Mid-infrared reflection signatures for trace chemicals on surfaces

Results are presented for the detection of trace explosive residues on real-world surfaces using active mid-infrared (MIR) hyperspectral imaging. The target surface is illuminated using miniature, rapidly tunable, external-cavity quantum cascade lasers (EC-QCLs) and the reflected light is imaged using a HgCdTe camera with a spatial resolution of 70 μm. Hypercubes with 128x128 pixels are captured with more than 256 wavelengths that span 7.7 – 11.8 μm. The samples consisted of PETN residues which were applied to keyboard keys at various levels of chemical loading. We estimate a limit of detection of less than 6 ng per pixel for the as-deposited chemical. The explosive residue remains detectable by HSI even after wiping the surface several times using isopropyl alcohol. Simple signature models for solid particles (i.e., Mie scattering) and thin-films account for the many of the spectral features observed in the chemical signatures.