Illuminant A

The CIE defines illuminant A in these terms:

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 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, CIE Standard Illuminants for Colorimetry

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

${\displaystyle M_{e,\lambda }(\lambda ,T)={\frac {c_{1}\lambda ^{-5}}{\exp \left({\frac {c_{2}}{\lambda T}}\right)-1}}.}$

At the time of standardizing illuminant A, both ${\displaystyle c_{1}=2\pi \cdot h\cdot c^{2}}$ (which does not affect the relative SPD) and ${\displaystyle c_{2}=h\cdot c/k}$ were different. In 1968, the estimate of c₂ was revised from 0.01438 m·K to 0.014388 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 K to 2856 K:

${\displaystyle T_{new}=T_{old}\times {\frac {1.4388}{1.435}}=2848\ {\text{K}}\times 1.002648=2855.54\ {\text{K}}.}$

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:^[1]

${\displaystyle S_{A}(\lambda )=100\left({\frac {560}{\lambda }}\right)^{5}{\frac {\exp {\frac {1.435\times 10^{7}}{2848\times 560}}-1}{\exp {\frac {1.435\times 10^{7}}{2848\lambda }}-1}}.}$

The coefficients have been selected to achieve a normalized 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 easily achieved daylight simulations. They modify illuminant A by using 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. Unfortunately, they are poor approximations of any phase of natural daylight, particularly in the short-wave visible and in the ultraviolet spectral ranges. Once more realistic simulations were achievable, illuminants B and C were deprecated in favor of the D series.^[1]

Illuminant C does not have the status of CIE standard illuminants 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^[5]

Illuminant B was not so honored in 2004.

The liquid filters, designed by Raymond Davis, Jr. and Kasson S. Gibson in 1931,^[6] have a relatively high absorbance at the red end of the spectrum, effectively increasing the CCT of the incandescent lamp to daylight levels. This is similar in function to a CTB 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

The D series of illuminants are designed to represent natural daylight and lie along the daylight locus. They are difficult to produce artificially, but are easy to characterize mathematically.

By 1964, several spectral power distributions (SPDs) of daylight had been measured independently by 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,^[7] and S. T. Henderson and D. Hodgkiss of Thorn Electrical Industries in Enfield (north London),^[8] totaling among them 622 samples. Deane B. Judd, David MacAdam, and Günter Wyszecki analyzed these samples and found that the (x, y) chromaticity coordinates followed a simple, quadratic relation, later known as the daylight locus:^[9]

${\displaystyle y=2.870x-3.000x^{2}-0.275.}$

Characteristic vector analysis revealed that the SPDs could be satisfactorily approximated by using the mean (S₀) and first two characteristic vectors (S₁ and S₂):^[10][11]

${\displaystyle S_{D}(\lambda )=S_{0}(\lambda )+M_{1}S_{1}(\lambda )+M_{2}S_{2}(\lambda ).}$

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 (along the locus), accounting for changes in the correlated color temperature due to proportion of indirect to direct sunlight.^[9] The third vector (S₂) corresponds to pink–green variation (across the locus) caused by the presence of water in the form of vapor and haze.^[9]

By the time the D-series was formalized by the CIE,^[12] a computation of the chromaticity ${\displaystyle (x,y)}$ for a particular isotherm was included.^[13] Judd et al. then extended the reconstituted SPDs to 300 nm–330 nm and 700 nm–830 nm by using Moon's spectral absorbance data of the Earth's atmosphere.^[14] The tabulated SPDs presented by the CIE today are derived by linear interpolation of the 10 nm data set down to 5 nm.^[15] However, there is a proposal to use spline interpolation instead.^[16]

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.^{[17] [18]}

The CIE positions D65 as the standard daylight illuminant:

[D65] is intended to represent average daylight and has a correlated colour temperature of approximately 6500 K. CIE standard illuminant D65 should be used in all colorimetric calculations requiring representative daylight, unless there are specific reasons for using a different illuminant. Variations in the relative spectral power distribution of daylight are known to occur, particularly in the ultraviolet spectral region, as a function of season, time of day, and geographic location.

— ISO 10526:1999/CIE S005/E-1998, CIE Standard Illuminants for Colorimetry^[19]

D65 values

Using the standard 2° observer, the CIE 1931 color space chromaticity coordinates of D65 are^[21]

$${\displaystyle {\begin{aligned}x&=0.31272\\y&=0.32903\end{aligned}}}$$

and the XYZ tristimulus values (normalized to Y = 100), are

$${\displaystyle {\begin{alignedat}{2}X&={}&95.047\\Y&={}&100{\phantom {.000}}\\Z&={}&108.883\end{alignedat}}}$$

For the supplementary 10° observer,^{[citation needed]}

$${\displaystyle {\begin{aligned}x&=0.31382\\y&=0.33100\end{aligned}}}$$

and the corresponding XYZ tristimulus values are

$${\displaystyle {\begin{alignedat}{2}X&={}&94.811\\Y&={}&100{\phantom {.000}}\\Z&={}&107.304\end{alignedat}}}$$

Since D65 represents white light, its coordinates are also a white point, corresponding to a correlated color temperature of 6504 K. Rec. 709, used in HDTV systems, truncates the CIE 1931 coordinates to x=0.3127, y=0.329.

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. 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.^[1]

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.^[26]

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.^[27] 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.^[5][28]

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

Illuminant series LED

Publication 15:2018 introduces new illuminants for different white LED types with CCTs ranging from approx. 2700 K to 6600 K.

LED-B1 through B5 defines LEDs with phosphor-converted blue light. LED-BH1 defines a blend of phosphor-converted blue and a red LED. LED-RGB1 defines the white light produced by a tricolor LED mix. LED-V1 and V2 define LEDs with phosphor-converted violet light.

White points of standard illuminants

A list of standardized illuminants, their CIE chromaticity coordinates (x,y) of a perfectly 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).^[29]The color swatches represent the color of each white point, automatically calculated by Wikipedia using the Color temperature template.