What is D65?

The D class of illuminants specify relative energy distributions that closely correspond to the radiation emitted by a so-called black-body. As the tempertature of a black body is increased there is a shift in the emitted radiation to shorter wavelengths. A specific D illuminant is therefore notated with reference to the temperature (in Kelvin) of the black-body which it most closely matches. For example, the illuminant D65 has a spectral energy distribution that closely matches that of a black-body at 6500K. Illuminant D65 also closely resembles the relative spectral energy distribution of north-sky daylight and is accordingly important for colour specification in northern Europe. Other D illuminants, notably D55, are important in other parts of the world.