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Eye Health Glossary

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index of refraction

Ratio of the speed of light in a vacuum to the speed of light in a particular substance. The index of refraction of air is approximately 1. Light bends whenever it passes through substances of different indices of refraction. High index lenses are currently as high as 1.7 for plastic lenses.

inferior oblique

The extraocular muscle (EOM) responsible for rotating the top of the eye away from the nose as well as turning the eye upward.

inferior rectus

The extraocular muscle (EOM) responsible for turning the eye downward as well as rotating the top of the eye away from the nose.

intraocular lens (IOL)

Tiny plastic lens that is inserted into the eye following cataract surgery. The IOL has the necessary refractive power to cancel any refractive vision problems (besides presbyopia) so that following cataract surgery, there is no need to wear glasses or contacts for distance. Learn more about cataract surgery.

intraocular pressure (IOP)

Measure of the pressure inside the eye. Normal IOP usually ranges from about 5 mmHg to 21 mmHg with the average being about 16 mmHg. High IOP is a risk factor for glaucoma.

involutional

Progressive decrease in function of tissues and organs that occurs gradually with age.

iris

The colored part of the eye that acts similar to the shutter of a camera, opening (dilating) and closing (constricting) to regulate the amount of light that enters the eye.

iritis

An inflammation of the iris, resulting in a red, painful eye. Light sensitivity is another common symptom of iritis.

Ishihara color vision test

Color vision test that determines the type and to some degree the severity of red-green color blindness. Plates (like pages in a book with pictures) are presented to the patient, whose task it is to determine whether he/she sees a number or a symbol within a circle made up of many colored dots.

isometropia

A refractive condition in which the refractive error between the eyes is equal. For example, a person with a prescription of -2.50 D in the right eye and -2.50 D in the left eye is isometropic.

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