Pupillary Distance (PD) is a measurement used when fitting new prescription eyeglasses. This measurement ensures that your new lenses provide visual comfort and optimal clarity. PD is a key metric eye care professionals use to precisely align the optical centers of the lenses with your eyes.
Defining Pupillary Distance and Its Average Range
Pupillary Distance (PD) is the measurement in millimeters between the centers of the pupils in both eyes. This distance is used to manufacture glasses so the optical center of the lens is positioned directly in front of the center of your pupil.
The typical range for an adult’s PD falls between 54 millimeters and 74 millimeters. The average PD is around 63 millimeters, though individual measurements vary widely. For children, the PD measurement is smaller, generally ranging from 40 millimeters to 58 millimeters. Obtaining an individual measurement is important, as relying on an average would likely result in an inaccurate fit.
Why PD Measurement is Crucial for Vision Correction
An accurate PD measurement ensures the optical center of the prescription lens is positioned directly over the pupil. This alignment is necessary because light rays must pass through the lens at the correct angle to achieve the intended vision correction. If the optical center is not perfectly aligned, the eye is forced to look through a different section of the lens.
When the PD is incorrect, it causes an induced prismatic effect, meaning light is bent in a way that shifts the image from its actual location. This misalignment forces the eye muscles to work harder to compensate, leading to uncomfortable symptoms. Consequences of an inaccurate PD can include eye strain, headaches, blurred vision, or double vision. These negative effects are often more pronounced for individuals who have stronger prescriptions or wear multifocal lenses.
Types of Pupillary Distance Measurements
There are two primary ways PD is measured and recorded for prescription eyewear. The first is Binocular PD, which is the total distance measured from the center of one pupil to the center of the other. This results in a single number, such as “63 mm,” and is the standard measurement used for most basic single-vision distance glasses. Binocular PD is sometimes adjusted to a “Near PD” by subtracting about three millimeters for reading glasses, as the eyes converge slightly when focusing on close objects.
The second measurement is Monocular PD, often referred to as Dual PD. This measurement determines the distance from the center of the nose bridge to the center of each pupil separately. It is recorded as two distinct numbers, such as “32/30 mm,” one for the right eye and one for the left. Monocular PD is preferred for complex prescriptions, such as progressive lenses, because it accounts for the fact that the face and eyes are rarely perfectly symmetrical.
How PD is Measured (Professional vs. At-Home)
Eye care professionals typically obtain a PD measurement using specialized instruments. The most common tool used in a professional setting is a pupillometer, a noninvasive digital device that provides a precise measurement. Automated refractors used during a comprehensive eye exam can also generate a PD measurement. These professional methods are considered the most reliable because they minimize human error and account for near or far viewing distances.
It is also possible to measure PD at home, though this method is subject to a higher risk of error. At-home measurements usually involve using a millimeter ruler and a mirror or having a friend assist. While a self-measurement can provide a rough estimate, accurately measuring Monocular PD or the subtle shift required for Near PD is difficult without proper training and equipment. For individuals with a strong prescription or those ordering progressive lenses, relying on a professionally obtained measurement is advised.