A prism prescription for glasses includes two pieces of information beyond a standard eyeglass prescription: a prism power measured in prism diopters (written as Δ, “PD,” or simply “prism”) and a base direction that tells the lab which way to orient the thicker edge of the prism in each lens. Once you understand those two elements, the rest of the prescription reads much like any other glasses order. The details matter, though, because even small errors in prism power or base direction can make symptoms worse instead of better.
The Two Key Elements on a Prism Prescription
Every prism prescription boils down to “how much” and “which way.” The “how much” is the prism power, given in prism diopters. You might see it written as a number followed by the Greek delta symbol (Δ), or labeled “prism” on the prescription form with a number next to it. A prescription might read something like “2.00 Δ” or “prism: 2.00.” The number tells the lab how strongly the lens needs to bend light to redirect it toward the correct spot on your retina. Low values like 0.5 or 1.0 represent a mild correction; values above 10 are considered high and come with optical trade-offs.
The “which way” is the base direction. A prism is thicker on one edge and thinner on the other; the thick edge is called the base. The base direction tells the optician where to position that thick edge relative to your eye. You will see one of four abbreviations:
- BI (Base In): The thick edge sits closest to your nose.
- BO (Base Out): The thick edge sits closest to your temple.
- BU (Base Up): The thick edge sits at the top of the lens.
- BD (Base Down): The thick edge sits at the bottom of the lens.
Some prescriptions specify base direction in degrees instead of these abbreviations, using a 360-degree notation similar to how axis is recorded for astigmatism corrections. In that system, 0° or 180° corresponds to a horizontal direction, while 90° and 270° point vertically. Degree notation is more common when the prism correction is not purely horizontal or vertical but sits at an oblique angle. If your prescription lists a base direction as a degree value, the lab uses that angle to orient the prism precisely.
Where Prism Appears on the Prescription Form
A standard eyeglass prescription has columns for sphere (SPH), cylinder (CYL), axis, and add power. Prism, when present, usually appears in its own column to the right of these or in a separate row beneath the main correction. Some prescription pads have dedicated boxes labeled “Prism” and “Base” for each eye. Others fold both values into a single field, written as something like “1.50 BU” or “3.00 BI.”
The prescription will list prism values for each eye separately because the correction often differs between the right eye (OD) and the left eye (OS). It is common for only one eye to carry a prism correction, or for both eyes to share the total prism load. For instance, if you need 4 prism diopters of vertical correction, the prescriber might split it as 2 BU in the right eye and 2 BD in the left eye rather than putting the full 4 in one lens. Splitting the prism this way keeps each individual lens thinner, lighter, and more cosmetically appealing. It also distributes the optical distortion more evenly across both eyes.
Why Prism Gets Prescribed
Prism lenses redirect light so that images land on corresponding points of both retinas simultaneously, which is the brain’s requirement for fusing two eye-images into one clear picture. The most common reason you will see prism on a prescription is double vision, also called diplopia. When the eyes are slightly misaligned, each eye points at a marginally different spot, and the brain receives two offset images it cannot merge. Prism bends the light path entering one or both eyes just enough to bring those images back into alignment. Research confirms that prism correction can resolve double vision even in cases where the misalignment is large or varies depending on gaze direction.1PubMed. Prisms are effective in resolving diplopia from incomitant, large, and combined strabismus
Prism is also prescribed for a condition called decompensated heterophoria, where the eyes have a latent tendency to drift that the brain can usually compensate for but sometimes cannot keep up with. When compensation fails, you get eyestrain, headaches, and intermittent blurring. Studies on people with decompensated heterophoria show that their ability to adapt to induced misalignment is dramatically weaker than normal, which helps explain why their symptoms persist without optical correction.2PLOS ONE. Deficient vergence prism adaptation in subjects with decompensated heterophoria
Less commonly, prism can be prescribed for convergence insufficiency, where the eyes struggle to turn inward together for close-up tasks like reading. However, the evidence for prism in this situation is mixed. A randomized trial in children with convergence insufficiency found that base-in prism reading glasses performed no better than placebo glasses at relieving symptoms or improving the near point of convergence.3PubMed. Randomised clinical trial of the effectiveness of base-in prism reading glasses versus placebo reading glasses for symptomatic convergence insufficiency in children Vision therapy exercises are generally considered a more effective first-line treatment for convergence insufficiency.
Ground-In Prism Versus Fresnel (Stick-On) Prism
When your prescription is filled, the prism can be incorporated into the lens in two fundamentally different ways. The first is a ground-in (or conventional) prism, where the lab physically grinds the lens so that one edge is thicker than the other. The prism is permanently part of the lens and looks almost like a normal pair of glasses, though higher prism powers make one edge noticeably thicker. Ground-in prisms deliver the clearest vision and are the standard choice for long-term wear.
The second option is a Fresnel prism, a thin, flexible plastic membrane with tiny parallel ridges molded into it. This membrane sticks onto the back surface of an existing lens. Fresnel prisms are useful for temporary corrections, for trial periods before committing to a ground-in prescription, or for very high prism powers that would make a ground-in lens impractically thick. They are also easy to change, which matters if your alignment is still shifting.
The trade-off is visual quality. Fresnel prisms reduce sharpness more than ground-in prisms at the same power, and the gap widens as prism power increases. Conventional CR39 plastic prisms begin to measurably reduce visual acuity at around 10 prism diopters, while Fresnel prisms start causing noticeable blur at just 5 prism diopters.4PubMed. The effect of conventional CR39 and Fresnel prisms on high and low contrast acuity At higher powers, Fresnel membranes can disrupt the ability to fuse images from both eyes and can even eliminate depth perception, particularly above about 12 to 15 prism diopters.5PubMed Central. Fresnel prisms and their effects on visual acuity and binocularity Because of these limitations, when a Fresnel prism is prescribed at high power, it is often placed on only one eye to preserve clearer vision through the other.
How Base Direction Connects to Your Specific Problem
The base direction on your prescription is not arbitrary. It directly corresponds to the direction your eye drifts or the direction the image needs to be shifted. If your eye turns slightly inward (toward your nose), the prism base will point outward (BO) to redirect the light path and meet your misaligned eye. If your eye drifts outward, the base points inward (BI). The same logic applies vertically: an eye that sits slightly higher than the other gets a base-down prism, or the lower eye gets a base-up prism, or both.
This is why getting the base direction wrong is so consequential. A prism with the correct power but the wrong base direction will double the apparent misalignment instead of correcting it. If you pick up new prism glasses and your double vision is immediately worse, the most likely culprit is an error in base orientation during fabrication. It is worth checking the direction on your written prescription against what the optician’s work order says before the lenses are made.
Compound corrections are also possible. If your eye drifts both horizontally and vertically, the prescription might list both a horizontal and a vertical prism component for the same eye. Some prescribers write these as two separate values (for example, “2.00 BI and 1.50 BU”), while others combine them into a single resultant prism expressed with a base direction in degrees. Both notations describe the same correction; they are just different ways of specifying the same diagonal prism angle.
Prism Measurement Is Trickier Than It Looks
One counterintuitive aspect of prism lenses is that the actual light-bending effect depends on how the prism is held or positioned relative to the eye. A 40 prism diopter glass prism, for example, delivers only about 32 diopters of actual deviation when positioned with its back face in the frontal plane instead of in the calibrated position.6PubMed Central. Ophthalmic prisms. Measurement errors and how to minimize them. This discrepancy exists because glass prisms and plastic prisms are calibrated differently: glass prisms are calibrated for the Prentice position, while plastic prisms are calibrated for the frontal plane position. Most patients never need to worry about this because the lab handles calibration, but it does explain why your eye doctor might measure you with a trial prism set and arrive at a slightly different number than the prescription eventually calls for. The discrepancy grows with higher prism powers.
Another measurement quirk involves stacking prisms. When a clinician adds a small prism on top of a large one during an exam, the combined effect is not a simple sum of the two values. Adding a 5 diopter prism to a 40 diopter prism produces roughly 59 diopters of effect rather than the expected 45.6PubMed Central. Ophthalmic prisms. Measurement errors and how to minimize them. These nonlinear effects at higher powers are well known among practitioners, but they help explain why precise prism prescriptions require careful technique and why a do-it-yourself approach to verifying prism power is unreliable.
Prism for Neurological Conditions
Prism lenses are not only for people whose eye muscles are misaligned. They are also used in people who have lost part of their visual field, often after a stroke. When a stroke damages the brain’s visual processing areas, a person may lose sight on one entire side. Peripheral prism glasses can expand the remaining functional field by shifting images from the blind side into the sighted area. In simulated walking tests, peripheral prisms dramatically improved detection of obstacles on the blind side, boosting detection rates from about 26% to 92% in patients who also had left-sided spatial neglect, and from about 43% to 98% in those without neglect.7PubMed Central. Peripheral Prisms Improve Obstacle Detection during Simulated Walking for Patients with Left Hemispatial Neglect and Hemianopia
These results are encouraging, but the broader evidence for prism use after stroke remains limited. A Cochrane review found insufficient evidence to draw firm conclusions about whether prisms produce generalizable benefits for visual field defects in stroke patients.8Cochrane Database of Systematic Reviews. Interventions for visual field defects in patients with stroke Neurological prism prescriptions look different from the standard strabismus correction on your prescription form. They may specify the prism location within the lens (such as “temporal peripheral prism sector only”) or reference a specific device name rather than a simple base direction.
When Prism Shows Up in Contact Lenses
Most people think of prism as something that only appears in spectacle lenses, but prism actually plays a hidden role in many toric (astigmatism-correcting) soft contact lenses. These lenses need to stay rotationally stable on the eye so the astigmatism correction lines up correctly, and several stabilization designs achieve this by building a small amount of prism into the lens. The vertical prism present in the central zone of commercially available toric lenses ranges from virtually zero to about 1.15 prism diopters depending on the brand and design.9PubMed. Resultant vertical prism in toric soft contact lenses
For most wearers this built-in prism is too small to notice. But if you wear a toric contact lens in only one eye, or if you wear two different toric brands with different prism amounts, the vertical prism imbalance between your eyes could cause symptoms like eyestrain or subtle double vision. This is one reason your contact lens fitter may be particular about which brand goes on which eye, or may prefer a lens design with minimal built-in prism when vertical alignment is already borderline.
Practical Tips for Filling a Prism Prescription
If your prescription includes prism, you will notice a few practical differences compared to ordering standard glasses. First, expect the lenses to be thicker and heavier, especially at higher prism powers. The prism correction adds a wedge shape to the lens, so one edge will always be thicker than the other. Choosing a smaller frame reduces the overall lens diameter and limits how thick the heavy edge gets. High-index lens materials also help, since they achieve the same light-bending in a thinner profile.
Second, prism glasses usually cannot be ordered through most budget online retailers. The fabrication requires precise placement and orientation, and many low-cost labs do not handle prism orders. This is one of those areas where an experienced local optician who can measure the fitting height, verify the base direction, and check the finished product with a lensometer is worth the extra cost.
Third, adaptation takes time. Even a perfectly made prism lens shifts your visual world slightly. Your brain needs a few days to a couple of weeks to recalibrate. During this adjustment period, floors may seem tilted, doorways may look skewed, and you might feel mildly disoriented. These sensations are normal and typically fade. If they persist beyond two to three weeks, or if your double vision is not improved, go back to your prescriber rather than toughing it out.
Finally, check the prescription before the lenses are made. Confirm that the prism power and base direction match for each eye. A transposition error, such as swapping BU for BD, is rare but not unheard of, and it produces the exact opposite of the intended correction. Knowing what BI, BO, BU, and BD mean puts you in a position to catch obvious errors before they become expensive remakes.
How Prism Prescriptions Change Over Time
Unlike a standard glasses prescription that might stay stable for years, prism prescriptions can shift as underlying conditions evolve. A patient recovering from a cranial nerve palsy after an illness, for instance, may need progressively less prism over months as the nerve heals. Someone with a progressive neurological condition might need increasing prism. Thyroid eye disease can cause alignment shifts that fluctuate during the active inflammatory phase and then stabilize. This is one reason doctors sometimes start with a Fresnel prism: it can be swapped out cheaply as the numbers change, and a ground-in lens is ordered only after the prescription has been stable for a few months.
Your brain can also adapt to prism over time, partly neutralizing its effect. This adaptation is normal and varies between individuals. Some people find that their initial prism correction stops fully relieving their symptoms after several months, and a small adjustment is needed. Others remain stable indefinitely. Regular follow-up, especially in the first year of wearing prism, helps catch these shifts before symptoms creep back.