Interpupillary distance (PD) is a measurement of the distance between the centers of your two eyes and is dependent on whether you are buying eyewear for distance or close up. This measurement is used to properly align the center of your eyeglass lenses with the center of your eyes. If the centers of your eyeglass lenses are not aligned properly, then you may experience eyestrain, headaches, distorted vision, double vision, blurred vision and/or an inability to wear your eyeglasses.
A PD measurement is part of the dispensing service, not part of the eye examination. Other dispensing services include selection of an appropriate frame, recommendation of an appropriate lens type, thickness and coatings, adjustment of the frame, proper placement of the bifocal or progressive. It is the responsibility of the person or company providing the dispensing service to accurately measure your PD.
There are a number of measurements that your doctor of optometry will take when fitting you for multifocal lenses. As with all multifocal lenses, the two key measurements are the interpupillary distance and the segment height. Both of these determine where the multifocal lens will be orientated in the frame to give you proper vision. Each lens manufacturer has its own version of customized progressives.
Near-pupillary distances are also measured as the reading area in progressive addition lenses is customized instead of being standard. Other measurements that are done include the vertex distance (the distance between the frame and the cornea), the pantascopic tilt (the amount of tilt on the front of the frame) and the wrap angle (the amount the frame is curved towards your face). The newest types of progressives are more user friendly, giving a wider area of clear vision and are generally easier to adapt to, especially for those with stronger prescriptions.