What is your PD?
Your PD measurement is your pupillary distance measurement, or the distance between the center of each of your pupils. It may be represented as a single value (between left and right eye), or as two values – the distance each eye is away from the centerline of your face.
What is it used for?
If you’re ordering glasses, your lenses will be ground (with your glasses frame in mind) to match the center of your lens with the center of your eyes.
How is it measured
If done in a professional eye doctor place or eyeglasses place, it will be done with a device called a pupilometer, or a centrometer. They can be found on places like Amazon.com for between 150 and 300$.
If you do it yourself, you may use a ruler in combination with a mirror or a friend as per Zenni Optical’s quick guide.