Contrast-enhancement of LightBoost photograph, showing ultra-faint pixel persistence that's still seen by human eye. It's almost below the noise floor. Most of the pixel persistence (transitions and overshoots) are hidden during the time period the backlight is turned off.
Originally Posted by tgm1024
1. Is the camera a film or digital? This by itself doesn't necessarily matter at all, but I'd like to know. Discrete mechanisms (even when assuming a wide open shutter for 1/30th of a second) can cause beat frequencies against each other depending upon exactly what is being done to cause the 1/30th of a second effective shutter. Even CCD and CMOS arrays are scanned internally I think, so I'm simply curious about that.
The camera is a Casio EX-FC200S. Any common film or digital camera can be used; they all yield the same motion blur result at the same camera exposure, if you properly synchronize the camera motion to the moving-object onscreen (within accuracy of +/-1 pixel during the duration of the exposure). I've confirmed there's no difference. Also tried an old Panasonic Lumix.
2. When the backlight strobe length is measureed by TFT central is that for the entire frame, or is that for each of the individual pixels. That is, is it possible in this scenario for the pixels to be flashing for a much shorter time than the entire frame seems to be?
At the LCD panel level, the pixel transitions are taking longer than the backlight flash. However, the pixel transitions are taking place in the dark. Marc Repnow (scientist) reversed engineered
LightBoost, and found it uses a vertical blanking interval of approximately 5 milliseconds. See his discoveries
as well as his HardForum post
. (Frames are buffered in order to allow accelerated scanouts, in order to have a long blanking interval). The refresh is scanned out fast (top-to-bottom) in total darkness, to allow a bigger idle time period between refreshes.
5ms -- backlight turned off; LCD is being refreshed in total darkness. Pixel transitions is occuring in the dark.
2ms -- backlight turned off; idle period, to wait for the last pixel transitions to settle in total darkness.
2ms -- backlight flash; fully refreshed frame seen by human eye.
This corresponds to one refresh cycle (8.3ms) at the 120Hz refresh. During longer strobe flashes, the backlight flash actually overlaps the next refresh slightly, causing slightly increased ghosting at the edges of the screen (a few percent more visible, only on the top/bottom 10% of the screen, and not objectionable). During shorter strobe flashes, the backlight flash completely fits within artificially-lengthened blanking interval between refreshes. Thus, by this technique, a good strobe backlight on a fast LCD, bypasses greater-than-99 percent of pixel persistence from becoming visible to the human eye.
There is definitely some vertical non-linearity in ghosting; e.g. top and bottom edges have more remnant pixel persistence than the center of the screen. That's an unavoidable situation caused by the top-to-bottom scanout of LCD, versus the all-at-once stroboscopic flash. But this no longer a motion blur bottleneck, unlike scanning which still has unavoidable light leakage between backlight segments (on vs off segments). Only strobe backlights (given a sufficiently fast LCD) currently make possible unbounded improvement in motion clarity.
All that aside, isn't what you just shown proof that by limiting the pixel persistence you get a clearer picture?
Therein lies the confusion. The LCD has the same pixel persistence, during all these photographs. It's a photo of the same LCD. The LCD isn't accelerating pixel transitions by 10x. (But yes, it's accelerating the top-to-down refreshing, but the individual pixels still take the same amount of time to transition). The backlight is simply hiding the pixel transitions, simply by being turned off between refreshes.
There is little difference in the LCD panel's ability to change the speed of pixel transitions (overdrive tweaks) in the photographs between 60Hz versus 120Hz versus LightBoost. All of the photos were taken using the same camera on the same computer monitor. The law of physics in the LCD remain the same; it doesn't transition pixels faster between the 120Hz versus 120Hz LightBoost (aka the pixel persistence, from the point of view of the LCD law of physics
, is unchanged). Marc Repnow mentioned to me that if you open up a LightBoost computer monitor and force the backlight to continuously shine during LightBoost mode, the motion blur instantly comes back -- including pixel persistence/overdrive artifacts -- which immediately comes back and becomes far more visible to the human eye. Further proof that the backlight being turned off, is successfully hiding pixel persistence. This A/B test of overriding the strobing, proves this.
The fact that the backlight is able to eliminate motion blur in this A/B test, also proves the following:
Edited by Mark Rejhon - 5/19/13 at 7:18pm
Recently, with today’s faster LCD’s, pixel persistence now only has a minor factor in motion blur.