Dictionary of terms relating to
eyesight and vision

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A paralysis that when occurring in ocular muscles causes double vision when looking in some directions.


Medical specialist dealing with the development, health, and diseases of children. Pediatrics became a specialized area of study in the 18th century, when the first children's hospitals were founded. Early pediatricians studied childhood diseases but could do little to cure them. By the mid-20th century, when antibiotics and vaccines had controlled most of these diseases in the developed world and infant and child mortality had fallen, pediatrics changed its focus to normal growth and child development. Recently, behavioral and social aspects of children's health have been incorporated.


To prevent sight out of the good eye and force the weaker, amblyopic eye, to function. A filter, eye patch, or eye drops such as atropin or miotics are used on the good eye.

Perceptual Skills

Includes the identification, discrimination, spatial awareness, and visual-sensory integration. These are visual cognitive skills used to processes visual information to the brain to be organized and interpreted. (See "Visual Perceptual Disorder")

PTS Computerized Perceptual Home Vision Therapy System

A home-based computerized perceptual therapy program, which was designed to enhance visual information processing. The therapy procedures address simultaneous processing, sequential processing and/or speed of information processing. This computer program contains 6 activities that are specifically for the following problems: a weakness with visual information processing skills such as figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization skills, slow speed of information processing, and acquired brain injury with perceptual-cognitive deficits. This program is available only from a licensed eye care practitioner.


The measurement of a visual field function (the total area that can be seen while looking straight ahead) using targets of different sizes and brightness (light levels). The visual field is measured in degrees. In a normal eye the peripheral field of vision is about 180 degrees. An instrument called a perimeter is used for mapping all areas of a person's eyesight, including peripheral (side) vision. Visual field testing can help detect certain patterns of visual loss, indicating specific types of eye diseases or vision conditions. It is the single best test for diagnosing glaucoma.

Peripheral Vision

The ability to see or be aware of what is surrounding us, our side vision. (See "Visual Field".)


Fast, jump movement. (See "Fusional Vergence")


Unusual sensitivity to light.

Physiological Diplopia

A normal diplopia (double vision) that occurs when an individual is not pointing his/her eyes on a certain object.

Plano Lens

A lens that has no prescription. No variance between the curvature of the front and back lens surfaces. It is a flat lens.


A method of eye exercises created to stimulate and train an amblyopic eye. The goal is to have eyesight which is produced by the fovea. (See "Eccentric Fixation")

Plus (+) Lens

Convex lens (thicker in the middle) relaxes focusing and converges light. It is typically used in glasses or contact lenses for people who are farsighted (hyperopic). Although it may also be prescribed for other visual conditions as well.

Polaroid Lens

A lens used in sunglasses and sometimes 3D glasses which consists of two glass or plastic surfaces with a plastic lamination between the two surfaces, and designed to reduce reflected glare. In optometric vision therapy, these lens are used with 3D pictures such as vectograms and stereograms, which are also polarized.


Diplopia also known as double vision can occur when viewing with only one eye; this is called monocular diplopia. Where the patient perceives more than two images, it is called monocular polyopia. In this case, the multiple vision can be caused by a structural defect in the vision system, such as cataracts, subluxation of the crystalline lens or Keratoconus causing irregularities in the refraction of light within the eye.

Positive Relative Accommodation (PRA)

A measure of the maximum ability to stimulate accommodation while maintaining clear, single binocular vision.


Sometimes called the fourth refractive error, is not truly a refractive error. It is the natural process of the eye losing the ability to accommodate or change the shape of the natural crystalline lens inside the eye to see comfortably at near. This vision defect occurs with the advancement of age; the onset usually occurs between the ages of 40 to 45. Unlike the rest of the body, which stops growing by the age of twenty, the lens of the eye continues to grow throughout life. As the lens ages and grows, it becomes harder in consistency, loses its elasticity, and therefore is resistant to changes in shape. The result is a gradual reduction in accommodation (near eye focus), and a more dependence on reading glasses. A plus lens or multi-focal lens (such as a bifocal lens) is prescribed in the form of glasses or contact lenses.


A wedge-shaped lens which is thicker on one edge than the other. This plastic or glass lens bends light (opposite direction from its thicker end). Prisms can be used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). A prism is sometimes added to glasses to help improve eyesight due to an eye misalignment or visual field loss. (See "Base-Down Prism", "Base-In Prism", "Base-Out Prism", "Base-Up Prism", "Yoked Prism")

Prismatic Effect By Lens

When light goes through a wedge shaped lens which is called a prism, it bends. Light is also bent when it does not go through the center of a lens. This is an undesirable effect that can occur in glasses. It commonly occurs when the pupillary distance (PD) is not measured or made correctly.

Proximal Vergence

A convergence response attributed to the awareness of, or the impression of nearness of an object of regard. (See "Vergence")


The condition Accommodative Excess/Spasm causes an individual to experience blurry distance vision after prolonged near work such as reading or using a computer. The individual may appear to be nearsighted (myopia). Treatment options may include prescription lenses and/or vision therapy.


Droopy upper eyelid, causing the eye to remain partially closed.


The opening at the center of the iris of the eye. It contracts (dilates) in the dark and when the eye is focused on a distant object. (See diagram of the eye)

Pupillary Distance (PD

The distances between the pupils of the eyes, in millimeters -- a necessary measurement for proper lens prescription.

Pupillary Reflex

The automatic contraction or enlargement of the pupil when confronted with the presence or absence of light, accommodation, or emotional change.


A device used to measure the distance between the pupils of the eyes, in millimeters, which is a necessary measurement for proper lens prescription. It also measures the diameter of the pupil.

Pursuit Dysfunction

A condition in which the individual's ability to follow a moving target is inadequate. Vision therapy is an effective treatment option. (See "Ocular Motor Dysfunction")

Pursuit Test

Measures the eyes ability to follow a moving target.


The eye's ability to smoothly follow a moving target.