Retinal Scanning Leads HMD Race
Virtual retinal displays raster-scan images directly onto the human retina, allowing users to view images superimposed over a background.
By: Douglas Stoll, Microvision Inc.
•Virtual retinal displays
•System design
•Configurations and applications
•The Future of VRD Technology
The microdisplay field is rapidly expanding as new technologies are developed and older ones are pushed to new levels of performance. The fundamental driver for this growth is demand for compact, high-resolution, full-color portable personal information displays that will be used for cameras, personal video viewers, and head- or helmet-mounted displays (see sidebar: Synthetic Vision Information Systems)
Currently, a number of technologies compete for a share of this market, including active matrix liquid crystal display (AMLCD); ferro-electric liquid crystal display (FLCD); liquid crystal display (LCD); active matrix electro-luminescent (AMEL); cathode ray tube (CRT); or field-emission display (FED). Each of these technologies involves displaying an image on a physical screen of some sort, which is then viewed by the user. Drawbacks include insufficient brightness in daylight, and limited image resolution and yield.
Virtual retinal displays offer an alternative to conventional technology. Instead of creating an image on a screen, the retinal scanning display projects the image directly onto the human retina, allowing the user to "see" the image without viewing it on a secondary surface. The approach offers the advantages of color purity, brightness, and increased resolution. Instead of having to develop new chip technology for each increase in resolution, the virtual retinal display engineer just has to design a system capable of scanning more pixels.
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Virtual retinal displays
In a retinal scanning display, low-power beams of red, green, and blue light are modulated to form a stream of colored pulses that represent the individual pixels of an image. The pulse stream is scanned onto the retina in a raster pattern at video rates to form an image. The result is a raster-scan pattern of colored pixels, similar to that in a conventional television set but made with photons instead of electrons.
The image created is high-resolution, full-motion, and full-color. Most importantly, the technology eliminates the use of external screens. The eye reads the image directly. In certain applications, an image appears in the viewer's field of vision as if the viewer were an arm's length distant from a high quality video screen. In other applications, retinal scanning display technology superimposes an image on the viewer's field of vision for a see-through or "augmented vision" capability. This enables the viewer to see data or other information within the context of his or her natural surroundings.
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System design
A retinal scanning display consists of drive electronics, light-source module, scanner assembly, pupil expander, and viewing optics.
- System drive electronics: The drive electronics receive and process an incoming video signal, provide image compensation, and control image display. For VGA projection, the electronics process over 18 Mpix/s. The virtual retinal display is capable of providing UXGA resolution of 1600 x 1200 or 115 Mpix/s.
- Light source module: The light source module contains laser light sources, acousto-optic modulators to create the pulse stream, and a color combiner that multiplexes the pulse streams. To provide sufficient brightness, full-color displays suitable for outdoor, daylight applications incorporate red diode lasers (635 nm), green solid-state lasers (532 nm), and blue solid-state or argon gas lasers (450-470 nm range). Systems designed for indoor use can incorporate LEDs; red, blue, and green devices currently under development for such systems are being tested.
Generally, the energy levels are on the order of nanowatts to milliwatts, depending on display requirements. The levels of light involved are well within laser safety standards for viewing, as confirmed by analysis.
The drive electronics control the acousto-optic modulators that encode the image data into the pulse stream. The color combiner multiplexes the individually-modulated red, green, and blue beams to produce a serial stream of pixels, which is launched into a singlemode optical fiber to propagate to the scanner assembly.
- Scanner assembly: The scanner assembly contains two scanning mirrors. One 24 mm x 6 mm x 6 mm scanning mirror sweeps the beam horizontally at a high frequency, typically between 15.75 kHz and 18.9 kHz for 60-Hz non-interlaced refresh rates. This corresponds to one-half the VESA monitor-timing standard since the retinal scanning display can process and display pixels bidirectionally. A second scanning mirror sweeps the beam of laser light vertically at 60 Hz to complete the raster image.
Synchronizing signals generated by the system drive electronics control both scanning mirrors. Position feedback signals from both mirrors provide the drive electronics with instantaneous information on exact mirror position. For high resolution images, multiple "lines" of the image can be processed and presented simultaneously.
Microvision is currently developing a microelectromechanical system (MEMS-) based full image scanner capable of bi-directional scanning. Unlike MEMS-based spatial light modulators used for projection applications, the millimeter-sized mirror in the virtual retinal display will provide the scan angles and rates necessary to provide the full raster image. The need to fabricate only one mirror significantly reduces the development effort for higher resolutions and associated yield problems. It also eliminates the potential for "dead" pixels due to inoperative mirror elements.
- 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.
- Viewer optics: The viewer optics relay the scanned raster image to the oculars worn by the user. The optical system varies according to the application. In the case of military applications such as helmet mounted or head mounted display optics, the system incorporates glass and or plastic components; for medical applications such as image-guided surgery, head-mounted plastic optics are used. In industrial or personal displays, the optics might be a simple plastic lens.
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Configurations and applications
Retinal scanning display technology can be implemented in a variety of ways. In its simplest incarnation, the input beams are modulated with an image signal and combined as a full-color pixel "stream" into a single optical fiber conduit. The fiber may run a considerable distance to a head-worn, 15-g scanner assembly, which is unlike a matrix-addressed display in that there is no pixel structure or yield issue, and for which the only resolution limits are imposed by optical beam diffraction, scatter, and effective acousto-optic modulation frequency.
In late 1998 Microvision will deliver a monochrome green binocular HMD system to the Army with a total field of view of 52° horizontal by 30° vertical with 1716 horizontal pixels by 960 display lines, and 1470 ft-Lm displayed raster luminance at the eye. The system weighs approximately 2 lb, including two display engines (one for each eye), the helmet-mounting structure, and the optics to provide the large display format. The system will project the images in a "see through" fashion —the pilot will still be able to see the background scene, but has the option of focusing in on the presented information.
Microvision has modularized the system designed to permit delivery of a full-color 1280 x 1024-pixel biocular head-mounted system three months later, as well as an HDTV (1920 x 1080 pixels) HMD to the Air Force by the year 2000.
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The Future of VRD Technology
Future systems will be even more compact than present versions once the MEMS-based scanners are incorporated. Edge-emitting, super-luminescent light-emitting diodes (SLEDs) and miniature diode lasers under development will allow direct light modulation. In conjunction with application-specific integrated-circuit technology, these devices will permit the direct fabrication of a VRD display engine incorporating the electronics, light sources, and scanning assembly, all in a compact, hand-held, battery-operated package.
The ultimate goal for the retinal scanning technology is a lightweight eyewear package. The approach can also be adapted to image projection systems. The applications for VRD technology are varied—HUDs, color projections systems for entertainment or flight training simulators, etc. A key area for continued development is an image display system that can augment and enhance a person's task performance.
Douglas Stoll is director of engineering at Microvision, 2203 Airport Way South, Suite 100, Seattle, WA 98134. Tel: 206-623-7055.