 This formula is a simplified version of that in section 2.2 of Stansberry et al., 2007, where emissivity and beaming parameter were assumed equal unity, and formula_3 was replaced with 4 accounting for the difference between circle and sphere.All parameters mentioned above were taken from the same paper.F(\alpha,m)\right),\,\!\end{smallmatrix} where formula_5 is the modular angle, or angular eccentricity; formula_6 and formula_7, formula_8 are the incomplete elliptic integrals of the first and second kind, respectively.