Hello I put numbers to make it clear .. the system I am modelling consists of four layers of different materials (from a to d in order according to the dashed arrow direction) .. if I would like to know how to determine the surface incident radiation on the surface of material a; which point in the plot I should pick, a1, a2, a3 or a4? and what does each point of them represents here? .. also why each material has a line of four points?