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

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Clarence E. Rash and William E. McLean
Figure 5.12. Percent ANVIS distortion as a function
of angular position.
the source of any distortion present in HMDs using these sources. FP
images also can be predistorted to correct for the display optics. However,
this will require at least one additional frame of latency (Nelson, 1994).
In ANVIS, the optical system can produce barrel or pincushion
distortion and the fiber-optic inverter can cause shear and gross (or “S”)
distortion. Shear distortion in fiber optic bundles causes discrete lateral
displacements and is known also as incoherency. “S” distortion is due to
the residual effect of the twist used to invert the image, which causes a
straight line input to produce an “S” shape (Task, Hartman, and Zobel,
1993). Distortion requirements for ANVIS are cited in MIL-A-49425 (CR)
and limit total distortion to 4%. Distortion for ANVIS typically is given as
a function of angular position across the tube. Sample data from a single
tube are presented in Figure 5.12 (Harding et al., 1996).
As a historical note, in 1988, when AN/PVS-5's were still the most
common I 2 system, a number of reports from National Guard units surfaced
regarding “depression” and “hump” illusions during approaches and
landings (Markey, 1988). Suspect goggles were obtained and tested. The
final conclusion was that the distortion criteria were not sufficiently