Dictionary of terms relating to
eyesight and vision

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Requires the use of good visual skills, which are distance and near acuity, accommodation skills, binocularity skills (convergence), oculomotor skills (saccadic), peripheral vision, figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization.


With refraction we mean the deviation of light entering the eye.

Refraction Test

Determines the eye's refractive error and the best corrective lenses to be prescribed. There are several methods of performing refraction: Retinoscopy, Automated Refractor, and Subjective Refraction.

Refractive Error

Defects in vision caused by the eye's inability to bend, or refract light and focus it clearly on the retina. Astigmatism, hyperopia, and myopia are common conditions of refractive error, also called ametropia.

Refractive Power

A lens' ability to bend parallel light rays into focus, as measured by power diopters. In general, the greater the curvature of a lens and the greater the difference between center thickness and edge thickness, the higher the index of refraction and the greater its refractive power. Refractive power can also refer the strength of a person's contact lenses or glasses.

Refractive Media

The parts of the eye that light travels through before being focused on the retina includes the cornea, crystalline lens, aqueous, and vitreous. (See diagram of the eye)

Relative Amblyopia

Functional amblyopia can co-exit with a pathology abnormality. Treatment is possible.


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. (See diagram of the eye)

Retinal detachment

A separation of the retina from its connection at the back of the eye. The separation usually results from a tear (that is, a rent or rip, not a tear drop) in the retina. The tear often occurs when the vitreous gel pulls loose or separates from its attachment to the retina, usually in the outside edges of the eye. The vitreous is a clear gel that fills most of the inside of the eye between the retina and the lens. If the retina is weak when the vitreous gel pulls loose, the retina will tear. This rip is sometimes accompanied by bleeding, or hemorrhage, if a blood vessel is also torn. Once the retina has torn, liquid from the vitreous gel can then pass through the tear and accumulate behind the retina. The build-up of fluid behind the retina is what separates (detaches) the retina from the back of the eye. As more of the liquid vitreous collects behind the retina, the extent of the retinal detachment can progress and involve the entire retina, leading to a total retinal detachment. A retinal detachment almost always affects only one eye. The second eye, however, must be checked thoroughly for any signs of the problem.


This technique determines the eye's refractive error and the best corrective lenses to be prescribed. An instrument called a retinoscope which consists of a light, lens, mirror, and handle, is used to shine light into a patient's eye. There are two types of retinoscope: streak and spot retinoscope. When light is shone into patient's eye, the light is reflected back ("reflex"). If the reflection is in the same direction ("with movement") of the retinoscope then the refractive error is hyperopia (farsightedness) and a plus lens is prescribed. If the reflection is in the opposite direction ("against movement") of the retinoscope then the refractive error is myopia (nearsightedness) and a minus lens is prescribed. The strength of the prescription is determined when the pupil is suddenly filled with light ("neutralized") with the appropriate lens powers (strength). To learn more about the principles of retinoscopy, please click here. There are other types of retinoscopy such as Static retinoscopy or Dynamic retinoscopy. (See "Cycloplegic Refraction")


A receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for night vision (non-color vision in low level light). (See diagram of the eye)