Geometry of the model and the observation. The computational domain (black cube) is illuminated by the underlying Sun (gray surface). The limb darkening of the incident continuum radiation depends on the θ heliocentric angle (see the text for details). The LOS, which is in the direction of the z axis, and the local solar vertical direction (y axis) form a 90° angle. In this academic example, the solar surface is approximated by an infinite plane.