Characteristic TNT Differential Reflection Spectra on Common Substrates
This work describes an explosives detection technique utilizing differential reflectograms to identify the characteristic differential reflectivity spectra of TNT. It accomplishes this by measuring the characteristic differential reflectivity (essentially the absorption) of a specimen while being exposed to high intensity UV light. The differential reflectometer is able to achieve high sensitivity because it measures two adjacent parts of the specimen simultaneously. As a result, a normalized difference in reflectivity is recorded and trace quantities of materials become apparent. It is shown that traces of 2,4,6-trinitrotoluene (TNT) display a characteristic shoulder in differential reflectograms in the optical spectral range of 380-420 nm. This characteristic shoulder is not obscured if TNT is deposited on fabrics, luggage, office supplies, human skin, metallic foil, or various papers and plastics. These substrates were tested because they are typical materials seen by airport terminal security systems. Wood, particle board, and plywood showed characteristic features in the same spectral range as TNT. They are, however, considerably weaker than that for TNT. It is not anticipated that these substrates will conceal the TNT signature to such a degree as to camouflage it from detection or cause a false positive. In short, differential reflectometry is shown to be a contactless, portable, and inexpensive optical detection system which detects TNT (and other explosives) on a large number of common substrates and can therefore be used where a high degree of security is needed, such as in airport security scanning devices.