08/12/2021
INTERPUPILLARY DISTANCE ??
What does "pupillary distance" mean in prescription glasses?
The Pupillary Distance, (PD) - better called the Interpupillary Distance (IPD) - is the distance between the centres of your pupils when looking at the focal length the glasses are designed for. In humans, it's usually between about 50 to 80 mm.
The importance of this measurement is that the lenses in your glasses should usually be centred the same distance apart as the PD, so that lines of sight pass through the lens centres. There are exceptions, but generally this provides the most comfortable vision.
If the centres are wrongly set, then person can get headaches, double vision, or may not be able to see through your glasses at all. This is especially so for strong lenses; people with weak lenses may not notice so much difficulty.
For the most complex of modern lenses, varifocals, it needs an accuracy of half a millimetre, and is measured separately for each eye, being the distance of the pupil centre from the midline of the nose. Usually a specialised PD gauge is used to get the accuracy required.
Mistakes in PD measurement are the most common reason people cannot see with new varifocals. Just remember that if an optician or optometrist gets it wrong, you can go back and they will have to put it right at their expense.
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