Based on the work of Brandemuehl and Beckman (1980), the following equation integrates the effect of the Angle of Incidence (AOI) for all angles within the field of view to provide an overall AOI correction factor for the diffuse radiation, Fd:

 

where ω is the solid angle of the incident diffuse irradiance and A is the range of ω. The radiation’s F(AOI) is weighted by its contribution to the in-plane irradiance.

F(AOI) is in this case the tabular IAM values that are part of the module file. The equivalent numerical equation that is being used becomes:

 

Please see the below drawing for a more detailed explanation:

 

Reference:

B. Marion “Numerical method for angle-of-incidence correction factors for diffuse radiation incident photovoltaic modules”, Solar Energy, Volume 147, Pages 344-348. 2017. DOI: 10.1016/j.solener.2017.03.027