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Eye Health Glossary

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base curve

A measure of the curvature of a contact lens (and also eyeglass lenses). It is necessary to fit a contact lens with a base curve similar to the corneal curvature or an eye in order to ensure that the contact lens fits properly.

best corrected visual acuity (BCVA)

A measure of the best possible visual acuity that can be obtained with correction (eyeglasses or contact lenses), if needed, in place. Conditions that result in a reduction of BCVA include eye diseases such as macular degeneration and infection, as well as amblyopia.

bifocal

Bifocals are lenses that are made up of two different lens powers. They are almost exclusively used for the correction of presbyopia, where a different power is needed for far and for near vision. Bifocals are available as contact lenses, progressive addition lenses (PALs), or as the traditional line bifocals in eyeglasses.

bilateral

Pertaining to both right and left sides. For ophthalmic purposes, pertaining to both eyes.

binocular

Simultaneous use of both eyes to obtain one image. Stereopsis is not possible without binocular vision. Those who cannot obtain binocular vision instead have monocular vision, where the eyes either alternate in visual dominancy or one eye is exclusively used for vision.

binocular rivalry

A phenomenon in which the two eyes compete for dominance. A situation in which the eyes are presented with very different targets (eg. vertical lines for the left eye and horizontal lines for the right eye), the ocular image that will be perceived is not a solid grid of vertical and horizontal lines but rather an alternation of horizontal and vertical lines.

binocular vision

The ability to use both eyes simultaneously to produce a single image. Binocular vision gives rise to stereopsis, which in turn gives rise to depth perception.

blepharochalasis

A rare inflammatory condition in which recurrent episodes of upper eyelid edema (swelling) leads to excessive stretching and thinning of eyelid tissue with consequent lid laxity, resulting in ptosis.

blepharospasm

A rare eye condition in the elderly in which spontaneous contraction of the orbicularis oculi muscle forces the eyes to twitch or, in extreme cases, to be closed completely for a prolonged period of time. Treatment of choice for blepharospasm is botulinum toxin injection. Learn more about blepharospasm.

blind spot

A blind spot is known as a scotoma. Everyone has a physiological blind spot, which exists in the area of visual space that corresponds the the place in the retina where the optic nerve head resides (there are no photoreceptors in this area). Pathological blind spots can be a consequence of many different eye diseases, including glaucoma, macular degeneration, macular hole, and diabetic retinopathy. The position and shape of a blind spot can help optometrists and ophthalmologists to locate where a problem is occurring in the retina.

bowman's membrane

Bowman's membrane is the collagen layer located between the surface epithelium and the stroma of the cornea.

bruch's membrane

Bruch's membrane is an elastic layer within the eyeball that resides between the retinal pigment epithelium and the choroid. Bruch's membrane gives support to the eye. Breaks in Bruch's membrane may become visible via ophthalmoscopy and are known as angioid streaks.

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