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

Introductory Overview 21
direct viewing of an object. Examples of virtual images include those
produced by eyeglasses, telescopes, and microscopes (Kingslake, 1983).
However, real image HMD designs are rare. They would be direct view
systems requiring the image source (e.g., a miniature LC display) to be
located in front of the eye(s) at the typical reading distance of the eye. All
fielded aviation HMDs are virtual image systems.
Virtual image displays offer several advantages (Seeman et al., 1992).
At near optical infinity, virtual images theoretically allow the eye to relax
(reducing visual fatigue) and provide easier accommodation for older
aviators. By providing a virtual image, a greater number of aviators can use
the system without the use of corrective optics (but not all) (Seeman et al.,
1992). The collimated image also reduces effects of vibration producing
retinal blur.
Shontz and Trumm (1969) categorize HMDs based on the mode by
which the imagery is presented to the eyes. They define three categories:
One eye, occluded; one eye, see-through; and two eye, see-through. In the
one eye, occluded type, imagery is presented to only one eye, to which the
real world is blocked, with the remaining eye viewing only the real world.
The one eye, see-through type, while still providing imagery to one eye,
allows both eyes to view the real world. [Note: The optics in front of the
imagery eye will filter the real world to a lessor or greater degree.] The
AH-64 IHADSS is an example of this type. In the two eye, see-through
type, imagery is presented to both eyes, and the real world also is viewed
by both eyes. The RAH-66 HIDSS is an example of this type.
Another classification scheme, which parallels the three types
described above, uses the terms monocular, biocular, and binocular. These
terms refer to the presentation of the imagery by the HMD. For this book,
monocular means the HMD imagery is viewed by a single eye; biocular
means the HMD provides two visual images from a single sensor, i.e., each
eye sees exactly the same image from the same perspective; binocular
means the HMD provides two visual images from two sensors displaced in
space. [Note: A binocular HMD can use a single sensor, if the sensor is
somehow manipulated to provide two different perspectives of the object
scene.] A biocular HMD may use one or two image sources, but must have
two optical channels. A binocular HMD must have separate image sources
(one for each eye) and two optical channels.