Basics of LCD Operation

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Basics of LCD Operation Liquid crystal displays (LCDs) are a passive display technology. This means they do not emit light; instead, they use the ambient light in the environment. By manipulating this light, they display images using very little power. This has made LCDs the preferred technology whenever low power consumption and compact size are critical. Liquid crystal (LC) is an organic substance that has both a liquid form and a crystal molecular structure. In this liquid, the rod-shaped mol
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  Basics of LCD Operation  Liquid crystal displays (LCDs) are a passive display technology. This means they donot emit light; instead, they use the ambient light in the environment. By manipulatingthis light, they display images using very little power. This has made LCDs thepreferred technology whenever low power consumption and compact size are critical.   Liquid crystal (LC) is an organic substance that has both a liquid form and a crystal   molecular structure. In this liquid, the rod-shaped molecules are normally in a parallelarray, and an electric field can be used to control the molecules. Most LCDs today usea type of liquid crystal called Twisted Nematic (TN). See figure below to see a visualof the molecule alignment.A Liquid Crystal Display (LCD) consists of two substrates that form a flat bottle that contains the liquid crystal mixture. The inside surfaces of the bottle or cell arecoated with a polymer that is buffed to align the molecules of liquid crystal. Theliquid crystal molecules align on the surfaces in the direction of the buffing. ForTwisted Nematic devices, the two surfaces are buffed orthogonal to one another,forming a 90 degree twist from one surface to the other, see figure below.This helical structure has the ability to control light. A polarizer is applied to the frontand an analyzer/reflector is applied to the back of the cell. When randomly polarizedlight passes through the front polarizer it becomes linearly polarized. It then passes  through the front glass and is rotated by the liquid crystal molecules and passes   through the rear glass. If the analyzer is rotated 90 degrees to the polarizer, the light   will pass through the analyzer and be reflected back through the cell. The observerwill see the background of the display, which in this case is the silver gray of thereflector.The LCD glass has transparent electrical conductors plated onto each side of the glassin contact with the liquid crystal fluid and they are used as electrodes. Theseelectrodes are made of Indium-Tin Oxide (ITO). When an appropriate drive signal isapplied to the cell electrodes, an electric field is set up across the cell. The liquidcrystal molecules will rotate in the direction of the electric field. The incominglinearly polarized light passes through the cell unaffected and is absorbed by the rearanalyzer. The observer sees a black character on a sliver gray background, see figure2. When the electric field is turned off, the molecules relax back to their 90 degreetwist structure. This is referred to as a positive image, reflective viewing mode.Carrying this basic technology further, an LCD having multiple selectable electrodesand selectively applying voltage to the electrodes, a variety of patterns can beachieved.Many advances in TN LCDs have been produced. Super Twisted Nematic (STN)Liquid Crystal material offers a higher twist angle (>=200° vs. 90°) that provideshigher contrast and a better viewing angle. However, one negative feature is thebirefringence effect, which shifts the background color to yellow-green and thecharacter color to blue. This background color can be changed to a gray by using aspecial filter.The most recent advance has been the introduction of Film compensated Super   Twisted Nematic (FSTN) displays. This adds a retardation film to the STN displaythat compensates for the color added by the birefringence effect. This allows a black and white display to be produced.   
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