A common form of refractive error, caused when the cornea is not round but rather shaped like a football. The light rays entering the cornea focus on different planes, creating blurring and distortion.

The clear outer window through which light enters the eye. It covers the pupil, the colored iris, and other tissues at the front of the eye. The cornea is the site of most refractive surgeries.

A clouding of the inner layer of the cornea, occuring in under 2% of PRK and 0.5% of LASIK patients. It usually clears within three to six months after surgery. If it is noticeable to the patient, it may be treated with eye drops or an additional surgery.

A computer-generated map of thousands of points on the surface of the eye. It provides a clear picture of the cornea's contour and its relative curvatures. It is used to determine the cornea's focusing capabilities and to program the excimer laser for PRK and LASIK procedures.

A unit for measuring degrees of refractive error. The term diopter is also used to describe the refractive power of a lens. For example, nearsightedness can be classified into low (1.00 to 5.99 diopters), moderate (6.00 to 9.99 diopters), and high myopia (10.00 diopters or greater).

A second refractive surgery. This is usually done to touch up the results of a first laser vision correction surgery.

A precisely controlled laser that uses computer programming and cool ultraviolet light to perform refractive surgeries such as LASIK and PRK. The Casey Vision Correction Center performs procedures with both Summit Apex Plus excimer laser and the newer Nidek EC-5000 laser.

See hyperopia.

A refractive error which causes difficulty in focusing on near objects. This occurs when the cornea is too flat or the eye is too short, making light rays focus behind the retina. Hyperopia is often called farsightedness.

The colored part of the eye. The iris is actually a muscle which controls the size of the pupil and the amount of light that enters the eye.

The abbreviation given to Laser in-Situ Keratomileusis. This refractive surgery technique uses a microkeratome to create a flap of corneal tissue which is retracted, allowing the excimer laser to reshape the central corneal tissue.

The natural lens of the eye is designed to fine-focus light on the retina. It varies its shape for focusing on far or near objects.

A hand-held surgical instrument designed to make very precise incisions in the cornea. The microkeratome is used to create the corneal flap needed in a LASIK surgery.

A way to manage presbyopia. One eye is corrected for distance vision and the other is corrected for near vision. This can be achieved surgically or with corrective lenses.

The technical term for nearsightedness, myopia is the most common refractive error. It is caused when the cornea is too steep or the eye is too long, thereby bringing light rays to a focal point before they reach the retina. This makes it difficult to focus on distant objects, although near objects can usually be seen clearly.

See myopia.

The result of either too much correction with the laser or an inadequate healing response on the part of the cornea. Significantly overcorrected vision can be hyperopic and may require an enhancement.

The abbreviation given to Photorefractive Keratectomy. The original excimer laser refractive surgery, PRK reshapes the surface of the cornea to correct nearsightedness, farsightedness and astigmatism.

A common focusing problem in people over 40, presbyopia occurs when the eye's natural lens loses its flexibility, making it difficult to focus on near objects. Reading glasses or bifocals are usually needed to improve visual accommodation, but monovision can be a good alternative for some people.

One of the first effective refractive techniques, RK was invented by a Dutch ophthalmologist in the nineteenth century, but was not widely known until it was promoted in Russia in the 1970s. This surgery alters the shape of the cornea by making spoke-like incisions in the peripheral cornea. RK is effective in treating low to moderate myopia, but it has been replaced by more predictable and widely applicable excimer laser procedures.

Focusing problems caused by variations in the length of the eye or the shape of the cornea. Refractive errors occur when light rays entering the cornea are not brought to a focal point precisely on the retina, thus producing blurred vision.

Any surgical procedure designed to alter the way the eye refracts or focuses light.

The delicate, light sensitive tissue inside the eye. The retina converts light images into electrical impulses. These are sent to the visual cortex of the brain where they are converted into visual images.

Less correction than is needed for clear, sharp vision. It usually results from a deliberately under-targeted laser correction or from an unusually aggressive corneal healing response. In some cases, one eye may be deliberately undercorrected to create monovision and compensate for presbyopia.