VIRTUAL RETINAL DISPLAY
The technologies of virtual reality (VR) and augmented reality (AR) are the new paradigm for visual interaction with graphical environments. The features of VR are interactivity and immersion. To achieve these features, a visual display that is high resolution and wide field of view is necessary. For AR a visual display that allows ready viewing of the real world, with superimposition of the computer graphics is necessary. Current display technologies require compromises that prevent full implementation of VR and AR. A new display technology called the Virtual Retinal Display (VRD) has been created. The VRD has features that can be optimized for the human computer interfaces.
As one stares at a computer monitor, light is focused into a dime-sized image on the retina at the back of our eyeball. The retina converts the light into signals that enters our brain via the optic nerve. To eliminate the bulky, power-hungry monitor by painting the images themselves directly onto your retina. To do so, use tiny semiconductor lasers or special light-emitting diodes, one each for the three primary colors-red, green, and blue-and scan their light onto the retina, mixing the colors to produce the entire palette of human vision. Short of tapping into the optic nerve, there is no more efficient way to get an image into your brain. And they call it the Virtual Retinal Display, or generally a retinal scanning imaging system.
VRD readily creates images that can be easily seen in the ambient room light and it can create images that can be seen in ambient day light. All subjects are readily able to match the VRD brightness
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A virtual retinal display (VRD), also known as a retinal scan display (RSD) or retinal projector (RP), is a 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. (However, the portion of the visual area where imagery appears must still intersect with optical elements of the display system. It is not possible to display an image over a solid angle from a point source unless the projection system can bypass the lenses within the eye.)
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 glasses. 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 by Kazuo Yoshinaka of Nippon Electric Co. in 1986.  Later work at the University of Washington in the Human Interface Technology Lab resulted in a similar system in 1991. Most of the 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 also can be 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, beingsadsfc able to replicate a full-sized monitor on a small device.
Apart from the advantages mentioned before, the VRD system scanning light into only one of our eyes allows images to be laid over our view of real objects, and could give us an animated, X-ray like image of a car's engine or human body, for example.
VRD system also can show an image in each eye with a very little angle difference for simulating three-dimensional scenes with high fidelity spectral colours. If applied to video games, for instance, gamers could have an enhanced sense of reality that liquid-crystal-display glasses could never provide, because the VRD can refocus dynamically to simulate near and distant objects with a far superior level of realism.
This system only generates essentially needed photons, and as such it is more efficient for mobile devices that are only designed to serve a single user. A VRD could potentially use tens or hundreds of times less power for Mobile Telephone and Netbook based applications.
It is believed that VRD based Laser or LED displays are not harmful to the human eye, as they are of a far lower intensity than those that are deemed hazardous to vision, the beam is spread over a greater surface area, and does not rest on a single point for an extended period of time.
To ensure that VRD device is safe, rigorous safety standards from the American National Standards Institute and the International Electrotechnical Commission were applied to the development of such systems. Optical damage caused by lasers comes from its tendency to concentrate its power in a very narrow area. This problem is overcome in VRD systems as they are scanned, constantly shifting from point to point with the beams focus.
Although the power required is low, light must be collected and focused down in a point. This is an inherent property with lasers, but not so simple with an LED. Advances in LED technology will be needed to further concentrate the light coming from these devices.
VRDs have been investigated for military use as an alternative display system for Helmet Mounted Displays. However no VRD-based system has yet reached operational use and current military HMD development now appears focused on other technologies such as holographic waveguide optics.
A system similar to car repair procedures can be used by doctors for complex operations. While a surgeon is operating, he or she can keep track of vital patient data, such as blood pressure or heart rate, on a VRD. For procedures such as the placement of a catheter stent, overlaid images prepared from previously obtained magnetic resonance imaging or computed tomography scans assist in surgical navigation.
In the mass-market of digital cameras, scanned-beam displays provide better image quality at lower power and cost than liquid-crystal-on-silicon and organic LED displays.
Scanned-beam technology is capable of displaying images, the data channel through a digital-to-analog converter controlling the light source to paint a picture on a blank canvas in a display. When an image is captured, the light source is turned on, and the data channel looks at the reflections from the object through an analog-to-digital converter connected to a photodiode. The light source, beam optics, and scanner are essentially the same in both applications.
Virtual Retinal Display
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.
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 dime-sized 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, power-hungry monitor altogether by painting the images themselves directly onto your retina. To do so, use tiny semiconductor lasers or special light-emitting diodes, one each for the three primary colors-red, green, and blue-and scan their light onto the retina, mixing the colors to produce the entire palette of human vision. Short of tapping into the optic nerve, there is no more efficient way to get an image into your brain. And they call it the Virtual Retinal Display, or generally a retinal scanning imaging system.
The Virtual Retinal Display presents video information by scanning modulated light in a raster pattern directly onto the viewer's retina. As the light scans the eye, it is intensity modulated. On a basic level, as shown in the following figure, the VRD consists of a light source, a modulator, vertical and horizontal scanners, and imaging optics (to focus the light beam and optically condition the scan).
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.
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
SUBIN C ANTONY
VIRTUAL RETINAL DISPLAY
PDD under development in HIT lab,University of Washington.
Not a screen based technology.
Scans light directly on to retina of eye.(Hence the name )
Full colour, high resolution, high brightness, wide field of view virtual
display without flickering.
Potential applications include HMDs for mmilitary, aerospace, engineering, medical fields.
Substitute to conventional bulky and power hungry VDUs.
Light is focused to a miniscule image(frame)on retina.
SLD or LED triads of R,G&B emits light.Mixing in proportions can produce any colour.
As light scans the retina, it is intensity modulated by the video signal.
Scanning is performed directly onto retina in a raster pattern through collimating optics.
Above figure is the basic block diagram of VRD
As shown, the viewer perceives a wide field of view
image as if from a screen placed some distance away.
Key features of VRD
Size and Weight: Small size as no intermediate screen is
present. All components are small and light making it highly
portable. Appropriate for Hand held and Head mount displays.
Power consumption: Light sources consume very less
power in order of milli watts. Scanning is done with a resonant
device (MRS) with high figure of merit. Exit pupil of VRD has
very small aperture allowing generated light to enter eyes almost
completely. Hence high power efficiency.
Resolution: Limited only by diffraction and optical aberration
in the optical components, limits in scanning frequency and
modulation b/w of photon source. SLD is a coherent source and
offer high modulation b/w to give resolutions well over a million
pixels. State of the art scanners can scan over a1000 lines per
frame which is comparable to HDTV.
Brightness: Perceived brightness is only limited by power of the
light source. SLD sources can provide very good brightness levels
even for see through mode in day light.
Field of view: Inclusive systems provide horizontal field of
view b/w 60-100 degrees. See through mode systems have it
slightly over 40 degrees. These figures are far better than existing
Stereoscopic display: Supports stereoscopic display as both
eyes can be separately addressed. Thus provides a good
approximation to natural vision.
Inclusive & See through: See through works very well even at
very high illumination conditions of about 10000 candella per meter