I already have done a literature review. Derivations of the equations of radiation in general as well as the PN and P1 - Model model are described in detail in standard works such as "Radiative Heat Transfer" by Modest and "Thermal Radiation Heat Transfer" by Howell et al. I know how to derive all the equations on that topic in the theory guide which includes equation 8.8 and 8.9. But I cannot find anything on the equations 8.10 and 8.11 which is why i assume that they were derived from ansys for the implementation in fluent.