Remote Explosives Detection (RED) by Infrared Photothermal Imaging
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Remote Explosives Detection (RED) by Infrared Photothermal Imaging Christopher A. Kendziora1, Robert Furstenberg1, Robert M. Jones2, Michael Papantonakis1, Viet Nguyen1, and R. Andrew McGill1 1 U. S. Naval Research Laboratory, Code 6365, 4555 Overlook Ave. SW, Washington, DC 20375, U.S.A. 2 ITT Exelis, 5901 Indian School Road NE, Albuquerque, NM 87110 ABSTRACT RED is a technique we have developed for stand-off detection of trace explosives using infrared (IR) photo-thermal imaging [1,2,3]. RED incorporates compact IR quantum cascade lasers tuned to strong characteristic absorption bands and may be used to illuminate explosives present as particles on a surface. An IR focal plane array is used to image the surface and detect any small increase in the thermal emission upon laser illumination. We have previously demonstrated the technique at several meters to 10’s of meters of stand-off distance indoors and in field tests [4,5], while operating the lasers below the eye-safe intensity limit (100 mWcm2) [6]. Sensitivity to traces of explosives as small as a nanogram has been demonstrated. By varying the incident wavelength slightly, we can readily show selectivity between individual explosives such as TNT and RDX. Using a sequence of lasers at different wavelengths, we increase both sensitivity and selectivity. A complete detection protocol can be performed in a sub-second time domain. More recently, RED has been used to emphasize measurements with cooled detectors in addition to examining the utility of filtering the collected thermal emission signal which is rich in analyte-specific spectroscopic information. A next generation RED system and detection algorithm is being developed to take advantage of these more powerful features. This manuscript will include an overview of the approach and recent experimental results. INTRODUCTION We have developed resonant infrared photo-thermal imaging as a method for non-contact detection of trace residues of chemicals on surfaces. The concept for the method is illustrated in Figure 1. An IR quantum cascade laser is used to illuminate a surface potentially contaminated with trace residues. If the wavelength of the light is resonant with absorption features of the trace residues, the analyte heats slightly. This is observed using a long-wave IR (LWIR) camera which collects images at a video frame rate (>30Hz). Differential imaging (subtracting the image with the laser “off ” from the frame with the laser “on ”) removes the time-independent background and reveals the trace residues. In Figure 1, a “dual analyte” sample has been stenciled with the letters TNT (2, 4, 6-trinitrotolune) and RDX (cyclo-1, 3, 5-trimethylene-2, 4, 6-trinitramine) written in the respective, analytes. A differential image shows that within the laser beam coverage area (1 cm diameter) (red circle in panel b) both of the analytes are heated. The advantages to the RED technique over traditional detection approaches include eye-safe operation, significant standoff distance, sensitivity to small quantities of analy
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