Virtual Retinal Display
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Abstract
The Virtual Retinal Display (VRD) is a personal display device under
development at the University of Washington's Human Interface Technology Laboratory
in Seattle, Washington USA. The VRD scans light directly onto the viewer's retina. The
viewer perceives a wide field of view image. Because the VRD scans light directly on the
retina, the VRD is not a screen based technology.
The VRD was invented at the University of Washington in the Human Interface
Technology Lab (HIT) in 1991. The development began in November 1993. The aim was
to produce a full color, wide field-of-view, high resolution, high brightness, low cost
virtual display. Microvision Inc. has the exclusive license to commercialize the VRD
technology. This technology has many potential applications, from head-mounted
displays (HMDs) for military/aerospace applications to medical society.
The VRD projects a modulated beam of light (from an electronic source) directly
onto the retina of the eye producing a rasterized image. The viewer has the illusion of
seeing the source image as if he/she stands two feet away in front of a 14-inch monitor. In
reality, the image is on the retina of its eye and not on a screen. The quality of the image
he/she sees is excellent with stereo view, full color, wide field of view, no flickering
characteristics.
Introduction
Our window into the digital universe has long been a glowing screen perched on a
desk. It's called a computer monitor, and as you stare at it, light is focused into a dimesized
image on the retina at the back of your eyeball. The retina converts the light into
signals that percolate into your brain via the optic nerve.
Here's a better way to connect with that universe: eliminate that bulky, powerhungry
monitor altogether by painting the images themselves directly onto your retina.
To do so, use tiny semiconductor lasers or special light-emitting diodes, one each for the
three primary colors—red, green, and blue—and scan their light onto the retina, mixing
the colors to produce the entire palette of human vision. Short of tapping into the optic
nerve, there is no more efficient way to get an image into your brain. And they call it the
Virtual Retinal Display, or generally a retinal scanning imaging system.
The Virtual Retinal Display presents video information by scanning modulated
light in a raster pattern directly onto the viewer's retina. As the light scans the eye, it is
intensity modulated. On a basic level, as shown in the following figure, the VRD
consists of a light source, a modulator, vertical and horizontal scanners, and imaging
optics (to focus the light beam and optically condition the scan).
Potential Advantages of the Virtual Retinal Display
It is really interesting to note why this family of imaging systems score better than
the conventional display systems.
Brightness
One problem with conventional helmet mounted display image sources is the low
luminance levels they produce. Most liquid crystal array image sources have insufficient
luminance levels for operation in a see-through display. The VRD, however, does not
contain individual Lambertian (or nearly Lambertian) pixel emitters (liquid crystal cells
or phosphors) as do most LCD arrays and CRT's. The only light losses in the VRD result
from the optics (including the scanners and fiber coupling optics). There is no inherent
tradeoff, however, between resolution and luminance as is true with individual pixel
emitters. In individual pixel emitters, a smaller physical size increases resolution but
decreases luminance. In the Virtual Retinal Display, intensity of the beam entering the
eye and resolution are independent of each other. Consequently, the VRD represents a
major step away from the traditional limitations on display brightness.
Resolution
As mentioned in the previous section there is a tradeoff between resolution and
brightness in screen based displays. As resolution requirements increase, the number of
picture elements must increase in a screen based display. These greater packing densities
become increasingly difficult to manufacture successfully. The VRD overcomes this
problem because the resolution of the display is limited only by the spot size on the
retina. The spot size on the retina is determined primarily by the scanner speed, light
modulation bandwidth, and imaging optics.
Yield
One limiting aspect in the manufacture of liquid crystal array image generators is
the yield and reliability of the hundreds of thousands of individual liquid crystal cells
present in these displays. For a liquid crystal array display to function properly at all
times, each picture element must function properly. The Virtual Retinal Display requires
only constant functionality from the light sources and the scanners. As resolution
increases in virtual image displays, liquid crystal arrays will contain more and more
individual liquid crystal cells. The Virtual Retinal Display will gain an increasing
advantage over liquid crystal array image generators in terms of yield as resolution
demands increase in the future.
Size
The theoretical size for horizontal and vertical scanners plus light sources for the
VRD is smaller than the size of conventional liquid crystal array and CRT image sources.
A typical size for a liquid crystal array image generator for helmet mounted display
applications is one inch by one inch. The Mechanical Resonant Scanner used in this
project was approximately 1 [cm] by 2 [cm]. Furthermore, the problem of scanner size
has not been directly addressed. Further size reduction is certainly possible. It should be
noted that light sources for a smaller, usable full color VRD must be much smaller than
the sources used in this project. The potential size of light emitting diodes and diode
lasers indicate that these sources show greatest promise for future systems in terms of
size.
History of Virtual Retinal Display
The VRD display concept was initially conceived by Dr. Thomas A. Furness as a
means of eliminating large aperture optics and expensive high-resolution addressable
images sources such as CRTs. Soon after joining the HIT Lab in 1991, Joel Kollin
realized a key feature about the VRD - movements of the eye would not result in
perceived movement in the image. Therefore, eye tracking would not be necessary
beyond that what might be needed to ensure that the light beam entered the eye. He then
designed and constructed the original bench-mounted VRD, using an acousto-optic
device as the horizontal scanner. Electronics largely designed and built by Bob Burstein
then allowed it to be driven directly by a DEC workstation, although it was still
significantly lower in both contrast and resolution than a standard SVGA display and
offered an image only in uncalibrated shades of red. We subsequently began work on
patenting the display and brought on board David Melville to engineer the mechanical
design, especially a new scanning system.