Understanding LED televisions
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Understanding LED televisions

Local dimming

Here we go with our series of technical articles dedicated to LED televisions to help you understand all the complexities of flat screens, their functions and their main qualities to help you choose the TV that's right for you when you get to the store. Today, we're starting with the operation of an LED screen and one if its main components in particular, local dimming.

To understand how an LED screen works, we need to go back to the start. LED televisions are the direct descendants of LCD, or Liquid Crystal Display, screens, on whose basic principles they are built. The idea is a relatively simple one. Each flat LCD screen, whether LED-based or not has a light source, backlighting. This backlighting, which is located at the back of the television, produces the light source which will pass through the liquid crystal panels before it is seen by the viewer. For each pixel, a liquid crystal operates as a switch, letting through more or less light, depending on the intensity to be displayed on the screen for the corresponding colour. While it is a modern technology, it's not a brand-new one since the first liquid crystal displays were produced in 1971. But it is to Matsushita to whom we owe the first LCD screen which could be used to display simple black and white text. That was in 1985.


Backlighting, permanently changing

And just how far we've come since then! At the time, backlighting for LCD screens was composed of fluorescent tubes, similar to what we find in low consumption light bulbs or neon strip lighting. It provides the LED screen with a new, more compact type of backlighting, one which uses less energy and which in the end provides higher performance by replacing the old tubes with Light Emitting Diodes, or LEDs. This was a huge step forward for picture quality, as the LEDs offer a white light, with a wider spectrum, which enabled viewers to obtain richer colours, until the recent wide gamut evolution. We'll come back to this in a future article.


Dynamic backlighting, or local dimming

The adoption of LEDs in backlighting also enabled major steps to be made in screen contrast, thanks to local dimming. This involves controlling the local intensity of the backlighting independently according to the picture content in order to increase the intensity in light zones, or alternatively to intensify blacks in dark zones. In fact, the liquid crystal light switch for pixels is never perfect. When closed, no light is supposed to get through to produce black, but a small part of the light from the backlighting still manages to pass. This means that uniform backlit screens do not provide perfect black; they are more like dark grey, especially when the screen's brightness is increased. Dynamic backlighting compensates for this weakness by gradually switching off the LEDs in the shaded zones on the screen to produce more intense blacks. Of course, the higher the number of zones the finer the backlighting control. A television such as the TCL Xclusive.X1 for example, features 288 zones controlled independently of each other, which guarantees a perfect level of black without compromising the shading of dark colours.

Now local dimming is no secret to you! In future articles we will look at other innovations that effect backlighting, such as wide gamut or quantum dot. We'll keep you posted!