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

Visual Coupling 89
delay times used with HMDs and VCSs.
So and Griffin (1995) investigated the effects of lag on head tracking
performance using lag times between head movement and target image
movement of 0, 40, 80, 120, and 160 msec. They found that head tracking
performance was degraded significantly by lags greater than or equal to 40
msec (in addition to a 40 msec delay in the display system). A similar
study (Rogers, Spiker, and Fisher, 1997) which investigated the effect of
system lag on continuous head tracking accuracy for a task of positioning
a cursor on a stable target found performance effects for lags as short as 20
msec (plus 40 msec display system delay).
The studies cited above, and others (Whiteley, Lusk, and Middendorf,
1990; Boettcher, Schmidt, and Case, 1988; Crane, 1980), suggest that there
is some uncertainty in maximum allowable time delays, ranging from 40 to
300 msec, depending on task and system. Wildzunas, Barron, and Wiley
(1996) utilized a NUH-60 Blackhawk simulator to investigate the delay
issue under a more realistic military aviation scenario. They tested delays
of 0, 67, 133, 267, 400, and 533 msec. The delays were inserted into the
simulator’s visual display. However, while more representative of rotarywing
flight, the displays were panel-mounted, not head-mounted. While
finding some performance effects for delays less than or equal to 267 msec,
consistently significant effects were found for the 400 and 533 msec delays.
Data show that lags, attributed to the display and VCS, must be
minimized. Strategies to achieve this include improved engineering
designs, faster processing chip technology, and the use of predictive
algorithms (Nelson et al., 1995; So and Griffin, 1992). Failure to achieve
an acceptable maximum lag value has been shown to degrade visual
tracking performance, introduce image artifacts, and sometimes promote
motion sickness (Moffit, 1997; Kalawsky, 1993; Biocca, 1992).
Roll Compensation
Some tracking systems provide only head azimuth and elevation
information, as does the AH-64 Apache head tracker. However, there has
been a growing interest in providing 3-axis information, with head roll
added. The Comanche plans to provide this capability. The addition of roll
information provides the capability of keeping the imagery aligned with the
aircraft structure (Task and Kocian, 1995). The availability of roll
compensation is considered to be an advantage and should reduce
workload. After all, the human visual system acts this way, and roll
compensation is intrinsic to all HMDs with helmet-mounted sensors, such