What Are Prism Lenses?

Medically Reviewed by Dany Paul Baby, MD on June 16, 2022
Written by Parang Mehta
4 min read

Diplopia (double vision) is a rare but disturbing condition. It happens when the images of objects before you are formed in different parts of the retinas in your two eyes. Many disorders of the eye and nervous system cause you to see two of everything. This condition can make your daily activities difficult. Prism lenses have the property of changing the direction of light rays. They can be used to make sure both your eyes form images in the same place. Prism lenses are valuable in both assessing double vision and reversing it.  

Diplopia, or double vision, is a condition in which you see two of everything. The two images could be above and below each other, side to side, tilted, or any combination of these. 

The eyeball has six muscles attached to it, which are controlled by three cranial nerves. If any of these muscles don't work as they should, your eyes don't move as a pair and become misaligned. This causes you to see double. Diplopia makes it difficult to judge distances, and reading and everyday tasks may become impossible.

Double vision has many causes:

Double vision can be a sign of a serious disease, so you should always consult your doctor when it happens. They'll try to find the cause and begin treatment. They'll also refer you to an ophthalmologist or optometrist for relief of the double vision. 

Double vision can sometimes be treated with eye exercises to improve the strength of the weak muscle. Your doctor may give you a prism to stick on your glasses as a temporary measure. If you see better with these temporary prisms, the prisms can be added to your prescription glasses. Sometimes, your ophthalmologist may give you a prism to further separate the two images so that one is ignored. If none of these methods work, you can cover one eye at a time to stop seeing double.

Prism lenses are made of plastic or glass and are wedge or triangle shaped. Light rays change direction on entering the prism lens and again when exiting. This property is used to ensure that the images of objects are formed in the same area of the retina in both eyes. 

Prism lenses, despite their name, have no focusing power. They don't correct refractive errors like farsightedness (hypermetropia) or nearsightedness (myopia). Their purpose is to change the location of the image in your eye and improve eye alignment.

Temporary prism lenses are made of vinyl and are attached to your regular lenses. These temporary prisms are called Fresnel prisms. They let you try out prism lenses before getting permanent prisms. They're also useful if your diplopia is likely to be temporary or is changing. The vision through these temporary stick-on prisms isn't completely clear.

Prism glasses work by changing the direction of light rays. Usually, your eyes move together in all directions. Their axes are aligned, and the images in both eyes are formed on the same parts of their retinas. 

When your eyes are misaligned, they don't move accurately together. The images are formed on different parts of your retinas and you see double. Prism glasses compensate for eye misalignment by redirecting the light rays to make the two images align.

Prism glasses are measured and prescribed in prism diopters. The axis must also be specified — horizontal, vertical, or oblique. If the power required is high, it can be split into two since alignment is a function of both eyes. Your ophthalmologist will have you try out prism glasses by attaching temporary prisms to your existing glasses. Once you find them comfortable, they can be ground into new glasses.

Prism glasses have two uses when dealing with diplopia. Your doctor will measure your eyes' misalignment by placing prism glasses of various strengths before your eyes till your double vision disappears. 

The Krimsky test uses prisms to measure the degree of misalignment of the eyes. Your ophthalmologist will ask you to look at a penlight placed in front of you. They'll put prisms of different strengths in front of your eye till the reflection in both eyes is in the center. That tells your doctor your required prism power.

The other use, of course, is to correct your double vision. Your doctor will prescribe the strength and alignment of prism glasses you need. You can have these ground into your prescription glasses. 

When first you use a prism, your doctor will give you temporary prisms (Fresnel prisms) to stick to your glasses. These are slightly visible on your glasses. 

Once it's clear that you need prisms and that the prisms are correcting your double vision, your ophthalmologist will prescribe prism lenses for long-term use. These prisms will be ground into your regular eyeglasses. Your eyeglasses will look just the same as before, but one of the glasses might be thicker.

Any new glasses can cause eye strain for the first few days. Other symptoms are quite rare but can include:

  • Headache
  • Nausea
  • Eye pain
  • Double vision

Prism glasses are most likely to cause such problems when the prescription is incorrect for your eyes. This can happen because your misalignment changed after your eyes were tested. You should see your ophthalmologist again. They may test your eyes again and prepare a fresh prescription. You shouldn't ever wear prism glasses unless your ophthalmologist has prescribed them.

Double vision can be disabling. Driving is prohibited in some countries if you have it. Treatment with prisms can restore your vision and improve your quality of life. Prism glasses are successful in 68% to 88% of people with diplopia and are a valuable non-surgical treatment option. Strabismus surgery is an option if prism glasses don't work.