Pupillary distance
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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 your pupillary distance, or you can make an approximate measurement yourself using a ruler, with someone’s assistance or a mirror. The measurement will usually be taken twice: 1st with the patient focusing at a distance, and 2nd with the patient focusing at something close. The second measurement is used for reading glasses, and bifocals however the rule of thumb is that the so-called "near PD" will be 4 millimetres less than the "far PD" when the far PD is over 60mm, or 3 millimetres less than the far PD when the far PD is under 60mm. It is recommended, due to accuracy, to measure the PD with a pupilometer, however some practitioners take preliminary measurements with a millimeter ruler placed across the bridge of the nose. You can also measure your PD using a suitable photo and an online pupilometer.
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 this case, measurements are often inaccurate by a few millimetres.
Typical values
- 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).
- 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 Japanese people or US/European children are to be included, a lower IPD distance will be required.
- Most adult PD's are between 54-70
- Most children's PD's are between 41-55
In England, 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, however this is not the case in the USA.[1]
See also
References
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- PD measurement Method and Procedure
- Measuring your pupillary distance online
- Variation and extrema of human interpupillary distance (pdf document)
- How to Read a PD? (plus video)
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