Terahertz images containing spectral information ineach pixel are recorded in transmission mode using a fibercoupledtime-domain spectroscopy system. The images are acquiredby mounting a sample holder on an x–y stage, whichis stepped across the beam in the two transverse directions,while the transmitted THz waveform is captured. The materialsunder investigation consist of uncovered and hidden samples ofan explosive (RDX) and simulants (lactose and tartaric acid).Spectral angle mapping is used to identify the materials in theTerahertz images by comparing the spectrum in each pixel witha library of reference spectra for the different materials. Wetest the performance of several spectral characteristics derivedfrom the measured transmission spectra. Robustness is studiedby investigating the Receiver-Operating-Characteristics (ROCs).The ROCs are used to find which of the spectral characteristics ismost robust to different sample preparation conditions, withoutthe need for extensive pre-treatment of the data, such as baselinecorrection. Simple theoretical considerations are used to supportthe experimental results.
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