The retina of the human eye is covered with photosensitive cells called rods and cones. The cones are concentrated in the fovea of the retina. The fovea is the "sweet spot" of the eye because it has most of the 6 million cones and handles detail and hue. The rest of the retina has most of the 120 million rods and handles peripheral vision, night vision, and movement detection. Cones need tens or hundreds of photons to activate, but rods can respond to a single photon.

 The eye normally has three kinds of cones, each more suited for particular hues.

  1. "R" (Red) or "ρ" (lower case rho) or "L" (Long wavelengths actually peaking at 564 nm or yellowish-green) or "pro" (Latin for "first").
  2. "G" (Green) or "δ" (lower case gamma) or "M" (Medium wavelengths actually peaking at 534 nm or green) or "deu" (Latin for "second").
  3. "B" (Blue) or "β" (lower case beta) or "S" (Short wavelengths actually peaking at 420 nm or indigo) or "tri" (Latin for "third").

GRAPH: Wavelengths that the 3 kinds of cones in the eye are attuned to [W]

(Note that rods are shown as "R" in the graph. Rods peak at 498 nm or cyan-green but really only perceive value.)

Most people are Trichromats or have trichromatic vision because their cones see all three colors. Hear are the different kinds of color-blindness:

Roughly 8% of men and 0.4% of women have some form of color-blindness:

It is statistically most important to design for male dichromats, especially those who cannot distinguis red from green. Here are some examples:


Links that lead to off-site pages about color blindness.

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