One of the reasons for feeling dizzy when wearing glasses purchased online is an inaccurate PD (pupillary distance) measurement.
Abstract
In optometric dispensing and clinical practice, Pupillary Distance (PD) is a critical parameter that determines the alignment accuracy of a lens's optical center. For a long time, the public and even some basic dispensing practices have relied on a single binocular PD value. However, biological anatomy demonstrates that physiological facial asymmetry is virtually universal in humans.
This paper explores the fundamental differences between binocular and monocular PD, elucidates the mechanism of induced prismatic effects caused by optical decentration due to facial asymmetry, and demonstrates the clinical necessity and value of adopting monocular PD measurements in modern high-index and progressive addition lens dispensing.
1. Conceptual Distinction: Binocular PD vs. Monocular PD
Binocular Pupillary Distance (Binocular PD): The absolute linear distance between the centers of the pupils of both eyes.
Monocular Pupillary Distance (Monocular PD): The horizontal distance from the anatomical midline of the bridge of the nose (sagittal plane reference) to the pupil center of the right eye (PD_R) and the left eye (PD_L), respectively.
Total PD = PD-R + PD-L
Under an ideal symmetrical model, PD_R = PD_L = Total PD1/2. In clinical practice, however, anatomical variations—such as nasal septum deviation, differences in orbital spacing, and asymmetric ear height—result in PD_R≠ PD_L in the vast majority of individuals, typically exhibiting a physiological deviation of 0.5 to 2.5 mm.
2. The Optical Cost of Overlooking Asymmetry: Induced Prismatic Effects and Visual Fatigue
When lenses are manufactured based on an assumed "symmetrical split" of total PD, the Optical Center (OC) of the lens will horizontally deviate from the wearer's actual visual axis (△ d).
According to Prentice's Rule:
P: Induced prismatic power in prism diopters (△)
F: Lens refractive power in diopters (D)
△ d: Decentration between the optical center and the visual axis in millimeters (mm)
Clinical Implications
High Myopes and Hyperopes: The higher the refractive power (F), the greater the unwanted prismatic effect generated by even a minor decentration.
Increased Binocular Fusion Burden: For instance, an induced base-in (BI) prism in one eye combined with an induced base-out (BO) prism in the other forces the extraocular muscles to continuously compensate. This often manifests as dry eyes, asthenopia, dizziness, spatial distortion, and loss of concentration when wearing new spectacles.
Comparative Analysis of Measurement Approaches
| Measurement Method | Theoretical Assumption | Optical Alignment Quality | Clinical Risks / Outcomes |
| Binocular PD | Absolute facial symmetry (1:1) | Unilateral or bilateral visual axis decentration | Induced prism, asthenopia, binocular fusion dysfunction |
| Monocular PD | Acknowledges physiological facial asymmetry | Precise coaxial alignment of visual axis and optical center | Clear and stable vision, minimal adaptation period |
3. The Crucial Role of Monocular PD in Modern Customized Lenses
Progressive Addition Lenses (PALs)
The intermediate corridor and near-addition (ADD) zones of progressive lenses are often only a few millimeters wide. Inaccurate monocular PD measurements will cause the patient's line of sight to fall directly into the peripheral aberrational zones during near-gaze transitions. This results in narrow reading fields, swim effect, and ultimately dispensing failure.
High Refractive Powers & Large Frames
Oversized frames feature increased edge thickness and are highly sensitive to astigmatic axis alignment. Measuring accurate monocular PD in conjunction with Fitting Height (FH) is essential to eliminate peripheral optical aberrations and distortion.
4. Conclusion and Recommendations
Reducing pupillary distance to a "single number" is an outdated compromise from the era of manual lens dispensing. Modern optometric practice must fully transition to a personalized centration protocol based on monocular pupillary distances (PD_R / PD_L) combined with monocular fitting heights (PH_R / PH_L).
For Eye Care Practitioners & Dispensing Clinics: Prioritize the use of digital pupillometers or digital video centration systems to capture precise monocular data.
For Consumers & Patients: When receiving an optical prescription, actively request individualized monocular measurements (PD_R and PD_L)—this is fundamental to ensuring long-term visual health and wearing comfort.