Schema of the geometry of the disk irradiation by a central star. The disk “surface” element d S (with centralpoint B) is impinged by the stellar radiation coming along the line-of-sight vector d from the stellar surface element dS_⋆ (with centralpoint A). Here α is the angle between the position vector of the point A and the vector d, β′ is the angle between normal vector ν of the surface element dS and the line-of-sight vector d, β = π∕2 − β′, and γ is the angle between normals of the surface element dS and of the equatorial plane. The idea is adapted from Smak (1989).