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Glossary of Vision Terms

Astigmatism
A condition in which the surface of the cornea is not spherical; causes a blurred image to be received at the retina.
Binocular Vision
The blending of the separate images seen by each eye into a single image; allows images to be seen with depth.
Blind Spot
(1) A small area of the retina where the optic nerve enters the eye; occurs normally in all eyes.
(2) Any gap in the visual field corresponding to and area of the retina where no visual cells are present; associates with eye disease.
Contrast Sensitivity
The ability to perceive differences between an object and its background.
Cornea
The outer, transparent, dome-like structure that covers the iris, pupil, and anterior chamber; part of the eye's focusing system.
Dilation
A process by which the pupil is temporarily enlarged with special eye drops (mydriatic); allows the eye care specialist to better view the inside of the eye.
Fundus
The interior lining of the eyeball, including the retina, optic disc, and macula; portion of the inner eye that can be seen during an eye examination by looking through the pupil.
Hyperopia
Farsightedness; ability to see distant object more clearly than close object; may be corrected with glasses or contact lenses.
Iris
The colored ring of tissue suspended behind the cornea and immediately in front of the lens; regulates the amount of light entering the eye by adjusting the size of the pupil.
Legal Blindness
In the U.S., (1) visual acuity 20/200 or worse in the better eye with corrective lenses (20/200 means that a person must be at least 20 feet from an eye chart to see what a person with normal vision can see at 200 feet) or (2) visual field restricted to 20 degrees diameter or less (tunnel vision) in the better eye. NOTE: These criteria are used to determine eligibility for government disability benefits and do not necessarily indicate a person's ability to function.
Lens
The transparent, double convex (outward curve on both sides) structure suspended between the aqueous and vitreous; helps to focus light on the retina.
Low Vision
Visual loss that cannot be corrected with eyeglasses or contact lenses and interferes with daily living activities.
Myopia
Nearsightedness; ability to see close objects more clearly that distant objects; may be corrected with glasses or contact lenses.
Peripheral Vision
Side vision; ability to see objects and movement outside the direct line of vision.
Presbyopia
The gradual loss of the eye's ability to change focus (accommodation) for seeing near objects caused by the lens becoming less elastic; associated with aging; occurs in almost all people over age 45.
Pupil
The adjustable opening at the center of the iris that allows varying amounts of light to enter the eye.
Refraction
A test to determine the best eyeglasses or contact lenses to correct a refractive error (myopia, hyperopia, or astigmatism)
Retina
The light-sensitive layer of tissue that lines the back of the eyeball; sends visual messages through the optic nerve to the brain.
Tonometry
The standard to determine the fluid pressure inside the eye (intraocular pressure).
Visual Acuity
The ability to distinguish details and shapes of objects; also called central vision.
Visual Field
The entire area that can be seen when the eye is forward, including peripheral vision.

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