Suitably formulated spatial modulation of a scene imaged by a computed-tomography imaging spectrometer (CTIS) has been found to be useful as a means of improving the imaging performance of the CTIS. As used here, “spatial modulation” signifies the imposition of additional, artificial structure on a scene from within the CTIS optics.
The basic principles of a CTIS were described in “Improvements in Computed-Tomography Imaging Spectrometry” (NPO-20561) NASA Tech Briefs, Vol. 24, No. 12 (December 2000), page 38 and “All-Reflective Computed-Tomography Imaging Spectrometers” (NPO-20836), NASA Tech Briefs, Vol. 26, No. 11 (November 2002), page 7a. To recapitulate: A CTIS offers capabilities for imaging a scene with spatial, spectral, and temporal resolution. The spectral disperser in a CTIS is a two-dimensional diffraction grating. It is positioned between two relay lenses (or on one of two relay mirrors) in a video imaging system. If the disperser were removed, the system would produce ordinary images of the scene in its field of view. In the presence of the grating, the image on the focal plane of the system contains both spectral and spatial information because the multiple diffraction orders of the grating give rise to multiple, spectrally dispersed images of the scene. By use of algorithms adapted from computed tomography, the image on the focal plane can be processed into an “image cube” — a three-dimensional collection of data on the image intensity as a function of the two spatial dimensions (x and y) in the scene and of wavelength (λ). Thus, both spectrally and spatially resolved information on the scene at a given instant of time can be obtained, without scanning, from a single snapshot; this is what makes the CTIS such a potentially powerful tool for spatially, spectrally, and temporally resolved imaging.
A CTIS performs poorly in imaging some types of scenes — in particular, scenes that contain little spatial or spectral variation. The computed spectra of such scenes tend to approximate correct values to within acceptably small errors near the edges of the field of view but to be poor approximations away from the edges. The additional structure imposed on a scene according to the present method enables the CTIS algorithms to reconstruct acceptable approximations of the spectral data throughout the scene.
The structure can be imposed in any of a number of alternative ways. In preliminary experiments, the structure was imposed by means of a digital multi-mirror device at the field stop in an all-reflective-optics CTIS. Any of the mirrors could be turned on or off to make a desired pattern. The optimum pattern has not yet been determined; a checkerboard pattern was used in the experiments. (In one alternative, in the case of refractive optics, the structure could be imposed by use of a suitably patterned opaque mask at the field stop.) A full image could be acquired by shifting the pattern by use of software or by moving the mirror device or mask.
This work was done by Gregory H. Bearman, Daniel W. Wilson, and William R. Johnson of Caltech for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-41557, volume and number of this NASA Tech Briefs issue, and the page number.