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

Visual Coupling 85
advanced optical/visual HMD enhancements. For example, several HMD
designs (Fernie, 1995; Barrette, 1992) have explored the concept of
creating within the HMD’s FOV a small inset area of increased resolution
which is slaved to eye movement (Figure 4.3). Such “area of interest”
displays overcome the computational problems of trying to provide high
resolution, wide field-of-view imagery in real time. This is achieved by
mimicking the eye’s design of maximum visual acuity within a central high
resolution area (fovea - 2º diameter area) (Robinson and Wetzel, 1989).
Such designs would help the long standing conflict between wide FOV and
high resolution, currently design tradeoff parameters.
Eye tracking devices must be usable over the range in which the “area
of interest” inset can be positioned. They must have sufficient spatial and
temporal resolution to accommodate the high velocity and acceleration
rates associated with the saccadic movements of the eye, which can be >
800º/sec and > 2000º/sec 2 , respectively. They also must operate over a
wide range of illumination levels, pupil sizes, and other physical ocular
differences. And, they have to be able to address all these variations, in
real time, while ignoring meaningless artifacts (Robinson and Wetzel,
1989).
Eye trackers can be monocular or binocular and can measure
movements along both horizontal and vertical axes. There are a number of
techniques used in these devices for detecting eye movements. These
include the use of electrodes to measure minute electrical voltages in eye
muscles responsible for eye movements, the detection of Purkinje images
formed by reflections from the cornea and lens of the eye (Crane, 1994),
the Limbus reflection method using an infrared (IR) LED on the border
between the cornea and the sclera of the eye (Onishi et al., 1994), and a
method where a coil is attached to the eye and its coupling effect to another
stationary coil is measured. However, techniques which are adaptable to
HMDs use a principle of reflecting IR energy from an IR LED(s) off the
eye back into an IR detector(s). One design uses pulsed IR LEDs to
illuminate the orbital field of the eyes. The distribution of the reflected
energy, which changes with eye movement, is detected by an array of
photodiode detectors (Permobil Medtech, Inc., 1997).
For HMD applications, eye tracking would be used in conjunction with
head tracking.