Flat panel TVs use one of two main image-generating technologies: plasma or LCD. On a plasma TV, the screen itself generates light, as power causes pockets of gas to fluoresce into a plasma – thus the name.
An LCD, on the other hand, is a grid of liquid crystal elements that change colour based on an electrical signal. While each element can mix red, green and blue to create 16 million different colours, the LCD grid itself doesn’t emit light.
For you to be able to see the image on an LCD, the TV needs a backlight. Traditionally, this is a high-grade cold-cathode fluorescent lamp (CCFL), related to fluoro tube overhead lighting.
CCFL backlighting works brilliantly – LCD TVs are typically brighter than plasmas – but it’s bulky and because the TV has to use refractors to spread the light across the whole display, the backlighting can become uneven with age.
Light-emitting diodes (LEDs) are solid-state components that can’t burn out. They can be smaller than the eraser on a pencil, are hugely reliable, very bright, and consume far less power than CCFLs. And with a grid of dozens of LEDs across the display, light is spread every evenly.
There are actually two kinds of LED lighting: backlit and edge-lit. Backlit is most similar to old CCFL displays: the LEDs are behind the liquid crystal panel and shine their light directly forward.
Edge-lit LED systems, as the name suggests, fire light from around the edge. The real advantage of this system is that the display can be much, much thinner. While a backlit LED LCD shaves several centimetres off thickness, edge-lit can make the TV as thin as just 27 mm!
Backlit does have an advantage of its own though: it’s easier to more precisely control LEDs in a backlit array. This means the LEDs behind the dark parts of a scene can be turned down, and that equals better contrast ratios. Most LED LCD TVs have contrast ratios that are so high it’s hardly even worth mentioning them! Black is really black, put it that way.
Edge-lit systems won’t let backlit get away with this for long. The latest generation of edge-lit LED LCDs also have computer-controlled brightness systems to improve contrast.
White, red, green and blue
There’s another difference in LED backlighting systems. Some TVs use an array of white LEDs, while others use a mixture of red, green and blue LEDs.
White arrays provide very even illumination, and this helps improve the TV’s contrast. LCD has traditionally lagged behind plasma in contrast performance, but LED narrows the gap immensely.
A TV that uses a grid of red, green and blue LEDs for backlighting can bias sections of the display to appear bluer, or redder, or greener. The colour from the LED can also be added to the colour shown by the LCD, increasing the number of colours the TV can show.
Which type of LED delivers better performance is fairly subjective, but both approaches make for images with better contrast and better colour than LCD televisions employing conventional backlighting. As if that wasn’t enough, the TV will be only a few centimetres thick.
Amazingly, LED LCD TVs can still pack an HDTV tuner and the full range of inputs and outputs into their skinny cases. And the lastest TVs even have internet connectivity, the ability to display images directly off a digital camera via USB or wirelessly using the Digital Living Network Alliance (DLNA) standard, and some even come packed with games for the kids. It’s like teletext for the Wikipedia generation!
Not sure if the new TV you have your eye on is an LED LCD? Here’s a tip: look at it side-on. If it disappears, you’ve found the right one.