Inter-pupillary distance

The IPD inside of a VR headset, in this case, the Apple Vision Pro

Interpupillary distance (IPD) is the distance between the centers of the pupils of the eyes. It is also known as interocular distance (IOD).

Most adult humans have an IPD of about 63mm.

IPD can be measured in a number of ways.

Some VR headsets have an IPD adjustment. The Oculus Rift DK1, DK2, and HTC Vive have manual IPD adjustment. And the Valve index too.

Incorrect IPD is the number 1 cause of VR sickness.

Mirror measurement

IPD can be measured by standing close to a vertical mirror, and closing one eye at a time. Each eye looks directly into that eye on the mirror. Use a marker to mark a spot on the mirror where the pupil is. The distance between the dots is a measurement of IPD.

The different types of IPD

• Anatomical IPD (Pupillary Center): This is the distance between the centers of the pupils. It is the most common measurement used for standard eyeglasses and binoculars.

• Optical IPD (Visual Axis): This measures the distance between the lines of sight that hit the fovea (the center of the retina). Because the eyes have a slight inward tilt (Angle Kappa), the visual axis is usually a few millimeters narrower than the anatomical pupil center.

• Mechanical IPD (Center of Rotation): This measures the distance between the pivoting centers of the eyeballs. In VR/AR headsets, this can be used to ensure the lenses stay clear even as the eyes rotate to look in a virtual environment.

• Functional IPD (Far vs. Near): This distinguishes between the eyes looking at the horizon (Far IPD) versus converging to read a book (Near IPD). Near IPD is typically 3–4mm shorter because the eyes tilt inward to focus on close objects.