LCD screens still make up the bulk of the market, and presumably will remain so for the foreseeable future. The backlight is an important part of the LCD TV and has an impact on the image quality, among other things. We explain what the Backlighting for LCD LED TVs does, and what the advantages and disadvantages are of the different options.
What is a backlight and what Backlighting for LCD LED TVs does
Before we dive headlong into the different types, we would like to explain what Backlighting for LCD LED TVs does, how an LCD TV functions and what the background lighting is. The schematic below illustrates the main parts of a pixel on the LCD screen. The backlight creates the white light. The two polarizing filters have different polarization directions. That means no light is visible at the front of the screen. The LCD panel can determine for each sub-pixel whether the polarization is adjusted or not, so that the light still passes through the front polarization filter. Finally, the three sub-pixels of the pixel have a red, green and blue color filter.
So the backlight provides the light, and that light has to pass through a number of layers before it is visible or not. That is why we call an LCD panel a transmissive system. We call an OLED TV emissive because each pixel itself emits light and therefore no separate background lighting is required.
The backlight (Backlight Unit, BLU for short) is a module that consists of several parts: a number of light sources (a series of LEDs) shine their light into the ‘light guide plate’ which in turn distributes the light over the entire screen. On the diagram below that indicates an edge led, the leds can be found as the orange bar under the light guide plate. A rear reflector and a number of diffusers and optical films provide improved performance (uniformity, light output). We provide the construction for completeness, but in practice only the placement of the LEDs is important for further understanding.
Attention, backlighting used to be built up with CCFL lamps (very fine tube lamps). Current screens make almost exclusive use of LEDs. They have a number of important advantages. They can operate more efficiently, and above all they can be dimmed very quickly and accurately. All setups we discuss below are for LED backlights.
Because an LCD panel cannot shut off the light of a pixel perfectly, the black display and therefore the contrast is limited. To solve that, the very best LCD TVs use a ‘Full Array’ backlight. The backlight is built up as a matrix of LEDs behind the entire surface of the screen. The LEDs shine the light towards the viewer.
The big advantage of a ‘Full Array’ is that the television can control each LED (or zone) separately. For example, the LEDs behind the dark parts of the screen can be dimmed, while LEDs behind bright parts can shine in full. We call this ‘local dimming’, and it significantly increases the static contrast (the contrast within one image). This type of backlight is therefore often referred to as ‘FALD’ (Full Array Local Dimming).
When there are too few zones, you can sometimes see the zone boundaries on the screen. A bright object on a dark background then has a light halo that gives away the rounded rectangular shape of the zone. This problem can be solved by using more and therefore smaller zones, but that also makes the device more expensive. The exact number of LEDs or zones in a screen can vary widely. For example, the 2016 Philips 65PUS7601 had 128 zones (16 x 8), while the Panasonic TX-65DX900E had 512. The Sony KD-65ZD9 even had more than 600.
To make the television screen as thin as possible, manufacturers use a backlight with the LEDs in the edge of the screen, hence the name ‘Edge Led’ or ‘Edge Lit’. The LEDs do not shine directly at the viewer, but shine in the light guide plate that distributes the light over the entire screen.
Edge LED screens come in a variety of configurations. Some only use LEDs at the bottom, others use LEDs at the top and bottom, or left and right. Because the LEDs have to shine bright enough to illuminate the entire screen, you can sometimes see problems, such as ‘headlighting’ (also called ‘flashlighting’). This is the effect whereby a light appearance is discernible in one or more corners or somewhere on the side where the LEDs are located when the screen should be dark. For the same reason, edge LED screens can sometimes have problems with uniformity, which is sometimes reflected in ‘clouding’ or cloud formation. In addition, you see on a dark screen light swims of light as if they were clouds.
Edge LED screens can also use local dimming, but are of course much more limited than Full Array screens due to their construction. For example, an edge LED can generally only dim in columns (if the LEDs are positioned above and / or below) or in rows (if they are on the left and right). Since there are very few zones (from 12 to even 4) you may sometimes see a faint column / row, typically again in dark scenes.
An alternative that is also used is ‘global dimming’. All LEDs are dimmed together based on the average brightness of the screen. For example, a dark image with one bright object results in a better black reproduction, but at the same time also reduces the brightness of the bright object. Global dimming creates a higher dynamic contrast, but does not change the static contrast.
The cheapest way to create an LED backlight is ‘Direct LED’. It uses the same setup as a full array, but with a very limited number of LEDs. Because there are so few LEDs, they never use local dimming. Some models do use global dimming.
These screens generally also have a much thicker profile. And uniformity problems (clouding) can more easily occur.
Conclusion for Backlighting for LCD LED TVs
The construction of the Backlighting for LCD LED TVs affects the design and image quality. Full Array with Local Dimming delivers the best image quality, but is very expensive and creates a thicker profile. Edge Led results in ultra-slim screens, and can make limited use of local dimming, but sometimes suffers from uniformity problems. Direct LED is the cheapest solution, but has little potential to improve image quality and results in thicker screens.
Unfortunately, manufacturers rarely provide complete information about the type of backlight used in each device. Where possible, we always try to mention it.