Dictionary of terms relating to
eyesight and vision

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Base-Down (BD) Prism

A wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned down. Prisms bend light (opposite direction from its thicker end) so the base-down prism turns the light upward thus causing the eye to also move up. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem).Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-In (BI) Prism

A wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned inward, closest to the nose. Prisms bend light (opposite direction from its thicker end) so the base-in prism turns the light outward (toward the ear) thus causing the eye to also move outward. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-Out (BO) Prism

A wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned outward, closest to the ear. Prisms bend light (opposite direction from its thicker end) so the base-out prism turns the light inward (toward the nose) thus causing the eye to also move inward. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-Up (BU) Prism

A wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned up. Prisms bend light (opposite direction from its thicker end (base)) so the base-up prism turns the light downward thus causing the eye to also move down. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Behavioral Optometrist

Also called Functional Optometrist or Developmental Optometrist. An optometrist who specializes in all aspects of vision as it is related to an individual's development and to the role of vision in relation to reading, computer monitor use, and sports. The optometrist may use prescription lenses and/or vision therapy to improve an individual's visual function and performance. Behavioral optometry had its origins in orthoptics, which is a non-surgical treatment for strabismus, and in case analysis systems which were developed to resolve eyestrain symptoms that include excessive tearing, itching, burning, visual fatigue, and headache in nonstrabismic individuals. Behavioral optometry's emphasize of visual care is in prevention, remediation, rehabilitation, and enhancement.

Bifocal Glasses

Used to correct vision at two distances, composed of two ophthalmic lenses such as a plus lens for near vision and a minus lens for distance vision.

Bi-lateral Integration/ Gross Motor Coordination

Visual guidance of body movements and the coordination between both sides of the body.

Binocular Fusion Dysfunction

A clinical condition in which the eyes are not working as a team. Vision therapy is an effective treatment option. (See "General Binocular Vision Disorder")

Binocular Vision

The simultaneous use of the two eyes.

Binocularity

The ability to use both eyes as a team and to be able to fuse (unite) two visual images into one, three-dimensional image (See "Convergence" and "Divergence").

Bi – Ocularity

Using both eyes, but not together as a team.

Blurred Vision

Lack of visual clarity or acuity.

Botulinum Toxin Type A (Oculinum, Botox®)

An injection of this poison has been used as an alternative to conventional surgery in selected strabismic patients. It causes a temporary paralysis of an extraocular muscle that leads to a change in eye position. This change has been reported to result in long-lasting and permanent alteration in eye alignment. Although one injection is often sufficient to produce positive results, one-third to one-half of patients may require additional injections. This technique has been most successful when used in adults with small-angle misalignments. It is not commonly used in children. This treatment is also used in patients who have blepharospam (an uncontrollable eye lid spasm).

Break Point

The point at which a person can no longer fuse (unite) two images into one. A blur point will occur before the this point.

Brewster Stereoscope

An instrument used in orthoptics/vision therapy to improve eye teaming skills and near focusing skills. It consists of two parallel viewing tubes with a +5.00 D lens. The distance from the target can be adjusted as well as the pupillary distance. The Bernell-O-Scope and Keystone Ophthalmic Telebinocular are designed essentially the same.