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
imagers.
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.
The University of Hawaii has developed a concept to ruggedize an existing thermal infrared hyperspectral system for use in the NASA SIERRA UAV. The Hawaii Institute of Geophysics and Planetology has developed a suite of instruments that acquire high spectral resolution thermal infrared image data with low mass and power consumption by combining microbolometers with stationary interferometers, allowing us to achieve hyperspectral resolution (20 wavenumbers between 8 and 14 micrometers), with signal to noise ratios as high as 1500:1. Several similar instruments have been developed and flown by our research group. One recent iteration, developed under NASA EPSCoR funding and designed for inclusion on a microsatellite (Thermal Hyperspectral Imager; THI), has a mass of 11 kg. Making THI ready for deployment on the SIERRA will involve incorporating improvements made in building nine thermal interferometric hyperspectral systems for commercial and government sponsors as part of HIGP’s wider program. This includes: a) hardening the system for operation in the SIERRA environment, b) compact design for the calibration system, c) reconfiguring software for autonomous operation, d) incorporating HIGP-developed detectors with increased responsiveness at the 8 micron end of the TIR range, and e) an improved interferometer to increase SNR for imaging at the SIERRA’s air speed. UAVs provide a unique platform for science investigations that the proposed instrument, UAVTHI, will be well placed to facilitate (e.g. very high temporal resolution measurements of temporally dynamic phenomena, such as wildfires and volcanic ash clouds). Its spectral range is suited to measuring gas plumes, including sulfur dioxide and carbon dioxide, which exhibit absorption features in the 8 to 14 micron range.
A prototype long wave infrared Fourier transform spectral imaging system using a wedged Fabry-Perot interferometer
and a microbolometer array was designed and built. The instrument can be used at both short (cm) and long standoff
ranges (infinity focus). Signal to noise ratios are in the several hundred range for 30 C targets. The sensor is compact,
fitting in a volume about 12 x12 x 4 inches.
A new approach for the design and fabrication of a miniaturized Hyperspectral imager is described. A unique and compact instrument has been developed by taking advantage of light propagation within bonded solid blocks of fused silica. The resulting microHSI is a VNIR hyperspectral sensor capable of operating in the 400-1000 nm wavelength range developed, patented, and built by NovaSol. The microHSI spectrograph weighs 12.4 oz from slit input to camera output. The microHSI can accommodate either custom foreoptics or C-mount input lenses to adapt to a wide range of fields-of-view (FOV). The prototype microHSI uses a telecentric F2.8 foreoptic, with 36 mm focal length, to cover a 15 degree FOV. It can resolve 960 spatial pixels, resulting in a 280 μrad IFOV for this particular foreoptics implementation. With a 1 nm/unbinned pixel dispersion, the spectrometer spectral resolution is 3.5 nm. Results of field and laboratory testing of the prototype microHSI are presented and show that the sensor consistently meets technical performance predictions. The prototype microHSI employs a holographic diffraction grating embedded within the optical blocks resulting in a 19% diffraction efficiency. Future units are anticipated to incorporate a blazed grating for improved throughput and SNR. The microHSI concept can be extended to operation over other wavelength regions. Designs are nearing completion for a SWIR version of the device, and a miniaturized LWIR microHSI sensor is currently at the conceptual design stage.
The AHI sensor consists of a long-wave infrared pushbroom hyperspectral imager and a boresighted 3-color visible high resolution CCD linescan camera. The system used a background suppression system to achieve good noise characteristics (less than 1(mu) fl NESR). Work with AHI has shown the utility of the long-wave infrared a variety of applications. The AHI system has been used successfully in the detection of buried land mines using infrared absorption features of disturbed soil. Recently, the AHI has been used to examine the feasibility active and passive hyperspectral imaging under outdoor and laboratory conditions at three ranges. In addition, the AHI was flown over a coral reef ecosystem on the Hawaiian island of Molokai to study fresh water intrusion into coral reef ecosystems. Theoretical calculations have been done propose extensions to the AHI design in order to produce an instrument with a higher signal to noise ratio.
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