What Are Prism Glasses?

Prism glasses are prescription eyeglasses with special lenses for correcting double vision (also known as diplopia). In some cases, prism glasses may also be prescribed to help with eye strain (depending on the cause).

Standard prescription lenses correct for visual clarity—vision errors like nearsightedness, farsightedness, or astigmatism (also called refractive errors). Prism lenses, on the other hand, correct for eye alignment or movement issues, not refractive errors. Sometimes when eyes aren’t aligning correctly, they can’t work together properly to create a single image. And that’s where prism glasses come in.

We’ve put together this handy guide to walk through everything you might want to know about prism glasses—from how they work and who they help to what prism correction looks like on a prescription (and everything in between). 

How Do Prism Glasses Work?

Prism glasses work by redirecting light before it enters the eye so that it hits the retina at the correct focal point. The prism is shaped like a three-dimensional triangle with a base that’s thicker than the top. The prescription determines the orientation of the prism in the glasses lens (base up, base down, base in, or base out). The prism’s orientation affects how the lens directs light onto the retina.

A prism correction may be used for one or both eyes. When prism correction is needed as a long-term solution, it can be incorporated into the lens(es) of a person’s glasses. 

Sometimes a prism correction is needed only temporarily. In these cases, a Fresnel prism can serve as an alternative to prism glasses. Fresnel prisms are like thin stickers that can be applied directly to the surface of the lenses of standard glasses.

Prism Glasses for Double Vision

Double vision happens when a person’s eyes aren’t working together the way they should. Instead of seeing a single image, they see two separate images because the light is hitting each eye’s retina in a different spot. Prism eyeglasses correct for this to create a single, clear image.

Who Needs Prism Glasses

Many eye conditions or health issues can cause double vision, and not all of them can be corrected with prism glasses. With that said, some conditions that may benefit from prism correction include:

• Strabismus (commonly known as crossed eyes)
• Graves’ disease
• Myasthenia gravis
• Binocular vision conditions, such as convergence insufficiency

Neurological conditions (brain- or nerve-related problems) also can cause double vision that may be treatable with prism lenses. These conditions include:

• Stroke
• Head injury
• Tumor
• Migraine
• Multiple sclerosis
• Diabetes mellitus

What Do Prism Glasses Look Like?

When a prism is part of your prescription and worked into your lenses, your glasses will look like any other prescription eyeglasses. The only difference is that prism lenses are sometimes noticeably thicker than standard lenses. A Fresnel prism, on the other hand—the temporary option applied directly to a lens—may be noticeable.

If it’s your first time getting prism glasses, your eye doctor can answer any questions you may have about your prescription and the thickness of your lenses. If lens thickness is a concern,  you might consider choosing thicker frames for your glasses. Of course, if lens thickness doesn’t bother you, there’s no need at all to limit your frame choices—you can wear any frames you want with a prism glasses prescription.

Interested in seeing some of our favorite frames that are on the thicker side? We’d be delighted to share.

Understanding Your Prism Glasses Prescription 

If your eye prescription includes a prism correction, you will notice numbers in a column labeled “prism.” These numbers indicate the prism diopters—the measurement of the prism’s power or strength. You might see “PD” by these numbers, which stands for “prism diopters.”

You may also see abbreviations listed in a column labeled “base.” These letters correspond to the orientation of the prism’s thickest edge—its base. Here’s what these abbreviations mean:

• BU –  Base up (the prism’s base is at the top edge of the lens)
• BD – Base down (the prism’s base is at the bottom edge of the lens)
• BI – Base in (the prism’s base is on the inner edge of the lens)
• BO – Base out (the prism’s base is on the outer edge of the lens)

Aligning on Next Steps

We hope this guide has made things a little clearer. (Vision pun? Check.)

If you have questions about your own prescription, don’t hesitate to make an appointment with your eye doctor. You may also find answers to lingering questions in our FAQs below.