TOKYO Using microelectromechanical systems (MEMS) technology, Sony Corp. has developed a prototype front-projection display system based on grating-light-valve devices that it says boasts brilliant color reproduction and an unprecedented 3,000:1 contrast ratio.
As it put grating-light-valve (GLV) technology on its priority R&D list, Sony also announced that it will not invest in Fuijtsu Hitachi Plasma Display Ltd., a question that's been hanging fire for two years. Instead, Sony says it is now negotiating with NEC Corp. to establish a close relationship for a secure plasma-display panel supply.
The projector display system that Sony demonstrated last month consists of three GLV devices and red, green and blue lasers. The light from each laser is separated at the GLV device into diffraction light, which the display system uses to form images, and reflection light, which is blocked. This structure theoretically enables an infinitely large contrast ratio, the company said.
The prototype achieved 3,000:1, whereas the most advanced projectors on the market today top out at around 1,000:1. The MEMS GLV technology also yields high-speed response and high color reproduction thanks to the use of pure red, green and blue lasers, the company said.
The GLV device measures 34 x 6 x 2 mm, has 1,080 pixels and contains multiple ribbonlike structures 6,480 in all which are formed on the surface of a silicon wafer. About 70 chips can be fabricated on one 8-inch wafer, Sony said. To increase the contrast, the ribbons need to have a flat, highly uniform surface.
Flat ribbons are key
The GLV devices themselves can be built in a mature process technology, "but the process to make the surface of the ribbons flat at the nanometer order is really difficult," said Shigeo Kubota, a corporate research fellow at Sony. But the flatness directly affects the contrast ratio.
"The development of silicon-based GLV devices has almost been completed," Kubota said. "We are going to complete development of GLV projector engines, including lasers, within two years."
Although the prototype's 3,000:1 contrast ratio bodes well for display applications, "the introduction may be too late," said Isamu Yoshii, editor-in-chief of New Media, a monthly media-business magazine that often covers display technology. "If [GLV] had been announced at the same time as DLP [digital light processing] appeared, these MEMS displays might have shown interesting growth by stimulating each other." But with two more years' development time ahead, "the display market will have changed. Competing display formats may have grown enough [for commercial viability]. It's Sony's challenge to lower the cost and to commercialize it at a competitive price."
The microribbon array at the heart of the GLV device is a one-dimensional diffraction grating with a total of 1,080 pixels, or 6,480 (6 x 1,080) ribbons in a line. The lined mirror forms 1,080 pixels vertically.
Each ribbon functions as a tiny reflective element. When electrostatic forces are applied, the ribbons form diffraction gratings. In the display application, one pixel consists of six ribbons; three are fixed and the other three move up and down depending on applied electrostatic forces.
This one-dimensional image is then scanned horizontally using a scanning mirror to create a two-dimensional image. The device used in the prototype allows horizontal scanning of 1,920 pixels, providing capability for 1,920 (horizontal) x 1,080 (vertical), the same as full high-definition progressive-scan image reproduction.
The system can project images of any aspect ratio; even a 360° circular projection is theoretically possible, according to Sony. The company optimized the wavelength of the red, green and blue lasers to 642, 532 and 457 nanometers, respectively. "Since the light source of the GLV system is lasers, color reproduction is superior to displays that uses a lamp," said Naoya Eguchi, principal researcher at Sony's core-technology development center.
The color reproduction area on the prototype in terms of the CIE1931 (Commission Internationale de l'Eclairage) specification is more than twice as large as the color reproduction area defined for high-definition CRTs, said Eguchi.
The GLV device and the basic structure of the display were invented in the early 1990s by David Bloom, then a professor at Stanford University. Sony engineers had a close relationship with Bloom in laser technology, and in July 2000 Sony signed an exclusive licensing deal with a company Bloom had formed, Silicon Light Machines (Sunnyvale, Calif.). A month after Sony signed its contract, Cypress Semiconductor purchased SLM.
The license enables Sony to develop and implement the GLV technology in displays and to produce and sell display devices and systems based on it.
Silicon Light Machines believes that the GLV technology has the potential to revolutionize many optics products. "SLM is investigating other applications, but our focus is on applications in other areas of the imaging market such as print equipment and photofinishing," said a spokeswoman. "SLM also manufactures GLV devices for imaging and telecommunications applications. Our relationship with the Sony development group remains close, and either company can purchase GLV devices from the other if necessary."
In the next two years, Sony said it will improve the picture quality of its GLV projector and focus its efforts on lowering the cost of lasers. Currently, the system uses a red semiconductor laser and diode-pumped solid-state lasers, the latter a kind of second-harmonic generation laser. "If all lasers become semiconductors, it's ideal," said Eguchi.