Results of wavelength position and bandpass analysis

Measurements of reference materials were made on the Corescan© HCI-III to evaluate the supplied channel wavelength positions and bandpass values. Wavelength position and bandpass of channels in a spectrometer, referred to as full-width half max (FWHM) in the contractor's documentation (Corescan_Product_MetaData_v3.pdf), are two fundamental spectral characteristics that need to be known in order to spectrally identify minerals by comparison to a spectral library, like the Material Identification and Characterization Algorithm (MICA) analysis used to generate the mineral predominance maps. Spectrometers with finer bandpass can reveal greater spectral detail that can be related to a material’s chemical composition or physical structure: for example, kaolinite crystallinity. Imprecise knowledge of wavelength positions of channels could interfere with interpreting a material’s composition from its spectral feature positions: for example, interpreting Al composition in white mica from wavelength shifts in the absorption feature centered near 2,200 nm. Because, push broom imaging spectrometers, like the Corescan HCI-III, can sometimes have variable spectral characteristics across the field of view, the channel wavelength positions across the field of view were also evaluated. The imaging spectrometer raw data were collected with an average bandpass of approximately 6 nm across the Short Wave Infrared (SWIR) but smoothing functions applied by Corescan during the conversion of raw data to reflectance result in a relative bandpass of approximately 13 nm in the data delivered to the U.S. Geological Survey (USGS). Wavelength evaluations of the imaging spectrometer data revealed that the supplied wavelength values should be shifted and, thus, adjustments were made to the wavelength positions (Kokaly and others, 2017). The wavelength and bandpass evaluation results are provided in this section of the data release and were used to adjust the Corescan reflectance data.