accommodation (uh-kah-muh-DAY-shun). The autotic adjustment of optical power by the eye in order to maintain a clear image (focus) as objects are moved closer.
age-related macular degeneration (AMD, ARMD) (MAK-yu-lur). A condition that includes deterioration of the macula and resulting in loss of sharp central vision. Most common cause of decreased vision after age 60.
astigmatism (uh-STIG-muh-tiz-um). A Refractive Error of the eye in which refractive power is not uniform in all directions (meridians). Light rays entering the eye are refracted unequally through different meridians, which prevents formation of a sharp image focus on the retina. Correctable with a cylindrical lens.
bifocals. A lens having two separate and distinct points of focus (focal lengths) which incorporate two different powers in each lens, usually for near and distance corrections.
binocular vision. Focusing and fusing of the separate images seen by each eye into one single binocular image.
blind spot. Sightless area within the visual field of a normal eye, where the optic disc attaches the optic nerve to the eye. Caused by absence of light sensitive photoreceptors where the optic nerve enters the eye.
cataract. Clouding of the crystalline lens, which may prevent a clear image from forming on the retina. If visual loss becomes significant, surgical removal is required. Types of cataracts include traumatic, congenital and age-related.
central vision.An eye's best vision; used for reading and discriminating fine detail and color.
color blindness. Decreased ability to determine differences between colors, especially shades of red and green. Usually hereditary.
cornea (KOR-nee-uh). Transparent membrane in the front of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power.
crystalline lens. The natural lens inside of the eye. Transparent, biconvex intraocular tissue that converges light to helps bring rays of light to the retina.
diopter (D) (di-AHP-tur). Unit of measurement for lens power. It is the reciprocal of the focal length in Meters.
diplopia, double vision. A visualization of two images from one object; images may be horizontal, vertical or oblique.
emmetropia (em-uh-TROH-pee-uh). Absence of Refractive Error. Sometimes called “Normal 20/20 Vision.” Images at 20 feet focus sharply on the retina.
fovea (FOH-vee-uh). Central area in the macula that produces the sharpest focus. Contains a high concentration of cones which aid in clear central vision.
glaucoma (glaw-KOH-muh). A disease of the eye characterized by increased intraocular pressure. A common cause of preventable vision loss. May be treated by prescription drugs or surgery.
hyperopia (hi-pur-OH-pee-uh), farsightedness. Focusing defect in which an eye is underpowered. Light rays coming from a distant object strike the retina before coming to sharp focus, causing blurred vision. Corrected with plus (convex) lenses.
iris. Pigmented tissue lying behind the cornea that gives color to the eye (e.g., blue eyes) Controls light by contracting and constricting the opening (pupil).
lens, crystalline lens. The natural lens inside the eye. Transparent, biconvex intraocular tissue that helps refract rays of light to a point focus on the retina.
low vision. Term usually used to indicate vision of less than 20/200. May require additional optical aids, especially for near point tasks.
myopia (mi-OH-pee-uh), nearsightedness. A Refractive Error in which the eye over focuses light. Rays of light traveling from a distant object are brought to focus in front of the retina. Requires a minus (concave) lens to correct.
ophthalmologist (ahf-thal-MAH-loh-jist). Physician (MD) specializing in diagnosis and treatment of refractive, medical and surgical problems related to eye
optician (ahp-TISH-un). Professional who makes and adjusts optical aids, e.g., eyeglass lenses, from refraction prescriptions supplied by an ophthalmologist or optometrist.
optometrist (ahp-TAHM-uh-trist). Doctor of optometry (OD) specializing in vision problems, treating vision conditions with spectacles, contact lenses, low vision aids and vision therapy, and prescribing medications for certain eye diseases.
orthoptics. Optical specialty dealing with the diagnosis and treatment of defective eye coordination, binocular vision, and functional amblyopia by non-medical and non-surgical methods, e.g., glasses, prisms, exercises.
peripheral vision. Side vision; vision, caused by stimuli falling on retinal areas distant from the macula, toward the sides of the globe.
photophobia (foh-toh-FOH-bee-uh). Extreme sensitivity to, and discomfort from, light. May be associated with excessive tearing.
presbyopia (prez-bee-OH-pee-uh). Refractive condition in which there is a diminished power of accommodation arising from loss of elasticity of the crystalline lens, as occurs with aging. Usually becomes significant after age 40.
progressive addition lens (PAL), progressive-power lens.. Ophthalmic lens that utilizes a steepening of curves to incorporate corrections for distance vision through intermediate range, to near vision (usually in lower part of lens), with smooth transitions and no bifocal demarcation line.
pupil. An opening in the center of the iris, of variable sizes, that regulates the amount of light that enters the eye.
refraction. A test to determine the refractive state of the eye, and the best corrective lenses required to aid in clear vision. A series of lenses in graded powers are offered to determine which provide sharpest, clearest vision.
refractive error. An error in refraction of the eye. An optical defect in an unaccommodating eye in which parallel light rays do not focus sharply on the retina.
retina (RET-ih-nuh). Light sensitive with photoreceptors in the eye that converts images from the eye's optical system into electrical impulses that are sent to the brain where the image is formed.
trifocal (TRI-foh-kul). An ophthalmic lens that incorporates three lenses of different powers. The main portion is usually focused for distance (20 ft.), the center segment for about 2 ft., and the lower segment for near (40 cm, or 16 inches).
20/20. “Normal” vision. Upper number is the standard distance (20 feet) between an eye being tested and the eye chart; lower number indicates that a tested eye can see the same small standard-sized letters or symbols as an emmetropic eye at 20 feet.
visual acuity. Assessment of the eye's ability to distinguish object details and shape, using the smallest identifiable object that can be seen at a specified distance (usually 20 ft. or 16 in.) and letter height (8.87mm).
visual field. The area visible to an eye that is fixating straight ahead.