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.
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:[1] At present no artificial source is recommended to realize CIE standard illuminant D65 or any other illuminant D of different CCT.
It is hoped that new developments in light sources and filters will eventually offer sufficient basis for a CIE recommendation.Nevertheless, they do provide a measure, called the metamerism index, to assess the quality of daylight simulators.
In a manner similar to the color rendering index, the average difference between the metamers is calculated.
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.The spectral radiant exitance of a black body follows Planck's law: At the time of standardizing illuminant A, both
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:[1] The coefficients have been selected to achieve a normalized SPD of 100 at 560 nm.
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.
The liquid filters, designed by Raymond Davis 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.
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] 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] In simpler terms, the SPD of the studied daylight samples can be expressed as the linear combination of three, fixed SPDs.
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.
[16] Similar studies have been undertaken in other parts of the world, or repeating Judd et al.'s analysis with modern computational methods.
of a D series illuminant can be derived from its chromaticity coordinates in the CIE 1931 color space,
Note that the CCTs of the canonical illuminants, D50, D55, D65, and D75, differ slightly from what their names suggest.
[1] 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
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.
Constructing a practical light source that emulates a D-series illuminant is a difficult problem.
[22] However, the SPDs of these sources deviate from the D-series SPD, leading to bad performance on the CIE metamerism index.
[23][24] 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.
[25] Illuminant E is an equal-energy radiator; it has a constant SPD inside the visible spectrum.
This is by design; the XYZ color matching functions are normalized such that their integrals over the visible spectrum are the same.
Manufacturers sometimes compare light sources against illuminant E to calculate the excitation purity.
[5] These 15 standards are distributed in 5 groups: CIE 15:2004 introduced five new illuminants representing different kinds of high pressure discharge lamps and comprising series HP.
[28] ID50 and ID65 are equivalent to their outdoor counterparts, D50 and D65, filtered through window glass, thereby removing the ultraviolet contents.
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.
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.
