The challenge of changing signatures in infrared stand-off detection of trace explosives

We report our preliminary results on numerical modeling of IR back-scattering (reflectivity) and absorption (photothermal) IR signatures of micron-size (5-20 μm) particles. We use the Mie scattering theory which is an exact solution of the scattering problem for spherical particles of arbitrary size. In this paper, we approximate the particles as spheres with an equivalent volume. While we expect particle shape to influence IR spectra (albeit to a lesser extent), in this paper, we restrict ourselves to the effect of size (i.e. particle diameter) only. We also study the effect of air, solvent and other additive inclusions on the IR spectra. Finally, we address the effect of particle surface roughness. We show that all these parameters (size, inclusions, roughness) affect the scattering process which results in distortions to the IR spectra as compared to library values for bulk material. The effect of substrate on the IR spectra is not studied due to the limitations of the Mie scattering theory which was developed for isolated particles only. In addition to particle-related effects on IR spectra, the presence of substrate will have additional effects as well and this was studied previously by other research groups. We expect that the results of this study will help improve the performance of various detection algorithms by accounting for changing IR spectra.