Fundamentals of Spatial Vision

The adult human eye is aproximately 25mm in diameter and weighs about 7g. The anterior section of the eye contains the eye's optical system whose major structures are the cornea, lens, and iris. The cornea provides about two-thirds of the eye's refractive power, but the lens provides ne focus for targets at distances from 20 feet down to about 4 inches (Pugh88). The ciliary muscles attached to edge of the lens e ect focusing by changing the len's shape. The space between the lens and the cornea is lled with a uid known as the aqueous humor. The iris sits just in front of the lens and has a central aperture known as the pupil that admits light to the eye. The central cavity of the eyeball is lled with a uid called the vitreous humor. The posterior section of the eyeball contains its neural structures. This section is composed of three layers. The sclera is a tough covering that protects the interior from damage and helps to maintain the eye's shape. The choroid is a middle layer that provides the blood supply to the eye's internal structures. The retina is the innermost layer that contains light sensitive photoreceptors and associated neural tissue. About 4% of the light incident on the eye is absorbed or re ected by the cornea. Absorption or scattering by the internal structures of the eye means that only about 50% of the light coming to the eye actually reaches the photosensitive retinal surface.