Schematic representation of the placement of ray paths (arrows) of each p for computing the radiation field in spherical geometry. Dotted semicircles represent spherical atmospheric layers at a spatial grid of m − 1, m, m + 1, and NP. The solid semicircle represents a surface of a body where the spatial grid m = 0. Dashed semicircles represent shells inside the surface and dotted straight lines represent the tangent lines to the shells (see text for details).