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Hi, I've been comparing the rear irradiance model published by Marion et al. ("A Practical Irradiance Model for Bifacial PV Modules", 2017) to the PVsyst 2D bifacial model. There are some differences, such the fact that PVsyst doesn't include irradiance reflected from the front of a row to the rear of another row, and the fact that the Marion model considers 1-degree slices of surface (ground, sky, or module row) instead of discrete ground points when calculating view factor and IAM factors. One difference I'm trying to understand is the diffuse IAM correction factor as a function of ground point. Marion prescribes an IAM correction factor curve as a function of one degree surface slice, shown below. When using the values from this curve I obtain a view factor profile that is similar to, but noticeably different from, the profile shown in PVsyst. Below is a PVsyst view factor profile for a system with zero degree tilt, 0.5 GCR, and 1 m height (blue curve), overlaid with one calculated using the Marion correction factors from the curve above (red dashed line). The curves are similar but noticeably different. I was wondering how PVsyst is calculating the IAM correction factors that are used, as they appear to be different from what was derived from Marion et al.