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

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Clarence E. Rash and William E. McLean
1992), but fall short of definitive recommendations.
In applications where direct view displays are supplemented or
replaced by helmet-mounted displays, the task of defining minimum
contrast values is further complicated by optical and EO design
considerations. The U.S. Army’s most current HMD program is the
HIDSS, being designed for use in the RAH-66 Comanche helicopter. The
current version of this design is similar to that of Figure 5.5. The HIDSS
specification for contrast and shades of grey, as available at the eye,
addresses high ambient daylight (up to 10,000 fL background luminance)
requirements. A contrast value (Equation 5.1a) of > 4.66 with a minimum
of 6 shades of grey is required. This contrast value of 4.66 is equivalent to
a C r value of 5.66 which corresponds to 6 SOG. For day symbology, the
contrast ratio is required to equal or exceed a value of 1.5:1 for a 3000 fL
background and equal to or exceed 7:1 for a background of 100 fL; both
values are based on the use of a tinted visor. For nighttime viewing of
sensor imagery, a minimum contrast ratio value of 11.2 which corresponds
to 8 SOG is required.
Resolution
The most frequently asked HMD design question is “How much
resolution must the system have?” Resolution refers to the amount of
information (detail) which can be presented. This will define the fidelity
of the image. Spatial resolution is, perhaps, the most important parameter
in determining the image quality of a display system. An HMD’s
resolution delineates the smallest size target which can be displayed. An
image’s resolution usually is given as the number of vertical and horizontal
pixels which can be presented.
In HMDs using CRTs as the image source, the CRT’s resolution is the
limiting resolution of the system. The CRT’s horizontal resolution is
defined primarily by the bandwidth of the electronics and the spot size.
Vertical resolution is usually of greater interest and is defined mostly by the
beam current diameter and the spreading of light when the beam strikes the
phosphor, which defines the spot size (and line width). CRT vertical
resolution is usually expressed as the number of raster lines per display
height. However, a more meaningful number is the raster line width, the
smaller the line width, the better the resolution. From Table 2.1, it can be
seen that 20 �m is the current limit on line width in miniature CRTs. Task
and Kocian (1995) have expressed the opinion that CRT electron designs
will continue to improve for specific applications.