Model-based assistance system for confocal measurements of rough surfaces

Abstract

Confocal sensors are well established in optical surface metrology and their performance has been thoroughly studied both experimentally and theoretically. However most of the theoretical work has been based upon the assumption of locally flat or point like measurement objects. As confocal sensors have become increasingly popular in industrial inspection of rough surfaces in recent years, severe measurement artifacts have been observed in certain situations. The physical reason for these artifacts was not fully understood and therefore a systematic procedure to choose a set of sensor parameters, that minimizes the impact of these artifacts, has been missing. In fact planning measurements of rough surfaces has been a formidable task that even highly experienced experts approached on a trial and error basis. To make things even worse, different confocal measurement systems, e.g. from different manufacturers, and different sensor parameters, e.g. different numerical aperture objectives, typically give substantially differing results. A reliable interpretation of these results let alone a sound judgement of the remaining uncertainty in the measurement results is very difficult. Starting from a quick review of a recently developed signal model, we therefore present an attempt to systematically guide the user of confocal sensors through the planning of an inspection task. In order to support our proposal, we present measurements of two roughness calibration standards, that were conducted with varying numerical aperture objectives on a custom build confocal microscope with rotating micro lenses. The uncertainty in these measurements is then compared to the predictions of our assistance system.