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The New TV Technologies

A plasma display panel (PDP) is a type of flat panel display now commonly used for large TV displays (typically above 37-inch or 940 mm). Many tiny cells located between two panels of glass hold an inert mixture of noble gases (neon and xenon). The gas in the cells is electrically turned into a plasma which then excites phosphors to emit light.

Liquid Crystal Display Televisions (LCD TV) are televisions that use LCD technology for visual output (producing an image). The technology used is generally TFT, because this allows for size, especially depth reduction, lower weight and reduced energy consumption. These LCD screens are also used as computer monitors for desktop computers, as well as in laptops.

Plasma Technology

Plasma History
Although you may think plasma technology is a recent phenomenon, the science has been around since 1960, and the first plasma prototype appeared back in 1964. While a handful of major manufacturers were interested in plasma technology at the outset, the absence of industrial outlets caused the entire industry to nearly grind to a halt by the late 1980s. However, plasma research continued in Japan, where the first commercial models hit the market in the early 1990s. Today most major consumer electronics manufacturers offer plasma televisions.

Plasma screens, as the name suggests, use a matrix of tiny gas plasma bubbles coated by phosphor and charged by precise electrical voltages to create a picture. Plasma technology operates on the premise that each subpixel within a plasma display is a microscopic fluorescent lamp that emits one of the primary colors (red, green or blue). Technicians are able to create a multitude of tints by varying the intensity of the light from these three subpixels. When it’s time to display an image signal (RGB or video), a digitally controlled electric current flows through the flat screen, causing the plasma inside designated bubbles to give off ultraviolet rays. This light in turn causes the phosphor coatings to glow the appropriate color. The millions of RGB bubbles glowing and dimming combine to make a rich, vivid image. Because the light emitted by the plasma is ultraviolet radiation, which is invisible to humans, it must be changed into a visible form of energy. To achieve this transformation, the walls of the plasma tube are coated with a UV-sensitive powder that emits white light. This powder, often called a phosphor, is known as a scintillator - a material that converts one type of radiation to another. CRTs also contain scinitillators that convert the electron beams into visible red, green or blue light.

Plasma Pluses
The most striking advantage that plasmas currently over LCD televisions is their availability in the largest screen formats. However, it won’t be long before some manufacturers produce LCD screens that will be comparable in size to many plasmas. Currently, on an inch by inch basis, plasmas are less expensive than LCDs. Additionally, plasma contrasts are also superior to those of LCDs, and equal to the best CRT televisions. Plasma TVs are quite versatile; capable of displaying full HDTV and DTV signals as well as XGA, SVGA and VGA signals from a computer. Furthermore, plasma televisions present a much wider range of richer colors due to their huge choice of scintillators. High-end plasma screens can display 16.77 million colors, providing superb realism with exceptionally subtle gradations among colors. In fact, color saturation represents one of the most dramatic advantages that plasma screens have over other display technologies.

Plasma Minuses
On the negative side, the large size of the plasma pixels means that plasma televisions are restricted in size to at least 32-inches diagonal in order to achieve competitive resolutions. Plasma sets also encounter some image quality problems stemming from the nature of their pixels. Because plasma pixels need an electrical discharge in order to emit light, a pixel must be lit or unlit, but has no intermediate state. Consequently, plasma manufacturers employ a method called PCM (Pulse Code Modulation) to control brightness. With PCM, a pixel is lit frequently to attain brightness and less often to attain a darker shade. This works well for medium and bright colors, but it’s often difficult to distinguish between two similarly dark shades. PCM technology creates a uniform image if the viewer if far enough from the panel, but some discomfort at close distances. Plasma pixels are also prone to burn-in, a phenomenon also found in CRT screens. Burn-in occurs when the same image is projected too long and becomes permanently imprinted on the phosphor because of premature aging of the scintillators. This isn’t a problem under normal use, because the images projected change constantly. However, in certain business applications, where the same channel is used on the screen all the time, issues can arise. For example, a network’s logo can become burned into the display. And when a plasma screen is used for static advertising displays, a fixed image projected constantly can become burned into the panel.

Uses for Plasma TVs
Plasma displays are found mostly in high-quality, large-format video systems. Their big size and video performance make them excellent for viewing DVDs, high definition or otherwise. Plasma is traditionally positioned at the high-end sector of the market, where the issues of high cost, phosphor aging and high power consumption are secondary to performance and quality.

LCD Technology

How LCD's Work
The twisted nematic (TN) is the most common type liquid crystal used in display applications such as LCD televisions, monitors and projectors. It is so named because it has a naturally twisted crystalline structure. This crystal reacts to electric currents in predictable ways, such as untwisting to varying degrees depending on the voltage of the current to which it is exposed. The main difference between plasma and LCD technology is that LCD pixels don’t emit light. As with plasma technology, an LCD pixel is comprised of three sub-pixels in the elementary colors. Because they don’t emit light, LCD displays need white backlighting. The light emitted by the backlighting passes through the liquid crystal and is then colored by a filter. Each subpixel has the same characteristics; only the color of the filter changes depending on the pixel. The liquid crystal of each subpixel can be controlled electrically like a valve; the amount of light allowed to pass through the crystal governs how much red, green and blue is emitted for each pixel. Active matrix LCDs employ thin film transistors (TFTs),m or tiny switching transistors and capacitors arranged in a matrix on a glass substrate, to direct electric charges down columns to reach a particular pixel. In turn, this causes the liquid crystals to untwist and display a predetermined amount of light generated by the light source – usually a fluorescent bulb located in back of them. By exploiting a combination of red, green, and blue subpixels of various intensities (or gray scales), a single pixel triad can reproduce approximately 16.8 million colors.

LCD Pluses
LCDs offer higher resolutions than plasmas of the same size. They also have excellent image stability. In other words, you can sit close without experiencing eye fatigue. Additionally, LCDs boast a longer lifetime than plasma televisions – on average about 50,000 hours versus 30,000 hours. Also, If you're contemplating a home entertainment setup involving a PC--perhaps running Windows XP Media center Edition - or other activities involving text as well as graphics, you'll get a crisper, brighter image from an LCD. LCDs are also space-efficient and because they operate at much cooler temperatures cost less per hour than plasma televisions. The smaller and better transistors found in LCDs give them another advantage over plasma – higher resolution.

LCD Minuses
LCD viewing angles cannot match those of plasma displays. You tend to see some brightness and color shift when you’re sitting at too far an angle from your LCD, while a plasma’s picture remains fairly solid. LCDs also have lower contrast ratios than plasmas and are not as good at rendering deep blacks. Additionally, they are not as good as plasmas in tracking motion and fast-moving objects may exhibit what is called, lag artifacts.

LCD Uses
The area where LCD reigns supreme over any other flat-panel displays is, of course, computers. LCD monitors can now be used for most applications including games, office applications, and photo retouching. But it's another story for television. LCD is lagging behind plasma, but it's available in more reasonable display sizes. In terms of absolute video quality, plasma is still tops, because it offers blacks as good as what CRTs can display, exceptional viewing angles, and unmatched color. However, LCDs are closing the gap little by little with technologies that are constantly being refined.

Comparison Chart: Plasma vs. LCD

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Tags: LCD TV, Plasma TV, thin film transistors, TFTs, Plasma Technology, scintillator