Short answer
Daylight is not one spectrum. Measured daylight ranges from below 4,000 K under low sun to above 30,000 K in clear-sky shade, and its spectrum shifts with sun height, cloud, haze and ozone. The CIE models this family with three basis spectra; D65 (about 6,500 K) stands for average daylight and D50 (about 5,000 K) is the graphic-arts viewing standard.
In 1964 Judd, MacAdam, Wyszecki and colleagues analysed several hundred measured daylight spectra and showed that almost all the variation could be captured by a mean spectrum plus two characteristic components. The CIE adopted that result: every CIE daylight illuminant is the fixed curve S0 plus weighted amounts of S1 and S2, with the weights set by the chosen correlated colour temperature. The model is defined for roughly 4,000 to 25,000 K. D65, with a CCT of about 6,504 K, is the CIE's recommended representative of daylight for colorimetry; D50, about 5,003 K, is the reference for print viewing. The odd final digits come from a later revision of a physical constant in Planck's law after the illuminants were fixed.
A two-year study in Granada recorded about 2,600 daylight spectra in all weathers. It found that 5,700 K best typified the daylight there, but the measured CCT ran from under 4,000 K to above 30,000 K. At high CCTs the chromaticities sat noticeably away from the CIE daylight locus, a reminder that the model is an idealisation fitted to other places and times. Low CCTs come from low sun, whose beam crosses a long path of atmosphere; very high ones from shade lit only by clear blue sky. Overcast skies sit between the two and are the most stable.
Outdoors there are always at least two sources: the direct sun and the sky. Surfaces facing the sun receive a warm beam; those in shadow receive only skylight, which is Rayleigh-scattered and blue. That is why one object can show a warm lit face and a cool shadowed face at the same moment, and why compass orientation matters in the site's room guides. Near and after sunset a third effect enters: ozone absorption in the Chappuis band removes orange and yellow from sunlight grazing the upper atmosphere, and it is this, more than scattering, that deepens the blue of the twilight sky.
Window glass absorbs most ultraviolet, so indoor daylight lacks the short-wavelength tail that makes optical brighteners and some fluorescent dyes glow outdoors; the CIE's indoor daylight illuminants, ID50 and ID65, model this. Reflections then tint what is left: light that has bounced off a green lawn or a red brick wall arrives coloured. A 'daylight' viewing booth therefore stands in for an idealised outdoor spectrum that no real room receives, and the CIE grades how well such simulators do it by checking that sample pairs matching under true daylight still match under the simulator.
| Reference | Correlated colour temperature | Used for |
|---|---|---|
| CIE D50 | about 5,003 K | Graphic-arts and print viewing |
| CIE D65 | about 6,504 K | Representative average daylight in colorimetry; sRGB white |
| Granada measurements, most typical | about 5,700 K | One site's best single summary of real daylight |
| Granada measurements, range | under 4,000 K to over 30,000 K | Low sun to clear-sky shade |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
StandardStrong evidence
CIE daylight illuminants of any correlated colour temperature are computed from three basis spectra, S0, S1 and S2, published by the CIE as part of CIE 015:2018.
Source: Components of the relative spectral distribution of daylight (S0, S1, S2), CIE 015:2018 dataset
FactStrong evidence
The CIE daylight model derives from Judd, MacAdam, Wyszecki and colleagues' 1964 analysis of measured daylight spectra as a function of correlated colour temperature.
Source: Spectral Distribution of Typical Daylight as a Function of Correlated Color Temperature
FactModerate evidence· Granada, Spain
In about 2,600 daylight spectra measured in Granada over two years, a CCT of 5,700 K best typified the daylight, measured CCTs spanned from under 4,000 K to over 30,000 K, and chromaticities lay above the CIE daylight locus at CCTs over 9,000 K.
Caveat: One site and climate; the range endpoints come from the paper's table as summarised, not re-measured.
Source: Color and spectral analysis of daylight in southern Europe
FactModerate evidence
D65 has a correlated colour temperature of about 6,504 K and D50 about 5,003 K; the D-series model is defined from about 4,000 to 25,000 K.
Source: Standard illuminant (Wikipedia); CIE datasets (colour-matching functions, illuminants)
FactStrong evidence
With the sun on the horizon, radiative-transfer modelling attributes about two thirds of the zenith sky's blue colour to ozone absorption in the Chappuis bands, against a few per cent when the sun is high.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.