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TV tech terms demystified, part four: LED backlighting

Jon L. Jacobi | April 27, 2016
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.

Somewhere along the line, thin and light supplanted picture quality as the secondary consideration 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 LCD TVs. But when the first lightweight, thin-panel LCD TVs showed up, they disappeared from store shelves in a hurry.

To create these thinner and lighter flat-panel TVs, as well as to conserve energy, TV manufacturers turned to LEDs, which produce an enormous amount of light for their size. They were deployed as distributed side-lighting at first, and then 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 blue end of the spectrum.

filter spectrum 
The heavy skew towards the blue with white LED back lighting is the reason for the inaccurate color on cheaper LED/LCD TVs. Credit: Unknown

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 white light at a layer of color filters. When this was done using older wide-spectrum CCFLs (Cold Cathode Fluorescent Lamp), the results were pretty darn good. Reds were red, yellows yellow, etc. With white LEDs... Not so much. Some vendors held on to CCFL backlighting for their professional lines for that very reason.

Obviously, the problem isn’t bad enough to overwhelm thin-and-light’s appeal. We are talking about a phenomenon subtle enough that many people still don’t realize what they'e missing. But the whole “blue” thing did not go unnoticed by the industry. It has spent the last six or seven years gradually remedying the color situation, while of course charging more for the TVs with the fixes. This year they’re even started to address another common LED/LCD problem: Contrast. This is being done with 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 flat-panel TVs. Sadly, RGB and even wRGB OLED have remained extremely difficult to produce in large sizes, and are therefore quite expensive.

 

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