If you’re not an eye care professional, reading a contact lens prescription can be enough to make your eyes glaze over. But it’s really just a simple list of instructions for making contact lenses that are right for you. Once you understand what it all means, your prescription will be crystal clear. Read on for an explanation and background on those terms.
Most contact lens prescriptions look like a chart. There are columns for each eye. The letters below, in upper case, are examples of what you’ll see on a prescription.
A Quick Lesson in Latin
OD, OS and OU: you’ll see these letters at the top of your prescription. They are abbreviations for Latin terms. OD stands for oculus dexter, which means right eye. OS stands for oculus sinister, which means left eye. OU stands for oculus uterque which means both eyes.1
How Much Power Do You Need to Reach 20/20?
PWR: this refers to the power of your lens. That’s the level of correction your lens has to provide to your eye, so it can give you 20/20 vision. If you’re farsighted in that eye, the number is positive. If you’re nearsighted in that eye the number is negative. The strength of the lens corresponds to the number, the farther from zero, the stronger the lens.2
Contacts that Fit Well Are All About the Base (Curve)
BC: stands for base curve. That’s the inside curve of your contact lens; the part of the lens that goes on top of your eye. The base curve of the contact lens should match or come close to the curve of your cornea, the dome-shaped outer layer at the front of your eye.3 This will ensure a snug fit.
Just the Right Length
DIA: means diameter — the length of your contact lens. Imagine drawing a line from one edge of your contact lens to the other, going through the center of the lens. This number determines where the edge of the lens will rest on your eye. The diameter of your lens has to be spot on, otherwise irritation or abrasion can happen.4
Fixing Eyes with Astigmatism
The next two terms on your prescription apply to people with astigmatism. That’s a common condition where your cornea does not properly focus the light that passes through it. This can distort your vision. A cornea with astigmatism is shaped like a football. A normal cornea is shaped like a basketball.5
CYL: stands for cylinder. This number measures the severity of your astigmatism.6
AXIS: pinpoints where on your cornea your astigmatism is, so your contact lens can compensate for it.7
Bifocal Wearers Need More Power
ADD: means added power. This adjustment is for people who need bifocal or multifocal lenses. As people age into their 40s, they may lose their ability to see small print clearly. It’s believed this is caused by the natural lens in your eye stiffening.8 A bifocal or multifocal contact lens can have two or more prescriptions in the same lens.9
Be Careful About Costume Contact Lenses
COLOR: refers to whether your lens will make your eyes a different color. That field may also include a special effect such as “vampire eyes.” Costume contact lenses that make your eyes look like the eyes of a cat, vampire or zombie might be a hit at your next Halloween party, but they don’t correct your vision. Still, you need an eye exam and a prescription to get these costume lenses.10
The Food and Drug Administration (FDA) says it’s against the law to sell cosmetic or costume contact lenses without a prescription. The FDA considers costume contact lenses medical devices. Be wary about costume contact lenses that tout “one size fits all” because that claim is not true. If costume contact lenses aren’t used and maintained the right way, they can damage your eye.11
BRAND: The last line of the prescription lists the brand of contact lenses your doctor recommends.