IEEE Spectrum July, 2007 - 40

TErAhErTz ANd ThE SpECTrAl fiNgErpriNT

HERTZ

TERAHERTZ

Microwaves

Infrared Visible Ultraviolet

X-rays

Gamma rays

crystal-such as zinc telluride-will cause the crystal's index camera's speed is the time it takes the computer to digitize the
of refraction to change. The result is that the polarization of data from the detectors and perform the needed calculations. The
infrared light passing through the crystal will rotate. Place a resolution of a reverse-transformed image comes from the numpolarization filter between the crystal and a camera chip so ber and arrangement of spatial frequencies represented in the
that it blocks out any infrared light that hasn't been rotated transform. Imagine trying to represent the picture of the footand you get an infrared facsimile of the terahertz image. In a ball referee in a Fourier transform, restricting yourself only to
sense, the terahertz radiation has been shifted up the electro- the jersey's frequency and one or two others. When you reverse
magnetic spectrum into the infrared. Such cameras can produce the transformation you might be able to make out the bars in the
pictures in less than one-sixtieth of a second, far quicker than jersey, but the rest of the picture would be a blur. But a transform
raster scanners and fast enough to produce video. Unfortunately, having dozens, or-better yet-hundreds of spatial frequencies
they also erase the spectral information that lets you chemi- represented would reconstruct the picture reasonably well.
cally fingerprint objects. All that information is wrapped up
One of the imagers we're developing is made up of 12 detectors
in the mix of T-ray wavelengths that strike the crystal, but the arranged in a spiral pattern on a 1-meter disk; it can measure
crystal's change in refractive index, which produces the image, 66 spatial frequencies. The imager is good enough to resolve a
is relatively insensitive to color.
2.5-centimeter square of RDX plastic explosive at 50 meters.
In an effort to get both speed and spectroscopy at a reasonable
price, our team at New Jersey Institute of Technology, in Newark, Powerful as TeraherTz imaging is, no imaging or detection
has been developing an imager that, with only a dozen detectors, technology can reliably find every threat to security. Each techcan produce complete images quickly enough for video and at nology has its own strengths and weaknesses. But when several
a resolution comparable to what you'd expect from a kilopixel are used as part of a sensor suite, their collective strengths are
camera chip. The method, called interferometric imaging, relies integrated. As an example, consider how you would screen trucks
on a common mathematical concept used in image processing, at a port or other checkpoint. There are detection systems for
the spatial Fourier transform. According to Fourier theory, any monitoring nuclear radiation, and X-ray systems could penetrate
signal can be broken down into the sum of many sine waves of through the truck's metal walls to probe the contents inside. But
different frequencies, phases, and amplitudes. Though it is less how would the driver himself be screened? The driver may be
intuitive, the same can be said of any image. To get a grasp of carrying concealed weapons, a bomb, a trigger mechanism for
spatial frequency in an image, imagine a photo of an American a weapon hidden in the truck, or other threats that would be
football referee in the traditional black-and-white vertical-stripe overlooked if only the vehicle were screened. You can't X-ray
jersey. The spatial Fourier transform of that image would be the driver, but you can subject him to a suite of safer imagers
dominated by the frequency that matches the jersey's stripes.
including a T-ray imager.
Instead of obtaining the image itself, an interferometric imager
That said, security screening is a more challenging applicaderives a picture's spatial Fourier transform from a measure of tion than it might seem at first. It has to work in real-world
the electric field when a terahertz wave reaches a pair of detec- conditions, has to be small enough to fit in already crowded
tors. Then a computer reverses the transformation to give the spaces such as an airport security checkpoint, and has to have
real image. In an interferometric imager,
a very low rate of false alarms. Otherwise
spatial frequencies are represented by the
it would become a bottleneck in the flow
ABout thE AuthorS
distance between two detectors on an
of people or goods.
JohN F. FEdErIcI and dAlE GAry
array [see illustration, "T-Ray Scanner"].
T-ray imagers are close to meeting all
are professors of physics at New
The imager takes advantage of the fact
those
requirements for short-range scanJersey Institute of technology, in
that when you increase the number of
ners, as long as the additional feature
Newark. roBErt BArAt is a
detectors, the number of spatial freof spectroscopy is excluded. That feaprofessor of chemical engineering
quencies you can measure increases by
ture may take a few more years of work,
and ZoI-hElENI mIchAloPoulu is a
a greater amount. For example, imagine
requiring smaller, brighter T-ray sources
professor of mathematics, also at NJIt.
three detectors spaced unevenly along a
and more sensitive detectors.
line. There are three different distances
As for imagers that can see a suspito ProBE FurthEr
between pairs of detectors-1 to 2, 2 to 3,
cious object in your shopping bag from
the u.S. National research council
and 1 to 3-and thus three spatial fre50 meters, those are more like five years
commissioned a report on the use of
quencies. Now, add a fourth detector faraway at our current rate of progress.
millimeter-wave and terahertz techther out on the line. The pairs are now
Adding the ability to tell whether that
nologies in security. See Assessment
1 to 2, 2 to 3, 3 to 4, 1 to 3, 1 to 4, and 2 to 4,
object is a taco or some TNT is an addiof Millimeter-Wave and Terahertz
or six spatial frequencies. In general, for
tional
five years away, at least. And getTechnology for Detection and
n detectors, the number of spatial freting them small enough to wear like
Identification of Concealed Explosives
quencies is n(n-1)/2.
spectacles? We won't even hazard a
and Weapons (National Academies
The Fourier image is acquired almost
guess. So if you see them advertised in
Press, 2007).
instantly, and the main limit on the
a comic book, trust us: they're fake. n

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Table of Contents for the Digital Edition of IEEE Spectrum July, 2007