When we think about the LED TV, we should really place them in two different categories.
At present, there are two main ways of building LED televisions and each has a big impact on the performance.
So, if you want to understand the difference between back-lit and edge-lit LED televisions, then you've come to the right place.
Firstly, let us summarise what an LED screen actually is.
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 back-lighting an LCD TV has been to use a cold cathode fluorescent lamp (CCFL).
This type of lamp has managed to produce high-quality televisions. But, there have always been disadvantages to this technology, such as poor contrast ratios and color 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 the same 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:
RGB LEDs allow a more accurate color reproduction compared TVs made with white LEDs.
So, now we understand the basics, we can group these TVs into two types depending on how they are made:
So what is the difference between the two - and why should we care?
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.
By lighting the screen from behind, there are a number of advantages. The most important is that there is more control over where the light occurs on the screen.
It should also allow for a more even spread of light across the picture. Although, that isn't always the case.
There are two main variations of this technique, back-lit with local dimming - and back-lit without any local dimming.
This type is usually called a full-array LED TV.
With this type of TV, the LED lights are grouped into blocks. Each block can be switched on or off independently of the other blocks.
If a block of LEDs is switched off then we can get a true black signal. This isn't possible with fluorescent lamps.
This technique of switching off, or reducing, the backlight is called local dimming.
Different parts of the screen can have the blocks of lights switched on or off at any one time. This enables the TV to have a much better contrast ratio than a traditional LCD TV.
A poor contrast ratio and a limited color spectrum have always been an issue with LCD TVs.
Therefore, a back-lit LED TV with local dimming produces a much better picture. This helps it to compare more favorably with other technologies such as plasma and OLED TVs.
To understand the differences between OLED and LED TVs, you might want to take a look at the LED and OLED TV buying guide.
An example of an LED TV with local dimming is the Sony XBR-65X900E 65-Inch 4K LED TV pictured above.
The downside of this technique is when you get an image which is a mixture of bright and dark areas.
In this case, the chances are there will be some blocks of LEDs which cover a light bit and a dark bit.
In this instance, you will need the block switched on to reproduce the bright area, and so the dark part will be affected too.
This can create a halo-like effect in dark areas of the picture - also called blooming.
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.
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 an OLED 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.
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.
These are often referred to as direct-lit televisions.
Some budget models may strike a compromise between edge-lit and full array designs.
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 usually be more uniform across the screen. Eliminating the light-pooling of edge-lit screens.
Unfortunately, they don't have the advantage of local dimming. So, the picture won't have the improved contrast ratios that a dimming TV will have.
Also, don't assume that a back-lit screen will always have a more uniform light across the screen. Some models aren't that great in this respect. Even though they should.
Guess where the lights of an edge-lit LED HDTV 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'.
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 XBR75X850E 75-Inch 4K Ultra HD Smart LED TV which is just 2-inches wide.
Another advantage of an edge-lit version is the power consumption is less than a standard LCD TV.
It should also be less than a back-lit LED screen.
Aesthetically, these TVs look absolutely fantastic and would look great in any room in the house. It will usually be cheaper than a back-lit model, however, this type of TV does have its issues.
The main downside is the picture quality. The improvement over standard CCFL LCD TVs isn't as great as with back-lit LED TVs.
An edge-lit TV will often have an inconsistency in the spread of the back-light 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.
This is where dark scenes will highlight brighter areas around the edge of the screen.
In the early days of LED televisions, it was only back-lit TVs that had local dimming technology.
It made more sense here, as individual blocks could isolate parts of the screen more easily.
As the technique became known for improving the picture quality, it was introduced into edge-lit televisions too.
The problem is that local dimming just doesn't work that well on edge-lit screens.
As you don't have blocks of light behind the screen, you can only alter the light from the sides. This limits the effect because there is less control across the whole screen.
So, I wouldn't buy an edge-lit LED TV on the strength of its local dimming.
There are advantages of edge-lit screens - but local dimming isn't one of them.
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.
Paul started the Home Cinema Guide to help less-experienced users get the most out of today's audio-visual technology. He has worked as a sound, lighting and audio-visual engineer for around 20 years. At home, he has spent more time than is probably healthy installing, configuring, testing, de-rigging, fixing, tweaking, re-installing again (and sometimes using) various pieces of hi-fi and home cinema equipment.