A slope-height integrated method for discontinuous specular object measurement using stereo deflectometry

Deflectometry can be used for continuous specular surface measurement with high measurement accuracy due to it is sensitive to slope variation of the measured surface and the surface is reconstructed through slope integration. For the three-dimensional measurement of discontinuous specular objects, the existing phase measurement deflectometry methods use the established relationship between phase information and height value through system calibration. However, the surface form measurement accuracy is low because deflectometry is less sensitive to surface height changes compare to surface slope changes. To overcome this problem, we present a slope-height integrated method in stereo deflectometry. In this approach the measured discontinuous specular object is separated into several continuous regions. Two texture images are captured from two cameras and then the feature points of each sub-region are extracted and matched to obtain the depth data as the height reference of the area. To
obtain the gradient field of the measured continuous regions, sinusoidal fringe patterns are projected onto the screen and the deformed patterns reflected via the surface are captured. The phase-shifting method and the optimum three-number fringe selection method are used to calculate the absolute phase. Thus the surface normal of the object under test is acquired. A slope integration algorithm based on gradient field and height reference is implemented in each sub-region respectively, and the threedimensional shape of the discontinuous specular object is reconstructed. The proposed method combines the advantages of high accuracy slope integral process and the absolute height measurement through texture extraction. This approach leads to high measurement accuracy for the discontinuous specular object. Experimental results show the proposed method is feasible and effective.