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* The viewing angle of a LCD is usually less than that of most other display technologies thus reducing the number of people who can conveniently view the same image. However, this negative has actually been capitalised upon in two ways. Some vendors offer portables with intentionally reduced viewing angle, to provide additional privacy for example when using the PC in airplanes. Secondly, it allows multiple TV outputs from the same LCD screen just by changing the angle from where the TV is seen. Such a set can also show two different images to one viewer, providing 3-D. Techniques for color graphics 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. When an electrical charge is applied to the electrodes, the molecules of the liquid crystal align themselves parallel to the electric field, thus limiting the rotation of entering light. If the liquid crystals are completely untwisted, light passing through them will be polarized perpendicular to the second filter, and thus be completely blocked. The pixel will appear unlit. By controlling the twist of the liquid crystals in each pixel, light can be allowed to pass though in varying amounts, correspondingly illuminating the pixel. Techniques for color graphics Enlarge Construction Reflective LCDs, often found in digital watches and calculators, are illuminated by external light reflected by a (sometimes) diffusing reflector behind the display. This type of LCD can produce darker 'blacks' than the transmissive type since light must pass through the liquid crystal layer twice and thus is attenuated twice, however because the reflected light is also attenuated twice in the translucent parts of the display image contrast is usually poorer than a transmissive display. The absence of a lamp significantly reduces power consumption, allowing for longer battery life in battery-powered devices; small reflective LCDs consume so little power that they can rely on a photovoltaic cell, as often found in pocket calculators.
TN+Film A French company, Nemoptic, has developed another zero-power, paper-like LCD technology which has been mass-produced in Taiwan since July 2003. This technology is intended for use in low-power mobile applications such as e-books and wearable computers. Zero-power LCDs are in competition with electronic paper. MVA (Multi-domain Vertical Alignment) was originally developed in 1998 by Fujitsu as a compromise between TN and IPS. It achieved fast pixel response (at the time), wide viewing angles, and high contrast at the cost of brightness and color reproduction. When an electrical charge is applied to the electrodes, the molecules of the liquid crystal align themselves parallel to the electric field, thus limiting the rotation of entering light. If the liquid crystals are completely untwisted, light passing through them will be polarized perpendicular to the second filter, and thus be completely blocked. The pixel will appear unlit. By controlling the twist of the liquid crystals in each pixel, light can be allowed to pass though in varying amounts, correspondingly illuminating the pixel. Value TFT screens and most 38 cm (~15 in.) sized LCDs usually fail to include a digital signal compatible DVI interface, thus their future proofing may be limited. The upper end of 43 cm (~17 in.) or 48 cm (~19 in.) gamer and office TFT screens may have dual analog-VGA and DVI sockets; almost all professional screens have DVI and pivot mode for letter-mode display. However, the use of a DVI video signal does not automatically guarantee better image quality: a good video card RAMDAC and properly shielded analogue VGA cable may produce a better display than a bad video card and DVI. TN+Film Twisted Nematic displays contain liquid crystal elements which twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, the light is polarized to pass through the cell. In proportion to the voltage applied, the LC cells twist up to 90 degrees changing the polarization and blocking the light's path. By properly adjusting the level of the voltage most any grey level or transmission can be achieved.
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IPS has since been superseded by S-IPS (Super-IPS), which has all the benefits of IPS technology with the addition of improved pixel refresh timing. Though color reproduction approaches that of CRTs, the contrast ratio remains relatively weak. S-IPS technology only appears in larger displays aimed at professionals, though pricing has come down to the reach of the typical consumer. Competing displays include the Cathode ray tube, OLED, AMLCD, DLP, SED-tv and field emission flat panel displays. The main advantage of plasma display technology is that a very wide screen can be produced using extremely thin materials. Since each pixel is lit individually, the image is very bright and looks good from almost every angle. Because many plasma displays still have a lower resolution the image quality is often not quite up to the standards of good LCD displays or cathode ray tube sets, but it certainly meets most people's expectations. Also, most cheaper consumer displays appear to have an insufficient color depth - a moving dithering pattern may be easily noticible for a discerning viewer over flat areas or smooth gradients; expensive high-res panels are much better at managing the problem. For high-resolution color displays such as modern LCD computer monitors and televisions, an active matrix structure is used. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, which allows each column line to access one pixel. When a row line is activated, all of the column lines are connected to a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation. Active-matrix displays are much brighter and sharper than passive-matrix displays of the same size, and generally have quicker response times. Enlarge General characteristics A French company, Nemoptic, has developed another zero-power, paper-like LCD technology which has been mass-produced in Taiwan since July 2003. This technology is intended for use in low-power mobile applications such as e-books and wearable computers. Zero-power LCDs are in competition with electronic paper. MVA
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. Main article: Color LCD 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. Enlarge 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.