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The 24-channel Geoscan scanner includes six grating-dispersed thermal infrared channels, co- registered with the 18 VNIR and SWIR bands. Considerable silicate (hydroxyl) discrimination can be seen in the SWIR band images but will not be treated further in this paper. Each of the TIR bands has a bandpass of 530 nm, centered between 8.64 and 11.28 micrometers band centers dispersed onto a linear detector array of HgCdTe. The system has been flown over Ludwig, Nevada three times (May 1989/June 1990), and twice over Virginia City, Nevada (June 1989/Aug. 1990). Pixel sizes ranged from 3 m to 6 m in this ''research-mode'' flying. In flight the scanner is operated in a noncalibrated, relative-radiance mode in all of the 24 channels. A sample of the terrain to be mapped is overflown, during which the offsets of each channel are set to mid-range (DN equals 127), with the data spread by the gain setting so as to occupy all of the 8-bit range. The recording therefore is of the 8 bits of data spread about the average (relative) brightness of the terrain in that band. In this manner, this scanner differs from almost every other unit either in airborne use or in the Landsat satellites. These units record absolute brightness from which, by use of the calibration parameters, the absolute radiance of the terrain may be reconstructed. The problem is to perform a transformation of the imagery to apparent reflectance allow comparison of the spectra extracted from the airborne imagery to ground-measured spectra. TIR spectra have also been obtained in ground-based stationary laboratory-type operation of the aircraft scanner, viewing warm samples (heated by the sun to about 45 C) for fifteen of the major rock-type assemblages. These laboratory-scanner match the airborne-scanner spectra abstracted from the imagery data as well as direct exitance spectra obtained previously from other sun-warmed samples.
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