The Big Picture
Many TVs with screens bigger than about 42 inches diagonal are rear-projection designs. They project a video image inside a box and direct it by means of lenses and mirrors onto the inner surface of a translucent screen. Originally, all used cathode-ray tubes, or CRTs, to create the image. Although this technique works well, the chassis is heavy and requires a relatively large, usually floor-standing cabinet.
As screens have grown and TV has begun migrating to the new, wider 16:9 aspect ratio (ratio of width to height), CRT-based designs have been displaced by new technologies that offer similar or better performance in lighter, more compact enclosures.
Regardless of technology, today's rear-projection TVs are all high-definition displays, able to take full advantage of digital HDTV broadcasts and discs. (HDTV provides wide-screen pictures with much greater clarity and detail than conventional television.) Built-in HD tuners are now standard, and all sets include tuners for regular analog broadcast and cable TV as well; the ability to receive unencrypted digital cable without a set-top box has also become a near-universal feature.
Digital light processing projectors are based on Texas Instruments' Digital Micromirror Device, which is actually a chip manufactured using semiconductor-fabrication techniques. Its method of operation sounds preposterous--thousands of microscopic mirrors flipping back and forth under microprocessor control--but is spectacularly effective. As with LCD sets, the light source is normally a user-replaceable, high-intensity lamp that will last for years in normal use, although some models now use LED light engines instead for quicker turn-on, much longer life, and better color reproduction. In fact, DLP and LCD rear-projection TVs are very similar in nearly every aspect of their physical construction, sharing the light weight and shallow cabinet depth that are such a big part of their appeal. Screens range from 46 inches to 73 inches diagonal; prices run from about $1200 to $5900, though relatively few have list prices higher than about $3500.
Also like LCD sets, DLP projectors are not subject to burn-in or declining brightness with age. Their handling of blacks and dark grays is superior, however--close to the performance of CRTs. A very small number of people occasionally notice a fleeting separation of colors ("rainbows") on moving objects when they move their heads or eyes quickly, but for most this is not an issue. (Projectors that use LED light sources instead of a white lamp with a color wheel to generate colors from a single DMD chip are immune to this effect.) All current DLP rear-projection TVs are wide-screen displays with a resolution of 1280 by 720 (720p) or 1920 by 1080 (1080p), and almost all are tabletop designs.
The oldest alternative imaging device in RPTVs is the liquid-crystal display, or LCD. Several manufacturers now use small LCD panels and high-intensity lamps to produce lightweight, high-performance rear-projection TVs. Even a 61-inch set will be less than 20 inches deep and weigh less than 100 pounds. And LCD sets can go on a shelf; floor stands are optional. Screens range from 37 inches to 61 inches diagonal, and prices range from about $1000 to $2400.
LCDs are not susceptible to burn-in and do not gradually lose brightness over time as CRTs do. On the other hand, they have a harder time producing deep blacks and gradations of dark gray, which can make LCD sets look a little washed out compared with CRTs. Manufacturers have worked hard to mitigate this weakness, however, and the best LCD projectors tend not to be as obviously challenged in this respect as they once were. All LCD rear-projection TVs are wide-screen displays, usually with a resolution of either 1280 by 720 (720p) or 1920 by 1080 (1080p).
Liquid crystal on silicon, or LCoS (also known as D-ILA, for direct-drive image light amplifier, or SXRD, for silicon x-tal reflective display), is a liquid-crystal display technology. But unlike conventional transmissive LCD panels, LCoS chips use a reflective silicon substrate, so that light bounces off them rather than passing through. Like DLP and LCD, LCoS allows production of shallow, lightweight, high-performance displays. A big appeal of this technology has always been the tight pixel packing it allows, which helps keep the picture smooth on very big screens. Screens range from 50 inches to 70 inches diagonal; prices range from about $2300 to $6000.
The strengths of current LCoS sets are similar to those of DLP models, though the best models may provide even better blacks and dark grays. In addition, all current implementations are three-chip designs (one for each primary color), which eliminates the possibility of the "rainbow" effect that some people observe with conventional DLP models. All current LCoS sets have 1920 by 1080 (1080p) resolution and are built for table or stand mounting.