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A plasma display is an emissive flat panel display where light is created by phosphors excited by a plasma discharge between two flat panels of glass. The gas discharge contains no mercury (contrary to the backlights of an AMLCD); a mixture of noble gases (neon and xenon) is used instead. This gas mixture is inert and entirely harmless. A general purpose alphanumeric LCD, with two lines of 16 characters. Many LCDs are driven to darkness by an alternating current, which disrupts the twisting effect, and become faint or transparent when no current is applied. Raw LCD TFT panels are usually factory-sorted into three categories, with regard to the number of dead pixels, backlight evenness and general product quality. Additionally, there may be up to +/- 2ms maximum response time differences between individual panels that came off the same assembly line on the same day. The poorest-performing screens are then sold to no-name vendors or used in "value" TFT monitors (often marked with letter V behind the type number), the medium performers are incorporated in gamer-oriented or home office bound TFT displays (sometimes marked with the capital letter S), and the best screens are usually reserved for use in "professional" grade TFT monitors (usually marked with letter P or S after their type number). In color LCDs each individual pixel is divided into three cells, or subpixels, which are colored red, green, and blue, respectively, by additional filters. Each subpixel can be controlled independently to yield thousands or millions of possible colors for each pixel. Older CRT monitors employ a similar method for displaying color. Color components may be arrayed in various pixel geometries, depending on the monitor's usage. Before applying an electrical charge, the liquid crystal molecules are in a relaxed state. Charges on the molecules cause these molecules to align themselves in a helical structure, or twist (the "crystal"). In some LCDs, the electrode may have a chemical surface that seeds the crystal, so it crystallizes at the needed angle. Light passing through one filter is rotated as it passes through the liquid crystal, allowing it to pass through the second polarized filter. A small amount of light is absorbed by the polarizing filters, but otherwise the entire assembly is transparent.
Until quite recently (c. 2004) the superior brightness and viewing angle of color plasma panels, when compared to LCD, made them one of the most popular forms of display for HDTV. However since that time improvements in LCD technology have closed the gap dramatically. The much lower weight, price and power consumption of LCDs have seen them make large inroads into the former plasma market. Sony now only sell a very limited range of Plasma screens and appear set to quit this market altogether. The first operational LCD was based on the Dynamic Scattering Mode (DSM) and was introduced in 1968 by a group at RCA in the USA headed by George Heilmeier. Heilmeier founded Optel, which introduced a number of LCDs based on this technology. Some LCD panels have defective transistors, causing permanently lit or unlit pixels. Unlike integrated circuits, LCD panels with a few defective pixels are usually still usable. It is also economically prohibitive to discard a panel with just a few bad pixels because LCD panels are much larger than ICs. Manufacturers have different standards for determining a maximum acceptable number of defective pixels. The following table presents the maximum acceptable number of defective pixels for IBM's ThinkPad laptop line.
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* LCD screens occasionally suffer from image persistence, which is similar to screen burn on CRT displays. Important factors to consider when evaluating an LCD monitor include resolution, viewable size, response time (sync rate), matrix type (passive or active), viewing angle, color support, brightness and contrast ratio, aspect ratio, and input ports (e.g. DVI or VGA). In 1969, the twisted nematic field effect in liquid crystals was discovered by James Fergason at Kent State University in the USA, and in 1971 his company ILIXCO (now LXD Incorporated) produced the first LCDs based on it, which soon superseded the poor-quality DSM types. The xenon and neon gas in a plasma television is contained in hundreds of thousands of tiny cells positioned between two plates of glass. Long electrodes are also sandwiched between the glass plates, on both sides of the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes, which are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer, are mounted above the cell, along the front glass plate. LCD panels are more likely to have defects than most ICs due to their larger size. In this example, a 12" SVGA LCD has 8 defects and a 6" wafer has only 3 defects. However, 134 of the 137 dies on the wafer will be acceptable, whereas rejection of the LCD panel would be a 0% yield. The standard is much higher now due to fierce competition between manufacturers and improved quality control. An SVGA LCD panel with 4 defective pixels is usually considered defective and customers can request an exchange for a new one. The location of defective pixels is also important. A display with only a few defective pixels may be unacceptable if the defective pixels are near each other. Manufacturers may also relax their replacement criteria when defective pixels are in the center of the viewing area. Types
In 1969, the twisted nematic field effect in liquid crystals was discovered by James Fergason at Kent State University in the USA, and in 1971 his company ILIXCO (now LXD Incorporated) produced the first LCDs based on it, which soon superseded the poor-quality DSM types. In-Plane Switching (IPS) LCD technology still has a few drawbacks in comparison to some other display technologies: The xenon and neon gas in a plasma television is contained in hundreds of thousands of tiny cells positioned between two plates of glass. Long electrodes are also sandwiched between the glass plates, on both sides of the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes, which are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer, are mounted above the cell, along the front glass plate. Enlarge