Light Source

Color changes with the light source. Therefore, standard illuminants have to be agreed upon and used. The prerequisite of a light source to be usable for color evaluation is to continuously emit energy throughout the visible spectrum (400 to 700 nm).

The CIE (Commission Internationale de l’Eclairage) standardized light sources by the amount of emitted energy at each wavelength (= relative spectral power distribution).

In practice, important illuminants are:
Daylight D65, C
Incandescent light A
Fluorescent light F2, F11

Observer

Without an observer there would be no color.
Reflected light from a colored object enters the human eye through the lens and strikes the retina. The retina is populated with three different types of light-sensitive receptors: one which reacts to red light, another to green light, and a third to blue light.

Light source and observer are defined by the CIE and their spectral functions are stored within color instruments. Optical properties of an object are the only variables that need to be measured.

Modern color instruments measure the amount of light that is reflected by a colored sample. This is done at each wavelength and is called the spectral data.

For example, a black object reflects no light across the complete spectrum (0% reflection), whereas an ideal white specimen reflects nearly all light (100% reflection).

All other colors reflect light only in selected parts of the spectrum. Therefore, they have specific curve shapes or fingerprints, which are their spectral curves.

In the following graphs, typical spectral curves for a red, blue and green sample are shown.