How Pixel Qi makes screens efficient

Computex 2010: OLPC spinoff shows high rez panels

Pixel Qi LogoPIXEL QI HAD some of their screens on display at Computex, and a chat with some of their engineers shed a lot of light on not just what they do but how. The screens may be made on the same tools as mainstream LCDs, but they are far from ordinary.

Pixel Qi is a for-profit spinoff of the One Laptop Per Child (OLPC) effort, the transflective screens of that machine are effectively the first generation Pixel Qi device. The current company was formed to commercialize the non-profit OLPC program’s efforts to make very low power screens, and bring them to the mainstream computer world. This drives up volume and drives down costs for the OLPC screens, hopefully making the world a better place.

While Pixel Qi screens are generally touted as being low power, and they are, there are some very unique properties that their process brings to the screen as well, like three times the resolution in monochrome with the backlight off. It looks gorgeous.

To understand how they do it, you need to know a little about how a ‘normal’ LCD works. The vastly simplified version is that it starts with either a fluorescent or LED light source at the very rear of the screen. This is called, wait for it, the backlight, and it makes all the light you see.

From there, there is an LCD screen on top that forms three distinct rectangles for each pixel. These control the pixel itself, if they are on/transmissive, the light comes through, and you see it as a bright white spot. Turn it off, and there is no light transmitted, and you ‘see black’, or at least don’t see light.

On top of that, there is a color filter. Each of the three colored rectangles that make up a pixel have a red, green, or blue filter on top of them, and those filter the light passing through them. If you have a blue filter, the pixel looks blue, etc etc.

The main problem here is that the filters filter out 2/3rds of the light going through them, best case. In reality, much less than that gets through, so you have to have a really bright light source as a backlight if you don’t want a dull and washed out colors on your screen. You then block the majority of it to get colors, so the energy wasted is horrendous, only a tiny fraction of the light gets to your eyes.

What Pixel Qi does is to take the light source and cover it with a reflective layer, likely silver or aluminum, and poke pinholes in each pixel. Think of a mirror with microscopic holes, but those are big enough to let the light through, reflection is your friend here. The holes may be small, but they get the job done. If you look at the backlight, it looks like a shiny mirror.

Pixel Qi relflector

This backlight is much shinier in person

From there, Pixel Qi puts a color filter over each pinhole, so the mirror ends up silver with microscopic RGB dots all over it. They are so small that you can’t see them, but since all the light coming through the silver layer goes through them, you get a fully colored pixel. On top of this you place an LCD screen, nothing exceptional but there is probably some magic lurking in there. The stack looks like a very ordinary LCD panel.

Pixel Qi panel

Magic lurks beneath the surface

What you end up with is a screen that works fairly normally when the backlight is on. Some of the light is absorbed by the silver layer, some by the color filter, and some by the LCD panel. When working in this mode, Pixel Qi screens are about as efficient as other LCDs, and look like almost any other screen.

If you turn off the backlight however, things get interesting. At that point, light coming from the screen is effectively zero, so all you see is light that hits the panel, goes through the LCD layer, reflects off the silver backing, goes back through the LCD, and then hits your eyes. If you are paying attention, you will see that there are no color filters absorbing light in this mode, so the light reflected is a fairly high percentage of the light that hits the panel.

Each pixel is now three monochrome pixels instead of one red, one green, and one blue pixel. If you want to watch a movie, as long as it was made before WWII, you are good to go, a 1080p panel has becomes a 3240p panel with 3x HD resolution. Anything, say, post Wizard of Oz loses a bit though. Since most people tend to watch movies in a darkened room with the backlight on, this isn’t a dealbreaker, you can always just turn the backlight on if you want.

For text however, things are amazingly clear. You get astoundingly high rez black text on a white background with decent contrast. Unlike normal LCDs, especially those damnable shiny screen monstrosities, the more background light there is, the better things look. I you can live with greyscale for word processing, spreadsheets, or anything else you would do in Open Office, the screen is really great.

One OS actually takes advantage of this, Ubuntu. It has a sub-pixel smoothing algorithm that takes the Pixel Qi screen properties into account, so you end up with astounding looking text when the backlight is off, and you remove the single largest power user on your laptop. How can you argue with better looks AND vastly longer battery life?

Text closeup

Closeup of the text

If you aren’t convinced, take a look at the text above. That is a 7 point font, and lower case ‘e’ is only 5 pixels high but completely readable. If you tried this on a normal LCD screen, or turned the backlight on, you would effectively have a 2-3 point font that looked like a blurry mess. Try seeing how well a 3 point font renders on your LCD with no zoom for comparison.

Dropping the backlight will drop screen power consumption of a laptop by over a third, more if you don’t refresh often and have somewhat static images. If you have a small 6″ screen optimized for low power use and slower refreshes like you would use in an ebook reader, you can get away with less than 50mW of power consumption.

Although it may sound a bit like funny math, the good folk at Pixel Qi were claiming that their screens could have lower power draw than an epaper screen. You might think that the claim is ludicrous because epaper takes up zero power when in a steady state, while an LCD needs some power to keep the pixels ‘on’. ‘Some’ is more than ‘zero’, so how does this claim hold water?

An average reader has a known rate at which they read text, and an average page holds enough text that the aforementioned person needs to flip an ebook reader’s page about two times a minute. Epaper takes a lot of power to first turn a page all black, then draw the correct text. A Pixel Qi screen needs much less power to change state.

Because of this, Pixel Qi claims that when reading, their screens will draw less power than an epaper screen. If the average page turn numbers are realistic, and they sound like they are in the right ballpark, Pixel Qi screens could be a good thing for ebook readers. Add in that you could turn on a backlight to watch a movie in full color, and I know which I would want for my next device.

In the end, you have a tradeoff with Pixel Qi screens, power vs performance. You can keep the backlighting on and use the panel like a fairly efficient color screen, simply never worrying about any other modes. On the other hand, you can turn off that backlighting and use the screen as an ultra-sharp monochrome or greyscale panel that sips power in ways that rival e-ink. Luckily, this choice is up to the user, and not just at the time of purchase.S|A

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Charlie Demerjian

Roving engine of chaos and snide remarks at SemiAccurate
Charlie Demerjian is the founder of Stone Arch Networking Services and SemiAccurate.com. SemiAccurate.com is a technology news site; addressing hardware design, software selection, customization, securing and maintenance, with over one million views per month. He is a technologist and analyst specializing in semiconductors, system and network architecture. As head writer of SemiAccurate.com, he regularly advises writers, analysts, and industry executives on technical matters and long lead industry trends. Charlie is also a council member with Gerson Lehman Group. FullyAccurate