Vision Diseases, Disorders & Definitions

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P Definitions

Peripheral vision:

Side vision; vision elicited by stimuli falling on retinal areas distant from the macula. Images are not well resolved.

Photochromic Lenses:

Lenses containing material that is photosensitive to ultraviolet light rays. Exposure to ultraviolet darkens the lens color. Conversely, a lack of ultraviolet rays will allow the lens to lighten in color.


Abnormal sensitivity to, and discomfort from light which may be associated with excessive tearing. This is a condition that is often due to inflammation of the iris or cornea.

Plano Lens:

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

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). 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.

Polycarbonate Lenses:

A plastic-like material used in eyeglass lenses that, because of its inherent softness, will not shatter or break in the same way that glass and other plastic materials may.


Refractive condition in which there is a diminished ability to accommodate (focus) arising from loss of elasticity of the crystalline lens, as occurs with aging. Usually becomes significant after age 45.

PRK (photorefractive keratectomy):

Use of high intensity laser (e.g., an excimer laser) to reshape the corneal curvature; for correcting refractive errors.


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.

Progressive Lenses:

Progressive power lenses are true "multifocal" lenses like bifocals or trifocals, but they provide a lineless, seamless progression of varied lens powers for different distances.


A pterygium (tur-RIDGE-ium) is a non-cancerous growth or thickening of hte conjunctiva (clear, thin tissue that lays over the white part of the eye) that grows onto the cornea. As a pterygium grows, it can take on an irregular shape and become red and irritated. Depending on teh size, the pterygium may cause visual impairments. In severe cases, a pterygium can block a patient's vision altogether or may cause an astigmatism due to the advanced distortion of the surface of hte cornea. A pterygium can affect one or both eyes.

UV radiation (usually from sunlight) is the most common cause of pterygiums. Other causes include continuous exposure to dry, windy, sandy, or dusty environments.

Pterygiums can be diagnosed by a physical examination of the eyes and eyelids by your eye care provider. Symptoms include:

  • Raised white tissue with blood vessels on the inner or outer edge of the cornea
  • Feeling like something foreign is in the eye
  • Blurred vision
  • Burning
  • Gritty feeling
  • Irritation
  • Itching

Wearing sunglasses with 100 percent UV protection is the best way to prevent a pterygium, with sunglasses that have a wrap-around design providing the best protection. Wearing a hat with a wide brim also provides valuable protection, but should be worn in addition to sunglesses, not instead of them.

Treatment of a pterygium is not needed unless the growth blocks vision or causes symptoms that are unable to be controlled. In mild cases, pterygium symptoms can be controlled with eye drops; however, when symptoms are resistant to treatment, or when vision is affected, surgery may be considered. Surgery would remove the abnormal tissue from the cornea and sclera (white of the eye). In traditional pterygium removal, the underlying sclera is left exposed. This approach can take two to four weeks to heal and cause mild to moderate discomfort. In addition, the pterygium may grow back in as many as half of all patients, and often, it will grow back larger than its original size.

More common today is a conjunctival autograft surgery because of a reduced risk of recurrence, and it allows most patients to return to work within one or two days of surgery.


Variable-sized, circular opening in the center of the iris that regulates the amount of light entering the eye. Appears black when looking at the eye.


Portions of this page are excerpted from Dictionary of Eye Terminology, copyright 1990-2006 by Barbara Cassin and Triad Communications. Reprinted with permission.

Some of the information above is reprinted from the National Eye Institute, National Institutes of Health, and is for educational purposes only. Superior Vision strongly recommends eye exams as the best way to diagnose eye conditions and learn more about potential vision problems.