Calibrating raw photometric measurements
If I understand the question correctly, you have raw data from a CCD sensor in arbitray units (which correlate with brightness) and your challenge is to callibrate this intensity for a certain frequency range.
Not knowing much about the SMEI sensor and where on the sky it has been looking at, I struggle to give you a full recipe. If you have for an object whose intensity (time series) from other observational instrument in the desired frequency band, you could use that non-SMEI data to correlate (if not even fit e.g. linearly) with the raw data time series for the same object as recorded by the SMEI sensor. This gives a calibration function which delivers intensity from raw data in arbitraty units.
References which sound possibly helpful

P. Hick, A. Buffington, B. V. Jackson: The SMEI real-time data pipeline: from raw CCD frames to photometrically accurate full-sky maps (2005)
A. Buffington et al.: Photometric Calibration for the Solar Mass Ejection Imager (SMEI)
Martin Clayton and Anthony Holloway: Simple Spectroscopy Reductions