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( TFT LCD ) |
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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 grey scale. Most displays today offer 256 levels of brightness per pixel.
An LCD that can show colours must have three subpixels with red, green and blue colour filters to create each colour pixel.
Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. Combining the subpixels produces a possible palette of 16.8 million colours (256 shades of red x 256 shades of green x 256 shades of blue), as shown below.
These colour displays take an enormous number of transistors. For example, a typical laptop computer supports 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.
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