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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 (or for the pedantic of you out there - three).

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

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

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

How does an LED TV work?

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?

Three scientists with an old TV

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

The traditional method of back-lighting 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 back-light 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 back-light.

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 back-lit LED televisions.

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

What are edge-lit LED TVs?

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

Want a clue?

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

That's right, the name is a bit of a give-away 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'.

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

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

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 Sony XBR65X900C Ultra HD 3D smart LED TV which is just 1.5 inches wide.

Sony XBR65X900C 65-Inch 4K Ultra HD 3D Smart LED HDTVSony XBR65X900C 65-Inch 4K Ultra HD 3D Smart LED TV

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

Aesthetically, these TVs look absolutely fantastic and would look great in any room in the house, and will usually cheaper than a back-lit model, however this type of TV does have its issues.

The main downside is the improvement in picture quality over standard CCFL LCD TVs isn't as great as with our second type - the back-lit LED TV.

An edge-lit TV will often have an inconsistency in the spread of the backlight across the entire screen.

In normal viewing conditions you will usually not notice this, but if you are watching in a darkened room then you may see light pooling - which is where dark scenes will  highlight brighter areas around the edge of the screen.

Some edge-lit TVs will claim to have local-dimming (explained further down), however the amount of local dimming they can actually do is fairly limited due to the design, so it often doesn't work very well.

What is a back-lit LED TV?

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

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

There are two main variations of this technique, back-lit with local dimming - and back-lit without any local dimming.

These TVs, with the lights positioned behind the screen, can be called by number of names such as full array or dynamic televisions.

Back-lit LED with local dimming

With this type of TV, 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.

A poor contrast ratio and a limited colour spectrum has always been an issue with LCDs. Therefore, a back-lit 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.

An example this type of TV is the  Samsung UN65KS9500 Curved 65-Inch 4K Ultra HD Smart LED TV pictured below.

Samsung UN65KS9500 Curved 65-Inch 4K Ultra HD Smart LED TVSamsung UN65KS9500 Curved 65-Inch 4K Ultra HD Smart LED 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 back-lighting 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 or edge-lit televisions, especially those models which use RGB LEDs - and also the fact that the light blocks are placed behind the screen means that a back-lit version won't be as thin as the edge-lit models.

Back-lit LED without local dimming (direct-lit TV)

These are often referred to as direct-lit televisions.

Some budget models may strike a compromise between edge-lit and local dimming.

This is where the LED lights are arranged behind the TV, but don't have the ability to dim certain parts of the screen.

The advantage of this over an edge-lit TV is that the lighting will be more uniform across the screen, eliminating the light-pooling of edge-lit.

However, they obviously don't have the advantage of local dimming so the picture won't have the improved contrast ratios that a dimming TV will have.

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

A back-lit LED TV with no local dimming provides a balance between the two - but just falls between two stools.

Obviously the main result of these differences will usually be the price you pay for a particular model.

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