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LED Backlit LCD TV

Plasma TVs do not use a backlight as plasma produces light from the red, green and blue phosphors in the display panel. Likewise for OLED TV panels as they are self-emissive. LCD TVs need a light in back of the LCD panel because the liquid crystals do not produce light themselves. Traditional LCD TVs have used fluorescent lights in back. On a typical LCD TV, fluorescent lights provide the backlighting through a special plastic sheet called a light guide that distributes light from a fluorescent tube evenly over the TV. On an LED-backlit LCD TV, fluorescent tubes are replaced with light-emitting diodes - LEDs. They can be either situated along the edges of the TV or arranged directly behind the screen in a grid.

The Benefit of LEDs

The most obvious reason LEDs have fallen into favor in LCD TVs: they're simply more efficient. Although fluorescent lights do a decent job, LEDs perform even better. Typically, manufacturers claim an efficiency improvement of up to 30 percent over fluorescent-based sets, which can add up significantly over the lifetime of a TV.

LEDs are also much smaller than tubes, even after accounting for the number of them needed to light an entire TV. That means LED-backlit televisions can be manufactured significantly thinner. Most of the ultra-thin televisions that measured under an inch thick use LED backlights, because they add very little depth to the profile. They're significantly skinnier than their fluorescent-backlit counterparts.

Image quality

Because fluorescent tubes must light the entire screen evenly, designers have no way to vary the backlighting intensity in different parts of the screen. Even if you want to show a single white pixel on an all-black screen, the light needs to be blazing away in back. But with some LED setups, lighting different parts of the screen separately becomes possible, allowing the lighting to actually improve the image.

Sony's 2005 Qualia KDX-46Q005 was the first LED-backlit LCD TV

It's made possible by a technique called local dimming, which can only occur on TVs that offer "full-array backlighting." These TVs arrange the individual LEDs up to 1,500 of them in a grid behind the LCD, rather than clustering them around the edges as you'll find on "edge-lit" screens. Because each LED lights a specific part of the screen, they can intelligently brighten or darken different zones of the screen to match the content being displayed on the LCD panel.

Not only does this improve efficiency, since not all the lights are running all the time, it improves contrast, producing blacker blacks and whiter whites on the same screen.

Keep in mind that not all LED TVs can achieve this effect. Many of the super-thin LED televisions you'll find use edge LED lighting to reduce their side profiles, making them slimmer and more efficient, but unable to "turn off" different parts of the screen intelligently the same way a full-array set can. Always make a point of discerning between edge-lit and full-array backlighting, and go with full-array, unless a thin profile is your number one priority.

LED backlighting boosts LCD performance in efficiency and image quality, while making TVs slimmer in the process. Some of the first full-array LED TVs were LG's 55LH90 and Toshiba's Regza 46SV670U. LED technology still carries a hefty price tag, making plasma an alternative for the less energy-conscious consumer.

LED TVs - Local dimming vs. edge-lit

When you see the words "LED TV", it refers to an LCD TV, but there's no guaranteeing that local-dimming technology is also on board.

All LCD-based TVs rely on a backlight of some kind to illuminate the LCD panel itself. Most utilize fluorescent backlights, known as CCFL, but a growing number employ LEDs instead. LEDs use less power but their main picture quality advantage is the capability to deliver deep black levels that rival, and in some circumstances surpass, those of the best plasma sets.

There are two major versions of LED backlights illuminating today's LCD TVs. One is called local dimming. It allows the backlight to dim or turn off in different areas across the screen. The other version, known as edge-lit, gets its name from the arrangement of the LEDs along the edge of the screen, allowing for extremely thin cabinet designs.

Branded simply as LED TVs, Samsung's 6000-, 7000- and 8000-series panels feature a new optical engine for their backlight unit. Available in up to 55-inch screen sizes, they're more advanced than their traditional fluorescent lamp- and LED-based counterparts through cleverly bending and diffusing light. The latest edge-lit LED backlight unit is Samsung's fourth design since 2006. The magic ingredient: Light guide plate

The Korean company's edge-lit LED backlight unit comprises two major components: A long LED module with a row of tiny white diodes and a thin screen-sized plastic sheet known as a light guide plate. Four LED modules will be deployed for the left, right, top and bottom of the panel. The combined light output is then funneled and redistributed evenly across the screen with a notable caveat. The current implementation does not support local dimming which selectively illuminates different sections of the display. Essentially, the edge-lit LED system lacks fine backlight control compared with its predecessor.

White light LEDs - There are two primary ways of producing high intensity white-light using LEDs. One is to use individual LEDs that emit three primary colors red, green, and blue, and then mix all the colors to produce white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.

LEDs can very easily be dimmed either by Pulse-width modulation or lowering the forward current.

Cool light:
In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.

Slow failure:
LEDs mostly fail by dimming over time, rather than the abrupt burn-out like incandescent bulbs.

LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000-2,000 hours.

Shock resistance:
LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.

The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.

LEDs do not contain mercury, unlike fluorescent lamps.

High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps.

Temperature dependence:
LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life.

Light quality:
Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism, red surfaces being rendered particularly badly by typical phosphor based cool-white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.

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