A Standard Illuminants that represents daylight.
An Illuminant is a mathematical description of a real or imaginary light source described by its spectral power distribution.
Illuminants normally are specified in terms of the relative energy for each wavelength.
Over the years, the CIE produced specific classes of illuminants such as D50, D65, known as CIE Standard Illuminants. Illuminant A was designated to represent tungsten light and Illuminant D was designated to represent daylight. Some illuminants are tied to the relative energy distributions that correspond to the radiation emitted by a so-called blackbody (see also Color Temperature). D illuminants are based on the daylight curve that runs above the blackbody. Therefore, D50 has a spectral energy distribution that closely matches that of a blackbody at 5000K, and D65 would closely match that of a blackbody at 6500K.
D50 is the illuminant specified in ISO 3664:2009.