LCD Display Technology
Most basically LCDs produce the image you see by blocking or emitting the light from a backlight using liquid crystals sandwiched in between two glass plates.
An LCD display is made up of a thin layer of liquid crystals arranged in a matrix (or grid) of a million or more pixels (picture elements), This layer is sandwiched between the two glass plates, which are covered in a matrix of electrodes and transistors (electronic switches), each coated with a polarising filter. The two polarising layers only allow light vibrating in one direction to pass through them, one allows horizontally vibrating light through and the other passes vertically vibrating light.
The light source in an LCD is its backlight so this unpolarized light becomes vertically polarized as it passes through the first polarizing filter at the back of the display. The other polarizing layer on the front sheet of glass is horizontally polarized, so ordinarily the now vertically polarized light coming from backlight can't pass through it. The role of the liquid crystal layer in the middle of the display is to rotate the vertically polarized light travelling through it by ninety degrees so it can pass through the front, horizontally polarized filter. By varying the voltage applied to the liquid crystal sub-pixels the amount they twist the light changes, allowing more light of each colour though as a greater voltage is applied.
Each pixel is made up of three sub-pixels aligned to a colour filter for each of the primary colours; red, green and blue. Individual pixel colours are produced by the combination of the primary colours produced by each sub pixel, with the pixel's overall brightness is produced by the sub-pixels relative intensities. Many thousands of these pixel units operating together in the display combine to produce the image you see.
What is LED TV?
The first thing to know about LED (Light Emitting Diode) TVs is that they are simply LCD TVs with a different kind of backlighting. The screen remains the same but LEDs are used for backlighting in place of Cold Cathode Fluorescent Lamps (CCFL) that are found in most LCD TVs.
The LEDs can come in two forms, Dynamic RGB LEDs which are positioned behind the panel, or white Edge-LEDs positioned around the rim of the screen which use a special diffusion panel to spread the light evenly behind the screen.
RGB Dynamic LED TV
This method of backlighting allows dimming to occur locally creating specific areas of darkness on the screen. This means you see truer blacks and much higher dynamic contrast ratios.
(Image courtesy of Sony)
Edge-LED TV
This method of backlighting allows for LED TVs to become extremely thin. The light is diffused across the screen by a special panel which produces a superb uniform colour range across the screen.
(Image courtesy of Sony)
Currently LEDs are not small enough to be used for individual pixels in domestic televisions, and so the use of true LED TVs is restricted to much larger screens in places such as sport stadia or above 40' sizes. There are some great benefits to choosing an LED TV over a standard LCD TV.
Improved brightness and contrast levels, with deeper blacks.
The use of Edge-LED lighting allows the TV to be thinner than standard LCD TVs.
LED TVs can consume up to 40% less power than a LCD TV of similar size.
They can offer a wider colour gamut, especially when RGB-LED backlighting is used.
LED TVs are also more environmentally friendly due to there being no mercury used during manufacture. Further longevity of LED TV's is claimed to be better.
Future of LED TV
The future of LED TV is expected to focus on the use of "Quantum Dots" as light emitting diodes to create QD-LED displays and QD-WLED (White LED) displays, which operate in a similar fashion to OLED displays in that light is supplied on demand. Quantum dots are valued for displays, because they emit light in very specific gaussian distributions. This can result in a display that more accurately renders the colors than the human eye can perceive. Quantum dots also require very little power since they are not color filtered. Research is still ongoing for this technology, and it is not expected to be put into use on commerical TVs until at least 2012.
How plasma displays work (little technical, difficult to simplify)
A panel typically has millions of tiny cells in compartmentalized space between two panels of glass. These compartments, or "bulbs" or "cells", hold a mixture of noble gases and a minuscule amount of mercury. Just as in the fluorescent lamps over an office desk, when the mercury is vaporized and a voltage is applied across the cell, the gas in the cells form a plasma. With flow of electricity (electrons), some of the electrons strike mercury particles as the electrons move through the plasma, momentarily increasing the energy level of the molecule until the excess energy is shed. Mercury sheds the energy as ultraviolet (UV) photons. The UV photons then strike phosphor that is painted on the inside of the cell. When the UV photon strikes a phosphor molecule, it momentarily raises the energy level of an outer orbit electron in the phosphor molecule, moving the electron from a stable to an unstable state; the electron then sheds the excess energy as a photon at a lower energy level than UV light; the lower energy photons are mostly in the infrared range but about 40% are in the visible light range. Thus the input energy is shed as mostly heat (infrared) but also as visible light. Depending on the phosphors used, different colors of visible light can be achieved. Each pixel in a plasma display is made up of three cells comprising the primary colors of visible light. Varying the voltage of the signals to the cells thus allows different perceived colors.
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• Great black levels • Produce exceptional color (up to 16.77 million) • Good for dark rooms • Cheapest of 3 TV types • Great for 3D | • Great in bright rooms • High native resolution • More energy efficient than Plasma | • Produce blacks similar to Plasma • Amazingly thin • Energy efficient • Good for 3D |
• Lower brightness than LCD/LED • Glass screens reflect light • Not slim or light as LCD/LED | • Do not produce true black • Narrower viewing angle than Plasma • Handles fast motion less efficient than Plasma • More expensive than Plasma | • Models with glass screens reflect light • Narrower viewing angle than Plasma • Not ideal for videogames • Most expensive TV technology |
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