Contamination influence on indirect optical geometry measurements

• Indirect geometry measurement using a fluorescent aerosol stream. • Influence of the aerosol flow on the uncertainty of the indirect geometry measurement. • Effects of the aerosol flow on the fluorophore contamination of the measuring surface. • Areal indirect optical geometry measurement on a microstructure. The article examines the influencing factors and challenges involved in indirect optical geometry measurement using fluorescent aerosol particles. The aim is to improve the implementation and accuracy of the method for surface topography measurements, as contamination of the measurement surface by aerosols has limited its use for long scanning times to date. Therefore, the spatiotemporal development of the surface contamination and its implication for the measurement error are analysed for a jet flow as particle supply flow that is inclined to a flat plate. As a result, the contamination increases rapidly after a critical period of time and then leads to severe measurement errors. In order to minimize the contamination, the measurement process is accelerated, the contamination rate is minimized by optimized flow conditions, and a surface cleaning cycle is introduced. In particular, the measurement time is reduced by a factor of 4 with the implementation of a sweep scanning and a fast photodetector, a low flow velocity and inflow angle is shown to be an ideal supply flow condition, and a surface cleaning with compressed air is shown to be highly effective. In addition, an ideal measurement region behind the supply flow impact point on the flat plate is identified to achieve a minimal measurement uncertainty of 4 µm. Based on these findings, a 2.5D topography measurement on a one-euro coin is demonstrated, confirming the method's abilities to achieve micrometre resolution and imaging capability. While systematic errors remain for complex geometries to be studied in future research, the experimental results show the potential of the indirect optical geometry measurement paradigm for precise and accurate measurements despite the contamination hazard. In order to get rid of any contamination in the future, a new approach is further presented as part of the outlook.