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Monocular DVE systems have been used on military helicopters and showed strong
physiological limitations (see USAARL Report on “Helmet-Mounted Displays:
Sensation, Perception and Cognition Issues”, C.E . Rash and al, 2009) like degraded
visual cues, visual illusions (static and dynamic), and visual discomfort. Bi-ocular DVE
systems (NVGs and HMSDs) are commonly used on military rotorcrafts and provide
If a monocular solution is proposed, the applicant will have to demonstrate that it is
suitable for the intended functions. Particularly the physiological effect of monocular
system, in particular binocular rivalry, and impact on pilot workload shall be
Binocular imaging systems are defined by their capability at both sensor and display
level to send different images to left and right eyes, in order to reproduce the capability
of human vision to perceive the third dimension by continuously analyzing the parallax
between the left and right vision. At display level, it means that separate displays and
optics are used and are dedicated to one eye. At sensor level, stereovision obviously
needs to duplicate sensors.
NOTE 1: When considering binocular HMDs, two kinds of solutions are possible:
with partial overlap or full overlap of left and right optical channels. Of
course full overlap is preferred, but systems with partial overlap (eg. 40°
of full overlap and 6° of partial overlap on each side resulting in a 52°
horizontal FOV) have been flight tested and considered as acceptable.
NOTE 2: Hyperstereoscopic vision occurs when the inter-pupillary distance
induced by the duplicated sensors is wider than human vision (typically
more than 15 cm). That allows improving the 3D perception (perception
of height and distances), particularly helpful during take-off, landing and
low altitude operations in DVE.
The visual display design, whether monocular, bi-ocular or binocular, should not cause
perceptual or cognitive problems or undue eye strain for the user in any lighting
conditions, including night or day operations, both in visual and instrument
Primary recommendation is monochrome, as many monochrome HMDs are already
flying on rotorcrafts, including in DVE conditions.
Four colors are recommended, in order to have one color for the sensor data, one
color for the symbology (piloting symbology, DVE symbology and SVS), and two
colors for the alarms (typically caution in amber and warning in red).
Additional colors are desirable, for instance to differentiate the conformal symbology
from the non-conformal one, or to “repeat” on the transparent display the set of colors
used on the HDD.
The choice of these colors has to be carefully validated through flight tests in order to
be sure that whatever the outside view is, there is no ambiguity in the perception and
interpretation of these colors.
Head-down display requirements
Point of view
The image may be generated from different viewing points of view. We distinguish
between ego-centric point of view and exo-centric point of view.
The ego-centric view is usually used for SVS systems, and is the only one that can be
conformal with the outside view from the pilot standpoint.
The exo-centric view may allow to visualize the aircraft within its environment, which
can be more convenient in certain phases of flight.
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