Prism and ‘Base In / Base Out’ on Your Glasses Prescription, Explained
Prism on Your Prescription at a Glance
A prism is a special part built into a lens for some people who see double. It bends light before it enters your eye so the two images your eyes send to your brain line up into one.1 On your prescription, prism shows up as a small number and a direction, such as base in or base out. This page explains what those words mean, how the number is measured, and when double vision needs a doctor rather than just new glasses.
Most glasses prescriptions have no prism at all. You get a prism line when your eyes do not aim at exactly the same point, which can cause double vision or eye strain. Prism is used to correct double vision that comes from the eyes being out of alignment.1 The prism does not change how sharp your vision is. It changes where the image lands in each eye so the two views match.
What Prism Correction Is and How It Works
A prism is shaped like a thin wedge, thicker on one edge than the other. A prism in your glasses bends light before it travels through the eye and redirects it to the right place on the retina in each eye.1 When both eyes land the image on matching spots, your brain fuses the two pictures into a single, steady image instead of two.
The strength of a prism is measured in prism diopters, written in steps such as 0.5, 1.0, and 1.5, and the number reflects how misaligned your eyes are.1 A larger number means a larger shift is needed.
One prism diopter shifts a beam of light 1 centimeter on a surface 1 meter away.2 You do not need to do this math yourself; your eye doctor measures the amount for you.
It helps to know what a prism cannot do. Prisms move the image to make it easier for the eyes to work as a team; they do not make the eye muscles stronger or move the eyes into place.3 Think of a prism as a tool that helps your eyes line up, not a treatment that fixes the underlying reason they drift.
The Exam That Measures Your Prism, and How to Prepare
Prism is measured as part of an eye exam that checks how your eyes work together, sometimes called a binocular vision assessment. This is often done when you report double vision, eye strain, or headaches with near work. Your eye doctor determines the prism amount and base direction using alignment tests such as the cover test and light-reflex tests.1
A little preparation makes the exam more accurate. Bring your current glasses and any older pairs, and be ready to describe when the double vision happens, whether it goes away when you cover one eye, and whether the images sit side by side or one above the other. Conditions such as head injury, stroke, and some nerve and muscle disorders can affect how the eyes align,1 so tell your eye doctor about any recent injury or new diagnosis.
How Your Eye Doctor Measures the Prism You Need
To find the amount and direction of prism, eye doctors use alignment tests such as the cover test, the Hirschberg light-reflex test, and the Krimsky test.1 In a cover test, the doctor covers and uncovers each eye in turn and watches for a small movement that shows the eyes are not pointing at the same spot. These tests do not hurt and take only a few minutes.
Once the doctor sees the misalignment, they measure how much prism cancels it out. The deviation is neutralized with loose prisms or a graded prism bar, and the amount is recorded in prism diopters.2 The doctor may also check the measurement for distance and for near, since the amount you need can differ between the two.
How to Read the Prism and Base Direction on Your Prescription
Prism is written as a number followed by a base direction, often marked with the Greek symbol delta, a small triangle.2 Reading a note like '10Δ BU OD' then just applies those parts together: ten prism diopters, base up, in the right eye. OD refers to the right eye and OS to the left.
Your prescription may have a separate field for prism, which not everyone has.4 If that field is blank, you do not need any prism.
The base is the thick edge of the wedge, and it points in the direction the light is shifted. Base out (BO) sits toward the outer edge of the lens, base in (BI) toward the inner edge, base up (BU) toward the top, and base down (BD) toward the bottom.1
Since the inner edge of the lens sits next to your nose and the outer edge sits toward your ear, base in points toward your nose and base out points toward your ear. The base up and base down directions handle a vertical difference between the two eyes.
Say your prescription reads '4Δ BO' for the right eye. That means a 4 prism diopter correction with the base toward your ear, which shifts the image to help it meet the other eye. Base directions are chosen to move each eye's image toward the point where the two can fuse. By bending the light before it reaches the eye, the prism merges two separate images into one and relieves double vision.2
What to Expect From Prism Glasses Over Time
Many people adjust to prism glasses within days as the brain gets used to the new image position. Sometimes a temporary Fresnel prism, a thin press-on plastic sticker fitted over the front or back of your glasses, is used first, either to test how the correction works or when the prescription is not yet stable.1
Press-on prisms are easy to change but can slightly blur vision and need regular cleaning.3 If your prescription settles, the prism can later be ground into the lens itself.
Higher amounts of prism can make one edge of the lens thicker and heavier, which is worth discussing with your optician when you choose frames. Prism relieves the double vision, but it does not fix the underlying reason the eyes are misaligned, so your doctor also looks for the cause.3 The amount you need can change over time, so keep your follow-up visits to have it rechecked.
When Double Vision Needs a Doctor, Not Just New Glasses
New or sudden double vision is different from a long-standing prism prescription and deserves prompt attention. Double vision can be caused by conditions such as stroke, head injury, brain tumor, migraine, multiple sclerosis, diabetes, and problems of the eye muscles or nerves.1 Contact an eye doctor or seek medical care the same day if you notice any of the following.
- Double vision that starts suddenly, especially with a severe headache.
- Double vision with weakness, numbness, a drooping eyelid, or trouble speaking.
- Double vision that begins after a head injury.
- Eye pain, or double vision that does not go away when you cover one eye.
If you have sudden weakness, face drooping, or trouble speaking, call 911, because these can be signs of a stroke.
For a stable, known prism prescription, routine care with your optometrist or ophthalmologist is usually enough; they measure the prism, write it, and recheck it over time. If your double vision is new, changing, or linked to another health condition, your eye doctor may refer you for further evaluation. Keep any scheduled follow-up so changes in your alignment can be caught and your prism adjusted.
Common Questions About Prism Prescriptions
The small triangle is the Greek letter delta, and it is shorthand for prism diopters. A note like '10Δ BU OD' then reads as ten prism diopters, base up, in the right eye. The number is the strength of the prism and the letters after it are the base direction. If your prescription has no triangle and no prism field filled in, you simply do not need prism, which is true for most people.
Both describe which way the thick edge of the prism points, which sets the direction the image shifts. Base in sits toward the inner edge of the lens and base out toward the outer edge.1 Because the inner edge sits next to your nose and the outer edge toward your ear, base in points toward your nose and base out points toward your ear.
Base up points toward the top of the lens and base down toward the bottom.1 Your doctor picks the direction that moves each eye's image toward the other so the two can fuse into one.
Not by itself. A prism written into a stable prescription is a routine correction. What matters is whether the double vision is new or changing. Double vision can come from many causes, including eye-muscle and nerve problems, stroke, head injury, and other neurological conditions.1 A long-standing prism you have worn comfortably is usually not an emergency, but new or sudden double vision should be checked promptly.
It depends on the cause. Prisms help the eyes line up but do not strengthen the muscles or move the eyes into position, so they manage the symptom rather than curing the cause.3 Some misalignments are stable and need the same prism for years; others change, and the amount is adjusted at follow-up visits. Your eye doctor can tell you what to expect for your situation.
Yes. Prism can be ground into ordinary single-vision or progressive lenses when your glasses are made. For a temporary or changing prescription, a thin press-on Fresnel prism can be applied to your existing glasses instead.1 Larger amounts of prism can add thickness to one edge of the lens, so it is worth talking with your optician about frame and lens choices.
More Questions, and What to Ask Your Doctor
Some people are prescribed prism for eye strain rather than obvious double vision. When the eyes have a tendency to drift, the effort of holding them aligned can cause tired eyes, headaches, or trouble with near work. Prism is used therapeutically for problems such as convergence insufficiency as well as to relieve double vision.2 A small amount of prism can ease that effort so reading feels more comfortable.
The amount is measured, not guessed. Your doctor finds the misalignment with alignment tests such as the cover test and light-reflex tests.1
The deviation is then neutralized with loose prisms or a prism bar and recorded in prism diopters.2 The measurement may be taken separately for distance and near, because the amount you need can differ between them.
A Fresnel prism is a thin plastic press-on sticker fitted over the front or back of your glasses, useful for testing how the prism works or when the prescription may still change.1
It is easy to swap out but can blur vision slightly and needs regular cleaning.3 If your prescription settles, the prism can then be ground permanently into your lenses.
Sometimes. A stable prism you already wear is routine, but new double vision can occasionally signal a serious problem. Causes of double vision include stroke, brain tumor, migraine, and head injury.1 Seek medical care the same day if double vision starts suddenly, follows a head injury, or comes with a severe headache, weakness, numbness, a drooping eyelid, or trouble speaking.
- What is causing my eyes to be misaligned?
- Is my prism prescription likely to stay the same or change over time?
- Should I start with a Fresnel press-on prism or have the prism ground into my lenses?
- Will this amount of prism make my lenses noticeably thicker?
- How often should I have my alignment and prism rechecked?
- Are there warning signs that mean I should be seen sooner?
- American Academy of Ophthalmology (EyeSmart) (2022). What Is Prism Correction in Eyeglasses?.
- StatPearls, NCBI Bookshelf (NIH) (2023). Prisms (StatPearls).
- American Association for Pediatric Ophthalmology and Strabismus (AAPOS) (2023). Prisms.
- American Academy of Ophthalmology (EyeSmart) (2022). How to Read an Eyeglasses Prescription.