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We have developed a multiwavelength Scanning Standoff Time-Resolved Raman spectroscopy (S2TR2S)
system to detect minerals and chemicals from a long distance (10-100 m) over a large area. The
multiwavelength SSTRRS system uses 532 and 785 nm pulsed lasers and two separate 5x beam expanders to
excite spontaneous Raman spectra of the chemicals with 10 mm diameter laser beams. The VIS-NIR system
employs a common Meade telescope (F/10, aperture 20.3 cm). In order to improve detection efficiency, the
light collected by the telescope is directly coupled into two f/1.8 transmission spectrograph covering the VIS
and NIR spectral regions by changing the volume Holographic Raman gratings for 532 and 785 nm laser lines,
respectively. The spectrograph is equipped with a gated intensified CCD camera and edge filters are used to
reject the reflected and Rayleigh scattered laser light. The S2TR2S system is operated using pan-tilt pointing
capability for precise measurements of selected distant points (under computer control). By making standoff
Raman measurements over a predefined grid array, a large area can be sampled and Raman composition maps
are constructed off the distant target area. This mapping capability of the instruments has been used to identify a
wide variety of minerals and hazardous chemicals from their Raman fingerprints and Raman images. The use of
pulsed laser and gated detection allow the measurement of the Raman spectra of minerals with minimum
interference from photoluminescence from transition metal ions and rare-earths ions, and ambient light.
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