As pointed out by Kotulak, vergence/accommodation mismatch in Night Vision Systems, usually due to
misadjustments of eyepiece focus, is sometimes a source of visual performance decrement. The increased separation
between sensors existing in some modern binocular Helmet Mounted Display systems, creating "hyperstereopsis",
was also identified to be potentially responsible for decreased performance at distances less than 5 meters.
Based upon basic knowledge pertaining to vergence and accommodation mechanisms, a study was performed using
a sensory approach, with the goal of better understanding the problem of dissociation between accommodation and
convergence. In this study, different conditions of interocular separation (nominal IPD, X3, X4) and viewing
distances (6m, 4m, 2m) were used. Six subjects participated in the experiment and were asked to view Landolt C
charts using NVGs and specially developed optical tools allowing changes to sensor separation.
The results show that, with a fixed eyepiece focus at 10m, the decrease in resolution performance is roughly
proportional to the interocular separation when looking at short distances. A fixed focus at a distance of 4m
considerably reduces the conflict and results in improved resolution for increased separation conditions.
An additional experiment was conducted to investigate the setting of objectives lenses focus at infinity (nominal
landing condition). With this setting, for visual acuity test, the decrease in resolution at short distance was such that
effects of the mismatch between accommodation and convergence are no longer apparent regardless of interocular
Distance judgments in virtual environments and Head-mounted Displays (HMD) systems are generally underestimated
compared with judgments in the real world. Some visual depth cues may be absent or modified, according to the
After a brief review of the literature pertaining to the representation of depth in Helmet-Mounted Displays, we explore
two possible causes for the reduced distance perception in virtual environments: the increased interocular separation (or
hyperstereopsis) and the reduction of the field of view.
Some laboratory and training ground data are reported. The effective influence of each factor on space perception is
Including night vision capabilities in Helmet Mounted Displays has been a serious challenge for many years. The use of
"see through" head mounted image intensifiers systems is particularly challenging as it introduces some peculiar visual
characteristics usually referred as "hyperstereopsis".
Flight testing of such systems has started in the early nineties, both in US and Europe. While the trials conducted in US
yielded quite controversial results, convergent positive ones were obtained from European testing, mainly in UK,
Germany and France. Subsequently, work on integrating optically coupled I<sup>2</sup> tubes on HMD was discontinued in the US,
while European manufacturers developed such HMDs for various rotary wings platforms like the TIGER.
Coping with hyperstereopsis raises physiological and cognitive human factors issues. Starting in the sixties, effects of
increased interocular separation and adaptation to such unusual vision conditions has been quite extensively studied by a
number of authors as Wallach, Schor, Judge and Miles, Fisher and Ciuffreda. A synthetic review of literature on this
subject will be presented.
According to users' reports, three successive phases will be described for habituation to such devices: initial exposure,
building compensation phase and behavioral adjustments phase.
An habituation model will be suggested to account for HMSD users' reports and literature data bearing on
hyperstereopsis, cue weighting for depth perception, adaptation and learning processes, task cognitive control.
Finally, some preliminary results on hyperstereopsis spatial and temporal adaptation coming from the survey of training
of TIGER pilots, currently conducted at the French-German Army Aviation Training Center, will be unveiled.