Glossary of vision-related terms

Age-Related Macular Degeneration (AMD): An acquired retinal disorder characterized by degeneration in the central (macular) area of the retina. This is the leading cause of blindness in persons over age 65.

ANSI: American National Standards Institute: a government-sponsored organization that sets standards in many industries including the optical industry.

A-Scan Ultrasonography: A noninvasive diagnostic test in which sound waves are used to determine the position of, and distances between, structures within the eye and orbit.

Asthenopia: Ocular fatigue, eyestrain.

Astigmatism: Refractive condition wherein light rays entering the eye do not focus at a single point, resulting in blurred or distorted vision. Corrected by cylinder lenses.

Bifocal: A lens with two refractive powers. Power in the upper portion is for distant vision, and the lower portion is for seeing close objects, e.g. reading.

Binocular: Pertaining to both eyes.

Blind Spot: The area on the retina where the optic nerve enters. It is not sensitive to light since it has no visual receptors.

Cataract: A partial or complete opacification, or clouding over, of the lens.

Cilia: Eyelashes

Color blindness: A term used to describe any color perception deficiency. Few people lack total color recognition, and the most common deficiency is the inability to distinguish between red and green.

Cones: Light-sensitive cells densely congregated in the central portion of the retina. They account for precise seeing and function at higher levels of illumination than the other light receptors, the rods. Color vision is a function of the cones.

Conjunctiva: The thin clear mucous membrane that covers the white portion of the eye (sclera) and lines the eyelids, providing protection to the eyeball.

Conjunctivitis: An inflammation of the conjunctiva.

Cornea: The transparent convex anterior portion of the eye. It covers the iris and lens and is continuous with the white sclera. It provides most of the eye's refracting power, focusing light as it passes onto the retina.

Depth Perception: The ability to judge distances; also called stereopsis.

DPA: Stands for Diagnostic Pharmaceutical Agents. A term used by eye doctors for eye drops used for diagnostic purposes during an eye examination.

Esotropia: The deviation of an eye inward, sometimes referred to as "cross-eyed."

Floaters: Cells of tissue particles that are suspended and float in the vitreous. When light enters the eye, they cast moving shadows on the retina.

Fundus: The back of the eyeball.

Glaucoma: A disease characterized by abnormally increased intraocular pressure resulting in optic nerve damage and progressive loss of vision, beginning in the periphery. A leading cause of blindness because of the absence of symptoms in the early stages.

High Index: A lens material with greater than normal refracting capacity that enables normally thick lenses to be lighter and thinner.

Hyperopia: Farsightedness. A refractive condition wherein light focuses behind the retina, which results in near vision being less clear than distant vision. Convex or "plus" lenses are prescribed for hyperopia.

Iris: The colored part of the eye. It is a circular disk that dilates and constricts, thereby controlling the amount of light entering the eye through its central opening, the pupil.

Myopia: Nearsightedness. A refractive condition wherein light focuses in front of the retina, which results in far vision being blurred. Concave or "minus" lenses are prescribed for myopia.

Ophthalmologist: A doctor of medicine who has completed a residency program in ophthalmology and specializes in treating diseases of the eye and performing ocular surgery.

Optician: A fabricator and dispenser of eyeglasses. Some opticians also fit contact lenses.

Optometrist: A doctor of optometry providing all ophthalmic services except surgery. In most states they are licensed to treat ocular diseases.

Photochromic Lenses: Lenses that darken when exposed to intense illumination (e.g. sunlight) and lighten in color when illumination is reduced.

Photorefractive Keratectomy (PRK): Refractive surgery to eliminate myopia by flattening the central portion of the cornea with a laser.

Polycarbonate: A high-index plastic that is used to make spectacle lenses that are very resistant to impact.

Presbyopia: The natural, age-related progressive loss of the accommodative (focusing) ability of the eye that usually begins in the early forties.

Progressive Lens: A no-line lens with a gradual blend of prescriptions ranging from the prescription for distance viewing at the top to the reading prescription at the bottom. Worn in lieu of bifocals and trifocals, which have visible lines.

Pupil: The opening in the center of the iris that appears as a black circle and admits light into the eye.

Radial Keratotomy: Refractive surgery to eliminate myopia by making a series of radial incisions in the corneal periphery. With healing, the cornea flattens.

Sclera: The white outer covering of the eye's surface.

Snellen Chart: The traditional eye chart used to measure visual acuity, consisting of rows of characters in decreasing size.

Stereoscopic: Seeing objects in three dimensions.

Stye: An infection of the eyelid at the base of an eyelash. A hordeolum.

Tonometer: The instrument used to determine the level of intraocular pressure.

Tonometry: Testing of the level of the intraocular pressure with a tonometer.

Twenty/Twenty (20/20) Vision: The designation for "perfect" distant vision. Indicates that a person tested at 20 feet sees what a normal eye should see at 20 feet.

Visual Acuity: A measurement of the ability to discern characters at a given distance.