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

Human Factors Engineering (HFE) Issues 241
from the edge of the eye box, the full FOV of the image will be
extinguished within the distance of the width of the eye pupil.
For NVGs, the displacements of the right and left oculars together or
relative to each other around the roll, tilt, and yaw axes will not displace the
viewed image when focused at infinity, since the sensor and display are
physically bound together and located near the eye. The individual FOV
will be displaced in the direction of movement, but not the image.
However, for HMDs with remote sensors, any relative movement between
oculars around the axes will displace the images and change the
convergence, divergence, or cyclo-rotation to the eyes. For the monocular
HDU of the IHADSS, the mechanical adjustments are fore-aft and roll.
The combiner can be moved up and down for eye alignment with the
optical axis of the HDU, but most of the alignment is obtained with proper
helmet fit to keep the combiner at the lowest position to obtain the
maximum eye clearance and FOV. Misalignment of the HDU and IHADSS
helmet outside a specific value will not allow a proper boresight with the
total system.
Activation, adjustment, or movement of any mechanism on the HMD
must be accomplished by the user through tactile identification and
activation through the aviator’s flight gloves, as well as, the chemical
protective over-glove currently used. Removing gloves for adjustments is
not a viable option.
Electronic adjustments
On present night vision imaging systems such as ANVIS, there are no
user electronic adjustments provided. The tube amplification and
automatic brightness control (ABC) level are set at the factory according
to specifications. Since the 2 nd and 3 rd generation intensifier tubes are
basically linear amplifiers with a gamma approaching unity (Allen and
Hebb, 1997; Kotulak and Morse, 1994a), the imaged contrast should
remain constant for changes in light level and between right and left tubes.
A field study at a U.S. Army NVG training facility measured the
differences in ANVIS luminance output between the right and left tubes for
20 pairs of ANVIS and found 15% of the sample had luminance differences
greater than 0.1 log unit (30%) below the ABC level and none had
differences greater than 0.1 log unit above the ABC level (McLean, 1997).
The recent AN/PVS-14 monocular night vision device for ground troops
has a user adjustable gain control, which may be incorporated in future
aviation NVG designs.