## 4 Conclusions  
Freeform optics can be seen as an enabling technology since they allow completely new designs of optical systems. Classical optical components have a rotational symmetric shape. The tolerances are therefore describable by simple parameters, e.g., radius. Since freeform optics are lacking symmetry, such simple tolerance parameters cannot be found. This makes the estimation of shape tolerances nearly impossible as well as the estimation of the impact of manufacturing tolerances on the optical performance. Doubtless, the use of PV and RMS data is very important to characterize and analyze the manufactured optics with respect to quality and accuracy of the manufacturing processes. However, a performance prediction of a fabricated optical system using freeform optics renders to be impossible if only based on PV and RMS data. For performance prediction of the manufactured freeform optics in an optical system, we propose the use of measurement data of the manufactured parts to enhance the optical model. The enhanced model will better resemble the real system hence simulation results can also be used to improve process optimization.  
The approach was shown using Alvarez-optics as freeform surfaces. As manufacturing process, a \(\upmu\)IM process was used. It turned out, that freeform surfaces with seemingly insignificant PV and RMS values could cause severe effects on the optical performance. This was shown for a freeform optics with a 5 mm clear aperture by way of simulation of an enhanced model. Optimization of the \(\upmu\)IM process leads to improved optics with a maximum deviation of only 2% from the nominal MTF values. These results show that enhancing the simulation model by means of metrology data will allow for performance prediction of the optical system. This approach can also be used to evaluate and optimize process parameters to improve the parts manufactured.  
[PERSON] developed the conceptualization of the design approach as well as the underlying methodology. Furthermore, he performed the optical simulation and formal analysis as well as the validation. He also wrote the original draft. A.Y.Y. conceived and designed the advanced microinjection molding process, did investigations with respect to the manufacturing parameters, manufactured the Alvarez-optics, and acquired funding. [PERSON] conceived the system integration of the optics, acquired funding, administrated the project, and contributed the respective text passages as well as reviewing and editing of the complete draft.  
German Federal Ministry of Education and Research (BMBF) (16 SV5472K).  
The work presented in this paper was conducted under the KD OptiMi2 programme (grant no. 16 SV5472K), subtopic B \"Fabrication of housing shells & measurement data\" funded by the German Federal Ministry of Education and Research. The authors would also like to acknowledge [PERSON] from the Ohio State University for his support in manufacturing of the freeform optics and [PERSON] and [PERSON] from Fraunhofer-Institut fur Angewandte Optik und Feinmechanik (IOF) for surface measurement. Furthermore, we acknowledge support by Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of Karlsruhe Institute of Technology.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.