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.
Like cold-cathode fluorescent lighting, LEDs are installed in the back of the TV and configured so their light passes through diffusers, creating an even spread of light.
Where LEDs can start to improve the image quality is when manufacturers use a Red/Green/Blue LED system. RGB systems consist of a grid of red, green and blue LEDs that combine their light to create white.
The advantage of RGB LEDs is that the temperature of white – the White Point of the display – can be set dynamically by the circuitry in the TV, or even by the user. Prefer a yellower look? Twiddle the remote, and it’s done.
RGB LED backlighting essentially boosts the colour-generating capabilities of the LCD screen. A TV with wide colour gamut support can harness its back-lighting system to further boost reds, blues and greens in scenes dominated by those colours. Said TV can also display more colours than its last-generation brethren, and by moving the red, green and blue ‘points’ can make these colours more vivid. (Colour ‘points’ have to do with the amount of light energy hitting your retina at short, medium and long wavelengths. For more detail on this, enrol in a degree in opthamology!)
The downsides of LED
Unfortunately it’s not all (extremely vivid) roses when it comes to LED backlighting. Like any kind of light, LEDs do age and their colour changes over time. What’s especially irritating about LEDs is that they don’t age uniformly. So when you put a bunch of them in a grid, different parts of the grid may age at different rates.
So you may find after several years that the upper right corner of your TV doesn’t produce as brilliant a red, or the bottom third doesn’t quite manage blue as well as the rest of the display. And because RGB LED systems use multiple colours to create white, the temperature of the display will almost definitely shift over time.
Still, the LEDs are solid state. This means it’s extremely unlikely for any of them to fail. They also have the potential to use much less power than a fluorescent backlight, though this does require clever circuitry.
In real terms though, there are very few disadvantages to LED backlighting, and plenty of reasons to look for it in your next TV purchase. And within the next few years, it won’t even be a matter of choice: all LCD TVs will use LEDs, as will your PC monitor and your notebook.
Flatter flats, more colourful colours, and less power consumption. All thanks to a component invented more than 100 years ago. Thanks LED!