With respect to the elements of visual perception, explain Brightness adaption & discrimination.
Answers
Figure 4.1 shows a simplified horizontal cross section of the human eye. The eye is nearly a
sphere, with an average diameter of approximately 20 mm. Three membranes enclose the eye:
the cornea and sclera outer cover; the choroid; and the retina. The cornea is a tough, transparent
tissue that covers the anterior surface of the eye. Continuous with the cornea, the sclera is an
opaque membrane that encloses the remainder of the optic globe. The choroid lies directly below
the sclera. This membrane contains a network of blood vessels that serve as the major source of
nutrition to the eye. Even superficial injury to the choroid, often not deemed serious, can lead to
severe eye damage as a result of inflammation that restricts blood flow. The choroid coat is
heavily pigmented and hence helps to reduce the amount of extraneous light entering the eye and
the backscatter within the optical globe. At its anterior extreme, the choroid is divided into the
ciliary body and the iris diaphragm. The latter contracts or expands to control the amount of light
that enters the eye. The central opening of the iris (the pupil) varies in diameter from
approximately 2 to 8 mm. The front of the iris contains the visible pigment of the eye, whereas
the back contains a black pigment.
The lens is made up of concentric layers of fibrous cells and is suspended by fibers that attach to
the ciliary body. It contains 60 to 70%water, about 6%fat, and more protein than any other tissue
in the eye. The lens is colored by a slightly yellow pigmentation that increases with age. In
extreme cases, excessive clouding of the lens, caused by the affliction commonly referred to as
cataracts, can lead to poor color discrimination and loss of clear vision. The lens absorbs
approximately 8% of the visible light spectrum, with relatively higher absorption at shorter
wavelengths. Both infrared and ultraviolet light are absorbed appreciably by proteins within the
lens structure and, in excessive amounts, can damage the eye.
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