## 6 Discussion and Conclusions  
The measurement system based on low coherence interferometry described here enables measurements to be taken directly at the point of ablation. This also requires a new method of controlling the machining process. The necessary synchronization between the machining process and the measurement increases the machining time for a workpiece. The increase in time depends on factors such as the number of measuring points or the possible recording frequency of the spectrometer used and is therefore dependent on the actual process. No removal of the workpiece is necessary, so there is no need for repositioning work. The ablation of a laser structuring process usually takes place in several cycles, with  
Figure 10: Integration unit for coupling the beam paths in the machine and for manipulating the measuring beam.  
material in the nm range being ablated in each cycle [32]. This is also a limitation of the LCI process, with a repeatability of 0.36 \(\upmu\)m an ablation cycle is not detectable, but a set of them. If the workpiece is inspected at the end of the machining process to determine target parameters or to adjust process parameters, the resulting delay is much smaller than if each cycle of the process is measured. If the machining process runs without the use of the measuring system, there is no delay or further influence on the process.  
The basic challenge of the spot offset was solved by integrating a separate scanner for the measuring system and a model for control. This integration also opens up new application possibilities. In this context, the goal was to achieve a spot overlap, which was successfully achieved with the control model. The model can be adapted to other optical systems. The generation of the correction data is very complex. There is a need for further research to reduce the amount of data and to find a more effective way to integrate the control into the process [33]. The independent control of the measuring and processing beam allows further investigations of the measuring strategy. For example, the measuring spot can be positioned in front of or behind the ablation point and new process data can be generated or interactions between the measuring light and the plasma created by the ablation can be bypassed. In addition to pure process monitoring, data volumes for AI approaches can also be recorded in this way. Spot offset compensation can be useful not only in the topographical case discussed here, but also in the application of classical OCT, e.g., for process monitoring in laser transmission welding [34]. However, further research is needed on these points.  
With the methods presented here, an extension of the LCI by a sport offset compensation was made and the system was developed to meet the specific project requirements. It could be shown that the target structures can be solved, and how a spot offset can be compensated, or which mechanical adjustments have to be made for a system integration into an existing machine. As the ESSIAL research project is still ongoing, the presented method will be further developed and adapted if necessary.  
Conceptualization, F.Z.; methodology, F.Z. and J.J.; software, F.Z. and J.J.; validation, F.Z. and J.J.; formal analysis, F.Z.; investigation, F.Z.; resources, F.Z.; data curation, F.Z. and J.J.; writing--original draft preparation, F.Z. and J.J.; writing--review and editing, F.Z., J.J. and R.H.S.; visualization, F.Z. and J.J.; supervision, R.H.S.; project administration, F.Z. and R.H.S.; funding acquisition, R.H.S. All authors have read and agreed to the published version of the manuscript.  
The research was funded by European Commission Project ESSIAL (Grant Agreement n. 766437, EU H2020 RIA FOF-06-2017).  
The authors declare no conflict of interest.