Design and Construction of a 30-Degree See-Through Head-Mounted Display
Douglas Holmgren
University of North Carolina, Department of Physics and Astronomy
This report describes the design and construction of a 30-degree see-through headmounted display (STHMD). The design goals for this STHMD were that it employ calibrated
means of adjusting the virtual image distance (focus) and inter-pupillary distance (IPD) and
provide for adjustment of exit pupil location with respect to individual wearers. The optics were
designed by Jannick Rolland to incorporate off-the-shelf optical components.
A head-on view of the STHMD optical assembly is shown in Figure 1. The liquid crystal
display (LCD) units are on the extreme right and left sides. The optical path for each side
proceeds from the LCD to a fold mirror, where a 90° downward turn occurs. It then passes
through a 2-element magnifying lens, followed by a partially (30%) transparent mirror. The
reflected (70%) LCD image is deflected 90° into the eye of the viewer, who simultaneously sees
the real world scene transmitted at 30% intensity through the semi-transparent mirror.
LCD Displays
The display elements for the STHMD are Sony LCD Color Monitors (Model FDL-330S).
Each display area measures 54 by 40 mm (67 mm diagonal) and consists of 240 rows with
approximately 340 single-primary pixels in each row. The actual LCD pixel plane is located 4
mm in optical path length (including refractive index effects) behind the front display plastic
window.
The Sony units come in three modules: the LCD monitor with associated drive
electronics, an rf video receiver assembly, and a battery/auxillary power pack. The rf video
receiver assembly was not needed in the STHMD application. The battery/auxillary power pack
was retained because it provided a convenient external power connection and mounting recess
for attaching the LCD unit to the STHMD mechanical structure without adding significant
weight.
Adjustable Mechanical Settings
The STHMD was designed to provide several opto-mechanical adjustments. Because of
the small 1 cm diameter exit pupil of the optical system, the STHMD has variable interpupillary
distance which can be set within a 55 to 75 mm range. To accomplish this adjustment, the left
and right display and optical assemblies move apart or together while maintaining parallel optical
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axes. Both assemblies ride in a dove-tail track and are moved with respect to a fixed center point
by means of a drive screw with a righthanded thread on one end and lefthanded thread on the
other. The desired IPD distance is set by turning a knurled adjustment knob. The IPD setting is
indicated by the position of the edge of the dove-tail track piece with respect to a calibration
scale.
A second adjustment sets the virtual display image distance by positioning the LCD
display an appropriate distance from the lens. The virtual image distance can be set anywhere
betweem 35 cm and infinity as measured from the system exit pupil. Each LCD display is
mounted on a precision linear positioning stage having 2.5 cm travel, which moves the LCD
toward or away from the fold mirror by turning a fine pitch screw. The virtual distance setting
can be read on a calibrated scale.
An additional adjustment that was considered for incorporation in the STHMD was
optical axis toe in/out. This adjustment would allow the left and right scene overlap to vary.
Because of the additional complexity in design that would be needed for this adjustment, it was
not included in the final design. The STHMD optical axes are fixed and remain parallel at all
IPD and virtual distance settings. The left and right scene overlap is approximately 30°.
Components and Design Details
The lenses for each eye are mounted in aluminum bases which also incorporate the dovetails for the IPD adjustment. The lens bases slide in a brass dove-tail track piece. The track
piece is the fixed anchor piece for the STHMD optical/display assembly. All other components
move with respect to it when IPD or virtual distance adjustments are made. The optical/display
assembly is attached to the headgear at the dove-tail track piece.
The 90 degree fold mirrors are front surface mirrors from Edmund Scientific (#32,221).
They are approximately 3 mm thick, 40 mm wide and were cut to a length of 77 mm. They are
mounted on 45 degree brackets attached to each lens base. The distance along the optical axis
between the upper flat lens surface and the mirror is 28 mm. The distance along the optical axis
from the mirror to the front surface of the LCD display window is 31 to 54 mm, depending on
the setting of the linear positioning stage.
The linear positioning stages were manufactured by Daedal-Parker Corporation. The left
side stage is Model 4054. The right side stage is a custom order mirror image version of the left
side stage.
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Focus Calibration
The virtual image distance (focus) calibration is based on known dimensions of the
STHMD optical stage and calculations using the effective focal length (81 mm) of the optics.
The minimum optical distance between the LCD pixel plane and the closest lens surface is 63
mm. At this distance the LCD housing is touching the lens mount. With the LCD in this
postion, the virtual image is located 350 mm from the exit pupil position, as determined by
CODE-V analysis of the lens. The calibration scale is attached to the STHMD such that the edge
of the Daedal stage aligns with the 35 cm mark when the LCD is at the minimum distance (63
mm) from the lens. When set at the infinity symbol, the LCD is 81 mm from the lens surface
and the virtual image is focused at infinity.
Head mounting
The STHMD optical assembly attaches to the headpiece structure by two aluminum
brackets screwed to the brass dove-tail track piece. These brackets in turn are attached to a gear
and rack assembly which allow vertical and forward/backward positioning of the STHMD
optical assembly with respect to the wearer's head. In this way the 10 mm diameter exit pupil of
the STHMD optics can be positioned coincident with the wearer's eye pupil or as needed to
accomodate eyeglasses. The gear and rack assembly attach to the ophthamologist headband
(manufactured by Keeler Instruments, Inc., Broomall, PA (800) 523-5620).
Weight
The weights in ounces of the individual components of the optical display assembly are
given in the following table:
Sony LCD Monitors (2)
7 oz (each)
Daedal Linear Stages (2)
7
(each)
Lenses
4.5 (total)
Lens bases (2)
2
(each)
Fold mirrors (2)
0.75 (each)
Mirror mounts (2)
1.7 (each)
Dove-tail track (1)
4.5
30T/70R mirrors (2)
0.5 (each)
The total assembly weighs including miscellaneous hardware weighs 47 ounces. A three
pound counterweight is attached to the back of the headmount assembly to provide overall
system balance.
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