Update: OLED and LED

To get straight to the point, no: OLED and LED aren't the same thing! To put things as simply as possible, LED screens are the missing link between today's LCD screens and the future generation of OLED screens that will arrive in a few years' time.

What they have in common

All three technologies display an image using a panel made up individual pixels, or, to be more precise, of a grid of red, green and blue sub-pixels.


Traditional LCDs

Light is provided by a fluorescent tube that runs behind the screen. There are usually four tubes, arranged either in parallel, or around the edges.


The backlight produces light behind the coloured pixels. On this simplified diagram, it's easy to see the main problem that this causes:

• the closer the pixels are to the backlight, the brighter they are. On our diagram, the top row of pixels will be brighter than the bottom row, even if they are showing the same colour.

In practice, this produces defects in the evenness of colour across the screen, which we find every time we test monitors. That's why we measure the homogeneity across the whole monitor for our Product Face-Offs, where we regularly find a discrepancy of 30% between any two points that should be displaying the same colour.

It's not the end of the world, and manufacturers of the best monitors pay careful attention to the placement of the backlights. On the very best (and most expensive) monitors, the maximum difference in brightness between any two points is 15%.

There's another problem, though:

• the light from the backlight can pass through the LCD panel and 'leak out', even if it's showing an entirely black image. That means that black appears as dark grey, or even light grey on the worst screens. That's why we always measure how deep the blacks are on the monitors and TVs that we test as well.

OLED: The Ideal Solution

Out go the backlights, and each sub-pixel becomes its own diode--either red, green or blue--which produces as much light as it needs:



The results are magnificent. Having tested the first TV to use this technology (the Sony XEL-1), as well as a digital photo frame from Kodak, and looked at countless prototypes, we really do think it's the ideal solution. The brightness remains perfectly even, as long as the screen is set up properly, and producing an absolute black at any one sub-pixel is as simple as turning it off. The result is that the colours can be controlled much precisely--right down to the level of the individual sub-pixel--unlike with a backlight, where increasing the brightness of the light source can affect a whole quarter of the screen, or 1, 500, 000 sub-pixels on a Full HD screen.

Getting rid of the backlighting to light each sub-pixel individually makes the screen itself much narrower. There are some that are no more than a few millimetres thick.

There are, though, a few problems:

• so far, we've mostly seen prototypes, and no finished products.
• the price remains very high. Sony's 11" TV, one of the very few products commercially available and no longer a prototype, is listed at £3, 500. Kodak's 7.6" photo frame, meanwhile, costs £700. How much would a 32" TV cost based on these prices? It would almost certainly be unaffordable ...

So, even though OLED technology has been fascinating us for years, there's still a lot more waiting to do before the first TVs to use this technology arrive on the market--and they'll be very expensive--you can be sure of it!

Because OLED simply isn't ready yet, we need a temporary solution. LED screens come to the rescue, forging the missing link between today's LCDs and tomorrow's OLEDs.

LED Screens: The Compromise

To make an LED screen, we remove the four fluorescent tubes and replace them with a grid of white LEDs spread out across the panel behind the sub-pixels. These screens are essentially the same as LCDs, but with a different backlighting system:


Once again, our simplified diagram illustrates the main problems of LED backlighting:

• the more white LEDs there are, the more even the brightness will be. However, it's impossible to get it perfect: the differences between dark and light areas might be smaller, but they're still there.
• just like LCD screens which use fluorescent tubes, light can escape from black areas of the screen. Black areas are never an absolute black, unless the screen detects that that one part of the image is darker, in which case it can reduce the brightness of the LEDs behind that part of the screen. However, that can lead to noticeable variations in brightness from one frame to the next.

The Secret: A Single Light Source for Every Pixel

Let's say that you need two hundred white LEDs to backlight a 1920 x 1080 pixel Full HD panel. That means that are are 1920 x 1080 = 1.778 Megapixels, or 5.33 M sub-pixels. The light from each of the 200 LEDs is divided amongst these 5.33 M to give 26, 667 sub-pixels per LED.

To recap:

• on a traditional LCD screen, one fluorescent tube provides the light source for 1, 500, 000 sub-pixels
• on an LED screen with 200 white LEDs, each diode provides the light source for about 27, 000 sub-pixels
• on an OLED screen, each sub-pixel is lit individually

There's clearly a natural progression, but things aren't quite that simple ...

• some LED screens have as few as 48 LEDs, not the 200 we've based our calculations on here. Obviously, they perform less well.
• we've also found LED screens where the LEDs aren't arranged in a grid behind the coloured sub-pixels, but around the outside. In this case, the only real benefit to using an LED backlight springs from marketing, rather than a genuine improvement.
• finally, the dynamic contrast on LED screens is still not up to scratch can produce uneven images high suffer from moving patches of grey, like clouds floating across the screen.

So, in theory, yes, it's better, but we'd still like to see more LEDs--there often aren't enough--and better dynamic contrasting to make the most of the improvements made by upgrading from screens backlit by fluorescent tubes. For the time being, though, we still recommend that whether you have a TV or a monitor, you should turn off dynamic contrast.

To find out more, you can read our tests of LED-backlit monitors and televisions:

> Test: Samsung UE55B7000
> Test: Sony Bravia KDL-46X4500
> Test: Apple LED Cinema Display 24"