Pupillary Distance, commonly abbreviated as PD, is a measurement that defines the distance between the centers of your two pupils. This small number, usually expressed in millimeters, is a required piece of information when manufacturing prescription eyeglasses. Despite its necessity, many patients are surprised to find this measurement absent from the prescription slip provided by their eye care professional. This omission is a common source of confusion, particularly for those who plan to purchase their eyewear from an online retailer.
Why PD Is Often Not Included
The primary reason for the absence of Pupillary Distance relates to the distinction between a refractive prescription and a fitting measurement. An eye doctor’s prescription, mandated for release by the Federal Trade Commission’s (FTC) Eyeglass Rule, details the lens power needed for vision correction. However, the PD is traditionally considered a dispensing or fitting measurement, which is separate from the refractive lens power data.
The FTC’s Eyeglass Rule requires the automatic release of the lens power prescription, but it does not explicitly mandate the inclusion of PD. Only a few states, such as Kansas, Massachusetts, Alaska, and New Mexico, require that PD be included by law. In the vast majority of locations, eye care providers are not legally obligated to provide this specific measurement to the patient at the time of the exam.
A significant number of practitioners view the PD as proprietary fitting data, which an optician typically measures when a patient selects frames. Since this measurement is an additional service performed to custom-fit the lenses to the chosen frame, some offices may withhold it to encourage patients to purchase their glasses directly from the practice’s optical shop.
The Importance of Pupillary Distance
The Pupillary Distance is a precise measurement that ensures the optical center of the lens aligns exactly with the center of the pupil. This alignment is necessary because the lens is engineered to provide the full corrective power only at its optical center. If the lens is not correctly centered, light is refracted incorrectly, which can cause a condition known as prism effect.
Wearing glasses with an incorrect PD can lead to eye strain, blurred or distorted vision, and persistent headaches. The eyes must work harder to compensate for the misaligned lenses, leading to fatigue. This misalignment also compromises depth perception, which can affect daily activities like driving.
The accuracy of the PD becomes more significant for stronger prescriptions or for multifocal lenses, such as progressives. While a single PD number measures the total distance between the pupils, a more precise measurement is the dual PD, or monocular PD. Dual PD provides two separate numbers, measuring the distance from the bridge of the nose to the center of each pupil individually. Since not all faces are perfectly symmetrical, dual PD ensures a more accurate placement for each lens.
Steps to Obtain Your Missing PD
The most straightforward way to get your Pupillary Distance is to contact the prescribing optometrist or ophthalmologist’s office. Even if the PD was not included on the original prescription slip, the measurement is typically on file and can be provided upon request. While some offices may still charge a small fee for releasing this information, many will provide it freely, especially if the patient explains the need for online ordering.
If the provider is unwilling to release the measurement, or if you prefer a quicker solution, you can measure your PD at home. This method involves using a millimeter ruler and a mirror. By standing about eight inches from a mirror, you can align the ruler’s zero mark with the center of one pupil and then read the measurement at the center of the other pupil.
Alternatively, you can ask a friend to assist, having them measure the distance while you focus on an object at least twenty feet away to keep your eyes steady. Many online eyewear retailers also offer digital PD measurement tools or smartphone applications. These tools use a camera and a standard-sized card to help determine the measurement, giving you the necessary number to complete your glasses order.