Advances in Hyperspectral Long Wave Infrared Imaging: System Modeling and Applications
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Abstract: This industrial PhD project represents a collaboration between Newtec Engineering A/S and NanoSYD at the University of Southern Denmark, with financial support from the Innovation Fund Denmark. The project focuses on the development and application of a hyperspectral imaging system that combines a microbolometer-based thermal camera and a low-order scanning Fabry-Pérot interferometer (SFPI). The system operates in the long-wave infrared (LWIR) spectral range, ranging from 8 µm to 15 µm wavelengths. The SFPI consists of two partially transmitting, mutually parallel mirrors placed only micrometers apart to form an optical cavity. The precise cavity length determines the distribution of transmitted wavelengths, and by capturing images at different cavity lengths, hyperspectral data cubes can be generated.
A substantial portion of this thesis is dedicated to describing and modeling the SFPI, which gives the imaging system its hyperspectral capabilities. Unlike conventional systems that provide spectral information in terms of wavelengths or wavenumbers, the SFPI acquires an interferogram with the spectral axis expressed in terms of the cavity length. This presents unique challenges in data processing and interpretation of results.
A key challenge is understanding how incident radiation transforms into recorded interferograms. An even more complex issue arises in the inverse process: converting the interferograms to the wavelength domain. This inverse problem is inherently underdetermined, which makes the reconstruction process non-trivial. Several potential solutions to this problem are explored in detail.
Finally, this thesis highlights several potential applications of the proposed technology, including classifying commercial plastics containing carbon black, mineral analysis, and detecting hydrocarbon fuel signatures through flame imaging. These applications demonstrate the versatility and promise of the hyperspectral imaging system within various scientific and industrial contexts.