The acquisition of a multi-spectral data set in a single FPA integration time (snapshot) with no moving parts or scanning is possible with a Computed Tomographic Imaging Spectrometer (CTIS). CTIS instruments employ specially designed computer generated holograms (CGH) etched in an appropriate media for the wavelength band of interest as the dispersing element. The replacement of current etched CGHs with an electronically tunable liquid crystal Optical Phase Array (OPA) extends the capabilities of the CTIS by adding the ability to change its configuration while maintaining its basic motivation as a non-scanning imaging spectrometer with no moving parts. This tunability allows the dispersion, number of diffraction orders, and diffraction efficiency of the orders to be changed affecting the instrument’s spectral resolution, data cube reconstruction quality and speed. This publication presents the results of characterizing the OPA phase vs. applied voltage profile and the feedback algorithm used to program the OPA as a CTIS disperser.
We have constructed a computed-tomography imaging spectrometer (CTIS) that uses two crossed phase-only computer generated holograms (CGH) as the dispersing elements. This imaging spectrometer collects the multiplexed spatial and spectral data simultaneously and can be used for flash spectral imaging. Previous CTIS instruments require a single CGH dispersing elements which were designed with the freedom of adjusting each element in the cell profile independently during the design process. The CHGs for this instrument are designed as identical crossed gratings to model the design parameters of a crossed 1D addressable liquid crystal spatial light modulator. Future integration of a liquid crystal spatial light modulator allows for the possibility of optical preprocessing of tomographic images. The CGH disperser pair has been designed to maintain nearly equal spectral diffraction efficiency among a 5x5 array of diffraction orders and to minimize the diffraction efficiency into higher orders. Reconstruction of the (x,y,(lambda) ) image cube from the raw, two-dimensional data is achieved by computed-tomography techniques.
This paper describes an algorithm based on the singular-value decomposition that converges to a solution for a computer-generated-hologram disperser from a random-phase starting diffuser. In this paper, we report on the application of this algorithm to the design of two-dimensional, surface-relief CGH dispersers for use in the Computed-Tomography Imaging Spectrometer (CTIS). The designed CGH's produce desired diffraction images at five wavelengths through a 1:1.67 wavelength band. Performance results are presented for a demonstration CGH designed by the SVD algorithm and fabricated in GaAs for use in the mid-wave infrared CTIS.
We report results from a demonstration of a midwave-infrared non-scanning, high speed imaging spectrometer capable of simultaneously recording spatial and spectral data from a rapidly varying target scene. High speed spectral imaging was demonstrated by collecting spectral and spatial snapshots of blackbody targets and combustion products. The instrument is based on computed tomography concepts and operates in a mid-wave infrared band of 3.0 to 5.0 micrometers . Raw images were recorded at a frame rate of 60 fps using a 512 x 512 InSb focal plane array. Reconstructed object cube estimates were sampled at 46x46x21 (x, y,(lambda) ) elements, or 0.1 micrometers spectral sampling. Reconstructions of several objects are presented.
The advent of imaging spectroscopy has enabled optical sensors to be constructed that provide hyperspectral imagery on scales previously unattainable. Whereas multiband imagery on several spectral bands have been available for some time, the new generation of instruments is capable of providing imagery in hundreds or thousands of spectral bands. The price of increased measurement resolution is both greater system complexity, and, increased data processing burden.
One of the new instrument designs for producing hyperspectral imagery is the Computed Tomographic Imaging Spectrometer (CTIS). This instrument relies on a computer generated holographic mask as a dispersing element with relatively conventional optical elements and arrays. Design philosophy is discussed relative to systems requirements for using hyperspectral imaging in missile and fire control systems. Issues of optical throughput, dispersion, mask complexity, and, producability are discussed. Results are shown for masks manufactured to operate in the visible and infrared regions.
In concert with the design issues of the Computed Tomographic Imaging Spectrometer, the data processing and reduction is discussed both for remote sensing, and, typical missile and fire control applications. System tradeoff between algorithm complexity and mission is presented with regard to current algorithms and their implementation.
Completed systems are presented and results from both first and second-generation instruments are displayed. Deviation of actual operation from expectations is discussed relative to plans for further development.
Our Imaging Spectrometer Simulation Software known under the name Eikon should improve and speed up the design of a Computed Tomography Imaging Spectrometer (CTIS). Eikon uses existing raytracing software to simulate a virtual instrument. Eikon enables designers to virtually run through the design, calibration and data acquisition, saving significant cost and time when designing an instrument. We anticipate that Eikon simulations will improve future designs of CTIS by allowing engineers to explore more instrument options.
We report results of experimentation with a new, high- resolution MWIR non-scanning, snapshot imaging spectrometer capable of simultaneously recording spatial and spectral data from a rapidly varying target scene. The instrument is based on computed tomography concepts and operates in a mid-wave infrared band of 3.0 to 5.0 micrometer. High speed spectral imaging was demonstrated by collecting spectro-spatial snapshots of an artificial target in the lab. Raw images were recorded using a 512 X 512 InSb focal plane array in snapshot mode.
We report results of experimentation with a MWIR non-scanning, high speed imaging spectrometer capable of simultaneously recording spatial and spectral data from a rapidly varying target scene. High speed spectral imaging was demonstrated by collecting spectral and spatial snapshots of filtered blackbodies, combustion products and a coffee cup. The instrument is based on computed tomography concepts and operates in a mid-wave infrared band of 3.0 to 4.6 micrometer. Raw images were recorded at a video frame rate of 30 fps using a 160 X 120 InSb focal plane array. Reconstructions of simple objects are presented.
Misalignment modes are combinations of rigid-body perturbations to the optical elements that comprise an optical system. Comparison of misalignment modes associated with metrology data and projection-camera performance can be used to increase the sensitivity of metrology measurements to specific camera-performance specification such as chief- ray distortion. Selection of measurable misalignment modes in the case of metrology and interesting misalignment modes in the case of camera performance is based on a determination of whether a mode can 'fit' into a projection camera given actuator-stroke and mirror tilt bounds. Measurement and interest subspaces are next compared using distance between subspaces. As an example of this type of analysis, we find that exit-pupil wavefront measurements can be made more sensitive to chief-ray distortion if these measurements are collected at field positions outside the ring field of view of an extreme ultraviolet lithography projection camera.
We report results from a demonstration of a MWIR non-scanning, high speed imaging spectrometer capable of simultaneously recording spatial and spectral data from a rapidly varying target scene. High speed spectral imaging was demonstrated by collecting spectral and spatial snapshots of filtered blackbodies and combustion products. The instrument is based on computed tomography concepts and operates in a mid-wave infrared band of 3.0 to 4.6 micrometer. Raw images were recorded at a video frame rate of 30 fps using a 160 X 120 InSb focal plane array. A reconstruction of a simple object is presented.
We describe fluorescence spectral-imaging results with the computed-tomography imaging spectrometer (CTIS). This imaging spectrometer is capable of recording spatial and spectral data simultaneously. Consequently, the CTIS can be used to image dynamic phenomena involving multiple, spectrally overlapping fluorescence probes. This system is also optimal for simultaneously monitoring changes in spectral characteristics of multiple probes from different locations within the same sample. This advantage will provide additional information about the physiological changes in function form populations of cells which respond in a heterogeneous manner. The results presented in this paper consist of proof-of-concept imaging results from the CTIS in combination with two different systems of fore- optics. In the first configuration, raw image data were collected using the CTIS coupled to an inverted fluorescence microscope. The second configuration combined the CTIS with a confocal microscope equipped with a fiber-optic imaging bundle, previously for in vivo imaging. Image data were collected at frame rates of 15 frame per second and emission spectra were sample at 10-nm intervals with a minimum of 29 spectral bands. The smallest spatial sampling interval presented in this paper is 0.7 micrometers .
We describe fluorescence spectral-imaging results with the microscope computed-tomography imaging spectrometer ((mu) CTIS). This imaging spectrometer is capable of recording spatial and spectral data simultaneously. Consequently, the (mu) CTIS can be used to image dynamic phenomena involving multiple, spectrally overlapping fluorescence probes. The result presented in this paper consists of proof-of-concept imaging result using two static targets. The first is composed of 6-micrometers fluorescing microspheres and the second consists of rat sinusoid epithelial cells loaded with 0.5-micrometers fluorescing microspheres. Image data were collected in integration times of 16 msec, comparable to video frame rate integration times. The emission spectra were sampled at 10-nm intervals between 420 nm and 710 nm. The smallest spatial sampling interval presented in this paper is 1.7 micrometers .
We report results from a simple signal-to-noise analysis of the computed tomography imaging spectrometer (CTIS). The CTIS is non-scanning high-sped imaging spectrometer capable of simultaneously recording spatial and spectral information about dynamic events. This instrument is based on computed- tomography concepts and operates in the visible. The numerical estimate of the noise equivalent spectral radiance for the CTIS is 1.6 nanowatts per centimeter squared per steradian per micrometer.
We discuss the calculation and effect of rigid-body- perturbation misalignment modes in two off-axis extreme- ultraviolet lithography projection cameras: a 4-mirror, 0.14-numerical-aperture (NA) design and a 6-mirror, NA equals 0.2 design. Two sets of modes are considered: (1) modes associated with camera distortion within the design ring field of view and (2) modes associated with camera exit- pupil wavefront 632.8-nm-light metrology data. We show that in the case of the 4-mirror design, a significant distortion misalignment mode coincides with a difficult-to-detect metrology misalignment mode.
We report results from a field demonstration of a non-scanning high-speed imaging spectrometer capable of simultaneously recording spatial and spectral information about a rapidly changing scene. High-speed spectral imaging was demonstrated by collecting spectral an spatial snapshots of a missile in flight. This instrument is based on computed-tomography concepts and operates in the visible (420 - 740 nm). Raw image data were recorded at video frame rate (30 fps) and an integration time of 2 msec. Reconstructions of the spatial and spectral scene information from the raw image data take considerably longer, on the order of 30 seconds. Comparisons of reconstructed spectra with spectra acquired by a non- imaging reference spectrometer have shown that extended-source spectra were reconstructed accurately. We present representative missile spectral-signature data and missile- tracking linear-classifier results from the missile firing.
We present results of a field demonstration of the computed tomography imaging spectrometer (CTIS). The CTIS was used to collect a sequence of image cubes of a missile in flight. This instrument is based on computed-tomography concepts and operates in the visible spectrum. Raw image data was recorded at video frame rate and an integration time of 2 msec. An iterative reconstruction of the spatial and spectral scene information from each raw image took 10 seconds. We present representative missile tracking-by- spectrum results.
We have constructed a portable computed tomography imaging spectrometer (CTIS) based on a previous laboratory-bench version. This spectrometer features a computer-generated phase-only hologram as the dispersive element and collects spatial and spectral data from the diffracted orders. CTIS is capable of flash spectral imaging. Reconstruction of the image cube from raw data is achieved by computed-tomography techniques. Other improvements from the original design include a more modular design, an automated and more precise calibration technique, and the inclusion of constraints in the image cube reconstruction. Reconstruction results compare favorably with measurements by a fiber spectrometer.
Keywords: Imaging spectrometry, computed tomography, flash spectral imaging.
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