An Ultrafast Imaging System

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. 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. Researchers modified 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, 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.


More information:

http://www.nytimes.com/2011/12/13/science/speed-of-light-lingers-in-face-of-mit-media-lab-camera.html?_r=1