Abstract

A Raman multispectral imaging technique is presented, which can be used for stand-off detection of single explosives particles. A frequency-doubled Nd:YAG laser operating at $10\mathrm{Hz}$ illuminates the surface under investigation. The backscattered Raman signal is collected by a receiver subsystem consisting of a $150\mathrm{mm}$ Schmidt–Cassegrain telescope, a laser line edge filter, a liquid-crystal tunable filter, and a gated intensified charge-coupled device (ICCD) detector. A sequence of images is recorded by the ICCD, where, for each recording, a different wavelength is selected by the tunable filter. By this, a Raman spectrum is recorded for each pixel, which makes it possible to detect even single particles when compared to known spectra for possible explosives. The comparison is made using correlation and least-square fitting. The system is relatively insensitive to environment and light variations. Multispectral Raman images of sulfur, ammonium nitrate, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene were acquired at a stand-off distance of $10\mathrm{m}$. Detection of sulfur particles was done at a distance of $10\mathrm{m}$.

© 2011 Optical Society of America

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