If the light source is the paint, Microvision's proprietary microelectromechanical systems (MEMS) biaxial scanner is the brush that applies the image to the retina. The scanner's main component is a mirror 1.5 millimeters in diameter that rapidly sweeps the light beam horizontally to position the pixels in a row, also moving the beam downward, to draw successive rows of pixels. This process continues until an entire field of rows has been placed and a full image appears to the user--quite similar to the process in a regular cathode-ray television, in which the magnetic deflection coils direct the electron beam to scan the phosphor-coated screen. But while a conventional display can create jagged edges on images because the pixels are fixed onto screen hardware, a scanned-beam display has no hard pixels: the continuously scanning beam creates a much smoother image.
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 15mm. One way to hit this target is to focus the scanned beam onto an optical element called an 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. For applications that require better image quality using less power, we can dispense with the exit pupil expander altogether either by using a larger scan mirror to make a larger exit pupil or by actively tracking the pupil to steer light into it.
The Simplicity And Elegance of the scanned-beam concept belies the underlying complexity of the enabling advancements over the past four decades in scanning, light-source, and image-processing technologies.
Early on, Microvision researchers identified the scanner as the crucial element in this emerging technology. Eight years ago, we scanned using a polished metal plate that combined the scan mirror and a stiff torsion spring that had a resonance of about 20 kilohertz. When driven by magnetic coils, the plate scans in a large, twisting, resonant motion. With that proof of principle in hand, we developed a MEMS version of the scanner. MEMS are electromechanical devices that are photolithographically defined on a silicon wafer, much as integrated circuits are made, and in quantities of more than 100 per wafer.
A typical MEMS scanner today measures about 5 mm across, with a 1.5-mm-diameter scan mirror capable of motion on two scan axes simultaneously [see photo, Image Painter].
Using MEMS allows us to integrate the scanner, coil windings, and angle-sensor functions all on one chip. Such a scanner provides SVGA (800-by-600) equivalent resolution at a 60-hertz refresh rate and is now in production and in products. We expect a higher performance per scanner as we more fully exploit the basic advantages of MEMS, which include the potential of very low costs in small packages. In addition, multiple scanners could provide higher-resolution images by each providing full detail in a tiled subarea. Eventually, costs will become low enough to make this practical, allowing the scanned-beam approach to surpass the equivalent pixel count of any other display technology.
With green laser diodes, we'll be able to build bright, full-color see-through displays
While the MEMS scanner is a relatively recent development, the laser, another indispensable element of the scanned-beam display, traces its origins back to 1960 and provides a compact source of spectrally pure, focused, virtually noise-free light. Microvision uses laser light sources in many of its see-through products because our customers' applications demand display performances with color-gamut and brightness levels far exceeding the capabilities of flat panel displays, notebook displays, and even higher-end desktop displays. For today's commercial products, only red laser diodes are small enough, efficient enough, and cheap enough to use in such see-through mobile devices as Nomad. Blue and green diode-pumped solid-state lasers are still too expensive for bright, full-color, head-up or projection displays for mainstream markets, but that could change soon. In the mid-1990s Shuji Nakamura of Nichia Chemical Industries Ltd. (now Nichia Corp., Tokushima, Japan) demonstrated efficient blue and green LEDs, and then blue laser diodes made of gallium nitride. When these designs and materials are extended to green laser diodes, we'll be able to build bright, full-color see-through displays.
On another front, Microvision recognized that 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 supply. Although the power required is low, the light must be collected and focused down to a pinpoint--easy to do with a laser, but not so easy with an LED. A scanned-beam display placed near the eye, such as a camera viewfinder, wastes little light, especially if it does not have to overcome a background scene. Even so, we've needed advances in LED technology to further concentrate the light coming from these devices.
Enter the edge-emitting LED. Unlike conventional LEDs, which 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 tenfold increase in 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 driven by huge 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.
On top of improvements in LEDs, lasers, and MEMS, memory density and processor power are expected to double every two years, translating directly into better performance from our displays. Increased memory and computational capacity boost the update rate of the light source and refine its control, increasing resolution even further.
In Addition To Displaying Images , the scanned-beam technology can capture them. In a display, the data channel through a digital-to-analog converter controls the light source to paint a picture on a blank canvas. In image capture, the light source is steadily 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.
Exploiting this versatility, we developed a design for an endoscope,the long, slender medical instrument that is used for examiningthe interior of a bodily organ or performing minor surgery.Composite red, green, and blue light from lasers travelsdown a single-mode fiber to the far tip of the endoscope. Thereat the tip, a simple lens collects the light into a singlebeam that then culminates in a fine point. A MEMS scannerdirects the fine point of light over an area that is 10-100mmdistant from the tip. Reflected light collected by fibersand conducted back to detectors contains the informationabout objects encountered. The detected light is digitized and,with software, reconstructed into an image of the objectencountered by the scanned beam.
Microvision just completed a study showing that a 2.5-mm-diameter MEMS chip scanning at large angles would provide resolution as good as that of the leading endoscopes. This MEMS chip is smaller than the sensor chip that is used in CMOS or charge-coupled-device imagers. Small size, of utmost importance for minimally invasive surgery, combined with simple optics, results in a disposable endoscope probe. Such a probe would reduce the cost of medical procedures by saving on the time and cost of sterilization while minimizing the risk of cross-contamination.
Medical-device applications will take more time to develop and to qualify for use. Meanwhile, to provide revenue and gain experience in high-volume manufacturing, Microvision is applying this rather exotic technology to the $1.8-billion bar-code-scanner market with the $99.95 Flic laser bar-code scanner, which we introduced in September 2002. The resolution and scan speeds can be much lower than those needed for display applications, so we can reduce costs by using a plastic multifaceted scan mirror operated by the energy harvested from pressing the scan button. NCR Corp., Dayton, Ohio, has recently introduced the Flic scanner under its own label, called the RealScan Companion. Consumer applications will take longer to come to market, but we expect that in the next five years, our displays will pop up in cellphones and cameras, giving users an HDTV experience on the go, and at a fraction of the power, weight, and cost required by today's devices.
