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.
One of the things that distinguishes the researchers' new system
from earlier high-speed imaging systems is that it can capture light
'scattering' below the surfaces of solid objects, such as the tomato
depicted here.
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.
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 math
After 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."
"It's very interesting work. I am very impressed," says Nils
Abramson, a professor of applied holography at Sweden's Royal Institute
of Technology. In the late 1970s, Abramson pioneered a technique called
light-in-flight holography, which ultimately proved able to capture
images of light waves at a rate of 100 billion frames per second.
But as Abramson points out, his technique requires so-called coherent
light, meaning that the troughs and crests of the light waves that
produce the image have to line up with each other. "If you happen to
destroy the coherence when the light is passing through different
objects, then it doesn't work," Abramson says. "So I think it's much
better if you can use ordinary light, which Ramesh does."
Indeed, Velten says, "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."
"I'm surprised that the method I've been using has not been more
popular," Abramson adds. "I've felt rather alone. I'm very glad that
someone else is doing something similar. Because I think there are many
interesting things to find when you can do this sort of study of the
light itself."
From sciencedaily
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