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How does it work

Flat panels LCD displays

By Peter H. Putman, etown.com
02/17/99

 

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You've probably heard about plasma displays -- those ultra-thin video monitors you can hang on the wall, stand on a tabletop or suspend from the ceiling. You certainly can't miss the commercials; one recent ad showed three industrious Gen-X types hauling a widescreen panel into a cramped little room to watch reruns of Flipper. Clearly, the vision of large, flat displays that was first articulated by science fiction writers in the 1930s has been realized.

Now let's get down to business. Admit it, the thought of winning PlayStation games and watching movies on a 16:9 screen that's not much thicker than a phone book is tempting, right? That's what manufacturers are hoping, and the consumer press has done their best to keep up the hype. The emergence of HDTV just adds more momentum to the plasma bandwagon, as does the availability of widescreen (particularly anamorphic) DVDs.

There's more to the story than hype, however. Plasma panels are indeed part of the "next wave" of video displays, along with liquid crystal display (LCD) screens and digital light processing (DLP) gear. Although they're not your standard TV set (few have built-in tuners), plasma display panels (PDPs) are capable of displaying everything from NTSC signals to computer graphics.

On the other hand, PDPs are not cheap. Current models (1999 ed.) costs from about $8000 to nearly $30,000. Moreover, as far as technology is concerned, PDPs are considered to be in their infancy. They are also fragile, which makes shipping them a major hassle. Most important, current models can't produce video images that look as good as your 27-inch or 32-inch direct-view TV sets without some help from external signal processors.

How it Works

Plasma displays are unique compared with other display technologies. So what's inside that thin panel, anyway? They use a color phosphor imaging system, like a TV set, and a fixed matrix of imaging pixels, just like an LCD. Their structure resembles two corrugated channels laid at right angles to each other to form tiny pixels at the channel intersections.
click to enlargeTake a microscope to those pixels and you'll see that they resemble small capacitors with three electrodes. An electrical discharge across these electrodes causes the rare gases sealed in the pixel to ionize and discharge ultraviolet light, which strikes the color phosphors along the face of each pixel, causing them to glow. The intensity and duration of that glow is determined by the charge, sustain and discharge cycle in each pixel.

Think of a low-voltage, lightweight TV monitor and you've got a plasma display panel. They have wide viewing angles (up to 160 degrees, which exceeds even conventional picture tubes) and are bright enough that you can watch them with the lights on. Models measuring 42 inches diagonally range in weight from 73 to 100 pounds, while 50-inch versions run from about 90 to 130 pounds. The thickest panels measure 6.2 inches, while the "slim jims" are almost half as thick (about 3.5 inches).

Be sure to read the fine print before you buy a plasma set. Plasma monitors are just coming into their own in terms of brightness and contrast; the brightest models are just now approaching the light output that the aforementioned 27-inch and 32-inch sets can produce. What's more, contrast numbers have been wildly exaggerated. Some manufacturers have claimed as much as 500:1 image contrast, but that's before the anti-glare glass is added to the raw panels.

The biggest obstacle that plasma panels have to overcome is their inability to achieve a smooth ramp from full white to dark black. Low shades of gray are particularly troublesome, and you'll often notice a posterized effect when watching movies or other video programming with dark scenes.

In technical terms, this problem is due to insufficient quantization, or digital sampling of brightness levels. In real world terms, it's the equivalent of trying to fit 10 gallons of water into a 7-gallon bucket -- water (with the plasma set, shades of gray) gets lost in the process. The bottom line is that black level is still an issue with PDPs. I'll address quantization in depth in each of the six plasma set review.

Quantization errors -- and the abrupt changes in luminance they create, which are known as false contours -- aren't as big a problem when viewing static computer images as they are when viewing video material. In fact, average brightness levels are pretty high. As a result, plasma panels look terrific if fed a steady diet of computer images. That's why they are adopted as "living" signs for such public areas as airports and railroad terminals. Merchants are using them in malls and stores, and some movie theaters are even displaying "dynamic" movie posters and coming attractions on PDPs.

 

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