To protect them against harsh weather conditions, exposed parts require a paint, lacquer or anodised coating of a certain thickness. Accurately inspecting the coating thickness is a challenge for the practitioner, especially on curved surfaces, because the geometry of the sample influences the measurement result.
Measuring on coated aluminium blinds clearly illustrates a quandary that arises in common practice: The coating thickness should be measured on both the convex and concave sides. While aluminium as substrate material calls for the eddy current principle, precisely this method is extremely sensitive to variant geometries: What is actually being measured here are the changes, induced by the thickness of the coating, in the alternating electromagnetic field generated by the probe. These changes, in turn, depend strongly on the sample shape. Measurement results for coatings on convex substrates are inflated, whereas on concave surfaces the thickness is underestimated.
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