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.
Imaging systems such as the Computed Tomographic Imaging Spectrometer (CTIS) are modeled by the matrix equation g = Hf, which is the discretized form of the general imaging integral equation.. The matrix H describes the contribution to each element of the image g from each element of the hyperspectral object cube f. The vector g is the image of the spatial/spectral projections of f on a focal plane array (FPA). The matrix H is enormous, sparse and rectangular. It is extremely difficult to discretize the integral operator to obtain the matrix operator H. Normally H is constructed empirically from a series of monochromatic calibration images, which is a time consuming process. However we have been able to synthetically construct H by numerically modeling how the optical and diffractive elements in the CTIS project monochromatic point source data onto the FPA. We can evaluate a CTIS system by solving the imaging equation for f using both the empirical and synthetic H from some test data g. Comparison between the two results provides a means to evaluate and improve CTIS system calibration procedures noting that the synthetic system matrix H represents a baseline ideal system.
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.
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