In the last years the requirement of special illumination optics increased in the course of developing specific optical systems for wide range of applications in industries and science. Standard components become continuously substituted by more complex freeform surfaces with higher efficiency. Therefore, other methods in evaluating optical systems are of special interest. In illumination design the classic way to check the performance of a system is to trace a huge number of rays through the system and analyze the radiance and irradiance distribution on the target surface. Another access to the most important illumination quantities like radiance is to look at the transformation of etendue in phase space. This offers a new perspective for the optical designer onto illumination systems. Another interesting aspect is the analysis of aberrations also for freeform elements where standard aberration theory for rotational symmetric systems fail.