William R. Johnson

Volume holographic spectral imaging

We report on a volume holographic imaging spectrometer (VHIS) system which allows retrieval of a scenes two-dimensional spatial information as well as its spectral information. This is performed using a transmission volume hologram and a simple rotary scanning mechanism. The system has the advantages of high spectral and spatial resolutions and the potential of single-shot, four-dimensional (3D spatial plus 1D spectral) imaging by recording multiple volume holograms in the same material. Also, due to the transmission diffraction geometry, the system automatically eliminates the stray excitation light from the captured signal. We give theoretical analysis of the performance and experimental demonstration using fluorescent CdSe/ZeS quantum dots. The measured quantum dots spectra agree well with the spectra obtained using a conventional spectrometer.

An all-reflective computed tomography imaging spectrometer

The computed tomographic imaging spectrometer (CTIS) is a passive non-scanning instrument which simultaneously records a scenes spectral content as well as its 2-D spatial. Simultaneously implies a time frame limited only by the frame rate and signal-to-noise of the imaging device. CTIS accomplishes this by feeding incident scene radiation through a computer generated hologram (CGH) in Fourier space. The resulting dispersion pattern is recorded on a conventional pixilated imager and is stored on a local computer for post processing using iterative reconstruction techniques. A virtual 3-D datacube is constructed with one dimension in terms of energy weights for each wavelength band. CTIS is ideal for observing rapidly varying targets and has found use in military, bio-medical and astronomical applications. For the first time we have built an entirely reflective design based on the popular Offner reflector using a computer generated hologram formed on a convex mirror surface. Furthermore, a micro electro-mechanical system (MEMS) has been uniquely incorporated as a dynamic field stop for smart scene selection. Both the MEMS and reflective design are discussed. The CTIS multiplexes spatial and spectral information, so the two quantities are interdependent and adjustments must be made to the design in order to allow adequate sampling for our given application. Optical aberrations arising from a tilted image plane are alleviated through design optimization.

Evaluations of classification and spectral unmixing algorithms using ground based satellite imaging

Abundances of material components in objects are usually computed using techniques such as linear spectral unmixing on individual pixels captured on hyperspectral imaging devices. However, algorithms such as unmixing have many flaws, some due to implementation, and others due to improper choices of the spectral library used in the unmixing (as well as classification). There may exist other methods for extraction of this hyperspectral abundance information. We propose the development of spatial ground truth data from which various unmixing algorithm analyses can be evaluated. This may be done by implementing a three-dimensional hyperpspectral discrete wavelet transform (HSDWT) with a low-complexity lifting method using the Haar basis. Spectral unmixing, or similar algorithms can then be evaluated, and their effectiveness can be measured by how well or poorly the spatial and spectral characteristics of the target are reproduced at full resolution (which becomes single object classification by pixel).

Remote gas plume sensing and imaging with NASA’s Hyperspectral Thermal Emission Spectrometer (HyTES).

The hyperspectral thermal emission spectrometer was developed under NASA’s instrument incubator program and has now completed three deployments. The scan head uses a state-of-the-art Dyson spectrometer cooled to 100K coupled to a quantum well infrared photodetector array held at 40K. The combination allows for 256 spectral channels between 7.5μm and 12μm with 512 cross track spatial pixels. Spectral features for many interesting gases fall within the instrument passband. We first review the pre-flight calibration and validation process for HyTES using a suite of instrumentation. This includes a smile measurement at two wavelengths (8.18μm and 10.6μm) as well as a concentration determination using large aperture gas cells. We then show positive gas plume detection at ranges >1000m for various cases: Ammonia gas detection from Salton Sea fumaroles, Methane detection from staged releases points in Wyoming as well as naturally occurring methane hot spots off the coast of Santa Barbara.

Multi-color QWIP FPAs for Hyperspectral thermal emission instruments

Infrared focal plane arrays (FPAs) covering broad mid- and long-IR spectral ranges are the central parts of the spectroscopic and imaging instruments in several Earth and planetary science missions. To be implemented in the space instrument these FPAs need to be large-format, uniform, reproducible, low-cost, low 1/f noise, and radiation hard. Quantum Well Infrared Photodetectors (QWIPs), which possess all needed characteristics, have a great potential for implementation in the space instruments. However a standard QWIP has only a relatively narrow spectral coverage. A multi-color QWIP, which is compromised of two or more detector stacks, can to be used to cover the broad spectral range of interest. We will discuss our recent work on development of multi-color QWIP for Hyperspectral Thermal Emission Spectrometer instruments. We developed QWIP compromising of two stacks centered at 9 and 10.5 μm, and featuring 9 grating regions optimized to maximize the responsivity in the individual subbands across the 7.5-12 μm spectral range. The demonstrated 1024x1024 QWIP FPA exhibited excellent performance with operability exceeding 99% and noise equivalent differential temperature of less than 15 mK across the entire 7.5-12 μm spectral range.

Thermal infrared spectral imager for airborne science applications

An airborne thermal hyperspectral imager is underdevelopment which utilizes the compact Dyson optical configuration and quantum well infrared photo detector (QWIP) focal plane array. The Dyson configuration uses a single monolithic prism-like grating design which allows for a high throughput instrument (F/1.6) with minimal ghosting, stray-light and large swath width. The configuration has the potential to be the optimal imaging spectroscopy solution unmanned aerial vehicles (UAV) due to its small form factor and relatively low power requirements. The planned instrument specifications are discussed as well as design trade-offs. Calibration testing results (noise equivalent temperature difference, spectral linearity and spectral bandwidth) and laboratory emissivity plots from samples are shown using an operational testbed unit which has similar specifications as the final airborne system. Field testing of the testbed unit was performed to acquire plots of emissivity for various known standard minerals (quartz). A comparison is made using data from the ASTER spectral library.

Towards HyTES: an airborne thermal imaging spectroscopy instrument

An airborne thermal hyperspectral imager is underdevelopment which utilizes the compact Dyson optical configuration and quantum well infrared photo detector (QWIP) focal plane array. The Dyson configuration uses a single monolithic prism-like grating design which allows for a high throughput instrument (F/1.6) with minimal ghosting, stray-light and large swath width. The configuration has the potential to be the optimal imaging spectroscopy solution unmanned aerial vehicles (UAV) due to its small form factor and relatively low power requirements. The planned instrument specifications are discussed as well as design trade-offs. Calibration testing results (noise equivalent temperature difference, spectral linearity and spectral bandwidth) and laboratory emissivity plots from samples are shown using an operational testbed unit which has similar specifications as the final airborne system. Field testing of the testbed unit was performed to acquire plots of emissivity for various known standard minerals (quartz). A comparison is made using data from the ASTER spectral library.

Novel calibration recovery technique for an expectation maximization tomographic reconstruction

An experiment is conducted that successfully reduces the amount of aberration detected in a reconstructed hyperspectral datacube. The correction alleviates an unnoticed rotational misalignment in the calibration of our near-infrared flash hyperspectral imager. This alteration induces a spherical-aberration-like artifact as well as spectral defects because of the three-dimensional hyperspectral quality of the data. This artifact, which would cause severe misregistration of the spatio-spectral information, is correctable post detection.

Infrared instrument support for HyspIRI-TIR

The Jet Propulsion Laboratory is currently developing an end-to-end instrument which will provide a proof of concept prototype vehicle for a high data rate, multi-channel, thermal instrument in support of the Hyperspectral Infrared Imager (HyspIRI)–Thermal Infrared (TIR) space mission. HyspIRI mission was recommended by the National Research Council Decadal Survey (DS). The HyspIRI mission includes a visible shortwave infrared (SWIR) pushboom spectrometer and a multispectral whiskbroom thermal infrared (TIR) imager. The prototype testbed instrument addressed in this effort will only support the TIR. Data from the HyspIRI mission will be used to address key science questions related to the Solid Earth and Carbon Cycle and Ecosystems focus areas of the NASA Science Mission Directorate. Current designs for the HyspIRI-TIR space borne imager utilize eight spectral bands delineated with filters. The system will have 60m ground resolution, 200mK NEDT, 0.5C absolute temperature resolution with a 5-day repeat from LEO orbit. The prototype instrument will use mercury cadmium telluride (MCT) technology at the focal plane array in time delay integration mode. A custom read out integrated circuit (ROIC) will provide the high speed readout hence high data rates needed for the 5 day repeat. The current HyspIRI requirements dictate a ground knowledge measurement of 30m, so the prototype instrument will tackle this problem with a newly developed interferometeric metrology system. This will provide an absolute measurement of the scanning mirror to an order of magnitude better than conventional optical encoders. This will minimize the reliance on ground control points hence minimizing post-processing (e.g. geo-rectification computations).

QWEST: Quantum Well Infrared Earth Science Testbed

Preliminary results are presented for an ultra compact long-wave infrared slit spectrometer based on the Dyson concentric design. The spectrometer has been integrated in a dewar environment with a quantum well infrared photodetecor (QWIP), concave electron beam fabricated diffraction grating and ultra precision slit. The entire system is cooled to cryogenic temperatures to maximize signal to noise ratio performance, hence eliminating thermal signal from transmissive elements and internal stray light. All of this is done while maintaining QWIP thermal control. A general description is given of the spectrometer, alignment technique and predicated performance. The spectrometer has been designed for optimal performance with respect to smile and keystone distortion. A spectral calibration is performed with NIST traceable targets. A 2-point non-uniformity correction is performed with a precision blackbody source to provide radiometric accuracy. Preliminary laboratory results show excellent agreement with modeled noise equivalent delta temperature and detector linearity over a broad temperature range.