Thread: New Chapter on Digital Reading (Sony)
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Old 09-16-2008, 08:41 PM   #131
BobVA
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Quote:
Originally Posted by EatingPie View Post
Thanks Valloric!

I'm still curious about the light though. My understanding (from a class I got a C- in, and that's only because I copied my A+ lab partner) all light bulbs flash at the 60hz rate electricity in the US uses. I thought backlights work the same way. Though I grew up in a CRT world, so flashing displays is just something I was always seeing (we STILL have CRTs at work).
It's sort of like that. In the case of the most common LCD backlight, a compact florescent lamp, it generally isn't fed from the 60 Hz line, but from a dedicated inverter that steps up a low voltage (like 12 volts DC) to a much higher square wave AC voltage that can light the florescent backlight tube. The frequency used is usually WAY above 60Hz since using a higher frequency allows use of a physically smaller transformer. Typical frequencies are in the 20 to 50 KHz range (i.e. 20,000 to 50,000 Hz). The ratio of "on" to "off" in the square wave can be varied to dim the light.

I'm not sure how LED backlit screens work, but if they are dimmable a high frequency square wave is certainly the most efficient way to power them, too.

That all said, I don't believe there's any noticeable perceptual difference between a continuous backlight and one running at a 50 KHz "flicker" rate. The persistence of the phosphor in the bulb will keep it lit through a large portion of the power cycle, so any visible "ripple" is a small portion of the total light. This is unlike a CRT, where the phosphor persistence must be carefully balanced against the need for response to a quick changing image. So, all this would allow me to restate what Valloric said as "For all practical purposes, the backlight doesn't flicker"

If you want a good example of a low power, non-flickering LCD display, take a look at a digital watch. That display is optimized for high contrast and minimal power consumption. Higher resolution displays can have a more visible refresh cycle.

I think eInk buys you few advantages, though: Higher contrast for a given resolution, wider viewing angles and no unpleasant interaction with polarized sunglasses. Another is zero power consumption when static. The Sony Reader doesn't really take full advantage of this (heck, it doesn't even take full advantage of being turned OFF!) but if you compare the size of the battery in the Rocketbook and the one in the Sony the difference is pretty impressive. Not all of the power difference is in the display, but I'm sure it's a significant part of that.

Cheers,
Bob
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