The roughness model assumes a continuous transition (error function) of the optical constants between two layers and can be altered by the roughness parameter σ.
Two modes implemented to calculate the spectra.
The slicing parameters can be changed in the “Adaptive Layer Segmentation” sheet.
The default model is “local” which means the method used in the Parratt32 software from the HMI. The global model is described in the diploma thesis of M. Harlander.
If the thickness is smaller than σ both models describe the interface almost like the roughness approximation by Nevot&Croce. However there are differences for the case σ>thickness. The local model describes the σ>thickness case like the Nevot&Croce approximation. The global models should give you physical more feasible results which cannot be modelled by the analytical approximation.