ABSTRACT
Standoff or noncontact techniques are needed for detection and identication of explosives and other materials when physical sampling is impossible, undesired, or not permitted. Reasons to avoid contact-based sampling include a need to remain a safe distance from a potential explosive threat, limited physical access to a surface, a desire for rapid in situ detection of explosives on a surface, or the need to conserve the analyte for additional laboratory methods. Optical techniques are inherently noncontact and nondestructive. A light source may be propagated over large distances before interacting with a sample and then may be propagated over a distance to a detector. The response of the material to optical radiation is very fast, as is the propagation time of the light to and from the sample. Material properties including transmission, reection, and emission may be probed using optical techniques. By varying the wavelength of incident or detected light to record a spectrum, it is possible to dene and measure a characteristic response of many materials for detection, identication, and sometimes quantication.
