Frontlighting, Examined
E Ink, Revisited
At its core, the Paperwhite remains an E Ink-based e-book reader. So, page refresh times are still significantly slower than what you’ll ever see on a tablet’s LCD.
E Ink employs charged microscopic capsules in order to generate images. So, the screen's refresh rate directly correlates to the time it takes to charge electrodes and move the capsules. This process isn't measured in single-digit milliseconds, as it might be on a tablet. Rather, the latency is quantified in the hundreds of milliseconds.
Paperwhite: What’s Different?
For starters, the Paperwhite employs a capacitive touch sensor, as opposed to the previous Kindle Touch's infrared sensor. This improves response times noticeably. Back in January of last year, we benchmarked screen refresh rates on the Kindle Touch and found them to be about 50-100 ms slower than the non-touch version and older Kindle Keyboard. The capacitive sensor yields a substantial speed-up; we can't detect a noticeable difference in the delay.
Perhaps more important, Amazon adds LED lights to the Paperwhite, allowing you to read the display in the dark. Backlight might be the wrong term to use, though. While tablet screens and computer monitors are illuminated by LEDs that emit light out towards the user, the Kindle Paperwhite shines light back down towards the screen. Ergo, the Paperwhite’s backlight is actually a frontlight.
The clip below is an excerpt from Amazon’s official Quick Tour video, which gives a breakdown of this technology:
In the next video, Amazon employees talk about the challenges they faced designing the Kindle Paperwhite.
It was actually very hard: We had to spend many hours in dark rooms to pick LEDs. We looked at samples and looked at for this batch here these are the best LEDS to match the characteristics of a white, very neutral color. We spent a lot of time looking at how many LEDs to put in, how hard to drive them, how bright we could get it without impacting the battery life. You don’t want someone to have to [consciously] manage their light, and say, I’d really like to have the light on but that’s gonna hurt my battery life.
In a nutshell, Amazon combined the extremely power-efficient characteristics of a non-glare display with a touch-sensitive screen, and managed to tack on illumination for good measure. This is nothing short of a tremendously impressive feat.
To be precise, the Paperwhite’s display employs a light guide that sits atop the capacitive touch sensor. The guide is made of a transparent material, and light from white LEDs enters at the bottom edge of the display, bouncing along nanoimprinted channels within the guide. Amazon says it operates in much the same way as a fiber optic cable, though the company doesn't share much more detail than that.
The nanoimprinted channels are responsible for an even distribution of light across the entire display. Because the LEDs provide the strongest illumination towards the bottom edge of the screen, the criss-cross pattern of these channels becomes denser towards the upper edge, intensifying the output to maintain even distribution of brightness throughout.
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