Remote sensing in the thermal infrared (TIR) band (3-14 μm) is an increasingly attractive tool for environmental sciences and security due to its chemical specificity and the large spectral superposition with the atmospheric transparency window. Currently, the technique is largely dominated by dispersive-type hyperspectral imagers, which usually require expensive, cumbersome and cooled quantum detectors setups to compensate for their low optical throughput. Here, we present a compact and low-cost hyperspectral camera based on the Fourier-transform approach. Its main element is a common-path birefringent interferometer made of TIR-transparent crystal calomel (Hg2Cl2). We characterize it with both coherent and incoherent sources in the TIR, determining high optical throughput, adjustable spectral resolution up to 4.5 cm-1, interferometric contrast higher than 90% even for incoherent radiation and a robust and long-term interferometric stability. By coupling the interferometer to an uncooled microbolometer detector with 640x480 pixels, we demonstrate hyperspectral imaging in the 8-14 μm spectral range for transmission and emission remote measurements.
We introduce a compact hyperspectral camera based on the time-domain Fourier-transform approach, equipped with an ultrastable birefringent interferometer. The time-domain approach enables hyperspectral imaging with shorter acquisition times and higher spectral accuracy compared to standard dispersive optics. We provide experimental proofs of the camera capability by performing remote-sensing measurements in the visible and near-infrared range. Recently we extended the spectral range to the thermal infrared, where vibrational transitions associated with chemical bonds have their absorption. Due to its compactness, lightweight and extreme stability even in harsh environments, the camera is a unique enabling technology for remote unambiguous chemical identification.
The extension of supercontinuum (SC) sources into the mid-infrared, via the use of uoride and chalcogenide optical fibers, potentially offers the high radiance of a laser combined with spectral coverage far exceeding that of typical tunable lasers and comparable to traditional black-body emitters. Together with advances in mid-IR imaging detectors and novel tunable filter designs, such supercontinua hold considerable potential as sources of illumination for spectrally-resolved microscopy targeting applications such as rapid histological screening. The ability to rapidly and arbitrarily select particular wavelengths of interest from a broad emission spectrum, covering a wide range of biologically relevant targets, lends itself to image acquisition only at key relevant wavelengths leading to more manageable datasets. However, in addition to offering new imaging modalities, SC sources also present a range of challenges to successful integration with typical spectral microscopy instrumentation, including appropriate utilisation of their high spatial coherence. In this paper the application of SC sources to spectrally-resolved microscopy in the mid-IR is discussed and systems-integration considerations specific to these sources highlighted. Preliminary results in the 3-5μm region, obtained within the European FP7 project MINERVA, are also presented here.
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