The diffraction loss model is described in Caldwell et al. (2000) and the curves within the SPIRE Spectrometer bands are derived and presented in Wu et al. (2013), Figure 4.
The diffraction loss correction is the gain factor needed for the conversion between point-source and extended-source calibration (see Valtchanov et al. 2017).
Ivan Valtchanov, Herschel Science Centre, Dec 2016