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

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Clarence E. Rash
performance, one can take the basic engineering approach of reducing the
amplitude of the identified vibration frequencies. Another approach is to
utilize active image stabilization techniques (Wells and Haas, 1992). One
such technique, adaptive noise-cancellation, acts as a low pass filter,
passing low frequency voluntary head motions, while dampening unwanted
higher vibrations (Velger, Merhav, and Grunwald, 1986). A less attractive
approach recommends increasing the size of the alphanumeric characters,
thereby reducing the effects of vibration (Lewis and Griffin, 1979).
However, this will increase cluster and reduce the amount of information
which can be displayed.
One final point regarding vibration: Most HMD designs are exit pupil
forming systems. They can, in a very loose analogy, be compared to
knotholes in a fence. To have an unobstructed view, you must put, and
keep, your eye in the knothole. The exit pupil is the HMD’s knothole. To
prevent vignetting of the full image, the aviator must keep his eye within
the exit pupil. If the exit pupil is large enough, additional vibrational
effects can be ignored. However, if the exit pupil is small, then the eye
may move out of it under the influence of vibration, reducing FOV.
Sensor Switching
The current version of the Comanche HIDSS expects to provide both
I 2 and FLIR imagery. While the final decision on whether the I 2 sensor(s)
will be aircraft- or head-mounted is yet to be made, the current HIDSS
design is based on all sensors being mounted on the aircraft. If at a later
date, a decision is made to mount the I 2 sensor(s) on the helmet, then
aviators will be in a situation where they will be switching back and forth
between sensor imagery originating from two different perspectives (Rash,
Verona, and Crowley, 1990). The human’s basic visual sensors are his/her
eyes. Prior to encountering aircraft-mounted sensors, his experience in
perception and interpretation of visual information has been referenced to
the eye’s position on the head. When flying the Apache, the imagery often
is from the FLIR sensor. This sensor is located on the nose of the aircraft
and is approximately 10 feet forward and 3 feet below the aviator’s design
position. This exocentric positioning of the imagery source can introduce
problems of apparent motion, parallax, and incorrect distance estimation
(Brickner, 1989). However, this mode of sensor location does offer the
advantage of allowing the aviator to have an unobstructed view of the area
directly in front of and under the aircraft. This “see-through” capability is
very useful when landing must be made in cluttered or unfamiliar landing