Photoreceptors: the rods and cones

There are basically two different types of photodetectors in the retina:

• Rods (over 100 milliion) are adapted to dim light (e.g. nightlight) and are therefore responsible for scotopic vision. They can respond to a single photon. Six closely spaced rods, each stimulated by a single photon, will produce a visible flash. But rods saturate quickly when the light intensity increases. Moreover, rods are not sensitive to different colors. This is why we have very poor color vision in the night. Their sensitivity to light as a function of wavelengths is a smooth curve that peaks at about 495 mm. Foveal area does not have rods. The density of rods peaks at about 20 degrees from fovea and monotonically decreases at higher eccentricities.

• Cones (about 5 million) are not sensitive to dim light, but they are adapted to bright light (e.g. daylight) and is responsible for photopic vision. They can respond to bright light and never saturate at steady light level at any intensity. There are three types of cones, called the L-, M- and S-cones because they are respectively sensitive to light of long, middle and short wavelength (or equivalently, low, middle and supra frequency). Their sensitivities as functions of the wavelength peak at 564 mm, 533 mm and 437 mm, respectively. The L, M and S cones are also referred to as the red, green and blue cones.