How to Read Prism in Glasses: A Complete Guide for Eyeglass Wearers [2024]

Do you wear eyeglasses? Have you ever noticed small numbers like 1.50 or 2.25 printed on your lens prescription or eyewear?

Those numbers indicate the amount of prism in your glasses lenses, measured in diopters. But what exactly does prism mean, and how do you read and understand prism measurements?

If you’re puzzled by prism in your eyeglasses, you’re not alone! Many eyeglass wearers have no idea what those little numbers represent or how prism impacts vision correction.

In this comprehensive guide, we’ll demystify prism in glasses once and for all. You’ll learn:

  • What prism means and how it works in glasses lenses
  • How to Read Prism in Glasses and interpret prism measurements
  • The differences between vertical, horizontal, and diagonal prism
  • When prism may be prescribed for certain vision conditions
  • Tips for adapting to prism in new glasses

So whether you’re curious, confused, or downright frustrated by prism values in your prescription, read on for in depth explanations of this complex optical topic!

 What Is Prism in Eyeglasses?

Before jumping into prism measurements, let’s start with the basics – what is prism and what does it do?

Prism is a corrective lens option that shifts or bends light rays entering the eye.

In glasses lenses, prism uses the principle of refraction to redirect light and change the way images appear to the eye. This optical adjustment helps correct certain vision problems caused by eye misalignment or muscle imbalances.

Here’s a quick overview of how prism lenses work:

  • Prism bends light rays entering the eye away from the thicker part of the lens and towards the thinner edge.
  • Bending light changes where it lands on the retina at the back of the eye, which shifts the perceived position of objects.
  • The prism “moves” images to a different spot on the retina, helping improve alignment of the eyes.
  • Different amounts or directions of prism placement cause different effects on vision.

So in summary, prism lenses use precise light manipulation to alter visual perception and address unique vision issues that regular glasses can’t correct on their own. Pretty cool!

How Is Prism Measured in Glasses?

Now that you know what prism is and does, let’s move on to those numbers – how do you measure and specify prism value in eyeglasses?

Prism strength is indicated in units called diopters (D). Most prism prescriptions range from 0.5 to 10 diopters.

Here are some key facts about diopter measurements:

  • Diopters quantify the amount or degree of light bending by the prism.
  • Higher diopter numbers equal stronger prism effects.
  • Diopters indicate the prismatic power at the center of the lens.
  • Prism strength may vary at different points across a single lens.
  • The magnitude and direction of prism is specified by diopter value and axis.

So in short, diopters designate the bending power of prism lenses – the highest number, the more the light rays are shifted or displaced.

How to Read Prism in Glasses

Now let’s get into the nitty gritty – how do you actually read and interpret the prism numbers written in a glasses prescription?

Prism values are indicated using a standard notation format, for example:

  • 1.50
  • 3^ BI
  • 1.25^ X090

Here’s how to decipher what the prism specifications mean:

  • First number – Diopter strength of the prism
  • Letters – Direction of the prism
    • BI = Base In (prism base is on nasal side)
    • BO = Base Out (base on temporal side)
    • BU = Base Up (base on lower edge)
    • BD = Base Down (base on upper edge)
  • X – Marks a diagonal prism
  • Last number – Axis orientation in degrees of the diagonal

So in the examples above:

  • 1.50 means 1.50 diopters of prism strength
  • 3^ BI means 3 diopters of Base In prism
  • 1.25^ X090 is 1.25D prism set diagonally at axis 090 degrees

Reading prism specs takes some practice, but once you get the hang of the notation system, it’s not too tricky.

Types of Prism in Glasses

There are four main types of prism used in eyeglasses, categorized by direction. Each corrects different vision issues:

Horizontal Prism

  • Base In (BI) – thick edge is on the nasal side, shifts images temporally
  • Base Out (BO) – thick edge is temporal, shifts images nasally
  • Used for certain eye misalignment issues like exotropia and esotropia

Vertical Prism

  • Base Up (BU) – thick edge is on lower edge, shifts images upward
  • Base Down (BD) – thick edge is upper, shifts images downward
  • Used for vertical strabismus, dissociated vertical deviation (DVD)

Diagonal Prism

  • Prism set at oblique axis, thick edge runs diagonally
  • Provides precise image shift in needed direction
  • Used for complex strabismus where vertical + horizontal prism combo needed

Slab Off Prism

  • High amount of horizontal prism tapered gradually
  • Provides smooth, unnoticeable transition
  • Used for large prism corrections (over 4Δ) to avoid image jump

So in summary, prism direction matches the desired displacement of the visual image to counteract specific eye misalignments or movement disorders.

Person putting on eyeglasses

Why Is Prism Prescribed?

Now that you understand the diopter system and prism types, you may be wondering – why do some glasses prescriptions include prism?

Here are some common vision conditions that call for prism lenses:

  • Strabismus – misaligned eyes, like crossed eyes (esotropia) or wandering eye (exotropia)
  • Double vision – seeing overlapping images, causing confusion
  • Binocular vision disorders – poor depth perception or 3D vision
  • Visual field defects – missing areas in peripheral or central vision
  • Abnormal head positions or postures adopted to improve visual perception
  • Traumatic brain injuries – disruption in visual processing
  • Post-stroke visual impairment – damage to nerves or muscles
  • Visual effects of MS, TBI, Parkinson’s disease, and other neurological conditions

In essence, any condition that disrupts optimal eye coordination, alignment, or comfort may benefit from prismatic correction.

Prism realigns the visual axis to reduce eyestrain, improve image perception, restore single vision, and allow for normal head positioning.

Person wearing eyeglasses and looking through prism

 Fitting Prism Glasses: What to Expect

Getting used to new glasses with prism can take some adjustment. Here’s what to expect with the fitting process:

  • Your optometrist will prescribe the optimal type and amount of prism correction based on your condition.
  • Prism requires custom lens fabrication. Glasses take 1-2 weeks to prepare with prism.
  • Initial prism may feel disorienting – images seem shifted and depth perception is distorted. This usually resolves within 1-2 weeks as your eyes/brain adapt.
  • Follow-up visits allow the optometrist to fine-tune the prism amount for maximum comfort and vision clarity.
  • Prism corrections may need to be updated every 1-2 years if your condition changes. Consistent follow-up is key.
  • If prescribed over 4Δ prism, a special slab-off grinding technique is used to prevent image jump.
  • Large prism corrections can mean thicker, heavier glasses lenses, but high-index materials minimize lens bulk.

With careful prescription and fitting, prism glasses can significantly improve eye alignment, visual symptoms, and daily function. Be patient during the adjustment phase.

Person sitting on couch wearing eyeglasses

FAQs About Prism Glasses:

Still have questions about prism eyeglass lenses and measurements?

Here are answers to some frequently asked questions:

How is prism different from my regular vision prescription?

Prism lenses alter the path of light entering your eye to shift images. Regular glasses just refract light to correct near/farsightedness and astigmatism but don’t displace images.

Do prism lenses make glasses thicker?

Prism requires thicker areas in the lens to bend light. But high-index plastic or glass materials minimize bulk. Large prism corrections can mean heavier glasses.

Can prism improve extreme double vision?

Yes, prism redirects each eye’s line of sight to overlap images if eyes don’t aim at the same spot naturally. This restores unified vision.

Will I notice the effects of prism when looking through glasses?

At first, you may feel slight dizziness, visual shifts, or poor depth perception. This subsides as your visual system adapts within 1-2 weeks. Well-fit prism should then feel comfortable.

Can children benefit from prism corrections?

Yes, prism can effectively treat common pediatric conditions like strabismus or amblyopia (lazy eye). It helps establish normal vision development.

Do prism lenses ever need to be replaced?

As your vision changes over time, the amount or direction of prism needed may need adjustment. Follow up every 1-2 years to ensure your prescription stays optimized.


Reading prism in your glasses prescription doesn’t need to be a confusing endeavor. With this guide, you now have a complete understanding of what prism means, how it’s measured, types of prism, when it’s prescribed, and what to expect with prism lenses.

While adapting to new prism glasses takes some time, this treatment can significantly improve eye alignment, visual clarity, comfort, and daily function if you have specific vision disorders. With the proper prescription and fitting from your eye doctor, prism lenses can be life-changing.

Now that you’re an expert on all things prism, you can optimally partner with your optometrist to maximize the benefits of prism in your glasses. Seeing clearly and comfortably is a priceless gift – for both eyes to enjoy together.

Walter Hendricks

Walter Hendricks is a well-known authority in the eyewear industry, specializing in a diverse range of products such as gaming glasses, swimming goggles, sunglasses, eyeglasses, computer glasses, and fashionable daily-wear eyewear.

Hendricks believes in empowering his readers with in-depth information to help them choose the right glasses that blend functionality and fashion, catering to their unique lifestyle requirements.

His comprehensive reviews and informative articles provide clear insights on everything from cutting-edge gaming glasses to the latest trends in eyewear fashion. Through his work, Hendricks has proven his dedication to helping consumers make informed eyewear decisions that support both their visual needs and style preferences.