LCD TV

Liquid crystal display television is, as indicated by its name, a television using LCD technology (generally TFT), as opposed to cathode ray or plasma for its visual output. Early LCD panel television had some difficulties displaying fast-moving action and had quite restricted viewing angles. These problems have largely been overcome in recent years, and the market for LCD televisions is booming, especially in Asia.

For a long time it was widely believed that LCD technology was suited only to smaller sized televisions, and could not compete with plasma technology at larger sizes. This belief has been undermined by the announcements of ever-larger panels by companies such as Sharp Corporation, Samsung and LG.Philips. In October 2004, 40" to 45" televisions were widely available and Sharp Corporation had announced the successful manufacture of a 65" panel. Also in 2004, Samsung and Sony joined forces to build a factory in South Korea, intended to produce 60,000 panels a month, and in March 2005, Samsung announced an 82" HDTV TFT Panel. The main manufacturers have all pledged to invest billions of dollars in LCD production over the next few years, with televisions expected to be a key market.

Modern LCD TV sets are geographically universal because they have a multisystem tuner, to display PAL, NTSC and SECAM norms. And they include an electronic (step-down & step-up) transformer that automatically can use 110/200 V AC indiferently and universal grounded adapter plugs. They are not only for old TV and HDTV (DVB), because they can display VGA/DVI and can be used like a computer monitor at the same time.

History

The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points. Because of his work, Friedrich Reinitzer is often credited for finding this phase of matter - the liquid crystal.

The most common application of liquid crystal technology is in liquid crystal displays (LCDs). LCD devices have become an important part of our lives, from the wrist watch, calculator to LCD monitors and now LCD TVs.

LCD Technology

LCD consists primarily of two glass plates with some liquid crystal material between them. Lack of bulky tubes makes LCD practical for many applications where size and weight is important like LCD TVs.

Passive Vs Active Matrix Displays

Passive Matrix:
A type of liquid crystal display which relies on persistence to maintain the state of each display element (pixel) between refresh scans. The resolution of such displays is limited by the ratio between the time to set a pixel and the time it takes to fade.

The passive matrix display is addressed by a set of transparent electrodes, parallel wires run both vertically and horizontally and pixels are turned on when the wires intersecting at that pixel are both energized

When there is a sufficient voltage across it to cause the liquid crystal molecules (in a pixel) to align parallel to the electric field, the passive pixel is addressed. A display can have more than one pixel on at any one time because of the response time of the liquid crystal material. When addressed, a pixel has a short turn-on time during which the liquid crystal molecules align in such a way as to make the pixel opaque. When the voltage is removed the pixel behaves similar to a discharging capacitor, slowly turning off as charge dissipates and the molecules return to their undeformed orientation.

Because of this response time, a display can scan across the matrix of pixels, turning on the appropriate ones to form an image. As long as the time to scan the entire matrix is shorter than the turn-off time, a multiple pixel image can be displayed. As the voltage is removed the cell remains opaque briefly before it becomes clear again.

Active Matrix:
A type of liquid crystal display where each display element (each pixel) includes an active component such as a thin film transistor (TFT) to maintain its state between scans. Considered superior to passive matrix

Active matrix displays can be found in computer monitors, laptops and LCD TVs. In this type of display, the addressing takes place completely behind the liquid crystal film. TFT acts as a switch for each pixel. The TFT is addressed by a set of electrodes, gate lines and source lines, running along the gaps between pixels. A pixel turned on by applying current to a gate line which allows charge from the source line to flow on to the rear electrode. This sets up a voltage across the pixel and turns it on. An image is created similar to the passive display as the addressing circuitry scans across the matrix. An active matrix display does not suffer from many of the limitations of the passive display. It can be viewed at an angle of up to 45 degrees. It does, however, require a more intense back lighting system because the TFT's and the gate and source lines are not very transparent and therefore block a fraction of the light.

Color Displays

To get full colour, each individual pixel is divided into three sub pixels: red, green and blue (RGB). If you go close enough to an LCD/Plasma/LCD Monitor you will notice that each pixel is infact made up of 3 smaller rectangular sub pixels. These sub pixels are created by applying colour filters which only allow certain wavelengths to pass through them while absorbing the rest. With a combination of red, blue and green sub pixels of various intensities, a pixel can be made to appear any number of different colours. If each red, green and blue sub pixel can display 256 different intensities of their respective colour, then 256x256x256 = 16777216 different colours can be emitted from the one pixel. RGB sub pixels are analogous to a colour cathode ray tube (CRT) like a television or computer monitor in which different phosphors glow red, green or blue when excited by an electron beam. The number of colours that can be made by mixing red, green and blue sub pixels depends on the number of distinct gray scales (intensities) that can be achieved by the display. The picture below shows an LCD pixels at close up. You will notice the 3 red, green and blue sub pixels that make a a pixel.

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