Prism correction is used in eyeglasses for some people with 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 image is seen.
Why Do People See Double?
Normally, we see one image through two eyes because light falls on the same part of the retina in each eye. With double vision, two images are seen because the light falls on different parts of the retina.
How Does Prism Fix Double Vision?
A prism added to eyeglasses bends light before it travels through the eye. The light is redirected to the right place on the retina in each eye. The brain then fuses the two images together to produce one, clear picture.
Will Prism Fix My Double Vision?
A prism may fix double vision from eye misalignment when caused by:
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 your double vision.
See a doctor immediately if your double vision:
- is new
- is caused by eye misalignment
- or happens for unknown reasons
How Do I Get Prism Put in My Glasses?
Just like regular eyeglasses, you need an in-person exam to test your vision. This is the only way to get the right prescription.
During the exam, your ophthalmologist may do one or more of these tests to evaluate whether you need prism:
Hirschberg test. This basic test helps check for strabismus. A pen light is aimed at the eyes. The light's 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 measures the offset (difference) between the two eyes to find the right prism to fix your double vision. With the Maddox Rod 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 the distance between the two images and provide the prescription value.
What Does a Prescription for Prism Look Like?
A prescription for prism typically has two numbers:
- Prism. Like normal eyeglasses, diopters measure the amount of prism correction (from 0.5PD, 1.0PD, 1.5PD, and so on) and indicate how misaligned your eyes are.
- Base. Depending on your double vision, the prism is placed vertically, horizontally or diagonally in one or both lenses of your eyeglasses. The “base” is the thickest edge of the prism and tells you where the light is redirected. It may be on the outer edge of the lens (Base Out, BO), the inner edge (Base In, BI), or at the top (Base Up, BU) or bottom edge (Base Down, BD).
What Do Prism Eyeglasses Look Like?
Sometimes, a temporary Fresnel prism (a thin press-on vinyl sticker) is fitted over the front or back of your eyeglasses. With a Fresnel prism lens, the prism is slightly visible. This isn't ideal for long-term use, but it allows 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.
Eyeglasses with a temporary Fresnel prism on left lens.
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. If this is a concern to you, consider a thicker frame to hide that portion of the lens.
Eyeglasses with prism built into the lens.
Do Prism Eyeglasses Cause Side Effects?
It may take a few days to get used to wearing new prism eyeglasses or those with an updated prescription. You may have some eye strain during this time.
Rare symptoms include:
- double vision
- eye pain
If you have any of these symptoms, call your eye care professional who prescribed the glasses. You may need the frame refit or a new prescription.