The new sets misleadingly referred to as LED TVs are in fact still LCD TVs. The difference is that these sets replace the cold cathode fluorescent lamp (CCFL) used as a backlight in most LCD TVs with LEDs instead. These LED driven HDTVs first appeared a couple of years ago and now comprise most of the high-end LCD TV market
LEDs have many advantages over the CCFL and solve many of the shortcomings of current LCD TVs. They produce more accurate colors, achieve better black levels, improve picture uniformity, last longer, allow for a slimmer chassis, use less power, and are more environmentally friendly since they have no mercury in them. LEDs will probably replace CCFL backlights in all LCD TVs in the next few years as costs come down and the technology improves.
There are two different varieties of LED backlit LCD TVs: Full-array and Edge-lit.
In an LCD TV with full-array LED backlighting, the LEDs are located directly behind the LCD screen approximately 1 to 3 inches apart and a diffuser panel spreads the light out creating a consistent area of illumination.
This setup allows for the use of a technology known as local-dimming (LED-LD) in which LEDs are dimmed individually or in groups behind dark areas of the picture, thus making blacks darker without affecting brighter elements of the picture. This creates a much greater increase in contrast (generally a weakness of LCD TVs vs plasma sets) and has allowed high-end LCDs with this technology to meet or even exceed the black levels and contrast ratios found in plasma TVs.
The only downside to this technology, besides cost, is that since each LED backlights multiple pixels, it is impossible to precisely transition between bright and dark areas of the picture. This results in blooming or white halos around the edges of bright objects cast against dark backgrounds. TV manufacturers have made improvements in local-dimming technology to reduce this blooming effect, but it still exists to a lesser degree.
LED edge-lit (LED-EL) LCD TVs are simpler and cheaper than full-array models. In an edge-lit set, the LEDs are positioned all around the edge or side of the screen and aimed sideways so the light beams toward the center of the screen. A large sheet of reflective material covered with small convex bumps bends this side-fired light at a 90-degree angle allowing it to pass through the LCD panel and illuminate the picture. Edge-lit LEDs can also be dimmed in order to produce deeper blacks, but only as a group so the effect isn't nearly as dramatic as local-dimming full array models.
One advantage edge-lit models have is that they can be made slimmer than both conventional CCFL and full-array LED TVs, although the difference is down to as little as a quarter-inch in some cases.
Next: Key Features to Look For When Buying an HDTV
