A prism prescription includes two pieces of information: a number measured in prism diopters (often written as PD or with the symbol Δ) and a base direction that tells you which way the thickest edge of the prism points. If your eyeglass prescription has a prism column, you’ll see something like “2 BO” or “1.5 BU,” and understanding what those numbers and abbreviations mean is straightforward once you know what to look for.
Where Prism Appears on Your Prescription
A standard eyeglass prescription lists values for each eye in columns: Sphere (for nearsightedness or farsightedness), Cylinder and Axis (for astigmatism), and then, if needed, Prism and Base. Not everyone has a prism value. It only appears when your eyes don’t align well enough on their own and need help pointing at the same spot.
Your right eye is labeled OD and your left eye OS. Each eye can have its own prism value, its own base direction, or both. In many cases, the total prism correction is split evenly between both eyes to keep the lenses thinner and more comfortable to wear. So if you need 4 prism diopters of correction total, you might see 2 listed for each eye.
The Two Parts: Power and Base Direction
The prism number tells you how much the lens redirects light, measured in prism diopters. A low value like 0.5 or 1 represents a small shift. Values up to about 6 prism diopters per eye can typically be ground into a standard lens. Higher amounts may require a stick-on prism (called a Fresnel prism) attached to the lens surface instead.
The base direction tells you where the thickest edge of the prism sits in your lens. There are four possible abbreviations:
- BI (Base In): The thick edge faces your nose.
- BO (Base Out): The thick edge faces your ear.
- BU (Base Up): The thick edge is at the top of the lens.
- BD (Base Down): The thick edge is at the bottom of the lens.
Some prescriptions write these out fully (“Base In”), while others use just the two-letter abbreviation. The direction matters because it determines which way light bends before reaching your eye.
What Each Base Direction Corrects
The base direction on your prescription reflects the specific way your eyes are misaligned. Prism lenses work by bending light so the image lands on the correct spot at the back of your eye, even when your eye itself is turned slightly off-target.
Base Out prism is prescribed when one or both eyes turn inward (toward the nose). This is common in a type of misalignment called esotropia. The prism shifts the image inward to meet the eye where it’s actually pointing. Base In prism does the opposite: it helps when the eyes drift outward or when you struggle to converge your eyes for close-up work like reading. It reduces the effort your eye muscles need to make when focusing on something nearby.
Base Up and Base Down correct vertical misalignment, where one eye sits higher than the other. If your left eye prescription says “BD” (base down), it means that eye is drifting upward and the prism is pulling the image down to compensate. Vertical prism is almost always written for just one eye, since vertical misalignment is a difference between the two eyes rather than a symmetrical problem.
Reading a Sample Prescription
Here’s what a prescription with prism might look like:
OD (right eye): -2.00 sphere, -0.75 cylinder, 180 axis, 2 BO
OS (left eye): -1.75 sphere, -0.50 cylinder, 175 axis, 2 BO, 1.5 BU
In this example, both eyes have 2 prism diopters of Base Out correction, likely splitting a total of 4 prism diopters needed for an outward eye turn. The left eye also has 1.5 prism diopters of Base Up, correcting a vertical misalignment where that eye tends to sit lower than the right.
Notice that a single eye can have both a horizontal and a vertical prism at the same time. When this happens, the lab combines both directions into one lens. You’ll see two entries for that eye’s prism: one horizontal (BI or BO) and one vertical (BU or BD).
Why Prism Is Prescribed
Prism correction addresses double vision caused by eye misalignment. The underlying causes vary widely. Some people have a nerve palsy affecting the muscles that move the eye. Others develop misalignment from thyroid eye disease, which causes swelling behind the eye that physically restricts its movement. Double vision can also follow eye surgeries, including cataract, glaucoma, and retinal procedures, or result from a long-standing subtle misalignment that worsens over time.
Fresnel prisms, the stick-on type, are often used first when double vision appears suddenly. They’re easy to adjust or remove as the condition changes. If the misalignment stabilizes, permanent prism ground directly into the lens gives clearer vision and looks like a normal pair of glasses.
What Higher Numbers Mean for Your Lenses
Because prism is created by making one edge of the lens thicker than the other, higher prism values produce thicker, heavier lenses. A prescription of 1 or 2 prism diopters adds barely noticeable thickness. Once you get above 4 or 5 per eye, the weight difference becomes more apparent, and choosing a high-index lens material or a smaller frame can help keep the glasses manageable.
Splitting the correction between both eyes is the standard approach for this reason. Putting all the prism in one lens would make that side noticeably thicker and heavier, while the other side stays thin. Dividing it evenly keeps the glasses balanced on your face and reduces distortion at the lens edges.