Face Off at the HDTV Corral Sidebar: LED Local Dimming
The LED local dimming feature is a revolutionary development. All LCD flat panels require a light source that’s separate from the LCD panel itself (in contrast to a plasma, in which the light comes from excited phosphors embedded in the panel). Until now, LCD backlights have largely consisted of cold cathode fluorescents (CCFLs) driven at a constant brightness. In such a display, the LCD panel itself does all the work in reproducing the black levels that the source requires. It does this by reorienting the LCDs as needed to pass or block the illumination from the fixed backlight.
But LCDs alone cannot completely block the unchanging output of the CCFL backlight. The result, in dark scenes, is a black level that never drops below a medium gray, rather than a true- or near-black. While the adjustable backlight setting that most LCDs offer can help, it’s no cure.
However, LEDs can be turned on and off almost instantaneously. You can substitute dozens of clusters of red, green, and blue LEDs (colored LEDs or white LEDs with colored filters) for the old, fixed-output CCFL and drive each of these clusters separately in response to the brightness the source requires in that area of the screen. To a degree, this can compensate for the light-blocking limitations of the LCD panel. Of course, the panel is still largely responsible for the brightness gradations in the image. It operates directly from the source on the level of nearly 2,000,000 separate pixels in a 1080p display, several orders of magnitude more than the number of LED clusters (dozens, or at best, hundreds). But the dynamic, zone-addressed LED backlights can nevertheless produce a dramatic improvement in an LCD’s black level—and shadow detail—with minimal side effects.