An MWIR spectral imaging sensor based on dual direct vision prism (DVP) architecture is described. This sensor represents a third generation of the Chromotomographic Hyperspectral Imaging Sensor (CTHIS). In the new sensor, a direct vision prism is synthesized by the vector addition of the spectral response of two matched, but independently aligned DVP's. The resulting sensor dispersion varies from zero to twice the single prism dispersion, as a function of the
angle between the dispersion axes of the two prisms. The number of resolved channels, and the related signal strength per channel, also adapts with this angle. The "synthesized prism" projects a spectral image onto the focal plane array of an infrared camera. The prism is rotated on the camera axis and the resulting spectral information is employed to form an image cube (x, y, λ), using tomographic techniques. The sensor resolves from 1 to 105 spectral channels, between 3.0μm and 5.2μm wavelength. Spectral image data and image reconstruction is provided for standard test sources and scenes.
A novel spectral imaging sensor based on dual direct vision prisms is described. The prisms project a spectral image onto
the focal plane array of an infrared camera. The prism set is rotated on the camera axis and the resulting spectral
information is extracted as an image cube (x, y, λ), using tomographic techniques. The sensor resolves more than 40
spectral bands (channels) at wavelengths between 1.2 μm and 2.5 μm wavelength. The sensor dispersion characteristic is
determined by the vector sum of the dispersions of the two prisms. The number of resolved channels, and the related
signal strength per channel, varies with the angle between the prism dispersion axes. This is a new capability for this
class of spectral imaging sensor. Reconstructed short-wave imagery and spectral data is presented for field and
laboratory scenes and for standard test sources.
Spectral imaging is the art of quantifying the spectral and spatial characteristics of a scene. The current state of the art in spectral imaging comprises a wide range of applications and sensor designs. At the extremes are spectrometers with high spectral sampling over a limited number of imaging pixels and those with little spectral sampling over a large number of pixels. The predominant technical issue concerns the acquisition of the three-dimensional spectral imagery (X,Y,l) using an inherently two-dimensional imaging array; consequently, some form of multiplexing must be implemented. This paper will discuss a new class of sensors, broadly referred to as Spectral Temporal Sensors (STS), which capture the position and spectra of uncued point sources anywhere in the optical field. These sensors have large numbers of pixels (>512x512) and colors (>50). They can be used to sense explosions, combustion, rocket plumes, LASERs, LEDs, LASER/LED excitations and the outputs of fiber optic cables. This paper will highlight recent developments on an STS that operates in a Pseudo-imaging (PI) mode, where the location of an uncued dynamic event and its spectral evolution in time are the data products. Here we focus on the sensor's ability to locate the event to within approximately 1/20th pixel, however we will also discuss its capabilities at fully characterizing event spectral temporal signature at rates greater than 100Hz over a large field of view (greater than 30°).
A very simple and fast technique for clustering/segmenting hyperspectral images is described. The technique is based on the histogram of divergence images; namely, single image reductions of the hyperspectral data cube whose values reflect spectral differences. Multi-value thresholds are set from the local extrema of such a histogram. Two methods are identified for combining the information of a pair of divergence images: a dual method of combining thresholds generated from 1D histograms; and a true 2D histogram method. These histogram-based segmentations have a built-in fine to coarse clustering depending on the extent of smoothing of the histogram before determining the extrema. The technique is useful at the fine scale as a powerful single image display summary of a data cube or at the coarser scales as a quick unsupervised classification or a good starting point for an operator-controlled supervised classification. Results will be shown for visible, SWIR, and MWIR hyperspectral imagery.
The thermionic thermal detector (TTD) sense IR radiation by temperature modulation of thermionic emission current within a silicon Schottky diode. The thermionic emission current is the well known Richardson dark current. The TTD operates in the LWIR band. The physics of TTD operation is distinct from that of silicon Schottky barrier MWIR detectors, such as PtSi/Si which are based on internal photoemission. In fact, the TTD has high detection efficiency. The architecture of a TTD array is very similar to that of microbolometer arrays, expect the detector elements are thermally isolated Schottky diodes, operating under reverse bias. When the TTD array is illuminated by an IR image, the temperature of individual detector elements will vary with the local incident power of the image. Under small signal conditions, the dark current of individual detectors will vary as temperature, resulting in an electronic image of the IR scene. The reverse bias dark current of a Schottky diode varies exponentially with temperature. For the small temperature variations observed on the focal pane of an uncooled sensor, this variation is approximately linear. The rate of temperature variation is determined by the Schottky barrier potential and, to a lesser extent by the applied bias potential. The operating temperature range of the detector can be designed into the device by selecting a metal with the appropriate Schottky barrier height. Experimental Schottky barrier heights were determined using Richardson dark current activation energy analysis. Devices optimized for operating at room ambient temperature have a 6 percent K temperature coefficient. The use of Schottky diode thermionic emission for uncooled IR imaging offers several advantages relative to current technology. TTD manufacture is 100 percent silicon processing compatible. Schottky barrier based thermionic emissions array have the same uniformity characteristics as MWIR Schottky barrier photoemissive arrays. Operating TTDs in reverse bias provides a high impedance 'current source' to the multiplexer, resulting in negligible Johnson noise. This mode of operation also results in negligible detector 1/f-noise and drift. In addition, the TTD thermionic emission detection process has high efficiency, fully comparable with the best current thermal detectors.
PtSi/Si/SiGe/Si Schottky diode IR detectors with extended and tunable cut-off wavelengths have been fabricated. Cut-off wavelengths depend on the SiGe composition and extend up to 10 micrometers for Si80Ge20. The cut-off wavelengths are also tunable by reverse bias. The tunability is due to the SiGe/Si offset serving as an additional potential barrier behind the Schottky barrier that can be varied in energy by a reverse bias. The sensitivity and range of the tunability is controlled by the SiGe thickness and composition. Cut-off wavelengths tunable from 4 micrometers at zero volts to 10 micrometers at 3 volts have been obtained. Quantum efficiency values are normal for operation at the long- wavelength end, but reduced over the rest of tunable range, because of the greater distance from the PtSi to the SiGe/Si offset.
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