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EE Times-Asia > Optoelectronics/Displays

Large TVs look toward liquid crystal on silicon

Posted: 16 Jan 2002 ?? ?Print Version ?Bookmark and Share

Keywords:flat panel display? hdtv? lcos? lcd? oled?

The large-screen TV world is due for a shake-up as various flat-panel display technologies vie to displace the heavy, bulky rear-projection CRT. The best bet for high-quality, relatively inexpensive TVs in the near term is the microdisplay applied in a rear-projection architecture and the best candidate in this field is the reflective LCOS microdisplay.

For HDTV in particular, rear-projection LCOS will offer advantages in size, resolution and cost. Rear-projection LCOS offers extremely high pixel counts at a relatively low cost. And because the LCOS's silicon backplane is the major cost driver of the display system, the technology can be scaled down easily to bring even lower cost per pixel over time.

In the future, for TVs that take advantage of the full resolution of HDTV, large direct-view LCDs and rear-projection LCOS displays may compete for TV and computer monitor applications in the 28-inch to 30-inch realm, but affordable LCDs in the 50-inch range are not yet visible on the horizon.

Consumer TV applications place a number of demands on display technology: high brightness, high contrast, full color, high resolution and low cost. All the candidates for rear-projection TV, besides LCOS, fall short in at least one of these categories.

For example, the conventional rear-projection CRT TV is very low cost, but it is very heavy and bulky. The HDTV-capable examples have the same size and weight drawbacks and they bear the added burden of high expense. Moreover, rear-projection CRT-based HDTVs are relatively dim and many do not provide full HDTV resolution on the screen, although they accept HDTV signals.

Three microdisplay types

There are basically three types of microdisplays suitable for rear-projection applications: reflective mechanical devices such as the digital micro-mirror devices (DMDs) of Texas Instruments; transmissive devices such as the active-matrix (AM) polysilicon LCDs of Epson and Sony and reflective devices such as the LCOS displays of Three-Five Systems, JVC, Displaytech, Aurora, Philips and others.

The microdisplay approach to rear-projection imaging provides the best value equation for consumer TVs, as well as flexibility. For example, the same microdisplay imager can be applied to TV and front projector applications. The same imager can be used with different sets of optics to yield HDTVs with different screen sizes. The LCOS alternative has more flexibility, given the rich opportunities to be leveraged in liquid-crystal engineering.

The key vectors for microdisplays are the development of an optical infrastructure and the innate ability to move to smaller pixels, smaller imagers and less expensive display devices.

The three candidate microdisplay technologiesLCOS, transmissive LCD and digital micro-mirror devicesall lead to very good projectors, but reflective displays have an inherent advantage over transmissive displays, especially at higher resolutions. With transmissive polysilicon microdisplays, some of the IC is located in the light-transmitting area (or aperture) of a pixel, pixel area, brightness, and at its worst, presenting a pixelated screen-door effect.

The effect grows worse as pixel density increases, with the circuitry blocking more and more of the light. With reflective microdisplays, in contrast, the circuitry lies below the lighting-reflecting area, resulting in very high apertures of 90 percent or better across a range of resolutions, providing a fill factor and smoothness of image that transmissive displays cannot match.

Moreover, the single-crystal silicon backplane of LCOS imagers has much higher electron mobility than polysilicon, which allows more electronic circuitry to be integrated directly into the display.

Among the reflective microdisplay alternatives, LCOS has several distinct advantages over DMDs, the most obvious being a significant edge in cost. The analog drive (at the LC cell level) of LCOS also gives richer gray scaling than the digitally driven DMD, as well as a less busy signal.

Much of the progress in LCOS rear projectors in the past few years has come from the optics industry in terms of improved and lower cost lamps, illumination systems and optical components.

Additionally, advances in splitter/combiner optics from such companies as Colorlink with its ColorQuad system and Nikon with its three-PBS (polarizing beam splitter) system have enabled on-axis projection architectures with excellent contrast and color saturation. In particular, the ColorQuad, which Thomson Multimedia Systems is using in its rear-projection LCOS TV, is particularly compact and has been effectively implemented by Thomson along with three of Three-Five's 1,920-by-1,200-pixel Brillian LCOS imagers, to achieve an on-screen contrast ratio of better than 450:1 and above 65 percent reflective efficiency.

Bob Melcher

Chief Technology Officer

Three-Five Systems Inc.

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