When you go to the eye doctor to get a new eyeglasses prescription, part of the measurements they perform involve something called your PD number. This term is short for pupillary distance, which is the measurement between the center of both of your pupils. In order to make a pair of prescription glasses, the eye doctor needs this information to pass on to the manufacturer. The eyeglass maker uses this measurement to place the optical center on each lens.
Why PD Is Important
If your PD measurement is incorrect, it will make focusing very difficult. The center of your eyes look directly through the middle of your eyeglass lenses in order to focus, so if this measurement is off, it can make it very hard for you to see clearly. The lens maker will place the very middle of the optical center on the area of the lens according to the PD measurement.
Some eye doctors do not provide this information to you because they know it can give you the option to purchase your glasses elsewhere. However, if you request a written prescription from your eye doctor, they must give it to you and include the PD measurement.
How To Measure Your Own PD
If you want to purchase a pair of eyeglasses online or from another vendor, there is an easy way to measure your own pupillary distance on your own. Follow these steps so you can get a correct measurement:
- Gather together a straight ruler, a sharpened pencil, and a hand mirror.
- Look straight into the mirror and place the ruler right onto the bridge of your nose. The start of the ruler should be right below the center of one of your eye's pupils.
- Hold the ruler steady, keeping it parallel to the floor. Then, mark the location on the ruler where you other pupil is located.
- Measure the distance between the mark on the ruler and the start of the ruler, and this should give you your PD measurement.
- It is best to repeat this a few times, just so you can be sure you've gotten the correct measurement for your eyes.
- If it is easier, you can ask someone to help you hold the ruler and make the markings for you while you look straight ahead.
Getting an accurate pupillary distance reading will ensure that your next pair of glasses work well for your eyesight needs. Contact an optometrist, such as one from All About Eyes, for more information.