Scanning backlighting for fast motion

Posted January 28, 2006

Philips has a nice technology they are using in their newer LCD television for getting crisper images during high-motion scenes. It's called ClearLCD or Aptura, and the way it works is that they illuminate only certain horizontal bands of the screen at a time, matching the way the video image is scanned.

Ordinarily the entire display is refreshed every 1/30 or 1/60 of a second, and is illuminated the entire time, so that the eye tracking a moving object will find it smeared across that length of time: even if the individual frames on the DVD are crisp, the eye is moving continuously and doing the actual smearing.

With Philips' system, each pixel is illuminated during a briefer period, although with a brighter backlight to make the overall illumination the same. In this case, the user's eye gets a crisp image "flashed" onto the retina, which persists until the next crisp frame in the new location. Since the intervening interval while the eye moves is dark, there's nothing to smear.

This has an additional bonus due to the way LCD panels behave: there's a finite time for a pixel to change from white to black, and if the backlight is illuminated all the time, you see that messy transition. With the new backlighting system, however, you can time it so that sections of the screen are totally dark until the pixels are "ready" with the new image.

It's really a win-win idea, with the only downside being that you have to have backlights which react quickly (i.e. don't need to warm up or anything) and which have peak brightnesses several times what is ordinarily required. They seem to have solved that, but even in the worst case you can gracefully degrade to the old behavior by keeping the backlights on for greater and greater portions of the time if you really need high brightness for certain scenes or situations.