Tuesday, November 1, 2011

Micro mirrors for 3D and color displays

I've been looking at some of Wallen Mphepö's work, after reading an article in the Economist. According to the article, Mphepö is a researcher at Taiwan's National Chiao Tung University. The research utilizes very tiny mirrors, and there are two applications for autostereoscopic 3D and passive color displays. By tiny, we are talking sub-pixel: many micro mirrors are packed into the space occupied by a single pixel extent.

In the first work on autostereoscopy, a projector is used. Now, usually, a projector projects an image onto a white, almost-lambertian screen. In this traditional scenario, the light from the projector would be uniformly reflected everywhere. But now imagine a "screen" made of tiny reflective micro prisms. Since the "screen" is a bank of tiny prisms, the light gets reflected non-uniformly, into specific directions. In particular, when a light ray from the projector, which corresponds to a single pixel, hits the "screen", it is reflected by a set of micro prisms that correspond to that pixel. If the projected image corresponds to concatenated left and right stereo images, you can specify the slope of the prisms so that the viewing space is filled up with places for an audience to see 3D without glasses. A nice diagram and explanation can be found here.

The second project follows a trend to create passive, reflection-based displays. This is in tune with the E-ink work and also the Mirasol displays from Qualcomm, both of which are easier to view in ambient light. Like the Mirasol displays, the micro mirror work is based on the same thin-film interference effects found in biology. Each micro mirror has two layers, each of which reflect some amount of the incoming light. The reflected light therefore has two components: light reflected from the first layer straightaway and light that is first refracted through the first layer and then reflected from the second layer. These two components are out of phase and interfere, giving rise to colors. The mirrors are controlled by MEMS devices, much like DMD chips, and so can change the incident viewing angle, which changes the perceived color. A nice overview is in the article.