The Secret Life of a Flat Panel Display
Commonly referred to as FPD, flat panel displays, compared with CRTs, or cathode ray tubes, take up less space, weigh less and need less power to operate. While CRTs are broadly accepted and in wide use because of their longevity and cost, FPDs are replacing some CRTs because of space, power or health requirements. FPDs are used widely in computers, transportation, medicine, aviation and the military.
A 1995 report from the now-defunct Office of Technology Assessment titled, "Flat Panel Displays in Perspective," explored the possible benefits for the United States of a high-volume FPD industry. The Office of Technology Assessment also assessed the role of government policies in developing such an industry. While concluding that an FPD industry would offer economic and national security benefits, the report noted the difficulty in determining the extent of the benefits. The reason cited was that trends in technology development and industry structure were making more displays available at lower prices. The report also noted that the barriers to setting up the industry were high and that the government lacked the necessary tools to lower them.
Today, nearly two years later, more than 90 percent of FPDs are produced overseas. According to the report, by value of sales, LCDs accounted for about 87 percent of the FPD market in 1995, evenly divided between active and passive matrix types. FPDs can be divided into transmissive (LCD) and emissive. Transmissive FPDs modulate an external light source, while emissive displays use materials that give off light when you apply an electric field. Four FPD types comprise the overwhelming majority currently in use:
1. Liquid crystal displays (LCDs)
With properties of both liquids and crystals, LCDs vary with temperature, stress, electric field and chemical environment. There are four types (a.) twisted nematic (TN-LCDs), a passive-matrix LCD; (b.) supertwisted nematic (STN-LCDs), a passive-matrix LCD; (c.) thin-film-transistor (TFT-LCDs) or active-matrix; and (d.) active-addressed (AA-LCDs), a hybrid of passive and active matrix.
On the plus side, LCDs have low power consumption, a very thin display, the lowest cost (TN-LCDs), capacity for full-color display, fast writing speed, legibility in direct sunlight and wide commercial availability. On the minus side, the passive matrix LCDs have a narrow viewing angle and other types have limited contrast.
2. Plasma display panels (PDPs)
This type works through the glow discharge that happens when ionized gas recombines. When removed from atoms, the electrons produce ions, which recombine with one another and release energy in the form of light. On the plus side, PDP is an established, rugged and reliable technology with many commercial sources. Its simple construction allows low-cost, high-volume production and it generally enjoys a long lifetime. On the minus side, it has high-voltage driver requirements, from 150 to 200 volts, washes out in bright sunlight and has limited gray-scale capacity.
3. Electroluminescent displays (ELs)
While similar to LCDs, ELs are doped (like a semiconductor) with impurities that add energy states that are below mobile electrons and above electrons orbiting atoms. These states effectively create transitions in which light is emitted. On the plus side, ELs are very thin and compact, with good legibility and brightness. They operate at low voltages. On the minus side, they require high-voltage drivers, from 170 to 200 volts, are expensive because of the difficulty of volume manufacturing, have high power consumption and wash out in bright light.
4.Vacuum fluorescent displays (VFDs)
This type operates at low voltages and is limited to a matrix of widely spaced phosphor dots. On the plus side, it demands less voltage than other display types and can be viewed in sunlight. On the minus side, large screen sizes involve high costs and full color is unavailable for home TV or computer monitors.