Colors – How Our Eyes See Them

The colors of a coral reef do not exist solely in the reef itself. They are created in our brain after light has been detected and processed by the visual system.

The first step takes place in the retina, the light-sensitive layer at the back of the eye. Here, specialised photoreceptor cells convert incoming light into electrical signals that are transmitted to the brain.

Rods and Cones

Normalized human photoreceptor absorbances for different wavelengths of light (Wikipedia).

The human retina contains two types of photoreceptors: rods and cones. Rod cells are extremely sensitive to light and allow us to see under dim conditions such as dusk or moonlight. Because all rods contain essentially the same visual pigment, they cannot distinguish between different wavelengths. As a result, night vision is almost entirely monochromatic, which is why colors disappear in very low light.

Color vision is provided by three types of cone cells, each containing a different light-sensitive pigment with a characteristic spectral sensitivity.

The three cone types are commonly referred to as:

S-cones (short wavelength), with maximum sensitivity near 420 nm (blue-violet),
M-cones (medium wavelength), peaking around 534 nm (green),
L-cones (long wavelength), peaking near 564 nm (yellow-green).

Together they allow humans to perceive the visible spectrum, extending approximately from 380 to 740 nanometres, although the exact limits vary between individuals.

How Three Receptors Create Millions of Colors

The color gamut of a calibrated monitor (blue triangle) compared to sRGB (green) and Adobe RGB 1998 (red).

Although sunlight contains a continuous spectrum of wavelengths, our eyes measure it using only these three types of cone cells.

Every color we perceive is determined by the relative stimulation of the S-, M-, and L-cones. Different spectral distributions can produce exactly the same pattern of cone responses and therefore appear identical to us. This phenomenon is known as metamerism and forms the basis of all modern color reproduction.

For this reason, most digital displays require only three primary colors—red, green, and blue (RGB). By varying the intensity of these three light sources, they can reproduce millions of colors that appear natural to the human visual system, even though the emitted spectrum differs substantially from that of the original object.

Strictly speaking, not every visible color can be reproduced in this way. Every display has a limited color gamut, represented by the familiar RGB triangle in the CIE chromaticity diagram. Colors lying outside this triangle cannot be displayed accurately, regardless of the screen quality.

The Language of Color

Photographers and lighting designers often describe light using terms such as color temperature, brightness, hue, saturation, and contrast rather than wavelength alone.

These quantities are not separate from physics; they are practical ways of describing how the human visual system interprets light. The underlying physical spectrum remains the same, but our perception compresses this enormous amount of information into a relatively small number of visual sensations.

This remarkable simplification explains why three types of cone cells are sufficient for us to appreciate the extraordinary diversity of colors found on a healthy coral reef.