“Spectacle Rx” is shorthand for “spectacle prescription,” where “Rx” is the universal medical symbol for a prescription. It refers to the specific set of numbers your eye doctor writes after an eye exam to correct your vision with eyeglasses. You’ll see this term on forms at optical shops, insurance paperwork, and medical records to distinguish your glasses prescription from a contact lens prescription, which uses different measurements.
If you’ve just received one of these and are staring at a grid of abbreviations and decimal numbers, here’s what every part means.
OD, OS, and OU: Which Eye Is Which
Every spectacle prescription is split into at least two rows, one for each eye. These are labeled with Latin abbreviations: OD (oculus dexter) is your right eye, and OS (oculus sinister) is your left eye. If you see OU, that refers to both eyes together. Some newer prescriptions simply use “Right” and “Left,” but the Latin abbreviations are still common.
Sphere, Cylinder, and Axis
The three core columns on a spectacle Rx are Sphere (SPH), Cylinder (CYL), and Axis. Together, they describe exactly how light needs to be bent to give you clear vision.
Sphere is the main correction power, measured in diopters. A negative number (like -2.50) means you’re nearsighted and need help seeing things far away. A positive number (like +1.75) means you’re farsighted and need help with close or intermediate distances. The further the number is from zero, the stronger the correction.
Cylinder corrects astigmatism, which happens when the front of your eye is curved more like a football than a basketball. Not everyone has a cylinder value. If yours is blank or says “SPH,” you don’t have significant astigmatism in that eye. Like sphere, cylinder is measured in diopters and can be positive or negative depending on which notation system your doctor uses.
Axis is a number between 1 and 180 that only appears when there’s a cylinder value. It describes the angle of your astigmatism, telling the lab exactly how to orient the corrective lens. An axis of 90, for example, means the correction needs to run vertically. This number doesn’t indicate severity; it’s purely directional.
ADD Power for Reading Vision
If you’re over 40, your prescription may include a value labeled “ADD” (short for addition). This is the extra magnifying power added to the bottom portion of your lenses so you can read up close. It compensates for presbyopia, the gradual loss of near-focusing ability that happens to virtually everyone with age.
ADD power is always a positive number, typically ranging from +0.75 to +3.00. The number tends to increase as you get older. In most cases, the ADD value is the same for both eyes, though your doctor may occasionally prescribe slightly different ADD powers for each eye to fine-tune the balance between near clarity and distance comfort. This value determines whether you get bifocals, progressives, or dedicated reading glasses.
DV and NV: Distance vs. Near
Some prescriptions are split into two sections labeled DV (distance vision) and NV (near vision) instead of using a single ADD column. The DV section contains your correction for seeing things far away, like driving or watching TV. The NV section shows the combined power you need for reading and close work. If your prescription is formatted this way, the difference between the NV and DV sphere values is essentially your ADD power.
Prism and Base
Most people won’t see this on their prescription, but if your eyes don’t align properly, your doctor may add prism correction. Prism lenses redirect light so that the two images your eyes produce merge into one, solving double vision.
A prism value has two parts. The first is the amount, measured in prism diopters (0.5PD, 1.0PD, 1.5PD, and so on), which indicates how misaligned your eyes are. The second is the base direction, which tells the lab where to place the thickest edge of the prism: Base In (BI), Base Out (BO), Base Up (BU), or Base Down (BD). If you get prism lenses for the first time, expect a few days of adjustment and possible mild eye strain while your brain adapts.
Pupillary Distance
Pupillary distance (PD) measures the space between the centers of your pupils, in millimeters. It’s not always printed on the prescription itself, but it’s essential when ordering glasses because it tells the lab where to position the optical center of each lens. If your PD is off, the lenses won’t line up with your eyes properly, which can cause eye strain, fatigue, and blurry vision. The effect is more noticeable with stronger prescriptions.
PD is sometimes written as a single number (like 63mm) for both eyes combined, or as two numbers (like 31/32) representing each eye’s distance from the bridge of your nose. You can ask your eye doctor’s office for this measurement if it’s not included on your printed prescription.
Why It Differs From a Contact Lens Rx
A spectacle Rx and a contact lens prescription are not interchangeable. Glasses sit about 12 millimeters in front of your eyes, while contacts rest directly on the eye’s surface. That difference in distance changes how much correction you need, especially at higher powers. Contact lens prescriptions also include measurements specific to the lens fitting, like the curve and diameter of the contact. You need a separate fitting and prescription for contacts even if you already have a current spectacle Rx.
How Long a Spectacle Rx Is Valid
Spectacle prescriptions expire, though the exact timeframe depends on your state’s laws. Most states set the expiration at one to two years from the date of your eye exam. Federal law requires your eye doctor to give you a copy of your prescription after the exam so you can fill it wherever you choose, but it must be filled before it expires. The expiration exists because your vision can change over time, and an outdated prescription may no longer correct your sight accurately.
Your prescription will include the exam date and expiration date, along with your doctor’s name and signature. If you’re ordering glasses online or at a new retailer, they’ll typically verify these details before processing your lenses.