A pupillary distance ruler measures the distance in millimeters between a subject's two pupils, a measurement which is used to customize prescription lenses. There are two different methods for taking this measurement: the binocular method and the monocular method.
The binocular method measures the distance between the center of both pupils. The monocular method measures from the center of the face to each pupil individually. The monocular method is used when the subject's face is particularly asymmetrical. Children typically have a pupillary distance of 41-55 millimeters, while adults range from 54-68 millimeters.
Pupillary distance is an important factor when creating lenses, as the lenses must be accurately centered over the pupils for maximum correction and clarity. For single-vision glasses, only the distance measurement is used. For bifocals, three is subtracted from the initial measurement, in order to adjust between near- and long-distance focusing.