Human vision is binocular, or involves two eyes, and near-eye viewing systems such as eyeglasses, binoculars and head mounted displays account for vision by two eyes wherein each line of sight originates from a different angle. Each eye is typically looking through its own lens or lens system. Ideally, every lens would provide perfectly collimated light providing a distortionless view no matter what portion of it the viewer looks through. However, that is not the case. Wearing a near-eye display device wherein the user's pupil is not aligned with a portion of the display providing collimated light causes visual discomfort because the eye muscles of the user's vergence system adapt to the display device optics.
Collimated light in a lens system is usually on its optical axis, and aligning each pupil with the respective optical axis relieves the visual discomfort. However, the distance between each person's pupils, his or her inter-pupillary distance (IPD), varies over about a 25 to 30 mm range in humans. A device with optical axes positioned at a distance suitable for one wearer will cause discomfort over time for another wearer having a different IPD.