Friday, July 28, 2006

Light-Sensing Fibers for Transparent Cameras

Semiconducting fiber webs could transform the way we make images.

By Kevin Bullis

MIT materials scientist Yoel Fink poses with a sphere of light-sensitive fibers. A system using these fibers could lead to transparent cameras that need no lenses. (Courtesy of Greg Hren Photography/Research Laboratory of Electronics, MIT)

You may be about to see the world in a whole new way. MIT researchers, reporting in this month's issue of Nature Materials, have demonstrated that nearly transparent webs made up of novel semiconducting fibers could replace lenses and sensors in cameras, and, among other things, lead to uniforms or automobile exteriors that give people a continuous view of their surroundings.

The fibers are made of a semiconducting glass core, lined along its full length by wires that act as positive and negative electrodes, and surrounded by a transparent polymer (see link to images below). When light hits the photosensitive core, an electrical current in the fiber changes, registering the hit.

[Click here for images of this light-sensing fiber.]

A mesh of these fibers can then be used to identify the location of the light on a surface. In the Nature Materials paper, the researchers, led by materials scientist Yoel Fink and physicist John Joannopoulos, demonstrate that the fibers, in addition to locating a point of light, can be used to determine the direction from which a light beam comes and can also sense light from a scene to form an image. "Here's a structure that's close to being invisible -- but can see," says one of the team members, Ayman Abouraddy, a research scientist at MIT.

For direction sensing, the researchers formed a grid of fibers into a sphere. A light beam from a flashlight first hits one side of the sphere and the grid registers the location. The light then passes through the sphere and out the other side, where it is detected again. Then an integrated circuit compares the entrance and exit points to calculate the path of the light.

Using a similar technique, the researchers were also able to record a scene, not just points of light. Light from a scene passes through first one, then another of two flat, parallel fiber grids, which register the intensity of light from the scene. However, because there's no lens, which in a camera focuses light from a given plane onto a light detector, the grids receive a blurry image. To compensate for the lack of a lens, the researchers wrote algorithms that compare slight differences between the images recorded by the two fiber grids. These differences allow them to trace the light back to its source -- and mathematically reconstruct an in-focus image. Because this "focusing" happens after the data has been recorded, it's also possible to refocus on various objects in a scene after a picture has been taken.