The illuminant defines the wavelength range of light produced by an emitting light source. The illuminant is therefore dependent on the color temperature and its emission spectrum, the spectral distribution of light energy.
The Commission Internationale d`Eclairage( CIE) has defined various artificial and natural types of light and designated them with letters. For example, the standard illuminant "A" corresponds to the light of an incandescent lamp. The color temperature for this light source is 2,856 Kelvin (K), and the wavelength range is between 380 nm and 770 nm.
For illuminant "C", which is bluer, the color temperature is 6,774 K, and illuminant "D" specifies daylight. This specification is reflected in projector lamps, among other things. For D light types, the first two digits of the color temperature are specified as an addition after the "D": DXX. For example, the standard illuminant D65 has a color temperature of 6,500 K, and D55, which is yellower than D65, has one of 5,500 K. This standard illuminant corresponds to daylight under overcast skies.
Standard illuminant "E" does not correspond to any real light source, and standard illuminant "F" corresponds to the color range offluorescent lamps.