Flowchart of the angular density optimization algorithm for the rays originating from a single source. All departing rays are collected in lists (high or low, see text) and their propagation is followed through the RT model. Depending on their beam size and contribution to the RFED, they are split or merged during the “select ray” phase. We note that, during step 1 of the RT algorithm (see Sect. 2), the decision condition δU>f_UU_LL is substituted by d_ray<R_pre with d_ray the distance crossed by the ray and R_pre the input maximum distance that can be crossed in this step.