Standard illuminant
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  represent variations 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: Nevertheless, they do provide a measure, called the metamerism index, to assess the quality of daylight simulators. 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 formula_2 (which does not affect the relative SPD) and formula_3 were different. In 1968, the estimate of c 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 c: The coefficients have been selected to achieve a normalized SPD of 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:. Lighting cabinets, such as the Spectralight III, that use filtered incandescent lamps have better fits to the D illuminants in the to range than do the fluorescent daylight simulators. The liquid filters, designed by Raymond Davis, Jr. 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 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. Derived by Judd, MacAdam, and Wyszecki , 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, and S. T. Henderson and D. Hodgkiss of Thorn Electrical Industries in Enfield had independently measured the spectral power distribution (SPD) of daylight from to , totaling among them 622 samples. Judd "et al." analyzed these samples and found that the ("x", "y") chromaticity coordinates had a simple, quadratic relation: Simonds  supervised the characteristic vector analysis of the SPDs. Application of his method revealed that the SPDs could be satisfactorily approximated by using the mean (S) and first two characteristic vectors (S and S): 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) 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) corresponds to yellow–blue variation, accounting for changes in the correlated color temperature due to  presence or absence of clouds or direct sunlight. The third vector (S) corresponds to pink–green variation caused by the presence of water in the form of vapor and haze. To construct a daylight simulator of a particular correlated color temperature one merely needs to know the coefficients M and M of the characteristic vectors S and S. Expressing the chromaticities x and y as: and making use of known tristimulus values for the mean vectors, they were able to express M and M as follows: The only problem is that this left unsolved the computation of the coordinate formula_12  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 K, 6500 K, and 7500 K. For other color temperatures, one could consult figures made by Kelly. 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 K to 25000 K. The y coordinate trivially followed from Judd's quadratic relation. 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 . 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." The relative spectral power distribution (SPD) formula_13 of a D series illuminant can be derived from its chromaticity coordinates in the CIE 1931 color space, formula_14: 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: where formula_21 are the mean and first two eigenvector SPDs, depicted above. The characteristic vectors both have a zero at , since all the relative SPDs have been normalized about this point. The CCTs of the canonical illuminants, D, D, D, and D, 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). 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 and M have to be rounded to three decimal places before calculation of "S." 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, D is the closest.) Manufacturers sometimes compare light sources against illuminant E to calculate the excitation purity. 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. 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. Illuminant series LED. Publication 15:2018 introduces new illuminants for different LED types with CCTs ranging from approx. 2700 K to 6600 K. White point. The spectrum of a standard illuminant, like any other profile of light, can be converted into tristimulus values. The set of three tristimulus coordinates of an illuminant is called a "white point". If the profile is normalized, then the white point can equivalently be expressed as a pair of chromaticity coordinates. 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.