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

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William E. McLean
translucent screen. Unlike the knot hole analogy for the nonpupil forming
device, the pupil forming system requires the pupil of the eye to be
positioned within a specific area to obtain the full FOV. If the eye is
moved closer than the exit pupil, the FOV will actually decrease. Also, if
the eye is moved laterally outside the exit pupil, the complete display
disappears where the nonpupil forming system merely vignettes the FOV
in the opposite direction of lateral movement outside the exit pupil. The
exit pupil for a pupil forming system is defined by the optical ray trace and
is shown in Figure 3.9a for the center of the FOV and Figure 3.9b for the
edge of the FOV. Note also the field lens, which is used to channel the
aerial image to the eyepiece and adjust the eye clearance.
The relay optics of pupil forming devices usually are determined after
the type eyepiece design, FOV, optical length, exit pupil diameter, and eye
clearance values have been defined. To minimize the size and weight of
the relay optics, the designer will attempt to use the shortest optical path
possible within mechanical constraints.
Partially Silvered, Dichroic, and Holographic Combiners
Partially silvered combiners are broadband reflectors of the visible
wavelengths. The advantages of partially silvered combiners are minimal
effects on color transmittance of the image source or see-through vision.
Increasing the reflectance of the combiners increases the luminance transfer
from the display, but proportionally reduces the see-through transmittance.
The sum of the display transfer and see-through vision for partially silvered
mirrors is always less than 100%.
Dichroic combiners reflect the primary wavelengths of the display and
transmit the other visible wavelengths. When using narrow band phosphors
such as P43 phosphors, the sum of the percent luminance transfer from the
display and the percent see-through vision can be greater than 100%. The
primary disadvantage of a dichroic combiner is the effects of color
perception with see-through vision. Typically, the wavelengths optimized
for reflection by the combiner are also one of the wavelengths of headdown
displays.
Holographic combiners are essentially diffraction gratings for use with
monochromic or very narrow band light. The primary advantages of a
holographic combiner are: a) it has a high luminance transmittance and
see-through vision; and b) the apparent shape and tilt of the holographic
reflective surface do not have to conform to the normal equal angles for