Use of binocular night vision devices by military aircrew has been associated with visual fatigue. Misalignment between the two binocular images may be one source of this fatigue and could degrade task performance. The eyes have some degree of tolerance to optical misalignment; however, published tolerance limits vary widely and may have limited relevance for military pilots flying long missions. We used a simulated flying task to investigate the effect of misalignment on task performance and visual fatigue. A simulated helicopter flying task was presented simultaneously with three secondary tasks relevant to the visual, auditory, and cognitive demands experience by military aircrew. Task performance was objectively assessed using tracking errors, response times and incorrect responses. Eight participants were exposed to a controlled level of optical misalignment by attaching customised lenses to a Tobii Pro 2 eye tracker. The misaligned condition was compared with an aligned condition with identical workload. Pupil diameter, peripheral skin temperature and ECG data were collected during the task as objective markers of fatigue. Our results showed that misalignment induced significant degradation of task performance both in terms of longer response times and an increased number of incorrect responses. Misalignment was also associated with significantly increased fatigue as measured by reduction of peripheral skin temperature and pupil diameter. Moreover, typical task related modulations in heart rate and heart rate variability were significantly impaired in the misaligned condition.
The additional and perhaps unnatural eye-movements required to fuse misaligned binocular images can lead to visual fatigue and decreased task performance. The eyes have some tolerance to optical misalignment. However, a survey of the scientific literature reveals a wide range of recommended tolerances but offers little supporting experimental evidence. Most experimental studies are based on small numbers of participants exposed to brief periods of optical misalignment. Therefore, these published tolerance limits might have limited relevance for long-duration exposure to misaligned binocular devices. Prolonged use of binocular devices may cause visual fatigue irrespective of binocular alignment especially for complex tasks such as night vision flying. This study attempts to identify measures most sensitive to misalignment in order to establish relevant tolerance limits for in-service binocular night vision devices. Firstly, we developed a rugged and deployable test bench that can measure binocular alignment with a reproducibility error of less than 1 arcmin. The bench was used to identify and investigate major factors affecting the stability of the optical misalignment over time. Our results indicated that the optical misalignment of a given device changed over time as a function of the in-service usage and thermal history of the device. Secondly, participants were exposed to experimentally controlled levels of optical misalignment typical of those measured on in-service binocular night vision devices. The visual fatigue of each participant was assessed via a set of oculomotor parameters. The oculomotor parameters showing high sensitivity to optical misalignment were compared for subjects exposed to extended periods of misalignment in a baseline reading task and a task using an actual night vision device.
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.
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.
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.
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.