Interpupillary distance: Difference between revisions - VR & AR Wiki

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[[File:Average IPD.png|thumb|250px|A bar chart of average IPD in humans]]

~~'''Interpupillary distance''' ('''IPD''') is the distance between the pupils of your eyes. There are two types of IPDs: Real IPD and Virtual IPD.~~

Most adult humans have an IPD of ~~about 63mm~~.

'''Interpupillary distance''' ('''IPD'''), also known as '''pupillary distance''' ('''PD''') or '''interocular distance''' ('''IOD'''), is the distance measured in millimeters between the centers of the pupils of the two eyes. In the context of [[Virtual Reality]] (VR) and [[Augmented Reality]] (AR), IPD is a critical measurement for achieving visual comfort, proper [[stereoscopic]] 3D perspective, and optimal [[binocular vision]]. Most adult humans have an interpupillary distance of approximately 63 mm on average, although individual IPD can vary widely, typically ranging from about 50 mm to 75 mm for adults.<ref name="Dodgson2004">Neil A. Dodgson (2004). "Variation and extrema of human interpupillary distance." Proceedings of SPIE Vol. 5291: Stereoscopic Displays and VR Systems XI, pp. 36–46. DOI: 10.1117/12.529999. https://www.researchgate.net/publication/229084829_Variation_and_extrema_of_human_interpupillary_distance</ref><ref name="ClevelandClinic">Pupillary Distance: What It Means & How To Measure. Cleveland Clinic (health library article). https://my.clevelandclinic.org/health/articles/pupillary-distance</ref><ref name="WebMD">Pupillary Distance: Types and How to Measure. WebMD. https://www.webmd.com/eye-health/pupillary-distance</ref>

IPD ~~can be measured in~~ a ~~number~~ of ~~ways~~.

VR and AR [[head-mounted display]]s (HMDs) must account for the user's IPD by adjusting the lenses or the rendered images for each eye to match that distance, ensuring the virtual stereoscopic images overlap correctly for a clear and comfortable experience.<ref name="RoadToVR-Measure">Ben Lang (Dec 5, 2023). "How to Measure Your IPD and Why It's Important for VR & AR Headsets." Road to VR. https://www.roadtovr.com/how-to-measure-ipd-vr-headset-ar-iphone-app/</ref> In VR terminology, there are two key notions of IPD: '''real IPD''' (the user's actual physical pupillary distance) and '''virtual IPD''' (the distance between the two virtual cameras or viewpoints in the 3D rendering).

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

==Types of IPD Measurement==

~~==Mirror measurement==~~

While often referred to as a single value, IPD can be categorized into several distinct types depending on the viewing distance, measurement method, and reference points used. These distinctions are critical for the correct fabrication of eyewear and the proper calibration of VR/AR devices.

IPD can be ~~measured using a mirror~~.

~~It can~~ be ~~measured~~ by ~~standing~~ close to a ~~vertical~~ mirror, ~~and~~ close ~~one eye at~~ a ~~time. Each eye look 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~~.<ref>How to ~~measure your~~ IPD ~~with~~ a ~~mirror at MTBS~~' ~~Immersipedia~~. https://~~web~~.~~archive~~.org/~~web~~/~~20140328140034/http~~://~~www~~.~~mtbs3d~~.~~com~~/~~wiki~~/~~index~~.~~php~~?~~title=How_to_measure_your_IPD_with_a_mirror~~</ref>

===Distance PD vs. Near PD===

The distance between the pupils changes depending on whether the eyes are focused on a distant or a near object, due to the mechanism of [[convergence (eye)|eye convergence]].<ref name="DistanceNearPD">Distance PD vs Near PD: Differences, Importance, and How to Measure. Low Cost Glasses. https://lowcostglasses.co.uk/distance-pd-vs-near-pd/</ref>

* '''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">Learn about IPD and lens spacing on Meta Quest. Meta Support. https://www.meta.com/help/quest/261777072346131/</ref><ref name="FittingBox">Is PD Measurement measures Distance or Near PD? FittingBox Help Center. https://fittingbox.com/en/resources/help-center/is-pd-measurement-measures-distance-or-near-pd</ref>

* '''Near PD''' is the measurement taken when the eyes converge to focus on a close object, such as a book or a smartphone (typically at a distance of about 40 cm). Because the eyes turn inward, the Near PD is always smaller than the Distance PD, typically by 3 to 4 millimeters.<ref name="WebMD"></ref><ref name="DistanceNearPD"></ref> This measurement is essential for fitting reading glasses or the near-vision segments of bifocal and [[progressive lens]]es.

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.

===Binocular PD vs. Monocular PD===

IPD can be expressed as either a single value for both eyes or as two separate values.

* '''Binocular PD''' (also called '''Single PD''') is the most common form of measurement. It is a single number representing the total distance from the center of the left pupil to the center of the right pupil.<ref name="WebMD"></ref>

* '''Monocular PD''' (also called '''Dual PD''') consists of two numbers, each representing the distance from the center of the bridge of the nose to the center of each pupil individually. It is typically written with the right eye (Oculus Dexter, OD) measurement first, followed by the left eye (Oculus Sinister, OS), for example, "32/30".<ref name="ZenniPD">How to Measure Your Pupillary Distance (PD). Zenni Optical. https://www.zennioptical.com/measuring-pd-infographic</ref> Monocular PD is considered a more precise measurement because it accounts for facial asymmetry, which is common. This level of precision is especially important for high-power corrective lenses and complex lens designs like progressive lenses, where even a small centering error can cause significant visual distortion.

===Anatomical vs. Physiological IPD===

In clinical settings, a further distinction is made based on the reference point for measurement.

* '''Anatomical IPD''' refers to the true physical distance between the geometric centers of the two pupils. This is what is typically approximated when using a manual ruler or "PD stick".<ref name="JournalOptometry">Agreement and inter-session repeatability of manual and automatic pupillometers for the measurement of interpupillary distance. Journal of Optometry. https://www.journalofoptometry.org/en-agreement-inter-session-repeatability-manual-automatic-articulo-resumen-S1888429620301205</ref>

* '''Physiological IPD''' is the distance between the centers of the corneal light reflexes of the two eyes. This is the measurement obtained by automated instruments like a [[pupillometer]], which works by projecting an internal light source onto the eyes and measuring the distance between the reflections.<ref name="AutomatedPupillometer">The Reliability, Validity, and Normative Data of an Automated Pupillometer. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC5497600/</ref><ref name="JournalOptometry"></ref> Because the corneal reflex is a better proxy for the eye's visual axis than the geometric center of the pupil, physiological IPD is often considered the more accurate measurement for precisely aligning ophthalmic lenses. The anatomical IPD averages 0.10 mm wider at distance and 0.30 mm wider at near than physiological IPD.

==Role in Binocular Vision and Stereopsis==

IPD is a cornerstone of [[binocular vision]] and the design of any device that presents separate images to each eye. Humans and many other animals have two forward-facing eyes separated by the interpupillary distance. This separation means that each eye captures a slightly different perspective of the same scene. This difference between the two retinal images is called '''[[binocular disparity]]'''.

The brain's visual cortex processes and fuses these two 2D images into a single, unified perception with an added dimension of depth. This process, known as '''[[stereopsis]]''', is the basis for high-fidelity depth perception, allowing for precise judgments of distance and the three-dimensional structure of objects.<ref name="StereoscopicVR">What is Stereoscopic VR Technology? Draw & Code. https://drawandcode.com/learning-zone/what-is-stereoscopic-vr-technology/</ref> The magnitude of the IPD directly influences the amount of binocular disparity; a wider IPD results in a greater difference between the two eyes' views, which can enhance the stereoscopic effect.

'''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.

==Real IPD==

Real IPD is the actual distance between the center of your eyes' pupils in real life. Any [[Head-mounted Display]] require this measurement to accurately position the virtual camera outputs on the screen in front of the user's eyes. The outputs have to be directly in front of both of the users' eyes. Any deviation can cause [[eye strain]]. Most [[HMD]]s allow users to adjust their camera output positions to match their IPD.

~~Additionally, some HMDs allow~~ the ~~physical position~~ of the lenses and ~~screens~~ to be ~~adjusted~~ to ~~match~~ the user's ~~physical~~ IPD.

'''Real IPD''' (also called physical IPD or true IPD) is the actual distance between the centers of a person's pupils in real life. This measurement is crucial for any [[head-mounted display]] (HMD) because the headset's lenses and images need to be positioned to align with the user's eyes. If the stereoscopic images or lenses in a headset are not aligned with the user's IPD, the user can experience [[eye strain]], blurriness, or double vision due to misfocus and disparity.<ref name="SimulaIPD">George Singer (Nov 13, 2022). "Simula One's Auto-IPD System." SimulaVR Blog. https://simulavr.com/blog/simula-one-auto-ipd-adjustment/</ref><ref name="ClevelandClinic"></ref>

The average IPD ~~for humans~~ is 64 ~~millimeter~~, with a range of 54 to ~~72 millimeter~~. ~~You~~ can ~~measure your~~ IPD by ~~measuring~~ the distance ~~between~~ the ~~center~~ of ~~your pupils~~ with a ~~ruler~~.

===Average Values and Population Range===

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><ref name="ANSUR">2012 Anthropometric Survey of U.S. Army Personnel (ANSUR II) – reported mean distance IPD ~64 mm for men and ~62 mm for women, with 95% of adults between approximately 55 mm and 72 mm.</ref> Most adults fall within roughly 50 mm to 75 mm, and only a small percentage have IPDs outside that range.<ref name="Dodgson2004"></ref><ref name="ClevelandClinic"></ref> In extreme cases, adult IPDs as low as ~45 mm or as high as ~80 mm have been recorded.<ref name="Dodgson2004"></ref>

The most comprehensive dataset comes from the 2012 ANSUR II U.S. Army Anthropometric Survey of 6,068 soldiers:

{| class="wikitable"

|+ IPD Statistics by Sex (2012 ANSUR II Data)

! Gender !! Sample Size !! Mean (mm) !! Std. Dev. (mm) !! Range (mm) !! 5th Percentile (mm) !! 95th Percentile (mm)

|-

| Female || 1,986 || 61.7 || 3.6 || 51.0–74.5 || 55.5 || 67.5

|-

| Male || 4,082 || 64.0 || 3.4 || 53.0–77.0 || 58.5 || 70.0

|}

===Variation Across Demographics===

'''By Sex:''' Males consistently have larger IPD than females by 1.58–2.5 mm on average, a difference that is statistically significant across all studied populations (p < 0.001).<ref name="JournalOptometry"></ref><ref name="TurkishIPD">Evaluation of interpupillary distance in the Turkish population. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC4529256/</ref>

'''By Age:''' IPD generally increases from childhood until it stabilizes in early adulthood. Children have significantly smaller IPDs that increase with age – for example, a five-year-old child may have an IPD of only around 40–50 mm.<ref name="Dodgson2004"></ref> Most of the growth in IPD occurs within the first few years of life, with gradual increases continuing into the late teens or early adulthood. Some studies suggest a continued slight increase up to age 30, followed by a small increase in the elderly population due to orbital expansion.<ref name="AutomatedPupillometer"></ref><ref name="TurkishIPD"></ref>

{| class="wikitable"

|+ Mean IPD Variation by Age Group and Sex (Turkish Population Study)

! Age Group (years) !! Female Mean ± SD (mm) !! Male Mean ± SD (mm) !! Total Mean ± SD (mm) !! Total Range (mm)

|-

| 20–30 || 59.2 ± 3.1 || 61.5 ± 3.8 || 60.3 ± 3.7 || 49–70

|-

| 31–50 || 62.0 ± 3.1 || 64.5 ± 3.6 || 63.0 ± 3.6 || 55–72

|-

| 51–70 || 62.3 ± 3.6 || 65.7 ± 4.3 || 63.8 ± 4.1 || 52–76

|-

| 71–89 || 62.1 ± 3.8 || 63.1 ± 4.3 || 62.7 ± 4.1 || 49–74

|}

'''By Ethnicity:''' Mean IPD also varies among different ethnic and racial groups, reflecting underlying differences in craniofacial morphology.<ref name="AutomatedPupillometer"></ref><ref name="PakistaniIPD">Interpupillary, Inner Canthal and Outer Canthal Distance in the Pakistani Population. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC6408655/</ref> Designing optical devices based on data from a single ethnic group may result in a poor ergonomic fit for other populations.

{| class="wikitable"

|+ Comparative Mean Distance IPD by Population

! Population Studied !! Male Mean (mm) !! Female Mean (mm) !! Overall Mean (mm)

|-

| U.S. Army (ANSUR II) || 64.0 || 61.7 || 63.0

|-

| Turkish || 63.9 || 61.4 || 62.5

|-

| Iranian || 63.6 || 61.1 || 62.1

|-

| Malaysian (KadazanDusun) || 64.6 || 61.2 || 63.2

|-

| South African (Black) || 68.8 || 65.9 || 67.2

|-

| Pakistani || ~64 || ~60 || 61.8

|}

===Pediatric IPD Development===

A comprehensive pediatric study established regression equations for calculating IPD by age:

'''Males:''' IPD (mm) = 43.36 + 1.663 × (age) - 0.034 × (age)²

'''Females:''' IPD (mm) = 41.76 + 1.891 × (age) - 0.052 × (age)²

Key developmental milestones include newborns at approximately 30 mm, 1-month-olds at approximately 43.4 mm (males) and 41.8 mm (females), 5-year-olds at approximately 50 mm, and 10-year-olds at approximately 57 mm. Females reach adult IPD by age 14, while males continue increasing until age 19 or later. This is one reason VR headsets (which typically do not adjust below ~55 mm) are often not recommended for young children, as the optics cannot accommodate their narrower eye spacing and the virtual images may not fuse comfortably for them.

===IPD Adjustment Mechanisms in VR/AR Headsets===

Most VR headsets therefore provide some mechanism to adjust for different real IPDs. Many headsets have physical adjustments (such as sliding lenses or dials) that move the lenses farther apart or closer together to match the user's IPD.

'''Continuous Mechanical Adjustment:''' These headsets feature a physical dial or slider that allows the user to smoothly and precisely adjust the lens spacing across a continuous range. An on-screen display typically shows the current setting in millimeters. This method offers the best fine-tuning and is found in many premium PC VR headsets.

'''Discrete Mechanical Presets:''' A cost-saving approach where the lenses can be manually pushed into a small number of fixed, predefined positions. This method is simpler but less precise, and may not provide a perfect fit for users whose IPD falls between the presets.

'''Software-Only Adjustment:''' Some headsets have fixed, non-moving lenses. IPD adjustment is handled entirely in software by shifting the rendered image on the displays. While this can correct for world scale, it cannot physically align the user's pupils with the lens sweet spot, making it an inferior solution for users whose IPD deviates significantly from the headset's fixed lens distance.<ref name="PalmerLuckey">Palmer Luckey (Apr 21, 2019). "I can't use Rift S, and neither can you." https://palmerluckey.com/i-cant-use-rift-s-and-neither-can-you/</ref>

'''Automatic Motorized Adjustment:''' The most advanced and user-friendly method. These systems use integrated [[eye tracking]] cameras to automatically measure the user's IPD upon putting on the headset. Small motors then physically move the lenses to the exact correct position without any manual input required.<ref name="SimulaIPD"></ref>

For example, the original [[Oculus Rift]] CV1 headset used two separate displays and a mechanical slider to support lens separations from about 58 mm to 72 mm, covering roughly the 5th to 95th percentile of adult IPDs.<ref name="RoadToVR-RiftS">Ben Lang (Mar 29, 2019). "Everything We Know (Officially) About the FOV and IPD of Rift S & Quest." Road to VR. https://www.roadtovr.com/oculus-rift-s-supported-ipd-range-fov-quest-go/</ref> The [[HTC Vive]] (2016) similarly included a knob to adjust IPD (approximately 60–73 mm range)<ref name="SimulaIPD"></ref>, and the later [[Valve Index]] (2019) features a slider with a range of about 58–70 mm.<ref name="VRExpert-Index">"What is the IPD range of the Valve Index?" – VR Expert Knowledge Base (April 21, 2023). https://knowledge.vr-expert.com/kb/what-is-the-ipd-range-of-the-valve-index/</ref> Some newer headsets, like the [[Meta Quest Pro]] (2022), offer an even wider hardware IPD range (around 55–75 mm) using a continuous wheel adjustment.<ref name="QuestProVRDB">"Meta Quest Pro" – Specifications (IPD range 55–75 mm hardware adjustable). VRDB (VR database). https://vrdb.app/headset/meta-quest-pro</ref>

On the other hand, some earlier or lower-cost devices lack physical lens adjustment and instead assume an average IPD, relying on software only. For example, the Oculus Rift development kits ([[Oculus Rift DK1|DK1]] and [[Oculus Rift DK2|DK2]]) and the standalone [[Oculus Go]] had fixed lens spacing (around 63–64 mm) and could only be optimized by entering the user's IPD in software. The later [[Oculus Rift S]] (2019) also used a fixed lens separation (approximately 63.5 mm), with only a software IPD setting to slightly adjust the rendered image for the user's IPD.<ref name="RoadToVR-RiftS"></ref> However, software-only adjustment cannot correct the physical lens misalignment, so users with IPD far from the fixed setting may still experience reduced clarity or comfort on such headsets (since their eyes won't be looking through the lens centers).<ref name="RoadToVR-RiftS"></ref><ref name="PalmerLuckey"></ref>

==Virtual IPD==

Virtual IPD, sometimes referred to as ICD (Inter Camera Distance) or VCS (Virtual Camera Separation), is the distance between the 2 virtual eyes (cameras) in [[Virtual Reality]]. This number can change the [[Sense of Scale]] of the objects in the virtual environment in relation to the user.

~~Developers~~ can ~~change~~ the sense of scale by ~~changing~~ virtual IPD. ~~They~~ can cause the ~~environment~~ to ~~increase~~ or decrease in ~~size~~ by ~~changing~~ the ~~virtual~~ IPD.

'''Virtual IPD''' (sometimes referred to as '''inter-camera distance''' (ICD) or '''virtual camera separation''' (VCS)) is the distance between the two virtual "eyes" (camera viewpoints) used by the rendering software in a [[stereoscopic]] 3D virtual environment. In most VR applications, the virtual IPD is set equal to the user's real IPD to mimic natural vision, which maintains the correct scale of the world and comfortable depth perception.<ref name="KholdScale">K. Kholdstare (Oct 6, 2013). "A Sense of Scale in VR." (Technical blog). https://kholdstare.github.io/technical/2013/10/06/sense-of-scale-vr.html</ref><ref name="XinRealityIPD">Interpupillary distance. XinReality Wiki. https://xinreality.com/wiki/Interpupillary_distance</ref>

If the virtual IPD does not match the real IPD, the brain's interpretation of depth and scale can be altered – a fact that developers can use intentionally to achieve certain effects. By adjusting the virtual IPD, the perceived scale of the VR world can be changed:

* '''Increasing''' the virtual IPD beyond the user's actual IPD (i.e., rendering the stereo cameras farther apart than the eyes are in reality) makes the user feel larger (as if the viewer is a giant) and causes the virtual world to appear smaller or miniaturized. This effect is known as [[hyperstereopsis]].

* '''Decreasing''' the virtual IPD (cameras closer than the real eye spacing) makes the user feel smaller, and the world appears magnified or larger around them.<ref name="DCSWorldScale">Forum discussion: "IPD and World Scale" – Digital Combat Simulator (DCS) Forums (April 2022). Explaining effects of changing in-game IPD on perceived world scale. https://forum.dcs.world/topic/299470-ipd-and-world-scale/</ref><ref name="RedditPSVRScale">Does the IPD (interpupillary distance) setting affect the sense of scale in games? Reddit r/PSVR. https://www.reddit.com/r/PSVR/comments/12wgxpj/does_the_ipd_interpupillary_distance_setting/</ref>

The mathematical relationship is calculated as: '''World Scale Factor = Default IPD / Virtual IPD'''

For example, if default IPD = 64 mm and virtual IPD = 46 mm (approximate 2-year-old child IPD), the scale factor = 64/46 = 1.39x, making the world appear 1.39 times larger.

In practice, most VR titles keep virtual IPD equal to real IPD for realism, but some simulators or games provide a "world scale" slider that essentially adjusts the virtual IPD to suit user preference. It is generally advised not to deviate too far from the true IPD, as a large mismatch can cause discomfort or visual distortion (each individual may tolerate it differently).<ref name="DCSWorldScale"></ref><ref name="KholdScale"></ref>

===Software IPD Implementation===

In game engines like [[Unity (game engine)|Unity]], virtual IPD is controlled via parameters like `Camera.stereoSeparation`, described as "the distance between the virtual eyes." Most VR devices provide this value automatically through their SDKs. In Oculus/Meta SDK, the default IPD is 64 mm (0.064 Unity units), with cameras positioned at ±0.032 (half IPD on each side). The [[OpenVR]] API uses projection matrices to adjust field-of-view based on IPD, synchronizing headset sensor data with GPU rendering.<ref name="UnityDynamicIPD">Dynamic IPD & Out Of Memory Problem. Meta Community Forums. https://communityforums.atmeta.com/discussions/dev-unity/dynamic-ipd--out-of-memory-problem/179101</ref>

===Use Cases for Adjusting Virtual IPD===

'''Game Design Applications:''' Some VR games intentionally use scaled virtual IPD for gameplay effects. For example, games like VR Giants use scaled IPD to make the VR player a giant while a non-VR player controls a tiny character in asymmetric co-op puzzle gameplay. Ghost Giant positions the player as a giant helping a small boy, manipulating miniature world environments.

'''Simulation and Training:''' Research has demonstrated virtual IPD adjustment for child perspective simulation (IPD 46 mm for 2-year-old, 54 mm for 8-year-old), wheelchair user perspective combined with adjusted eye height, and product design where designers creating furniture for children scaled items more accurately when experiencing child-scale IPD. Studies found that "when designer's perspective matched that of intended end-user, it yielded significantly lower variance among designs and more precise scales suitable for the end-user."

'''360° Video Playback:''' If viewer IPD is lower than camera IPD, everything looks too small in VR; if higher, things look too big. Playback applications allow "Stereo Separation" adjustment to solve double-vision problems, reduce eye strain, and make world-scale appear more realistic within comfort limits.

==IPD Measurement Methods==

Accurately measuring one's interpupillary distance is important for configuring a VR/AR headset properly. There are several ways to measure IPD, ranging from professional clinical tools to do-it-yourself techniques and modern smartphone applications.

===Professional Clinical Measurement===

The most accurate and reliable method for determining one's IPD is to have it measured by a trained professional, such as an [[optometrist]] or [[optician]]. The gold standard instrument is the '''[[pupillometer]]''', a handheld or desktop device that the professional uses to measure the patient's IPD. The patient looks at an internal light source, and the operator aligns markers with the corneal light reflexes seen in each eye to get a precise digital readout of the physiological IPD.<ref name="AutomatedPupillometer"></ref>

Professional pupillometers measure the distance between corneal light reflexes using a coaxially-mounted light source that eliminates parallax. These devices locate the visual axis rather than the anatomical pupil center. Clinical studies show mean differences versus manual measurement less than 1 mm for all conditions, with 77% of binocular measurements within ±2 mm clinical range and 63–74% of monocular measurements within ±1 mm clinical range.

An [[autorefractor]], a machine used to provide an approximate eyeglasses prescription, can also often provide an IPD measurement as part of its automated assessment.

===Mirror and Ruler Method===

A simple DIY method is to use a mirror and a millimeter ruler. Stand in front of a mirror (about arm's length away, approximately 20 cm) and hold a ruler just below your eyes. '''Crucially, to measure Distance PD, you must focus on a distant object that you can see in the mirror's reflection. Do not focus on your own eyes.''' Close one eye and align the ruler's "0" mark directly under the center of your open eye's pupil. Then, keeping the ruler steady, switch eyes (close the first eye and open the other) and note the millimeter mark that lines up under the second eye's pupil. The reading on the ruler is your IPD.<ref name="ImmersipediaMirror">"How to measure your IPD with a mirror" – MTBS3D Immersipedia (archived). https://web.archive.org/web/20140328140034/http://www.mtbs3d.com/wiki/index.php?title=How_to_measure_your_IPD_with_a_mirror</ref><ref name="RoadToVR-Measure"></ref>

A variation of this method is to make marks on the mirror itself: stand close to a mirror and close one eye at a time, marking the reflected position of each pupil on the mirror's surface with a non-permanent marker; the distance between the two marks on the mirror can then be measured as your IPD.<ref name="ImmersipediaMirror"></ref>

Accuracy: ±3 mm for 95% of users. The method suffers from parallax errors, requires steady hands, and experiences convergence issues since eyes naturally converge when focusing close.

===With Assistance from Another Person===

Another straightforward approach is to have someone else measure your PD with a ruler. The person helping you holds a ruler up to your face (just below the eyes) while you look straight ahead at a distant object (at least 10–20 feet away). They align the 0 mm mark under the center of one pupil and then read the millimeter mark that lines up under the center of your other pupil. This method allows you to keep both eyes open and focused (unlike the mirror method) and can be quicker for measuring other people (for instance, when setting up a VR demo for a new user).<ref name="RoadToVR-Measure"></ref><ref name="WikiHow">How to Measure Your Interpupillary Distance. wikiHow. https://www.wikihow.com/Measure-Your-Interpupillary-Distance</ref>

Accuracy: ±2–3 mm. The professional Viktorin method improves accuracy when performed by trained examiners, achieving clinical range of resolution within ±2 mm for binocular measurements and ±1 mm for monocular measurements.

===Smartphone Apps and Online Tools===

There are mobile apps and web-based tools that help measure IPD using a phone or computer camera. Modern smartphone applications leverage computer vision and depth-sensing technology for IPD measurement with varying accuracy.

* '''EyeMeasure (iOS, iPhone X or newer with TrueDepth camera)''': Uses 3D scanning technology to measure distance and near IPD plus segment height for progressive lenses without physical objects. Clinical studies found EyeMeasure achieved mean absolute error of 0.51 mm compared to digital pupillometers as gold standard.<ref name="SmartphoneApps">Comparing the Effectiveness of Smartphone Applications in the Measurement of Interpupillary Distance. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC10389117/</ref>

* '''Warby Parker App (iOS and Android)''': Achieved mean absolute error of 0.51 mm in clinical testing, tied for best performer with EyeMeasure. The app's superior performance is attributed to detailed stepwise instructions, active guidance during measurement, and prompts for adjustments.<ref name="SmartphoneApps"></ref>

* '''GlassesOn (iOS and Android)''': FDA, CE, Health Canada, and TGA listed medical-grade app using computer vision technology. Uses any magnetic card for scale reference. Meets ANSI Z80.17 industry guidelines with PD accuracy within 2 mm.

* '''PDCheck AR by EyeQue''': Clinical testing showed mean absolute error of 1.375 mm, significantly less accurate than EyeMeasure and Warby Parker.<ref name="SmartphoneApps"></ref>

While these digital methods are convenient, their accuracy can vary, so it's often recommended to repeat the measurement a few times or cross-check the result with a manual method. Some apps claim accuracy within about ±0.5 mm.<ref name="RoadToVR-Measure"></ref>

{| class="wikitable"

|+ Accuracy Comparison of IPD Measurement Methods

! Method !! Accuracy/Error !! Best Use Case

|-

| Professional pupillometer || Gold standard, <1 mm || Progressive lenses, high prescriptions

|-

| Manual ruler (professional) || ±1–2 mm || Standard eyewear fitting

|-

| Warby Parker app || 0.51 mm MAE || General eyewear, VR

|-

| EyeMeasure app || 0.51 mm MAE || General eyewear, VR

|-

| GlassesOn app || Within 2 mm || General eyewear

|-

| Mirror method (DIY) || ±3 mm || VR headset adjustment

|-

| Ruler with assistant || ±2–3 mm || Home measurement

|-

| PDCheck AR || 1.375 mm MAE || Not recommended

|}

===Built-in Headset Calibration===

Some advanced headsets with [[eye tracking]] can automatically assist or perform IPD measurement. For example, the [[Varjo Aero]] (2021) VR headset uses eye-tracking to automatically detect the user's IPD and then drives motorized lenses to the correct spacing, achieving an auto-IPD adjustment without user input.<ref name="SimulaIPD"></ref> Other devices like the [[PlayStation VR2]] (2023) use eye-tracking to guide a manual adjustment: the headset will display an on-screen prompt or overlay (such as two alignment circles or markers) and ask the user to turn the IPD dial until their eyes or pupils are properly centered in the lenses.

[[Microsoft HoloLens|HoloLens 2]] (Microsoft's AR headset) does not have adjustable lenses but instead runs a calibration routine using eye-tracking to profile each user's eye position and IPD, and then the system automatically corrects the hologram projection for that IPD in software.<ref name="HoloLensCalib">Microsoft Docs: "Calibrating your HoloLens 2" – HoloLens 2 uses eye-tracking to adjust to the user's IPD for correct visual alignment. (2022). https://learn.microsoft.com/hololens/hololens-calibration</ref> These built-in calibration systems make it easier to obtain an accurate IPD setting and ensure the best viewing experience without needing an external ruler or manual measurement.

==Importance in VR and AR==

In [[stereoscopy|stereoscopic]] displays like HMDs, IPD alignment ensures that each eye receives the appropriate image, mimicking natural binocular disparity for accurate depth cues.<ref name="VarjoIPD">IPD definition and explanation. Varjo Support. https://support.varjo.com/hc/en-us/ipd</ref> An accurate match between a user's IPD and the settings of a head-mounted display is essential for achieving a clear, comfortable, and immersive stereoscopic experience. A mismatch can lead to a range of negative effects.

===Aligning the User with the Virtual World===

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"></ref>

This alignment ensures that the user is looking through the lens's '''optical sweet spot''', also known as the '''[[eyebox]]'''. The eyebox is the three-dimensional volume where the eye can be positioned to receive a clear, full, and undistorted view of the virtual image.<ref name="RoadToVR-Measure"></ref><ref name="EyeboxVR">VR HMD eye box and sweet spot. https://blekenbleu.github.io/XR/eyespot.htm</ref> VR lenses have a central area of maximum clarity called the "sweet spot" or "optical center," typically 15–25 mm in diameter for [[Fresnel lens]]es and larger for [[pancake lens]]es.

===Consequences of IPD Mismatch===

When a user's IPD does not match the headset's lens spacing, their pupils are positioned outside of this optimal eyebox. This mismatch is a direct cause of numerous negative physiological and perceptual effects.

====Physiological Effects====

* '''Eye Strain and Headaches''': This is the most common symptom. When the images presented to the eyes are misaligned, the extrinsic eye muscles must work harder to fuse them into a single image, leading to fatigue, discomfort, and headaches that can persist for hours after VR sessions.<ref name="MaeckerIPD">How IPD Affects Your VR Experience And How To Adjust It. Maecker VR. https://maeckervr.com/blogs/news/how-ipd-affects-your-vr-experience-and-how-to-adjust-it</ref><ref name="ValveIndexIPD">Is it dangerous to use an IPD lower than your IPD? Reddit r/ValveIndex. https://www.reddit.com/r/ValveIndex/comments/kzmnh3/is_it_dangerous_to_use_an_ipd_lower_than_your_ipd/</ref>

* '''[[Cybersickness]]''': IPD mismatch contributes significantly to visually induced motion sickness (VIMS), or cybersickness. The distorted visual input creates a sensory conflict that can induce symptoms like dizziness, nausea, and disorientation.<ref name="IPDMismatchXR">Causes and Consequences of IPD Mismatch in XR Devices. ResearchGate. https://www.researchgate.net/publication/382700047_16-2_Invited_Paper_Causes_and_Consequences_of_IPD_Mismatch_in_XR_Devices</ref><ref name="PimaxIPD">How to Measure Your IPD. Pimax. https://pimax.com/blogs/blogs/how-to-measure-your-ipd</ref>

* '''Unnatural Eye Fixations''': In cases where the headset's rendered IPD is wider than the user's actual IPD, trying to fixate on a distant virtual object can force the eyes into a divergent (outward-pointing) gaze. This is an unnatural and uncomfortable state for the human visual system and can lead to a complete breakdown of binocular fusion.<ref name="HibbardIPD">The implications of interpupillary distance variability for virtual reality. Paul Hibbard et al. https://peterscarfe.com/papers/Hibbard_etal_2020.pdf</ref>

====Perceptual Distortions====

* '''Blurriness and Reduced Clarity''': The most immediate consequence of being outside the optical sweet spot is a blurry or unfocused image, especially in the periphery. Text becomes difficult to read, and fine details are lost.<ref name="PimaxIPD"></ref><ref name="ViveProIPD">Adjusting the IPD on the headset. HTC Vive Support. https://www.vive.com/us/support/vive-pro-hmd/category_howto/adjusting-the-ipd-on-the-headset.html</ref>

* '''Double Vision (Diplopia)''': With a severe mismatch, the brain may be unable to fuse the two disparate images, resulting in the user perceiving a distracting double or ghosted image.<ref name="DCSForumIPD">In-game eye distance is wrong, how to correct? DCS Forums. https://forum.dcs.world/topic/163912-in-game-eye-distance-is-wrong-how-to-correct/</ref>

* '''Distorted Depth Perception''': Stereoscopic depth perception relies on the brain correctly interpreting binocular disparity based on its learned understanding of the user's own IPD. When the rendered disparity does not match this expectation, the perception of depth becomes distorted, and objects may appear closer or farther than intended.<ref name="HibbardIPD"></ref>

* '''Incorrect Sense of Scale''': A direct consequence of distorted depth perception is an incorrect [[sense of scale]]. If the rendered IPD is wider than the user's IPD, the world can feel miniaturized, like a "dollhouse". Conversely, if the rendered IPD is narrower, the world can feel gigantic.<ref name="RedditPSVRScale"></ref><ref name="KholdScale"></ref>

Research indicates that even small mismatches (e.g., 5 mm) can reduce visual acuity and comfort, particularly in high-resolution HMDs.<ref name="IEEEVR">The Implications of Interpupillary Distance Variability for Virtual Reality. IEEE Transactions on Visualization and Computer Graphics. DOI: 10.1109/TVCG.2020.3038537</ref> For AR, IPD affects overlay alignment with the real world, impacting tasks like surgical simulation or navigation.

==IPD in Various VR/AR Headsets==

Different VR/AR headsets have varying methods and ranges for IPD adjustment. The table below summarizes a number of devices and their IPD specifications:

{| class="wikitable sortable"

|+ IPD Adjustment Mechanisms and Ranges for Major VR/AR Headsets

! Headset !! Type !! Adjustment Method !! IPD Range (mm) !! Notes

|-

| [[Apple Vision Pro]] || AR/VR || Automatic Motorized || 51–75 || Uses eye tracking to automatically measure and adjust IPD; manual fine-tuning via Digital Crown<ref name="TechGearLab">Best VR Headset of 2024. TechGearLab. https://www.techgearlab.com/topics/cool-gadgets/best-vr-headset</ref>

|-

| [[Valve Index]] || Tethered VR || Continuous Mechanical Slider || 58–70 || Physical slider with on-screen readout; adjustable eye relief for FOV optimization<ref name="VRExpert-Index"></ref><ref name="ValveSoftware">Valve Index Headset. Valve Software. https://www.valvesoftware.com/index/headset</ref>

|-

| [[Meta Quest 3]] || Standalone VR || Continuous Mechanical Wheel || 58–70 (53–75 effective) || Scroll wheel adjustment; pancake lenses provide larger sweet spot<ref name="TechGearLab"></ref><ref name="RoadToVRQuest3">Ben Lang (Sep 27, 2023). "Quest 3 Hands-on – An Impressive Leap..." Road to VR. https://www.roadtovr.com/meta-quest-3-oculus-preview-connect-2023/</ref>

|-

| [[Meta Quest Pro]] || Standalone VR || Continuous Mechanical Slider || 55–75 || Free-moving slider; eye tracking for guided setup; pancake lenses<ref name="SimulaIPD"></ref><ref name="QuestProVRDB"></ref>

|-

| [[Meta Quest 2]] || Standalone VR || 3 Discrete Mechanical Presets || 58, 63, 68 || Three fixed positions; lenses manually pushed into place<ref name="MetaIPD"></ref><ref name="VRExpertQuest2">"How to change the IPD on the Oculus Quest 2" – VR Expert Knowledge Base (Dec 13, 2022). https://knowledge.vr-expert.com/kb/how-to-change-the-ipd-on-the-oculus-quest-2/</ref>

|-

| [[Meta Quest]] (Original) || Standalone VR || Continuous Mechanical Slider || 58–72 || Smooth slider adjustment<ref name="RoadToVR-RiftS"></ref>

|-

| [[Oculus Rift S]] || Tethered VR || Software Only || 63.5 fixed (58–72 recommended) || Single LCD panel; no hardware adjustment; software offset only<ref name="RoadToVR-RiftS"></ref><ref name="PalmerLuckey"></ref>

|-

| [[Oculus Rift]] (CV1) || Tethered VR || Continuous Mechanical Slider || 58–72 || Dual OLED displays; physical slider with electronic feedback<ref name="RoadToVR-RiftS"></ref><ref name="RedditRiftCV1">Oculus Rift CV1 IPD adjustment range. Reddit. https://www.reddit.com/r/oculus/comments/4decyk/oculus_rift_cv1_ipd_adjustment_range_59_mm_70_mm/</ref>

|-

| [[Oculus Go]] || Standalone VR || Software Only || ~63 fixed || No hardware adjustment<ref name="RoadToVR-RiftS"></ref>

|-

| [[HTC Vive]] (Original) || Tethered VR || Continuous Mechanical Knob || 60–74 || Rotating knob on side of headset<ref name="SimulaIPD"></ref><ref name="RedditViveIPD">PSA: Best way to set your IPD correctly. Reddit r/Vive. https://www.reddit.com/r/Vive/comments/4e9roo/psa_best_way_to_set_your_ipd_correctly/</ref>

|-

| [[HTC Vive Pro]] || Tethered VR || Continuous Mechanical Knob || 60–74 || Knob with on-screen display<ref name="WindowsCentralIndex">Valve Index vs. HTC Vive Pro: Which should you buy? Windows Central. https://www.windowscentral.com/valve-index-vs-htc-vive-pro</ref><ref name="ViveProIPD"></ref>

|-

| [[HTC Vive Pro 2]] || Tethered VR || Continuous Mechanical Knob || 57–72 || Similar mechanism to Vive Pro

|-

| [[HTC Vive XR Elite]] || Standalone VR || Automatic (with tracker) || Variable || Automatic IPD adjustment via eye tracking when Full Face Tracker installed

|-

| [[HTC Vive Focus 3]] || Standalone VR || Continuous Mechanical Dial || 57–72 || Dial adjustment with on-screen feedback<ref name="ViveFocus3IPD">Adjusting the IPD on the headset. HTC Vive Support. https://www.vive.com/us/support/focus3/category_howto/adjusting-the-ipd-on-the-headset.html</ref>

|-

| [[PlayStation VR2]] || Tethered VR || Continuous Mechanical Dial || 58–72 || Physical dial with eye-tracking assisted calibration

|-

| [[PlayStation VR]] (Original) || Tethered VR || Software Only || ~63 fixed || No hardware IPD adjustment; software adjustment 48–78 mm

|-

| [[HP Reverb G2]] || Tethered VR || Continuous Mechanical Slider || 60–68 (57.5–70.5 effective) || Linear lens spacing adjustment slider; narrower range than competitors

|-

| [[Pico 4]] || Standalone VR || Automatic Motorized || 62–72 (58–72 supported) || 21 increments in 0.5 mm steps; automatic adjustment via Quick Settings; pancake lenses

|-

| [[Pico 4 Enterprise]] || Standalone VR || Automatic Motorized || 62–72 (58–72 supported) || Eye tracking measures IPD automatically on headset wear

|-

| [[Pico Neo 3 Link]] || Standalone VR || 3 Discrete Mechanical Presets || 58, 63.5, 69 || Three fixed positions; physical manual adjustment

|-

| [[Varjo Aero]] || Tethered VR || Automatic Motorized || 57–73 || Eye-tracking with sub-millimeter accuracy; real-time adjustment<ref name="SimulaIPD"></ref>

|-

| [[Varjo XR-4]] || Tethered AR/VR || Automatic Motorized || 56–72 || Fully automatic; professional/enterprise market

|-

| [[Microsoft HoloLens|HoloLens 2]] || AR || Eye-Tracking Calibration || N/A (software adjust) || Fixed lenses; automatic software adjustment to each user<ref name="HoloLensCalib"></ref>

|-

| [[Samsung Gear VR]] || Mobile VR || Software Only || ~63 fixed || No hardware adjustment

|-

| [[Google Cardboard]] || Mobile VR || None || Fixed || Entirely fixed design; no adjustment

|}

===Population Coverage by IPD Range===

Different IPD ranges accommodate varying portions of the adult population:

* '''58–68 mm''': ~80% of adults

* '''55–70 mm''': ~90–95% of adults

* '''52–72 mm''': ~95–98% of adults (covers 99% of men, 93% of women)

* '''50–75 mm''': ~99%+ of adults

* '''60–65 mm''': Only 54.1% of adults

These statistics demonstrate why fixed-IPD headsets at 63.5 mm accommodate only 43–46% of adults "best," while mechanical adjustment spanning 58–72 mm accommodates 93–99% of the population.

==Hardware vs. Software IPD==

In VR/AR systems, "IPD" can refer to two distinct but related parameters: the physical alignment of the hardware and the rendering parameters of the software. An optimal experience requires both to be set correctly.

===Hardware IPD (Lens Spacing)===

'''Hardware IPD''' refers to the physical distance between the optical centers of the two lenses inside the HMD.<ref name="MetaIPD"></ref> 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">How do you calibrate VR systems to accommodate different interpupillary distances (IPD)? Milvus. https://milvus.io/ai-quick-reference/how-do-you-calibrate-vr-systems-to-accommodate-different-interpupillary-distances-ipd</ref>

Hardware adjustment provides optical alignment where eyes look through the optical center, achieving maximum clarity, full resolution and sharpness, designed field of view specifications, reduced eye strain, and physical comfort.

===Software IPD (Virtual Camera Separation)===

'''Software IPD''', also known as '''Inter-Camera Distance''' (ICD) or '''Virtual Camera Separation''' (VCS), is a parameter within the [[game engine]] or rendering software that defines the distance between the two virtual cameras used to generate the stereoscopic images for the left and right eyes.<ref name="UnityDynamicIPD"></ref><ref name="XinRealityIPD"></ref>

For a realistic, 1:1 scale representation of the virtual world, the Software IPD should be set to be identical to the user's real-world IPD.<ref name="KholdScale"></ref> However, software adjustment cannot fix optical misalignment regardless of rendering adjustments. It corrects perceived world scale and depth perception only within a narrow tolerance range, but clarity issues, blur, reduced FOV, and distortion remain if physically misaligned.

Ideally, the hardware IPD adjustment should automatically inform the software, so that when a user sets their lens spacing to 65 mm, the game engine also sets the virtual camera separation to 65 mm. However, this link is not always guaranteed. On some platforms, the hardware adjustment may only move the lenses without passing that value to the running application.<ref name="RedditSoftwareIPD">On the importance of software IPD and what is it. Reddit r/ValveIndex. https://www.reddit.com/r/ValveIndex/comments/1b81983/on_the_importance_of_software_ipd_and_what_is/</ref> This can create a subtle but disorienting mismatch where the image appears sharp (correct hardware IPD) but the world scale feels incorrect (mismatched software IPD).

==Related Optical and Perceptual Concepts==

IPD does not exist in isolation; it is deeply interconnected with other key principles of optics and human perception that define the VR/AR experience.

===Vergence-Accommodation Conflict===

The '''[[Vergence-accommodation conflict]]''' (VAC) is one of the most significant human factors challenges in current-generation HMDs. It describes a mismatch between two normally linked functions of the eye: vergence and accommodation.<ref name="VACWikipedia">Vergence-accommodation conflict. Wikipedia. https://en.wikipedia.org/wiki/Vergence-accommodation_conflict</ref>

* '''Vergence''' is the rotation of the eyes to converge on an object at a specific distance.

* '''Accommodation''' is the focusing of the crystalline lens in the eye to bring that object's image into sharp focus on the retina.

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.

This decoupling of vergence and accommodation creates a sensory conflict that can lead to eye strain, fatigue, and nausea.<ref name="VACTaylor">Vergence-accommodation conflict. Taylor & Francis. https://taylorandfrancis.com/knowledge/Engineering_and_technology/Computer_science/Vergence-accommodation_conflict/</ref> While an incorrect IPD setting does not cause VAC, it adds another layer of strain to the visual system, exacerbating the discomfort caused by the conflict. IPD mismatch creates a "double burden" effect, forcing optical compensation while simultaneously struggling with VAC from fixed-focus displays.

===Binocular Overlap===

'''Binocular overlap''' is the area of the [[field of view]] (FOV) that is visible to both eyes simultaneously. This overlapping region is where true stereoscopic vision occurs. HMD designers can manipulate the amount of overlap to trade off between the stereoscopic area and the total horizontal FOV.

* '''100% Overlap''': The left and right displays show the exact same field of view. This provides a robust stereoscopic image across the entire visual field but limits the total FOV to that of a single display.

* '''Partial Overlap''': The displays are canted slightly outwards so that each eye sees a portion of the visual field that the other eye does not. This increases the total combined horizontal FOV, enhancing immersion, but it reduces the area where stereopsis is possible.<ref name="RoadToVROverlap">Understanding Binocular Overlap and Why It's Important for VR Headsets. Road to VR. https://www.roadtovr.com/understanding-binocular-overlap-and-why-its-important-for-vr-headsets/</ref>

A potential downside is '''binocular rivalry''', a visual artifact where the brain has difficulty fusing the image at the edges of the overlapping zone. The user's IPD and the headset's IPD setting are critical in determining how this overlap is perceived.

===Eyebox and Optical Sweet Spot===

The '''eyebox''' is the three-dimensional volume within which a user's eye pupil must be located to see the entire, un-vignetted, and clear image produced by the HMD's lens.<ref name="OpticaEyebox">A perceptual eyebox for near-eye displays. Optica Publishing Group. https://opg.optica.org/abstract.cfm?uri=oe-28-25-38008</ref> The center of this volume, where image quality is highest, is often called the '''optical sweet spot'''.<ref name="RoadToVR-Measure"></ref>

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">VR Optics: Lenses, Displays, and Performance. Avantier Inc. https://avantierinc.com/resources/knowledge-center/how-vr-optics-work/</ref>

==Lens Design Impact on IPD Requirements==

===Fresnel Lenses===

[[Fresnel lens]]es use concentric circular ridges to maintain large aperture with reduced thickness, enabling wider potential FOV and lighter weight for easier physical IPD adjustment mechanisms. However, they create a small sweet spot requiring precise IPD alignment within ±1–2 mm tolerance. God rays (light artifacts from ridges) worsen when viewing off-axis, chromatic aberration (color fringing) increases with IPD mismatch, and edge blur increases rapidly outside the optical center.

Headsets using Fresnel lenses include [[Valve Index]], [[Meta Quest 2]], [[PlayStation VR2]], [[HP Reverb G2]], and [[HTC Vive]] series.

===Pancake Lenses===

Pancake lenses use a folded optical path with polarizers and beam splitters, bouncing light multiple times within the lens assembly. This reduces thickness by 40–50% compared to Fresnel and creates a significantly larger sweet spot more forgiving of IPD mismatch (±2–3 mm tolerance). They provide better edge-to-edge clarity with more uniform sharpness across FOV, minimal god rays nearly eliminated by design, reduced chromatic aberration for better color accuracy, and a compact form factor enabling lighter, more comfortable headsets.

The trade-offs include 25–30% light loss through polarization requiring brighter displays with higher power consumption, potential ghosting from light bouncing creating faint double images, and slightly reduced FOV (typically 5–10° less than equivalent Fresnel design).

Headsets using pancake lenses include [[Meta Quest 3]], [[Meta Quest Pro]], [[Pico 4]], [[Apple Vision Pro]], [[Bigscreen Beyond]], and [[HTC Vive XR Elite]].

===IPD Tolerance Comparison by Lens Type===

* Simple convex: HIGH tolerance (±4 mm)

* Fresnel: LOW tolerance (±1–2 mm)

* Pancake: MEDIUM-HIGH tolerance (±2–3 mm)

* Aspheric: MEDIUM tolerance (±2 mm)

The industry is transitioning from Fresnel to pancake lenses specifically to improve IPD tolerance, reduce weight, and enhance comfort while maintaining image quality.

==Field of View Relationship to IPD==

[[Field of view]] (FOV) is measured from eye position to the edge of the visible display area. IPD settings change the effective eye position relative to lenses, influencing the achievable FOV. Wider IPD positions eyes further from display center, reducing inner binocular overlap but potentially increasing outer FOV. Narrower IPD increases binocular overlap but may reduce peripheral vision.

For example, on the [[Meta Quest 2]], FOV varies by IPD setting:

* Setting 1 (58 mm): ~92–94° horizontal

* Setting 2 (63 mm): ~89–91° horizontal

* Setting 3 (68 mm): ~86–89° horizontal

* Total variation: ±4–6° depending on IPD setting

The [[Valve Index]] achieves its advertised ~130° FOV through close eye relief (adjustable lens-to-eye distance), 5° canted lenses angling outward, large lens diameter, and proper IPD alignment. The canted display design optimizes the interior versus outer FOV trade-off.

Distance from lens to eye dramatically affects FOV, with every 1 mm closer adding approximately 2–3° FOV. IPD must be correct for eyes to align with the sweet spot at optimal distance, as IPD misalignment can limit FOV even with close eye relief.

==Historical Evolution of IPD Adjustment==

===Early VR Era (1960s–1990s)===

Early head-mounted displays like the 1960 Telesphere Mask and 1968 Sword of Damocles by [[Ivan Sutherland]] featured fixed lens spacing with no IPD consideration. The 1993 [[Sega VR]] and 1995 [[Nintendo Virtual Boy]] both had fixed IPD at approximately 63 mm, with lack of IPD adjustment cited as a comfort issue contributing to market failures.

By the late 1990s, research documented IPD variance in the 42–75 mm range with means of 61 mm (women) and 63 mm (men). IPD mismatch was identified as a major comfort issue, though technology and cost constraints prevented affordable adjustable optics.

===Modern VR Renaissance (2010–2016)===

Palmer Luckey's 2010 prototype demonstrated revolutionary wide FOV and proved market demand. The 2012 [[Oculus Rift]] DK1 Kickstarter raised $2.4 million, offering the headset for $300 with fixed lens separation and software IPD adjustment only. The 2014 [[Oculus Rift DK2]] remained fixed at 63.5 mm physically with software adjustment only, generating major complaints from users outside the 63.5 mm ±3 mm range.

The industry realized between 2014–2015 that consumer VR needs adjustable IPD. ANSUR II dataset analysis showed fixed 63.5 mm fits only ~45% of adults "best," launching a race for affordable mechanical IPD adjustment.

===Breakthrough Year: 2016===

'''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.

The significance of 2016 established the industry standard where mechanical IPD adjustment became expected, making VR a viable consumer product category.

===The Regression: Cost-Cutting Era (2019–2020)===

'''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"></ref>

'''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.

===Innovation and Automation Era (2021–Present)===

'''Varjo XR-4/Aero (2021–2022)''' introduced automatic IPD where eye tracking automatically detects IPD across 56–72 mm range with sub-millimeter accuracy, targeting enterprise/professional markets.

'''Meta Quest Pro (October 2022)''' launched with eye tracking and continuous mechanical adjustment across 55–75 mm.

'''Apple Vision Pro (February 2024)''' at $3,499 provides automatic IPD adjustment via advanced eye tracking with no user intervention required, following the seamless "it just works" design philosophy.

'''Meta Quest 3 (October 2023)''' features continuous mechanical wheel adjustment across 58–70 mm with an effective range of 53–75 mm due to pancake lens sweet spot characteristics.

Future directions point toward automatic IPD becoming standard as eye-tracking costs decrease, wider ranges targeting 50–80 mm, per-eye independent adjustment, and AI-driven optimization learning user preferences.

==Applications in Eyewear and Optical Instruments==

The principle of aligning optics with the user's IPD is fundamental to traditional optical instruments beyond VR/AR.

===Eyeglasses===

When fabricating prescription [[eyeglasses]], the [[optician]] must ensure that the optical center of each lens is precisely aligned with the center of the pupil it serves. If the PD is incorrect, the wearer will be looking through a part of the lens that is not the optical center, which induces an unwanted prismatic effect. This forces the eyes to work harder to fuse the images, leading to symptoms such as eye strain, headaches, blurred vision, double vision (diplopia), and a general inability to comfortably wear the glasses.<ref name="OptoCanada">Interpupillary Distance (PD). Opto.ca. https://opto.ca/eye-health-library/interpupillary-distance-pd</ref>

===Binoculars and Microscopes===

Binocular devices are designed to be used by many different people, so they require an adjustable mechanism to accommodate the wide range of human IPDs. Most [[binoculars]] use a central hinge that allows the two barrels to be folded inward or outward.<ref name="SWOpticsBinoculars">Why Interpupillary Distance (IPD) Matters When Choosing Binoculars. SW Optics. https://swoptics.co.uk/blogs/news/interpupillary-distance-binocular-fit</ref> The user adjusts this hinge while looking at a distant object until the two separate circular images they see merge into a single, perfectly round, and comfortable view.<ref name="OberwerkIPD">Adjusting IPD (Inter-Pupillary Distance). Oberwerk. https://oberwerk.com/support-article/adjusting-ipd-inter-pupillary-distance/</ref> If the IPD is set incorrectly, the user may see two overlapping circles, or the view may be partially obscured by black, crescent-shaped artifacts, leading to significant eye strain and a poor viewing experience.

==Best Practices for VR Usage==

To minimize discomfort and ensure optimal visual quality in VR/AR experiences:

* '''Measure IPD accurately''' within ±2 mm using professional methods, smartphone apps, or DIY techniques

* '''Set headset IPD''' before long sessions, adjusting until maximum clarity is achieved

* '''Follow the 20-20-20 rule''': Every 20 minutes, look 20 feet away for 20 seconds

* '''Start with shorter sessions''' of 20–30 minutes, especially when new to VR

* '''Clean lenses regularly''' to maintain optical clarity

* '''Use ambient lighting''' rather than pitch-black rooms to reduce eye strain

* '''Blink consciously''' to counter reduced natural blink rate in VR

* '''Take breaks immediately''' if discomfort occurs

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.

==See Also==

* [[Binocular vision]]

* [[Stereopsis]]

* [[Vergence-accommodation conflict]]

* [[Cybersickness]]

* [[Head-mounted display]]

* [[Field of view]]

* [[Eye tracking]]

* [[Sense of scale]]

* [[Anthropometry]]

* [[Pupillometer]]

==References==

~~{{Reflist}}~~

<ref name="Dodgson2004">Neil A. Dodgson (2004). "Variation and extrema of human interpupillary distance." Proceedings of SPIE Vol. 5291: Stereoscopic Displays and VR Systems XI, pp. 36–46. DOI: 10.1117/12.529999. https://www.researchgate.net/publication/229084829_Variation_and_extrema_of_human_interpupillary_distance</ref>

<ref name="ClevelandClinic">Pupillary Distance: What It Means & How To Measure. Cleveland Clinic (health library article). https://my.clevelandclinic.org/health/articles/pupillary-distance</ref>

<ref name="WebMD">Pupillary Distance: Types and How to Measure. WebMD. https://www.webmd.com/eye-health/pupillary-distance</ref>

<ref name="RoadToVR-Measure">Ben Lang (Dec 5, 2023). "How to Measure Your IPD and Why It's Important for VR & AR Headsets." Road to VR. https://www.roadtovr.com/how-to-measure-ipd-vr-headset-ar-iphone-app/</ref>

<ref name="RoadToVR-RiftS">Ben Lang (Mar 29, 2019). "Everything We Know (Officially) About the FOV and IPD of Rift S & Quest." Road to VR. https://www.roadtovr.com/oculus-rift-s-supported-ipd-range-fov-quest-go/</ref>

<ref name="ImmersipediaMirror">"How to measure your IPD with a mirror" – MTBS3D Immersipedia (archived). https://web.archive.org/web/20140328140034/http://www.mtbs3d.com/wiki/index.php?title=How_to_measure_your_IPD_with_a_mirror</ref>

<ref name="VRExpert-Quest2">"How to change the IPD on the Oculus Quest 2" – VR Expert Knowledge Base (Dec 13, 2022). https://knowledge.vr-expert.com/kb/how-to-change-the-ipd-on-the-oculus-quest-2/</ref>

<ref name="VRExpert-Index">"What is the IPD range of the Valve Index?" – VR Expert Knowledge Base (April 21, 2023). https://knowledge.vr-expert.com/kb/what-is-the-ipd-range-of-the-valve-index/</ref>

<ref name="SimulaIPD">George Singer (Nov 13, 2022). "Simula One's Auto-IPD System." SimulaVR Blog. https://simulavr.com/blog/simula-one-auto-ipd-adjustment/</ref>

<ref name="KholdScale">K. Kholdstare (Oct 6, 2013). "A Sense of Scale in VR." (Technical blog article). https://kholdstare.github.io/technical/2013/10/06/sense-of-scale-vr.html</ref>

<ref name="DCSWorldScale">Forum discussion: "IPD and World Scale" – DCS (Digital Combat Simulator) Forums, April 2022. (Explains how altering in-game IPD/ICD affects perceived world scale.) https://forum.dcs.world/topic/299470-ipd-and-world-scale/</ref>

<ref name="QuestProVRDB">"Meta Quest Pro" – Specifications (IPD range 55–75 mm, hardware adjustable). VRDB (VR/AR database). https://vrdb.app/headset/meta-quest-pro</ref>

<ref name="RoadToVRQuest3">Ben Lang (Sep 27, 2023). "Quest 3 Hands-on – An Impressive Leap That's Still Held Back by Software Struggles." Road to VR. (Includes Quest 3 IPD details: continuous 58–70 mm dial; supports ~53–75 mm IPD range given lens design.) https://www.roadtovr.com/meta-quest-3-oculus-preview-connect-2023/</ref>

<ref name="HoloLensCalib">Microsoft Docs: "Calibrating your HoloLens 2" – HoloLens 2 uses eye-tracking to adjust to the user's IPD for correct visual alignment and comfort. (Microsoft Learn, updated 2022.) https://learn.microsoft.com/hololens/hololens-calibration</ref>

<ref name="DistanceNearPD">Distance PD vs Near PD: Differences, Importance, and How to Measure. Low Cost Glasses. https://lowcostglasses.co.uk/distance-pd-vs-near-pd/</ref>

<ref name="MetaIPD">Learn about IPD and lens spacing on Meta Quest. Meta Support. https://www.meta.com/help/quest/261777072346131/</ref>

<ref name="FittingBox">Is PD Measurement measures Distance or Near PD? FittingBox Help Center. https://fittingbox.com/en/resources/help-center/is-pd-measurement-measures-distance-or-near-pd</ref>

<ref name="ZenniPD">How to Measure Your Pupillary Distance (PD). Zenni Optical. https://www.zennioptical.com/measuring-pd-infographic</ref>

<ref name="JournalOptometry">Agreement and inter-session repeatability of manual and automatic pupillometers for the measurement of interpupillary distance. Journal of Optometry. https://www.journalofoptometry.org/en-agreement-inter-session-repeatability-manual-automatic-articulo-resumen-S1888429620301205</ref>

<ref name="AutomatedPupillometer">The Reliability, Validity, and Normative Data of an Automated Pupillometer. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC5497600/</ref>

<ref name="StereoscopicVR">What is Stereoscopic VR Technology? Draw & Code. https://drawandcode.com/learning-zone/what-is-stereoscopic-vr-technology/</ref>

<ref name="ANSUR">2012 Anthropometric Survey of U.S. Army Personnel (ANSUR II) – reported mean distance IPD ~64 mm for men and ~62 mm for women, with 95% of adults between approximately 55 mm and 72 mm.</ref>

<ref name="TurkishIPD">Evaluation of interpupillary distance in the Turkish population. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC4529256/</ref>

<ref name="PakistaniIPD">Interpupillary, Inner Canthal and Outer Canthal Distance in the Pakistani Population. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC6408655/</ref>

<ref name="PalmerLuckey">Palmer Luckey (Apr 21, 2019). "I can't use Rift S, and neither can you." https://palmerluckey.com/i-cant-use-rift-s-and-neither-can-you/</ref>

<ref name="XinRealityIPD">Interpupillary distance. XinReality Wiki. https://xinreality.com/wiki/Interpupillary_distance</ref>

<ref name="RedditPSVRScale">Does the IPD (interpupillary distance) setting affect the sense of scale in games? Reddit r/PSVR. https://www.reddit.com/r/PSVR/comments/12wgxpj/does_the_ipd_interpupillary_distance_setting/</ref>

<ref name="UnityDynamicIPD">Dynamic IPD & Out Of Memory Problem. Meta Community Forums. https://communityforums.atmeta.com/discussions/dev-unity/dynamic-ipd--out-of-memory-problem/179101</ref>

<ref name="WikiHow">How to Measure Your Interpupillary Distance. wikiHow. https://www.wikihow.com/Measure-Your-Interpupillary-Distance</ref>

<ref name="SmartphoneApps">Comparing the Effectiveness of Smartphone Applications in the Measurement of Interpupillary Distance. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC10389117/</ref>

<ref name="VarjoIPD">IPD definition and explanation. Varjo Support. https://support.varjo.com/hc/en-us/ipd</ref>

<ref name="EyeboxVR">VR HMD eye box and sweet spot. https://blekenbleu.github.io/XR/eyespot.htm</ref>

<ref name="MaeckerIPD">How IPD Affects Your VR Experience And How To Adjust It. Maecker VR. https://maeckervr.com/blogs/news/how-ipd-affects-your-vr-experience-and-how-to-adjust-it</ref>

<ref name="ValveIndexIPD">Is it dangerous to use an IPD lower than your IPD? Reddit r/ValveIndex. https://www.reddit.com/r/ValveIndex/comments/kzmnh3/is_it_dangerous_to_use_an_ipd_lower_than_your_ipd/</ref>

<ref name="IPDMismatchXR">Causes and Consequences of IPD Mismatch in XR Devices. ResearchGate. https://www.researchgate.net/publication/382700047_16-2_Invited_Paper_Causes_and_Consequences_of_IPD_Mismatch_in_XR_Devices</ref>

<ref name="PimaxIPD">How to Measure Your IPD. Pimax. https://pimax.com/blogs/blogs/how-to-measure-your-ipd</ref>

<ref name="HibbardIPD">The implications of interpupillary distance variability for virtual reality. Paul Hibbard et al. https://peterscarfe.com/papers/Hibbard_etal_2020.pdf</ref>

<ref name="ViveProIPD">Adjusting the IPD on the headset. HTC Vive Support. https://www.vive.com/us/support/vive-pro-hmd/category_howto/adjusting-the-ipd-on-the-headset.html</ref>

<ref name="DCSForumIPD">In-game eye distance is wrong, how to correct? DCS Forums. https://forum.dcs.world/topic/163912-in-game-eye-distance-is-wrong-how-to-correct/</ref>

<ref name="IEEEVR">The Implications of Interpupillary Distance Variability for Virtual Reality. IEEE Transactions on Visualization and Computer Graphics. DOI: 10.1109/TVCG.2020.3038537</ref>

<ref name="TechGearLab">Best VR Headset of 2024. TechGearLab. https://www.techgearlab.com/topics/cool-gadgets/best-vr-headset</ref>

<ref name="ValveSoftware">Valve Index Headset. Valve Software. https://www.valvesoftware.com/index/headset</ref>

<ref name="RedditRiftCV1">Oculus Rift CV1 IPD adjustment range. Reddit. https://www.reddit.com/r/oculus/comments/4decyk/oculus_rift_cv1_ipd_adjustment_range_59_mm_70_mm/</ref>

<ref name="RedditViveIPD">PSA: Best way to set your IPD correctly. Reddit r/Vive. https://www.reddit.com/r/Vive/comments/4e9roo/psa_best_way_to_set_your_ipd_correctly/</ref>

<ref name="WindowsCentralIndex">Valve Index vs. HTC Vive Pro: Which should you buy? Windows Central. https://www.windowscentral.com/valve-index-vs-htc-vive-pro</ref>

<ref name="ViveFocus3IPD">Adjusting the IPD on the headset. HTC Vive Support. https://www.vive.com/us/support/focus3/category_howto/adjusting-the-ipd-on-the-headset.html</ref>

<ref name="MilvusIPD">How do you calibrate VR systems to accommodate different interpupillary distances (IPD)? Milvus. https://milvus.io/ai-quick-reference/how-do-you-calibrate-vr-systems-to-accommodate-different-interpupillary-distances-ipd</ref>

<ref name="RedditSoftwareIPD">On the importance of software IPD and what is it. Reddit r/ValveIndex. https://www.reddit.com/r/ValveIndex/comments/1b81983/on_the_importance_of_software_ipd_and_what_is/</ref>

<ref name="VACWikipedia">Vergence-accommodation conflict. Wikipedia. https://en.wikipedia.org/wiki/Vergence-accommodation_conflict</ref>

<ref name="VACTaylor">Vergence-accommodation conflict. Taylor & Francis. https://taylorandfrancis.com/knowledge/Engineering_and_technology/Computer_science/Vergence-accommodation_conflict/</ref>

<ref name="RoadToVROverlap">Understanding Binocular Overlap and Why It's Important for VR Headsets. Road to VR. https://www.roadtovr.com/understanding-binocular-overlap-and-why-its-important-for-vr-headsets/</ref>

<ref name="OpticaEyebox">A perceptual eyebox for near-eye displays. Optica Publishing Group. https://opg.optica.org/abstract.cfm?uri=oe-28-25-38008</ref>

<ref name="AvantierVR">VR Optics: Lenses, Displays, and Performance. Avantier Inc. https://avantierinc.com/resources/knowledge-center/how-vr-optics-work/</ref>

<ref name="OptoCanada">Interpupillary Distance (PD). Opto.ca. https://opto.ca/eye-health-library/interpupillary-distance-pd</ref>

<ref name="SWOpticsBinoculars">Why Interpupillary Distance (IPD) Matters When Choosing Binoculars. SW Optics. https://swoptics.co.uk/blogs/news/interpupillary-distance-binocular-fit</ref>

<ref name="OberwerkIPD">Adjusting IPD (Inter-Pupillary Distance). Oberwerk. https://oberwerk.com/support-article/adjusting-ipd-inter-pupillary-distance/</ref>

</references>

[[Category:Terms]]

[[Category:Human visual system]]

[[Category:Virtual reality hardware]]

[[Category:Augmented reality]]

[[Category:Anthropometry]]

[[Category:Ophthalmology]]

[[Category:Optometry]]