LED backlight
LED backlight or LED backlighting or LED lower light is a variant of the illumination of liquid crystal screens ( LCDs ). It is used in LED televisions and TFT screens . White or colored light-emitting diodes (LEDs) that can be mixed into white and are arranged behind the liquid crystal elements are used.
Structure and functionality
Big technical problems with liquid crystal screens (LC displays / LCDs) with conventional cold cathode ray lighting (CCFL) are the relatively low contrast value and the uneven illumination compared to plasma and tube devices. The reason for this is the construction of the LCD technology: the individual image-generating liquid crystal elements are, like a film, illuminated with the help of fluorescent tubes. Usually several CCFL tubes are required for this, which are arranged next to one another or one below the other. The difficulty is in distributing the light evenly. A light-conducting plastic is used for this purpose, which is supposed to generate a uniform light source. Despite this technology, areas of different brightness can appear on the monitor, mostly at the points where the tubes are located due to the design.
With the help of the LED backlight, this problem can be largely avoided: Many small LEDs distributed over the entire surface of the monitor provide better illumination. There is also the option of using a large number of LED elements that can be controlled separately. This means that individual parts of the illuminated area that are supposed to display a deep black can be dimmed separately ( local dimming ) or switched off. That makes the picture more contrasty. Thus, the advantages of LED and LCD are used.
Another advantage of LED backlit monitors is the long service life of LEDs. With CCFLs there is often a flickering or failure of the light source. In addition, LED elements are more energy-efficient, which allows notebooks to run on batteries for longer . Practically all new notebook models with LCD screens as well as corresponding desktop monitors and televisions now have LED backlighting.
Two different techniques are currently used with television screens:
- With the side arrangement (English: edge LED , from edge : edge) a few white LEDs are attached to the sides of the monitor and illuminate the entire surface from there. The advantage is a small housing depth, but problems arise with the uniform illumination. For reasons of cost, this is the most commonly used technique today.
- In the case of the flat arrangement ( direct LED or full array with local dimming ), light-emitting diodes illuminate the image from behind over the entire image area. In the best and most expensive televisions to date, combinations of three light-emitting diodes each with the colors blue, red and green are combined to form white light. The image can be illuminated more evenly and the contrast can be increased by local dimming of individual LEDs (those behind dark areas of the image).
Since 2013, white LEDs have been replaced by blue LEDs for the first time, with subsequent conversion of a light component into green and red colors in order to achieve better use of the color gamut . This is done with quantum dots (ger .: Quantum Dots (QD)). At the Consumer Electronics Show 2015, several television manufacturers presented such optimized devices with significantly improved color rendering.
Advantages of LED backlight
- Low power consumption
- Possibility of a flatter design of the devices
- Higher contrast through local dimming
- Long lifetime
Disadvantages of LED backlight
- The increased contrast only applies to devices with so-called local dimming . There are no differences in terms of static contrast for devices with Edge LEDs.
- If the PWM frequency selected by the manufacturer is too low , the image may be perceived as flickering by sensitive people.
Web links
- Explanation of the functional principle
- Edge-type LED Backlight Thins Large LCD TVs
- Video on how LED backlights work
- How Edge-LED and Full-LED TVs work