A standard illuminant is a theoretical source of visible light with a spectral power distribution that 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 (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:
Nevertheless, they do provide a measure, called the metamerism index, to assess the quality of daylight simulators. Prepared by TC 1-53 "A Standard Method for Assessing the Quality of Daylight Simulators". 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.
Illuminant A The CIE defines illuminant A in these terms:
The spectral radiant exitance of a black body follows Planck's law:
At the time of standardizing illuminant A, both (which does not affect the relative SPD) and 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:
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:
for a wavelength λ (nm). The coefficients have been selected to achieve a normalized SPD of 100 at . The tristimulus values are , and the chromaticity coordinates using the standard observer are .
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.
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:
Characteristic vector analysis revealed that the SPDs could be satisfactorily approximated by using the mean (S0) and first two characteristic vectors (S1 and S2):
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. 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.
By the time the D-series was formalized by the CIE, a computation of the chromaticity 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 .CIE 1931 and 1964 Standard Colorimetric Observers from to in increments of . 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.Studies from the 1960s and 1970s include: Analyses using the faster computation of the 1990s and 2000s include:
The CIE positions D65 as the standard daylight illuminant:
Computation The relative spectral power distribution (SPD) of a D series illuminant can be derived from its chromaticity coordinates in the CIE 1931 color space, .The coefficients differ from those in the original paper due to the change in the constants in Planck's law. See Lindbloom for the current version, and Planckian locus for details. First, the chromaticity coordinates must be determined:
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. The same discrepancy applies to all illuminants in the D series—D50, D55, D65, D75—and can be "rectified" by multiplying the nominal color temperature by ; for example for D65.
To determine the D-series SPD (SD) that corresponds to those coordinates, the coefficients M1 and M2 of the characteristic vectors S1 and S2 are determined:
where are the mean and first two eigenvector SPDs, depicted in figure. The characteristic vectors both have a zero at , since all the relative SPDs have been normalized about this point. In order to match all significant digits of the published data of the canonical illuminants the values of M1 and M2 have to be rounded to three decimal places before calculation of SD.D65 values Using the standard 2° observer, the CIE 1931 color space chromaticity coordinates of D65 are
and the XYZ tristimulus values (normalized to ), are
For the supplementary 10° observer,
and the corresponding XYZ tristimulus values are
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
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 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.
Illuminant series FL CIE Publication 15.2 introduced twelve new illuminants representing several fluorescent lamps and comprising series F,Spectral power distribution of Illuminants Series FL (CSV with metadata), in increments from to . 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.
CIE 15:2004 also introduced fifteen new fluorescent illuminants representing different kinds of fluorescent lamps and comprising subseries FL3. These 15 standards are distributed in 5 groups: Standards FL3.1-FL3.3 represent standard halophosphate lamps (similar to FL1-6) Standards FL3.4-FL3.6 represent DeLuxe type lamps (similar to FL7-9) Standards FL3.7-FL3.11 represent three-band lamps (similar to FL10-12) Standards FL3.12-FL3.14 represent multi-band lamps Standard FL3.15 represents a D65 simulating fluorescent lamp
Illuminant series HP CIE 15:2004 introduced five new illuminants representing different kinds of high pressure discharge lamps and comprising series HP.: Standard HP1 for standard high-pressure sodium lamps Standard HP2 for color-enhanced high-pressure sodium lamps Standards HP3-HP5 for metal halide lamps.
Illuminant series LED