Pupillary Distance (PD) is the precise measurement of the distance between the centers of the two pupils. This figure is necessary for manufacturing prescription eyewear because it ensures the optical center of the lenses aligns perfectly with the wearer’s line of sight. Proper lens centering is important for visual comfort and clarity, especially when ordering new frames online. Without this measurement, lenses may be positioned incorrectly, potentially causing strain or distortion.
Types of Pupillary Distance
Pupillary Distance is categorized into two types: Single or Dual. Single PD, also called binocular PD, is the total distance measured between the centers of the two pupils. This combined number is used for prescriptions with lower power and standard lens types.
Dual PD, known as monocular PD, separates the measurement into two numbers (e.g., 32/30), indicating the distance from the nose bridge to each pupil individually. This method offers superior accuracy by accounting for minor asymmetries between the eyes. Dual PD is recommended for individuals with high-power prescriptions or those selecting progressive lenses, where precise alignment is important.
Measuring PD Using a Mirror
A common method for self-measurement involves using a mirror and a millimeter ruler. Stand approximately 8 to 12 inches away from the mirror, ensuring you can clearly see your reflection without leaning forward. Hold the ruler horizontally above the eyes, pressing it gently against the forehead or brow line for stability. Align the zero mark on the ruler precisely over the center of the right pupil.
To minimize parallax, a visual error occurring when the eye is not directly over the measurement point, close one eye while reading the result. Close the left eye while aligning the zero mark, then open the left eye and close the right eye to read the measurement aligning with the center of the left pupil. This final number represents the total, or Single, Pupillary Distance. Repeat the process a few times to ensure consistent readings.
The Most Accurate Measurement Method
The most reliable method involves enlisting the help of another person, which eliminates self-measurement errors like parallax. The subject and the measurer should sit or stand facing each other at eye level, maintaining a comfortable distance of about 18 to 24 inches. The subject must focus their gaze on a distant object positioned over the measurer’s head. This prevents the eyes from converging inward, which would result in an artificially smaller PD reading.
The measurer holds the millimeter ruler horizontally against the subject’s brow.
Measuring Single PD
To find the Single PD, the zero mark is aligned with the center of the subject’s right pupil, and the measurer reads the measurement corresponding to the center of the left pupil.
Measuring Dual PD
For the Dual PD, the measurer aligns the zero mark over the center of the nose bridge. They record the distance to the center of the right pupil, and then repeat the process for the left pupil. Recording these two separate numbers provides the necessary precision for lens manufacturing.