Design and first results of a Fourier Transform imaging spectrometer in the 3-5 μm range

Abstract : An imaging spectrometer in the 3-5 μm wavelength range is presented. This wavelength range reveals important information about scenes such as gas or landmine detection, but the amount of light is usually low and signal to noise ratio is a real issue. We selected a Fourier transform (FT) configuration, expecting an advantage in signal to noise ratio in the presence of detector noise. Radiometric and noise models are summarized. A Michelson interferometer with its mirrors replaced by twin mirrors arranged at right angles in a hollow roof was chosen for its nearly straight equidistant fringes localized at infinity. Because in such FT-based spectral imagers, the interferogram is acquired over the whole field of the camera while the scene of interest scans the path difference range, vignetting should be strongly limited while keeping the size of the interferometer as small as possible for manufacturability and cost reasons. The key point for that purpose is to put the entrance pupil of the imaging lens inside the interferometer and to make careful trade-offs between lens F number and angular field of view. The resulting system has a spectral resolution of about 25cm^-1 that fulfils the requirement for most targeted applications. Examples of absorption bands detection are shown.