The process of transforming light energy into neural impulses that can then be interpreted by the brain.
The human eye is sensitive to only a limited range of radiation, consisting of wavelengths between approximately 400 to 750 nanometers (billionths of a meter).
The full spectrum of visible color is contained within this range, with violet at the low end and red at the high end. Light is converted into neural impulses by the eye, whose spherical shape is maintained by its outermost layer, the sclera. When a beam of light is reflected off an object, it first enters the eye through the cornea, a rounded transparent portion of the sclera that covers the pigmented iris. The iris constricts to control the amount of light entering the pupil, a round opening at the front of the eye. A short distance beyond the pupil, the light passes through the lens, a transparent oval structure whose curved surface bends and focuses the light wave into a narrower beam, which is received by the retina. When the retina receives an image, it is upside down because light rays from the top of the object are focused at the bottom of the retina, and vice versa. This upside-down image must be rearranged by the brain so that objects can be seen right side up. In order for the image to be focused properly, light rays from each of its points must converge at a point on the retina, rather than in front of or behind it. Aided by the surrounding muscles, the lens of the eye adjusts its shape to focus images properly on the retina so that objects viewed at different distances can be brought into focus, a process known as accommodation. As people age, this process is impaired because the lens loses flexibility, and it becomes difficult to read or do close work without glasses.
The retina, lining the back of the eye, consists of ten layers of cells containing photoreceptors (rods and cones) that convert the light waves to neural impulses through a photochemical reaction. Aside from the differences in shape suggested by their names, rod and cone cells contain different light-processing chemicals (photopigments), perform different functions, and are distributed differently within the retina. Cone cells, which provide color vision and enable us to distinguish details, adapt quickly to light and are most useful in adequate lighting. Rod cells, which can pick up very small amounts of light but are not color-sensitive, are best suited for situations in which lighting is minimal. Because the rod cells are active at night or in dim lighting, it is difficult to distinguish colors under these circumstances. Cones are concentrated in the fovea, an area at the center of the retina, whereas rods are found only outside this area and become more numerous the farther they are from it. Thus, it is more difficult to distinguish colors when viewing objects at the periphery of one's visual field.
The photoreceptor cells of the retina generate an electrical force that triggers impulses in neighboring bipolar and ganglion cells. These impulses flow from the back layer of retinal cells to the front layer containing the fibers of the optic nerve, which leaves the eye though a part of the retina known as the optic disk. This area, which contains no receptor cells, creates a blind spot in each eye, whose effects are offset by using both eyes together and also by an illusion the brain creates to fill in this area when one eye is used alone. Branches of the optic nerve cross at a junction in the brain in front of the pituitary gland and underneath the frontal lobes called the optic chiasm and ascend into the brain itself. The nerve fibers extend to a part of the thalamus called the lateral geniculate nucleus (LGN), and neurons from the LGN relay their visual input to the primary visual cortex of both the left and right hemispheres of the brain, where the impulses are transformed into simple visual sensations. (Objects in the left visual field are viewed only through the right brain hemisphere, and vice versa.) The primary visual cortex then sends the impulses to neighboring association areas which add meaning or "associations" to them.
Hubel, David. Eye, Brain, and Vision. New York: Scientific American Library, 1987.