For anyone who has spent a significant amount of time in an urban setting, the scene of a bomb squad responding to a report of a suspicious package might be all too familiar. But just how is it determined that the lunchbox left under the park bench is just leftovers - or a lethal weapon? The most common way is spectroscopy.
"Spectroscopy is good, but it only gets you so far," says Eric Houser, a program manager in the Explosives Division of the Department of Homeland Security's Science and Technology Directorate (S&T). The wave of the future may lie in a technology called optimal dynamic detection (ODD), which overcomes many of spectroscopy's limitations.
Spectroscopy uses the color spectrum to shed light on a package's makeup. Since it uses visible light only, spectroscopy can't see through a lunchbox, but what it can see is microscopic residue on the box's outer layer, which can provide telltale clues about what's inside.
Using spectroscopy, bomb squad personnel will beam a laser at the package, then compare the reflected "light signature"-an optical fingerprint-against a library of known signatures for chemical compounds, such as nitroglycerin. If there is nitro inside, chances are that some of it will be found in the package's residue.
This method presents two problems. First, there's distance. Many threat detection methods require either the person or the detector to be physically near the bomb, making spectroscopy extremely dangerous.
Second, approaches like spectroscopy, which rely on reflected light, often are not sensitive or selective enough, especially in the real world where chemical signatures may overlap or be contaminated. Think of light signatures as fingerprints. Capturing a fingerprint from a clean surface is not especially difficult. But in real life, surfaces are anything but clean, and dust, grease, or even ink stains can cause a backpack or lunch pail to bear small deposits of several different chemicals, each with a unique optical fingerprint. To minimize false alarms, a detector must be both sensitive and selective.
Source: Forensic Report
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