Interpupillary distance: Difference between revisions

* '''Distance PD''' (also called '''Far PD''') is the measurement taken when the eyes are looking at a distant object, causing the lines of sight to be effectively parallel. This is the standard measurement required for single-vision distance glasses, the upper portion of [[bifocal]]s, and most importantly, for setting up VR and AR headsets, which typically have a fixed optical focus set at a distance (e.g., 2 meters).<ref name="MetaIPD" /><ref name="FittingBox" />

A common source of error for VR users is inadvertently measuring their Near PD when they require their Distance PD. Many self-measurement guides instruct the user to stand close to a mirror (e.g., 8 inches away). If the user focuses on their own reflection at this close range, their eyes will converge, yielding an inaccurate Near PD measurement. To correctly measure Distance PD using a mirror, the user must focus on a distant object reflected in the mirror, ensuring their eyes remain parallel.

'''Horizontal disparity''', defined as the difference between viewing angles from each eye to an object, drives stereoscopic depth sensation. The '''horopter''' depicts points with zero disparity relative to fixation—points at the same depth as the fixation point project onto corresponding locations in both retinas. Objects closer than the horopter have '''crossed disparity''' (negative), while objects farther have '''parallel disparity''' (positive). Within '''Panum's fusional area'''—the region of binocular single vision—points off the horopter have disparity but are still seen as single and in depth relative to fixation. Outside this area, physiological [[diplopia]] (double vision) occurs.

The average adult real IPD is around 63–64 mm, with males tending to have slightly larger IPDs on average than females (e.g., about 64.0 mm in adult males vs 61.7 mm in adult females in large surveys).<ref name="Dodgson2004" /><ref name="ANSUR" /> Most adults fall within roughly 50 mm to 75 mm, and only a small percentage have IPDs outside that range.<ref name="Dodgson2004" /><ref name="ClevelandClinic" /> In extreme cases, adult IPDs as low as ~45 mm or as high as ~80 mm have been recorded.<ref name="Dodgson2004" />

A VR/AR headset works by presenting a separate, slightly different image to each eye, simulating binocular disparity to create the illusion of depth. To achieve this effectively, the optical system—composed of displays and lenses—must be precisely aligned with the user's visual system. The primary goal of IPD adjustment is to horizontally position the optical center of each lens directly in front of the center of each pupil.<ref name="MetaIPD" />

Research indicates that even small mismatches (e.g., 5 mm) can reduce visual acuity and comfort, particularly in high-resolution HMDs.<ref name="IEEEVR" /> For AR, IPD affects overlay alignment with the real world, impacting tasks like surgical simulation or navigation.

'''Hardware IPD''' refers to the physical distance between the optical centers of the two lenses inside the HMD.<ref name="MetaIPD" /> The adjustment mechanism on a headset—whether it's a slider, knob, or automated motor—directly changes this lens spacing. The goal of adjusting the hardware IPD is to physically align the lenses with the user's pupils, placing them in the center of the eyebox to achieve maximum image clarity and minimize optical aberrations.<ref name="MilvusIPD" />

In the real world, these two actions are tightly coupled by a neurological reflex; when you look at a near object, your eyes both converge and refocus simultaneously. In most VR headsets, however, the displays are at a fixed physical distance, and the lenses place the virtual image at a fixed optical distance (e.g., 2 meters). This means your eyes must ''accommodate'' to this fixed distance at all times. Yet, to view virtual objects that are rendered at different depths (e.g., an object 30 cm away), your eyes must ''verge'' to that closer distance.

Moving the eye outside the eyebox—either horizontally, vertically, or in depth ([[eye relief]])—will result in a degraded image, with effects like blurring, [[chromatic aberration]], or vignetting (the edges of the image being cut off). The purpose of hardware IPD adjustment is to horizontally position the user's pupils within the eyeboxes of the two lenses. A headset with a larger eyebox is more forgiving of small IPD misalignments and headset movement on the user's face, contributing to a more comfortable and consistent experience.<ref name="AvantierVR" />

'''March 2016—Oculus Rift CV1''' launched with mechanical adjustment via physical slider offering smooth continuous adjustment across approximately 58–72 mm range, making it the first mainstream consumer headset with hardware IPD adjustment. Over 500,000 units sold in the first year.

'''April 2016—HTC Vive''' launched with mechanical adjustment knob providing rotary continuous adjustment across approximately 60–75 mm range, setting the standard for enthusiast VR with room-scale tracking.

'''October 2016—PlayStation VR''' launched with mechanical adjustment via slider (~58–70 mm), selling millions of units to achieve the largest installed base at the time.

'''March 2019—Oculus Rift S''' represented a significant backward step, launching with fixed IPD at 63.5 mm—identical to DK2 from 2014—with software-only adjustment. The "best fit" range spanned only 61.5–65.5 mm (4 mm total). ANSUR II analysis showed only 46% of men and 43% of women fit "best" compared to 99%/93% with mechanical adjustment. Massive community backlash ensued, with many refusing to buy.<ref name="PalmerLuckey" />

'''September 2020—Oculus Quest 2''' at $299 offered a three-position compromise with discrete positions at 58 mm, 63 mm, and 68 mm. While better than Rift S, it was worse than Quest 1, yet became the best-selling VR headset ever due to aggressive pricing.

Medical consensus indicates that short-term effects—eye fatigue, blurred vision, headaches—are common but resolve within minutes to hours. No proven permanent damage occurs in healthy adults with proper use, though children under 6 should avoid displays causing vergence-accommodation conflict. Red flags requiring medical attention include symptoms lasting more than 24 hours, progressive worsening, or persistent double vision.