Helmet-Mounted Displays: - USAARL - The - U.S. Army

Optical Performance 137
A number of studies have been conducted in an attempt to understand
the role of FOV in pilotage and targeting tasks. Sandor and Leger (1991)
looked at tracking with two restricted FOVs (20º and 70º). They found that
tracking performance appeared to be “moderately” impaired for both FOVs.
Further investigation on FOV targeting effects found negative impacts on
coordinated head and eye movements (Venturino and Wells, 1990) and
reinforced decreased tracking performance with decreasing FOV size
(Kenyon and Kneller, 1992; Wells and Venturino, 1989). Kasper et al.
(1997) also examined the effect of restricted FOVs on rotary-wing aviator
head movement and found that aviators respond to such restrictions by
making significant changes in head movement patterns. These changes
consist of shifts in the center of the aviator’s horizontal scan patterns and
movements through larger angles of azimuth. They also concluded that
these pattern shifts are highly individualized and change as the restrictions
on FOV change. This work was an extension of Haworth et al. (1996)
which looked at FOV effects on flight performance, aircraft handling, and
visual cue rating.
Perhaps the most important FOV study to rotary-wing aviation is the
Center for Night Vision and Electro-Optics, Fort Belvior, VA, investigation
of the tradeoff between FOV and resolution (Greene, 1988). In this study,
five aviators using binocular simulation goggles, performed terrain flights
in an AH-1S Cobra helicopter. Seven combinations of FOV (40º circular
to 60º x 75º), resolutions (20/20 to 20/70), and overlap percentages (50%
to 100%) were studied. They reported the lowest and fastest terrain flights
were achieved using the 40º - 20/60 - 100% and 40º - 20/40 - 100%
conditions, with the aviators preferring the wider (60º) condition.
However, the author did not feel that the results justified increasing FOV
without also increasing resolution.
In spite of this research, the question of how large a FOV is required
still has not been fully answered. Aviators want it to be as large as
possible. HMD designers must perform tradeoffs between FOV, resolution,
weight, size, and cost. The task of determining FOV required for flying is
not a simple one. Obviously, the selected FOV should reflect the aircraft’s
mission, providing optimal visual search performance, object recognition,
and spatial orientation (Lohman and Weisz, 1989). Therefore, first the
minimal FOV required is highly task dependent. Consider the different
sensory cues used for high-speed flight across a desert floor (narrow FOV)
versus a confined-area hovering turn (wide FOV). Second, the FOV
required to maintain orientation depends on workload. A small attitude
indicator bar (or cue), occupying only a few degrees on the display image,