Recent developments in reducing the cost and mass of hyperspectral sensors have enabled more widespread use for short range compositional imaging applications. HSI in the long wave infrared (LWIR) is of interest because it is sensitive to spectral phenomena not accessible to other wavelengths, and because of its inherent thermal imaging capability. At Spectrum Photonics we have pursued compact LWIR hyperspectral sensors both using microbolometer arrays and compact cryogenic detector cameras. Our microbolometer-based systems are principally aimed at short standoff applications, currently weigh 10-15 lbs and feature sizes approximately 20x20x10 cm, with sensitivity in the 1-2 microflick range, and imaging times on the order of 30 seconds. Our systems that employ cryogenic arrays are aimed at medium standoff ranges such as nadir looking missions from UAVs. Recent work with cooled sensors has focused on Strained Layer Superlattice (SLS) technology, as these detector arrays are undergoing rapid improvements, and have some advantages compared to HgCdTe detectors in terms of calibration stability. These sensors include full on-board processing sensor stabilization so are somewhat larger than the microbolometer systems, but could be adapted to much more compact form factors. We will review our recent progress in both these application areas.
Fourier transform spectroscopy is a widely employed method for obtaining visible and infrared spectral imagery, with
applications ranging from the desktop to remote sensing. Most fielded Fourier transform spectrometers (FTS) employ the
Michelson interferometer and measure the spectrum encoded in a time-varying signal imposed by the source spectrum
interaction with the interferometer. A second, less widely used form of FTS is the spatial FTS, where the spectrum is
encoded in a pattern sampled by a detector array.
Recently we described using a Fabry-Perot interferometer, with a deliberately wedged gap geometry and engineered
surface reflectivities, to produce an imaging spatial FTS. The Fabry-Perot interferometer can be much lighter and more
compact than a conventional interferometer configuration, thereby making them suitable for portable and handheld
applications. This approach is suitable for use over many spectral regimes of interest, including visible and infrared
regions. Primary efforts to date have focused on development and demonstration of long wave infrared (LWIR) spectral
The LWIR version of the miniaturized Fabry-Perot has been shown to be effective for various applications including
spectral imaging-based chemical detection. The compact LWIR spectral imager employs uncooled optics and a
microbolometer camera; a handheld version is envisioned for future development. Recent advancements associated with
the spatial Fourier Transform imaging spectrometer system are described.
Chemical micro-imaging is a powerful tool for the detection and identification of analytes of interest against a
cluttered background (i.e. trace explosive particles left behind in a fingerprint). While a variety of groups have
demonstrated the efficacy of Raman instruments for these applications, point by point or line by line acquisition of a
targeted field of view (FOV) is a time consuming process if it is to be accomplished with useful spatial resolutions.
Spectrum Photonics has developed and demonstrated a prototype system utilizing long wave infrared hyperspectral
microscopy, which enables the simultaneous collection of LWIR reflectance spectra from 8-14 μm in a 30 x 7 mm
FOV with 30 μm spatial resolution in 30 s. An overview of the uncooled Sagnac-based LWIR HSM system will be
given, emphasizing the benefits of this approach. Laboratory Hyperspectral data collected from custom mixtures
and fingerprint residues is shown, focusing on the ability of the LWIR chemical micro-imager to detect chemicals of
interest out of a cluttered background.