Pupillary Distance (PD), measured in millimeters, is the space between the centers of your two pupils. This measurement is necessary for manufacturing prescription eyeglasses, ensuring the optical center of each lens aligns perfectly with your pupils. Without an accurate PD, the corrective power in the lens will be misplaced, potentially causing visual discomfort. PD is often not included on the prescription slip provided by the eye doctor, requiring you to obtain it before ordering new eyewear.
Obtaining Your PD Through Professional Services
The most accurate way to obtain your Pupillary Distance is to have it measured by a trained eye care professional, such as an optician or optometrist. Professionals typically use a specialized instrument called a pupillometer, which provides a precise measurement. This guarantees that the optical centers of your new lenses are correctly positioned, maximizing visual clarity and comfort.
You can visit an optical shop or an eye doctor’s office and request a PD measurement, even if you are not purchasing glasses from them. Some providers may charge a small fee for this service, but the precision offered by a professional is valuable, particularly for higher prescriptions.
If you have purchased glasses previously, your PD is likely recorded in your patient file. Contact the eye doctor or dispensing optician who handled your last fitting and ask them to provide the measurement. Most professionals will release this information upon request, even if it is not printed on the prescription. Accessing existing records saves time and ensures you are using a professionally verified number.
Utilizing At-Home Measurement Methods
For those ordering glasses online, at-home methods offer a convenient alternative to a professional visit. The simplest technique involves using a millimeter ruler and a mirror or a helper. To measure yourself, stand about 8 inches away from a mirror, hold the ruler horizontally, and align the 0mm mark with the center of one pupil. Read the measurement that aligns with the center of your other pupil, repeating the process for consistency.
If a friend or family member is available, they can provide a more accurate reading by measuring your PD while you look straight ahead at a distant object. This ensures your eyes are in a relaxed, parallel position, which is necessary for a distance PD measurement. Your helper will place the ruler across the bridge of your nose and measure the distance between the center of your pupils.
A more modern approach involves using dedicated smartphone applications or online tools that utilize your device’s camera or webcam. These technologies often require holding a standard object, like a credit card, against your face to calibrate the measurement. While these digital tools are convenient, it is advisable to double-check the result or take the measurement multiple times for reliability. Self-measurement carries a higher risk of error than a professional measurement, so exercise caution when relying on the number for your final order.
Interpreting Your Pupillary Distance
When submitting your PD for an online order, you will encounter two main formats: single and dual Pupillary Distance. A single PD (binocular PD) is one number representing the total distance between the centers of the pupils. Dual PD (monocular PD) is expressed as two numbers, measuring the distance from the center of the nose bridge to the center of each pupil separately (e.g., 32/30).
The dual PD format is preferred because it accounts for facial asymmetry, as one pupil may be slightly further from the nose than the other. This separate measurement is useful for complex prescriptions or when ordering progressive lenses, where precise lens centering is necessary. If you only have a single PD, it can be mathematically divided in half to approximate the dual PD, but this assumes perfect symmetry.
The measurement you obtain is usually your distance PD, used for seeing objects far away. For reading glasses or the near-vision portion of bifocals and progressives, a slightly smaller near PD is required. This difference occurs because your eyes naturally converge when focusing on close objects. The near PD is typically 2 to 4 millimeters less than the distance PD. If using a distance PD for reading glasses, subtract approximately 3mm from the total number. Using an incorrect PD can cause an unintended prismatic effect, leading to symptoms like eye strain or headaches.