Hardware & Display Technology

Head-Mounted Displays (HMDs) are worn on the body; Head-Tracked Displays (HTDs) are fixed in the room.

HMDs — Oculus Rift, Oculus Quest, HTC Vive (Pro), Valve Index, Windows Mixed Reality — move with the user's head, so the display itself must be tracked. HTDs — single-screen setups, surround-screen rooms, workbenches, arbitrary-surface projections, and autostereoscopic displays — stay in a fixed location, so instead the user's head must be tracked to keep the perspective correct.

Display technologies include liquid crystals (LCD), light-emitting diodes (LED/OLED), digital light processing (DLP), and older CRTs — each with a different way of turning an electrical signal into light.

Spatial resolution is a measure of visual quality, and depends on both the number of pixels and the size of the screen.

It's commonly measured in dots per square inch (dpi) or cycles per degree — the latter matters more for HMDs, since the same pixel count spread over a wider field of view looks less sharp.

Refresh rate (Hz) is how fast a display refreshes the image already in its buffer.

It's commonly confused with frame rate, which is how fast new images are generated and placed into that buffer in the first place — a display can refresh at 90Hz while the application only manages 45 new frames per second.

Grounding is the point of contact between the display screen and the world and/or the participant.

Two simultaneous visual display channels with separate views are required for stereopsis.

Screens come in a variety of shapes — rectangular, circular, L-shaped, hemispherical, and hybrids. Projection mapping extends this further, supporting display on any surface at all.

Field of view is the maximum visual angle seen instantaneously; field of regard is the total physical space around the user in which images can be displayed.

An HMD can offer a 100% field of regard simply by covering the eyes wherever the head turns — a fixed projection screen cannot, since turning away from it exposes empty space.

Focal distance is the apparent optical distance of the images from the viewer's eyes.

With most current technology, all images sit on the same focal plane regardless of their intended virtual distance from the viewer — a major source of the accommodation-convergence mismatch covered later in this unit.

Latency is the delay between user movements and display updates — the "motion to photon" delay every VR system has some amount of.

High latency is stressful for the viewer's perceptual system, and can manifest as nausea or headaches.

Images display directly in front of the user's eyes on one or two small screens; refractive lenses and/or mirrors magnify them into a larger virtual image.

A Fresnel lens is a concentric series of simple lens sections — thin, with a short focal length and large diameter.

This gives more even resolution distribution and less distortion than a single thick lens, at a fraction of the weight, which is why most consumer HMDs use them.

Spherical aberration distorts the shape of the image — straight lines render curved — so the graphics system must pre-distort the image to compensate.

Chromatic aberration warps different color wavelengths by different angles, creating a ghost-like fringe where one color continues past an object's edge.

It's most visible at the edges of the lens and least visible in the center of the view.

LEEP (Large Expanse, Extra Perspective) optics trade a very wide field of view and higher central resolution for lower peripheral resolution and intentional pincushion distortion.

If the image source sits exactly at the lens's focal point, the virtual image appears at optical infinity — useful for flight simulators, where distant objects should stay in focus without eye strain.

Optical see-through displays place partially transparent, partially reflective combiners in front of the eyes.

These give a direct, full-resolution, zero-delay view of the real world, but registration problems between real and virtual content are highly visible.

Video see-through displays instead stream real-time video from head-mounted cameras into the graphics pipeline.

This is easier to achieve with a regular VR HMD and supports a wider field of view, but the real world typically ends up at lower visual quality and with added latency.

Immersive displays should be comfortable, unobtrusive, and lightweight. Mobility — whether a tethered cable requires management, or a headset is fully untethered — directly affects both immersion and usefulness.

The type of visual display can also influence integrated motion tracking — tracking systems with a limited operating range or that require cables will restrict the user's movement.

Light pollution — excess visible light from an external or internal source — can interfere with the visual presentation of the virtual world, though most consumer headsets seal tightly enough against the face that it's a non-issue. Projection-based displays, by contrast, generally need to limit external light to maintain contrast, and the size of the physical space becomes a major factor for any room-scale display.

Portability ranges from a folding handheld stereoscope to a room-scale rig that takes a team hours to assemble.

Head-mounted displays are inherently single-user; projection-based systems can support multiple people physically present, but only one can be the "driver" who receives the geometrically correct perspective — everyone else sees a distorted view. Safety-wise, the most obvious HMD risk is tripping or colliding with real-world objects while unable to see them, and eye fatigue or nausea afterward is common enough that users are advised not to drive immediately after a session.

IPD is the horizontal distance between a user's eyes — a display set too high causes eye strain and fusion problems; too low reduces the stereoscopic volume.

Always adjust the IPD setting when putting on a VR headset.

Fidelity is the degree to which a display's sensory stimuli correspond to what would be present in the real world.

It's more precise and quantifiable than the broad concept of "immersion," comprising many components — spatial resolution, FOV, FOR, and more — and is a useful lens for picking the right display for a given application.

In a head-tracked display, the screen sits at a fixed location in physical space, so head tracking is required to keep imagery perspective-correct as the viewer moves.

To render a single view in almost any graphics software, you must specify an eyepoint, a look-at point, a view-up direction, and either a field of view (for HMDs) or the location of the projection plane (for HTDs).

Because focal distance is usually fixed at the screen while convergence changes with a virtual object's rendered depth, the eyes' focus and their aim disagree — the accommodation-convergence mismatch behind much of VR's eye strain.

Linear polarization causes ghosting when the head tilts, since the two views only partially separate, and cuts brightness by about half. Circular polarization avoids the tilt-dependent ghosting at the cost of further brightness and crispness. Linear polarization has an added problem in multi-screen rooms: a floor image can't stay aligned with both the front screen and the side screens at once.