Illustration of the model space. The planet with radius R_p is located at the center of the model space, i.e., the origin of the coordinate system. The dashed lines represent the radial cell boundaries that divide the atmosphere above the surface into individual layers. The star, located at distance d_* along the positive z-axis, has a radius of R_*. The first two emission angles are sampled and define the starting position of a photon package on the stellar surface. The position is described by the polar angle θ₁ with respect to the z-axis and the azimuthal ϕ₁ in the (x, y)-plane. The second polar angle θ₂ is defined with respect to the surface normal and the azimuthal angle ϕ₂ is defined inthe surface plane at this location.