The response of a spectrometer channel depends on the transmission of the detector window (as a function of its length), the detector linearity, the crystal curvature and the transmission of the thermal filter. All three of these factors were independently measured during construction of the BCS and an all-up check was then performed at RAL. The response files for each of the channels are in the directory defined by ``$DIR_BCS_CAL'' under the names BCSA1.nnn, BCSA2.nnn, BCSB3.nnn and BCSB4.nnn, where nnn is the file version number. The sensitivity functions contained in these files are listed in tables in an appendix of Lang et al., 1993.
These functions represent the departure from the nominal efficiency, or effective area of the spectrometers. The measured effective areas, with uncertainties, for the flight wavelength ranges are 0.1040.009 cm, 0.1140.012 cm, 0.3030.032 cm and 0.0710.010 cm for channels 1 to 4 respectively when the relative response within a channel is taken to be uniform.