We present initial results on the performance of a compressive sensing setup for Raman imaging spectroscopy for standoff trace explosives detection. Hyperspectral image reconstruction is demonstrated under low signal conditions and successful spatial separation of substances with close lying Raman peaks is shown.
Imaging Raman spectroscopy based on tunable filters is an established technique for detecting single explosives particles
at stand-off distances. However, large light losses are inherent in the design due to sequential imaging at different
wavelengths, leading to effective transmission often well below 1 %.
The use of digital micromirror devices (DMD) and compressive sensing (CS) in imaging Raman explosives trace
detection can improve light throughput and add significant flexibility compared to existing systems. DMDs are based on
mature microelectronics technology, and are compact, scalable, and can be customized for specific tasks, including new
functions not available with current technologies.
This paper has been focusing on investigating how a DMD can be used when applying CS-based imaging Raman
spectroscopy on stand-off explosives trace detection, and evaluating the performance in terms of light throughput, image
reconstruction ability and potential detection limits. This type of setup also gives the possibility to combine imaging
Raman with non-spatially resolved fluorescence suppression techniques, such as Kerr gating.
The system used consists of a 2nd harmonics Nd:YAG laser for sample excitation, collection optics, DMD, CMOScamera
and a spectrometer with ICCD camera for signal gating and detection.
Initial results for compressive sensing imaging Raman shows a stable reconstruction procedure even at low signals and
in presence of interfering background signal. It is also shown to give increased effective light transmission without
sacrificing molecular specificity or area coverage compared to filter based imaging Raman. At the same time it adds
flexibility so the setup can be customized for new functionality.
Standoff Raman imaging systems have shown the ability to detect single explosives particles. However, in many cases, the laser intensities needed restrict the applications where they can be safely used. A new generation imaging Raman system has been developed based on a 355 nm UV laser that, in addition to eye safety, allows discrete and invisible measurements. Non-dangerous exposure levels for the eye are several orders of magnitude higher in UVA than in the visible range that previously has been used. The UV Raman system has been built based on an UV Fabry-Perot Interferometer (UV-FPI) developed by VTT. The design allows for precise selection of Raman shifts in combination with high out-of-band blocking. The stable operation of the UV-FPI module under varying environmental conditions is arranged by controlling the temperature of the module and using a closed loop control of the FPI air gap based on capacitive measurement. The system presented consists of a 3rd harmonics Nd:YAG laser with 1.5 W average output at 1000 Hz, a 200 mm Schmidt-Cassegrain telescope, UV-FPI filter and an ICCD camera for signal gating and detection. The design principal leads to a Raman spectrum in each image pixel. The system is designed for field use and easy manoeuvring. Preliminary results show that in measurements of <60 s on 10 m distance, single AN particles of <300 μm diameter can be identified.