Vision Diseases, Disorders & Definitions

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R Definitions

Radial keratotomy (RK)

Type of refractive surgical procedure. Series of spoke-like (radial) cuts made in the cornea to flatten it, reducing its refractive power and thereby correcting myopia (nearsightedness). Now, mostly replaced by LASIK.


A test performed by optometrist or ophthalmologist to determine an eye's refractive error and the best corrective lenses to be prescribed. Series of lenses in graded powers are used to determine which provide sharpest, clearest vision.

Refractive error

Optical defect in an eye. Parallel light rays are not brought to a sharp focus precisely on the retina, producing a blurred retinal image. Can be corrected by eyeglasses (spectacles), contact lenses, or refractive surgery.


The innermost layer of the eye, a neurological tissue, which receives light rays focused on it by the lens. This tissue contains receptor cells (rods and cones) that send electrical impulses to the brain via the optic nerve when the light rays are present.

Retinitis pigmentosa (RP)

Set of inherited retinal degenerations. Death of photoreceptors gradually leads to night-blindness and loss of peripheral vision causing tunnel vision.


Portions of this page are excerpted from Dictionary of Eye Terminology, copyright 1990-2006 by Barbara Cassin and Triad Communications. Reprinted with permission.

Some of the information above is reprinted from the National Eye Institute, National Institutes of Health, and is for educational purposes only. Superior Vision strongly recommends eye exams as the best way to diagnose eye conditions and learn more about potential vision problems.