Geometry of the incident radiation. A point S is at a distance h above the solar surface. The segment SC, where C is the solar centre, meets the surface at P(x_D,y_D). The point B is at the largest distance from where photons can reach S. The projected distance on the image is D_max and θ_max the angle at S. The point A(x,y) represents an intermediate position from where light can reach S , and γ is the angle between SA and the normal to the solar surface.