Color is made by the observer
Light consists of electromagnetic waves, and the portion we can see spans roughly 400 to 700 nanometers in wavelength, from violet through blue, green, yellow and orange to red. A physicist can measure wavelengths, but color as an experience arises only in a nervous system. A ray of light of one wavelength has no color in itself; it is the visual system’s response to it that produces the sensation. For this reason, color perception is as much about the brain as about physics.
Two kinds of photoreceptor
The retina contains two types of light-sensing cell with different roles.
- Rods are highly sensitive and work in dim light. They contain a single type of light-sensitive pigment, so they cannot signal differences in wavelength on their own. Vision at night is therefore mostly colorless. Rods are concentrated away from the center of the retina, which is why a faint star can sometimes be seen better by looking slightly to one side of it.
- Cones need more light. In most people there are three types, each containing a different pigment. They are concentrated at the center of the retina, giving sharp, colorful central vision.
The three cone types are usually called short-, medium- and long-wavelength cones (S, M and L). Their sensitivities peak in roughly the blue, green and yellow-green parts of the spectrum respectively, and the curves overlap broadly. The long-wavelength cone is often loosely called the “red” cone, but its peak is not at red.
Why three cone types are enough
A single cone type cannot tell wavelength from brightness. A dim light at a wavelength it likes can produce the same response as a brighter light at a wavelength it likes less. With three types, however, the brain can compare their relative responses. Each wavelength produces a characteristic ratio, and the brain reads this ratio as color. This idea, known as trichromacy, explains why a display with just three kinds of colored pixels can mimic a huge range of colors. Different mixtures of light can produce identical cone responses, so they look the same, a phenomenon called metamerism.
This also means we do not see wavelengths directly. Purple, for example, has no single wavelength of its own; it is a perception produced by combining responses to both ends of the spectrum.
Processing beyond the cones
Signals from the cones are not sent to the brain as three separate channels. In the retina and the next relay stations, they are recombined into opponent channels: one compares long- versus medium-wavelength cone signals (loosely red versus green), another compares short-wavelength signals with the sum of the others (loosely blue versus yellow), and a third carries overall brightness. Opponent processing helps explain why we never describe a color as reddish-green or yellowish-blue, and why staring at one color can leave an afterimage of the opposite.
The visual cortex then builds on this. Our experience of color is also shaped by context. The brain tends to adjust for the lighting, so a white sheet looks white in both yellowish indoor light and bluish daylight, even though the light reaching the eye differs a lot. This is called color constancy. Illusions in which the same patch looks different against different backgrounds show how much the brain’s interpretation matters, and how artists exploit it when looking at art and in the art movements built around color.
Differences between people
Color vision varies.
- Color vision deficiency, often called color blindness, occurs when one cone type is missing or has altered sensitivity. The most common forms affect distinguishing reds from greens and are more frequent in males because the genes involved are on the X chromosome. This is a classic example in medical genetics. Complete absence of color vision is rare.
- Normal variation: Even among people with typical vision, the exact cone pigments differ slightly, so two people may not draw the boundary between, say, blue and green in exactly the same place.
- Rare enhanced color vision: Some people may have an extra cone type, though how much this changes perception is still being studied.
- Language and culture influence how colors are named, and may subtly affect how quickly they are distinguished, but all humans share the same basic cone biology.
Aging also affects color vision, partly because the lens yellows over time and absorbs more short-wavelength light.
Beyond the visible
Humans cannot see ultraviolet or infrared light, because the lens and the cones are not sensitive there. Other animals differ: some birds, insects and fish have more or different cone types, and some see into the ultraviolet. Devices can map invisible light into visible colors, which is how astronomers’ images of the sky are made, as in optical telescopes and other instruments.
This article is a general science overview and not medical advice. Anyone with concerns about their color vision should consult a qualified eye-care professional.