For many years, effort has been concentrated on designing and manufacturing better and better IR imagers. These devices have become very good, but their images are “black and white.” Monochrome images can provide much information, but clearly that information is greatly enhanced if the different wavelengths can be distinguished. Dr. LeVan's novel design is a dual-band imaging spectrometer that works over two octaves of wavelength in the infrared (visible light is essentially one octave). This approach to hyperspectral imaging utilizes a dual-band focal plane array (FPA), and thereby greatly reduces the mass, power requirements, and optical complexity—all of which are big issues for space-based applications.

The spectrometer design utilizes a grating blaze chosen to be efficient at both 3.75–6.05 and 7.5–12.1 microns. The spectrometer has been calibrated with flooded blackbody illumination and the mercury cadmium telluride FPA was found to have cutoff wavelengths near 5.2 and 10.5 microns. We have also obtained images of distant objects in order to test the instruments capability. One was a simple test blackbody set up 100 m away. Of far greater interest were spectral images of the Sun and the Moon.

The spectrometer works by using a telescope to cast an image on its slit. The grating then disperses the different wavelengths across the FPA. Each column then corresponds to two particular wavelengths—one in each of the wavelength bands. The result is a multispectral image of what is viewed through the slit. Figure 1 is such an image of the Sun. To see more than one slice of the object, either the object must move or the image must be scanned. After quite a bit of data analysis, a composite picture of the Sun can be obtained. Figure 2 is one such picture.

So far four Bethel students have made significant contributions to this work: Mike Munson (‘06), Ben Todt (’07), Cory Lindh (’08), and Randy Johnson (’08). Three are co-authors on some of the published papers that have resulted.  Links to the published papers and the corresponding PowerPoint presentations are listed below.

Progress on characterization of a dualband IR imaging spectrometer, Brian P. Beecken, Paul D. LeVan, Cory W. Lindh, and Randall S. Johnson, Proceedings of SPIE 6940, (2008).

“Progress on characterization of a dualband IR imaging spectrometer,” Infrared Technology and Applications XXXIV, SPIE Defense+Security, Orlando, FL, 18 March 2008.

Demonstration of a dual-band IR imaging spectrometer, Brian P. Beecken, Paul D. LeVan, and Benjamin D. Todt, Proceedings of SPIE 6660, (2007).

 “Demonstration of a dual-band IR imaging spectrometer,” Infrared Detectors and Focal Plane Arrays IX, SPIE Optics+Photonics, San Diego, CA, 27 August 2007.