Normalised absorption spectra of human cone (S,M,L) and rod (R) cells

Cone cells, or cones, are cells in the retina which only function in relatively bright light. There are about 6 million in the human eye, concentrated at the fovea and gradually becoming sparser towards the outside of the retina.

Cones are less sensitive than the rod cells in the retina (which support vision at low light levels), but allow the perception of color because there are [normally] three kinds of cones, with different photopsins, which have different response curves (that is, they respond to variation in color in different ways).

The three kinds of cones typically respond most to yellowish-green (long wavelength or L), green (medium or M), and bluish-violet (short or S) light (peak wavelengths of 564 nm, 534 nm, and 420 nm respectively). The difference in the signals received from the three kinds allows the brain to perceive a wide gamut of different colors.

The color yellow, for example, is perceived when the yellowish-green receptor is stimulated slightly more than the green receptor, and the color red is perceived when the yellowish-green receptor is stimulated significantly more than the green receptor. Similarly, blues are perceived when the bluish-violet receptor is stimulated more than the other two.

Structurally, cone cells have a cone-like shape at one end where the pigment that filters incoming light, giving them their different response curves. They are typically 50 µm long, and their diameter varies from 1.0 to 4.0 µm, being smallest and most tightly packed at the centre of the eye (the fovea). The blue-sensitive cells are a little larger than the others, and an order of magnitude less common.

The S (bluish-violet) cones are sensitive to light at wavelengths shorter than 400 nm, but the lens and cornea of the human eye are increasingly absorbative to these wavelengths, and this sets the lower wavelength limit of human-visible light to approximately 380 nm (the onset of ultraviolet light).

See Webvision’s Photoreceptors (http://webvision.umh.es/webvision/photo1.html)