MIT researchers have created a new imaging system that can acquire visual data
at a rate of one trillion exposures per second. That’s fast enough to produce a
slow-motion video of a burst of light traveling the length of a one-liter
bottle, bouncing off the cap and reflecting back to the bottle’s bottom.
Media Lab postdoc Andreas Velten, one of the system’s developers, calls
it the “ultimate” in slow motion: “There’s nothing in the universe that looks
fast to this camera,” he says.
The system relies on a recent technology called a streak camera, deployed in a
totally unexpected way. The aperture of the streak camera is a narrow slit.
Particles of light — photons — enter the camera through the slit and pass
through an electric field that deflects them in a direction perpendicular to the
slit. Because the electric field is changing very rapidly, it deflects
late-arriving photons more than it does early-arriving ones.
The image
produced by the camera is thus two-dimensional, but only one of the dimensions —
the one corresponding to the direction of the slit — is spatial. The other
dimension, corresponding to the degree of deflection, is time. The image thus
represents the time of arrival of photons passing through a one-dimensional
slice of space.
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| Ripples of Waves: A time-lapse visualization of the spherical fronts of advancing light reflected by surfaces in the scene. |
The camera was intended for use in experiments where
light passes through or is emitted by a chemical sample. Since chemists are
chiefly interested in the wavelengths of light that a sample absorbs, or in how
the intensity of the emitted light changes over time, the fact that the camera
registers only one spatial dimension is irrelevant.
But it’s a serious
drawback in a video camera. To produce their super-slow-mo videos, Velten, Media
Lab Associate Professor Ramesh Raskar and Moungi Bawendi, the Lester Wolfe
Professor of Chemistry, must perform the same experiment — such as passing a
light pulse through a bottle — over and over, continually repositioning the
streak camera to gradually build up a two-dimensional image. Synchronizing the
camera and the laser that generates the pulse, so that the timing of every
exposure is the same, requires a battery of sophisticated optical equipment and
exquisite mechanical control. It takes only a nanosecond — a billionth of a
second — for light to scatter through a bottle, but it takes about an hour to
collect all the data necessary for the final video. For that reason, Raskar
calls the new system “the world’s slowest fastest camera.”
Doing the
mathAfter an hour, the researchers accumulate hundreds of
thousands of data sets, each of which plots the one-dimensional positions of
photons against their times of arrival. Raskar, Velten and other members of
Raskar’s Camera Culture group at the Media Lab developed algorithms that can
stitch that raw data into a set of sequential two-dimensional images.
The
streak camera and the laser that generates the light pulses — both cutting-edge
devices with a cumulative price tag of $250,000 — were provided by Bawendi, a
pioneer in research on quantum dots: tiny, light-emitting clusters of
semiconductor particles that have potential applications in quantum computing,
video-display technology, biological imaging, solar cells and a host of other
areas.
The trillion-frame-per-second imaging system, which the
researchers have presented both at the Optical Society's Computational Optical
Sensing and Imaging conference and at Siggraph, is a spinoff of another Camera
Culture project, a camera that can see around corners. That camera works by
bouncing light off a reflective surface — say, the wall opposite a doorway — and
measuring the time it takes different photons to return. But while both systems
use ultrashort bursts of laser light and streak cameras, the arrangement of
their other optical components and their reconstruction algorithms are tailored
to their disparate tasks.
Because the ultrafast-imaging system requires
multiple passes to produce its videos, it can’t record events that aren’t
exactly repeatable. Any practical applications will probably involve cases where
the way in which light scatters — or bounces around as it strikes different
surfaces — is itself a source of useful information. Those cases may, however,
include analyses of the physical structure of both manufactured materials and
biological tissues — “like ultrasound with light,” as Raskar puts it.
As
a longtime camera researcher, Raskar also sees a potential application in the
development of better camera flashes. “An ultimate dream is, how do you create
studio-like lighting from a compact flash? How can I take a portable camera that
has a tiny flash and create the illusion that I have all these umbrellas, and
sport lights, and so on?” asks Raskar, the NEC Career Development Associate
Professor of Media Arts and Sciences. “With our ultrafast imaging, we can
actually analyze how the photons are traveling through the world. And then we
can recreate a new photo by creating the illusion that the photons started
somewhere else.”
Velten adds, “As photons bounce around in the scene or
inside objects, they lose coherence. Only an incoherent detection method like
ours can see those photons.” And those photons, Velten says, could let
researchers “learn more about the material properties of the objects, about what
is under their surface and about the layout of the scene. Because we can see
those photons, we could use them to look inside objects — for example, for
medical imaging, or to identify materials.”
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