Curved surface scattering geometry in the shooting and bouncing rays method

In geometrical optics (GO), the divergence factor (DF) is an explicit concept in maintaining the size and field density of ray tubes after they interact with curved surfaces. There is no corresponding concept in physical optics (PO), as the effect emerges implicitly. The shooting-and-bouncing ray method uses GO to extend PO to multiple bounces. In this paper, we examine the significance of the divergence factor in SBR. The results have implications for the use of flat-surface CAD models in SBR codes.