Suppression of contrast-related artefacts in phase-measuring structured light techniques

The advantages of the phase-measuring approach in optical metrology have helped it spread from interferometry to fringe projection and fringe reflection. The latter two techniques generally use a spatiotemporal phase-shifting approach: A sequence of fringe patterns with varying spacing is used, whose phase maps allow the calculation of unique data sets by mapping camera pixels to projector or monitor pixels. One ubiquitous problem with phase-shifting structured-light techniques is that phase artefacts appear near regions of the image where the fringe modulation changes abruptly, e.g. near dirt/dust particles on the surface in deflectometry or reflectivity transitions in fringe projection. The phenomenon has been known for a long time but is usually ignored because it does not compromise the overall reliability of results. In deflectometry, however, often the objective is to find and classify small defects, so that phase artefacts must be distinguished from actual surface features. We present an analytical derivation of the error terms, study the parameters influencing the phase artefacts (in particular the fringe period), and suggest some simple algorithms to reduce them.