Using lasers to look through container walls from a distance and detect explosives
By Darren Quick
February 27, 2012
Contrary to what some cartoons might have led you to believe, explosives aren't always emblazoned with the letters TNT making them easy to identify. Some people will actually go to the trouble of disguising explosives by placing them in nondescript containers. This means that to analyze them, some close quarter examination that puts someone at risk is usually required. Researchers at the Vienna University of Technology (TU Vienna) have developed a detection method using laser light that allows explosives to be detected not only from distances of over 100 meters (328 ft), but works even when the explosives are hidden inside an opaque container.
Like a similar system developed at Michigan State University, the technology developed at TU Vienna relies on Raman spectroscopy, in which molecules can be identified by exposing them to laser light and analyzing the color spectrum of the resulting scattered light bouncing off individual molecules.
Going the distanceAccording to TU Vienna's Professor Bernhard Lendl, because only a few of the hundreds of millions of photons hitting a sample trigger a Raman scattering process and these scattered particles of light are scattered uniformly in all directions, only a tiny fraction travel back to the detector. This meant that any sample to be analyzed had to be placed very close to the laser and the light detector.
By using a highly efficient telescope and extremely sensitive light detectors, the TU Vienna team has been able to extract as much information as possible from a very weak signal, enabling samples to now be analyzed from a distance. In tests on frequently used explosives, including TNT, ANFO and RDX, the team says their new method was able to reliably detect substances even at distances of over 100 meters (328 ft).
X-ray visionJust as impressive - and potentially life-saving - is the ability to use Raman spectroscopy to see through the walls of a nontransparent container. Although the laser beam is scattered by the container walls, a small portion of the beam will penetrate the container where it can trigger the Raman scattering process. The tricky part is then distinguishing the light signal of the container from that of the sample inside.
The researchers solve this problem by using a simple geometric trick. Because the laser beam hits the container on a small, well-defined spot, the light signal emitted from the container stems from a very small region. In contrast, the light that enters the container is scattered into a much larger region. Aiming the detector telescope just a few centimeters away from the point at which the laser hits the container ensures that the signal being measured is coming from the contents rather than the container itself.
Although the system has obvious applications for the detection of explosives for airport security and the military, the TU Vienna researchers say their method could be used to identify substances in cases where it is hard to get close to the subject - studying icebergs or for geological analysis on a Mars mission, for example.
Source: TU Vienna