This invention relates to a method of designing an illumination device, wherein a light source unit is modeled with a set of rays. Therein, each ray is assigned a light power and is further characterized by a light volume, which specifies how “spread out” the light is in area and angle. In this way, a selection of rays with respect to their “density” becomes possible, and an optical system can be optimized for a transmission of those rays providing the most light per volume.