Research Of Calibration Method Of Optical 3D Measurement System For Specular Objects

The specular objects are difficult to be measured because of their high reflective surface.Using the coordinate measuring machine and spraying method to change the surface characteristics can not satisfy the demand of the modern industry.As a result,the measurement of the specular objects gets the high attention and a lot of research from all over the world.The fast development of the optical measurement technology provides the direction of obtaining the three-dimensional shape of specular objects.System calibration is to determine all the parameters of the measurement modal,which is the most important step and determines the accuracy of the measurement results.Due to the limited depth of field of the imaging lens,the displaying screen and the measured specular objects cannot be clearly captured.Therefore,the existing calibration methods have the drawbacks of low accuracy and complicated procedure.In order to solve these problems,this paper proposes a new calibration method to obtain the system parameters by using digital phase target.As a result,it can build the relationship between the absolute phase map and depth data.Firstly,the orientation between CCD camera and reference mirror should be determined according to a matrix of discrete hollow ring markers on a mirror surface.Then horizontal and vertical sinusoidal fringe patterns with the optimum fringe numbers are subsequently generated and displayed on two LCD screens.The fringe patterns are reflected by the reference mirror and captured by the CCD camera from another position for post-processing.Absolute phase data are calculated using the optimum three-fringe number selection method and a four-step phase shifting algorithm.The orientation between CCD camera and the LCD screens can be determined according to phase matching principle.Finally,the distance between reference mirror and the LCD screens could be obtained for finishing the calibration of system parameters.An experiment has been carried out for comparing the obtained extrinsic parameters of the LCD screen by using phase data and the checkerboard.Finally,the results of two methods should be compared with the actual distance stepped through a translating stage.The results show that the method using phase data is more accurate when images are out of focus.In addition,the paper analyzes various errors in the process of calibration in detail and provides the corresponding correction method.Using the calibrated system,an artificial specular step with discontinuous reflective surface and a concave mirror have been tested with the average error of 22 um.Experimental results show that the proposed method precisely determines the system parameters,so that 3D shape ofspecular objects can be measured with a high accuracy.Besides,the calibration method is simple and easy to be accomplished on site,so it has an high application value.