Computed-tomography imaging spectrometers (CTISs) that contain only reflective optics (instead of at least some refractive optics) have been proposed. As is the case for other types of optical instruments, the change to all-reflective designs for CTISs would eliminate the chromatic aberration exhibited by refractive optics and would relieve designers of the task of finding optical materials with adequate transmissivity over wavelength ranges of interest. As a result, it would be easier to scale CTIS designs to different wavelength ranges from ultraviolet through long-wavelength infrared.
The CTIS concept and transmissive implementation were described previously in "Improvements in Computed-Tomography Imaging Spectrometry" (NPO-20561) NASA Tech Briefs, Vol. 24, No. 12 (December 2000), page 38. To recapitulate: A CTIS offers capabilities for imaging a scene with spatial, spectral, and temporal resolution. In the case of a CTIS that contains refractive optics, the spectral disperser is a two-dimensional diffraction grating made of a transparent material and positioned between two relay lenses in a video imaging system. In the absence of the grating, 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 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 (l). Thus, both spectrally and spatially resolved information on the scene at a given instant of time can be obtained, without scanning, from a single video snapshot.
The figure shows the basic optical layout of one of many possible all-reflective CTIS designs. This design features the Offner configuration, which provides for relay of images by two concentric spherical mirrors. Traditional slit-type imaging spectrometers (not CTISs) in the Offner configuration have become popular in the past several years because they can be very compact and exhibit excellent imaging. In order to implement a CTIS in this Offner configuration, it will be necessary to fabricate the two-dimensional diffraction grating on the convex mirror surface. Heretofore, this would have been a formidable or even an impossible task; however, it is now feasible, thanks to recent advances in analog-relief electron-beam lithography on curved surfaces.
This work was done by Daniel Wilson, Paul D. Maker, Richard Muller, and Pantazis Mouroulis of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.
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Refer to NPO-20836.