A virtual retinal display (VRD) is a head-mounted display system that projects an image directly onto the human retina with low-energy lasers or LCDs. VRDs can give the user the illusion of viewing a typical screen-sized display hovering in the air several feet away.The user sees what appears to be a conventional display floating in space in front of them.
A VRD unit consists of 4 modules; drive electronics to break down an incoming source image into an information stream, a light source made up of laser(s) or LED(s), a scanner bank made up of horizontal and vertical scanners, and a lens to expand the image that projects through the scanners. As in a television, the scanners rapidly oscillate left-to-right or down-to-up, selectively permitting colors through in precise configurations that produce a high-resolution 2mm x 2mm field of pixels. Then a lens acting as an expander boosts the size of the image to something like 18mm x 18mm, allowing for a larger and more natural image. The pixel field is then projected onto the eye, where the eye’s lens focuses the image onto the retina. Aside from tapping into the optic nerve itself, there may be no more effective way to display an image.
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 realitysystems. 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 usingmotion compensation. In this role the VRD offers unique advantages, being able to replicate a full-sized monitor on a small device.
How does eye form image?
A brief review of how the eye forms an image will aid in understanding the VRD.
A point source emits waves of light which radiate in ever-expanding circles about the point. The pupil of an eye, looking at the source, will see a small portion of the wavefront. The curvature of the wavefront as it enters the pupil is determined by the distance of the eye from the source. As the source moves farther away, less curvature is exhibited by the wavefronts. It is the wavefront curvature which determines where the eye must focus in order to create a sharp image.If the eye is an infinite distance from the source, plane waves enter the pupil. The lens of the eye images the plane waves to a spot on the retina. The spot size is limited by the aberrations in the lens of the eye and by the diffraction of the light through the pupil. It is the angle at which the plane wave enters the eye that determines where on the retina the spot is formed. Two points focus to different spots on the retina because the wavefronts from the points are intersecting the pupil at different angles.
The virtual retinal display is highly efficient with respect to power consumption, requiring far less power than the postage-stamp LCD screens used commonly in today’s mobile devices. A VRD display uses about a microwatt of power. Since VRD displays project images directly onto the retina, they provide a sharp, clear image regardless of external lighting conditions. VRD displays require a fraction of the hardware of conventional display devices, allowing for lighter and more elegant mobile devices, in high demand for today’s electronics market. VRD shows strong potential to replace LCD screens in cell phones, handheld computers, handheld gaming systems, and eventually even larger computers such as laptops.
Apart from the advantages mentioned before, the VRD system scanning light into only one eye allows images to be laid over one’s view of real objects. For example, it could project an animated, X-ray-like image of a car’s engine or the human body.
VRD system also can show an image in each eye with a enough angle difference to simulate three-dimensional scenes with high fidelity. 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.
VRD technology is being exclusively commercialized by the Seattle-based tech company MicroVision, Inc. Two products available so far include Nomad(tm), a head-mounted VRD system that displays a monochromatic overlay of relevant information to a task at hand, and Flic(tm), a laser bar code scanner. Nomad uses Windows CE and the 802.11b wireless protocol. As the components of VRD displays decrease in cost and the manufacturing processes used to create them improve, distribution of the product will surely expand to a very large market.
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.
If the laser stops scanning, permanent damage to the eye will result because the beam stays focused in one spot. This can be prevented by an emergency safety system to detect the situation and shut it off.