Research On Multi-view Fringe Projection 3D Micro-measurement System

The demands for three-dimensional(3D)shape measurement at the scale of centimeter and millimeter are quickly growing along with the trends of precision and miniaturization for electronic and mechanical system.The classic techniques for optical 3D microscopic shape measurement are briefly reviewed.Then the applications of the fringe projection technique to the 3D measurement of micro objects(with the scale of mm to cm)are surveyed.When the scale of the object to be measured is relative small,the limited depth of focus(DOF)and shiny surface are the main factors which impact the quality of measurement.To address these challenges,the multi-view fringe projection 3D microscopy(MVFP-3DM)system is designed and established.Related problems are investigated,which include system modeling and calibration,data synthesis and analysis.Finally,the high-quality 3D measurement of micro objects is achieved.The main research contents of this thesis are as follows:(1)A basic monocular 3D micro measurement is built based on a stereo vision microscope,which provides a preliminary validity for the feasibility of applying fringe projection to the 3D measurement of micro objects.To reduce the side effect of limited DOF,the Scheimpflug principle is introduced into the imaging branch.Experiments demonstrate that this configuration will maximize the measurable volume of the system by well overlap the focusing area of different imaging branches.To improve the accuracy and robustness of measuring shiny surface,the MVFP-3DM system with 4 imaging branches and 1 projection branch is established by using telecentric lens with long work distance.(2)Due to the telecentric lens and Scheimpflug structure,the classic perspective projection model is not suitable for the proposed MVFP-3DM.Therefore,an approach of non-parametric calibration based on the general imaging model is proposed.Any special structure of the system can be implicitly contained in the corresponding relationship between imaging pixels and incoming chief rays.Considering the collinearity of the ray,proposed system is calibration based on three different poses of the calibration target.(3)Another approach of parametric calibration is proposed based on the special design of the MVFP-3DM system.The telecentric lens is described with orthographic projection,and the distortion model is modified to fit the Scheimpflug structure,which lead to a parametric model of the system.Then the calibration starts form estimating the initial values of the parameters and ends with a non-linear optimization.The calibration approach is verified with the reprojection errors and measurement results.(4)To avoid the influence of defocusing on the locating of markers,a micro organic light-emitting diode(OLED)display is proposed to serve as an active calibration target,and the edge intersections of orthogonal binary fringes serve as the markers.By using complementary binary fringes,the edge extraction escapes from the limitation of focusing level,which improve the flexibility for changing poses of the calibration target.(5)The redundancy and complementarity arising from the multi-view configuration are fully utilized to promote the performances of 3D measurement.The accuracy of 3D reconstruction is improved by calculating the intersection of lights from multiple viewpoints.The measurable range in depth is extended after making each imaging branch have in turn raised focusing positions.The missing areas due to occlusion are greatly reduced by merging range data from different viewpoints.Factors that affect the quality of phase reconstruction are analyzed,and then the preprocess via intensity normalization and smooth filtering is proposed to improve the accuracy of phase reconstruction and subsequent 3D reconstruction.