Virtual retinal display

Not to be confused with Retina Display.

A diagram showing the workings of the virtual retinal display

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.

Mechanics

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. In a VRD no real image is ever produced. 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 above.

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. 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 (vector) mode, in which the lines that form the image are drawn directly, or in a raster mode, much like standard computer monitors or television. Use of the raster method of image scanning 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.

Comparison to LCDs and other display devices

Liquid crystal displays (LCDs) currently are the primary active display devices for the presentation of entertainment and information. An image that is generated electronically is viewed with the optical system of the eye. The image you see is subject not only to the quality of the optical system of the eye, but also to the quality of the display and the environment in which the display is located.

With a VRD, defects in the eye's optical system, such as damaged cornea and lens and reduced retinal sensitivity could be bypassed, as well as the problems of the display environment, such as ambient brightness, angle-of-view and display brightness. Additionally, the seen image could be augmented with other information and brightness of the system doesn't affect the image formed on the retina

Although the VRD is an output device, the technology lends itself to augmentation with eye tracking or eyegaze systems for input. Eye tracking is currently used in advanced still and video cameras for focusing on the object you wish to record.

This approach produces several advantages over conventional display devices:

• Potentially very small and lightweight, glasses mountable
• Large field and angle of view, greater than 120 degrees
• High resolution, approaching that of human vision
• Full color with better potential color resolution than conventional displays
• Brightness and contrast ratio sufficient for outdoor use
• True stereo 3D display with depth modulation
• Bypasses many of the eye's optical and retinal defects

Eye

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.

Neglecting the aberrations in the lens of the eye, one can determine the limit of the eye's resolution based on diffraction through the pupil. Using Rayleigh's criteria the minimum angular resolution is computed as follows:

$\mathrm{angular\ resolution} = \frac{1.22 \lambda}{D}$

Where
D = diameter of the pupil
lambda = wavelength of light

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 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.

Advantages

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 an 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.

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.

Another important advantage is privacy: Only the intended user (in the usual case of single-user devices) is able to see the image displayed. This kind of device is also less vulnerable to TEMPEST type side-channel leak of information.

Safety

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.

Damage to the eye could result if the laser stopped scanning with the beam focused on a single point. This can be prevented by an emergency safety system to detect the situation and shut it off.

LED enhancements

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 a LED. Advances in LED technology will be needed to further concentrate the light coming from these devices.

Utilities

Military utilities

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.

Manufacturers and commercial uses

• Brother Industries from Japan exhibited VRD system named AirScouter in September 2010.

It was subsequently commercialised in August 2011.

See also

• Augmented reality
• Bionic contact lens
• Google Glass
• Head-mounted display
• Head-up display
• List of emerging technologies
• Magic Leap
• Optical head-mounted display
• Physics of the Future
• Smartglasses
• Visual prosthetic

External links

• Animations of how a VRD works
• AirScouter VRD system from Brother Industries – Sep 2010 (Engadget)
• YouTube video for AirScouter VRD system from Brother Industries – Sep 2010