What Are Prism Glasses?

double vision of hand holding wrist

You just had an eye exam and the doctor has prescribed you a pair of glasses with a prism. What does that even mean?

Hopefully, your doctor has explained why you need this correction in your new lenses, but if not, I will break it down as simply as possible. Prism glasses are prescribed to correct double vision, otherwise known as diplopia.

This article will help you understand the basics of why you need prism lens glasses and how they work to correct this refractive error.

What is Prism?

Prism is used in eyeglasses for some people with double vision. Prism glasses are mainly used for individuals who experience the types of double vision caused by eye movement problems such as strabismus.

Prism lenses help align the two images so that only one image is visible. When your vision functions normally, two separate images from each of your eyes are combined into one single image in the brain.

illustrations of strabismus condition

This is known as binocular vision.

Other possible causes of double vision are:

Traumatic Brain Injury (TBI) – an injury that affects how the brain works i.e., head injuries, car accident, or a fall

Neurological (Brain-Related) Issues – such as stroke, aneurysm, increased brain pressure due to injury or tumors

Medical Issues like diabetes can cause nerve-related problems that can lead to double vision. Weak eye muscles caused by diseases like Graves’ Disease can also contribute to double vision

Another reason you might need a prism prescription is if you are diagnosed with Binocular Vision Dysfunction or BVD.

BVD is a condition where the eyes are slightly misaligned and struggle to send one clear image to the brain. This condition can cause a variety of symptoms such as dizziness, motion sickness, headaches, facial pain and nausea.

According to Optometrist.org, BVD is very often misdiagnosed as dyslexia or ADD/ADHD in children. However, small amounts of prism correction can alleviate many of the symptoms of BVD.

How Does a Prism Work?

Prisms are, by definition triangles, where the base is wider than the apex or pointy top.

Prism glasses work to refract the light before it enters the eye. When light travels through a prism, it is reflected away from the base towards the apex.

Prism in glasses is written using two things, the prism power and the base direction. Some eye doctors also use a triangle symbol to indicate a prism when writing a prescription.

light shining through a prism
Light reflecting through a prism


The unit of measurement used to measure prism power is called a diopter. The amount of visual misalignment, or how much the image needs to be moved, will determine the degree of prism built into your prescription.

The higher the degree of misalignment, the higher the prism diopter.


The “base” is the thickest edge of the prism and tells you which direction the light will be redirected.

Depending on your binocular issue, the prism is placed vertically, horizontally, or diagonally in one or both lenses of your eyeglasses. 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).

It’s important to note that your lens will be thicker wherever the base is placed. For example, if you have Base Down (BD) prism, your lens will be slightly thicker at the bottom.

Keep lens thickness in mind when selecting your eyeglass frame.

The base’s position determines which way the image will be shifted to allow you to see clearly.

Remember, the image will shift towards the apex of the prism; therefore, Base Up (BU) prism will shift the image down toward your chin, Base In (BI) prism will shift the image over towards your ear, and so on.

How is a Prescription for Prism Written?

Prism usually has its own column on an eyeglass prescription. The amount of prism, in diopters, is written in number form, and the base is noted, indicating the direction it should be positioned for you to see clearly.

Here are some examples:

rx sample with prism

This is a standard prescription to correct nearsightedness or myopia. You can also see that there is one diopter of Base Up prism prescribed for the right eye and half a diopter of Base Down prism on the left eye.

prism rx in one eye

This example shows a prism indicated by a triangle symbol. This prescription is for a farsighted person who also requires a reading correction. (Hyperopia/Presbyopia).

In addition, two diopters of Base Down prism are prescribed for the right eye only. This prescription lets us know double vision is only being experienced in the right eye.

When you have been prescribed eyeglasses with a prismatic correction, it is advised that you wear them whenever you would wear your normal glasses. Your vision might require an adjustment period before you are comfortable with your new prism glasses.

Whether you have a slight eye misalignment or need a more substantial correction, prism lenses are a simple solution to get you seeing clearly again.