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

Lasers
Image Sources 49
Another novel imaging source, which has recently gained recognition
as having a potential for application to HMDs, is the laser. Lasers as image
generators have been designed and investigated on a large physical scale
(Bohannon, 1997). Based on projection, these devices produce imagery on
a screen using the basic scanning method of CRTs. Rather than an electron
beam, a laser beam is scanned in two dimensions, with the beam intensity
modulated at every pixel. If scanned at frequencies of 60 Hz or greater, a
flicker-free image which the eye can see is produced. Laser projectors are
claimed to produce images with: sufficient luminance, color gamut, and
color saturation.
An image source based on the scanning laser which generates an image
directly onto the retina of the eye has been proposed for HMD application
(Proctor, 1996; Johnston and Willey, 1995; Kollin, 1993) (Figure 1.12).
One version of this device, called the Virtual Retinal Display, has been
developed at the Human Interface Technology Laboratory, University of
Washington, Seattle, Washington. Its basic principle is the same as used
in the scanning laser ophthalmoscope (Webb, Hughes, and Delori, 1987).
A laser diode (or three laser diodes for color) is intensity modulated as it
is scanned vertically and horizontally. An optical interface is used to
project the scanning beam onto the retina. The exit pupil of the optics is
designed to be coplanar with the entrance pupil of the eye. The eye’s
natural focusing then forms the image on the retina. It is claimed that the
device will be able to provide high (diffraction limited) resolution, high
luminance, and monochromatic or color imagery within the small weight
and volume requirements of HMD designs. Disadvantages, at least
potentially, include scanning complexity, susceptibility to degradation in
high vibration environments (as in Army aviation), limited exit pupil size,
and safety concerns.
An adaptation on the laser scan virtual retinal display described above
is a system being developed by Microvision, Inc., Seattle, WA. Rather than
scanning directly onto the retina, the Microvision system takes “video
modulated” laser beams and couples them via fiber optic cables to
mechanical scanners to create a raster image which is delivered to the eyes
by an HMD optical relay system.
References