Many people find contact lenses an ideal alternative to eyeglasses. Unfortunately, getting contact lenses is not just a matter of walking into the store and requesting a box. Contact lenses require a prescription, just like medication.
Even if you know exactly what you need, no one can sell you contact lenses without a prescription. A contact lens prescription typically lasts a year, after which you will need a doctor to reexamine your eyes. What exactly does a contact lens prescription say?
Your prescription may have different numbers for each eye. OS refers to the left eye (from the Latin "oculus sinister"). OD refers to the right eye ("oculus dexter"). OU ("oculus uterque") refers to either eye.
One set of numbers will appear under the designation "PWR." This number, measured in diopters, refers to refractive power. It represents the amount of refraction needed to give the patient 20/20 distance vision. A negative number means nearsightedness and a positive number means farsightedness. The bigger the number, the more correction is necessary.
The base curve millimeters show how much the lens should curve to match your cornea’s back curvature. The diameter is how far in millimeters it is across the lens from one edge to the other.
This cylinder measures the extent of astigmatism, in diopters. If you have hyperopic astigmatism, the number is positive. With myopic astigmatism, it is negative. The axis is the number, in degrees -- the cylinder must rotate to compensate for the astigmatic shape of the lens.
Add power is a prescription added for reading or close-up seeing for those with bifocal lenses.
There may be other information on your prescription, such as color for colored lenses, brand of your contact lenses and date of the prescription. Remember that getting a prescription for lenses is not the same as having lenses fitted. Different eyes may require different sized lenses for ideal fitting. Since the shape of the eyes may change over time, you should not be surprised if some of these measurement numbers are different for each examination, especially if you only get your eyes examined once a year.