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

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Joseph R. Licina
1994b) that will show reduced resolution with decreasing light levels which
increase the pupil size and blur circle on the retina. The eyepiece lens
power that provides most users with the best resolution with NVGs and
HMDs appears to be slightly minus power between approximately -0.25
and -0.75 diopter. To ensure that optimum resolution is obtained by the
aviation population of all of the nonspectacle wearing and spectacle
wearing personnel using night imaging devices, a small range of adjustment
would be desired, and better training in focusing procedures, to include a
binocular focusing method to control accommodation with vergence. A
problem found with some fixed-focused viewing devices such as the "Cats
eyes NVGs" has been the ability of the factory to precisely set the eyepiece
focus within a 0.12 diopter tolerance. The zero position on the diopter
scale of newly received ANVIS was found to vary by up to 1.25 diopters
on 10 sets of NVGs. The military specification for the zero scale tolerance
for NVGs is 0.50 diopter, which would result in blurred vision for
emmetropic users if the error were on the plus lens power side. With the
newer generation of image intensifiers and thermal sensors, the resolution
has improved to approximately 20/25 for optimum conditions. Therefore,
the focus adjustments for both the objective and eyepiece will be more
critical than previous night imaging devices. Therefore, we recommend a
small range of user adjustable eyepiece and objective lens focus for the
image intensifier systems and for the eyepieces of HMDs.
Anthropometry
Since the head is being used as the basic support platform for the
HMD, it is important to understand its anthropometry. This point was well
illustrated in the initial fielding of the IHADSS. The helmet and fitting
system were designed to the parameters of the SPH-4 series helmet. The
fit of the SPH-4 to the Army aviator population had been proven
satisfactorily. This is attributed to the fact that the manufacturer
deliberately built a helmet which exceeded the basic sizing requirements.
When the IHADSS helmet was built to specifications, the Army test pilots
found the helmet to be “tight” to “unacceptable.” A quick survey (Sippo,
Licina, and Noehl, 1988) of 500 Army attack helicopter aviators revealed
head sizes exceeding existing design specifications. These data, coupled
with continuing fielding fit problems, led to a follow-on $1.6 million effort
in the design and fielding of an extra-large IHADSS helmet size.
Subsequent helmet designs, such as the HGU-56/P, have taken into
consideration and accommodated the small evolving female aviator