LCD, Liquid Crystal Display or also known as Liquid Crystal Diode is one of the most popular display technologies currently. LCD monitors are lightweight, compact, occupy less space, consume low power and are available in a reasonable price. Currently there are two types of LCD technology in use – Active matrix LCD technology or TFT and Passive matrix technology. The TFT technology is more reliable with better image quality while the passive matrix technology has a slower response and gradually becoming outdated.
As the name indicates, liquid crystals are the key elements of the display screen. By manipulating the crystal we can change the way they interacts with the light. There is a display controller in the monitor which receives the display signals from the video adaptor in the motherboard. The display controller controls two things – the electric signals to the liquid crystals and the back light. Structure of an LCD is shown in the below images (Also see how LCD works).
The liquid crystals used in the LCD are Twisted Nemantic (TN), a type of liquid crystals that are twisted at 90o with the surface. In this state, crystals allow the light to pass through the polarizer but on applying a voltage, they get untwisted and block the light to passing through the polarizer. The display controller starts the backlight that passes through the first piece of the glass. At the same time the display controller also send the electrical currents to the liquid crystal molecules to align and allowing the varying level of light to pass through the second piece of glass, forming the desired picture on the screen. In color monitors, each pixel is made of three liquid crystal cells fronted with red, green and blue filters. The light passing through the filtered screen forms the color what you see on the monitor. A wide range of colors are formed by varying the intensity of colored pixels.
The backlight is made of cathodes, and depending on the quality of the monitor, there may be a single cathode at the top or one at the top and one at the bottom, or two at the top and two at the bottom to improve the brightness and clarity of the monitor. These cathodes are diffused through a layer of plastic and diffusing materials.
Resolution – Unlike the CRT monitors there is no complex equation for the dot pitch and the resolution. The resolution of a monitor is simply the number of pixels contained in the matrix. Typically a 17 inch monitor has a resolution of 1280 x 1024 pixels.
In the below video Bill Hammack explains how a TFT monitor works, how it uses liquid crystals, thin film transistors and polarizers to display information.
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