Realization of a Low-Cost, Spectroscopic, Mid-Infrared Imaging Platform for Explosives Detection and Thermal Time-Harmonic Imaging

Mid-infrared (MIR) spectroscopic imaging platforms have been an integral part in the development of stand-off based methods for detecting substances, especially organics. This is due to the unique and strong absorption characteristics of many materials in this region. However, MIR stand-off systems have remained bulky, expensive, power-hungry, and often require liquid nitrogen cooling to operate; this has made them unable to be used in many applications for instance explosives detection at the checkpoint (e.g. airports). I will show in this presentation that the detectors in these systems, often liquid nitrogen cooled mercury-cadmium-telluride (MCT) arrays (>$5,000) detectors, can be replaced with extremely inexpensive, commercially available VOx microbolometers arrays ($250) as long as one does not require real-time imaging. While restricting the scope of applications, the integration time is still sub-second and lends itself well to many process control and detection environments. However, due to the extremely low-cost of these detectors brand new applications become economically feasible and I will touch upon using these detectors for distributed imaging and optimal sensor selection paradigms.

The second part of this presentation will outline the development of thermal time-harmonic (phasor) imaging. The “laser-flash method” is one method commonly used for measuring thermal properties of materials. This method works by utilizing a laser to excite a material’s surface and detecting the propagated thermal signal on the back of the material. From this thermal signal one can deduce the diffusivity and conductivity. This method, however, is limited to extremely thin samples (1-4mm) and does not lend itself to imaging applications as one has to excite the front-side of the target and view the back. In my analysis, I investigate exciting the sample from the front with a time-harmonic laser signal and viewing the heat signal from the front side showing that, under certain conditions, that thermal properties can also be extracted. The idea of this method setting the foundation for thermal-based diffuse tomography similar to diffuse optical tomography, a well developed field, is then explored.