Optical see-through still shows the real world on power loss; video see-through blinds

What it is

The frame the battery hits zero, optical see-through still shows the corridor through the lenses; video see-through becomes two black panels. The optical path does not need power to carry real photons; the video path seals the eyes inside a shell whose picture is computed. The distinction is whether the world remains visible on failure (fail-safe see-through): OST after a power cut, crash, or driver death is still a pair of sunglasses; VST on the same event is a locked blindfold.

“Still visible” is not a quality win. It is a different failure mode.

Why it happens

An OST combiner is passive. When the microdisplay dies, real light keeps its path; at worst the virtual layer is gone and contrast drops through the coatings. VST cuts that path: an opaque housing plus a single camera–display visual chain. Camera stop, compositor death, display-driver reset, battery-protection trip — any one of the four leaves no scene on the retina. Meeting that frame while moving is sudden blindfolding.

Doffing takes time. Clasps, a rear pad, a cable, hair in the strap stretch “black screen to seeing” into seconds. During those seconds the vestibular system still reports motion and vision is zero; balance and obstacle avoidance run blind. This is not slightly higher latency. It is a hard cut of the visual channel.

Studying it

Measure time-to-reseeing: cut power or kill the passthrough process during walking or reaching, and record time until the naked eye sees the environment, plus collisions, missed steps, and empty grasps in between. Compare non-flip VST, flip-up VST, and OST.

Events: battery exhaustion, watchdog reset, compositor freeze. Records: reseeing time, collision count, peak body sway, whether people reach to doff rather than to a safety referent.

Seated, stationary, assisted lab sessions systematically understate the risk. Test standing, walking, alone, with an unannounced cut.

Where it stops holding

A VST that flips up or doffs in one motion turns blinding from “locked” into “needs an action.” Dark-waveguide OST in a very dark room is almost as blind after power loss; fail-safety is then limited by ambient light. Fully enclosed VR headsets have no passthrough to fall back to; blinding is their default and is managed with a spotter, boundary systems, and a ban on moving — not by borrowing OST fail-safety. Visual interruptions from sleep, seizure, or a sudden blood-pressure drop sit outside the device failure model, but they stack with a black screen in the wearer’s experience.

Applying it

• Enclosed video see-through used for walking, stairs, or a shop floor must offer a flip-up or quick-release completable in about a second, taught in first use.
• At a battery threshold, drop to passthrough-only and kill the virtual layer first. Do not lose passthrough and UI on the same last frame.
• A compositor watchdog timeout should cut to camera-direct, not freeze on the last virtual frame — a frozen virtual layer keeps blocking the path.
• How to check: pull the battery or kill passthrough while walking through a doorway. OST should still show the frame; VST should flip or doff within the budgeted seconds and clear the frame. Still taking a step inside a black screen past that budget means the failure path is unfinished.

Related

• Same group：N5.08.1 In optical see-through, virtual content can only add light and cannot truly occlude real objects · N5.08.2 Video see-through re-digitizes the real world, so a virtual object can occlude a real one · N5.08.3 Camera position is offset from the eye and needs extra correction · N5.08.4 Video see-through still undershoots the eye in dynamic range and resolution; highlights and shadows both lose
• Nearby：N5.01 See-through Modes · N5.02 Virtual–Real Occlusion · N5.13 Safety Risks from Occluding Reality
• Search terms：fail-safe see-through · power-loss blindness · passthrough failure