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 M1 and M2 have to be rounded to three decimal places before calculation of SD. 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 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.Spectral power distribution of Illuminants Series F (Excel), in increments from to . 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