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

14
Clarence E. Rash
� Hazard free
� Maximum crashworthiness
� 24-hour, all weather operation
� Minimum training requirements
� Low maintenance
� Low design cost and minimum schedule
From this abridged list of requirements, it becomes apparent that the
design of an HMD requires the careful consideration of a multitude of
physical parameters and performance factors. This results in two different
design approaches. The first emphasizes careful analysis and control of the
individual subsystems’ physical characteristics. The identified subsystems
are those in the basic description given earlier: image source, display
optics, helmet, and tracking system. This approach is presented in Table
1.2 and as an Ishikawa (Fishbone) diagram (Figure 1.6). The second
approach, which focuses on performance, is presented in Table 1.3 and
Figure 1.7. In the latter approach, which allows for subsystem interaction,
physical characteristics are replaced by performance figures of merit
(FOMs). These FOMs are grouped into natural performance categories:
optical system, visual, helmet (with tracking system), and human factors
engineering. As expected, there can be considerable overlap both between
and within the two approaches. The performance approach (Table 1.3) is
adopted in this book.
Types
There are several classification schemes which can be applied to
HMDs. These include imagery type, imagery presentation mode, and
optical design approach. Strictly speaking, HMDs can produce either real
or virtual images. Images are the regions of concentration of light rays
originating from the source, called the object (Levi, 1968). When these
rays actually intersect, the resulting image is real; when only the extensions
of the rays intersect, the resulting image is virtual. More practically, the
image formed by an optical system, e.g., an HMD, is a real image if it is
formed outside the optical system, where it falls onto a surface such as a
Table 1.2.
HMD subsystem physical characteristics.