3D Shape Measurement For Discontinuous Specular Object Based On Telecentric System

There are a large number of specular surfaces in optics,precision instruments and other devices.And it is very important to accurately measure the 3D shape of the specular objects.However,the research of shape measurement for these objects is still in the early stage.The main methods for measuring 3D shape of those objects are using coordinate measuring machine or changing the surface characteristics by spraying paint.Therefore,it is urgent to study an effective non-contact optical shape measurement method for specular objects.This paper presents a full-field 3D shape measurement method based on telecentric system for measuring discontinuous specular surfaces by building the direct relationship between absolute phase and depth data.Two LCD monitor planes are located at different positions during the procedure of calibration and measurement.Fringe pattern sets are generated by software and displayed on LCD monitor planes.The reflected fringe patterns are deformed with respect to slope and depth of the specular surfaces and captured by a camera with telecentric lens.Telecentric lens shows a very low distortion and offers a large depth of field.Therefore,it has obvious advantages to calibrate the system and measure the specular surface with large depth of changes.Wrapped phase map and absolute phase map are obtained from the captured fringe patterns by using four-step phase-shifting algorithm and the optimum three-fringe numbers selection method,respectively.And this paper proposes a novel method to calibrate system parameters.The method can directly obtain the rotation matrix between LCD monitor plane and the reference specular plane.After the parameter of the system has been calibrated,depth information can be directly obtained from the obtained absolute phase information.Because depth directly relates to absolute phase without needing gradient integration,the proposed method can measure discontinuous specular objects.Initial experimental results on measuring a concave mirror and a monolithic multi-mirror arrays with multiple specular surfaces show that the system effectively obtains full-field 3D shape information of discontinuous specular object.