Speed of Light Lingers in Face of New Camera

http://www.nytimes.com/2011/12/13/science/speed-of-light-lingers-in-face-of-mit-media-lab-camera.html?_r=3&pagewanted=all1

By JOHN MARKOFF

More than 70 years ago, the M.I.T. electrical engineer Harold (Doc) Edgerton began using strobe lights to create remarkable photographs: a bullet stopped in flight as it pierced an apple, the coronet created by the splash of a drop of milk.
Enlarge This Image
Di Wu and Andreas Velten, MIT Media Lab

SLOW DOWN M.I.T.'s camera captures light particles seemingly in motion by using repeated exposures, creating a “movie” of a nanosecond-long event.

Now scientists at M.I.T.’s Media Lab are using an ultrafast imaging system to capture light itself as it passes through liquids and objects, in effect snapping a picture in less than two-trillionths of a second.

The project began as a whimsical effort to literally see around corners — by capturing reflected light and then computing the paths of the returning light, thereby building images coming from rooms that would otherwise not be directly visible.

“When I said I wanted to build a camera that looks around corners, my colleagues said, ‘Pick something that is more safe for your tenure,’ ” said Ramesh Raskar, an associate professor of media arts and sciences at the Media Lab. “Now I have tenure, so I can say this is not so crazy.”

Dr. Raskar enlisted colleagues from the chemistry department to modify a “streak tube,” a supersensitive piece of laboratory equipment that scans and captures light. Streak tubes are generally used to intensify streams of photons into streams of electrons. They are fast enough to record the progress of packets of laser light fired repeatedly into a bottle filled with a cloudy fluid.

The instrument is normally used to measure laboratory phenomena that take place in an ultra-short timeframe. Typically, it offers researchers information on intensity, position and wavelength in the form of data, not an image.

By modifying the equipment, the researchers were able to create slow-motion movies, showing what appears to be a bullet of light that moves from one end of the bottle to the other. The pulses of laser light enter through the bottom and travel to the cap, generating a conical shock wave that bounces off the sides of the bottle as the bullet passes.

The streak tube scans and captures light in much the same way a cathode ray tube emits and paints an image on the inside of a computer monitor. Each horizontal line is exposed for just 1.71 picoseconds, or trillionths of a second, Dr. Raskar said — enough time for the laser beam to travel less than half a millimeter through the fluid inside the bottle.

To create a movie of the event, the researchers record about 500 frames in just under a nanosecond, or a billionth of a second. Because each individual movie has a very narrow field of view, they repeat the process a number of times, scanning it vertically to build a complete scene that shows the beam moving from one end of the bottle, bouncing off the cap and then scattering back through the fluid. If a bullet were tracked in the same fashion moving through the same fluid, the resulting movie would last three years.

“You can think of it as slow motion,” Andreas Velten, a postdoctoral researcher who is a member of the design team, said during a recent technical presentation. “It is so much slow motion you can see the light itself move. This is the speed of light: there’s nothing in the universe that moves faster.”

Dr. Raskar says the technology has a variety of promising commercial applications. Last year, for example, one of his graduate students, Jaewon Kim, published a thesis envisioning portable CAT-scanning devices.

Dr. Raskar said he could also envision smartphone software that would capture and interpret reflections from, say, fruit. “Imagine if you have this in your phone about 10 years from now,” he said. “You will be able to go to your supermarket and tell if your fruit is ripe.”

Until now, picosecond speeds have largely been the province of an elite group of scientists clustered at the nation’s weapons laboratories.

At Lawrence Livermore National Laboratory, Gary Jones is an optical physicist who builds ultrafast imaging systems that help characterize the first microseconds of events like laser fusion and nuclear explosions. “To get a two-dimensional image within a picosecond means you have to have a lot of electronics moving really fast,” he said.

For Dr. Raskar — who optimistically calls the project “femto photography,” using the term for quadrillionths of a second — it is about more than just engineering or science. “We were inspired by looking at the world in a unique way just because we could,” he said.

The system allows the naked eye to see information that has until now been rendered as data and charts. The proper analogy is to the way astronomers use instruments like radiotelescopes to create images with “fake” colors to see things in new ways — or to the original inspiration of Eadweard Muybridge, the 19th-century British photographer who achieved a new understanding of a horse’s gait by creating a camera array with electromagnetic shutters set off by tripwires.

“We’re still trying to get our heads around what this means,” Dr. Raskar said, “because no one has been able to see the world in this way before.”