Curtis Earl Volin

Demonstration of a computed-tomography imaging spectrometer using a computer-generated hologram disperser.

We have constructed a computed-tomography imaging spectrometer that uses a phase-only computer-generated hologram (CGH) array illuminator as the disperser. This imaging spectrometer collects multiplexed spatial and spectral data simultaneously and can be used for flash spectral imaging. The CGH disperser has been designed to maintain nearly equal spectral diffraction efficiency among a 5 x 5 array of diffraction orders and to minimize 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. The reconstructed image and spectral-signature data compare favorably with measurements by other spectrometric methods.

Demonstration of a high-speed nonscanning imaging spectrometer

We report results from a field demonstration of a nonscanning high-speed imaging spectrometer [computed-tomography imaging spectrometer (CTIS)] capable of simultaneously recording spatial and spectral information about a rapidly changing scene. High-speed spectral imaging was demonstrated by collection of spectral and spatial snapshots of a missile in flight. This instrument is based on computed-tomography concepts and operates in the visible spectrum (430-710nm). Raw image data were recorded at video frame rate (30frames / s) and an integration time of 2ms. An iterative reconstruction of the spatial and spectral scene information from each raw image took 10s. We present representative missile spectral signatures from the missile firing. The accuracy of the high-speed spectrometer is demonstrated by comparison of extended-source static-scene spectra acquired by a nonimaging reference spectrometer with spectra acquired by use of CTIS imaging of the same static scenes.

Computed tomography-based spectral imaging for fluorescence microscopy

The computed tomography imaging spectrometer (CTIS) is a non-scanning instrument capable of simultaneously acquiring full spectral information (450-750 nm) from every position element within its field of view (75 microm x 75 microm). The current spatial and spectral sampling intervals of the spectrometer are 1.0 microm and 10 nm, respectively. This level of resolution is adequate to resolve signal responses from multiple fluorescence probes located within individual cells or different locations within the same cell. Spectral imaging results are presented from the CTIS combined with a commercial inverted fluorescence microscope. Results demonstrate the capability of the CTIS to monitor the spatiotemporal evolution of pH in rat insulinoma cells loaded with SNARF-1. The ability to analyze full spectral information for two-dimensional (x, y) images allows precise evaluation of heterogeneous physiological responses within cell populations. Due to low signal levels, integration times up to 2 s were required. However, reasonable modifications to the instrument design will provide higher system transmission efficiency with increased temporal and spatial resolution. Specifically, a custom optical design including the use of a larger format detector array is under development for a second-generation system.

Midwave-infrared snapshot imaging spectrometer

We report results from a demonstration of a midwave-infrared, nonscanning, high-speed imaging spectrometer capable of simultaneously recording spatial and spectral data from a rapidly varying target scene. We demonstrated high-speed spectral imaging by collecting spectral and spatial snapshots of blackbody targets and combustion products. The instrument is based on computed tomography concepts and operates in a midwave-infrared band of 3.0-5.0 mum. We record raw images at a frame rate of 60 frames/s, using a 512 x 512 InSb focal-plane array. Reconstructed object cube estimates were sampled at 46 x 46 x 21 (x, y, lambda) elements, or 0.1-mum spectral sampling. Reconstructions of several objects are presented.

High-speed spectral imager for imaging transient fluorescence phenomena

We describe fluorescence spectral imaging results with the microscope computed-tomography imaging spectrometer (muCTIS). This imaging spectrometer is capable of recording spatial and spectral data simultaneously. Consequently, muCTIS can be used to image dynamic phenomena. The results presented consist of proof-of-concept imaging results with static targets composed of 6-mum fluorescing microspheres. Image data were collected with integration times of 16 ms, comparable with video-frame-rate integration times. Conversion of raw data acquired by the muCTIS to spatial and spectral data requires postprocessing. The emission spectra were sampled at 10-nm intervals between 420 and 710 nm. The smallest spatial sampling interval presented is 1.7 mum.

Design of broadband-optimized computer-generated hologram dispersers for the computed-tomography imaging spectrometer

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 CGHs 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.

MWIR computed-tomography imaging spectrometer: calibration and imaging experiments

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.

Video-rate spectral imaging system for fluorescence microscopy

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 .

Diffractive optical elements for spectral imaging

Diffractive optical elements fabricated on flat and non-flat substrates frequently act as dispersive elements in imaging spectrometers. We describe the design and electron-beam fabrication of blazed and computer-generated-hologram gratings for slit and tomographic imaging spectrometers.

Portable computed-tomography imaging spectrometer

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.