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How LCD screens work.

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How LCD screens work

Although the term liquid crystal sounds like an oxy moron, it is in fact describing a solid whose particles are free to move around each other almost like a heavy molecular level.

Crystals are either categorised as thermotropic or lyotropic. Thermotropic crystals are affected by changes in temperature and some times pressure. They are either isotropic (with no particular arrangement) or nematic (with a pattern).image00.jpg

Ferro electric crystals use nematic crystals with a spiral pattern to allow for microsecond switching. It is necessary to have a layer of glass to maintain a certain pressure and to alow for even quicker switching.

There are four facts that allow for liquid crystals to work in the way they do

  • Light can be polarized.
  • Liquid crystals can transmit and change polarized light.
  • The structure of liquid crystals can be changed by electric current.
  • There are transparent substances that can conduct electricity.
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Active-matrix LCDs depend on thin film transistors (TFT). Basically, TFTs are tiny switching transistors and capacitors. They are arranged in a matrix on a glass substrate. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. Since all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. And if we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through. By doing this in very exact, very small increments, LCDs can create a gray scale. Most displays today offer 256 levels of brightness per pixel.

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resolutions up to 1,024x768. If we multiply 1,024 columns by 768 rows by 3 subpixels, we get 2,359,296 transistors etched onto the glass! If there is a problem with any of these transistors, it creates a "bad pixel" on the display. Most active matrix displays have a few bad pixels scattered across the screen.



Display size is limited by the quality-control problems faced by manufacturers. Simply put, to increase display size, manufacturers must add more pixels and transistors. As they increase the number of pixels and transistors, they also increase the chance of including a bad transistor in a display. Manufacturers of existing large LCDs often reject about 40 percent of the panels that come off the assembly line. The level of rejection directly affects LCD price since the sales of the good LCDs must cover the cost of manufacturing both the good and bad ones. Only advances in manufacturing can lead to affordable displays in bigger sizes.

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