A virtual retinal display (VRD), also known as a retinal scan display (RSD), is a new display technology that draws a raster display (like a television) directly onto the retina of the eye. The user sees what appears to be a conventional display floating in space in front of them. In the past similar systems have been made by projecting a defocused image directly in front of the user's eye on a small "screen", normally in the form of large sunglasses. The user focused their eyes on the background, where the screen appeared to be floating. The disadvantage of these systems was the limited area covered by the "screen", the high weight of the small televisions used to project the display, and the fact that the image would appear focused only if the user was focusing at a particular "depth". Limited brightness made them useful only in indoor settings as well. Only recently a number of developments have made a true VRD system practical. In particular the development of high-brightness LEDs have made the displays bright enough to be used during the day, and adaptive optics have allowed systems to dynamically correct for irregularities in the eye (although this is not always needed). The result is a high-resolution screenless display with excellent color gamut and brightness, far better than the best television technologies. The VRD was invented at the University of Washington in the Human Interface Technology Lab in 1991. Most of this research into VRDs to date has been in combination with various virtual reality systems. In this role VRDs have the potential advantage of being much smaller than existing television-based systems. They share some of the same disadvantages however, requiring some sort of optics to send the image into the eye, typically similar to the sunglasses system used with previous technologies. It can be also used as part of a wearable computer system. More recently, there has been some interest in VRDs as a display system for portable devices such as cell phones, PDAs and various media players. In this role the device would be placed in front of the user, perhaps on a desk, and aimed in the general direction of the eyes. The system would then detect the eye using facial scanning techniques and keep the image in place using motion compensation. In this role the VRD offers unique advantages, being able to replicate a full-sized monitor on a small device.
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Virtual retinal display
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A virtual retinal display (VRD), also known as a retinal scan display (RSD), is a new display technology that draws a raster display (like a television) directly onto the retina of the eye. The user sees what appears to be a conventional display floating in space in front of them. In the past similar systems have been made by projecting a defocused image directly in front of the user's eye on a small "screen", normally in the form of large sunglasses. The user focused their eyes on the background, where the screen appeared to be floating. The disadvantage of these systems was the limited area covered by the "screen", the high weight of the small televisions used to project the display, and the fact that the image would appear focused only if the user was focusing at a particular "depth". Limited brightness made them useful only in indoor settings as well. Only recently a number of developments have made a true VRD system practical. In particular the development of high-brightness LEDs have made the displays bright enough to be used during the day, and adaptive optics have allowed systems to dynamically correct for irregularities in the eye (although this is not always needed). The result is a high-resolution screenless display with excellent color gamut and brightness, far better than the best television technologies. The VRD was invented at the University of Washington in the Human Interface Technology Lab in 1991. Most of this research into VRDs to date has been in combination with various virtual reality systems. In this role VRDs have the potential advantage of being much smaller than existing television-based systems. They share some of the same disadvantages however, requiring some sort of optics to send the image into the eye, typically similar to the sunglasses system used with previous technologies. It can be also used as part of a wearable computer system. More recently, there has been some interest in VRDs as a display system for portable devices such as cell phones, PDAs and various media players. In this role the device would be placed in front of the user, perhaps on a desk, and aimed in the general direction of the eyes. The system would then detect the eye using facial scanning techniques and keep the image in place using motion compensation. In this role the VRD offers unique advantages, being able to replicate a full-sized monitor on a small device.
17-07-2009, 06:48 PM
can u plz post the page link for the full report
31-01-2010, 11:58 AM
[attachment=1590]
Virtual Retinal Display
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.1
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 colorsâ€red, green,
and blueâ€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).
Fig1. Basic block diagram of the
Virtual Retinal Display. The resultant imaged formed on the retina is
perceived as a wide field of view image originating from some viewing
distance in space. The following figure illustrates the light raster
on the retina and the resultant image perceived in space.
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Fig2.Illustration of light raster imaged onto the retina and the
resultant perceived image. In general, a scanner (with magnifying
optics) scans a beam of collimated light through an angle. Each
individual collimated beam is focused to a point on the retina. As
the angle of the scan changes over time, the location of the
corresponding focused spot moves across the retina. The collection of
intensity modulated spots forms the raster image as shown above
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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.
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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. Moreover, it will be quite
surprising to know that the original stereographic display, or the
three dimensional view as the eye means it, can be accomplished only
by an imaging system like the one proposed above.
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Fundamentals of human eye
The eye is a specialized organ that is
capable of light reception, and in the case of vertebrates, is able
to receive visual images and then carry it to the visual centre in
the brain. The horizontal sectional view of human eye is as follows
(courtesy Encyclopedia Britannica 2002)
Fig3. The cross sectional view
of the human eye The eyeball is generally described as a globe or a
sphere, but it is oval, not circular. It is about an inch in
diameter, transparent in front, and composed of three layers. 1) The
outer fibrous, the supporting layer 2) Middle, vascular, and 3) Inner
nervous layer.
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Six muscles move the eye, four
straight and two oblique. These lie inside the orbit passing from the
bony walls of the orbit to be attached to the sclerotic coat of the
eye behind the cornea. The movements of the eyes are combined, both
eyes move to right or left, up, and down, etc. Normally the axes of
both the eyes converge simultaneously on the same point; when owing
to paralysis of one or more muscles, they fail to do so squint
exists. The Sclera is the tough outer fibrous coat. It forms the
white of the eye and is continuous in front with the transparent
window membrane, the cornea. The sclera protects the delicate
structures of the eye and helps to maintain the shape of the eyeball.
The Choroid or middle vascular coat contains the blood vessels, which
are the ramifications of the ophthalmic artery, a branch of the
internal carotid. The vascular coat forms the iris with the central
opening or pupil of the eye. The pigmented layer behind the iris
gives its colour and determines whether the eye is blue, brown, grey
etc. The horoids is continuous in the front with the iris and just
behind the iris this coat is thickened to form the ciliary body, thus
the ciliary body lies between the choroids and the iris. It contains
circular muscle fibres and radiating fibres; contraction of the
former contracts the pupil of the eye.
The Retina is the inner nervous
coat of the eye, composed of a number of layers of fibres, nerve
cells, rods and cones, all of which are included in the construction
of the retina, the delicate nerve tissue conducting the nerve
impulses from without inwards to the optic disc, the point where the
optic nerve leaves the eyeball. This is the blind spot, as it
possesses no retina. The most acutely sensitive part of the retina is
the macula, which lies just external to the optic disc, and exactly
opposite the centre of the pupil.
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Fig4.The layered
view of retina showing blood vessels The retina is the nervous
mechanism of sight. It contains the endings of the optic nerves, and
is comparable to a sensitive photographic plate. When an image is
perceived, rays of light from the object seen pass through the
cornea, aqueous humour, lens, and vitreous body to stimulate the
nerve endings in the retina. The stimuli received by the retina pass
along the optic tracts to the visual areas of the brain, to be
interpreted. Both areas receive the message from both eyes, thus
giving perspective and contour. In ordinary camera one lens is
provided. In the eye, whilst the crystalline lens is very important
in focusing the image on the retina, there are in all four structures
acting as lenses: the cornea, the aqueous humour, the crystalline
lens, and the vitreous body. As in all interpretations of sensation
from the surface, a number of relaying stations are concerned with
the transmission of the senses which in this case is the sight. A
number of these relaying stations are in the retina. Internal to the
periphery of the retina are layers of rods and cones which are highly
specialized sight cells sensitive to light. The circular
interruptions in these are termed as granules. The proximal ends of
the rods and cones form the first synapse with a layer if bipolar
cells, still in the retina. The second processes of these cells form
the second nerve synapse with large ganglion cells,
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also in the retina. The axons of these cells form the fibres of
the optic nerve. These pass backwards, first reaching the lower
centre in special bodies near the thalamus, and finally reaching the
special visual centre in occipital lobe of cerebral hemisphere where
sight is interpreted.
Fig5. The Human visual pathway Each retina
includes multiple mosaics of neurons that separately represent the
visual field. Image transduction uses two systems of photoreceptors:
the rods and cones. Each system comprises a separate sampling mosaic
of retinal image. The rods encode the data for a system with low
spatial resolution but high quantum efficiency. The cones encode the
image data at much higher spatial resolution and lower quantum
efficiency. Rods and cones generally operate under different viewing
conditions, but there are also many cases in which multiple
representations of the image are obtained under a single viewing
condition. For example, the cones can be subdivided into three
sampling mosaics that expand the spectral encoding. The three cone
mosaics also differ in their spatial sampling properties.
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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. In 1993, a newly formed corporation, MicroVision
Inc., licensed the VRD technology and signed a 4 year, $5.1 million
development contract with the University. Rich Johnston was hired
specifically to manage the VRD and other hardware products of the
Lab. By forming relationships with other researchers in the College
of Engineering, he has orchestrated a program to solve the challenges
and bottlenecks of the project. In late 1993 and 1994, Mike Tidwell
redesigned the VRD to maximize the resolution possible with the A-O
scanner while David Melville designed a new Mechanical Resonant
Scanner (MRS) which would be capable of the high rates of horizontal
scanning without the costs and other limitations of the A-O devices.
The MRS was then utilized in full-color inclusive and "see-through"
systems.
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Virtual Retinal Display- A system overview
The VRD can be considered a portable system that creates the
perception of an image by scanning a beam of light directly into the
eye. Most displays directly address a real image plane (typically a
CRT or matrix-addressed LCD) which might be relayed to form a larger,
more distant image for a head-mounted display (HMD). The VRD uses a
scanned, modulated light beam to treat the retina as a projection
screen, much as a laser light show would use the ceiling of a
planetarium. The closest previously existing device would be the
scanning laser opthalmoscope (SLO) which scans the retina to examine
it; the SLO is designed to capture light returning from the eye
whereas the VRD is designed as a portable display.. The VRD has
several advantages over CRTs, LCD, and other addressable-screen
displays:
¢
Resolution is limited by beam diffraction and optical
aberrations, not by the size of an addressable pixel in a matrix.
Very high resolution images are therefore possible without extensive
advances in micro-fabrication technology. Also, the VRD does not
suffer from pixel defects.
¢
The display can be made as bright as
desired simply by controlling the intensity of the scanned beam. This
makes it much easier to use the display in "see-though" configuration
on a bright day.
¢
The scanning technology in the current display
requires only simple, well understood manufacturing technology and
can therefore be manufactured inexpensively.
¢
Because the light is
projected into the eye and the scanner is electro-mechanically
efficient, the display uses very little power. In theory, the VRD
allows for accommodation to be modulated pixel by pixel as the image
is being scanned.
¢
All components in the VRD are small and light,
making them ideal for use in a portable display.
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The Basic System In a conventional display a real image is
produced. The real image is either viewed directly or, as in the case
with most head-mounted displays, projected through an optical system
and the resulting virtual image is viewed. The projection moves the
virtual image to a distance that allows the eye to focus comfortably.
No real image is ever produced with the VRD. Rather, an image is
formed directly on the retina of the user's eye. A block diagram of
the VRD is shown in the figure below.
Fig6. The functional block
diagram of a VRD system
To create an image with the VRD a photon
source (or three sources in the case of a color display) is used to
generate a coherent beam of light. The use of a coherent source (such
as a laser diode) allows the system to draw a diffraction limited
spot on the retina. The light beam is intensity modulated to match
the intensity of the image being rendered.
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The
modulation can be accomplished after the beam is generated. If the
source has enough modulation bandwidth, as in the case of a laser
diode, the source can be modulated directly. The resulting modulated
beam is then scanned to place each image point, or pixel, at the
proper position on the retina. A variety of scan patterns are
possible. The scanner could be used in a calligraphic mode, in which
the lines that form the image are drawn directly, or in a raster
mode, much like standard computer monitors or television. Our
development focuses on the raster method of image scanning and allows
the VRD to be driven by standard video sources. To draw the raster, a
horizontal scanner moves the beam to draw a row of pixels. The
vertical scanner then moves the beam to the next line where another
row of pixels is drawn. After scanning, the optical beam must be
properly projected into the eye. The goal is for the exit pupil of
the VRD to be coplanar with the entrance pupil of the eye. The lens
and cornea of the eye will then focus the beam on the retina, forming
a spot. The position on the retina where the eye focuses the spot is
determined by the angle at which light enters the eye. This angle is
determined by the scanners and is continually varying in a raster
pattern. The brightness of the focused spot is determined by the
intensity modulation of the light beam. The intensity modulated
moving spot, focused through the eye, draws an image on the retina.
The eye's persistence allows the image to appear continuous and
stable. Finally, the drive electronics synchronize the scanners and
intensity modulator with the incoming video signal in such a manner
that a stable image is formed
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VRD Features The
following sections detail some of the advantages of using the VRD as
a personal display. Size and Weight The VRD does not require an
intermediate image on a screen as do systems using LCD or CRT
technology. The only required components are the photon source
(preferably one that is directly modulatable), the scanners, and the
optical projection system. Small photon sources such as a laser diode
can be used. As described below the scanning can be accomplished with
a small mechanical resonant device developed in the HITL. The
projection optics could be incorporated as the front, reflecting,
surface of a pair of glasses in a head mount configuration or as a
simple lens in a hand held configuration. HITL engineers have
experimented with single piece Fresnel lenses with encouraging
results. The small number of components and lack of an intermediate
screen will yield a system that can be comfortably head mounted or
hand held. Resolution Resolution of the current generation of head
mounted and hand held display devices is limited by the physical
parameters associated with manufacturing the LCDs or CRTs used to
create the image. No such limit exists in the VRD. The limiting
factors in the VRD are diffraction and optical aberrations from the
optical components of the system, limits in scanning frequency, and
the modulation bandwidth of the photon source. A photon source such
as a laser diode has a sufficient modulation bandwidth to handle
displays with well over a million pixels. If greater resolution is
required multiple sources can be used.
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Currently
developed scanners will allow displays over 1000 lines allowing for
the HDTV resolution systems. If higher resolutions are desired
multiple sources, each striking the scanning surface at a different
angle, can be used. . Field of View The field of view of the VRD is
controlled by the scan angle of the primary scanner and the power of
the optical system. Initial inclusive systems with greater than 60
degree horizontal fields of view have been demonstrated. Inclusive
systems with 100 degree fields of view are feasible. See through
systems will have somewhat smaller fields of view. Current see
through systems with over 40 degree horizontal fields of view have
been demonstrated. Color and Intensity Resolution Color will be
generated in a VRD by using three photon sources, a red, a green, and
a blue. The three colors will be combined such that they overlap in
space. This will yield a single spot color pixel, as compared to the
traditional method of closely spacing a triad, improving spatial
resolution. The intensity seen by the viewer of the VRD is directly
related to the intensity emitted by the photon source. Intensity of a
photon source such as a laser diode is controlled by the current
driving the device. Proper control of the current will allow greater
than ten bits of intensity resolution per color. Brightness
Brightness may be the biggest advantage of the VRD concept. The
current generations of personal displays do not perform well in high
illumination environments. This can cause significant problems when
the system is to be used by a soldier outdoors or by a doctor in a
well lit operating room. The common solution is to block out as much
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ambient light as possible. Unfortunately, this
does not work well when a see through mode is required. The VRD
creates an image by scanning a light source directly on the retina.
The perceived brightness is only limited by the power of the light
source. Through experimentation it has been determined that a bright
image can be created with under one microwatt of laser light. Laser
diodes in the several milliwatt range are common. As a result,
systems created with laser diode sources will operate at low laser
output levels or with significant beam attenuation. Power Consumption
The VRD delivers light to the retina efficiently. The exit pupil of
the system can be made relatively small allowing most of the
generated light to enter the eye. In addition, the scanning is done
with a resonant device which is operating with a high figure of
merit, or Q, and is also very efficient. The result is a system that
needs very little power to operate. A True Stereoscopic Display The
traditional head-mounted display used for creating three dimensional
views projects different images into each of the viewer's eyes. Each
image is created from a slightly different view point creating a
stereo pair. This method allows one important depth cue to be used,
but also creates a conflict. The human uses many different cues to
perceive depth. In addition to stereo vision, accommodation is an
important element in judging depth. Accommodation refers to the
distance at which the eye is focused to see a clear image. The
virtual imaging optics used in current head-mounted displays place
the image at a comfortable, and fixed, focal distance. As the image
originates from a flat screen, everything in the virtual image, in
terms of accommodation, is located at the same focal distance.
Therefore, while the stereo cues tell the viewer an object is
positioned at one distance, the accommodation cue indicates it is
positioned at a different distance.
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With the VRD it
is theoretically (this is currently in the development stage)
possible to generate a more natural three dimensional image. The VRD
has an individual wavefront generated for each pixel. It is possible
to vary the curvature of the wavefronts. Note that it is the
wavefront curvature which determines the focus depth. This variation
of the image focus distance on a pixel by pixel basis, combined with
the projection of stereo images, allows for the creation of a more
natural three-dimensional environment. Inclusive and See Through
Systems have been produced that operate in both an inclusive and a
see through mode. The see through mode is generally a more difficult
system to build as most displays are not bright enough to work in a
see through mode when used in a medium to high illumination
environment where the luminance can reach ten thousand candela per
meter squared. As discussed above, this is not a problem with the
VRD.
In the VRD a light source is modulated with image information,
either by direct power ("internal") modulation or by an external
modulator. The light is passed through an x-y scanning system,
currently the MRS and a galvanometer. Light from the scanner pair
enters an optical system, which in present implementations of the VRD
forms an aerial image and then uses and eyepiece to magnify and relay
this image to infinity.
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Components of the Virtual
Retinal Display
Video Electronics In its current form, the video
electronics of the VRD controls the light intensity modulation,
scanner deflection, and the synchronization between modulation and
scanning. The horizontal and vertical synchronization signals in the
video signal are used to determine scanner synchronization. A user
selectable delay of up to one full line is incorporated into the
video electronics to allow for phase difference between the
horizontal scanner position and the modulation timing. Also, the
respective drive levels for intensity modulation of each light source
are output from the electronics. The drive electronics control the
acousto-optic modulators that encode the image data into the pulse
stream. The color combiner multiplexes the individually-modulated
red, green, and blue beams to produce a serial stream of pixels,
which is launched into a single mode optical fiber to propagate to
the scanner assembly. The drive electronics receive and process an
incoming video signal, provide image compensation, and control image
display. For VGA projection, the electronics process over 18 Mpix/s.
The virtual retinal display is capable of providing UXGA resolution
of 1600 x 1200 or 115 Mpix/s. Light Sources and Modulators The light
sources for the VRD generate the photons which eventually enter the
eye and stimulate the photo receptors in the retina. The modulation
of the light source determines the intensity of each picture element.
The size of the scanning spot and the rate at which it can be
modulated determine the effective size of each picture element on the
retina. As the light is scanned across the retina, the intensity is
synchronized with the instantaneous position of the spot thereby
producing a two dimensional pattern of modulated light that is
perceived as a picture.
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According to conventional
additive color theory, any color can be represented as a mixture of
three appropriately chosen primaries. The three ideal VRD light
sources would be monochromatic for maximum possible color
saturation.. Spatial coherence is also important - larger source
spots will correspond to larger spots on the retina, decreasing
resolution. The primary cause of the real (if sometimes exaggerated)
hazards of laser light are the result of spatially coherent light
focusing to a small area on the retina, causing highly localized
heating and ablation of tissue. In the VRD the spot is traveling in
two directions and even when stationary is not at a power level that
would cause damage. We are working with ophthalmologists and will
publish a definitive article on this in the near future.
Incidentally, polychromatic sources can be shown to form spots
comparable to monochromatic ones of the same spatial extent.
Therefore spatial coherence is responsible for the small spot size
which leads to both high resolution and (given enough power) retinal
hazard. To achieve the desired resolution, all current VRD prototypes
have used lasers for their superior spatial coherence
characteristics. In order to use a point source such as an LED, the
image of the source should be smaller than the diffraction limit of
the scanner. Using the lens magnification, one can determine the
maximum source size that can be used before degrading the diffraction
limited spot size at the image plane. The angular divergence of the
source is effectively limited by treating the scanner as a stop.
Light which does not hit the mirror does not contribute to the image
plane spot size. From this geometric argument we can derive an
equivalent point source size between 4 and 5 microns for a VGA
resolution image in our current system. For a system where the
scanner is illuminated with a collimated Gaussian beam, similar
arguments can be made to determine the required divergence and beam
waist from the equations for image plane spot size. The light source
module contains laser light sources, acousto-optic modulators to
create the pulse stream, and a color combiner that multiplexes the
pulse streams. To provide sufficient brightness, full-color displays
suitable for outdoor, daylight applications incorporate red diode
lasers (635nm), green solid-state lasers (532 nm), and blue solid-
state or argon gas lasers (450-470 nm range). Systems designed
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for indoor use can incorporate LEDs; red, blue,
and green devices currently under development for such systems are
being tested. Generally, the energy levels are on the order of
nanowatts to milliwatts, depending on display requirements. The
levels of light involved are well within laser safety standards for
viewing, as confirmed by analysis. Generally two types of intensity
modulation of lasers are done in existing designs. They are Laser
diode modulation and acousto-optical modulation. The laser diode
modulation is generally used for red laser. The small rise time of
the solid state diode laser device allows high bandwidth (up to 100
[MHz]) analog modulation. The video electronics regulate the voltage
seen by the laser current driver and it controls the current passing
through the laser which in turn controls the light output power from
the laser. The laser diode is operated between amplitudes of 0.0 and
80.0[mA]. Acousto-optic (A-O) modulators intensity modulate the green
and blue laser beams. Acousto-optic modulators create a sound wave
grating in a crystal through which a light beam passes. The sound
wave creates alternate regions of compression and rarefaction inside
the crystal. These alternating regions locally change the refractive
index of the material. Areas of compression correspond to higher
refractive indices and areas of rarefaction correspond to lower
refractive indices. The alternating areas of refractive index act as
a grating and diffract the light. As the sound wave traverses the
light beam, the diffracted beam is intensity modulated according to
the amplitude modulated envelope on the carrier signal. Scanners The
scanners of the VRD scan the raster pattern on the retina. The
angular deviation of the horizontal scanner combined with the angular
magnification of the imaging optics determines the horizontal field
of view. The angular deviation of the vertical scanner combined with
the angular magnification of the imaging optics determines the
vertical field of view. The horizontal scanner speed and the frame
rate determine the number of horizontal lines in the display,
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Number of horizontal lines = horizontal scanner
frequency / frame rate, where frame rate is the number of times per
second the entire picture (or frame) is generated. The modulation
rate and the horizontal scanner frequency determine the number of
pixels per line in the display, Number of pixels per line =
modulation frequency / horizontal scanner frequency, where the
modulation frequency is the number of times per second the pixels are
created (or modulated). The horizontal scanning mechanism of the VRD
must be capable of both relatively high scan rates (15 kHz-90+ kHz)
and high resolution (500-2000+ pixels) for NTSC to HDTV formats,
respectively. SVGA format systems (80 kHz) in monochrome/greyscale
using an A-O scanner and 30 kHz in full-color with a mechanical
resonant one have been built. The scanning device consists of a
mechanical resonant scanner and galvanometer mirror configuration.
The horizontal scanner is the mechanical resonant scanner (MRS)]. The
MRS has a flux circuit induced by coils which are beneath a spring
plate. The flux circuit runs through the coils and the spring plate
and alternately attracts opposite sides of the spring plate and
thereby moves the scanner mirror through an angle over time. In a
design developed at the HITL the vertical deflection mirror was
chosen as the galvanometer mirror. The galvanometer deflection can be
selected according to the aspect ratio of the display and a typical
ratio of 4:3 can be chosen. The galvanometer frequency is controlled
by the video electronics to match the video frame rate. The
galvanometer and horizontal scanner are arranged in what is believed
to be a novel configuration such that the horizontal scan is
multiplied. The scanners are arranged, as shown in the following
figures. Such that the beam entering the scanner assembly first
strikes the horizontal scanner then strikes the vertical scanner. The
beam is reflected by the vertical scanner back to the horizontal
scanner before exiting the scanner assembly. The beam therefore
strikes the horizontal scanner twice before exiting the scanner
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configuration. In such an arrangement, the first
scan (corresponding to the first bounce or reflection) is doubled by
the second scan (corresponding to the second bounce or reflection).
The case shown is for = 45 [deg.] wherein the exit beam returns
parallel to the horizontal incident beam. In the first figure the MRS
is undeflected and in the latter the MRS is deflected by d [deg.].
Fig7. MRS/Galvanometer scanner assembly showing incident and exit
beam paths for the MRS in an undeflected position.
Fig8.
MRS/Galvanometer scanner assembly showing incident and exit beam
paths for the MRS in a deflected position.
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The
result of arranging the scanners as in the above figures is a
doubling of the horizontal optical scan angle. Other configurations
have been applied to this approach to achieve a tripling in the
horizontal direction and simultaneously a doubling in the vertical
direction. For more compact designs, techniques from micro electro-
mechanical systems maybe utilized in the fabrication of scanners. The
electrostatic actuation of a MEMS scanner had been developed. By
etching thin layers from a sliver of silicon, the researchers were
able to build a scanner that weighs a mere 5 grams and measures less
than 1 square centimeter. The mirror, too, is much smaller at 1
millimeter across and is mounted on the end of a thin, flexible, bar
which is anchored to the silicon. The mirror is turned into one plate
of a capacitor, with the other plate formed by a small area of
silicon beneath it. Put a rapidly varying voltage across the two
plates and then the mirror will be first repelled and then attracted.
The mirror can move up or down more than 30,000 times each second.
Fig9. A MEMS mirror Micro-Electro-Mechanical Systems (MEMS) is the
integration of mechanical elements, sensors, actuators, and
electronics on a common silicon substrate through the utilization of
microfabrication technology. The electronics are fabricated using
integrated circuit (IC) process sequences, while the micromechanical
components are fabricated using compatible "micromachining" processes
that selectively etch away parts of the
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silicon
wafer or add new structural layers to form the mechanical and
electromechanical devices. The electromagnetic actuation of the
scanners yields more life to the system and imparts more torque. Such
designs have also been developed for retinal scanning displays.
Pupil
expander Nominally the entire image would be contained in an area of
2 mm2. The exitpupil expander is an optical device that increases the
natural output angle of the image and enlarges it up to 18 mm on a
side for ease of viewing. The raster image created by the horizontal
and vertical scanners passes through the pupil expander and on to the
viewer optics. For applications in which the scanned-beam display is
to be worn on the head or held closely to the eye, we need to deliver
the light beam into what is basically a moving target: the human eye.
Constantly darting around in its socket, the eye has a range of
motion that covers some 10 to 15 mm. One way to hit this target is to
focus the scanned beam onto exit pupil expander. When light from the
expander is collected by a lens, and guided by a mirror and a see-
through monocle to the eye, it covers the entire area over which the
pupil may roam. For applications that require better image quality
using less power, we can dispense with the exit pupil expander
altogether either by using a larger scan mirror to make a larger exit
pupil or by actively tracking the pupil to steer light into it.
Viewer optics The viewer optics relay the scanned raster image to the
oculars worn by the user. The optical system varies according to the
application. In the case of military applications such as helmet
mounted or head mounted display optics, the system incorporates glass
and or plastic components; for medical applications such as image-
guided surgery, headmounted plastic optics are used. In industrial or
personal displays, the optics might be a
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simple
plastic lens. A typical viewing system that was employed in a VRD
developed at HITL is as follows. The viewing optics, or the optics
through which the user sees the intended image, are diagrammed in the
following figure. The convergent tri-color beams emanating from the
scanner pass (partially) through a beamsplitter. The beamsplitter (or
beamsplitter/combiner) is coated such that 40% of any light striking
it is reflected and 60% is transmitted. The transmittance/reflectance
is somewhat angle dependent but this dependence is not severe. On
first pass, 60% of the energy in the scan is transmitted through the
splitter/combiner to a concave spherical mirror. The mirror is
actually a rectangular section of a spherical mirror with radius of
curvature -100 [mm]. The negative sign denotes concavity .
Fig10. The
viewing optics system of VRD
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Holographic Optical
Element One of the problems with the VRD only becomes apparent when
you put it on. It can be likened to looking through a pair of high-
magnification binoculars that one must line his eyes precisely with
the beam or the image disappears. Since we rarely fix our eyes on a
single point for more than few seconds, using VRD becomes difficult.
So en eye-tracking system that follows the movements of the pupil by
monitoring the reflections from the cornea had to be developed. The
tracker calculates where the eye is looking and moves the laser
around to compensate. But this system is complex and expensive. A
better solution may lie with a special kind of lens known as a
holographic optical element. An HOE is actually a diffraction grating
made by recording a hologram inside a thin layer of polymer. It works
by converting a single beam of laser into a circular array of 15
bright spots. Place the HOE between the scanning mirrors and the eye,
and the array of beams that forms will illuminate the region round
your pupil. Move your eyes slightly and one of the beams will still
strike the cornea and be focused to form an image on the retina. HOEs
have a big advantage over eye tracking systems: because they are made
from a thin layer of polymer, they weigh next to nothing. All of the
action takes place in a layer just a fraction of millimeter thick,
says a researcher.
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Estimated Retinal Illuminance
The
relationship between estimated retinal illuminance and scene
luminance is important in understanding the display operating on this
principle. As the display in this thesis contains no screen or real
object, it is impossible to discuss the brightness of the display in
terms of luminance. In terms of brightness, estimated retinal
illuminance is a common denominator, so to speak, of screen based
display systems and retinal scanning displays systems. The estimated
retinal illuminance is [36]: I (trolands) = R x pupil area (mm2) x
scene luminance (cd/m2) where I = retinal illuminance, "pupil area"
refers to the area of the pupil of the eye, and R = the effectivity
ratio. The effectivity ratio, R, allows for the Stiles-Crawford
effect and is, R = 1 - 0.0106d2 + 0.0000416d4. where d = the eye's
pupil diameter in millimeters. As shown by dimensional analysis on
the equation for I , trolands reduce effectively to the units of
optical power per unit steradian. The Stiles-Crawford effect
describes the contribution to brightness sensation of light entering
different points of the pupil (i.e. light entering the center of the
pupil contributes more to the sensation of brightness than does light
entering farther from the pupil center). Some standard scene
luminance values, L, and their corresponding Stiles-Crawford
corrected estimated retinal illuminance values, I, are given in Table
II.1 [36,37]. Approximate Luminance [cd/m ] 104 103 102
2
Type of Scene
Clear day Overcast day Heavily overcast day
Estimated
Retinal
Illuminance [trolands] 3.0 x 104 4.5 x 103 9.5 x 102
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Sunset, overcast day 1/4 hour after sunset, clear 1/2 hour after
sunset, clear Fairly bright moonlight Moonless, clear night sky
Moonless, overcast night sky
10 1 10-1 10
-2
1.5 x 102 20 2.0 0.23 2.7 x
10-2 3.0 x 10-3
10-3 10-4
Table1. Standard scene luminance values and
corresponding estimated retinal illuminance values. Transmission
Characteristics of the Ocular Media Transmission losses in the eye
result from scattering and absorption in the cornea, lens, aqueous
humor, and vitreous humor. The transmittance of the ocular media is a
function of the wavelength of the light traveling through the media.
Figure 2.2 shows a plot of the total transmittance of the ocular
media as a function of wavelength [38].
Fig11. Transmittance of the
ocular media vs. wavelength.
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Image Quality as Related
to the Eye
Introduction Measurements of display image quality depend
heavily on two display characteristics, resolution and "contrast"
(see subsequent sections). It is virtually fruitless to discuss image
quality in terms of either resolution or "contrast" without including
the other. Definitions for display resolution, contrast, contrast
ratio, and modulation contrast are given in the following discussion.
Whenever possible, the meanings of the terms are related to the
effect or result at the retina. Display Resolution and the Eye The
resolution of a display can be defined as the angle subtended by each
display resolution element. For a screen (CRT or LCD) based display,
the angular extent of each pixel element determines the resolution.
For the VRD, the angular extent of each spot on the retina dictates
the system resolution. A spot of extent h on the retina allows for an
angular resolution of,
tan-1[h/feye]
where feye is the focal length
of the eye. Display resolution is often measured in cycles per degree
for periodic gratings such as bar patterns or sinusoidal gratings.
Display Contrast and the Eye The contrast, C, of a display is the
ratio of the difference between the maximum display intensity and the
minimum display intensity divided by the maximum. In other terms
[40], C = (LDmax - LDmin) / LDmax where LDmax = the maximum display
luminance and LDmin = the minimum display luminance. Extending the
definition of contrast in terms of estimated retinal illuminance
gives
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C = (IDmax - IDmin) / IDmax. where IDmax =
the maximum estimated retinal illuminance due to the display and
IDmin = the minimum estimated retinal illuminance due to the display.
In other words, the values of IDmax and IDmin correspond to the
estimated retinal illuminance values of displays with luminance
values of LDmax and LDmin respectively. In the case of a retinal
scanning display, as in this thesis, estimated retinal illuminance is
a preferable measure of display brightness as there is no screen in
the system. Display Contrast Ratio and the Eye The contrast ratio,
CR, of a display is the ratio of the maximum display intensity to the
minimum display intensity. In other terms [40], CR = (LDmax/LDmin)
where LDmax = the maximum display luminance and LDmin = the minimum
display luminance. Extending the definition of contrast in terms of
estimated retinal illuminance gives CR = (IDmax/IDmin) where IDmax =
the maximum estimated retinal illuminance due to the display and
IDmin = the minimum estimated retinal illuminance due to the display.
The values of IDmax and IDmin correspond to the estimated retinal
illuminance values for displays with luminance values of LDmax and
LDmin respectively. Display Modulation Contrast and the Eye The
modulation contrast, CM, of a display is the ratio of the difference
between the maximum display intensity and the minimum display
intensity divided by the sum of the minimum and maximum intensities.
In other terms [40], CM = (LDmax - LDmin) / (LDmax + LDmin)
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where LDmax = the maximum display luminance and LDmin = the
minimum display luminance. Extending the definition of contrast in
terms of estimated retinal illuminance gives CM = (IDmax - IDmin) /
(IDmax + IDmin) where IDmax = the maximum estimated retinal
illuminance due to the display and IDmin = the minimum estimated
retinal illuminance due to the display. In other words, the values of
IDmax and IDmin correspond to the estimated retinal illuminance
values of displays with luminance values of LDmax and LDmin
respectively.
Stereographic Displays using VRD
As discussed previously
while treating the possibility of three-dimensional imaging systems
using VRD there are two cues by which the human beings perceive the
real world namely the accommodation cue and the stereo cue. There is
a mismatch of the information conveyed by the two cues in projection
systems so that prolonged viewing can lead to some sort of
psychological disorientation. In VRD we can generate individual
wavefronts for each pixel and hence it is possible to vary the
curvature of individual wavefronts which determines the focal depth,
so what we get is a true stereographic view. The Virtual Retinal
Display (VRD) developed at the University of Washington Human
Interface Technology Lab (HIT Lab) is being modified from a fixed
plane of focus display to a variable focus display.. By integrating a
deformable mirror into the VRD, the wavefront of light being scanned
onto the retina can be changed and various fixation planes created
depending on the divergence of the light entering the eye. Previous
embodiments of 3D displays allowing for natural accommodation and
vergence responses include the use of a varifocal mylar mirror and
the use of a liquid-crystal varifocal lens. In the former, a
reflective mylar surface was deformed by air pressure
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using a loudspeaker behind the mylar mirror frame. A CRT screen
was positioned so that the viewers saw the reflection of the CRT in
the mirror at various virtual image depths. In the latter, an
electrically-controllable liquid-crystal varifocal lens was
synchronized with a 2-D display to provide a 3D image with a display
range of “1.2 to +1.5 diopters (1/focal length in meters). Although
these systems provided for a 3D volumetric image allowing for natural
human eye response, they are large and cumbersome benchtop systems.
Deformable Membrane Mirror The deformable membrane mirror is a MEMS
device that is used in adaptive optics applications. The mirror is
bulk micromachined and consists of a thin, circular membrane of
silicon nitride coated with aluminum and suspended over an electrode.
When a voltage is applied to the electrode, the mirror membrane
surface deforms in a parabolic manner above the electrode. The
wavefront of a beam of light hitting the mirror membrane surface can
be changed by varying the voltage applied to the electrode. With no
voltage applied, the mirror membrane surface remains flat. With a
certain amount of voltage applied, the reflecting beam will be made
more converging. By integrating the deformable mirror into the VRD
scanning system, a three-dimensional picture can be created by
quickly changing the scanned beamâ„¢s degree of collimation entering
the eye. Optical Design The HeNe laser beam is spatially filtered and
expanded before striking the deformable mirror. When the mirror is
grounded, the beam is at maximal divergence when entering the eye.
Conversely when the mirror voltage is at maximum, the resultant beam
is collimated when entering the eye. The beam is reflected off a
scanning galvanometer and through an ocular lens to form a viewing
exit pupil. A viewer putting his eye at the exit pupil would see a 1
-D image at a focal plane determined by the amount of beam
divergence. With no voltage on the mirror this image is located at
close range; with maximum voltage on the mirror the image is at
optical infinity. In this way the optical setup provides a range of
focal planes from near to far which can be manipulated by changing
the voltage on the mirror.
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Evolution of VRD systems
The
project's initial goal was to prove the viability of forming an image
on the retina using a scanned laser. As a result of the work, a
patent application was filed and the technology licensed to a Seattle
based start up company, Micro Vision, Inc. Under terms of the
agreement, Micro Vision is funding a four-year effort in the HITL to
develop the technologies that will lead to a commercially viable VRD
product. This development work began in November 1993. Prototype #1
The original prototype had very low effective resolution, a small
field of view, limited gray scale, and was difficult to align with
the eye. One objective of the current development effort was to
quickly produce a bench-mounted system with improved performance.
Prototype #1 uses a directly modulated red laser diode at a wave
length of 635 nanometers as the light source. The required horizontal
scanning rate of 73,728 Hertz could not be accomplished with a simple
galvanometer or similar commercially available moving mirror scanner.
The use of a rotating polygon was deemed impractical because of the
polygon size and rotational velocity required. It was thus decided to
perform the horizontal scan with an acousto-optical scanner. The
vertical scanning rate of 72 Hertz is within the range of
commercially available moving mirrors and is accomplished with a
galvanometer. The use of the acousto-optical scanner comes with a
number of drawbacks: * It requires optics to shape the input beam for
deflection and then additional optics to reform the output beam to
the desired shape. * It requires complex drive electronics that
operate at frequencies between 1.2 GHz and 1.8 GHz.
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* Its total scan angle is 4 degrees. Thus, additional optics are
needed to increase the angle to the desired field-of-view. Due to the
optical invariant, this optical increase in angle comes with the
penalty of decreased beam diameter which leads to a small exit pupil.
The small exit pupil necessitates precise alignment with the eye for
an image to be visible. * It is expensive and will not, in the
foreseeable future, allow the producers to reach the cost goals for a
complete VRD system. Prototype #2 To overcome the limitations of the
acousto-optical scanner, HITL engineers have developed a miniature
mechanical resonant scanner. This scanner, in conjunction with a
conventional galvanometer, provides both horizontal and vertical
scanning with large scan angles, in a compact package. The estimated
recurring cost of this scanner will allow the VRD system to be priced
competitively with other displays. Prototype #2 of the VRD uses the
mechanical resonant scanner.. The system was built and demonstrated
during the summer of 1994. The VGA resolution images produced are
sharp and spatially stable. The mechanical resonant scanner is used
in conjunction with a conventional galvanometer in a combination
which allows for an increase in the optical scan angle. When the
mirrors of the two scanners are arranged in such a manner that a
light beam undergoes multiple reflections off the mirrors, then the
optical scan is multiplied by the number of reflections off that
mirror. Optical scan multiplication factors of 2X, 3X and 4X have
been realized. Prototype #2 uses a system with 2X scan multiplication
in the horizontal axis. Prototype #3 The third prototype system
developed uses the same scanning hardware as Prototype #2 but uses
three light sources to produce a full color image. In addition the
eyepiece optics have been modified to allow for see through
operation. In the see through mode the image produced by the VRD is
overlaid on the external world.
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Present Scenario
In the current version, a wireless computer with a touch-pad control
is worn on the belt. Such units are largely used by the production
units of many industries, most of them automobile manufacturers. Like
a high-tech monocle, a clear, flat window angled in front of the
technician's eye reflects scanned laser light to the eye. That lets
the user view automobile diagnostics, as well as repair, service, and
assembly instructions superimposed onto the field of vision. The
information that the device displays comes from an automaker's
service-information Web site through a computer running Microsoft
Windows Server 2003 in the dealership or repair shop. The data gets
to the display via an ordinary IEEE 802.11b Wi-Fi network, and all
the technicians in the service center are able to access different
information simultaneously from one server. Typical MEMS scanner
today measures about 5 mm across, with a 1.5-mmdiameter scan mirror
capable of motion on two scan axes simultaneously Using MEMS allows
us to integrate the scanner, coil windings, and angle-sensor
functions all on one chip. Such a scanner provides SVGA (800-by-600)
equivalent resolution at a 60-hertz refresh rate and is now in
production and in products. In addition, multiple scanners could
provide higher-resolution images by each providing full detail in a
tiled subarea. Eventually, costs will become low enough to make this
practical, allowing the scannedbeam approach to surpass the
equivalent pixel count of any other display technology. With green
laser diodes, it will be possible to build bright, full-color see-
through displays . Microvision uses laser light sources in many of
its see-through products because our customers' applications demand
display performances with color-gamut and brightness levels far
exceeding the capabilities of flat panel displays, notebook displays,
and even higher-end desktop displays. For today's commercial
products, only red laser diodes are small enough, efficient enough,
and cheap enough to use in such see-through mobile devices as Nomad.
Blue and green diode-pumped solid-state lasers are still too
expensive for bright, full-color, head-up or projection displays for
mainstream markets, but that could change soon. In the mid-1990s
Shuji Nakamura of Nichia Chemical Industries Ltd. (now Nichia Corp.,
Tokushima, Japan) demonstrated efficient blue and
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green LEDs, and then blue laser diodes made of gallium nitride.
When these designs and materials are extended to green laser diodes,
it will be possible to build bright, full-color see-through displays.
. As an alternative, small green laser are now being produced which
use a crystal to frequency double a neodymium YAG laser. These
devices are larger than desired and are not directly modulatable at
the required frequency. They do however, offer a short term solution.
In the HITL researchers are investigating a number of alternatives to
blue and green laser diodes. One frequency doubling technique being
researched uses rare earth doped fibers as the doubling medium. A
second technique uses wave guides placed in a lithium niobate
substrate for the doubling. The above methods all utilize a laser as
the light source. Additional work is directed at using non-lazing,
light-emitting diodes (LEDs) as the light source. In order for this
to be successful two primary issues are being addressed. The first
issue is how to focus the LED output to the desired spot size. The
second issue is the development of fabrication techniques that will
allow us to directly modulate the LEDs at the desired frequency.
Enter the edge-emitting LED. Unlike conventional LEDs, which emit
light from the surface of the chip, an edge-emitting LED has a
sandwich-like physical structure similar to that of an injection-
laser diode, but it operates below the lasing threshold. These LEDs
emit incoherent beams of light that, while not so fine as a laser's
beam, provide a tenfold increase in brightness. We also use multiple
inexpensive surfaceemitting LEDs, each contributing a portion of the
overall power, to achieve high brightness. Further performance
improvements of LED materials driven by huge investments aimed at
general lighting applications will increase the brightness and range
of applications for scanned-beam displays based on green and blue
gallium nitride devices and aluminum gallium indium phosphide red
LEDs.
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In addition to displaying images, the
scanned-beam technology can capture them. In a display, the data
channel through a digital-to-analog converter controls the light
source to paint a picture on a blank canvas. In image capture, the
light source is steadily on, and the data channel looks at the
reflections from the object through an analog-todigital converter
connected to a photodiode. The light source, beam optics, and scanner
are essentially the same in both applications
Laser safety analysis
Maximum Permissible Exposures (MPE) have been calculated for the VRD
in both normal viewing and possible failure modes. The MPE power
levels are compared to the measured power that enters the eye while
viewing images with the VRD. The power levels indicate that the VRD
is safe in normal operating mode and failure modes. The scanned beam
is passed through a lens system which forms an exit pupil about which
the scanned beam pivots. The user places themselves such that their
pupil is positioned at the exit pupil of the system. This is called a
Maxwellian view optical system. The lens of the eye focuses the light
beam on the retina, forming a pixel image. The following figure
(fig.10) compares the illumination of the retina by a pixelbased
display versus the VRD. Inset figures show schematized light
intensity over any given retinal area in the image. Typical pixel-
based displays such as CRTs have persistence of light emission over
the frame refresh cycle, whereas the VRD illuminates in brief
exposures.
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Figure.12 Preliminary tests and
calculations of VRD images demonstrated that the system's power
output with typical images is below the maximum permissible exposure
(MPE) limits established for various lighting schemes. Measures of
power output with typical images indicate that the VRD generates
power on the order of 200 nanowatts during normal operation. This is
below the Class 1 laser power limit of 400 nanowatts. If failure were
to occur, i.e. if scanning were to stop in one or both dimensions,
the power limits indicate the mechanism is still safe. To use t
Virtual Retinal Display
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.1
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 colorsâ€red, green,
and blueâ€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).
Fig1. Basic block diagram of the
Virtual Retinal Display. The resultant imaged formed on the retina is
perceived as a wide field of view image originating from some viewing
distance in space. The following figure illustrates the light raster
on the retina and the resultant image perceived in space.
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Fig2.Illustration of light raster imaged onto the retina and the
resultant perceived image. In general, a scanner (with magnifying
optics) scans a beam of collimated light through an angle. Each
individual collimated beam is focused to a point on the retina. As
the angle of the scan changes over time, the location of the
corresponding focused spot moves across the retina. The collection of
intensity modulated spots forms the raster image as shown above
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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.
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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. Moreover, it will be quite
surprising to know that the original stereographic display, or the
three dimensional view as the eye means it, can be accomplished only
by an imaging system like the one proposed above.
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Fundamentals of human eye
The eye is a specialized organ that is
capable of light reception, and in the case of vertebrates, is able
to receive visual images and then carry it to the visual centre in
the brain. The horizontal sectional view of human eye is as follows
(courtesy Encyclopedia Britannica 2002)
Fig3. The cross sectional view
of the human eye The eyeball is generally described as a globe or a
sphere, but it is oval, not circular. It is about an inch in
diameter, transparent in front, and composed of three layers. 1) The
outer fibrous, the supporting layer 2) Middle, vascular, and 3) Inner
nervous layer.
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Six muscles move the eye, four
straight and two oblique. These lie inside the orbit passing from the
bony walls of the orbit to be attached to the sclerotic coat of the
eye behind the cornea. The movements of the eyes are combined, both
eyes move to right or left, up, and down, etc. Normally the axes of
both the eyes converge simultaneously on the same point; when owing
to paralysis of one or more muscles, they fail to do so squint
exists. The Sclera is the tough outer fibrous coat. It forms the
white of the eye and is continuous in front with the transparent
window membrane, the cornea. The sclera protects the delicate
structures of the eye and helps to maintain the shape of the eyeball.
The Choroid or middle vascular coat contains the blood vessels, which
are the ramifications of the ophthalmic artery, a branch of the
internal carotid. The vascular coat forms the iris with the central
opening or pupil of the eye. The pigmented layer behind the iris
gives its colour and determines whether the eye is blue, brown, grey
etc. The horoids is continuous in the front with the iris and just
behind the iris this coat is thickened to form the ciliary body, thus
the ciliary body lies between the choroids and the iris. It contains
circular muscle fibres and radiating fibres; contraction of the
former contracts the pupil of the eye.
The Retina is the inner nervous
coat of the eye, composed of a number of layers of fibres, nerve
cells, rods and cones, all of which are included in the construction
of the retina, the delicate nerve tissue conducting the nerve
impulses from without inwards to the optic disc, the point where the
optic nerve leaves the eyeball. This is the blind spot, as it
possesses no retina. The most acutely sensitive part of the retina is
the macula, which lies just external to the optic disc, and exactly
opposite the centre of the pupil.
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Fig4.The layered
view of retina showing blood vessels The retina is the nervous
mechanism of sight. It contains the endings of the optic nerves, and
is comparable to a sensitive photographic plate. When an image is
perceived, rays of light from the object seen pass through the
cornea, aqueous humour, lens, and vitreous body to stimulate the
nerve endings in the retina. The stimuli received by the retina pass
along the optic tracts to the visual areas of the brain, to be
interpreted. Both areas receive the message from both eyes, thus
giving perspective and contour. In ordinary camera one lens is
provided. In the eye, whilst the crystalline lens is very important
in focusing the image on the retina, there are in all four structures
acting as lenses: the cornea, the aqueous humour, the crystalline
lens, and the vitreous body. As in all interpretations of sensation
from the surface, a number of relaying stations are concerned with
the transmission of the senses which in this case is the sight. A
number of these relaying stations are in the retina. Internal to the
periphery of the retina are layers of rods and cones which are highly
specialized sight cells sensitive to light. The circular
interruptions in these are termed as granules. The proximal ends of
the rods and cones form the first synapse with a layer if bipolar
cells, still in the retina. The second processes of these cells form
the second nerve synapse with large ganglion cells,
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also in the retina. The axons of these cells form the fibres of
the optic nerve. These pass backwards, first reaching the lower
centre in special bodies near the thalamus, and finally reaching the
special visual centre in occipital lobe of cerebral hemisphere where
sight is interpreted.
Fig5. The Human visual pathway Each retina
includes multiple mosaics of neurons that separately represent the
visual field. Image transduction uses two systems of photoreceptors:
the rods and cones. Each system comprises a separate sampling mosaic
of retinal image. The rods encode the data for a system with low
spatial resolution but high quantum efficiency. The cones encode the
image data at much higher spatial resolution and lower quantum
efficiency. Rods and cones generally operate under different viewing
conditions, but there are also many cases in which multiple
representations of the image are obtained under a single viewing
condition. For example, the cones can be subdivided into three
sampling mosaics that expand the spectral encoding. The three cone
mosaics also differ in their spatial sampling properties.
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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. In 1993, a newly formed corporation, MicroVision
Inc., licensed the VRD technology and signed a 4 year, $5.1 million
development contract with the University. Rich Johnston was hired
specifically to manage the VRD and other hardware products of the
Lab. By forming relationships with other researchers in the College
of Engineering, he has orchestrated a program to solve the challenges
and bottlenecks of the project. In late 1993 and 1994, Mike Tidwell
redesigned the VRD to maximize the resolution possible with the A-O
scanner while David Melville designed a new Mechanical Resonant
Scanner (MRS) which would be capable of the high rates of horizontal
scanning without the costs and other limitations of the A-O devices.
The MRS was then utilized in full-color inclusive and "see-through"
systems.
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Virtual Retinal Display- A system overview
The VRD can be considered a portable system that creates the
perception of an image by scanning a beam of light directly into the
eye. Most displays directly address a real image plane (typically a
CRT or matrix-addressed LCD) which might be relayed to form a larger,
more distant image for a head-mounted display (HMD). The VRD uses a
scanned, modulated light beam to treat the retina as a projection
screen, much as a laser light show would use the ceiling of a
planetarium. The closest previously existing device would be the
scanning laser opthalmoscope (SLO) which scans the retina to examine
it; the SLO is designed to capture light returning from the eye
whereas the VRD is designed as a portable display.. The VRD has
several advantages over CRTs, LCD, and other addressable-screen
displays:
¢
Resolution is limited by beam diffraction and optical
aberrations, not by the size of an addressable pixel in a matrix.
Very high resolution images are therefore possible without extensive
advances in micro-fabrication technology. Also, the VRD does not
suffer from pixel defects.
¢
The display can be made as bright as
desired simply by controlling the intensity of the scanned beam. This
makes it much easier to use the display in "see-though" configuration
on a bright day.
¢
The scanning technology in the current display
requires only simple, well understood manufacturing technology and
can therefore be manufactured inexpensively.
¢
Because the light is
projected into the eye and the scanner is electro-mechanically
efficient, the display uses very little power. In theory, the VRD
allows for accommodation to be modulated pixel by pixel as the image
is being scanned.
¢
All components in the VRD are small and light,
making them ideal for use in a portable display.
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The Basic System In a conventional display a real image is
produced. The real image is either viewed directly or, as in the case
with most head-mounted displays, projected through an optical system
and the resulting virtual image is viewed. The projection moves the
virtual image to a distance that allows the eye to focus comfortably.
No real image is ever produced with the VRD. Rather, an image is
formed directly on the retina of the user's eye. A block diagram of
the VRD is shown in the figure below.
Fig6. The functional block
diagram of a VRD system
To create an image with the VRD a photon
source (or three sources in the case of a color display) is used to
generate a coherent beam of light. The use of a coherent source (such
as a laser diode) allows the system to draw a diffraction limited
spot on the retina. The light beam is intensity modulated to match
the intensity of the image being rendered.
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The
modulation can be accomplished after the beam is generated. If the
source has enough modulation bandwidth, as in the case of a laser
diode, the source can be modulated directly. The resulting modulated
beam is then scanned to place each image point, or pixel, at the
proper position on the retina. A variety of scan patterns are
possible. The scanner could be used in a calligraphic mode, in which
the lines that form the image are drawn directly, or in a raster
mode, much like standard computer monitors or television. Our
development focuses on the raster method of image scanning and allows
the VRD to be driven by standard video sources. To draw the raster, a
horizontal scanner moves the beam to draw a row of pixels. The
vertical scanner then moves the beam to the next line where another
row of pixels is drawn. After scanning, the optical beam must be
properly projected into the eye. The goal is for the exit pupil of
the VRD to be coplanar with the entrance pupil of the eye. The lens
and cornea of the eye will then focus the beam on the retina, forming
a spot. The position on the retina where the eye focuses the spot is
determined by the angle at which light enters the eye. This angle is
determined by the scanners and is continually varying in a raster
pattern. The brightness of the focused spot is determined by the
intensity modulation of the light beam. The intensity modulated
moving spot, focused through the eye, draws an image on the retina.
The eye's persistence allows the image to appear continuous and
stable. Finally, the drive electronics synchronize the scanners and
intensity modulator with the incoming video signal in such a manner
that a stable image is formed
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VRD Features The
following sections detail some of the advantages of using the VRD as
a personal display. Size and Weight The VRD does not require an
intermediate image on a screen as do systems using LCD or CRT
technology. The only required components are the photon source
(preferably one that is directly modulatable), the scanners, and the
optical projection system. Small photon sources such as a laser diode
can be used. As described below the scanning can be accomplished with
a small mechanical resonant device developed in the HITL. The
projection optics could be incorporated as the front, reflecting,
surface of a pair of glasses in a head mount configuration or as a
simple lens in a hand held configuration. HITL engineers have
experimented with single piece Fresnel lenses with encouraging
results. The small number of components and lack of an intermediate
screen will yield a system that can be comfortably head mounted or
hand held. Resolution Resolution of the current generation of head
mounted and hand held display devices is limited by the physical
parameters associated with manufacturing the LCDs or CRTs used to
create the image. No such limit exists in the VRD. The limiting
factors in the VRD are diffraction and optical aberrations from the
optical components of the system, limits in scanning frequency, and
the modulation bandwidth of the photon source. A photon source such
as a laser diode has a sufficient modulation bandwidth to handle
displays with well over a million pixels. If greater resolution is
required multiple sources can be used.
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Currently
developed scanners will allow displays over 1000 lines allowing for
the HDTV resolution systems. If higher resolutions are desired
multiple sources, each striking the scanning surface at a different
angle, can be used. . Field of View The field of view of the VRD is
controlled by the scan angle of the primary scanner and the power of
the optical system. Initial inclusive systems with greater than 60
degree horizontal fields of view have been demonstrated. Inclusive
systems with 100 degree fields of view are feasible. See through
systems will have somewhat smaller fields of view. Current see
through systems with over 40 degree horizontal fields of view have
been demonstrated. Color and Intensity Resolution Color will be
generated in a VRD by using three photon sources, a red, a green, and
a blue. The three colors will be combined such that they overlap in
space. This will yield a single spot color pixel, as compared to the
traditional method of closely spacing a triad, improving spatial
resolution. The intensity seen by the viewer of the VRD is directly
related to the intensity emitted by the photon source. Intensity of a
photon source such as a laser diode is controlled by the current
driving the device. Proper control of the current will allow greater
than ten bits of intensity resolution per color. Brightness
Brightness may be the biggest advantage of the VRD concept. The
current generations of personal displays do not perform well in high
illumination environments. This can cause significant problems when
the system is to be used by a soldier outdoors or by a doctor in a
well lit operating room. The common solution is to block out as much
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ambient light as possible. Unfortunately, this
does not work well when a see through mode is required. The VRD
creates an image by scanning a light source directly on the retina.
The perceived brightness is only limited by the power of the light
source. Through experimentation it has been determined that a bright
image can be created with under one microwatt of laser light. Laser
diodes in the several milliwatt range are common. As a result,
systems created with laser diode sources will operate at low laser
output levels or with significant beam attenuation. Power Consumption
The VRD delivers light to the retina efficiently. The exit pupil of
the system can be made relatively small allowing most of the
generated light to enter the eye. In addition, the scanning is done
with a resonant device which is operating with a high figure of
merit, or Q, and is also very efficient. The result is a system that
needs very little power to operate. A True Stereoscopic Display The
traditional head-mounted display used for creating three dimensional
views projects different images into each of the viewer's eyes. Each
image is created from a slightly different view point creating a
stereo pair. This method allows one important depth cue to be used,
but also creates a conflict. The human uses many different cues to
perceive depth. In addition to stereo vision, accommodation is an
important element in judging depth. Accommodation refers to the
distance at which the eye is focused to see a clear image. The
virtual imaging optics used in current head-mounted displays place
the image at a comfortable, and fixed, focal distance. As the image
originates from a flat screen, everything in the virtual image, in
terms of accommodation, is located at the same focal distance.
Therefore, while the stereo cues tell the viewer an object is
positioned at one distance, the accommodation cue indicates it is
positioned at a different distance.
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With the VRD it
is theoretically (this is currently in the development stage)
possible to generate a more natural three dimensional image. The VRD
has an individual wavefront generated for each pixel. It is possible
to vary the curvature of the wavefronts. Note that it is the
wavefront curvature which determines the focus depth. This variation
of the image focus distance on a pixel by pixel basis, combined with
the projection of stereo images, allows for the creation of a more
natural three-dimensional environment. Inclusive and See Through
Systems have been produced that operate in both an inclusive and a
see through mode. The see through mode is generally a more difficult
system to build as most displays are not bright enough to work in a
see through mode when used in a medium to high illumination
environment where the luminance can reach ten thousand candela per
meter squared. As discussed above, this is not a problem with the
VRD.
In the VRD a light source is modulated with image information,
either by direct power ("internal") modulation or by an external
modulator. The light is passed through an x-y scanning system,
currently the MRS and a galvanometer. Light from the scanner pair
enters an optical system, which in present implementations of the VRD
forms an aerial image and then uses and eyepiece to magnify and relay
this image to infinity.
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Components of the Virtual
Retinal Display
Video Electronics In its current form, the video
electronics of the VRD controls the light intensity modulation,
scanner deflection, and the synchronization between modulation and
scanning. The horizontal and vertical synchronization signals in the
video signal are used to determine scanner synchronization. A user
selectable delay of up to one full line is incorporated into the
video electronics to allow for phase difference between the
horizontal scanner position and the modulation timing. Also, the
respective drive levels for intensity modulation of each light source
are output from the electronics. The drive electronics control the
acousto-optic modulators that encode the image data into the pulse
stream. The color combiner multiplexes the individually-modulated
red, green, and blue beams to produce a serial stream of pixels,
which is launched into a single mode optical fiber to propagate to
the scanner assembly. The drive electronics receive and process an
incoming video signal, provide image compensation, and control image
display. For VGA projection, the electronics process over 18 Mpix/s.
The virtual retinal display is capable of providing UXGA resolution
of 1600 x 1200 or 115 Mpix/s. Light Sources and Modulators The light
sources for the VRD generate the photons which eventually enter the
eye and stimulate the photo receptors in the retina. The modulation
of the light source determines the intensity of each picture element.
The size of the scanning spot and the rate at which it can be
modulated determine the effective size of each picture element on the
retina. As the light is scanned across the retina, the intensity is
synchronized with the instantaneous position of the spot thereby
producing a two dimensional pattern of modulated light that is
perceived as a picture.
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According to conventional
additive color theory, any color can be represented as a mixture of
three appropriately chosen primaries. The three ideal VRD light
sources would be monochromatic for maximum possible color
saturation.. Spatial coherence is also important - larger source
spots will correspond to larger spots on the retina, decreasing
resolution. The primary cause of the real (if sometimes exaggerated)
hazards of laser light are the result of spatially coherent light
focusing to a small area on the retina, causing highly localized
heating and ablation of tissue. In the VRD the spot is traveling in
two directions and even when stationary is not at a power level that
would cause damage. We are working with ophthalmologists and will
publish a definitive article on this in the near future.
Incidentally, polychromatic sources can be shown to form spots
comparable to monochromatic ones of the same spatial extent.
Therefore spatial coherence is responsible for the small spot size
which leads to both high resolution and (given enough power) retinal
hazard. To achieve the desired resolution, all current VRD prototypes
have used lasers for their superior spatial coherence
characteristics. In order to use a point source such as an LED, the
image of the source should be smaller than the diffraction limit of
the scanner. Using the lens magnification, one can determine the
maximum source size that can be used before degrading the diffraction
limited spot size at the image plane. The angular divergence of the
source is effectively limited by treating the scanner as a stop.
Light which does not hit the mirror does not contribute to the image
plane spot size. From this geometric argument we can derive an
equivalent point source size between 4 and 5 microns for a VGA
resolution image in our current system. For a system where the
scanner is illuminated with a collimated Gaussian beam, similar
arguments can be made to determine the required divergence and beam
waist from the equations for image plane spot size. The light source
module contains laser light sources, acousto-optic modulators to
create the pulse stream, and a color combiner that multiplexes the
pulse streams. To provide sufficient brightness, full-color displays
suitable for outdoor, daylight applications incorporate red diode
lasers (635nm), green solid-state lasers (532 nm), and blue solid-
state or argon gas lasers (450-470 nm range). Systems designed
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for indoor use can incorporate LEDs; red, blue,
and green devices currently under development for such systems are
being tested. Generally, the energy levels are on the order of
nanowatts to milliwatts, depending on display requirements. The
levels of light involved are well within laser safety standards for
viewing, as confirmed by analysis. Generally two types of intensity
modulation of lasers are done in existing designs. They are Laser
diode modulation and acousto-optical modulation. The laser diode
modulation is generally used for red laser. The small rise time of
the solid state diode laser device allows high bandwidth (up to 100
[MHz]) analog modulation. The video electronics regulate the voltage
seen by the laser current driver and it controls the current passing
through the laser which in turn controls the light output power from
the laser. The laser diode is operated between amplitudes of 0.0 and
80.0[mA]. Acousto-optic (A-O) modulators intensity modulate the green
and blue laser beams. Acousto-optic modulators create a sound wave
grating in a crystal through which a light beam passes. The sound
wave creates alternate regions of compression and rarefaction inside
the crystal. These alternating regions locally change the refractive
index of the material. Areas of compression correspond to higher
refractive indices and areas of rarefaction correspond to lower
refractive indices. The alternating areas of refractive index act as
a grating and diffract the light. As the sound wave traverses the
light beam, the diffracted beam is intensity modulated according to
the amplitude modulated envelope on the carrier signal. Scanners The
scanners of the VRD scan the raster pattern on the retina. The
angular deviation of the horizontal scanner combined with the angular
magnification of the imaging optics determines the horizontal field
of view. The angular deviation of the vertical scanner combined with
the angular magnification of the imaging optics determines the
vertical field of view. The horizontal scanner speed and the frame
rate determine the number of horizontal lines in the display,
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Number of horizontal lines = horizontal scanner
frequency / frame rate, where frame rate is the number of times per
second the entire picture (or frame) is generated. The modulation
rate and the horizontal scanner frequency determine the number of
pixels per line in the display, Number of pixels per line =
modulation frequency / horizontal scanner frequency, where the
modulation frequency is the number of times per second the pixels are
created (or modulated). The horizontal scanning mechanism of the VRD
must be capable of both relatively high scan rates (15 kHz-90+ kHz)
and high resolution (500-2000+ pixels) for NTSC to HDTV formats,
respectively. SVGA format systems (80 kHz) in monochrome/greyscale
using an A-O scanner and 30 kHz in full-color with a mechanical
resonant one have been built. The scanning device consists of a
mechanical resonant scanner and galvanometer mirror configuration.
The horizontal scanner is the mechanical resonant scanner (MRS)]. The
MRS has a flux circuit induced by coils which are beneath a spring
plate. The flux circuit runs through the coils and the spring plate
and alternately attracts opposite sides of the spring plate and
thereby moves the scanner mirror through an angle over time. In a
design developed at the HITL the vertical deflection mirror was
chosen as the galvanometer mirror. The galvanometer deflection can be
selected according to the aspect ratio of the display and a typical
ratio of 4:3 can be chosen. The galvanometer frequency is controlled
by the video electronics to match the video frame rate. The
galvanometer and horizontal scanner are arranged in what is believed
to be a novel configuration such that the horizontal scan is
multiplied. The scanners are arranged, as shown in the following
figures. Such that the beam entering the scanner assembly first
strikes the horizontal scanner then strikes the vertical scanner. The
beam is reflected by the vertical scanner back to the horizontal
scanner before exiting the scanner assembly. The beam therefore
strikes the horizontal scanner twice before exiting the scanner
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configuration. In such an arrangement, the first
scan (corresponding to the first bounce or reflection) is doubled by
the second scan (corresponding to the second bounce or reflection).
The case shown is for = 45 [deg.] wherein the exit beam returns
parallel to the horizontal incident beam. In the first figure the MRS
is undeflected and in the latter the MRS is deflected by d [deg.].
Fig7. MRS/Galvanometer scanner assembly showing incident and exit
beam paths for the MRS in an undeflected position.
Fig8.
MRS/Galvanometer scanner assembly showing incident and exit beam
paths for the MRS in a deflected position.
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The
result of arranging the scanners as in the above figures is a
doubling of the horizontal optical scan angle. Other configurations
have been applied to this approach to achieve a tripling in the
horizontal direction and simultaneously a doubling in the vertical
direction. For more compact designs, techniques from micro electro-
mechanical systems maybe utilized in the fabrication of scanners. The
electrostatic actuation of a MEMS scanner had been developed. By
etching thin layers from a sliver of silicon, the researchers were
able to build a scanner that weighs a mere 5 grams and measures less
than 1 square centimeter. The mirror, too, is much smaller at 1
millimeter across and is mounted on the end of a thin, flexible, bar
which is anchored to the silicon. The mirror is turned into one plate
of a capacitor, with the other plate formed by a small area of
silicon beneath it. Put a rapidly varying voltage across the two
plates and then the mirror will be first repelled and then attracted.
The mirror can move up or down more than 30,000 times each second.
Fig9. A MEMS mirror Micro-Electro-Mechanical Systems (MEMS) is the
integration of mechanical elements, sensors, actuators, and
electronics on a common silicon substrate through the utilization of
microfabrication technology. The electronics are fabricated using
integrated circuit (IC) process sequences, while the micromechanical
components are fabricated using compatible "micromachining" processes
that selectively etch away parts of the
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silicon
wafer or add new structural layers to form the mechanical and
electromechanical devices. The electromagnetic actuation of the
scanners yields more life to the system and imparts more torque. Such
designs have also been developed for retinal scanning displays.
Pupil
expander Nominally the entire image would be contained in an area of
2 mm2. The exitpupil expander is an optical device that increases the
natural output angle of the image and enlarges it up to 18 mm on a
side for ease of viewing. The raster image created by the horizontal
and vertical scanners passes through the pupil expander and on to the
viewer optics. For applications in which the scanned-beam display is
to be worn on the head or held closely to the eye, we need to deliver
the light beam into what is basically a moving target: the human eye.
Constantly darting around in its socket, the eye has a range of
motion that covers some 10 to 15 mm. One way to hit this target is to
focus the scanned beam onto exit pupil expander. When light from the
expander is collected by a lens, and guided by a mirror and a see-
through monocle to the eye, it covers the entire area over which the
pupil may roam. For applications that require better image quality
using less power, we can dispense with the exit pupil expander
altogether either by using a larger scan mirror to make a larger exit
pupil or by actively tracking the pupil to steer light into it.
Viewer optics The viewer optics relay the scanned raster image to the
oculars worn by the user. The optical system varies according to the
application. In the case of military applications such as helmet
mounted or head mounted display optics, the system incorporates glass
and or plastic components; for medical applications such as image-
guided surgery, headmounted plastic optics are used. In industrial or
personal displays, the optics might be a
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simple
plastic lens. A typical viewing system that was employed in a VRD
developed at HITL is as follows. The viewing optics, or the optics
through which the user sees the intended image, are diagrammed in the
following figure. The convergent tri-color beams emanating from the
scanner pass (partially) through a beamsplitter. The beamsplitter (or
beamsplitter/combiner) is coated such that 40% of any light striking
it is reflected and 60% is transmitted. The transmittance/reflectance
is somewhat angle dependent but this dependence is not severe. On
first pass, 60% of the energy in the scan is transmitted through the
splitter/combiner to a concave spherical mirror. The mirror is
actually a rectangular section of a spherical mirror with radius of
curvature -100 [mm]. The negative sign denotes concavity .
Fig10. The
viewing optics system of VRD
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Holographic Optical
Element One of the problems with the VRD only becomes apparent when
you put it on. It can be likened to looking through a pair of high-
magnification binoculars that one must line his eyes precisely with
the beam or the image disappears. Since we rarely fix our eyes on a
single point for more than few seconds, using VRD becomes difficult.
So en eye-tracking system that follows the movements of the pupil by
monitoring the reflections from the cornea had to be developed. The
tracker calculates where the eye is looking and moves the laser
around to compensate. But this system is complex and expensive. A
better solution may lie with a special kind of lens known as a
holographic optical element. An HOE is actually a diffraction grating
made by recording a hologram inside a thin layer of polymer. It works
by converting a single beam of laser into a circular array of 15
bright spots. Place the HOE between the scanning mirrors and the eye,
and the array of beams that forms will illuminate the region round
your pupil. Move your eyes slightly and one of the beams will still
strike the cornea and be focused to form an image on the retina. HOEs
have a big advantage over eye tracking systems: because they are made
from a thin layer of polymer, they weigh next to nothing. All of the
action takes place in a layer just a fraction of millimeter thick,
says a researcher.
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Estimated Retinal Illuminance
The
relationship between estimated retinal illuminance and scene
luminance is important in understanding the display operating on this
principle. As the display in this thesis contains no screen or real
object, it is impossible to discuss the brightness of the display in
terms of luminance. In terms of brightness, estimated retinal
illuminance is a common denominator, so to speak, of screen based
display systems and retinal scanning displays systems. The estimated
retinal illuminance is [36]: I (trolands) = R x pupil area (mm2) x
scene luminance (cd/m2) where I = retinal illuminance, "pupil area"
refers to the area of the pupil of the eye, and R = the effectivity
ratio. The effectivity ratio, R, allows for the Stiles-Crawford
effect and is, R = 1 - 0.0106d2 + 0.0000416d4. where d = the eye's
pupil diameter in millimeters. As shown by dimensional analysis on
the equation for I , trolands reduce effectively to the units of
optical power per unit steradian. The Stiles-Crawford effect
describes the contribution to brightness sensation of light entering
different points of the pupil (i.e. light entering the center of the
pupil contributes more to the sensation of brightness than does light
entering farther from the pupil center). Some standard scene
luminance values, L, and their corresponding Stiles-Crawford
corrected estimated retinal illuminance values, I, are given in Table
II.1 [36,37]. Approximate Luminance [cd/m ] 104 103 102
2
Type of Scene
Clear day Overcast day Heavily overcast day
Estimated
Retinal
Illuminance [trolands] 3.0 x 104 4.5 x 103 9.5 x 102
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Sunset, overcast day 1/4 hour after sunset, clear 1/2 hour after
sunset, clear Fairly bright moonlight Moonless, clear night sky
Moonless, overcast night sky
10 1 10-1 10
-2
1.5 x 102 20 2.0 0.23 2.7 x
10-2 3.0 x 10-3
10-3 10-4
Table1. Standard scene luminance values and
corresponding estimated retinal illuminance values. Transmission
Characteristics of the Ocular Media Transmission losses in the eye
result from scattering and absorption in the cornea, lens, aqueous
humor, and vitreous humor. The transmittance of the ocular media is a
function of the wavelength of the light traveling through the media.
Figure 2.2 shows a plot of the total transmittance of the ocular
media as a function of wavelength [38].
Fig11. Transmittance of the
ocular media vs. wavelength.
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Image Quality as Related
to the Eye
Introduction Measurements of display image quality depend
heavily on two display characteristics, resolution and "contrast"
(see subsequent sections). It is virtually fruitless to discuss image
quality in terms of either resolution or "contrast" without including
the other. Definitions for display resolution, contrast, contrast
ratio, and modulation contrast are given in the following discussion.
Whenever possible, the meanings of the terms are related to the
effect or result at the retina. Display Resolution and the Eye The
resolution of a display can be defined as the angle subtended by each
display resolution element. For a screen (CRT or LCD) based display,
the angular extent of each pixel element determines the resolution.
For the VRD, the angular extent of each spot on the retina dictates
the system resolution. A spot of extent h on the retina allows for an
angular resolution of,
tan-1[h/feye]
where feye is the focal length
of the eye. Display resolution is often measured in cycles per degree
for periodic gratings such as bar patterns or sinusoidal gratings.
Display Contrast and the Eye The contrast, C, of a display is the
ratio of the difference between the maximum display intensity and the
minimum display intensity divided by the maximum. In other terms
[40], C = (LDmax - LDmin) / LDmax where LDmax = the maximum display
luminance and LDmin = the minimum display luminance. Extending the
definition of contrast in terms of estimated retinal illuminance
gives
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C = (IDmax - IDmin) / IDmax. where IDmax =
the maximum estimated retinal illuminance due to the display and
IDmin = the minimum estimated retinal illuminance due to the display.
In other words, the values of IDmax and IDmin correspond to the
estimated retinal illuminance values of displays with luminance
values of LDmax and LDmin respectively. In the case of a retinal
scanning display, as in this thesis, estimated retinal illuminance is
a preferable measure of display brightness as there is no screen in
the system. Display Contrast Ratio and the Eye The contrast ratio,
CR, of a display is the ratio of the maximum display intensity to the
minimum display intensity. In other terms [40], CR = (LDmax/LDmin)
where LDmax = the maximum display luminance and LDmin = the minimum
display luminance. Extending the definition of contrast in terms of
estimated retinal illuminance gives CR = (IDmax/IDmin) where IDmax =
the maximum estimated retinal illuminance due to the display and
IDmin = the minimum estimated retinal illuminance due to the display.
The values of IDmax and IDmin correspond to the estimated retinal
illuminance values for displays with luminance values of LDmax and
LDmin respectively. Display Modulation Contrast and the Eye The
modulation contrast, CM, of a display is the ratio of the difference
between the maximum display intensity and the minimum display
intensity divided by the sum of the minimum and maximum intensities.
In other terms [40], CM = (LDmax - LDmin) / (LDmax + LDmin)
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where LDmax = the maximum display luminance and LDmin = the
minimum display luminance. Extending the definition of contrast in
terms of estimated retinal illuminance gives CM = (IDmax - IDmin) /
(IDmax + IDmin) where IDmax = the maximum estimated retinal
illuminance due to the display and IDmin = the minimum estimated
retinal illuminance due to the display. In other words, the values of
IDmax and IDmin correspond to the estimated retinal illuminance
values of displays with luminance values of LDmax and LDmin
respectively.
Stereographic Displays using VRD
As discussed previously
while treating the possibility of three-dimensional imaging systems
using VRD there are two cues by which the human beings perceive the
real world namely the accommodation cue and the stereo cue. There is
a mismatch of the information conveyed by the two cues in projection
systems so that prolonged viewing can lead to some sort of
psychological disorientation. In VRD we can generate individual
wavefronts for each pixel and hence it is possible to vary the
curvature of individual wavefronts which determines the focal depth,
so what we get is a true stereographic view. The Virtual Retinal
Display (VRD) developed at the University of Washington Human
Interface Technology Lab (HIT Lab) is being modified from a fixed
plane of focus display to a variable focus display.. By integrating a
deformable mirror into the VRD, the wavefront of light being scanned
onto the retina can be changed and various fixation planes created
depending on the divergence of the light entering the eye. Previous
embodiments of 3D displays allowing for natural accommodation and
vergence responses include the use of a varifocal mylar mirror and
the use of a liquid-crystal varifocal lens. In the former, a
reflective mylar surface was deformed by air pressure
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using a loudspeaker behind the mylar mirror frame. A CRT screen
was positioned so that the viewers saw the reflection of the CRT in
the mirror at various virtual image depths. In the latter, an
electrically-controllable liquid-crystal varifocal lens was
synchronized with a 2-D display to provide a 3D image with a display
range of “1.2 to +1.5 diopters (1/focal length in meters). Although
these systems provided for a 3D volumetric image allowing for natural
human eye response, they are large and cumbersome benchtop systems.
Deformable Membrane Mirror The deformable membrane mirror is a MEMS
device that is used in adaptive optics applications. The mirror is
bulk micromachined and consists of a thin, circular membrane of
silicon nitride coated with aluminum and suspended over an electrode.
When a voltage is applied to the electrode, the mirror membrane
surface deforms in a parabolic manner above the electrode. The
wavefront of a beam of light hitting the mirror membrane surface can
be changed by varying the voltage applied to the electrode. With no
voltage applied, the mirror membrane surface remains flat. With a
certain amount of voltage applied, the reflecting beam will be made
more converging. By integrating the deformable mirror into the VRD
scanning system, a three-dimensional picture can be created by
quickly changing the scanned beamâ„¢s degree of collimation entering
the eye. Optical Design The HeNe laser beam is spatially filtered and
expanded before striking the deformable mirror. When the mirror is
grounded, the beam is at maximal divergence when entering the eye.
Conversely when the mirror voltage is at maximum, the resultant beam
is collimated when entering the eye. The beam is reflected off a
scanning galvanometer and through an ocular lens to form a viewing
exit pupil. A viewer putting his eye at the exit pupil would see a 1
-D image at a focal plane determined by the amount of beam
divergence. With no voltage on the mirror this image is located at
close range; with maximum voltage on the mirror the image is at
optical infinity. In this way the optical setup provides a range of
focal planes from near to far which can be manipulated by changing
the voltage on the mirror.
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Evolution of VRD systems
The
project's initial goal was to prove the viability of forming an image
on the retina using a scanned laser. As a result of the work, a
patent application was filed and the technology licensed to a Seattle
based start up company, Micro Vision, Inc. Under terms of the
agreement, Micro Vision is funding a four-year effort in the HITL to
develop the technologies that will lead to a commercially viable VRD
product. This development work began in November 1993. Prototype #1
The original prototype had very low effective resolution, a small
field of view, limited gray scale, and was difficult to align with
the eye. One objective of the current development effort was to
quickly produce a bench-mounted system with improved performance.
Prototype #1 uses a directly modulated red laser diode at a wave
length of 635 nanometers as the light source. The required horizontal
scanning rate of 73,728 Hertz could not be accomplished with a simple
galvanometer or similar commercially available moving mirror scanner.
The use of a rotating polygon was deemed impractical because of the
polygon size and rotational velocity required. It was thus decided to
perform the horizontal scan with an acousto-optical scanner. The
vertical scanning rate of 72 Hertz is within the range of
commercially available moving mirrors and is accomplished with a
galvanometer. The use of the acousto-optical scanner comes with a
number of drawbacks: * It requires optics to shape the input beam for
deflection and then additional optics to reform the output beam to
the desired shape. * It requires complex drive electronics that
operate at frequencies between 1.2 GHz and 1.8 GHz.
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* Its total scan angle is 4 degrees. Thus, additional optics are
needed to increase the angle to the desired field-of-view. Due to the
optical invariant, this optical increase in angle comes with the
penalty of decreased beam diameter which leads to a small exit pupil.
The small exit pupil necessitates precise alignment with the eye for
an image to be visible. * It is expensive and will not, in the
foreseeable future, allow the producers to reach the cost goals for a
complete VRD system. Prototype #2 To overcome the limitations of the
acousto-optical scanner, HITL engineers have developed a miniature
mechanical resonant scanner. This scanner, in conjunction with a
conventional galvanometer, provides both horizontal and vertical
scanning with large scan angles, in a compact package. The estimated
recurring cost of this scanner will allow the VRD system to be priced
competitively with other displays. Prototype #2 of the VRD uses the
mechanical resonant scanner.. The system was built and demonstrated
during the summer of 1994. The VGA resolution images produced are
sharp and spatially stable. The mechanical resonant scanner is used
in conjunction with a conventional galvanometer in a combination
which allows for an increase in the optical scan angle. When the
mirrors of the two scanners are arranged in such a manner that a
light beam undergoes multiple reflections off the mirrors, then the
optical scan is multiplied by the number of reflections off that
mirror. Optical scan multiplication factors of 2X, 3X and 4X have
been realized. Prototype #2 uses a system with 2X scan multiplication
in the horizontal axis. Prototype #3 The third prototype system
developed uses the same scanning hardware as Prototype #2 but uses
three light sources to produce a full color image. In addition the
eyepiece optics have been modified to allow for see through
operation. In the see through mode the image produced by the VRD is
overlaid on the external world.
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Present Scenario
In the current version, a wireless computer with a touch-pad control
is worn on the belt. Such units are largely used by the production
units of many industries, most of them automobile manufacturers. Like
a high-tech monocle, a clear, flat window angled in front of the
technician's eye reflects scanned laser light to the eye. That lets
the user view automobile diagnostics, as well as repair, service, and
assembly instructions superimposed onto the field of vision. The
information that the device displays comes from an automaker's
service-information Web site through a computer running Microsoft
Windows Server 2003 in the dealership or repair shop. The data gets
to the display via an ordinary IEEE 802.11b Wi-Fi network, and all
the technicians in the service center are able to access different
information simultaneously from one server. Typical MEMS scanner
today measures about 5 mm across, with a 1.5-mmdiameter scan mirror
capable of motion on two scan axes simultaneously Using MEMS allows
us to integrate the scanner, coil windings, and angle-sensor
functions all on one chip. Such a scanner provides SVGA (800-by-600)
equivalent resolution at a 60-hertz refresh rate and is now in
production and in products. In addition, multiple scanners could
provide higher-resolution images by each providing full detail in a
tiled subarea. Eventually, costs will become low enough to make this
practical, allowing the scannedbeam approach to surpass the
equivalent pixel count of any other display technology. With green
laser diodes, it will be possible to build bright, full-color see-
through displays . Microvision uses laser light sources in many of
its see-through products because our customers' applications demand
display performances with color-gamut and brightness levels far
exceeding the capabilities of flat panel displays, notebook displays,
and even higher-end desktop displays. For today's commercial
products, only red laser diodes are small enough, efficient enough,
and cheap enough to use in such see-through mobile devices as Nomad.
Blue and green diode-pumped solid-state lasers are still too
expensive for bright, full-color, head-up or projection displays for
mainstream markets, but that could change soon. In the mid-1990s
Shuji Nakamura of Nichia Chemical Industries Ltd. (now Nichia Corp.,
Tokushima, Japan) demonstrated efficient blue and
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green LEDs, and then blue laser diodes made of gallium nitride.
When these designs and materials are extended to green laser diodes,
it will be possible to build bright, full-color see-through displays.
. As an alternative, small green laser are now being produced which
use a crystal to frequency double a neodymium YAG laser. These
devices are larger than desired and are not directly modulatable at
the required frequency. They do however, offer a short term solution.
In the HITL researchers are investigating a number of alternatives to
blue and green laser diodes. One frequency doubling technique being
researched uses rare earth doped fibers as the doubling medium. A
second technique uses wave guides placed in a lithium niobate
substrate for the doubling. The above methods all utilize a laser as
the light source. Additional work is directed at using non-lazing,
light-emitting diodes (LEDs) as the light source. In order for this
to be successful two primary issues are being addressed. The first
issue is how to focus the LED output to the desired spot size. The
second issue is the development of fabrication techniques that will
allow us to directly modulate the LEDs at the desired frequency.
Enter the edge-emitting LED. Unlike conventional LEDs, which emit
light from the surface of the chip, an edge-emitting LED has a
sandwich-like physical structure similar to that of an injection-
laser diode, but it operates below the lasing threshold. These LEDs
emit incoherent beams of light that, while not so fine as a laser's
beam, provide a tenfold increase in brightness. We also use multiple
inexpensive surfaceemitting LEDs, each contributing a portion of the
overall power, to achieve high brightness. Further performance
improvements of LED materials driven by huge investments aimed at
general lighting applications will increase the brightness and range
of applications for scanned-beam displays based on green and blue
gallium nitride devices and aluminum gallium indium phosphide red
LEDs.
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In addition to displaying images, the
scanned-beam technology can capture them. In a display, the data
channel through a digital-to-analog converter controls the light
source to paint a picture on a blank canvas. In image capture, the
light source is steadily on, and the data channel looks at the
reflections from the object through an analog-todigital converter
connected to a photodiode. The light source, beam optics, and scanner
are essentially the same in both applications
Laser safety analysis
Maximum Permissible Exposures (MPE) have been calculated for the VRD
in both normal viewing and possible failure modes. The MPE power
levels are compared to the measured power that enters the eye while
viewing images with the VRD. The power levels indicate that the VRD
is safe in normal operating mode and failure modes. The scanned beam
is passed through a lens system which forms an exit pupil about which
the scanned beam pivots. The user places themselves such that their
pupil is positioned at the exit pupil of the system. This is called a
Maxwellian view optical system. The lens of the eye focuses the light
beam on the retina, forming a pixel image. The following figure
(fig.10) compares the illumination of the retina by a pixelbased
display versus the VRD. Inset figures show schematized light
intensity over any given retinal area in the image. Typical pixel-
based displays such as CRTs have persistence of light emission over
the frame refresh cycle, whereas the VRD illuminates in brief
exposures.
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Figure.12 Preliminary tests and
calculations of VRD images demonstrated that the system's power
output with typical images is below the maximum permissible exposure
(MPE) limits established for various lighting schemes. Measures of
power output with typical images indicate that the VRD generates
power on the order of 200 nanowatts during normal operation. This is
below the Class 1 laser power limit of 400 nanowatts. If failure were
to occur, i.e. if scanning were to stop in one or both dimensions,
the power limits indicate the mechanism is still safe. To use t
can u show the diagrams of this presentation fully its urgent
08-02-2010, 10:06 PM
please download the attachment above post .There every diagrams of this presentation available
18-03-2010, 06:23 PM
can u provide ppt of the above topic pls.. make t very soon pls
18-03-2010, 07:17 PM
Sorry,
ppt of this topic is not available as of now. We will upload it as soon as it is available.
ppt of this topic is not available as of now. We will upload it as soon as it is available.
22-03-2010, 11:35 PM
[attachment=2775]
How good human and computers collaborate with each other depends on the display to some degree . In a Virtual Retinal Display (VRD) a tiny laser beam scans an image from a VGA source directly on the retina. To project the image exactly on the retina two mirror scanners, a horizontal and a vertical one, are used to deflect the beam. Using the VRD technology allows to build a high resolution, wide field-of-view personal display device that is light weight and will operate in a high brightness environment.
Scanning
This project was a further work with the objective of improvement of the existing prototype. One of the main problems of the prototype was its instability. The horizontal resonance-scanner had been operated with the VGA clock instead of its own resonance frequency. An exactly adjustment of the VGA signal with the resonance frequency of the horizontal scanner is pretty difficult to achieve. Applying the VRD system to different VGA sources (PC, TV, etc.) makes the adjustment even impossible. The remaining frequency mismatch leads to an instable operation of the horizontal scanner.
Our goal was to decouple the VGA source and the scanner. This can be done with an image buffer where the VGA data are stored with an other frequency than the data are read out and transfered to the laser. The image buffer is controlled by a FPGA (Field Programmable Gate Array) where also the rest of the implementation is done. The FPGA makes the complete system very flexible. A reprogramming of the FPGA can change the system to a new application.
Concept
There is still continuative work to do. One disadvantage is that the laser does not follow the eye. To guarantee a proper work of the VRD the laser beam should enter the eye in the middle of the pupil. Hence, a combination of the VRD with a pupil tracking system would be an interesting issue. Another work to do is the miniaturisation of the whole system so that it can be used as a wearable system.
Presented By:
Cyril Stutz, Andreas Grüter
How good human and computers collaborate with each other depends on the display to some degree . In a Virtual Retinal Display (VRD) a tiny laser beam scans an image from a VGA source directly on the retina. To project the image exactly on the retina two mirror scanners, a horizontal and a vertical one, are used to deflect the beam. Using the VRD technology allows to build a high resolution, wide field-of-view personal display device that is light weight and will operate in a high brightness environment.
Scanning
This project was a further work with the objective of improvement of the existing prototype. One of the main problems of the prototype was its instability. The horizontal resonance-scanner had been operated with the VGA clock instead of its own resonance frequency. An exactly adjustment of the VGA signal with the resonance frequency of the horizontal scanner is pretty difficult to achieve. Applying the VRD system to different VGA sources (PC, TV, etc.) makes the adjustment even impossible. The remaining frequency mismatch leads to an instable operation of the horizontal scanner.
Our goal was to decouple the VGA source and the scanner. This can be done with an image buffer where the VGA data are stored with an other frequency than the data are read out and transfered to the laser. The image buffer is controlled by a FPGA (Field Programmable Gate Array) where also the rest of the implementation is done. The FPGA makes the complete system very flexible. A reprogramming of the FPGA can change the system to a new application.
Concept
There is still continuative work to do. One disadvantage is that the laser does not follow the eye. To guarantee a proper work of the VRD the laser beam should enter the eye in the middle of the pupil. Hence, a combination of the VRD with a pupil tracking system would be an interesting issue. Another work to do is the miniaturisation of the whole system so that it can be used as a wearable system.
Presented By:
Cyril Stutz, Andreas Grüter
02-04-2010, 07:19 PM
hey please read http://studentbank.in/report-virtual-ret...play--1628 and http://studentbank.in/report-virtual-ret...628?page=2 and http://studentbank.in/report-a-retinal-d...plications and http://studentbank.in/report-virtual-ret...isplay-vrd for getting entire document of virtual retinal display information
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