Correction of curved screen in deflectometry

摘要

Deflectometry has been widely used in topography measurement of specular surface due to the merits of full-field data acquisition, automatic data processing, high accuracy, and large dynamic range. Existing deflectometry-based methods, such as model phase measuring deflectometry, direct phase measuring deflectometry, stereo deflectometry, employ a plane screen. Due to the size and shape of the used screen, reconstruction of specular surface with large curvature can't be carried out in single measurement. The characteristics of curved screens effectively expand the ranges of the gradient and height field of mirror. Ideally, curved screens are a part of normal cylinders. However, initial experiment shows that the commercialized curved screen changes in radius, instead of a constant value. In order to obtain the model of the curved screen, two CCD cameras capture fringes displayed on the curved screen. Stereo technique is applied to compute screen's point cloud containing curved screen's location and phase. The stereo technique is rectifying pairs of fringe map based on epipolar constraint to the same rotating matrix. The epipolar line of fringe map pair lies in the same row. Then disparity is computed through block match in column coordinate and refined through curve fitting. Some experiments have been carried out and results show that rectifying fringe map helps get more accurate and stable point cloud of curved screen to be suitably employed in deflectometry by a curved screen.