TV Tech Terms Demystified

TV tech terms demystified, part four: LED backlighting

They enable thinner flat-panel TVs and save energy, but until just recently, the color has been inferior to what older technologies delivered, leaving us to sing the LED backlight blues.

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Vizio

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Somewhere along the line, thin and light supplanted picture quality as the secondary priority for TV purchases; the primary consideration for most of us being price. Perhaps it was the memory of the CRT behemoths that preceded the LCD revolution, or maybe the difficulty in wall-mounting the first generation of heavier CCFL-backlit LCD TVs. But when the first lightweight, thin-panel LCD TVs showed up, CCFL types disappeared from store shelves in a hurry.

The creation of these thinner and lighter flat-panel TVs was made possible by LEDs, which produce an enormous amount of light for their tiny size. They were deployed as distributed side-lighting at first, and increasingly in arrays directly behind the panel, as shown below.

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A typical LED backlight array shown with local dimming engaged.

But there was a casualty: Color. Instead of relatively pure, saturated colors, you got light orange-ish reds, yellow-ish greens, and so on. If you’ve seen a LED flashlight, especially those from a few years ago, you probably have an inkling why. The light they produce is cold, harsh, icy…. Pick your adjective. That’s because “white” LED light skews heavily toward the cool blue end of the spectrum.

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The heavy skew towards the blue with white LED backlighting is the reason for the inaccurate color on cheaper LED-backlit LCD TVs.

There are of course red, green, and blue LEDs, but implementing a system using them is complex and expensive. It was far easier and cheaper to use the old method of shining a bright broad-spectrum light at a layer of color filters. When this was done using older relatively even-spectrum CCFLs (Cold Cathode Fluorescent Lamp), the results were pretty darn good. Reds were red, yellows yellow, etc.. In fact, some manufacturers held on to CCFL backlighting for their professional display lines for that very reason.

The problem wasn’t bad enough to overwhelm the appeal of thinner, lighter, and cheaper-to-operate (LEDs consume far less juice than CCFLs); we’re talking about a phenomenon subtle enough that many people still don’t realize what they’re missing. But the whole “blue” thing was certainly noticed by the industry, which spent a vast amount of time and money to remedy the problem. The fixes rendered the higher-end TVs they first appeared in more expensive, of course, but better color has trickled down to the lower-end models as R&D costs have been recouped. Lately, the industry has begun to address another common LED-backlit LCD problem—lack of contrast—using HDR (high dynamic range). But I digress.

The efforts towards better color might have moved a little faster if OLED hadn’t been hyped as the future of all flat-panel TVs. RGB and even wRGB OLED have remained difficult to produce in large sizes, and are therefore still relatively expensive though far cheaper than they used so be (LG’s $30,000 88ZN being a notable exception). More typical OLED TVs are on par with high-end LED-backlit LCD TVs these days.

sony bvm x300 Sony

Pure RGB OLED TVs (most are wRGB, with a white element added to increase brightness) are rare and very pricey. This Sony BVM-X300 is just 30 inches wide, but costs $30,000. Mainstream wRGB OLED TVs start at $3,000.

Getting better all the time

There are basically two methods vendors use to improve color accuracy in their TVs. One is to improve the balance of the emanated spectrum by varying the color of the LEDs and filters. Sony has advanced this technique quite a ways, but not as far as the method described next.

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Sony’s XBR-930D with improved color and HDR.

Many vendors have opted to use tiny crystalline semiconductors called quantum dots that intake light and re-emit it at a specific wavelength. The beauty of quantum dots is that the implementation requires only interdicting the light source with a bevy of the little beauties. That is, placing a layer of quantum dots between the backlight and the filters and LCDs.

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Samsung’s SUHD TVs use quantum dots to improve color.

Implementing quantum dots is said to be relatively inexpensive, but it has taken several years for them to migrate down to lower-mid-range TVs. Now they’re showing up in even lower mid-range models.

The upshot

Significantly better LED-backlit LCD color and contrast are here, even in lower-cost models. It hasn’t reached the bargain-basement models quite yet, but it will get there. I highly recommend that you spend the relatively little extra cash to get a TV that uses quantum dots. And if that’s too rich for your budget, move up just a rung or two in the non-quantum-dot realm. It’s worth it.

Don't miss the other three installments in this series. In part one, we cover screen size, resolution, and speed; part two goes over display types and display technologies; part three dispels the mysteries of color spaces and standards, and we wrap up the series with the lowdown on LCD backlighting tech.

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