The Pupillary Distance (PD) is the distance between pupils. This is typically measured in millimeters and can be written as Monocular or Binocular. This measurement is used to place the Optical Center of prescription lenses directly in front of the wearers eyes.
Monocular: The distance from the pupil of each eye to the center of the bridge.
Binocular: The total distance from one pupil to the other.
Binocular PD: 65
Monocular PD: R = 32.5 L = 32.5
In this example the mono measurement is perfectly even but it is actually more common for the right and left PD to be different.
There are two types of Pupillary Distance that can be measured.
Far PD: This is the standard Binocular or Monocular measurement used for all glasses except reading and computer glasses.
Near PD: Our eyes naturally converge when focusing on something up close and measuring the PD while in this converged position is called the Near PD. This number is generally 3-4 millimeters less than the Binocular Far PD.