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

48
Clarence E. Rash, Melissa H. Ledford, and John C. Mora
sky (0.00001 fL) to that of a sunlit white cloud (10,000 fL). The image
source must have high enough luminance to provide (after losses through
the optics which can be as high as 80%) sufficient contrast (SOGs) to allow
adequate vision for successful completion of all mission tasks. Current
commercially available miniature FP image sources are limited to
luminances of only slightly better than 200 fL.
There are two leading candidate FP technologies for the miniature
image sources needed for Army aviation: AMEL and AMLCD. As with all
FP displays, these two display types do not have a mature and reliable
manufacturing history, do not provide for sufficient symbology luminance,
and have limited distortion correction schemes (Belt et al., 1997). AMELs
additionally suffer from insufficient video luminance; AMLCDs ( because
of their low structure transmission) require extremely high backlight
luminances and have limited temporal response for presenting the dynamic
imagery required for the military rotary-wing environment.
A FP technology display which has recently gained considerable
attention because it offers CRT-like characteristics in a thin, flat package
is the FED (Jones and Jones, 1995). FEDs are considered by some HMD
designers to be the best of both worlds and a hands-down choice for future
aviation applications. Their potential performance advantages include very
low power requirements, wide viewing angle, excellent resolution, and
high contrast (>100:1); and they can withstand the harsh aviation
environment, including temperature and vibration requirements. However,
FED displays have yet to meet their full potential, still attempting to
overcome problems with high density patterning, switching voltages,
luminance uniformity, driver electronics, production, reliability, and others
(Jones et al., 1996; Giri, 1995.). While considered as the most promising
display technology for advanced cockpit applications (Marticello and
Hopper, 1996), for now, FEDs will have to settle for being the “holy grail”
of image sources.
An excellent bibliography for the technical characteristics of currently
available miniature FP image sources is provided by Ferrin (1997). The FP
manufacturing community is actively seeking to expand the performance
of current displays. Through these efforts, these displays are slowly
overcoming the limitations briefly described here. It is imperative that
HMD developers maintain awareness of such improvements. The authors
have found the two major sources of information on FP development and
HMD design to be the annual conferences held by the Society of
Information Display (SID), Santa Ana, California, and by the Society of
Photo-Optical Instrumentation Engineers (SPIE), Bellingham, Washington.