Written By: Daniel Porter and published with AAO.org
Prism correction is used in eyeglasses for some people who have diplopia, or double vision. This is when someone sees two separate images of one object. The prism helps align the two images, so that only one is image is seen.
How does prism correction work?
To see clearly, light enters each eye passing through the cornea to fall on the retina, the light sensitive tissue lining the back of the eye. With normal binocular vision, the eyes work together, and the brain sees a single image. This is because light falls on the same part of the retina of each eye.
With double vision, two images are seen because the light falls in different places on the retina of each eye.
A prism used in eyeglasses bends light before it travels through the eye. The light is redirected so it will fall correctly on the retina in each eye. The brain then does its usual work of fusing the two retina images together to produce one, clear picture.
Who might benefit from prism correction?
A prism can be used for double vision from eye misalignment when caused by:
- Eye muscle problems, such as myasthenia gravis, Graves’ disease, or strabismus (crossed eyes or wall eyes)
- Neurological (brain-related) issues, such as head injuries, stroke, migraine, or tumor.
- Nerve-related problems, like multiple sclerosis or diabetes mellitus
Double vision has many causes, and a prism isn’t helpful for all types of double vision. Because it can be a sign of a serious health condition, it’s important to find out what’s causing double vision.
See a doctor immediately if your double vision:
- is new,
- is caused by eye misalignment,
- or happens for unknown reasons.
How is a prism prescribed?
You may need to do one or more of these tests to get a prism for your glasses:
Hirschberg test. This basic test helps check for strabismus. A pen light is aimed at the eyes. When the patient looks at the light, the corneal reflection should appear in the center of the pupil in each eye. If the reflection is off-center in either or both eyes, there may be strabismus.
Off-center corneal reflection of light in left eye, showing possible strabismus
Krimsky test. This test helps measure the degree of misalignment. This is like the Hirschberg test, but with a prism. While looking at the pen light, prisms of different strengths are placed in front of the eye. When a prism brings the reflection in each eye to the center, the correct prism measurement has been found.
Cover tests. There are three types of cover tests. With the cover/uncover test, one eye is covered and one is left uncovered. The doctor looks for any movement in the eye that is uncovered. The prism and alternate cover test alternates the cover over both eyes while a prism is placed in front of one eye. This helps measure the offset (difference) between the two eyes and determine what prism is needed to fix your double vision. With the Maddox test, each eye sees a vertical or horizontal line. The offset of these images allows the doctor to determine what type of double vision you have. Prisms are also used to measure distance between the two images.
Your prism prescription and eyeglasses
Like normal eyeglasses, a prism is measured and prescribed with a unit of measurement called prism diopters (from 0.5PD, 1.0PD, 1.5PD, and so on). Depending on your double vision, the prism is placed vertically, horizontally or diagonally in one or both lenses of your eyeglasses.
Sometimes, a temporary Fresnel prism (a thin press-on vinyl sticker) is fitted over the front of your eyeglasses. With a Fresnel prism lens, the prism is slightly visible. This isn’t ideal for long-term use, but it does allow you to test drive a prism and see how it works. Fresnel prisms are also used when the prism prescription isn’t stable and may change soon.
If prism correction is needed for a longer period, it can be ground into (made part of) the lens of your eyeglasses. Eyeglasses with a prism look like any other glasses without a prism, although the lens on one side may be thicker and more noticeable.