A flight study was conducted to assess the impact of hyperstereopsis on helicopter handling proficiency, workload and
pilot acceptance. Three pilots with varying levels of night vision goggle and hyperstereo helmet-mounted display
experience participated in the test. The pilots carried out a series of flights consisting of low-level maneuvers over a
period of two weeks. Four of the test maneuvers, The turn around the tail, the hard surface landing, the hover height
estimation and the tree-line following were analysed in detail. At the end of the testing period, no significant difference
was observed in the performance data, between maneuvers performed with the TopOwl helmet and maneuvers
performed with the standard night vision goggle. This study addressed only the image intensification display aspects of
the TopOwl helmet system. The tests did not assess the added benefits of overlaid symbology or head slaved infrared
camera imagery. These capabilities need to be taken into account when assessing the overall usefulness of the TopOwl
system. Even so, this test showed that pilots can utilize the image intensification imagery displayed on the TopOwl to
perform benign night flying tasks to an equivalent level as pilots using ANVIS. The study should be extended to
investigate more dynamic and aggressive low level flying, slope landings and ship deck landings. While there may be
concerns regarding the effect of hyperstereopsis on piloting, this initial study suggests that pilots can either adapt or
compensate for hyperstereo effects with sufficient exposure and training. Further analysis and testing is required to
determine the extent of training required.
Helmet-mounted display (HMD) designs have faced persistent
head-supported mass and center of mass (CM) problems,
especially HMD designs like night vision goggles (NVG) that utilize image intensification (I2) sensors mounted forward
in front of the user's eyes. Relocating I2 sensors from the front to the sides of the helmet, at or below the transverse plane
through the user's head CM, can resolve most of the CM problems. However, the resulting increase in the separation
between the two I2 channels effectively increases the user's interpupillary distance (IPD). This HMD design is referred to
as a hyperstero design and introduces the phenomenon of hyperstereopsis, a type of visual distortion where stereoscopic
depth perception is exaggerated, particularly at distances under 200 feet (~60 meters). The presence of hyperstereopsis
has been a concern regarding implementation of hyperstereo HMDs for rotary-wing aircraft. To address this concern, a
flight study was conducted to assess the impact of hyperstereopsis on aircraft handling proficiency and pilot acceptance.
Three rated aviators with differing levels of I2 and hyperstereo HMD experience conducted a series of flights that
concentrated on low-level maneuvers over a two-week period. Initial and final flights were flown with a standard issue
I2 device and a production hyperstereo design HMD. Interim flights were flown only with the hyperstereo HMD. Two
aviators accumulated 8 hours of flight time with the hyperstereo HMD, while the third accumulated 6.9 hours. This paper
presents data collected via written questionnaires completed by the aviators during the post-flight debriefings. These data
are compared to questionnaire data from a previous flight investigation in which aviators in a copilot capacity, hands not
on the flight controls, accumulated 8 flight hours of flight time using a hyperstereo HMD.
KEYWORDS: Visualization, Sensors, Night vision, Head-mounted displays, Eye, Visual system, Night vision systems, Night vision goggles, 3D modeling, Defense and security
Modern helmet-mounted night vision devices, such as the Thales TopOwlTM helmet, project imagery from intensifiers
mounted on the sides of the helmet onto the helmet visor. This increased effective inter-ocular separation distorts several
cues to depth and distance that are grouped under the term "hyperstereopsis". Stereoscopic depth perception, at near to
moderate distances (several hundred metres), is subject to magnification of binocular disparities. Absolute distance
perception at near distances (a few metres) is affected by increased "differential perspective" as well as an increased
requirement for convergence of the eyes to achieve binocular fixation. These distortions result in visual illusions such as
the "bowl effect" where the ground appears to rise up near the observer. Previous reports have indicated that pilots can
adapt to these distortions after several hours of exposure. The present study was concerned with both the time course and
the mechanisms involved in this adaptation. Three test pilots flew five sorties with a hyperstereo night vision device.
Initially, pilots reported that they were compensating for the effects of hyperstereopsis, but on the third and subsequent
sorties all reported perceptual adaptation, that is, a reduction in illusory perception. Given that this adaptation was the
result of intermittent exposure, and did not produce visual aftereffects, it was not due to the recalibration of the
relationship between binocular cues and depth/distance. A more likely explanation of the observed visual adaptation is
that it results from a discounting of distorted binocular cues in favour of veridical monocular cues, such as familiar size,
motion parallax and linear perspective.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.