Color Deficiency

Photoreceptor Mechanisms

The four types of photoreceptors in the human retina initiate the process of vision, and these types can be separated into two main classes (rods and cones). Rods are effective only at low light levels and can detect even a single photon of light, thus affording our scotopic vision (i.e., nighttime vision). Cones are less sensitive than rods are and therefore operate at higher light levels (serving photopic, or daytime, vision), and are responsible for our ability to perceive color. Thus, most of our visual experience is mediated by the cone photoreceptors (paradoxically, however, the cones constitute only about 5% of the total number of photoreceptors). There are three different classes of cone photore-ceptor, each one with a different spectral sensitivity (i.e., plot of sensitivity of the photopigment vs. wavelength). Each of the four types of photo-receptor contains a single type of photopigment molecule, which gives the photoreceptor its characteristic spectral sensitivity.

As hypothesized 200 years ago by Thomas Young, we now know that trichromatic color vision is afforded by the presence of three spectrally distinct cone photopigments in the retina. Each cone type can be classified by the wavelength of light that the photopigment it contains is most sensitive to: short-, middle-, and long-wavelength sensitive (abbreviated S, M, and L). All humans with normal color vision have the same S-cone pigment, with peak absorption around 417 nanometers (nm). The M-cone pigment varies slightly between individuals, but peaks around 530 nm. Interestingly, there is widespread variation of the L-cone pigment among humans with normal color vision, but the peak of the L-cone pigment is approximately 559 nm. Rods are maximally sensitive to 500 nm light, but do not contribute significantly to our color vision.

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