" People catergorized as visually impaired range from individuals with mild visual losses to those with low vision to those who are totally blind" (Winzer,370). Vision impairments does not necessarily mean a total loss of sight. Some visually impaired person can detect light, others can see shapes and forms, and others can see nothing at all. Generally, many different problems that interfere the retina to form image or the transmission of retinal images to the brain can cause blindness or vision impairment. It can be heredity or caused by disease or damage after birth. In some cases a specific reason cannot be determined.
There are four major types of vision impairments. The first type are refractive errors. It is cause by the change of shape or size of the eyeball, conera or the lens. Myopia (short-sighted) and hyperopia (long-sighted) are two common examples. People with myopia or hyperopia cannot focus image accurately on the retina. Another example is astigmatism, it is caused by the abnormal curve shape of the cornera or the lens. And it can cause distorted or blurred vision.
The second type are ocular motor problems. It is caused by irregular movement of the ocular muscles that control the movement of the eyeballs. It leads to the inability for the eyes to control focusing objects. Strasbismus is an example. This condition effects about 2 percent (2 out of 100) of all young children. Strabismus is a lack of coordinated muscle movement or focusing ability between the eyes, causing the eyes to point in different directions. One or both eyes may turn inward (crossed eyes) or outward ("walleye"). The eyes, the brain area that controls vision, and the muscles attached to the eyeball are involved. It result in the child that looks in certain directions, double vision (sometimes), vision in one eye only, with loss of depth perception. Strabismus can affect both sexes, all ages, but it usually begins during early childhood, frequently before age 5. If not corrected through therapy or surgery, crossed eyes can result in permanent loss of vision.
The third type are eye diseases, it is "caused by damage or disease before or after birth to one or more structure of the eye" (Winzer, 374). One off the most common causes of blindness is cataracts. Cataracts are a clouding of the lens of the eye that keeps light from reaching retina. Resulting in badly blurred vision, double vision, sensitivity to bright lights and change in color vision. One eye is often more seriously affected than the other. It may be cause by chemical change in the lens. The exact cause is still unknown, but there are many factors that can accelerate cataract formation.These include an injury, either from a blow to the head or direct eye injury, other diseases exposure to radiation of any kind (x-rays, microwaves, or infrared rays) long-term use of a corticosteroid drug. A child may be born with cataracts or develop them at an early age. These cataracts, called congenital cataracts, may be caused by a genetic disorder such as down syndrome or from a condition the mother had during pregnancy such as German measles.
The fourth type are other defects such as color blindness, photophobia or albinism. Color blindness is a inherited vision disorder which is often found in male rather than in female. Persons who have color blindness do not have one of the three cone cells which are responsible for receiving either blue, green and red. Therefore, people with color blindness are likely to confuse with red and green. But the condition is not serious enough to connect with visual impairments. Albinism is a hereditary disease. It must be pass thorugh from both parents, "each of whom either has albinism or is a carrier of that trait. Albinism affects the production of melanin"(Winzer,379). A lack of melanin in the retina, iris and choroid will cause the pupil to become in deep red color, the iris will become in grey, light blue or pink. A person with albinism will also suffer from photophobia, which is the extreme sensitivity to light. Bright light may decreases the person's visual acuity and may blinds the persons with albinism.