Why do hyperspectral HOCR sensors require two calibration files?

HOCR sensors output two distinct frame types (light and dark).  Thermal dark current changes that occur within the spectrograph are corrected across the full spectrum with the use of a mechanical dark shutter that closes periodically in the radiometer.  This creates a unique frame of data that must be collected separately from the light data.  SATView requires both calibration files so that it collects both data outputs.