Since the color of objects looks different for different light sources, the type of light source always needs to be defined.
|Since the color of objects looks different for different light sources, the type of light source always needs to be defined.|
For unambiguous definition of the illumination conditions during the determination of colors, the spectral composition of the light source must be known.
In absolute spectral measurements, the real light source must physically emulate one of the defined standard illuminants. In the more common relative spectral measurements (versus a white standard), one of the standard light sources must be included in the color calculation as a defined spectral curve. The color coordinates thus calculated then identify the color stimuli that would be generated by the sample at the selected illumination setting.
In 1931, CIE defined the first standards of typical light sources, including standard illuminants A and C. This was expanded in 1964 to include the now most commonly used standard illuminants for daylight at various color temperatures, D55, D65, D75, etc.
Standard illuminant A = standardized incandescent light (2856 K)
Standard illuminant C = average daylight, no UV fraction (6750 K)
Standard illuminant C = average daylight, including UV fraction (6500 K)
Standard illuminants D65 and D75 are used very commonly. They correspond to the spectral composition of average daylight (color temperature 6500 K) or of a sunny day with blue sky (7500 K) and take into consideration the corresponding UV fraction of this light.
Nowadays, standard illuminant D65 in combination with CIE 1964 10° standard observer is used most commonly. This is often used as the default setting for color calculations from spectral data in the corresponding software.