Space-based hyperspectral technologies for the thermal infrared

Various approaches now exist for obtaining spectral imagery over a broad range of infrared wavelengths. One involves use of a single grating element in two grating orders with dualband focal plane array (FPA) technology - an approach offering high efficiency over both the MWIR & LWIR, and obviating the need for separate focal plane arrays, dispersing elements, and optical beamsplitters. Another approach achieves similar results by exploiting an FPA having broad wavelength response with an innovative grating having useable efficiency extending beyond the single octave limits of traditional gratings. Significant advantages result in either case for space-based hyperspectral imagers, for which a reduction in cryo-cooled mass translates into prodigious savings in overall payload mass, cryo-cooling requirements, and waste heat removal. By contrast, longer term approaches might realize infrared "hyperspectral pixels" in 2-D imaging focal plane arrays. In this case, each pixel would detect different wavelengths of radiation at different depths, and the resulting "spectral photocurrents" would be transported to read-out circuitry through a vertical grid of electrical contacts. Although not yet realized in practice, the conceptual basis for accomplishing this with the widely-available HgCdTe detector material has been described. With regard to employment, space-based thermal hyperspectral imaging (HSI) is characterized by coarser ground resolution as a result of aperture diameter limitations and diffraction considerations at the longer infrared wavelengths. The resulting sub-pixel detections based on spectral signature are often complementary with higher resolution, shorter wavelength, panchromatic imagery. Overlapping fields-of-view between the two sensor types on the dayside of the earth enable simultaneous correlation of infrared spectral signatures with spatially-resolved scene features; data collects on the night-side are limited to the thermal hyperspectral images and would await correlations with high resolution visible imagery at the next daytime opportunity.