The energy capture feature lets you calculate the amount of energy captured from a point source as a function of the position of the point source in the field of view. It is useful when analyzing collecting optics for experiments in, e.g., atomic physics.
The algorithm for searching over ray emission directions gives rise to interesting structures. Presently LensForge does not display these structures, only a summary graph. But we can not resist sharing this picture of a recursive search structure with you: