How eyes see color
Three kinds of cone cells, and every color you've ever seen is a ratio of their signals.
- Cone types
- 3 (short, medium, long)
- Proposed
- Thomas Young, 1802
- Peaks measured
- Bowmaker & Dartnall, 1980
Your retina has three kinds of color-sensing cells called cones. One responds best to short wavelengths (peaking near 420 nm, violet-blue), one to medium (near 534 nm, green) and one to long (near 564 nm, which is yellow-green, not red). A color is the brain's reading of how strongly each type fires compared with the others. That's trichromacy, 'three colors'.
Thomas Young guessed this in 1802, long before anyone could see a cone. He reasoned the eye couldn't hold a separate sensor for every color, so it must mix a few. Hermann von Helmholtz developed the idea in the 1850s, and James Clerk Maxwell showed it by matching colors with three lights. In 1861 he projected the first color photograph, a tartan ribbon, from three pictures taken through red, green and blue filters.
Trichromacy is why screens need only red, green and blue pixels, the trick Maxwell used for the first color photograph. The yellow on your phone contains no yellow light, just red and green light that excite your cones the way real yellow light does. Two lights that differ physically but look identical are called metamers. It's also why a paint can match a swatch in the shop and fail at home: different light, different match (see color constancy).
The three signals don't stay three. Further along, the visual system recodes them as opposites, red against green and blue against yellow, as Ewald Hering proposed in 1878. That's why no one sees a 'reddish green' (see complementary colors). If one cone type is missing or shifted, the result is color blindness.
Swatches
- S cone peak · ~420 nm
- M cone peak · ~534 nm
- L cone peak · ~564 nm
Colors
Sources
- Bowmaker, J. K. & Dartnall, H. J. A. (1980). Visual pigments of rods and cones in a human retina. Journal of Physiology 298: 501–511.
- Young, T. (1802). On the theory of light and colours. Philosophical Transactions 92: 12–48.
- Maxwell, J. C. (1861). On the theory of three primary colours. Royal Institution lecture, 17 May 1861.
Hex values are screen approximations. Every page lists its sources.