Editors' note: Updated June 3, 2010
I've written articles in the past explaining various TV technologies, including the differences between 720p and 1080p and 120Hz and 240Hz LCD TVs. But with Samsung, LG, Sony, and other manufacturers pushing so-called LED TVs these days, it's high time that I--with an assist from our resident video guru, David Katzmaier--sort through all the marketing mumbo jumbo and provide some insight into just what an LED TV is. Here goes.
1. An LED TV is not a new kind of TV.
I appreciate a good marketing ploy as much as the next guy, but an LED TV is just an LCD TV that's backlit with light-emitting diodes (LEDs) instead of standard cold-cathode fluorescent lights (or CCFLs). And though they became well-known last year with Samsung's ultrathin models, LED-backlit LCDs have been on mainstream store shelves since 2007, when Samsung's LN-T4681F debuted.
Unlike plasma and OLED, which are emissive technologies where each pixel is its own discrete light source, LCD is a transmissive technology where each pixel has to be illuminated from behind, or backlit.
2.There are two LED backlight configurations
Initially, LED-based displays like the Samung LN-T4681F were backlit by what's referred to as a "full array" of LEDs behind the LCD, across the back of the panel--just like a standard CCFL backlight. But to create even thinner TVs, engineers needed to eliminate that extra layer of LEDs and move it to the sides of the display. With this form of backlighting, the LEDs are affixed to all four sides of the TV and light is projected inward to the middle of the TV via "lightguides." These types of TVs are commonly referred to as "edge-lit" LED-based LCDs, and are by far the most common available today.
3. Each configuration may also offer "local dimming."
All current LED-based LCDs with rear-placed, full-array LED backlighting--except the Sharp LC-LE700UN series from 2009--feature a technology called "local dimming." With local dimming, portions of the backlight can be dimmed or brightened independently when different areas of the picture get darker or brighter. For example, the LEDs behind the words in a credit sequence can illuminate while the ones behind the black background remain dim.
Being able to dim portions of the screen helps reduce the amount of light that leaks through to darkened pixels, and the end result is blacks that appear darker and more realistic. Since black levels are crucial to contrast ratio, the deeper the blacks, the more the picture--and colors--appear to pop. Also, the image as a whole will seem crisper. A couple of examples of local dimming done right are Samsung's UNB8500 series and LG's LH8500 series--respectively the best and second-best LCDs we've ever tested.
One downside to local dimming is an effect called "blooming," where brighter areas bleed into darker ones and lighten adjacent black levels.… Read more
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