Surface profile measurement of metal objects by use of a fringe projection system with polarized dual projectors

When measuring surface profiles of metal objects by use of the fringe projection technique, a “high dynamic range” issue usually occurs and affects measurement accuracy and efficiency. To alleviate this problem, this paper suggests a fringe projection measurement system that consists of dual projectors having orthogonal polarization directions and a polarization camera with four polarized channels. The projectors simultaneously cast fringe patterns with opposite phases onto the measured metal objects, and the camera grabs four composite patterns at a single shot. By analyzing these composite patterns, two separate fringe patterns that correspond to the projected fringe patterns can be reconstructed. In this procedure, because redundant fringe patterns have been collected, the oversaturated areas that are caused by the “high dynamic range” issue and appear in some of the four composite patterns can be excluded and will not affect the reconstructed fringes. As a result, the reconstructed fringe patterns can have a larger dynamic range over the camera capacity, thus helping to alleviate the effects of the “high dynamic range” issue. By using the phase-shifting technique, the fringe phases and, further, the depth map of the measured object, are calculated. Additionally, this method has an improved measurement efficiency in comparison with others using a single projector because we reconstruct two phase-shifted fringe patterns from a single shot. The validity of this method is demonstrated by using experimental results.