due to presence or absence of clouds or direct sunlight. The third vector (S2) 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 M1 and M2 of the characteristic vectors S1 and S2. Expressing the chromaticities x and y as: and making use of known tristimulus values for the mean vectors, they were able to express M1 and M2 as follows: The only problem is that this left unsolved the computation of the coordinate 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. 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, : 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 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, 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). As explained in a previous section, this is