When white light is broken into its component wavelengths, say by means of a prism, all the color of the spectrum are separated and spread out. Monochromatic or spectral light is an ideal concept of light that has only one wavelength. The closer light gets to this state, the more pure or saturated it is said to be. A very saturated color generally has almost all its intensity fairly close to the dominant wavelength, while an unsaturated color would have contributions from many other wavelengths.
From FALK Seeing the light, pp 238-239.
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