If you've been shopping for an HDTV lately then you'll notice that while 1080p is now pretty much a standard feature in a decently sized TV, the new selling point is "LED". What is LED? What's the difference between LED and LCD? What are the advantages and what are the disadvantages of LED TVs? We'll cut through some of the hype and explain it all, and if a word seems confusing, click it to see the definition in our HDTV glossary.
What is LED TV?
In its most basic form, LED (short for "Light-Emitting Diode") is a type of highly efficient light bulb. You've seen them on everything from electronic control panels to Christmas lights to torches and even car headlights. LED TVs are simply LCD TVs that use LEDs as a backlight instead of CCFL (Cold Cathode Fluorescent Lamp). If you want to pick nits, they're correctly referred to as LED-backlit LCD TVs. And that's it.
Edge-lit Versus Back-lit LED TV
There are two basic varieties of LED backlights: edge-lit and back-lit. Back-lit LED LCD TVs simply replace the CCFL tubes found in earlier LCD TVs. Edge-lit LED LCD TVs surround the actual screen with LEDs hidden in the bezel (or frame) and use filters to spread the light evenly. An advantage of this is that the TV can be made a lot thinner and lighter than a back-lit LCD TV. How thin? Some Sony LCDs are only 9.9mm thick. Edge-lighting also means that less LEDs can be used to get a uniform spread of light, which makes it somewhat cheaper.
But there's one thing that state-of-the-art back-lit LED TVs can do that edge-lit ones can't, and it's a biggie.
Local-dimming or dynamic LED TVs are capable of analysing the image and dimming the backlight directly behind the darkest areas of the picture and amplifying it behind the brightest areas. This means that a TV with locally dimming LEDs can achieve true black where required. LED TVs with local-dimming are the first LCD screens that can achieve blacks as deep as plasma or CRT.
At the higher end of the price spectrum, you'll find the LED TVs further split into "White LED" and "RGB-LED". RGB-LED TVs have backlights consisting of clusters of discrete red, green and blue LEDs that can dim individually. RGB-LEDs can achieve deeper colours than White-LED models because the backlight is capable of true primary colours. While this wide colour gamut sounds like a great feature—and it is for certain applications—the kind of colours it's capable of generating actually extend beyond of what's considered necessary for current video standards (even Blu-ray) and so RGB-LED is not a must-have feature in the way local-dimming is. That said, set up properly, the colour reproduction of RGB-LED is spectacular and may be worth the extra money if you like to look at digital photography in your home theatre. And sometimes the cost of futureproofing now is often lower than upgrading later.
What About OLED/AMOLED?
OLED/AMOLED (Active Matrix Organic Light-Emitting Diode) is a completely different technology that is not related to LED-LCD. AMOLED screens have no backlight and consist of individually powered plastic pixels. AMOLED currently represents the pinnacle of flat-screen technology, but is difficult to make in large sizes and so is currently only found at tradeshows and on portable devices like media players and cellphones.
LEDs are very efficient light sources. LED LCD TVs consume less power than CCFL LCD TVs and far less than plasma TVs. LED TVs with local dimming can compete with plasma sets for true blacks, and RGB-LED sets can display more colours than either technology. Edge-lit LED sets can be made almost shockingly thin and light compared to other technologies, so if you're style conscious, these might be the ones to go for.
Firstly, at the moment, it's cost. LED TVs are a lot pricier than similarly-sized CCFL models. This may be a moot point, as LEDs are likely to last quite a lot longer than a CCFL backlight and use considerably less power. The second drawback is that local dimming is not perfect in some models. It can leave haloes around light objects on dark backgrounds, and sometimes it "pumps"—the backlight doesn't dim or brighten in pace with the image. So, especially considering the cost, it's essential that you audition an LED TV before you consider buying it. 'Cos when it's good, LED-backlit LCD is astounding.