Stand-off detection of trace explosives by infrared photo-thermal spectroscopy

We have developed a technique for stand-off detection of trace explosives using infrared photo-thermal imaging. Compact infrared quantum cascade lasers tuned to strong absorption bands in the explosive traces illuminate a surface of interest while an infrared camera detects the small increase in thermal signal. We have demonstrated the technique at several meters of stand-off distance under laboratory conditions using TNT and RDX traces, while operating the lasers below the eye-safe limit (100 mW/cm²). Sensitivity to explosive traces as small as 1 ng has been demonstrated, using a micro-bolometer array. We show the viability of this approach on a variety of surfaces which transmit, reflect or absorb the infrared laser light. By varying the incident wavelength slightly, we show selectivity between TNT and RDX. Using several laser wavelengths, we increase both sensitivity and selectivity while reducing the false alarm rate. We have developed a prototype system for outdoor testing at longer stand-offs.