Focus variation – a robust technology for high resolution optical 3D surface metrology – Reinhard Danzl, Franz Helmli, Stefan Scherer

This article describes and evaluates the focus variation method, an optical 3D measurement
technique. The goal is to analyse the performance of the method on a series of typical measurement tasks
including roughness measurements, form and wear measurements. First, a comparison of roughness
measurements between the proposed method and a tactile device on a newly developed roughness standard
is made.
Results show that both systems deliver Ra values that are comparable to each other with differences
of a few nanometers. Afterwards form measurements are performed on a calibration standard with
hemi-spherical calottes, showing a repeatability of sphere measurements < 100 nm. Finally, two typical
engineering applications are provided. The first is wear measurement of cutting tools, the second the
inspection and classification of welding spots. Both applications demonstrate the ability of the method to
measure steep surface flanks up to 80° and surfaces with difficult reflectance behaviour.