interpupillary

Interpupillary refers to the distance between the centers of the pupils of the eyes. The term is used in ophthalmology, optometry, and optics to describe measurements important for the proper alignment of binocular devices and corrective lenses. When combined with the noun distance, as in interpupillary distance (IPD), it denotes the spatial separation between the two pupils.

Interpupillary distance is a key factor in fitting eyewear and in the design of stereoscopic displays and optical instruments. Proper IPD ensures that the optical centers of lenses align with the wearer's pupils, reducing prismatic effects and eye strain. In vision correction, incorrect PD can degrade visual acuity, cause binocular double vision, or reduce the effectiveness of prism corrections.

Measurement methods include direct measurement by a clinician using a pupillometer or PD ruler, over-the-distance methods with patient-facing markers, and digital or smartphone-based techniques. IPD is normally measured in millimeters and can vary widely among individuals, with adults typically ranging roughly from 50 to 75 mm. On average, men tend toward the higher end and women toward the lower end, but there is substantial overlap. IPD is relatively stable in adulthood, though it may change slightly with aging, significant facial changes, or after certain eye surgeries.

Applications extend beyond corrective lenses to devices such as virtual reality headsets, where adjustable IPD settings help ensure a comfortable, immersive visual experience. Clinicians also consider IPD in diagnosing and managing binocular vision disorders and in planning refractive or surgical interventions.

See also: binocular vision, pupil, pupillometer, stereoscopy.