Pupillary distance (PD), also known as interpupillary distance, measures the space between the centers of your pupils, typically in millimeters. This measurement is fundamental for crafting prescription eyeglasses. It plays a significant role in ensuring your new eyewear provides clear and comfortable vision.
The Role of Pupillary Distance in Eyeglass Lenses
Pupillary distance dictates the precise placement of the optical centers within your eyeglass lenses. The optical center is the point on each lens through which your eye should look for optimal clarity and minimal distortion. Proper alignment ensures light enters your eyes correctly, allowing your prescription to function as intended.
When lens optical centers do not align with your pupils, your eyes may work harder to compensate. This misalignment can lead to visual discomfort like eye strain, headaches, and blurred vision. An incorrect PD can also introduce a “prism effect,” bending light in unintended ways. For stronger prescriptions, an inaccurate PD’s impact is more pronounced, making precise measurement crucial for visual comfort.
Why Pupillary Distance Is Often Not Included
Pupillary distance is often not explicitly listed on an eyeglass prescription for several reasons. In many places, including most U.S. states, eye care providers are not legally required to include PD on the prescription itself. The Federal Trade Commission’s Eyeglass Rule focuses on ensuring patients receive their refractive error measurements, the core of the prescription. Only a few states, such as Alaska, Massachusetts, and New Mexico, mandate its inclusion.
Traditionally, opticians measure pupillary distance during the eyeglass fitting process. This accounts for slight variations based on the chosen frame and its fit on an individual’s face, ensuring accurate lens positioning. Some eye care practices also view PD as a separate fitting measurement, distinct from the refractive prescription.
Another factor is the variability in PD measurements. Different types exist, including distance PD (for far vision), near PD (for reading), monocular PD (each eye’s distance from the nose bridge), and binocular PD (total distance between both pupils). Monocular PD, measuring each eye separately, is often more precise, especially for progressive lenses or higher prescriptions, as faces are rarely perfectly symmetrical. This complexity means a single number on a prescription may not capture all necessary details for a precise fitting.
How to Get Your Pupillary Distance Measurement
Obtaining your pupillary distance measurement is generally straightforward, even if not on your initial prescription. The most reliable method is to request it from your eye care professional. They typically have this measurement on file or can take it quickly during your visit.
Another option is to have a licensed optician measure your PD. Opticians are skilled in taking accurate measurements for eyeglass fittings and often provide this service as part of their fitting process, especially when purchasing glasses from their establishment. They can also determine monocular PD, measuring the distance from the center of your nose to each pupil individually, which is beneficial for complex prescriptions like progressive or multifocal lenses.
For those needing their PD quickly or ordering glasses online, self-measurement or online tools are viable alternatives. Methods include using a ruler and a mirror, or having a friend assist. Many online eyewear retailers offer digital tools or smartphone applications that estimate your PD using a webcam or phone camera. While convenient, it is important to exercise caution with self-measurements, as they may not be as accurate as professional ones, potentially leading to visual discomfort if off by even a millimeter.