Method of locating the mirror center of laser tracker target ball

When using laser tracker to conduct global calibration of multi-sensor 3D vision measuring system, to accurately locate the mirror center of target ball is a challenge due to the complexity of calibration field environment since the spot appearance of target ball is heavily influenced by environment light intensity, visual observation angle of measuring system and projection angle of laser. A method of locating the mirror center of laser tracker target ball is presented, which is free to illumination changes, no requirement of laser spot shape or the distribution of light intensity. First, the image segmentation algorithm is used to accurately locate the target ball area and obtain a located area. The located area is then filtered to reduce the image noise as well as enhance the visibility of the target ball and its background. After that, a mirror area of target ball is obtained by segmenting the target ball, removing strong reflective area and using morphological image processing in sequence. Finally, a mirror center position coordinates of target ball are calculated by performing the statistics of the pixel coordinates of the mirror area. Experiments on 2048×2048 pixel complex spot images are carried out. Experimental results show that the correctly positioned rate is 91.2%, and the average position RMS errors is no more than 0.74 pixel, which indicates that the proposed method can basically satisfy the requirements of the mirror positioning accuracy in global calibration.