Comment: Qualcomms MEMS display is smart but is it bright enough?

Comment: Qualcomms MEMS display is smart but is it bright enough?.

Peter Clarke

11/24/2009 11:32 AM EST

LONDON — Last week I was able to look at examples of the Qualcomm MEMS display, known as Mirasol.

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.
Sticking point
But for me there is one sticking point that could inhibit take up of the Mirasol display. It is more to do with human behavior and preferences than technology, and the Pike Research report does touch upon it.

“Pike Research estimates that reflective display technology could capture greater than 20 percent market share in mini-display sizes between 2.5 and 10 inches over the next five years with an increasing percentage over time if the operational characteristics of the displays are perceived, first by OEMs and then by potential customers, as equivalent to an LCD,” the report states — but the italics are mine.

My experience is that the Mirasol display is nice to look at but nowhere near as impactful as a backlit liquid crystal display. Qualcomm claims Mirasol displays offer reflectivity on the order of 50 percent and contrast ratios greater than 8:1. It, of course, consumes far less power than a backlit LCD, but the eye does not see that.

So a big challenge for Qualcomm is that many consumer buying decisions are made not in the head as the result of trading off power consumption versus readability metrics but in the heart after seeing a glossy presentation slide across a display. Except for the most ardent of eco-warriors, most consumers expect the battery life of their toys and gadgets to be significantly improved WITHOUT reducing the brightness and attractiveness of the display. And liquid crystal displays have set the bar high.

And the designers and creators of mobile phone and netbook computer specifications know this and, on many occasions, will select the display technology that gives them the best chance of making that heart-led sale. On the other hand, if you are an eco-warrior your heart will seek out the low-power solution.

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