Illuminant

Once we separate the illuminant and colorant, characteristics unique to each appear. Most illuminants are broad-spectrum light sources—they emit electromagnetic energy in a broad band across the visible spectrum. The sun, incandescent lights, fluorescent lights, neon and mercury-vapor lights all emit different spectra. Some illuminants are nonuniform or even emit only discrete wavelengths.

In almost all cases, the eye response to a particular broadband illuminant is set as the perceptual whitepoint, the reference point which the brain accepts as the purest, brightest color upon which to base other color perception. This phenomenon allows the eye to adapt to slight variations in hue or intensity of the illuminant such that color perception remains little changed under various illuminants. The background white provided by a piece of paper may establish a local whitepoint which may offer a somewhat different response than the illuminant itself. Note that non-adaptive color receptors, such as video and photographic recording equipment, do not inherently support this phenomenon.

In an effort to standardize illuminants, the CIE (Commission Internationale de l'Eclairage) mathematically defined a series of Daylight Illuminants covering the range of color produced when a blackbody is heated to temperatures from 5,000°K to 9,300°K. The 6,500°K illuminant is popular, often referred to as D65. Higher color temperatures generally move from yellow towards blue (towards lower wavelengths).