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 while they've become best-known this 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 types of LED backlighting.
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. But to create superthin 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.
Samsung is the main maker of such sets this year with three series of edge-lit sets, although Sony did release one model earlier this year, the KDL-40ZX1M, and has another flagship series, the KDL-XBR10 models, waiting in the wings. Samsung, Sony, Sharp Toshiba, LG, and Vizio all have non-edge-lit, or "full array" models, available today. See our comparison of edge-lit vs. local dimming for more info.
3. Of the two, local dimming can produce deeper black levels, but also creates "blooming."
Local dimming LED backlights can dim or turn off individually as needed.
The type of backlighting can impact how deep a shade of black a TV can produce. All current LED-based LCDs with rear-placed, "full-array" LED backlighting--except the Sharp LC-LE700UN series--feature a technology called "local dimming." With local dimming, specific areas of the backlight can be dimmed or brightened when different areas of the picture get darker or brighter.
With fluorescent backlighting and edge-lit LED backlighting, by contrast, the entire backlight dims or brightens at once, if at all.
Being able to dim specific quadrants 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 great example of local dimming done right is Samsung's UNB8500 series, which is one of the best-performing TVs 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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