Large-curvature specular surface phase measuring deflectometry with a curved screen

Abstract

Deflectometry has been widely used in topography measurement of the 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, and stereo deflectometry, employ a plane screen, so they have a small field view. The characteristics of curved screens can effectively expand the range of the gradient and height field of the measured specular surface. This paper presents a novel deflectometric technique by using a curved screen to measure large-curvature specular surface. In order to obtain a model of the curved screen, a stereo vision method has been proposed to capture fringes displayed on the curved screen. After phase correction along epipolar lines and homologous points matching, the point cloud containing location and phase is obtained. A virtual plane fixed on the screen is assumed to gain posture and position of the screen. The model of the curved screen is transferred to the deflectometric system through the virtual plane. After calibrating the deflectometric system, height of specular objects can be obtained. Simulated and actual experiments on measuring specular objects have been carried out and the results show high accuracy and large field view for large-curvature specular surface of the proposed technique.