PHOTONIC FRONTIERS: Large-scale display technologies: Large displays rise to the challenge of high-definition TV

LCDs, plasma displays, and rear-projection TVs based on micromirrors are consigning the venerable CRT to the fate of the dinosaurs.

Mar 1st, 2006
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LCDs, plasma displays, and rear-projection TVs based on micromirrors are consigning the venerable CRT to the fate of the dinosaurs.

Jeff Hecht, contributing editor


After years of anticipation, the market for large digital displays is finally taking off. The U.S. Congress gave the final push late last year when it set a hard deadline for the shutdown of analog television broadcasts. Digital television does not require a large screen, but the U.S. digital broadcast standard does offer a high-resolution format tailored for large-screen viewing. And last year Hollywood finished a new digital cinema standard that the studios hope will finally push film off the big screen, requiring even larger digital displays.

The electronics industry has been pushing for big-screen television for more than 20 years. Back in 1983, plans for high-definition television (HDTV) called for analog transmission at double the vertical and horizontal resolution of the old NTSC broadcast standard. Developers of standards turned to digital technology in the 1990s, which allowed one HD channel to be squeezed into the same 6‑MHz bandwidth as one low-definition analog channel. Yet big-screen technology remained a stumbling block.


FIGURE 1. In the structure of a plasma display, with a single element and an array, the elements are addressed by arrays of conductors on front and back of the panel. When a pixel is addressed, an internal discharge excites xenon atoms to emit ultraviolet light, which excites a red, green, or blue phosphor.
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The venerable cathode-ray-tube (CRT) display can offer high resolution, but large sizes are impractical. New tubes can deflect the electron beam at sharper angles, reducing their depths. However, price and bulk both increase steeply with size. The largest direct-view CRT-based televisions measure 40 in. diagonally and weigh in at more than 300 lb, but today virtually all television screens larger than 32 in. are flat-panel or projection displays.

Flat-panel displays

Today’s flat-panel TV market is divided between plasma and liquid-crystal displays, with plasma holding the edge in size. Invented in the 1960s, plasma panels are arrays of tiny tubes or cells containing a mixture of xenon and neon sandwiched between glass sheets. An electric discharge excites the xenon atoms to emit UV light, which in turn excites a phosphor; the neon is a buffer gas. Separate cells contain red, green, and blue phosphors; the three are grouped together to make color pixels. Electronics address individual pixels through an array of horizontal and vertical conductors (see Fig. 1).

Plasma panels had an early lead in size and remain the largest flat-panel displays available. The largest at last year’s Society for Information Display show (SID; May 22-27, 2005; Boston, MA) was an impressive 102-in. full-color HD display from Samsung. The largest plasma TV screens are about 50 in., with prices topping $4000, and are only a few inches thick.

The size of plasma displays is now limited by the size of glass sheets that meet fabrication requirements. Plasma displays are bright, but are not efficient, with luminous efficiency typically below 2 lumens per watt. One advantage is darker blacks, because pixels generate their own light instead of being backlit.


FIGURE 2. LCD panels require light from an external source. Crossed polarizers transmit light from the backlight only when the liquid crystal is turned on by bias on electrodes (not shown). A red, green, or blue filter colors each pixel.
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Liquid-crystal displays started small, and took off with the spread of laptop computers. They are now the standard monitors for desktop computers, in sizes to 30 in., and are transmissive arrays of pixels with red, green, and blue filters that are modulated individually and illuminated by white light (see Fig. 2). Like plasma panels, they are a few inches thick, with the active elements sandwiched between glass plates, but tolerances on the glass sheets are tighter for LCDs.

Liquid-crystal displays can’t quite match the size of the biggest plasma displays; at last year’s SID and this year’s Consumer Electronics Show (CES; Jan. 5-8; Las Vegas, NV) Samsung showed an 82-in. HD LCD next to its 102-in. plasma display. Such giants are “vanity displays” for corporate lobbies costing around $150,000, says Chris Chinnock of Insight Media (Norwalk, CT). Liquid-crystal-display TV monitors range up to 46 in., and dominate the flat-panel market at smaller sizes where they are less expensive than plasma displays.

Developers are still refining the internal structures of plasma and liquid-crystal displays for optimum performance as flat panels. A third option for flat displays are organic LEDs, which have already made great strides in small formats. However, technical problems remain in scaling to large sizes, and competition with LCDs and plasmas is likely a few years away, says Matt Brennesholtz of Insight Media.

Rear-projection displays

Although flat-panel displays have captured most of the attention in the technical press, the largest consumer TV monitors on the market are rear-projection sets measuring up to 73 in. Although laser projectors have been developed, (see www.laserfocusworld.com/articles/243300), today’s products use xenon lamps to project small images onto large screens. The most common types on the TV market are based on the micromirror arrays developed by Texas Instruments, called “digital light processing” (DLP). Typically a filter wheel shines each primary color on the array in sequence; the eye blends the colors together. Some manufacturers also project images from transmissive or reflective LCDs. A new twist that appeared at CES in January is to substitute bright LEDs for the lamps, which last only a few thousand hours and cost hundreds of dollars to replace (see “Laser- and LED-based TVs are within reach,” www.laserfocusworld.com/articles/248114).


FIGURE 3. A thin rear-projection television uses a folded optical system to project an image on the back of a screen; the viewer sees the image as if it was projected from in back of the television cabinet. At last year’s SID, Shinsuke Shikama of Mitsubishi’s Information Technology R&D Center (Kanagawa, Japan) said this design allowed the cabinet to be less than 20 cm deep.
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Manufacturers fold the rear-projection optics and package them into a box that resembles a standard console TV (see Fig. 3). The lamp and projection lens are at the lower rear of the box, with mirrors folding the light path and projecting the image onto the back of the screen. Like a CRT or flat-panel set, the rear-projection set is usable in an illuminated room.

Front-projection displays work like movie projectors, focusing an image onto a screen that reflects light to the viewer. This normally produces a low-intensity image that is best seen in a dark room, so the main applications of these displays have been in business and conference presentations.

Technology challenges for HDTV

The biggest challenge facing display manufacturers is to scale up production while reducing costs. At last year’s SID, Samsung said it expected sales of LCD televisions to rise from 17 million in 2005 to 100 million in 2010. To meet that soaring demand, Samsung is building massive new plants and hopes to bring the price of a 32-in. LCD set below $1000.

Others are not ready to cede the market to LCDs. At a January HDTV market seminar sponsored by Insight Media, makers of LCD, plasma, and micromirror rear-projection TVs each forecast that their own technology would dominate by 2010-understandably confusing attendees. Chinnock says all three major HD display technologies “have achieved acceptable performance for 95% of consumers. That’s why we’re in the rapid adoption phase.”

All large display technologies are still evolving, as developers try to improve parameters such as power consumption, color, on-screen response to rapid motion, and contrast ratio. The task is confusing because many measurements are not standardized across the different types of displays. Surprisingly little hard information has been compiled on actual viewing angles and levels of ambient lighting in television rooms, which impact perceived image quality.

Digital cinema

The digital-display trend is also coming to theaters. Hollywood has worked to keep its screens more attractive than home screens since TV emerged after World War II, and HDTV has thrown down a new challenge. Now, Brennesholtz says, a homeowner willing to spend $5000 on a home theater built around a 60-in. screen could beat the sound and pictures in the local cinema multiplex-without the sticky floors, $5 popcorn, or teenagers chatting on cell phones.

Seven major studios last summer announced a Digital Cinema System Specification, which specifies two screening formats that would offer better pictures to keep box offices busy. One would approximate the maximum HDTV quality, with 2048 × 1080 pixels per frame at either 24 or 48 frames per second. The second would double the resolution, to 4096 × 2160 pixels, at 24 frames per second.

The Digital Cinema Initiatives do not specify a technology, but micromirror arrays are available at resolutions to 4000 × 2000 pixels. Theater systems could illuminate three arrays with different colors from one xenon lamp, and combine the colors in the projector. Laser displays are also in development, but Brennesholtz says that big theaters would need 20-W red, green, and blue lasers. The lamps aren’t cheap but “the cost of those lasers is just astounding,” he said.

Outlook

We’re still a long way from a 1-in.-thin 6-ft screen that can hang lightly on a wall and show crystal-clear images-and even further from one we can afford. But the massive Samsung plasma and LCD displays are not merely big, but also good enough to show bright colors in a well-lit part of the SID show floor. Prices are coming down in the sizes needed for HDTV. But the market has yet to pick a winning technology, and most reports say consumers at large still don’t understand the digital television transition or the many available display formats. The public confusion about format seems to overshadow discussion of content, bringing to mind a line from the late cartoonist Jeff MacNelly: “Billions for high-resolution TV [but] not a cent for high-intelligence TV.”

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