How does a LCD TV work?
June 24, 2007
As the name suggests, the display in a Liquid Crystal Display (LCD) TV is based on the inter-active properties of liquid crystals and light. The technology used in making a LCD is called Thin Film Transistor (TFT), which has made tremendous advancements during the last ten years.
What are liquid crystals?
Liquid crystals are cylindrical shaped molecules which allow light to pass through them in their natural, twisted phase. However, they start to untwist or relax, when they are placed in an electric field. The amount of electric current decides the degree of untwisting of crystalline molecules. The more the untwisting of molecules the more the blocking of light rays will be. In their extreme relaxed state, they completely block the light and become opaque. By varying the amount of current, the intensity of light can vary between extreme brightness and blackness.
How LCD TV works?
In a LCD TV, liquid crystals are sandwiched between two transparent electrodes and two polarizing filters. In other words, liquid crystals (most often in nematic phase) are packed between two glass plates. Two grids of electrodes (one is horizontally aligned and another is vertically aligned) contact either side of the crystals. The job of this network of electrodes is to access each pixel of the crystal layer and control the electric flow.
Liquid crystals are not luminious by themselves and it is the backlight device made of fluorescent lamps which give off the light behind them. The light of the backlight is then diffused by a white panel before passing through the liquid crystals. This white panel ensures that the entire surface of the screen gets uniform brightness. The front panel of LCD is made up of a grid of wires which are in contact with each pixel of the screen and activate it separately.
With in network of crystals, each molecule functions like a gate to open or close the passage of light through them depending on the voltage of the current. To produce bright display, the voltage is kept low while for dark details the voltage is increased. By nature, liquid crystals always allow a smaller amount of light to pass through them making the pure black levels somewhat impossible.
Passive Matrix structure and Active Matrix structure
There are two options for controlling voltage – using passive matrix structure or active matrix structure. In the passive matrix structure of electrodes, one pixel can be accessed at a time. LCD displays with lesser resolution like simple mobiles, wristwatches use this option. However, the refresh rate will be usually slow and the voltage control would not be proper. Picture may not have outstanding clarity.
Large LCD displays like flat panel LCD television use the advanced active matrix structure. The electrodes will have an additional template of thin film transistors (TFT) which is capable of addressing the exact pixel and changing the colour of sub-pixels without involving the nearby pixels, as a result, better clarity of details and more colour preciseness are achieved. TFT also function like capacitors storing electric charge during the refreshing cycle of pixels.
How does a LCD TV produce colours?
In earlier mono-chrome LCD displays, only two basic colours were available, most of them had green as their back-ground colour. In a colour LCD display, as in Plasma TV, a pixel (picture element) has three sub-pixels – red, blue and green. In addition, there is a colour filter placed before the crystal layer to produce the overall colour of the pixel. As the name indicates, colour filters filter the colours from the colour spectrum and produce innumerable combinations of basic colours (red, blue and green) to form coloured display
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