Phase-Stepping Fringe Analysis by Rotation of a Combined Projection and Camera Unit

We propose a novel phase-stepping technique that is suitable for an optical fringe projection unit mounted to a coordinate measurement machine (CMM). The fringe projection unit consists of a camera and a compact fringe projector that projects only a single fringe pattern on to the test surface. The projection of a single fringe pattern eliminates the need for delicate moving mechanical parts (e.g. shifting a grating) which reduces cost, is more dimensionally stable and eliminates the warm-up time associated with a data projector. The lightweight optical unit is moved around the test surface and its position is accurately reported by the CMM on which it is mounted. The relative motion of the object and the probe results in a shift in the phase of the pattern recorded by the camera. Therefore the projected fringe phase is calculated from a series of images with relative motion between the camera and projector unit and the surface. Furthermore, phase shifted images can be used to distinguish features on the object such as steps, edges and vertices. Results of calibrated surface shape measurements are presented for an object using multiple perspectives, and a full three dimensional model of an unknown object built up with minimal user intervention. The accuracy of the new phase stepping technique is compared to traditional phase stepped results. Initial measurements for an object with a curved surface and a step feature are presented with an accuracy of 60 mu m within the measurement volume.