The Human Eye, Its Functions, and Visual Impairment
How the Human Eye Works
The human eye can be compared to a camera which gathers, focuses, and transmits light through a lens to create an image of the environment. In a camera, the image is created on film; in the eye, the image is created on the retina, a thin layer of light sensitive cells at the back of the eye. The lens of the eye bends, or refracts, light that enters the eye.
The cornea, which is a clear, transparent covering in the front portion of the eye also contributes to focusing light on the retina. Nerve fibers extending back from the retina's nerve cells come together behind the retina to form the optic nerve, a "cable" of nerve fibers connecting the eye with the brain.
The optic nerve transmits messages about what we see from the eye to the brain. Like a camera, the human eye controls the amount of light that enters the eye through the lens under various lighting conditions.
Visual Acuity and 20/20 Vision
Visual acuity is the sharpness of vision determined by a person's ability to discriminate fine details, and is measured by using specially devised tests and charts. One chart that is commonly used for measuring visual acuity is the Snellen chart, which contains letters of the alphabet arranged by line, with each line of letters from the bottom up increasing in size. The letters on the lowest line are the smallest letters on the chart, and the letter at the top is the largest. The character on the bottom line represent 20/20 vision; the single large letter at the top represents 20/200, the designation of legal blindness.
When the Snellen chart is used, visual acuity is generally measured with a person seated 20 feet away from the chart. A person who has normal visual acuity has 20/20 vision. This means that at 20 feet the person can see the line of letters that people with normal sight see from 20 feet.
Why People Need Glasses—Refractive Errors
Most often people need to wear eyeglasses to correct blurred or distorted vision caused by imperfections in the eyes' focusing mechanism. These imperfections, which occur because light entering the eye is not brought into sharp focus on the retina, are known as common errors of refraction or refractive errors.
Refractive errors occur as a result of irregularities in the shape of the cornea, the actual size or shape of the eyeball itself, or the focusing capacity of the lens. Common refractive errors that are fully corrected with eyeglasses or contact lenses are not visual impairments because sight can be corrected to normal. Nearly every person is likely to have a refractive error at some point in life, especially after age 40, and perhaps need to wear eyeglasses or contact lenses. The common refractive errors are:
- Myopia (Nearsightedness)
Myopia is blurred vision that occurs when the eye's focusing mechanism brings light to a focus in front of the retina, usually because the eyeball is very elongated in shape. Eyeglasses or contact lenses correct myopia but do not slow or alter its progression.
- Hyperopia (Farsightedness)
Hyperopia is blurred vision that occurs when light is focused behind the retina,usually because the eyeball is short or small. Eyeglasses or contact lenses correct hyperopia but do not slow or alter its progression.
Astigmatism refers to an irregularly curved cornea that distorts the focus of light entering the eye. Generally corrective lenses restore clear vision.
Presbyopia refers to the eye's loss of accommodation, the eye's focusing power and ability to adjust the focus of the eye on the distance between the individual and the object. People with presbyopia, typically those age 40 and older, experience a progressive inability to focus for near vision viewing as the lens becomes less elastic with age. Lenses with magnification are used to provide the correction needed. These lenses are commonly referred to as "reading glasses," or necessary magnification can be added to a person's regular eyeglasses as bifocals, or trifocals. Variable focus lenses are also available to correct presbyopia.
Visual impairment describes vision that cannot be fully corrected by ordinary prescription lenses, medical treatment, or surgery. The term visual impairment includes conditions ranging from the presence of good usable vision, low vision, or to the absence of any sight at all--total blindness. Many terms are used when people refer to visual impairment. These terms are explained below.
- Legal Blindness
Legal blindness defines visual conditions that, when present, connote eligibility for government or other benefits and services. An individual who is legally blind has a visual acuity of 20/200 in the better eye with the best correction or a visual field of no more than 20 degrees.
- Severe Visual Impairment
Severe visual impairment is a term used by researchers at the National Center for Health Statistics (NCHS) to describe visual impairment in people who are unable to read ordinary newsprint even with correction. This term, used primarily for studying visual impairment in the population, is not used in clinical references by eye care professionals. People with a severe visual impairment may or may not be legally blind.
- Visually Impaired
The term visually impaired, also used by the National Center for Health Statistics for studying visual impairment in the population, describes visual impairment in people who have difficulty reading ordinary newsprint even with correction. Like the term severe visual impairment, visual impairment is used by researchers who study the population, and is not used in clinical references.
- Low Vision
Low vision is a clinical diagnostic term used to describe impaired vision that cannot be improved by conventional eyeglasses, contact lenses, medications, or surgery in which some good usable vision remains. People with low vision can learn to make the best use of the vision available to them.
American Academy of Ophthalmology
655 Beach Street, P.O. Box 7424
San Francisco, CA 94109-7424
Web site: www.eyenet.org
American Optometric Association
243 Lindbergh Boulevard
St. Louis, MO 63141
Telephone: Toll-free 888-396-EYES (3937); 314-991-4100
Web site: www.aoanet.org
American Foundation for the Blind
National Aging Program
11 Penn Plaza, Suite 300
New York, NY 10001
Telephone: 800-232-5463; 212-502-7634
Web site: www.afb.org
Association for Macular Diseases
210 E. 64th Street
New York, NY 10021
Council of Citizens with Low Vision International
c/o American Council of the Blind
1155 15th Street N.W., Suite 720
Washington, DC 20005
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