When is a flat panel TV not really that flat? When it doesn’t have an LED backlight.

Dart into the shops these days and you’ll see a new breed of displays. Compared to a CRT, an-old school LCD TV is pretty flat. But compared to an LED-backlit display, it’s positively chunky.

These new TVs are typically less than 10 cm thick – compare that to the 20 or 30 cm thickness of a regular LCD.

Enter the LED

The secret to this sveltness is the Light Emitting Diode. It’s not a new technology (it was invented in 1907) but its presence in big displays is a new application. Let’s start with the electronics.

A regular light globe generates light when the element in the globe resists an electrical current passing through it. A by-product of this resistance is light. The more resistance (measured in watts) the brighter the light.

But last-generation flat panel TVs don’t use incandescent backlights. Instead, they use fluorescent lights. These pass electricity into a cathode that spews electrons into the lamp, which react with a low-pressure gas, creating light.

Light Emitting Diodes (LEDs) are something else entirely. They use two semi-conductors, which interact in such a way as to produce light when electricity is passed through them. The real advantage of LEDs is that they are extremely efficient at generating light, compared to how much power you pump into them.

Currently LEDs are roughly four times as efficient as an incandescent bulb. They’re not as efficient as a fluorescent tube, but they do light up much faster and they can be very small. They also solder directly onto a circuit board: they don’t need ballasts. (Ballasts are chunks of electronics, mainly capacitors, that regulate the flow of electricity to the tube. They run hot, they can fail, and they generally make the whole system more complex and bulky.)

Importantly, they can be built to emit a specific colour of light, instead of needing a colour filter. A blue LED really is blue, not a white light in a blue case.

They’re also almost completely shock resistance and don’t contain toxic mercury. There’s no element, so if you drop one, it won’t break or ‘blow’ like a regular light globe.

You can destroy an LED by running electricity through it at a reverse polarity. As most LEDs are soldered onto circuit boards though, this would require the average user to crack open their device and get busy with a soldering iron.

LEDs in the world at large are almost everywhere. In fact, wherever you need a bright point of light that needs to switch on and off regularly and last thousands or even tens of thousands of hours. Many new cars now uses LED brake lights and the snazzier models have a line of LEDs under the headlights which work as ‘running lights’ and improve the visibility of the vehicle during daylight: the bright white LEDs stand out even in direct sunlight.

What makes LED perfect for TVs?

For TV users, it’s the compact size and low power consumption that make LEDs so perfect for TVs. The technology is already proven, as LEDs have been backlighting small electronic devices such as handheld games consoles for years. They’re also making their way into the latest generation of notebook PCs – Apple uses LED backlights in many of its MacBook computers.

LCD TVs need a backlight because the LCD grid itself creates only colour; it doesn’t generate its own light. A backlight has to shine through it, so you can see the image.