PD is an abbreviation for "Pupillary Distance"
A PD is the distance between a person°Øs right and left pupil. This measurement is usually taken when the person is looking to infinity or at a far away object. Every person will have a different PD measurement for long distance and near objects. This is because when a person focuses on an object which is close to the eye, their eyes converge so a near PD will be smaller than a distance PD.
If you are ordering spectacles, the laboratory cannot make them without this information being supplied. Often, a PD measurement will not be included on your spectacle prescription. This measurement needs to be taken by an eye care professional such as an Optical Dispenser or Dispensing Optician or an Optometrist. It is very specific and cannot be guessed. If you are wanting to purchase a pair of spectacles online rather than from an Optometric Dispensary, you will need to get an eye care professional to write your PD on your prescription.
An adult PD measurement usually ranges from 57mm - 68mm. In some cases the PD measurement may be as small as 52mm or as large as 74mm with the average adult PD being about 60-62mm.
All lenses have an °∞Optical Centre°±. The optical centre of a lens usually needs to sit directly in front of the pupil to give the spectacle wearer the most comfortable vision. If the prescription is more than +1.00D or -1.00D it is very important to have the correct measurement to avoid unwanted prismatic effects. The importance of this measurement grows with increasing lens powers. The higher the lens power, the more prism will be induced by not matching the optical center of the lenses with the PD of the spectacle wearer. If the prescription is only +0.50D or -0.50D, the optical centre of the lenses could be quite a long way away from the pupil and it would not cause much prism at all so the wearer would not notice. If the prescription is +5.00D or -5.00D, the wearer would notice if the optical centre is off by only a few mm.
Looking through a prism will cause an image to be displaced (the image will appear to be in a different position to what it is in reality). You can induce prism with a prescription spectacle lens by fitting the optical centre so that it is not sitting directly in front of the pupils. Some people need to have prism added to their spectacles to avoid double vision but this is not very common. If you have NOT been prescribed prism, the optical centres of your lenses must be fitted directly in front of the pupils. If the lenses are not fitted correctly, they could cause headaches and in some cases diplopia (double vision). Diplopia can be either horizontal (you will see 2 objects side by side) or vertical (you will see 2 objects above and below eachother).
If a person has a distance PD of 65mm, on average, the near PD will be 60mm. This can vary but most commonly, the near PD is 5mm less than the distance PD. This means that a pair of glasses for driving will be fitted with the optical centres of the lenses 65mm apart and a pair of glasses for reading books will be set with the optical centres of the lenses 60mm apart.
Often your eye care professional will provide only a distance PD so if you want to order glasses for reading you need to provide a smaller number. You can use the recommendations below as a guide and you shouldn°Øt have too many problems unless you have convergence problems (your eyes do not converge/move closer together when you focus on a near object). If you have this problem your Optometrist should have discussed this with you so make sure that your eye care professional takes both your distance and your near PD measurement for you.
Usually, If your distance PD is 62mm or higher, you should be pretty safe to order reading lenses with a near PD which is 5mm less than your distance PD. If your distance PD is 61mm or less, you should minus 4mm for your near PD. If your distance PD is very large (70mm or higher) you can deduct 6mm to get your near PD. If your distance PD is very small (56mm or less) you can deduct 3mm to get your near PD.
“PD” is an abbreviation for “Pupillary Distance”
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23 September 2009
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