SAN DIEGO, CA--This year`s annual meeting of the Society for Information Display (SID, May 14-16) featured expanded sessions on liquid-crystal-display (LCD) technology, active-matrix liquid-crystal displays, emissive displays, large and miniaturized displays, and the most traditional displays--cathode-ray tubes. The conference included an international exhibit, and technical papers were presented from all over the world in both research and applications sessions. Manufacturing technologies were
SID meeting highlights new display technology
SAN DIEGO, CA--This year`s annual meeting of the Society for Information Display (SID, May 14-16) featured expanded sessions on liquid-crystal-display (LCD) technology, active-matrix liquid-crystal displays, emissive displays, large and miniaturized displays, and the most traditional displays--cathode-ray tubes. The conference included an international exhibit, and technical papers were presented from all over the world in both research and applications sessions. Manufacturing technologies were also covered, by paper and on the exhibit floor, where several new products were launched.
US-based manufacturers are leading development of miniature devices, and several showed their products at SID. Part of the driving force behind such displays has been military requirements for head-u¥and helmet-mounted displays. Funding under the US government`s Defense Advanced Research Projects Agency programs initially helped stimulate these developments. Further commercial applications, such as hand-held fax machines and pagers, have provided additional stimulus.
One of the impressive large-screen displays exhibited was the digital light-processing system developed by Texas Instruments (TI; Dallas, TX). The system comprises a display engine and a rear-projection monitor or screen. Both components are still prototypes, but this year the displays were readily viewable under ambient lighting on the show floor.
The core of the TI system is a digital micromirror device (DMD)--a mechanical, electro-optical switch. Each mirror in the device is 16 ¥ 16 µm, fabricated on hinges ato¥a static random-access memory. The mirror can occupy one of three possible states: "on" (tilted +10°), "off" (tilted at -10°), and parked at 0°. The position of each mirror is controlled by electrically addressing an underlying memory cell with the binary bit-plane signal. Pulsewidth modulation controls the length of time that each mirror remains in a particular position. Modulation rates are more than 1000 movements per second, allowing digital gray scale and color reproduction.
The mirrors are fabricated using traditional semiconductor-manufacturing techniques, building on the underlying CMOS addressable circuit. Although there are mechanical aspects to this semiconductor device (the mirrors move on the hinges), the DMD is built much like an integrated circuit. In an 848 ¥ 600 device, there are 508,800 mirrors, each individually controlled.
One, two, or three of the DMDs are incorporated into a projection engine, along with light source, condensing optics, color filter system, and a projection lens. The engine is then incorporated as an OEM component into a projection system. According to TI, the system has a fill factor of u¥to 90% and a light efficiency of greater than 60%, making this projection system more efficient than traditional LCD projection displays.
Component optical materials
Liquid-crystal displays have a common limitation in that the usable viewing cone is narrower than that of a cathode-ray tube. Colors shift or invert, and contrast decreases at high incident angles because of the differences in light path length through the liquid-crystal material. The problem is even more pronounced with color images than with monochrome images. Researchers from AlliedSignal MicroOptic Devices (Los Gatos, CA) described a system to enhance the viewing angle on LCDs.
AlliedSignal adds two pieces to a typical LCD assembly--a collimating sheet and a diffusing screen. The collimating sheet is a two-sided array of micro-optic elements, bonded to the backlight assembly. In a backlit LCD, waveguided light from the fluorescent edge-lit lightpipe is coupled into micro-optics, collimated by total internal reflection within the prismatic optical structure, and further collimated by refraction at a lensing micro-optic structure on the second side of the sheet. The result is two-axis collimated light in the liquid-crystal cell.
Once the light exits the cell, it must be spread out by the diffusing screen to provide a suitable viewing cone. AlliedSignal discussed its nonscattering diffusing screen, composed of three-dimensional micro-optic structures, each of which is a tapered pyramid (see figure). The amount of diffusion is a function of the angle and height of the pyramid as well as the indices of refraction of the polymer. The structure can be customized by application. Because both sheets are molded, volume applications are possible.