Modern helmet-mounted night vision devices, such as the Thales TopOwl helmet, project imagery from intensifiers
mounted on the side of the helmet onto the helmet faceplate. The increased separation of the cameras induces
hyperstereopsis - the exaggeration of the stereoscopic disparities that support the perception of relative depth around the
point of fixation. Increased camera separation may also affect absolute depth perception, because it increases the amount
of vergence (crossing) of the eyes required for binocular fusion, and because the differential perspective from the
viewpoints of the two eyes is increased. The effect of hyperstereopsis on the perception of absolute distance was
investigated using a large-scale stereoscopic display system. A fronto-parallel textured surface was projected at a
distance of 6 metres. Three stereoscopic viewing conditions were simulated - hyperstereopsis (four times magnification),
normal stereopsis, and hypostereopsis (one quarter magnification). The apparent distance of the surface was measured
relative to a grid placed in a virtual "leaf room" that provided rich monocular cues, such as texture gradients and linear
perspective, to absolute distance as well as veridical sterescopic disparity cues. The different stereoscopic viewing
conditions had no differential effect on the apparent distance of the textured surface at this viewing distance.
The side mounting of the night-vision sensors on some helmet-mounted systems creates a situation of hyperstereopsis in
which the binocular cues available to the operator are exaggerated such that distances around the point of fixation are
increased. For a moving surface approaching the observer, the increased apparent distance created by hyperstereopsis
should result in greater apparent speed of approach towards the surface and so an operator will have the impression they
have reached the surface before contact actually occurs. We simulated motion towards a surface with hyperstereopsis
and compared judgements of time to contact with that under normal stereopsis as well as under binocular viewing
without stereopsis. We simulated approach of a large, random-field textured and found that time to contact estimates
were shorter under the hyperstereoscopic condition than those under normal stereo and no stereo, indicating that
hyperstereopsis may cause observers to underestimate time to contact leading operators to undershoot the ground plane
when landing.
Modern helmet-mounted night vision devices, such as the Thales TopOwl helmet, project imagery from intensifiers
mounted on the sides of the helmet onto the helmet faceplate. This produces a situation of hyperstereopsis in which
binocular disparities are magnified. This has the potential to distort the perception of slope in depth (an important cue to
landing), because the slope cue provided by binocular disparity conflicts with veridical cues to slope, such as texture
gradients and motion parallax. In the experiments, eight observers viewed sparse and dense textured surfaces tilted in
depth under three viewing conditions: normal stereo hyper-stereo (4 times magnification), and hypostereo (1/4
magnification). The surfaces were either stationary, or rotated slowly around a central vertical axis. Stimuli were
projected at 6 metres to minimise conflict between accommodation and convergence, and stereo viewing was provided
by a Z-screen and passive polarised glasses. Observers matched perceived visual slope using a small tilt table set by
hand. We found that slope estimates were distorted by hyperstereopsis, but to a much lesser degree than predicted by
disparity magnification. The distortion was almost completely eliminated when motion parallax was present.
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