Diffractive Optical Element Design For Generating Point-array Structured Light

With the development of 3D digital technology,the demand for low-cost,low-power,small-sized three-dimensional measuring equipment is increasing,which has led to the development of a micro-structured light three-dimensional measuring device based on diffractive optical elements,which has been widely used in industry.Measurement,medical diagnosis,simulation training and many other fields.This new type of three-dimensional measuring device based on diffractive optical elements has a short development time.There are still some problems in structural optical coding and device design that need to be improved:(1)Traditional structured light uses use multiple symbols or multi-level color coding to increase information redundancy.Quantity,reduce the length of the code to improve the measurement speed,accuracy and anti-interference ability,while the micro-structured light three-dimensional measuring device is limited by the diffractive optical element t o generate a simple coding pattern,so a suitable design of high amount of information code scheme for the diffractive optical element is of great significance.(2)The three-dimensional measurement of point array coded structure light mainly depends on the matching of neighborhood feature points,and the relative position of the projected feature points.Therefore,how to realize the clear and accurate projection of the coded point array to the surface of the object is the main problem of designing the diffractive optical element.Aiming at the above two problems,this thesis mainly studies the optimization algorithm of point array structured optical coding and diffractive optical element design,and proposes a large size M array coding scheme with binary,small window,high density and long average Hamming distance.An improved G-S algorithm is used to verify the design algorithm through processing experiments.The point array structure light projection conforming to the coding scheme is obtained,which is of great significance for improving the accuracy and speed of the micro-structured light three-dimensional measurement system.The main work of the thesis includes the following:(1)A binary coding method based on formula method and exhaustive method coding is proposed.This method solves the problems of excessive length-to-width ratio and non-central symmetry of the formula method by point-by-point optimization.Besides,it breaks through the size limitiation of exhanstive method and reduces computation time.Compared with Kinect's coding scheme,the proposed coding method has longer code point density,smaller single sub-window and longer average Hamming distance,which lays foudation for structured light 3D measurement with stronger anti-interference ability,faster decoding speed and higher precision.(2)In order to improve the uniformity of point array projection,we propose a new improved algorithm based on G-S algorithm.Compared with classical algorithm,this algorithm greatly alleviates the initial phase sensitivity of G-S algorithm while maintaining the convergence speed of G-S algorithm.At the same time,the proposed algorithm has smaller amount of computations than that of SA algorithm,and the feasibility of the algorithm is verified by strict Rayleigh-Sommerfeld diffraction integral.(3)Multiple sets of diffractive optical elements were fabricated by micro-nano fabrication technology.The optical measurement system was built to verify that the relative position of the projected point array is completely consistent with the code designed in(1),the overall deformation of the projected point array is small,and The outline of the projection point is clear and non-adhesive.Finally,the error sources of the strong zero-order diffraction and cross-shaped diffraction spots in the projected point array are analyzed.It is determined that such problems are mainly caused by the etching depth error and the gap between the structure and the pupil.