FAQ: How LCD TV backlights work

Cold-cathode is the oldest technology, and the simplest to implement. It's also the worst technology when it comes to delivering high-contrast images with strong black levels. The problem is that it's not possible to dim the CCFL backlights in specific areas of the TV. So, if you have a scene that's a mixture of light and dark elements, the dark areas will always look washed-out. Additionally, there's a minimum level at which a CCFL backlight can be driven before it will shut off completely.

Here you can see the individual strips of lamps clearly. The diagonal striped areas are the different layers of light diffuser. These are essential, because, without them, there would be areas on the screen that are distinctly brighter than others. These layers do the trick -- most LCD TVs that use this technology have a very even amount of illumination.

With a direct LED backlight, you're essentially replacing the cold-cathode tubes with a series of LEDs. This means that you get a bright image on the screen, and get more control over the amount of light passed through the LCD panel. This is good for contrast levels overall, but it can't help when you've got light and dark areas on the screen at the same time. Compared to CCFL technology, LED backlights can be dimmed to a greater extent, so you can crank your set down good and low when needed.

LED edge lighting is a popular technology because it enables manufacturers to make tellies with the whisper-thin profile of a supermodel after a particularly minimal lettuce salad. That makes them suited to hanging on your wall. It's a good technology for the most part, although it's not as bright as a direct LED backlight system, and, on TVs larger than 50 inches, you're likely to see less light in the centre of the image than at the sides. We were recently told by Toshiba that the British public would rather have slender sets than high-quality pictures. We nearly vomited in disgust. Although better than CCFL backlights, LED edge lighting can require a fair amount of power, because the process of getting light across the whole of the LCD panel means you need higher brightness levels than in a direct system.

Here you can see how compact the LED lighting at the edge of the screen is. You can also get a sense of how the light is diffused over the entire surface of the LCD panel.

The most sophisticated and best-performing technology is direct LED backlighting with local dimming capability. Unfortunately, it's also the most expensive to implement. Such backlights offer black levels closer to those of plasma TVs than LCD sets have traditionally been able to offer. This is because the LEDs are arranged in groups which can be turned off or dimmed when not needed. This sort of backlight can easily cope with bright and dark areas in the same image, by dimming the dark region while keeping the light area at maximum brightness.