Hyperspectral imaging using acousto-optic tunable filters

Hyperspectral imaging holds great promise for object detection and recognition due to the richness of the spectral content in images from such objects. Ordinary broad-band imagers integrate the spectral information over the entire spectral band of coverage when used without any spectral filtering. In general, the spectral details in the images can be obtained by using an optical filtering element such as a filter wheel, a grating, or an acousto-optic tunable filter (AOTF). Since each task of detection and object recognition may require only a limited set of specific spectral bands based on the object as well as the background, it is best to choose a filtering optical element that has high-speed spectral selectivity with high resolution. Of all the optical filtering elements available, only an AOTF offers this capability. Such capability greatly reduces the amount of data collection and processing. In this paper, we present hyperspectral images obtained in the laboratory and from field tests, using visible-to-near-IR (VNIR) AOTF imagers. The imagers use a tellurium dioxide, TeO2, AOTF cell that covers the spectral band from 450 to 1000 nm with a spectral resolution of 10 nm at 600 nm, a charged coupled device (CCD) camera, image-forming optics, frame grabber board, rf electronics, and control and processing software. The imager used for outdoor testing is equipped with a variable phase retardation plate to obtain images with polarimetric signatures (patent pending). The spectral and polarimetric imaging capabilities of the AOTF imager were successfully tested to discriminate targets and backgrounds in various environments.