How do Prism Lenses Work?

Editor: Dr. Russel Lazarus, Published April 20, 2021

Your eye doctor may have prescribed prism lenses, but you’re not sure why?

With healthy binocular vision, light enters the two eyes by passing through the cornea and focusing onto the retina in the back of the eye. The brain then interprets the information it receives from the retina of each eye, and produces a single image.

A single image can only be produced if the eyes are working together, and the light is focused on the same retinal area in each eye.

An eye condition, known as Binocular Visual Dysfunction, is a common reason for the eye doctor prescribing prism lenses.

Double vision occurs when the light is focused on different retinal areas in each eye, and is generally caused by a misalignment of the two eyes.

Prism lenses bend light before it passes through the eye, to redirect the light onto the correct retinal area in each eye. The brain can then accurately interpret the information and produce a single image. 

How is a prism prescribed?

Hirschberg test. This test is used to identify the presence of strabismus, or a turned eye. Your doctor will aim a small light at your eyes and look for a corneal reflection in the center of each pupil. If your reflection is off-center in either one or both eyes, you may have strabismus.

Krimsky test. This test is used to measure the degree of eye misalignment. Your doctor will aim a small light at your eyes, and place prisms of varying strengths in front of your eye. To determine which prism is appropriate for you, your doctor will look for the prism that is able to bring the corneal reflection to the center of your pupil.

Cover tests. There are three types of cover tests:

  1. Cover/uncover test: One eye is covered while your doctor looks for any movement in the eye that is uncovered.
  2. Prism and alternate cover test: A prism is placed in front of one eye while your doctor alternates the cover over your two eyes. This test measures the difference between the two eyes to determine what type of prism is appropriate for you.
  3. Maddox test: Each eye sees a vertical or horizontal line. Your doctor will use prisms to measure the difference between each of the images. This test is used to determine which type of double vision you have (vertical or horizontal).

What is my prism prescription?

Prisms are measured and prescribed with a unit of measurement called prism diopters, for example, 0.5PD or 1.5PD. The prism is added to your regular eyeglass prescription, in one or both of your lenses. The way the prism is situated depends on the type of double vision you have— either horizontal or vertical.

When the prism is added to your regular lens prescription, you will not be able to see it, though it may cause your lenses to appear thicker.

Fresnel prisms

Sometimes, fresnel prisms are prescribed as a temporary solution if your prism prescription is not stable, or you would like to try the prism before purchasing a new pair of lenses. This type of prism is a vinyl sticker that is applied to the front of your eyeglasses.

Are prism lenses a long term solution?

While prism lenses will help to relieve double vision while you are wearing your glasses, they cannot help your double vision when you remove your glasses. Therefore, prisms may only be a temporary solution.

Vision therapy

If you are searching for a long term solution for your double vision or binocular visual dysfunction, speak with your doctor about vision therapy.

Vision therapy retrains your eye-brain connections to improve your visual skills for clearer, single vision, with or without your eyeglasses.

If you suffer from double vision or binocular vision difficulties, prism lenses can be prescribed to improve your clarity of vision— enabling a more comfortable way of seeing.

Contact an eye doctor near you to find out if prism lenses could be the solution you have looking for.