fixed points. A recent CIE recommendation deals with two possible smoothing algorithms, one that keeps the value of the SPD at the fixed points constant [9] and one that smoothes the curves further, so that they can be realized more accurately with physical sources [10, 11]. For further details, see entry on “ Daylight Illuminants.”

Further CIE Illuminants

Often it is necessary to perform a colorimetric calculation using light source spectra of commercial lamps. To make such calculations more transparent, CIE published the spectra of some commercial gas-discharge lamps [4]: The collection of fluorescent lamp spectra includes 12 older constructions (six halophosphate, three broadband, and three narrowband lamp spectra); a second series contains, besides halophosphate lamps (three spectra), three Deluxe lamp spectra, five three-band lamp spectra, three multiband spectra, and a D65 simulator spectrum. Further spectra show typical high-pressure sodium lamp and high-pressure metal halide lamp spectra.

CIE Standard Source

The spectrum of coiled coil tungsten incandescent lamps is very near to the SPD of a black-body radiator; thus, CIE standard illuminant A can be realized by a gas-filled tungsten-filament lamp operating at a correlated color temperature of 2,856 K (c2 = 14,388 × 10−2 m · K). If the source is also to be used in the UV region, a lamp having an envelope or window made of fused quartz or silica must be used because glass absorbs the UV component of the radiation from the filament.

The spectrum of CIE standard illuminant D65 is too complicated to be realized with small enough error to represent a standard D65 source. Real sources to be used in visual observations as daylight sources are called “simulators,” and readers are directed to the entry “ CIE Method of Assessing Daylight Simulators” for further details.

Cross-References

References

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    CIE: Official recommendations, committee E-1.3.1 – colorimetry. In: Proceedings of the 15th Session, Vienna, 1963, vol. A, p. 35. CIE Publication 11 A (1964)Google Scholar
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    CIE Technical Report: Colorimetry, 3rd edn. Publication 15:2004, CIE Central Bureau, Vienna (2004)Google Scholar
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    Joint CIE/ISO Standard: Colorimetry – Part 2: CIE Standard Illuminants for Colorimetry. CIE S 014-2/E:2006/ISO 11664-2:2007(E)Google Scholar
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    Wyszecki, G., Stiles, W.S.: Color Science, Concepts and Methods, Quantitative Data and Formulae, 2nd edn, p. 145. Wiley, New York (1982)Google Scholar
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    Judd DB, MacAdam DL, Wyszecki G (with the collaboration of Budde HW, Condit HR, Henderson ST, Simonds JL): Spectral distribution of typical daylight as function of correlated color temperature. J. Opt. Soc. Am. 54, 1031–1040 (1964)Google Scholar
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    CIE: Recommendations on standard illuminants for colorimetry. In: Proceedings of the CIE Washington Session, vol. A, pp. 95–97, CIE Publ. 14. (1967), Vienna.Google Scholar
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© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.VeszprémHungary