Laser sight: NYU's real-life tricorder

Why didn’t anyone think of this before? The basis for imaging this way is called Lorenz-Mie theory. It’s all based on a single equation, which has been well known for a century. But doing the calculations to reconstruct the position, velocity, size, and refractive index of the object is complicated at best. Grier notes that on older machines, the calculations can take a long time.

“Before, you’d have been old before the calculation was done,” he jokes. That all changed with the advent of more powerful PCs and digital imaging technology.

“People had been satisfied for years with ‘back of the envelope’ calculations,” he says. Those older attempts mostly produced snapshots, not precise images.

Developing the new software was the tough part. Grier says the problem is that there were too many moving parts. The laser produces a scatter pattern that changes with time, and you have to track the target’s position, refractive index, and size from the constantly changing image. That’s a total of six parameters (position is three for three dimensions, plus velocity).

At first, Grier’s team tried to standardize two or three of those numbers in the hopes that the others would then come more easily. That didn’t work. “More or less out of resignation we just let all of them vary,” he says. “That worked.”

Joseph Katz, professor of mechanical engineering at Johns Hopkins University in Baltimore, says the big difference in Grier’s technique is the ability to study dynamic systems, instead of having to deal with the narrow depth of field an ordinary microscope offers. (It was one of Katz’s original papers that inspired Grier’s work.)

Because you can tell where small particles are moving, you can track how they diffuse through other substances. One recent experiment – designed by a student – involved eggs.

“The administrator looked at us a little funny when we said we needed to buy eggs,” Cheong says.
The team used organic eggs, commercial eggs, and even ostrich eggs (available at the local Whole Foods supermarket). The result? Certain kinds of particles diffuse less well through organic eggs. They haven’t figured out why that is yet.

Another application is dentistry. Cavities happen because bacteria get on your teeth and live in a film that sticks on the surface. If the film could be analyzed and disrupted, the bacteria would die off.

Cheong notes that because the imager can “see” very small particles, it can show exactly how nutrients affect the bacteria.

“It turns out the bacteria feed on both sugar and starch,” he says. “So it’s worse to eat cake than it is to eat candy.” The dream? Use results from this device to create a chewing gum that would eliminate the need for toothbrushes.

One of the reasons Grier started the work was frustration with conventional imaging. Much of the work in his lab deals with finding out how very small particles interact, as well as how to manipulate them. He wanted, he says, to “see what he was doing.”

A similar laser setup can be used to make “light traps” that use focused laser light (slightly different from a laser beam) to hold individual particles in place. The technique is well known, but Grier’s imaging technology lets experimenters see the results as they happen.

With a setup not unlike a video game, Cheong shows how to move tiny glass beads into place, using just a mouse and looking at the image on a screen. So far, there haven’t been any “serious” applications. But for Cheong, that isn’t a problem.

“We try to encourage play here,” he says, adding that it is where good ideas sometimes come from.

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