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

Optical Performance 133
stringent. Based on testing, a recommendation was made to tighten both
shear and “S” distortion specifications. Distortion requirements generally
apply to single tubes. However, distortion differences between tubes in a
pair of NVGs are more important. In fact, care should be taken to match
tubes in pairs based on other characteristics; e.g, luminance, as well as
distortion.
In Crowley’s (1991) investigation of visual illusions with night vision
devices, he cites examples of where aviators reported having the illusion of
landing in a hole or depression when approaching a flat landing sight.
Aviators also reported that normal scanning head movement with some
pairs of ANVIS caused the illusion of trees bending.
In general, for monocular, as well as for biocular/binocular, optical
systems with fully overlapped fields of view, an overall 4% distortion value
has usually been considered acceptable. That is, a deviation in image
mapping towards the periphery of the display could be off by 4%, providing
the deviation is gradual with no noticeable irregular waviness of vertical or
horizontal lines. For a projected display with a 40-degree circular field-ofview
and 4% distortion, this would mean an object at the edge of the visible
FOV could appear at 40 x 1.04 (41.6º pincushion distortion) or 40/1.04
(38.5º barrel distortion). For binocular displays, differences in distortion
between the images presented to the two eyes are more serious than the
amount of distortion (Farrell and Booth, 1984.) Distortion is better
tolerated in static images than in moving images, and therefore is of
increased concern in HMDs.
Biocular/binocular HMDs having overlapping symbology will have to
meet head-up display specifications of 1 milliradian or less difference
between the right and left image channels for symbology within the
binocular overlapped area if the symbology is seen by both eyes.
Otherwise, diplopia and/or eye strain will be induced. However, with seethrough
vision, this criterion can not be met when viewing at less than 60
meters due to eye convergence (McLean and Smith, 1987).
When imagery is used with a minimum see-through requirement, the
maximum displacement between the right and left image points within the
biocular/binocular region should not exceed 3 milliradians (0.3 prism
diopter) for vertical, 1 milliradian (0.1 prism diopter) for divergence, and
5 milliradians (0.5 prism diopter) for convergence.
Distortion can be particularly important in aviation. For example, the
apparent velocity of a target having a relative motion will change in
proportion to the magnitude of the distortion (Fischer, 1997).