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

58
William E. McLean
given FOV. To demonstrate the importance and contribution of the eye
clearance on FOV, recalculating the above equation with an eye clearance
of 30 mm and the eyepiece and exit pupil the same, the FOV is reduced to
23.3 degrees. Similarly, to obtain a 40 degree FOV with 30 mm of eye
clearance, the diameter of the eyepiece would be 31.0 mm. Also note that
reducing the exit pupil size reduces the eyepiece diameter the same amount.
With angular eye movements, the eye is displaced perpendicular to the
optical axis and will require the optical exit pupil to be located ideally
approximately 2 to 3 mm behind the pupil of the eye, particularly for pupil
forming imaging systems (Shenker, 1987).
The primary purpose of this first order optical exercise was to show
how the variables of FOV, exit pupil size, eye clearance, image source size,
and f/# interact with the simplest of optical designs for a flat display with
nonsee-through vision. When see-through vision is desired with an added
combiner, the calculations become more complex, but can be solved with
multiple trigonometry steps. The optical designer can also increase the
FOV for a given eyepiece focal length by using a concave display or image
plane, inducing barrel distortion for the objective lens and neutralizing the
barrel distortion with an equivalent pincushion distortion for the eyepiece.
This technique is used for ANVIS. Graphs of the diameter of the eyepiece
for the various HMD designs will be shown in the next section for
comparison purposes.
Optical Aberrations
In addition to just focusing or collimating the display, additional optical
elements are usually required to compensate for chromatic and spherical
aberrations, distortion, field of curvature, etc. Because the additional
elements add undesirable mass to electro-optical devices, a short discussion
of these optical characteristics will be included (Smith, 1990).
Chromatic aberration
All lens elements with refractive power act like a prism by refracting
(bending) wavelengths of different colors by slightly different amounts. To
compensate for this and to reduce the rainbow effects from the lens
elements, the optical designer uses lenses in pairs (usually fused), opposite
in lens power with different refractive characteristics (index of refraction
and dispersion). These lenses are called achromats. Other methods to
reduce chromatic aberrations are to (a) use narrow band light sources or