Humans normally have three kinds of cones. The first responds the most to light of long wavelengths, peaking at a reddish colour; this type is sometimes designated L for long. The second type responds the most to light of medium-wavelength, peaking at a green colour, and is abbreviated M for medium. The third type responds the most to short-wavelength light, of a bluish colour, and is designated S for short. The three types have peak wavelengths near 564–580 nm, 534–545 nm, and 420–440 nm, respectively. The difference in the signals received from the three cone types allows the brain to perceive all possible colours, through the opponent process of colour vision. (Rod cells have a peak sensitivity at 498 nm, roughly halfway between the peak sensitivities of the S and M cones.)
All of the receptors contain the pigment photopsin, with variations in its conformation causing differences in the optimum wavelengths absorbed.
The colour yellow, for example, is perceived when the L cones are stimulated slightly more than the M cones, and the colour red is perceived when the L cones are stimulated significantly more than the M cones. Similarly, blue and violet hues are perceived when the S receptor is stimulated more than the other two.
The S cones are most sensitive to light at wavelengths around 420 nm. However, the lens and cornea of the human eye are increasingly absorptive to shorter wavelengths, and this sets the short wavelength limit of human-visible light to approximately 380 nm, which is therefore called 'ultraviolet' light. People with aphakia, a condition where the eye lacks a lens, sometimes report the ability to see into the ultraviolet range. At moderate to bright light levels where the cones function, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common (M and L) of the three kinds of cones almost equally. At lower light levels, where only the rod cells function, the sensitivity is greatest at a blueish-green wavelength.
Cones also tend to possess a significantly elevated visual acuity because each cone cell has a lone connection to the optic nerve, therefore, the cones have an easier time telling that two stimuli are isolated.
Read more about this topic: Cone Cell
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