Calibration of a combined system with phase measuring deflectometry and fringe projection

There is a variety of well developed methods to measure diffusely reflecting free-form surfaces. For instance fringe projection based systems are commonly used for the contactless optical measurement of such surfaces. The calibration procedures used in these systems are well understood and state of the art [10, 4]. However, contactless measurement of specular free-form surfaces requires new measurement techniques along with the corresponding calibration methods. In this paper a multi-sensor approach for measuring specular free-form surfaces using stereo based phase measuring deflectometry (PMD) combined with a fringe projection sensor unit will be presented. With the stereo enhancement over classical PMD techniques it is possible to measure the shape of the specular free-form surface and not only the slope of the surface. However, the major challenge is the necessary calibration. We present a new calibration method to obtain the orientation information of the deflectome try screen and the measurement cameras needed to calculate the shape of the object from the observed phase values. To solve this task we combined the calibration/ measurement principles of fringe projection and phase measuring deflectometry into a single measurement cycle. The developed calibration algorithm will be described in detail along with an analysis of the calibration accuracy using simulated data.