TechReport (UK)

LED Backlight Arrangement

All LCD panels require some kind of lighting behind them in order to produce a visible TV picture.

This is necessary as LCDs can only either block or allow the passage of light – they do not provide any light themselves.

In modern TVs, this lighting is generally provided by an ‘LED Backlight’ - a symetrical array of LEDs arranged either behind the LCD panel, or along the upper & lower or right & left edges of the TV (typically hidden behind the bezel).

No doubt you are wondering why you should be concerned about the type of LED backlight the TV you are considering may have, but please bear with us on this one, as it can actually make quite a difference to the picture quality, specifically in the correct representation of both very bright and very dark areas of the TV picture.

The first point to note, however, is that this need not be of concern to owners of OLED TVs – OLEDs are an ‘emissive’ technology – each and every OLED within a TV screen (typically 4 per pixel) emits its own light and can be fully controlled, from completely off up to full brightness. With and OLED TV, there is no need for backlighting and the these TVs can provide a very accurate representation of any image, no matter how dramatic the variations from dark to light.

With normal LED / LCD displays, the implementation of the backlight is always a compromise – the more separately controllable (and placed in widely varying locations) LEDs used, the more accurately the picture can be lit, but inevitably the greater the cost and the more space taken up behind and around the screen (so the bulkier the TV) – the less LEDs used, the more cost effective the solution and the slimmer the TV can be.

The main options are:

Back-lit:

Here, an array of LEDs is placed behind the screen and they are all permanently on, much like the old CCFL backlights. This is the method used on lower cost TVs as it is cheap and simple. The disadvantages are that as there is no ability to control the brightness of individual areas of the backlight, the maximum contrast of the TV is limited and the placement of the LEDs behind the main display adds a lot to the depth of the TV.

Edge-lit (+ local dimming):

This tends to be the norm on most mid to high-end LED / LCD TVs. Basically, the LEDs are arranged around two or more edges (left and right or top and bottom) of the screen and ‘light guides’ are used to reflect a varying proportion of the light forwards onto the screen (through the LCD) – the light guides are arranged in such a way that despite the fall-off in light from the LED beam nearer the centre of the screen, more of the light is reflected forwards, thus ensuring even lighting across the width / height of the screen.

The light output of each LED can be fully controlled depending upon the level of brightness required on the screen, thus allowing higher contrast levels – this is called ‘local-dimming’.

There can be issues with this technology in an effect called ‘blooming’ – the creation of an unwanted ‘halo’ of light around particularly bright objects surrounded by a much darker background:

         

           Moon against dark background,                   Moon against dark background,

                OLED TV                                   LED Edge-lit (top and bottom) TV

Fortunately, clever signal processing has greatly reduced the visibility of this effect on modern TVs – in reality the effect would rarely appear as visible as in the above image.

Many retailers will specify the type of LED backlighting technology (at least in very generic terms) – to get the best picture, go for an OLED TV if you can afford it, but in most cases modern Edge-lit LED / LCDs offer very good levels of contrast with only very limited side-effects (e.g. blooming) and can also reach much higher levels of brightness than OLED TVs.

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