Quality of image is far better in TFT monitor than the LCD monitor, This is because TFT monitor uses Thin Film Transistor (TFT) technology to enhance the contrast and addressability of the image. It is a variant of the Liquid Crystal Display (LCD) that uses matrix LED technology to provide clarity, brightness etc of the monitor. TFT monitors are now common in TV, Computer, Mobile phone etc.
Liquid Crystal Display
Liquid Crystal is a form matter that has the properties between liquid and solid. The material can flow like a liquid and its molecules can orient to form a crystal. LCD is a flat display unit made up of liquid crystals with light modulating property. Unlike CRT monitors, the LCD does not emit light directly. LCD has numerous Pixels filled with liquid crystals arrayed in front of a light source called Backlight. It gives images either in colour or monochrome. The pixels in the LCD are addressed in rows and columns which are connected to transistor switches one for each pixel. Each pixel works like a small capacitor having a layer of liquid crystal sandwiched between two transparent conductive layers.
How TFT Works?
TFT monitors deliver crisp text, improved response time and vibrant colours. The transistors in TFT are made from a thin film of amorphous silicon deposited on a glass panel through the PECVD process. In each pixel, the transistor occupies only a small area leaving the remaining part to pass light. Each transistor in TFT is very small so that the charge needed to operate it is very low. As a result, the image redrawing is very fast and the image is refreshed many times per second. In a typical TFT monitor, there will be around 1.3 million pixels and 1.3 million tiny transistors. The transistors are highly sensitive so that they may fail occasionally leading to “Dead Pixels” creating tiny dots of colours on a solid back ground. Dead pixel means the pixel of TFT in which the transistor is not working. In High performance TFT, Polycrystalline silicon is used to make the transistors.
TFT – LCD Structure
In structure, the TFT has two glass substrates sandwiching a layer of liquid crystal. The front glass substrate has a colour filter while the back glass substrate has transistors arranged in rows and columns. At the back of the back glass substrate, there is a light source called Backlight unit. When voltage is applied to the liquid crystal layer, the molecules bend and allows light to pass through to create a pixel. The color filter on the front glass substrate gives colour to each pixel and forms the image.
Image formation in TFT
The TFT –LCD display is controlled by electric signals. The LCD is located between two conducting layers of ITO electrodes. By applying varying voltage, the liquid crystal molecules can be aligned in different directions. The direction of alignment of the liquid crystals produces regions of lightness and darkness in the image. This is the Grey scale image formation, while in colour image formation; a colour filter substrate gives different colour in the image. The pixel of color or grey scale depends on the voltage level applied from the data driver.
The back glass substrate is fabricated with around 1.3 million switching transistors and the formation of pixel depends on the On / Off these transistors. The switching controls the flow of electrons into the ITO electrode zone. The flow of electrons controls each pixel on or off. The alignment occurs by the rotation of the LC molecules according to the charge applied to each pixel. When millions of pixels are charged differently, millions of LC angles are produced with in the area.
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