Understanding the LED HDTV - Two Different Types of Screen

When we think about the LED HDTV, we should really place them in two different categories.

At present there are two different ways of building LED televisions, and each has a big impact on the performance.

Here we will call them edge-lit and backlit LED TVs - although there doesn't appear to be a definitive name that is used by everybody.

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Firstly, let us summarise what an LED screen actually is.

How Does an LED TV Work?

Boffins with a TV

So here we are, another new TV to spend our money on - so what have the boffins at the TV manufacturers been up to this time?

Well, an LED TV is essentially an LCD TV - but with a different type of back-lighting.

The traditional method of backlighting an LCD TV has been to use a CCFL (cold cathode fluorescent lamp).

Whilst this type of lamp has managed to produce high-quality televisions, there have always been disadvantages to this technology such as poor contrast ratios and colour reproduction.

The aim of using a different type of backlight is to fix some of these issues.

Therefore, televisions are now being built using LCD screens, but with light-emitting diodes (LEDs) to create the backlight.

There are two different types of LEDs that are used in making this type of television - white or RGB (red green blue) LEDs. The RGB LEDs are the ones that allow a more accurate colour reproduction compared to other LCD TV technology - and less so with white LEDs.

Now we understand the basics, we can group these TVs into two types depending on how they are made - edge-lit and backlit LED televisions.

So what is the difference between the two - and why should we care?

Edge-lit LED TVs

Guess where the lights of an edge-lit LED TV are positioned?

Want a clue?

Ok, I'll tell you......it's around the edge of the screen!

That's right, the name is a bit of a giveaway isn't it - but this is important when we are looking at the performance of the TV.

The lights in edge-lit LED televisions are placed around the perimeter of the LCD screen, and the light they emit is spread out across the back of the panel by a series of 'light guides'.

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We can summarise the main advantage of an edge-lit LED screen in one word.

Thin .....skinny, slender, slim and lean. Ok, five words. smiley

If you see a model that almost disappears when you look at it from the side, then you can be sure it is an edge-lit screen.

An example of this is the Samsung UN40B7000 40 inch 1080p LED HDTV - which is just 1.2 inches thick (without the stand) - or the Sony KLV-40ZX1M 40 inch BRAVIA screen.

Another advantage of an edge-lit version is the power consumption is less than a standard LCD TV - and is also less than a backlit LED screen (see below).

Aesthetically, these TVs look absolutely fantastic and would look great in any room in the house, however this type of TV does have its issues.

They are relatively expensive and the improvement in picture quality over standard CCFL LCD TVs isn't as great as with our second type - the backlit LED TV.

Backlit LED TVs

So what is there to know about this type of TV?

Well, backlit LED televisions have the lights arranged behind the screen - similar to a traditional LCD with a fluorescent lamp.

However, the LEDs are grouped into blocks - and each block can be switched on or off independently of the other blocks. If a block of LEDs are switched off, then we can get a true black signal which isn't possible with fluorescent lamps.

This technique is called local dimming, where different parts of the screen can have the blocks of lights switched on or off at any one time - and this enables the TV to have a much better contrast ratio than a traditional LCD TV.

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A poor contrast ratio and a limited colour spectrum has always been an issue with LCDs. Therefore, a backlit LED TV with local dimming - and with RGB LED lights to improve the colour spectrum - produces a much better picture which can be favourably compared with that of a plasma TV.

The downside of this technique is that when you get an image which is a mixture of bright and dark areas, the chances are there will be some blocks of lights which cover a light bit and a dark bit.

In this instance you will have to have the block switched on to reproduce the bright area, and so the dark part will be affected too.

The only way to completely get around this problem is to have one LED per pixel on the screen - but this would be far too expensive to manufacture.

However, having said that, this backlighting technique is widely regarded as providing the best images ever seen on TVs with LCD screens - so it is definitely worth considering this type of television if you want something which can get close to the picture quality of a plasma TV.

You may find the power consumption of these TVs can be greater than standard LCD televisions, especially those models which use RGB LEDs - and also the fact that the light blocks are placed behind the screen means that a backlit version won't be as thin as the edge-lit models.

Understanding The LED HDTV Summary

So now we can see why it is important to understand the two different types of LED HDTV.

Each design has its advantages and disadvantages.

An edge-lit LED TV can be extremely thin, sexy and easy install, but at the expense of picture quality compared to other TV types. However, they are more economical to run than other types of television.

A backlit LED screen has a better picture quality, but at the expense of thickness and power consumption - compared to an edge-lit model.

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