A dispersive transform spectral imager named FAROS (FAst Reconfigurable Optical Sensor) has been developed for high frame rate, moderate-to-high resolution hyperspectral imaging. A programmable digital micromirror array (DMA) modulator makes it possible to adjust spectral, temporal and spatial resolution in real time to achieve optimum tradeoff for dynamic monitoring requirements. The system's F/2.8 collection optics produces diffraction-limited images in the mid-wave infrared (MWIR) spectral region. The optical system is based on a proprietary dual-pass Offner configuration with a single spherical mirror and a confocal spherical diffraction grating. FAROS fulfills two functions simultaneously: one output produces two-dimensional polychromatic imagery at the full focal plane array (FPA) frame rate for fast object acquisition and tracking, while the other output operates in parallel and produces variable-resolution spectral images via Hadamard transform encoding to assist in object discrimination and classification. The current version of the FAROS spectral imager is a multispectral technology demonstrator that operates in the MWIR with a 320 x 256 pixel InSb FPA running at 478 frames per second resulting in time resolution of several tens of milliseconds per hypercube. The instrument has been tested by monitoring small-scale rocket engine firings in outdoor environments. The instrument has no macro-scale moving parts, and conforms to a robust, small-volume and lightweight package, suitable for integration with small surveillance vehicles. The technology is also applicable to multispectral/hyperspectral imaging applications in diverse areas such as atmospheric contamination monitoring, agriculture, process control, and biomedical imaging, and can be adapted for use in any spectral domain from the ultraviolet (UV) to the LWIR region.

Remotely sensed spectral imagery of the earth's surface can be used to fullest advantage when the influence of the atmosphere has been removed and the measurements are reduced to units of reflectance. Here, we provide a comprehensive summary of the latest version of the Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes atmospheric correction algorithm. We also report some new code improvements for speed and accuracy. These include the re-working of the original algorithm in C-language code parallelized with message passing interface and containing a new radiative transfer look-up table option, which replaces executions of the MODTRAN (R) model. With computation times now as low as 10 s per image per computer processor, automated, real-time, on-board atmospheric correction of hyper- and multi-spectral imagery is within reach. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.OE.51.11.111707]

We present preliminary results for a new passive optical sensor that is the basis for a temperature pattern factor control system. The sensor uses the natural combustion radiance in several spectral bands to deduce the local pre-combustion equivalence ratio, which is correlated with local combustor exit temperature. The sensor was applied in an arc-sector combustor and in a closed-loop control system in a four-way laboratory burner.

Dispersive transform spectral imagers with both one- and two-dimensional spatial coverage have been demonstrated and characterized for applications in remote sensing, target classification and process monitoring. Programmable spatial light modulators make it possible to adjust spectral, temporal and spatial resolution in real time, as well as implement detection algorithms directly in the digitally controlled sensor hardware. Operating parameters can be optimized in real time, in order to capture changing background and target evolution. Preliminary results are presented for short wave, mid-wave, and long-wave infrared sensors that demonstrate the spatial and spectral versatility and rapid adaptability of this new sensor technology.

First-principles atmospheric correction that converts Visible-NIR-SWIR spectral imagery to surface reflectance requires an estimate of the scene visibility / aerosol optical depth. This paper describes aerosol upgrades to FLAASH, a first-principles atmospheric correction algorithm developed by Spectral Sciences, Inc. and the US Air Force Research Laboratory. FLAASH utilizes an automated band ratio method for retrieving an average scene visibility from dark pixels. The visibility estimate is combined with a MODTRANTM aerosol representation to describe the atmosphere. Recent FLAASH upgrades improve both visibility retrieval and atmospheric correction accuracy. These result in better handling of highly off-nadir viewing geometries, high aerosol optical depths and sensors lacking infrared spectral channels.

An innovative passive standoff system for the detection of chemical/biological agents is described. The spectral, temporal and spatial resolution of the data collected are all adjustable in real time, making it possible to keep the tradeoff between the sensor operating parameters at optimum at all times. The instrument contains no macro-scale moving parts and is therefore an excellent candidate for the development of a robust, compact, lightweight and low-power-consumption sensor. The design can also serve as a basis for a wide variety of spectral instruments operating in the visible, NIR, MWIR, and LWIR to be used for surveillance, process control, and biomedical applications.

The MODTRAN (TM) 5 radiation transport (RT) model is a major advancement over earlier versions of the MODTRAN (TM) atmospheric transmittance and radiance model. New model features include (1) finer spectral resolution via the Spectrally Enhanced Resolution MODTRAN (TM) (SERTRAN) molecular band model, (2) a fully coupled treatment of auxiliary molecular species, and (3) a rapid, high fidelity multiple scattering (MS) option. The finer spectral resolution improves model accuracy especially in the mid- and long-wave infrared atmospheric windows; the auxiliary species option permits the addition of any or all of the suite of HITRAN molecular line species, along with default and user-defined profile specification; and the MS option makes feasible the calculation of Vis-NIR databases that include high-fidelity scattered radiances. Validations of the new band model algorithms against line-by-line (LBL) codes have proven successful.

Passive optical probes and high-resolution emission spectroscopy are used to provide a general-purpose real-time temperature and chemical species sensing capability. Probes can be inserted in the combustor, at the turbine inlet, in the augmenter, or at the engine exit with application as an engine development diagnostic tool that provides spatially resolved measurements of the key combustion parameters: temperature, CO concentration, and H2O concentration Multiple probes are arrayed to collect the emitted infrared radiation over different views of the hot gas path. Line-of-sight averaged concentrations and temperatures are determined by spectral analysis of the emitted radiation along each line of sight (LOS). Spatial profiles may also be determined by simultaneous analysis of overlapping lines of sight. The collected infrared spectra contain optically thin and optically thick features that reflect the effects of emission and absorption within the combustion region. The known spectral structure of the component spectra can be used for the automated interpretation of the observed radiance spectra in terms of concentrations and temperatures along the line of sight, and in specific volume elements of overlapping lines of sight. In this work, we present measurements of atmosphericpressure flames and high-pressure combustors and describe the formalism for fitting the observed spectra to a basis of simulated spectra to extract estimates of concentrations and temperatures. The spectral basis is constructed using a multilayer radiation transport model, in which each line-of-sight or measurement volume is divided into segments of uniform concentration and temperature. The observed radiance emanating from each segment is calculated as a function of the local physical variables. The collection of observed data, which contains a highly structured emission spectrum over each line of sight, is fit to the spectral basis to extract line-ofsight averaged physical properties, or in the case of spatial reconstruction, volume-averaged properties for each of the overlap regions.

The MODTRAN5 radiation transport (RT) model is a major advancement over earlier versions of the MODTRAN atmospheric transmittance and radiance model. New model features include (1) finer spectral resolution via the Spectrally Enhanced Resolution MODTRAN (SERTRAN) molecular band model, (2) a fully coupled treatment of auxiliary molecular species, and (3) a rapid, high fidelity multiple scattering (MS) option. The finer spectral resolution improves model accuracy especially in the mid- and long-wave infrared atmospheric windows; the auxiliary species option permits the addition of any or all of the suite of HITRAN molecular line species, along with default and user-defined profile specification; and the MS option makes feasible the calculation of Vis-NIR databases that include high-fidelity scattered radiances. Validations of the new band model algorithms against line-by-line (LBL) codes have proven successful.

A new end-member analysis method based on convex cones has been developed. The method finds extreme points in a convex set. Unlike convex methods that rely on a simplex, the number of end-members is not restricted by the number of spectral channels. The algorithm simultaneously finds fractional abundance maps. The fractional abundances are the fractions of the total spectrally integrated radiance of a pixel that are contributed by the end-members. A physical model of the hyper-spectral or multi-spectral scene is obtained by combining subsets of the end-members into bundles of spectra for each scene material. The bundle spectra represent the spectral variability of the material in the scene induced by illumination, shadowing, weathering and other environmental effects. The method offers advantages in multi-spectral data sets where the limited number of channels impairs material un-mixing by standard techniques. The method can also be applied to compress hyper-spectral data. The fractional abundance matrices are sparse and offer an additional compression capability over standard matrix factorization techniques. A description of the method and applications to real and synthetic hyper-spectral and multi-spectral data sets will be presented.

Structured Emission Thermometry (SET) is a new optical technique for deriving temperature and species concentrations in high-temperature regions of a combustor flow-path from measurements of water and soot emission spectra in the 1 micron region. A prototype instrument has been built for gas- and liquid-fuel combustors with temperatures ranging from 1400–2500K (2100–4000F). This instrument can be used for test stand instrumentation and routine monitoring in fixed installations. With further evolution, the technique may be suitable for an aircraft engine control system. The emissions are measured along narrow lines of sight using small, simple passive fiber probes placed at a short standoff distance from the hot gas flow-path. With proper placement of multiple probes, data may be collected over overlapping lines of sight and inverted to produce a low-resolution spatial map of the temperature and emitter density in the flow-path. In this work, we describe a series of combustor measurements, including cross-validation measurements in a well-controlled laboratory flame and measurements in combustor test stands. Both line-of-sight measurements and reconstructed temperature profiles are presented and discussed.

A combination of good spatial and spectral resolution make visible to shortwave infrared spectral imaging from aircraft or spacecraft a highly valuable technology for remote sensing of the Earth's surface. Many applications require the elimination of atmospheric effects caused by molecular and particulate scattering; a process known as atmospheric correction, compensation, or removal. The Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes (FLAASH) atmospheric correction code derives its physics-based algorithm from the MODTRAN4 radiative transfer code. A new spectra; recalibration algorithm, which has been incorporated into FLAASH, is described. Results from processing Hyperion data with FLAASH are discussed.

Model-based atmospheric correction of multi-spectral and hyperspectral imagery (MSI/HSI) typically involves searching through a look-up table (LUT) of potential atmospheric representations for a best fit, based on some fit criterion. These representations are generated using a radiation transport model such as MODTRAN. The parameter space covered by the LUT is defined to cover the likely atmospheric conditions encountered by the sensor that affect observed radiance over the spectral region covered by the sensor. For instance, aerosols play an important role in the visible through SWIR (450-2500 nm) but a minor role in the thermal IR, where water column content and atmospheric temperature are critical. We investigate the sampling and representation of the atmospheric parameter space in the thermal IR as it effects retrieval of the atmosphere. Using the SMACC convex projection technique we evaluate selection of significant basis members from a broadly-based LUT. We apply SMACC selected endmembers to solve for an arbitrary atmosphere.

The feasibility of an innovative minimally intrusive sensor for monitoring the hot gas stream at the turbine inlet in high performance aircraft gas turbine engines was demonstrated. The sensor uses passive fiber-optical probes and a remote readout device to collect and analyze the spatially resolved spectral signature of the hot gas in the combustor/turbine flowpaths. Advanced information processing techniques are used to extract the average temperature, temperature pattern factor, and chemical composition on a sub-second time scale. Temperatures and flame composition were measured in a variety of combustion systems including a high pressure, high temperature combustion cell. Algorithms for real-time temperature measurements were developed and demonstrated. This approach should provide a real-time temperature profile, temperature pattern factor, and chemical species sensing capability for multi-point monitoring of high temperature and high pressure flow at the combustor exit with application as an engine development diagnostic tool, and ultimately, as a real-time active control component for high performance gas turbines.

The atmospheric correction of thermal infrared (TIR) imagery involves the combined tasks of separation of atmospheric transmittance, downwelling flux and upwelling radiance from the surface material spectral emissivity and temperature. The problem is ill posed and is thus hampered by spectral ambiguity among several possible feasible combinations of atmospheric temperature, constituent profiles, and surface material emissivities and temperatures. For many materials, their reflectance spectra in the Vis-SWIR provide a means of identification or at least classification into generic material types, vegetation, soil, etc. If Vis-SWIR data can be registered to TIR data or collected simultaneously as in sensors like the MASTER sensor, then the additional information on material type can be utilized to help lower the ambiguities in the TIR data. If the Vis-SWIR and TIR are collected simultaneously the water column amounts obtained form the atmospheric correction of the Vis-SWIR can also be utilized in reducing the ambiguity in the atmospheric quantities. The TIR atmospheric correction involves expansions in atmospheric and material emissivity basis sets. The method can be applied to hyperspectral and ultraspectral data, however it is particularly useful for multispectral TIR, where spectral smoothness techniques cannot be readily applied. The algorithm is described, and the approach applied to a MASTER sensor data set.