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

Image Sources 41
AMLCDs employ an array of individual pixels, each controlled by an
electronic switch (Tannas, 1985). The most successful active matrix
approach to addressing pixels uses thin film transistors (TFTs). In this
approach, a TFT and a capacitor are used to switch each LC cell on and off.
LCDs can be monochrome or full color. Monochrome LCDs usually
use a backlight consisting of one or more fluorescent lamps, a reflector, and
a diffuser. Less frequently used is a backlight where the light source is an
electroluminescent panel. [See following section.] Approaches to
achieving color LCDs are numerous and increasing every day. One
approach is similar to the additive color method employed in modern CRT
displays. In this approach, pixels are composed of three or more color
subpixels. By activating combinations of these subpixels and controlling
the transmission through each, a relatively large color gamut can be
achieved.
Backlighting is an important issue with LCDs and even more important
for HMD designs based on LCDs. In general, backlights must be efficient
(> 40 lumens/watt), produce high luminance (> 20,000 fL) (critical due to
the poor through-put of LCDs), have high luminance uniformity (< 20%
variation), have long life (>30,000 hours), and have a wide dimming range
(> 5000:1) (Allen et al., 1995). Research is ongoing to achieve and/or
exceed these requirements for military aviation applications (Altadonna,
1996; Jiang, 1996; Kalmanask and Sundraresan, 1996). For HMDs, two
options are possible: (a) The backlight is physically located away from the
LCD, i.e., elsewhere in the aircraft or (b) a miniature backlight must be
used. Mounting the backlight in the aircraft places additional luminance
requirements on the backlight and the aircraft designers, since space,
weight, and power demands must be addressed. Integrating the backlight
with the display requires the development of subminiature (