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

Optical Designs 61
optics. Examples are the ANVIS (Figure 3.1a) with no see-through vision
and a reflex HUD (Figure 3.1b) with a 45º angle combiner and see-through
vision. The see-through vision is provided with a partial reflective beam
splitter or plano combiner. IHADSS HDU (Figure 3.2a), which is an HMD
with see-through vision in the AH-64 aircraft for night pilotage, tilts the
combiner to 38º from the last optical lens to improve eye relief. Refractive
optical designs use lenses for imaging. The IHADSS HDU provides
imagery and symbology from remote sensors, where the two night imaging
sensors (I 2 tubes) are contained in the ANVIS. The primary advantage of
the refractive design with a plano combiner is the high percent luminance
transfer from the display to the eye. The primary disadvantages for
refractive HMDs with see-through vision are excessive weight with limited
fields of view and eye clearance.
The ANVIS eyepiece is a simple well corrected magnifier with no seethrough
vision. Other NVG designs such as the Eagle Eye TM or the Cat’s
Eyes TM use prism combiners for see-through vision with I 2 , but the seethrough
combiners with intensifier tubes have been used primarily by fixedwing
fighter type aircraft with HUDs. These see-through plano combiners
are enclosed or sandwiched between two prisms which, when combined,
form a plano refractive media with minimal prismatic deviation. The
purpose of the prism combiners is to increase the combiner stability and
increase the eye clearances for a given FOV and eyepiece diameter. Figure
3.2b shows a prism combiner using the IHADSS design. The prism
combiners can also be used with power reflective combiners. Figure 3.2c
shows a catadioptric eyepiece design without the prism combiner and
Figure 3.2d with a prism combiner.
Catadioptric
Catadioptric optical designs use curved reflective mirrors with or
without lenses for imaging (Figures 3.2c and d). The primary advantage of
catadioptric designs is larger diameter optics with less weight and without
induced chromatic aberrations. By coating transmissive curved surfaces
with partial reflective materials to provide see-through vision, the beam
splitter is referred to as a power combiner. Figure 3.2d shows the
catadioptric design with a prism combiner to increase the eye clearance for
a given FOV. The primary disadvantages are reduced luminance transfer