Virtual Retinal Display

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.

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.

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.

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.

Pupil Expander 

Nominally the entire image would be contained in an area of 2 mm2. The exit-pupil 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.

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.

Manufacturing

The same characteristics that make the VRD suitable for medical applications, high luminance and high resolution, make it also very suitable for a manufacturing environment. In similar fashion to a surgery, a factory worker can use a high luminance display, in conjunction with head tracking, to obtain visual information on part or placement locations. Drawings and blueprints could also be more easily brought to a factory floor if done electronically to a Virtual Retinal Display (with the option of see-through mode). Operator interface terminals on factory floors relay information about machines and processes to workers and engineers. Thermocouple temperatures, alarms, and valve positions are just a few examples of the kind of information displayed on operator interface terminals. Eyeglass type see-through Virtual Retinal Displays could replace operator interface terminals. A high luminance eyeglass display would make the factory workers and engineers more mobile on the factory floor as they could be independent of the interface terminal location.

Conclusion

Various strategic agencies have already started working with the VRD and with so much at stake, status reports on progress are not readily available. Nevertheless we can say that right now, all those engineers, fighter pilots and partially sighted people working with VRD will be struggling with different facets of the same problem.