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Why You Might Need Prism Glasses

What Are Prism Glasses?


Prism glasses are one form of treatment for some people who have diplopia or double vision. Find out how prism glasses work (and whether you may need them) using this informative guide.


How Do Prism Glasses Work?


With normal vision, light enters each eye through the cornea. It then falls on the retina, the light-sensitive tissue at the back of the eye. As light falls on the same part of the retina of each eye, the eyes are able to work together, causing the brain to see a single image.

With double vision, two images are seen. This is because the light falls in different places on the retina of each eye, and the brain is unable to account for the difference in images. 

When wearing prism glasses, light is bent before it travels through the eye and redirected so it falls correctly on the retina. The brain does the rest, fusing the two retina images together to produce one clear picture.


Who Might Benefit From Prism Correction?


Sufferers of these conditions often experience double vision:

  • Eye muscle problems, such as myasthenia gravis, Graves’ disease, or strabismus (crossed eyes or wall eyes).

  • Neurological issues, such as head injuries, stroke, migraine, or tumors.

  • Nerve-related problems, like multiple sclerosis or diabetes mellitus.

It should be noted that double vision has many causes, and a prism is not an all-encompassing solution. As it can be a sign of a serious health condition, it's important to find out what's causing double vision before going straight to prism glasses.

See a doctor immediately if your double vision is new, caused by eye misalignment, or happens for unknown reasons.


Your Prism Prescription and Eyeglasses


Like normal eyeglasses, a prism is measured and prescribed with a unit of measurement called prism diopters (presented as 0.5PD, 1.0PD or 1.5PD, for example). Depending on the nature of 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 soon change.


Can You Drive With Prism Glasses?


If you have prism glasses, you will have to inform your national driving authority, such as the DVA in the USA, or DVLA in the UK. They will need you to confirm that your double vision has been rectified - if it has, you will be able to drive, but only with your prism glasses on.


What Do Prism Glasses Look Like?


Eyeglasses with a prism are not hugely distinguishable from regular glasses, although the lens on one side may be thicker and more noticeable, with multiple lines crossing the lens.


How Long Do Prism Glasses Take To Work?


According to eye care professionals, generally it only takes 2 to 3 days for the eyes to get used to prism lenses. However, not everyone’s eyes are the same; in some cases, it could take up to 2 weeks for the eyes to properly adjust. Read more about how to adjust to new glasses here.

Read more about eye health from our article selection here, or consult our ‘Ask the Optician’ eye health FAQs here.

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