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Pupillary Distance (PD) or interpupillary distance (IPD) is the distance (the industry standard is to measure in millimeters) between the centers of the pupils in each eye. This measurement is used when preparing to make prescription eyeglasses. Positioning lenses correctly in relation to the centre of the pupils is especially important for higher powered lenses due to the location of the optical centre of the lenses. It can also be relevant to binoculars: they must be adjusted to suit the user's IPD; and the minimum allowed by some binoculars is still too great for people with a small IPD.
Measuring pupillary distance
Someone with training in the field of optics can accurately measure a person's pupillary distance. This is normally done with a small millimeter ruler referred to as a "PD stick" or with a corneal reflex pupillometer, which is a machine calibrated to help the optical professional more accurately measure the pupillary distance. There are also mobile phone and web apps that can also measure one's pupillary distance.
In very young children and babies, where the patient is unlikely to stay still, the optical professional will typically measure from one medial canthus to the other lateral canthus.
- The typical pupillary distance for adults is around 54–68 mm, while measurements generally fall between 48 and 73 mm.
- For children the measurement usually ranges from 41 to 55 mm.
- The 95 percentile adult male in the USA has an IPD of 70 mm (2.8 in) and the 5 percentile, 55 mm (2.17 in). Those are each the 5% of USA men at the far range and short range, respectively.
- For adult females in the USA the figures are 65 and 53 mm (2.6 and 2.1 in).
- For Europeans the figures are 1 mm smaller than the above.
- The IPD is one of the many measurements used in anthropometry, measurements of the human body. The statistical spread of these measurements, usually expressed as percentiles, is used for many purposes. In the case of IPD, it is used in specifying the size range for eyeglasses, binoculars and other optics such as head-mounted display systems.
- From the above figures, if a 95 percentile US male and a 5 percentile European female are to be accommodated, the IPD adjustment must be at least 52 – 70 mm, but if adult Asian people or US/European children are to be included, a lower IPD distance will be required.
In both the UK and most of Canada (excluding British Columbia), the PD measurement is classed as a dispensing tool rather than a part of the actual prescription of the person whose eyes were tested, thus there is no obligation for a PD to be provided on patient request.