Virtual retinal display



A virtual retinal display (VRD), also known as a retinal scan display (RSD), 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.)

History
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 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.

Advantages
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 can also 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.

Safety
It is believed that VDR 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 DRV 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.

LED enhancements
The fact that LEDs are able to provide needed light for VRD, makes cheaper and easier VRD manufacturing. The total amount of light that enters your eye from a desktop display is actually less than a microwatt, and that this is small compared with what an LED can contribute.

On the other hand, although the power required is low, light must be collected and focused down in a point. That is easy to do with a laser, but not so easy with a LED. A even so, we have needed advances in LED technology to further concentrate the light coming from these devices.

On the contrary of conventional LEDs, that 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 ten times more brightness. We also use multiple inexpensive surface-emitting LEDs, each contributing a portion of the overall power, to achieve high brightness. Further performance improvements of LED materials made by 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.

Military utilities
Like other technology enhancements, VRD was developed for military use. Nowadays several military units like U.S. army's Stryker Brigade. The commander of a Stryker, a tank, can view its onboard battlefield computer with a helmet- mounted daylight-readable display. So, the commander can observe better the surroundings, drive the Stryker, choose the best path, and share tactical information. A similar device is used by fighter and helicopters pilots.

Medic utilities
A system similar to car repair procures is used by doctors for complex operations. While surgeon is operating, he can watch in the display vital patient data, like blood pressure or heart rate. And in such procedures as the placement of a catheter stent, overlaid images prepared from previously obtained magnetic resonance imaging or computed tomography scans assist in surgical navigation.

Cameras
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 can capture displaying images, the data channel through a digital-to-analog converter controls the light source to paint a picture on a blank canvas in a display. When an image is captured, the light source is regularly 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.