Model of the eye
Every color model starts in the eye. That color is described by three numbers does not follow from the physics of light but from the structure of the retina: we have three kinds of color-sensitive cells.
The retina
Light passes through the cornea, the lens and the vitreous body, and the image forms on the retina, a thin layer of neural tissue at the back of the eye. It holds two kinds of photoreceptors:
- rods, about 120 million: very sensitive, working in dim light, but not distinguishing colors,
- cones, about 6 million: they need more light and are responsible for color vision and sharpness.
Cones are concentrated in the fovea, a small area on the visual axis covering a few degrees of the field of view. That is why the 1931 CIE standard observer applies to a 2° field (see CIE XYZ).
Three kinds of cones
Cones differ in their photopigment, and so in the wavelengths they respond to most strongly:
| Cone | Wavelengths | Peak sensitivity | Share |
|---|---|---|---|
| S | short | about 420-440 nm (violet, blue) | about 5-10% |
| M | medium | about 530-545 nm (green) | about 30-35% |
| L | long | about 560-580 nm (yellow-green) | about 60% |
The M and L sensitivity curves overlap heavily. Calling the cones "red, green and blue" is therefore a big simplification: the L cone responds most to yellow-green light, and we recognize red because it stimulates L much more than M.
Each cone passes on only one piece of information: how strongly it was stimulated. It does not know which wavelength caused it. That is why different spectra can produce the same color (metamerism), and why three well-chosen RGB lights can imitate most colors.
Opponent channels
Cone signals do not reach the brain separately. Already in the retina they are combined into three channels:
This explains why we never see a "reddish green" or a "yellowish blue", as Ewald Hering noticed (see history). CIELAB has the same split: L* is lightness, a* the red-green axis, b* the yellow-blue axis. Television YUV also separates brightness from two color components, because the eye is far more sensitive to detail in brightness than in color.
Brightness sensitivity
In daylight vision the eye is most sensitive to light around 555 nm (yellow-green). That sensitivity curve, V(λ), is the ȳ(λ) function of CIE XYZ and so the basis of luminance Y. That is why in sRGB green contributes over 71% of luminance and blue just over 7%.
At dusk, when mainly rods are working, the peak shifts to about 507 nm. Reds darken faster than blues: the Purkinje effect.
Color vision deficiency
When one kind of cone works abnormally or is missing, the color space shrinks to fewer than three dimensions. Red-green deficiencies (involving the L or M cones) affect about 8% of men and about 0.5% of women of European descent, because the genes for these pigments lie on the X chromosome. Deficiencies involving S cones are rare.
When designing charts and interfaces, avoid carrying information with a red-green pair alone. It is safer to vary lightness (L*) too, or shape and labels.