LWIR polarization sensing: investigation of liquids and solids with MoDDIFS

摘要

MoDDIFS (Multi-option Differential Detection and Imaging Fourier Spectrometer) is a DRDC Valcartier technology built around a differential Fourier Transform Infrared (FTIR) spectrometer optimized for optical subtraction in the long wave infrared (LWIR). MoDDIFS is a dual use hyperspectral prototype offering two fore-optics configurations: "long range", specialized for the detection of small quantities of gaseous substances, and "polarization", built to investigate liquids and powders spills. We report and present a preliminary analysis of a series of measurement tests made with the polarization configuration. The tests were performed under indoor and outdoor environments. Different liquid and solid substances were deposited on different types of surfaces. Many liquid targets and some solid materials produce a noticeable linearly-polarized signal, with a more or less characteristic spectral modulation. For the liquids, the behavior of the observed radiance spectrum seems more predictable when the liquid is thick, or when it is deposited at any thickness on non-absorbing and weakly-reflective substrates. The behavior of the radiance spectrum observed becomes more complex when a thin layer of the liquid is deposited on a smooth and strongly-reflective substrate, or on an absorbing substrate. The parameter chosen to analyze the relative amount of polarization is the degree of linear polarization. When its value is noticeable, the polarized hyperspectral radiance measurements bring additional information on both targets and the backgrounds, as compared to standard unpolarized hyperspectral measurements. The tests performed can then help assess the materials for which the detection and the identification will be improved with polarized measurements.