Numbers on a contact lens prescription tell the lens maker customize your lens.
In the U.S., you need an examination by an eye care practitioner and a written prescription before you can buy contact lenses. The measurements on the prescription will be preceded by either a positive (+) or a negative (-) sign.
Types
There are six types of numerical measurements on a contact lens prescription: PWR, BC, DIA, CYL, AXIS and ADD.
Functions
PWR, which stands for power, indicates the amount of correction you need to have 20/20 vision, while BC, which stands for base curve, indicates the back curvature of your eye in millimeters. DIA, which stands for diameter, is the distance from one edge of your contact lens to the opposite side. Both CYL, which stands for cylinder, and AXIS are measurements to correct for astigmatism, an abnormally curved cornea. ADD is a number that tells the lens maker create a bifocal lens by adding a second area of focus.
Features
A positive number in the PWR section indicates farsightedness, while a negative one signals nearsightedness.
Format
A contact lens prescription has two columns labeled OS (left eye) and OD (right eye), generally with different numbers in each column.
Tip
As a contact lens user, you have a legal right to obtain a copy of your contact lens prescription from the eye care professional who ordered the lenses.
Tags: contact lens, contact lens prescription, lens prescription, which stands, lens maker, your contact