Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 87

Fig. 4.5 Diagram of sensitivity in nanometers (nm) of rods and cones (reprinted with permission
from Bowmaker and Dartnall 1980 )
They are much more sensitive to light than the cones and are active in dark
conditions; in bright conditions they become overloaded. The distributions of rods
and cones overlap in the parafovea. The rods are mostly located beyond the
parafovea, in the periphery of the retina, and are very sensitive to movement.
The visual receptors respond to light waves. The term light is used to describe
electromagnetic energy in the visible range of wavelengths approximately
400-700 nm (see Fig. 4.5 ). The wavelengths that we see are determined by the
physics of the eye and the chemistry of the photoreceptor pigments in the eye.
Electromagnetic energy with wavelengths below the visible spectrum includes
ultraviolet rays, X-rays, and gamma rays; electromagnetic energy with wavelengths
above the visible spectrum includes infrared rays, microwaves, and radio waves.
Our sensitivity to spectral radiation is not constant across the electromagnetic
spectrum. Instead, we sample it through a pair of filters. The first filter is provided by
the spectral sensitivity of the rods, which have a maximum sensitivity around
500 nm (498 in Fig. 4.5 ). The second is provided by the pooled responses of the three
types of cones (the leftmost and two rightmost curves in Fig. 4.5 ) and has maximum
sensitivity around 555 nm. Below 380 nm (infrared) and above 700 nm (ultraviolet)
we are effectively blind to electromagnetic waves. Therefore, there is a wide range of
information that meets our eyes which falls outside our window of visibility.
Although we sense infrared energy as heat, other wavelengths outside the
visible spectrum are imperceptible without the aid of devices such as radios or
infrared goggles. Ultraviolet energy is destructive to living tissue, so it is filtered
out by the yellow pigment in the lens of the eye. People who have had their lenses
removed because of cataracts, however, can see ultraviolet energy as light. Rather
than experiencing it as a new color sensation, which would require another type of
cone, they see it as the same color that people with normal vision would see violet.