Three Cones, a Million Colors

Three Lamps Make White

Shine red, green and blue lamps on a white wall. Where red and green overlap you get yellow; green and blue make cyan; red and blue make magenta; where all three meet, white.

This is the opposite of mixing paint: mixing paints usually makes them darker; adding lights always makes them brighter.

Red, green and blue lamps · a white wall

Colored Shadows

Put a white ball in front of the lamps and it casts three shadows, each one colored. In the shadow that blocks the red lamp, only green and blue light remain: cyan. The one blocking green is magenta; the one blocking blue is yellow.

Sunlight Isn’t Three Colors

Yet sunlight is not built from three colors. Send it through a prism and it spreads into a continuous band — violet, blue, green, yellow, orange, red — with countless colors in between, each with its own wavelength. But you won’t find magenta, pink or white in the band: they have no wavelength of their own, and appear only when several lights reach the eye together.

So why can three lamps make so many colors? The answer is not in the light. It is in your eyes.

A continuous spectrum · wavelength in nanometers

Three Kinds of Cones

The retina at the back of your eye holds about five million cone cells of three kinds: one most sensitive to long waves (the red end), one to medium, one to short (the violet end). Below is how strongly each responds to every wavelength.

The three curves overlap a great deal: the kind often called the “red cone” is actually most sensitive to yellow-green light. Any light that enters your eye gets three answers, one from each kind of cone, and the color you see starts from those three numbers.

Sensitivity of the three cones · Stockman & Sharpe (2000)

Fooling the Eye

Take a pure yellow light, 580 nanometers. Now mix red light and green light in just the right amounts. The two beams are completely different, yet they make the three kinds of cones answer almost exactly alike, so to us they are the same yellow.

This is how TV and phone screens work: each pixel has only tiny red, green and blue dots, yet they fool us into seeing millions of colors. If our eyes had four kinds of cones, a screen with only three kinds of dots would have a much harder time fooling us.