The light that is illuminating an object. The perceived color of an object (the light that reflects off of an object) is the product of the object's reflectance (its inherent color) and the color of the illuminant. When the illuminant…

Impossible Color

A color that cannot be seen through normal functioning of the visual system, but can hypothetically be seen by interfering with the visual transduction pathway. Examples include Stygian Blue, Hyperbolic Orange or Hyper-Green

Inherited CVD

CVD that is passed from parent to offspring. It is always genetic.

Inverted Spectrum

A thought experiment postulated by John Locke. An individual wakes up one morning, and finds that for some unknown reason all the colors in the world have been inverted, i.e. swapped with its complementary color (the color on the opposite…


Iridescence is a property of a reflective surface, which changes color depending on the viewing angle. It is an effect (or side effect) of structural color. Examples include thin films of oil (pictured), soap bubbles, feathers, butterfly wings, and certain…

Ishihara Test

A Colorblind test using Pseudoisochromatic Plates that detects for red-green Colorblindness. Blue-yellow Colorblindness is not detected. The test consists of a number of Ishihara plates, each of which depicts a solid circle of colored dots appearing randomized in color and…

Island of the Colorblind

A book by Oliver Sacks chronicling his visit to the Micronesian island of Pingelap, famous for its extremely high prevalence of Achromats. The visit was also made into a BBC Documentary by the same name.


Two colors that appear to be the same chromaticity to a given observer. Colors within a MacAdam Ellipse are isochromatic.Metamers are isochromatic.Confusion colors are isochromatic. To be seen as the exact same color side-by-side, isochromatic colors must also be isoluminant.…


Having uniform subjective light intensity, where two or more colors appear to be the same luminance. Colorblind individuals have a different Luminance Curve from color normals, so the relative brightness of different colors will differ. The Flicker Test is used…