Colour Science & Perception
what the eye settles, and what it cannot
Concept

Hering's opponent theory — where it appears

The nineteenth-century account of colour vision as three opponent pairs rather than three independent channels, which explains why no colour is reddish green. It was long treated as a rival to trichromacy and turned out to describe the stage after it.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

One palette under normal vision and two dichromacies. The same 7 colours simulated by the Brettel–Viénot–Mollon construction at severity 1.0. protanopia and deuteranopia collapse the red-green distinctions. This shows which discriminations survive, not what anybody sees.

Three cones, two axes

The retina does not send three receptor signals down the optic nerve. It sends a sum and two differences, and the reason falls out of the statistics of natural light rather than out of anything the eye intended.

eye · Cones The four unique hues, and the axes they are said to define. A constant-lightness, constant-chroma ring in CIECAM16, with the four unique hue anchors marked and CIELAB's a and b axes drawn through the same circle. If a* really were the red-green axis the anchors would fall on the crosshairs. Unique red sits 25° off, and the four are not 90° apart in any case. Hatched sectors are hues this display cannot reach at this chroma.

Why there are four unique hues

Observers agree that four hues are elementary and that no colour is reddish green. Nothing in the three receptors predicts either fact, and the axes of every standard colour space miss the four by tens of degrees.

brain · Appearance

Named alongside it

The objects these essays reach for when they reach for this one.

CIELABOpponent processingUnique huesCIECAM16Cone correlationDecorrelationEfficient codingElementary coloursHue quadratureLightnessNamingReflectance

All concepts