What Is Prism on a Glasses Prescription?

Most glasses prescriptions contain familiar measurements such as SPH and, where required, CYL and AXIS.

Some prescriptions, however, also contain prism.

Prism is less commonly encountered, so seeing it written on your prescription can understandably raise a few questions.

What is prism in glasses?

Prism can be prescribed when the eyes need help working together comfortably.

Unlike ordinary lens power, which primarily corrects refractive errors such as short-sightedness or long-sightedness, prism changes the apparent position of an image.

This can help the eyes coordinate more effectively in certain circumstances.

Why might somebody need prism?

Normally, both eyes work together so that the brain can combine the images received from each eye into a single image.

If the eyes aren't aligning or coordinating as required, somebody may experience symptoms such as double vision.

Prism may sometimes be prescribed to help compensate for an eye alignment problem.

Does prism correct eyesight?

Prism performs a different job from the SPH and CYL portions of your prescription.

Your prescription may therefore contain ordinary lens powers to correct your vision as well as prism to address an alignment requirement.

How is prism written on a prescription?

Prism is usually measured in prism dioptres.

Alongside the amount of prism, you'll normally find a base direction.

You may see terms such as:

Base In

Base Out

Base Up

Base Down

The amount and direction both matter.

What does the base direction mean?

The base describes the orientation of the prism within the lens.

For example, Base In and Base Out describe horizontal directions, while Base Up and Base Down describe vertical directions.

You don't need to calculate this yourself. The important thing when ordering glasses is to provide the information exactly as it has been prescribed.

Is prism the same in both eyes?

Not necessarily.

Prism can be prescribed differently between the two eyes depending on the individual's requirements.

Always treat the right and left eye measurements separately and follow the prescription exactly as written.

Can you leave prism out of your glasses?

If prism has been prescribed, don't simply remove it from an order because the other prescription values appear correct.

It forms part of the spectacle prescription that has been determined for you.

If you're unsure whether prism is required in the glasses you're ordering, seek advice before placing the order.

Can every pair of glasses be made with prism?

The answer depends on factors including the amount of prism, the prescription, lens options and the capabilities of the company making the glasses.

If your prescription contains prism, check that the retailer can accommodate it before completing your purchase.

Check carefully when ordering online

Prism prescriptions contain additional information compared with many standard prescriptions.

When entering one online, check:

The correct prism amount has been entered.

The correct base direction has been selected.

The measurements haven't been swapped between the right and left eyes.

A quick check before submitting an order can prevent an important prescription detail from being entered incorrectly.

A specialised but important part of some prescriptions

Not everybody will ever see prism written on their spectacle prescription.

For those who do require it, however, it serves a specific purpose in helping the eyes work together.

Understanding what prism and its base direction represent makes another seemingly complicated part of your glasses prescription much easier to recognise.