Cornea: The cornea is the crystal clear dome that covers the front of the eye. The majority (70%) of the bending (refracting) of light rays is accomplished by the cornea.

Diopter: the measurement of refractive error. When used in Excimer refractive surgery, diopter is a measurement of the refractive power of the eye. In LASIK and other refractive procedures, a negative diopter value signifies an eye with myopia and positive diopter value signifies an eye with hyperopia.

Farsightedness: the common term for hyperopia.

Glare: scatter from bright light that decreases vision.

Halos: rings around lights.

Hyperopia: when the eye sees distant objects more clearly than near objects.

Iris: The iris controls the amount of light entering into the eye and is the part of the eye that gives it color (i.e., blue, green, brown).

Laser: an acronym for light amplification by stimulated emission of radiation. A laser is an instrument that produces a powerful beam of light and can vaporize tissue.

LASIK: the acronym for laser assisted in situ keratomileusis which refers to creating a flap in the cornea with a microkeratome and using a laser to reshape the underlying cornea.

Lens: The crystalline lens finishes the focusing of light. It helps to "fine tune" vision, and it is able to change shape to allow the eye to focus on near objects. When it becomes cloudy, it is called a cataract. Unlike the cornea, the lens can be made to change its shape (and, therefore, its refractive power) rapidly and voluntarily. Using its ability to change shape, the lens allows the eye to change its focal point. Changes in the shape of the lens will allow a normal eye to focus on near objects.

Microkeratome: a surgical device that is affixed to the eye by use of a vacuum ring. When secured, a very sharp blade shaves a small amount of the cornea at a predetermined depth.

Monovision: the purposeful adjustment of one eye for near vision and the other eye for distance vision.

Myopia: when the eye sees near objects more clearly than distant objects.

Nearsightedness: the common term for myopia.

Optic nerve: The optic nerve is the nerve that runs from the eyeball to the brain. It carries information from the retina to the brain for interpretation.

Overcorrection: a complication of refractive surgery where the expected amount of correction is more than desired.

Presbyopia: part of the normal process of aging. As a person becomes older, one begins to lose the flexibility of the lens of the eye which limits the ability of the eye to change its point of focus from distance to near.

PRK: the acronym for photorefractive keratectomy which is a procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping and use of a computer-controlled Excimer laser to reshape the stroma.

Pupil: The pupil is the opening in the middle of the iris. It functions like the diaphragm in a camera, controlling the amount of light that enters the eye. The pupil is small in bright light and large in dim light.

Refraction: a test to determine the best eye glasses or contact lenses to correct a refractive error (myopia, hyperopia, astigmatism) OR the bending of light by the use of a lens or other material.

Retina: The retina is a thin layer of nerve tissue that senses light. Specialized cells called rods and cones convert light energy into nerve signals that travel through the optic nerve to the brain. The retina is analogous to the film in a camera.

Undercorrection: a complication of refractive surgery where the expected amount of correction is less than desired.