Spiral optical designs for nonimaging applications Pablo Zamora a , Pablo Benítez a,b , Juan C. Miñano a,b , Juan Vilaplana b , Marina Buljan a a CEDINT, Technical University of Madrid (UPM), Campus de Montegancedo 28223 Pozuelo de Alarcón, Madrid, Spain b LPI-LLC, 2400 Lincoln Ave., Altadena, CA 91001, USA ABSTRACT Manufacturing technologies as injection molding or embossing specify their production limits for minimum radii of the vertices or draft angle for demolding, for instance. In some demanding nonimaging applications, these restrictions may limit the system optical efficiency or affect the generation of undesired artifacts on the illumination pattern. A novel manufacturing concept is presented here, in which the optical surfaces are not obtained from the usual revolution symmetry with respect to a central axis (z axis), but they are calculated as free-form surfaces describing a spiral trajectory around z axis. The main advantage of this new concept lies in the manufacturing process: a molded piece can be easily separated from its mold just by applying a combination of rotational movement around axis z and linear movement along axis z, even for negative draft angles. Some of these spiral symmetry examples will be shown here, as well as their simulated results. Keywords: geometric optical design, nonimaging optics, illumination, manufacturing process 1. INTRODUCTION When dealing with nonimaging optical designs based on plastic refractive elements (for both, illumination and concentration photovoltaics), the first idea and the simpler one is to develop continuous lenses. Nevertheless this type of solutions always implies two vital drawbacks, both of them related to the lens thickness. The first of them is an increase on the system global cost: the thicker the lens is the more material quantity we will need and the more expensive the system will be. Figure 1 Optical efficiency losses due to draft angle. (Left) 0º draft angle flat Fresnel lens, where every single normal incidence ray focuses on a single point at the focal distance. (Right) 4º draft angle flat Fresnel lens, where bold black lines denote rays hitting draft angle and thus being wrongly directed. Red circle points at the draft angles where rays hit.