What Are Eyeglass Prisms?

Optometrists prescribe prisms for eyeglasses and contacts to a variety of individuals with all sorts of different eye conditions. Muscular conditions in which an individual's eyes refuse to work together often require prisms to ease the accompanying eye strain. Other conditions, such as partial peripheral vision loss, sometimes benefit from the light-bending properties of prescription prisms, as well.

Prism Design

• Standard prisms have a pyramidal shape, while prisms in eyeglasses and contact lenses have a conical shape. The "apex" of a prism refers to the top point or thin edge of a lens, and the "base" refers to the flat bottom or thick edge of a lens. A lens with a prism has a slightly rounded edge at the top. Light hits the rounded edge, bends and exits through the other side of the lens at an angle.

Prism Use

• Optometrists prescribe prisms for eyeglasses and contact lenses to treat certain eye conditions that prevent the patient's eyes from working together. "Fixation disparities," or muscle imbalances, cause some eyes to pull apart while the person tries to focus on something. For example, one eye may go up while the other moves down. Such imbalances create additional eyestrain.

  The manner in which prisms bend light tricks the brain into thinking that the eyes work together, thus easing the symptoms of the muscle imbalance and reducing the effect of strain.

Double Vision

• Optometrists typically prescribe prisms to treat diplopia, more commonly referred to as double vision. Some people with diplopia see two separate images, which the brain must merge into one image. Others see a single, blurred image. Others still have one eye that sees flat images, rather than three-dimensional images.

  Prisms help correct each of these occurrences of diplopia by bending light together in a way that repositions images into a single, focused picture. Prisms only work, however, with mild cases of double vision. Severe cases often require surgery to repair the damaged eye muscles.

Peripheral Vision Correction

• Special eyeglasses containing prisms have also helped individuals with homonymous hemianopia, a condition in which an individual loses half of her visual field that results in a peripheral vision blind spot. Optometrists mount plastic prisms on the top and bottom edges of one lens in a pair of eyeglasses. Light from the blind spot strikes one side of the prism, bends and reaches the eye, restoring the otherwise lost images from the individual's blind spot.

Prism Measurements

• Optometrists measure prisms in a measurement called the "diopter" and calculate prisms using the Prentice's Rule. The power of prism equals the distance in centimeters multiplied by the dioptric power of the lens. For an easier, more accurate calculation, however, opticians generally use a manual or automatic lens meter to decide the correct prism prescription.