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

Visual Performance 181
0 to -5.25 diopters ( mean of -2.28 diopters) was obtained. It was
concluded that the required positive accommodation by the eye to offset
these negative focus settings was a likely source of headaches and visual
discomfort during and following long flights. No correlation was found
between the focus settings and aviator age or experience; nor were there
differences between instructor pilots and students, or day versus night.
Table 6.1.
Apache aviator reports of visual complaints during and after flight.
(Behar et al., 1990)
Complaint During flight After flight
Visual discomfort
Headache
Double vision
Blurred vision
Disorientation
Afterimages
Never Sometimes Always
49 % 51 % --
65 % 35 % --
86 % 12 % 2%
79 % 21 % --
81 % 19 % --
NA NA NA
Never Sometimes Always
70 % 28 % 2 %
67 % 32 % 2 %
89 % 9 % 2 %
72 % 24 % 3 %
95 % 5 % --
79 % 19 % 2 %
In another survey (Crowley, 1991) of 242 aviators flying either ANVIS
(rotary- and fixed-wing) or IHADSS, a very small percentage of the rotarywing
ANVIS users (n = 212) reported physiological effects to include
eyestrain (3%), headache (2%), motion sickness/vomiting (2%), postflight
blurred vision (1%), and dizziness (1%); only 5% of Apache aviators (n =
21) reported any visual problems (that of dark adaptation effects).
The move towards two-eyed (binocular) wide FOV HMDs may result
in adverse visual effects if care is not taken in their design. Mon-Williams,
Wann, and Rushton (1995) point out that conflicts between accommodation
and vergence, focal error, and prismatic errors may result in “unstable
binocular vision.” As previously discussed, failure to maintain strict
binocular alignment may introduce serious performance problems.
Currently, HMDs intended for use in the Army aviation community are
required to provide some measure of look-under, look-around, and/or lookthrough
capability. However, future HMD designs may employ fullimmersion
displays in the form of virtual reality display systems. There is
considerable ongoing effort in investigating a phenomenon known as
“cybersickness” associated with such systems. Cybersickness is similar to
simulator sickness in that symptoms of motion sickness (e.g., nausea,