HDTV BUYING TIPS

The light sources for these panels are actually tiny fluorescent lamps, constructed in such a way as to ensure a uniform distribution of light across the screen. LCD panels have been manufactured in sizes as large as 40 inches, and 30-inch models are now sold with integrated TV tuners. AM LCD panels can also be used to shutter light from high-intensity projection lamps, as in many of the first LCD projectors.

AM LCDs are versatile, and their manufacturing costs have come way down. It's now possible to buy a 15-inch, 4:3, flat-screen computer monitor for a few hundred dollars with XGA (1024x768) resolution yet weighing less than 15 pounds. Even the larger, 30-inch panels often come in under 60 pounds, or about half the weight of a 32-inch, 4:3 CRT-based TV.

But large AM LCD panels are impractical for small projectors, because they can't be made small enough to fit. Instead, a variation of the technology is used: high-temperature polysilicon (HTPS), manufactured primarily by Epson and secondarily by Sony and Panasonic.

Polysilicon panels are much smaller and harder to manufacture, but they're available in very high resolutions for their size—a typical XGA (1024x768) panel might measure 0.7–0.9 inch diagonally, while an SXGA (1280x1024) panel is just under two inches diagonally.

Like their larger brothers, polysilicon LCDs shutter light as pixels are turned on and off. Combined with small, high-pressure arc lamps, they can make some amazingly bright images in packages weighing as little as six pounds. Unlike AM LCDs, separate color filters known as dichroic mirrors must be used with each of three panels to blend the monochrome LCD images into full RGB color.

The downside? Neither AM LCDs nor polysilicon LCDs can completely shut off the light passing through them. The result is a dark gray instead of the deep blacks we've gown accustomed to with CRTs. Unless properly set up, LCD panels can also crush gray scales and compress high-luminance details. To top it off, those high-intensity arc lamps are spectrally deficient, giving the resulting images a greenish-blue cast.

At present, a handful of companies are producing front projectors and rear-projection TVs with polysilicon LCD panels specifically for the home-theater market. Sony, Sanyo, and Epson all offer front projectors that use 16:9 panels with 1366x768 or 1280x720 resolution. Sony, Zenith, and Panasonic also manufacture rear-projection TVs with LCD imaging, in sizes from 50 to 60 inches.

In all cases, the LCD panels are acting as transmissive light shutters. They work just like a Venetian blind, and shutter light passing through the panel. But there's another way to control light with LCDs.

The levels of gray are produced by the Digital Micromirror Device (DMD), comprised of thousands of tiny mirrors. In a single-chip projector, the color is produced by a rotating color wheel, timed to coincide with the mirrors and, by relying on the eye's persistence of vision, producing a full color image. Illustration courtesy of Texas Instruments.

DMD mirrors (two shown). Responding to the signal source, each rotating mirror/pixel directs the light from the projection bulb either into the lens or away from it to be absorbed within the case. As the number of times each mirror flashes per second varies, our vision sees the result as levels of gray. Illustration courtesy of Texas Instruments.

LCoS Operation
The silicon active matrix applies a voltage between the common, transparent electrode and the reflective electrodes that comprise each pixel. The voltage on each pixel excites the electro-optical properties of the liquid crystal material and modulates the incoming light by altering its polarization. This re-polarized light is reflected back, optically separated from the incident light, and projected onto a screen. Typically, the output of three chips, one each for red, green, and blue, are combined to produce a full-color image. Illustration courtesy of JVC Professional.

LCoS and D-ILA: With their fixed-pixel structures and tiny liquid crystals whipping around in response to control voltages, LCoS and LCD devices have a lot in common. But the similarity ends there.

Instead of shuttering light as it passes through them like LCDs, an LCoS device actually shutters light as it is reflected from the mirrored rear surface, which sits on an integrated-circuit chip (hence the name Liquid Crystal on Silicon). This trick is accomplished by polarizing the light and increasing the reflection angle from 45° to 90°. This makes things a bit more complicated optically, but it does improve the efficiency of the light shutter, because the controlling transistors are behind the liquid-crystal surface of the LCoS chip, and not in the way of any beams of light. The space between pixels (known as the fill factor) is smaller, further improving illumination efficiency. And for a given size of chip, the fixed resolution of LCoS can be a lot higher than that of polysilicon LCD.

However, the black levels and gray scales obtainable from LCoS devices are just as limited as those from transmissive LCDs. Managing contrast-lowering light scatter inside the polarized beam splitter is a tricky process, and three panels must be used with red, green, and blue filters to get full-color imaging.

LCoS chips can be used with any kind of short-arc projection lamp (in which the gap between the electrodes is short, which is better for illuminating a small panel), although most models seen so far use small xenon arc lamps. These lamps have more evenly distributed spectral output, and their color is somewhat easier to correct. However, xenon lamps are more expensive to operate, and don't last nearly as long as metal-halide short-arc lamps. Some models cost almost $1000 new and last about 1000 hours before they reach half luminance.

The largest manufacturer of LCoS displays right now is Japan Victor Corporation, otherwise known as JVC. Their implementation of LCoS is called the Direct Drive Image Light Amplifier, or D-ILA for short. The original image light amplifier was developed by Hughes Electronics for the US Navy in the 1980s, and used a super-small CRT harnessed to a 2-inch (diagonal) LCD panel. When JVC acquired Hughes in the 1990s, they phased out the ILA in favor of a newer, smaller version, introduced in 1997.