## 5 Conclusions and Outlook  
Optical metrology for industrial applications needs to be precise, fast, robust and capable of coping with complex geometrical conditions. The paper emphasizes on progress in dealing with robustness and complexity as prime areas of improvement. The development starting from traditional phase front measurements to the measurement of the full complex amplitude of a wave field comes to its limits. Tailoring the coherence properties of light according to sample requirements has been demonstrated in the Multi View Digital Holography (MuVie) microscope approach, which employs careful adjustment of the coherence properties of the light in order to separate holograms created simultaneously during the measurement. Already during the work using CoSI, but finally using Multiple Aperture Shear Interferometry (MArS), a new paradigm becomes mandatory, as the use of a multitude of independent light sources requires a description of light fields based on the coherence function. Substantial progress is also demonstrated for measurement methods based on geometrical optics such as deflectometry and fringe projection. Due to faster and more accurate calibration, these techniques are now much better suited for metrology applications in an industrial environment. The examples shown in this paper demonstrate that optical metrology based on fringe projection can deliver reliable results even under such harsh conditions as inside a chamber for powder-based metal melting processes. Further work quantifying the improvements of robustness and complexity is outside the scope of this paper as such a quantification depends on a large number of technical parameters. Further work is also needed in order to achieve faster measurement and data evaluation. Progress in Computer Vision [65], Compressed Sensing [66] and Machine Learning [67] are expected to lead to significantly improved performance. Conceptualization: [PERSON]; methodology: [PERSON], M.K. ; software: [PERSON], M.K. ; validation, C.F., M.K. ; formal analysis: [PERSON], M.K. ; investigation: [PERSON], M.K. ; resources: R.B.B. ; data curation: [PERSON], M.K. ; writing--original draft preparation: [PERSON], [PERSON], M.K. ; writing--review and editing: [PERSON]; visualization: [PERSON], C.F., M.K. ; supervision: [PERSON]; project administration: [PERSON], C.F., M.K. ; funding acquisition, R.B.B. All authors have read and agreed to the published version of the manuscript. **Funding:** Research based on holography and shear interferometry has been funded by the projects _Quality Inspection and Logistic Quality Assurance of Micro Technical Manufacturing Processes_ (B5) and _Inspection of Functional Surfaces on Micro Components in the Interior of Cavities_ (T3) within the collaborative research center _Cold Micro Metal Forming_ (SFB 747), the project _Optical surface metrology with spatially and temporally partially coherent light wave fields_ (OPAL) project no. 258565427, and the project _Verfahren zur vollstandigen Bestimmung koharenter Wellenfelder nach dem Prinzip der Selbstferenzen_ (WeSer), project no. 60045034, all funded by the Deutsche Forschungsgemeinschaft (DFG), and project _Robuste Digitale Holografie in der Produktionsumebeung_ (RobuHoP), project no. ZF4587001 LT8, funded by the Federal Ministry for Economic Affairs and Energy (BMWi) via the German Federation of Industrial Research Associations (AiF). Work concerning BLADE was carried out within the EU's CleanSky program under JTI-CS-2010-5-SFWA-03-004 and Grant Agreement 286745 and 641489 as well. We sincerely thank the EU for support of this research. We would like to thank [PERSON] (EU Project Officer CSJU) and [PERSON] (Project Lead, Metrology and Assembly, Airbus UK) for collaboration and support within BLADE. The work concerning PBF-LB/M is funded in the framework of the IGF-Project No. : 21.178 N / DVS-No. : 13.3214 of the \"Forschungsveeringung Schweisen und verwandte Verfahren e.V.\" of the German Welding Society (DVS), Aachener Str. 172, 40223 Dusseldorf.