September 25, 2008
| by Steven Castle
Local dimming: It sounds like something a lighting control system might do, or a utility might enforce when there isn’t enough power.
Not quite, though it’s a term you’re bound to hear a lot more of. Local dimming is used often today to describe video displays that can dim part of a picture that is dark, for instance, and the term is used most often with today’s new LCD TVs with LED backlights.
New and forthcoming LCDs like Samsung’s LN55A950 Series 9 ($4,500), Sharp’s Limited Edition 65-inch LC-65XS1U-S and 52-inch LC-52XS1U-S, Sony’s Triluminous KDL-46XBR8 ($5,000) and KDL-55XBR8 ($7,000) and LG’s 47LG70 ($3,900) all use LED backlighting instead of the traditional fluorescent backlighting. And yes, they’re all 1080p, 120 Hz. A big drawback with fluorescent-lit LCDs is that the backlights never turn off completely during viewing, so some light always leaks through in dark scenes.
LED backlighting, by contrast, allows local dimming in certain regions of the picture. By shutting off regions of the screen at certain times, energy is saved, and the blacks are deeper and better.
I saw all the aforementioned displays at the recent CEDIA (Custom Electronic Design & Installation Association) Expo, and they all looked far better than traditional LCDs with fluorescent backlighting. They’ll save energy as well. LEDs are far more efficient than fluorescent lighting, and switching some of them off at times can save even more energy. Look for sets units to meet new Energy Star standards for “on mode” power efficiency.
Manufacturers differ in their LED backlighting, but in most cases there are more than a thousand LEDs behind the screen, and they’re divided into sections. If one part of the image is dark, certain sections shut off, making the picture darker and saving more energy.
The LG TV uses white LEDs, while Sony and Sharp with use red, green and blue LEDs, which are said to provide better color rendition. Either way, you get a more efficient and higher contrast picture. And with more LED TVs becoming available, you’ll hear more and more about local dimming.
The technology is bound to improve, as well. Dolby plans to license its HDR (high-dynamic range) local dimming technology that can modulate individual LED backlights, not just groups of them.
For now, we just have to get past those whopping price tags, which will happen as LED costs decrease and production of LED-backlit TVs increase.
Steven Castle is Electronic House's managing editor. he has been writing about consumer electronics, homes and energy efficiency topics for two decades. He is also the co-founder of GreenTech Advocates