Eye Care Glossary

The central optical zone

The central optical zone is a 3-4mm diameter area around the center of the cornea through which visual images pass. The central optical zone is critical for clear vision.


The clear, dome-shaped "window" at the front of the eye that covers the iris and pupil. The cornea plays an important role in vision because it is the main focusing part of the eye, accounting for approximately 75 per cent of the eye's light-focusing power. It is made of hundreds of layers of tissue.

Corneal Periphery

The area of the cornea outside the central optical zone, also where Intacs are placed.


Unit used to measure the amount of refractive error (nearsightedness, farsightedness and astigmatism) of an eye. Intacs are intended for people with a refraction between -1.0 and -3.0 diopters.

Eximer Laser

A medical device to emit laser energy created by a mixture of argonfluoride gases. Used in photorefractive kerotectomy (PRK) and LASIK (laser- assisted intrastromal keratoplasty) to reshape corneal curvature by ablating, or burning off, tissue from the center of the cornea.


Two clear, ultra-thin, precision-engineered crescents, each with an arc length of 150º, that can be placed in the periphery of the cornea of the eye to correct myopia. Placed outside of the central optical zone, Intacs are designed to reshape corneal curvature without cutting or removing tissue from the optical zone, which is the critical area for clear vision. This subtle reshaping is intended to make nearsighted corneas flatter, thereby correcting vision. In the United States, Intacs have been approved for commercial use by the Food and Drug Administration.


The pigmented tissue behind the cornea color to the eye (blue, brown, hazel, etc.) and controls the amount of light entering the eye by varying the size of the pupillary opening.


A structure inside the eye that helps to focus light on the retina. It is curved on both sides and acts similar to a camera lens.

Optic Nerve

Acts as a cable to carry images received by the eye to the processing center in the brain, making sense of what is seen.

Polymethylmethacrylate (PMMA)

This material is the clear medical polymer used to make Intacs. This material has been used since 1952 in the intraocular lenses used in cataract surgery and for contact lenses. PMMA has been proven to be biocompatible (i.e., it does not cause a reaction when placed in the eye).


The blade, circular opening in the center of the iris (the colored portion of the eye). The pupil controls the amount of light that enters the eye.


A measure of the eye's focusing power.

Refractive error

A defect in the ability to focus light rays precisely on the retina. Common refractive errors are myopia, hyperopia and astigmatism, affecting approximately half the world's population.


Like the film in a camera, the retina records the images received by the eye and sends them to the brain through the optic nerve.


The middle bisque layer that comprises approximately 90 percent of the cornea. The stroma is composed of densely packed layers similar to the pages of a book. Intacs are designed to be placed within these layers.

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