Pupillary distance

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.

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 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.

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.

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