Liquid-crystal display televisions ( LCD TV ) are television sets that use LCD technology to produce images. LCD televisions are thinner and lighter than CRTs of similar display size, and are available in much larger sizes as well. This combination of features made LCDs more practical than CRTs for many roles, and as manufacturing costs fell their eventual dominance of the television market was all but guaranteed.
In 2007, LCD televisions surpassed sales of CRT-based televisions worldwide for the first time, and its sales figures relative to other technologies is accelerating. LCD TVs are quickly displacing the only major competitors in the large-screen market, the plasma display panel and rear-projection television. LCDs are, by far, the most widely produced and sold television technology today, pushing all other technologies into niche roles.
In spite of the LCD's many advantages over the CRT technology they displaced, LCDs also have a variety of disadvantages as well. A number of other technologies are vying to enter the large-screen television market by taking advantage of these weaknesses, including OLEDs, FED and SED, but none of these have entered widespread production.
Description
Basic LCD concepts
LCD televisions produce a colored image by selectively filtering a white light. The light is typically provided by a series of cold cathode fluorescent lamps (CCFLs) at the back of the screen, although some displays use white or colored LEDs instead, a method that is being increasingly used by the likes of Samsung and Sony on many of their latest TVs. Millions of individual LCD shutters, arranged in a grid, open and close to allow a metered amount of the white light through. Each shutter is paired with a colored filter to remove all but the red, green or blue (RGB) portion of the light from the original white source. Each shutter–filter pair forms a single sub-pixel . The sub-pixels are so small that when the display is viewed from even a short distance, the individual colors blend together to produce a single spot of color, a pixel . The shade of color is controlled by changing the relative intensity of the light passing through the sub-pixels.
Liquid crystals encompass a wide range of (typically) rod-shaped polymers that naturally form into thin layers, as opposed to the more random alignment of a normal liquid. Some of these, the nematic liquid crystals , also show an alignment effect between the layers. The particular direction of the alignment of a nematic liquid crystal can be set by placing it in contact with an alignment layer or director , which is essentially a material with microscopic groves in it. When placed on a director, the layer in contact will align itself with the grooves, and the layers above will subsequently align themselves with the layers below, the bulk material taking on the director's alignment. In the case of an LCD, this effect is utilized by using two directors arranged at right angles and placed close together with the liquid crystal between them. This forces the layers to align themselves in two directions, creating a twisted structure with each layer aligned at a slightly different angle to the ones on either side.
LCD shutters consist of a stack of three primary elements. On the bottom and top of the shutter are polarizer plates set at (typically) right angles. Normally light cannot travel through a pair of polarizers arranged in this fashion, and the display would be black. The polarizers also carry the directors to create the twisted structure aligned with the polarizers on either side. As the light flows out of the rear polarizer, it will naturally follow the liquid crystal's twist, exiting the front of the liquid crystal having been rotated through the correct angle that allows it to pass through the front polarizer. LCDs are normally transparent.
To turn a shutter off, an electrical voltage is applied across it from front to back. When this happens, the rod-shaped molecules align themselves with the electric field instead of the directors, destroying the twisted structure. The light no longer changes polarization as it flows through the liquid crystal, and can no longer pass through the front polarizer. By controlling the voltage applied across the crystal, the amount of remaining twist can be finely selected. This allows the transparency or opacity of the shutter to be accurately controlled. In order to improve switching time, the cells are placed under pressure, which increases the force to re-align themselves with the directors when the field is turned off.
Several other variations and modifications have been used in order to improve performance in certain applications. In-Plane Switching displays (IPS and S-IPS) offer wider viewing angles and better color reproduction, but are more difficult to construct and have slightly slower response times. IPS displays are used primarily for computer monitors. Vertical Alignment (VA, S-PVA and MVA) offer higher contrast ratios and good response times, but suffer from color shifting when viewed from the side. In general, all of these displays work in a similar fashion by controlling the polarization of the light source.
Addressing sub-pixels
In order to address a single shutter on the display, a series of electrodes is deposited on the plates on either side of the liquid crystal. One side has horizontal stripes that form rows, the other has vertical stripes that form columns. By supplying voltage to one row and one column, a field will be generated at the point where they cross. Since a metal electrode would be opaque, LCDs use electrodes made of a transparent conductor, typically indium tin oxide.
Since addressing a single shutter requires power to be supplied to an entire row and column, some of the field always leaks out into the surrounding shutters. Liquid crystals are quite sensitive, and even small amounts of leaked field will cause some level of switching to occur. This partial switching of the surrounding shutters blurs the resulting image. Another problem in early LCD systems was the voltages needed to set the shutters to a particular twist was very low, but that voltage was too low to make the crystals re-align with reasonable performance. This resulted in slow response times and led to easily visible "ghosting" on these displays on fast-moving images, like a mouse cursor on a computer screen. Even scrolling text often rendered as an unreadable blur, and the switching speed was far too slow to use as a useful television display.
In order to attack these problems, modern LCDs use an active matrix design. Instead of powering both electrodes, one set, typically the front, is attached to a common ground. On the rear, each shutter is paired with a thin-film transistor that switches on in response to widely separated voltage levels, say 0 and +5 volts. A new addressing line, the gate line , is added as a separate switch for the transistors. The rows and columns are addressed as before, but the transistors ensure that only the single shutter at the crossing point is addressed; any leaked field is too small to switch the surrounding transistors. When switched on, a constant and relatively high amount of charge flows from the source line through the transistor and into an associated capacitor. The capacitor is charged up until it holds the correct control voltage, slowly leaking this through the crystal to the common ground. The current is very fast and not suitable for fine control of the resulting store charge, so pulse code modulation is used to accurately control the overall flow. Not only does this allow for very accurate control over the shutters, since the capacitor can be filled or drained quickly, but the response time of the shutter is dramatically improved as well.
Building a display
A typical shutter assembly consists of a sandwich of several layers deposited on two thin glass sheets forming the front and back of the display. For smaller display sizes (under 30 inches), the glass sheets can be replaced with plastic.
The rear sheet starts with a polarizing film, the glass sheet, the active matrix components and addressing electrodes, and then the director. The front sheet is similar, but lacks the active matrix components, replacing those with the patterned color filters. Using a multi-step construction process, both sheets can be produced on the same assembly line. The liquid crystal is placed between the two sheets in a patterned plastic sheet that divides the liquid into individual shutters and keeps the sheets at a precise distance from each other.
The critical step in the manufacturing process is the deposition of the active matrix components. These have a relatively high failure rate, which renders those pixels on the screen "always on". If there are enough broken pixels, the screen has to be discarded. The number of discarded panels has a strong effect on the price of the resulting television sets, and the major downward fall in pricing between 2006 and 2008 was due mostly to improved processes.
To produce a complete television, the shutter assembly is combined with control electronics and backlight. The backlight for small sets can b
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