Dictionary of terms relating to
eyesight and vision

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Ill-Sustained Accommodation

This clinical condition is also called Accommodative Fatigue. It is the inability of the eye to adequately sustain sufficient focusing over an extended time period. The most common sign or symptom is blurred vision after prolonged near work such as reading and using a computer. In addition, such patients often have asthenopia (eyestrain). Clinical signs include: normal amplitude of accommodation, decreased PRA, and the patient generally fails the +/-2.00 D flipper test. Plus lenses (glasses or contacts) and vision therapy are effective in treating this condition.

Incomitant Strabismus

A condition also known as Noncomitant Strabismus. It occurs when the magnitude of deviation is not the same in the different positions of gaze or with either eye fixating. There is an abnormal restriction to movement or an over-action of one or more of the extraocular muscles. Generally, the magnitude must change by at least 5 PD to be incomitant (nonconcomitant).


Plastic inserts that are placed just under the eye's surface in the periphery of the cornea, which helps achieve a flatter cornea leading to clearer vision. Intacs may be needed when keratoconus patients no longer can obtain functional vision with contact lenses or eyeglasses.

Inverse Occlusion

Covering the amblyopic eye. (See "Direct Occlusion" and "Occlusion")


The colored part of the eye located between the lens and cornea; it regulates the entrance of light. (See diagram of the eye)


A keratometer, also known as a ophthalmometer, is a surgical device for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It was invented by the French ophthalmologist Louis Émile Javal in 1880.


a device consisting of alternate black or white concentric circles and used for examining corneal curvature; called also Placido's Disk.


The sensation of bodily position, presence, or movement resulting chiefly from stimulation of sensory nerve ending in muscles, tendons, and joints.

Lag of Accommodation

A measure of the eye's ability to focus accurately on a given target. The dioptric difference between the eye's focusing response and the stimulus to focus.

Latent Hyperopia

Hyperopia (farsightedness) is compensated by accommodation and the tonicity (tension) of the ciliary muscle; identified by cycloplegic refraction. In mild cases of hyperopia (farsightedness), the eyes are able to compensate without corrective lenses; otherwise a plus lens (glasses or contacts) is prescribed.

Learning Disability (LD)

A disorder that affects people's ability to either interpret what they see and hear or to link information from different parts of the brain. Learning disabilities can be divided into five broad categories: speech and language disorders, reading disorder, arithmetic disorder, writing disorder, and attention disorders. The term learning disability does not include children who have learning problems that are primarily the result of visual, hearing, or motor disorders.


The lens or crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to focus on the retina. Its function is thus similar to a man-made optical lens.

In humans, the refractive power of the lens in its natural environment is approximately 15 dioptres, roughly one-fourth of the eye's total power.

The lens is flexible and its curvature is controlled by ciliary muscles through the Zonules. By changing the curvature of the lens, one can focus the eye on objects at different distances from it. This process is called accommodation. The lens continually grows throughout life, laying new cells over the old cells resulting in a stiffer lens. The loss of the individual's focusing ability is called Presbyopia.

The lens is made of transparent proteins called crystallins. The average concentration of lens proteins is about twice than that of other intracellular proteins and is thought to play a structural role in the lens. It is about 5 mm thick and has a diameter of about 9 mm for an adult human (though these figures can vary). The proteins are arranged in approximately 20,000 thin concentric layers, with a refractive index (for visible wavelengths) varying from approximately 1.406 in the central layers down to 1.386 in less dense cortex of the lens. This index gradient enhances the optical power of the lens. The lens is included into the capsular bag, maintained by the zonules of Zinn.

During the fetal stage, the development of the lens is aided by the hyaloid artery. In adults, the lens depends entirely upon the aqueous and vitreous humors for nourishment.


Also called Verometer, is a device used to measure the refractive power of eyeglasses and contact lenses.