Prism glasses are specialized eyeglasses with prism lenses to correct diplopia (double vision) and other eye movement visual disorders. Double vision describes the simultaneous perception of two images of a single object, which may appear one on top of the other or next to each other.

Prism correction is utilized as a component of some eyeglass prescriptions to provide clear vision if you experience diplopia.

What Are Prism Glasses?

Prism glasses, or prism lenses, are special plastic or glass optics that eye doctors prescribe to treat binocular vision defects, including double vision. They change the direction where light is reflected, directing it to the correct place on the retina. They do not contain any focusing ability and are ineffective at correcting refractive errors.

Prism eyeglasses help your eyes discern images so that single vision results in correction of binocular vision, double vision, reading difficulties, vision-related headaches and more.

They may be base in, base out, base up, or base down, depending on the direction of the eye misalignment.boy in prism glasses

How Do They Work?

When your vision is clear and you have normal binocular sight, light enters each eye through the cornea and falls on the retina. Your eyes work in tandem to enable your brain to visualize a single image.

Light hits the same spot of the retina in each eye. Each eye collects visual information that’s sent to your brain for processing. Your brain combines this visual information from each eye to produce a single clear image.

With double vision, light entering each eye falls on different spots on the retina, making it so your brain can’t process the visual information accurately to create a single clear image. The result is the production of two images.

The introduction of a prism in from of the eyes bends light before it travels through your eye, redirecting it such that it falls on the correct spot on the retinas in each eye. That allows the brain can then fuse the two images to produce a single clear picture.

Conditions Prism Glasses Treat

While eye doctors use prism glasses to correct double vision and binocular visual dysfunction, the lenses are also viable for other conditions. Doctors prescribe them vision problems associated with:

  • Strabismus: esotropia, exotropia and hyperopia
  • Eye muscle imbalances: convergence excess, convergence insufficiency, and strabismus
  • Visual defects: poor depth perception and visual field loss
  • Muscular problems: Grave’s disease, Myasthenia gravis, among other conditions
  • Suppression: subconscious adaptation to eliminate the effects of binocular vision disorders
  • Eyestrain: to eradicate related headaches
  • Neurological and nerve-related problems: stroke, brain injuries, migraines, concussions, tumors, diabetes, and multiple sclerosis

What Prism Lenses Look Like

For prolonged treatment, lens manufacturers incorporate prisms into prescription lenses. The lenses, and glasses, look like non-prism eyeglasses with one exception. The lens on one side of the eyeglasses may appear thicker and more noticeable.

To hide the prism portion of the lens, some glasses may have thicker frames.

A prism eyeglass can be fitted with a Fresnel prism, a thin press-on vinyl sticker, over the front or back of the lens. This prism will be visible to others, but it is typically employed for temporary prism prescriptions.

Cost

The cost of permanently ground prism glasses ranges between $600 and upwards of $1,500, depending on prescription requirements and frame preferences. Vision insurance covers prism eyeglasses since they are prescribed. The addition of the prism element in the lens makes the lenses more expensive than regular prescription glasses.

The price tag of Fresnel prism eyeglasses ranges between $250 and $500.

Several factors determine the ultimate cost:

  • Type of prism lens
  • Frames you select
  • Where you buy your glasses
  • Your vision insurance plan

How Are They Measured and Added to Glasses?

Prism correction is measured in prism diopters. A prism correction prescription will also specify the “base,” the thickest part of the lens, and the apex, the thinnest part of the lens.

Light is bent towards the base, and the image shifts towards the apex. The base may be specified in, out, up or down.

Your eye doctor determines the type of prism correction necessary by conducting several tests, including Hirschberg, Krimsky and cover tests.

How to Calculate the Amount of Prism (Prentice’s Rule)

Prism diopters specify prism correction. They refer to a unit of angular measurement represented by the Greek symbol delta in superscript.

Technicians use Prentice’s rule to calculate the amount of prism in a lens. The formula was named after Charles F. Prentice, who is considered the father of American optometry. He created (and promoted) the diopter measuring system in the late 1800s.

The amount of prism correction is the product of decentration (distance between the pupil center and a lens’s optical center, in centimeters) and lens power (in diopters).

References

What is Prism Correction in Eyeglasses? (March 2021). American Academy of Ophthalmology.

What's the Deal with Prism Lenses? (November 2020). Optometrists Network.

Perfecting Prism (August 2019). Review of Optometry.

Prism use in adult diplopia (September 2012). National Center for Biotechnology Information.

Diplopia (August 2021). National Center for Biotechnology Information.

Prisms( December 2021). American Association for Pediatric Ophthalmology and Strabismus.

Double Vision (Diplopia) (July 2019). Harvard Health Publishing Harvard Medical School

What are Prism Lenses? (August 2020). Optometrists Network.

Notes on Optics: Prentice's Rule and More (November 2015). Optician Notes.

History of Optometry. American Optometry Association.

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