diopter

A diopter is a unit of measurement that describes the power in a pair of glasses or contacts.  For example, weak reading glasses have a diopter power of +1.00 while stronger readers have a power of +3.00 diopters.  Farsighted people require positive (+) diopter glasses to improve their vision, while nearsightedness requires negative (-) diopter power.  Many people have a small amount of astigmatism correction built into their glasses prescription, and this astigmatism correction is also measured in diopters.  Finally, prism glasses are used to fix ocular alignment problems in people with double vision (for example, if you are cross-eyed). The amount of prism ground into a pair of spectacles is also measured in “prism diopters.”