pigmented colorants: dependence on media and time - Cornell ...

Not all areas have the same sensitivity and resolution to light. As light passes
through the retina, some of it is absorbed or scattered before it reaches the pho-
toreceptors. In the central portion of the retina, the fovea, some of the overlying
structures are pushed to the margins. This allows more light to reach this area
unimpeded, resulting in a much higher spatial resolution than in the periphery of
the eye.
In addition, the human eye contains two classes of photoreceptors: rods and
cones, their names derived from their respective shapes. Rods and cones are not
uniformly distributed throughout the fovea, as seen in Figure 3.12. Cones pre-
dominate in central vision, while rods are abundant in the periphery. In the very
center, not only do only cones dominate, but they are even thinner and more heav-
ily packed than they are elsewhere. Notice the interruption of the plot in the figure
where there is a complete absence of receptors. Technically, this “blind spot” is
the optic disk, where the nerve cells of the eye form the optic nerve carrying the
visual information to the brain. Surprisingly, we almost never notice the large gap
that the optic disks create on our retinas.
Figure 3.13: Pictomicrographs of the nasal side of the human retina at
different distances from the center. The large cells are cones and the
small ones are rods. The photos are each about 44µm in width. (a)
1.35 mm from the center of the retina. (b) 5mm from the center of the
retina. (c) 8mm from the center of the retina. Adapted from [G95a].
Since the density of the 100 million photoreceptors varies spatially on the retina,
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