I saw small monochrome displays in cell phones and a 5.3-inch full-color unit, posing as an e-reader display. It is a nice looking and technically elegant piece of work. It is a thin, bi-stable, reflective display that therefore theoretically only needs power for changing the image. And it is available in full color with spatial dithering of sub-pixels to provide grey-scale.

A 5.3-inch diagonal full-color Mirasol display

It uses ambient light rather than a backlight, which is its main source of its power saving. Liquid crystal displays after all use a powerful backlight and electro-optic shutters to prevent more or less of the light getting through the panel. The use of ambient light is good outdoors, where LCDs perform badly, but does mean that in low-light conditions the display is hard to read, just like it is hard to read from paper when the lights are out. Qualcomm's solution is an occasional-use front light.

So what is not to like about this "green" full-color display?

Such a display obviously has great potential in mobile applications where power consumption and battery life are key issues. Depending on the application, a bi-stable non-volatile display can more than halve power consumption in applications such as a smart phone or an e-reader.

However, Qualcomm documentation illustrates a Mirasol scheme that uses spatial dithering to achieve only 3-bits of grey scale or 512 colors. It discusses the addition of temporal dithering to extend the grey-scale to 6-bits per primary color pixel. This gives 64 levels for each of the red, green and blue sub-pixels or a total of 256-k colors. But such a scheme would require the pixels to be continually moving and therefore consuming power.

Qualcomm MEMS Technologies Inc. commissioned a report by Pike Research LLC (Boulder, Colo.) which found that a mobile device using an IMOD (interferometric modulator display), such as Mirasol, would consume 33.7 percent less energy compared with a similar mobile device that uses a conventional LCD display. However, this was based on a 2.4-inch to 2.8-inch diagonal display and it is not clear whether it took into account any temporal dithering of the display. The report goes on to give extensive "green" and sustainable characteristics to the IMOD display.