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 (1967) represent variations of daylight, illuminant E is the equal-energy illuminant, while illuminants F (2004) 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: 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.
Illuminant A The CIE defines illuminant A in these terms: The
spectral radiant exitance of a
black body follows
Planck's law: 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 c_1 = 2\pi \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 0.01438 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: T_\text{new} = T_\text{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: Illuminant B was not so honored in 2004. The liquid filters, designed by Raymond Davis and Kasson S. Gibson in 1931, 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, and S. T. Henderson and D. Hodgkiss of
Thorn Electrical Industries in
Enfield (north London), 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: :y = 2.870 x - 3.000 x^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): :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. a computation of the chromaticity (x,y) for a particular isotherm was included. Judd
et al. then extended the reconstituted SPDs to – and – by using Moon's spectral absorbance data of the Earth's atmosphere. The tabulated SPDs presented by the CIE today are derived by
linear interpolation of the data set down to . However, there is a proposal to use
spline interpolation instead. 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. First, the chromaticity coordinates must be determined: : 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} & 4000\ \mathrm{K} \leq T \leq 7000\ \ \mathrm{K} \\ 0.237040 + 0.24748 \frac{10^3}{T} + 1.9018 \frac{10^6}{T^2} - 2.0064 \frac{10^9}{T^3} & 7000\ \mathrm{K} :y_D = -3.000 x_D^2 + 2.870 x_D - 0.275 where
T is the illuminant's CCT. Note that 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). This is because the originally experimentally determined values of the constants in Planck's law have become more accurately known (and now take on fixed values in the international system of units and measurements) since the definition of these canonical illuminants, whose SPDs are based on the original values in Planck's law. \begin{align} x &= 0.31272 \\ y &= 0.32903 \end{align} and the
XYZ tristimulus values (normalized to ), are \begin{alignat}{2} X &={}& 95.047 \\ Y &={}& 100\phantom{.000} \\ Z &={}& 108.883 \end{alignat} For the
supplementary 10° observer, \begin{align} x &= 0.31382 \\ y &= 0.33100 \end{align} and the corresponding XYZ tristimulus values are \begin{alignat}{2} X &={}& 94.811 \\ Y &={}& 100\phantom{.000} \\ Z &={}& 107.304 \end{alignat} 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.
Daylight simulator There are no actual daylight light sources, only simulators. Constructing a practical light source that emulates a D-series illuminant is a difficult problem. The chromaticity can be replicated simply by taking a well known light source and applying filters, such as the Spectralight III, that used filtered incandescent lamps. However, the SPDs of these sources deviate from the D-series SPD, leading to bad performance on the CIE
metamerism index. Better sources were achieved in the 2010s with
phosphor-coated white LEDs that can easily emulate the A, D, and E illuminants with high CRI.
Illuminant E , and roughly at the
CCT of D55. 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.
Illuminant series FL CIE Publication 15.2 introduced twelve new illuminants representing several
fluorescent lamps and comprising
series F, later renamed to
series FL from CIE Publication 15:2004 onward. The original 12 standards are distributed to 3 groups: • Standards FL1–FL6 represent "standard" fluorescent lamps consisting of two semi-broadband emissions of
antimony and
manganese activations in calcium halophosphate
phosphor. FL4 is of particular interest since it was used for calibrating the CIE
color rendering index (the CRI formula was chosen such that FL4 would have a CRI of 51). • Standards FL7–FL9 represent "broadband" (
full-spectrum light) fluorescent lamps with multiple phosphors, and higher CRIs. • Standards FL10–FL12 represent 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, which leads to poor CRI. The members within a group represent different CCTs, such that the phosphor weights can be tuned to achieve the desired CCT. In each of these three groups, CIE states that FL2, FL7, and FL11 "take priority" to be representative of their respective groups. which were later included as
series ID in CIE 15:2018. ID50 and ID65 are equivalent to their outdoor counterparts, D50 and D65, filtered through window glass, thereby removing the ultraviolet contents. The indoor CCTs are about 100K higher (cooler) relative to their outdoor counterparts. File:CIE illuminants D65 and ID65.svg|
CIE standard illuminants D65 and ID65 in comparison. File:CIE illuminants D50 and ID50.svg|
CIE standard illuminants D50 and ID50 in comparison. File:CIE glass transmission.svg|
Relative transmission of 10mm glass sheet based on CIE illuminants ID65 and D65. ==White point==