pixels             …everywhere.

Energy use in the pixels everywhere future

by on Aug.08, 2011, under energy use

Long term readers (it’s been all of two months, now!) may have noticed that I mention power consumption a lot. That’s because right now many of the obvious forms of digital light aren’t very power efficient. To be blunt, a world illuminated by modulated digital light could consume a lot more energy than today’s world of ordinary light. That’s a problem –a big problem. Can this be solved?  (Warning…. this is a longer than normal post, and still just scratches the surface of this important topic.)

Take projection, for instance. Most forms of projection simply block (i.e., waste) light energy in the dark areas of scenes. In other words, today it takes the same amount of energy to make a bright white scene as one that is essentially black. Solid state light sources where the brightness of red, green, and blue light sources can be independently controlled, frame by frame, promise to improve the efficiency somewhat but pixel modulation still depends on blocking light. Further improvements may be possible with scanning laser projection (e.g. MicroVision) and with digital holographic projection (e.g. Light Blue Optics) but they’ll need to become brighter to be useable as digital light.

Transmissive LCD panels (like the one on your monitor) are just as bad… light gets wasted in dark areas. LED backlights, especially “zonal” ones, will help but there’s no escaping the fact that modulation happens by blocking light. LED panels are better — each pixel only emits the amount of light needed –that’s good– but the images from LED displays are useless close-up because the pixels (individual LEDs) are spaced too far apart (3mm or more). Maybe OLED technology can  have closely spaced pixels and be scaled up in display area, but that’s not yet proven.

Part of the answer may be to use the ambient light around us. A number of companies have developed reflective technologies like the kinds used on eBook readers. New forms of LCD and electrowetting technologies are two

Pixel Qi display

examples. The good news is that they use very low power; the bad news is that they are unusable in the dark. Some companies, like Pixel Qi and Liquavista   get around this by taking a hybrid ‘transreflective’ approach: they can be used either (a) in a reflective mode, or (b) in a transmissive mode, blocking light from a backlight, or (c) combining reflective light with backlighting.

There are a lot of issues to consider about these reflective / transreflective approaches when thinking about using them for digital light:

Liquavista prototype

  • will the contrast be good enough? This may not be a huge issue in high ambient lighting but it needs to be considered.
  • can they be big enough? Right now, they’re being targeted at eBook and tablet sizes around 10″ or so. For digital light/pixels everywhere, we”ll need surface areas on the order of square meters.
  • will they be bright enough in low light?
  • will they be fast enough for video (you’re Kindle isn’t). Some claim they are.
  • will the video quality be good enough? Several are boasting 6-bits per color today — that is below marginal for most people in my opinion.
  • can they be tiled to make larger displays? That begs the question bezel sizes, uniformity, viewing angles, ruggedness, lifetimes and so on.

As a very brief case study, I found the specs for the Pixel Qi 10.1″ PQ3Qi-01 display on their website. This display is available in single-unit quantities from MakerShed for $275. Here are some back-of-the-envelope points. All the points are mine alone; I haven’t ran them by Pixel Qi as of this writing:

  • active area: 0.0279 sq. meters which implies 34.84 units per sq. meter. Not practical (yet) for digital light / pixels everywhere. More further on.
  • at $275 qty.1 list, the price is $9855 per sq. meter. Seems way too high, but don’t be misled — it may be more promising than it seems at first glance. More further on.
  • 0.4watts per tile in reflective mode. That’s 14 watts per square meter. Impressive!
  • bezel widths are 8.86 mm (right and left sides) and 3.64 mm (top and bottom). The top/bottom bezels aren’t bad for tiling (wish they were smaller) but the side bezels are too big. I’ll bet that this really wasn’t a design consideration for Pixel Qi’s engineers, so maybe the bezels could be smaller.
  • Transmissive mode brightness: 60 nits.  That’s probably OK for eBooks, but not nearly bright enough for many digital light uses. But again, I’ll bet higher brightness was not a design consideration. It could be solved with a brighter back light unit with no penalty in reflective mode, but higher energy use in transmissive and transreflective modes.
  • display uniformity and viewing angles not spec’d. This are important parameters for large area tiled applications.
  • Contrast ratio (CR) not spec’d. In high ambient reflective mode, low CR may be OK, but high CR is more important for transreflective and transmissive modes.

So what does one conclude from this? Being a glass-is-half-full kind of person, I think there may actually be potential for Pixel Qi and similar technologies in the digital light future, but there needs to be some important progress firs:

  • the real price per square meter may actually be promising. No one will need to pay $249 in high quantities for 10.1″ displays. If the manufacturing process is scalable to larger displays, then the cost per square meter will go down — a lot.
  • the power consumption is very, very, attractive in reflective mode
  • If the size could be increased to, say, 30″ diagonal or more and the bezels shrunk a little, this could be a thinkable tiled display if (a big if) the uniformity and viewing angles are good, and (a big and) the individual tiles can be matched to each other (not likely possible with only 6 bits to play with –more control will be needed).

So, we’re not there yet. But we’ll get there and maybe reflective/transreflective technologies like these will be part of the solution.

What other candidates are there for a low energy-use pixels everywhere future? Add your comments below.



Related posts:

  1. Pixels, pixels, on the wall…
  2. Digital light… showing the way
  3. Energy Use for Large Outdoor Displays Improving
:, , , , , ,

Leave a Reply