The Future of LCD HDTV Is LED
By Adrienne Maxwell
In the plasma vs. LCD debate, LED backlighting could be the great equalizer for LCD. In the past, many theaterphiles considered LCD to be a fine technology for everyday-use TVs but felt it lacked the higher-end performance they craved for serious movie watching. LED backlighting has the potential to change that perception; when properly executed, this technology helps LCD produce a gorgeous, theater-worthy picture that rivals the best plasmas.
To understand what LED backlighting brings to the table, you must first understand the issues that can hinder a standard, non-LED-based LCD television. Unlike a plasma pixel that generates its own light, an LCD pixel relies on an external light source; in a traditional LCD, that light source is a fluorescent backlight--more specifically, a cold cathode fluorescent lamp (CCFL). While you can oftentimes adjust the brightness of this backlight to suit your viewing environment, the light is always on, which makes it difficult for the TV to display truly deep blacks and precisely render the finest black details. LCD blacks often look grey, and the picture can look flat or washed out in a dark room. To get the best possible black level, you're forced to turn down the TV's backlight to its dimmest level, thus limiting overall brightness.
The always-on backlight creates a second problem: motion blur. This occurs because the constant backlight doesn't give your eyes the break they need between frames; your eyes blend the frames, which causes you to perceive image blurring. This is especially noticeable with faster-moving action scenes and sports content, although admittedly some people are more sensitive to the effect than others. (Because plasma doesn't use a constant light source, motion blur is not a problem.)
In an LED-based TV, the CCFL backlight is replaced with many small LEDs, or light-emitting diodes. These LEDs can be all white (a la Samsung) or a combination of red, green, and blue clusters (a la Sony). Of course, it's not just the type of backlight that matters; it's how you use it. If these individual LEDs were to function in an all-on/all-off manner, you wouldn't see much of an improvement in the TV's black level, except with a totally black image. The real benefit comes through the use of local dimming technology, in which the LEDs respond dynamically to the source content being displayed. If the source contains black areas, such as a night sky, the LEDs around that part of the screen can remain turned off, while the rest of the LEDs light up as needed. This helps black parts of the image look truly black, while brighter portions of the image can still be very bright, which produces excellent overall contrast.
The more LEDs the TV uses, the more precise this local-dimming effect can be. At this point, manufacturers are using hundreds of LEDs to illuminate a 1080p panel, but it's still not a 1:1 ratio between LEDs and pixels. In other words, an LCD with a 1920 x 1080 resolution does not use 2,073,600 LEDs. So, the illumination effects may not be quite as precise as you'll find with the best plasmas, in which each pixel generates its own light. For instance, if you were looking at white text against a black background, the area around the text might seem to glow a little. This is a minor issue that will surely become even less noticeable as the technology evolves.
We should point out that not every LED-based LCD uses local dimming. Some LED-based TVs only place the LEDs around the edges of the panel and then direct the light inward toward the center. These "edge-lit" models bring their own set of benefits, allowing for slimmer cabinet depths, reduced power consumption, and lower cost; however, they don't offer the same performance benefits as a local-dimming model.
To address LCD's motion-blur issues, some manufacturers have designed an LED backlighting system that cycles quickly through the illuminated LEDs, providing the needed break in light output to reduce or prevent motion blur. Of course, 120Hz and 240Hz technologies are also used to address motion blur, and these technologies can appear in both LED- and CCFL-based LCDs.
All in all, the improvement that local-dimming LED technology offers compared with a traditional CCFL backlight is dramatic, both in overall black level and the ability to highlight fine black details. Not surprisingly, these TVs currently demand a price premium over standard CCFL models. As the technology becomes more ubiquitous, prices will surely fall. A recent DisplaySearch report estimated that the penetration rate of LED-based LCDs will grow from less than 3 percent in 2009 to 40 percent in 2013 and will surpass CCFL backlights in 2014. In all likelihood, the CCFL-based LCD will eventually go the way of Beta and disappear from production lines entirely. The question is, will plasma TVs still be around when that day comes?
