The RSRF of the SWS Flight Model was intensively measured in the laboratory prior to launch. The calibration source used was a cryogenic black-body source with temperatures between 30 and 300K. After the launch of ISO, the recalibration of the RSRF was performed on standard stars and asteroids using calibration uplink procedures. These scan more than the allowed wavelength range for each AOT band and ensure each scanner position is scanned.
The in-flight RSRFs are derived using the SWS Interactive Analysis package. The measurements are processed as in the standard pipeline up until the flux conversion steps. The RSRF is then obtained by dividing this `raw' spectrum by a model of the observed source. For stellar sources containing spectral features, small inaccuracies in the model can introduce false features in the RSRF, and thus in every spectrum processed with this RSRF. Therefore, extreme care has to be taken before `correcting' the RSRF.