Calibration error for dual-camera digital image correlation at microscale

Digital image correlation (DIC) has been widely conducted in many engineering applications. This paper describes a dual-camera system which is mounted on a stereo light microscope to achieve 3D displacement measurement at microscale. A glass plate etched with precision grids was used as the calibration plate and a translation calibration procedure was introduced to obtain the intrinsic and extrinsic parameters of the cameras as well as the aberration of the imaging system. Two main error sources, including grid positioning and stage translation, were discussed. It was found that the subpixel positioning errors had limited influences on displacement measurement, while the incorrect grid positioning can be avoided by analyzing the standard deviation between the grid spacing. The systematic translation error of the stage must be eliminated to achieve accurate displacement measurement. Based on the above analysis, a precisely controlled motorized calibration stage was developed to fulfill fully automatic calibration for the microscopic dual-camera system. An application for measuring the surface texture of the human incisor has been presented. It is concluded that the microscopic dual-camera system is an economic, precise system for 3D profilometry and deformation measurement.

► A stereo microscope camera system incorporated with the motorized translation stage and the Tsai's calibration method was presented to achieve 3D profile measurement.
► The error introduced by the motorized stage in calibration step could cause severe errors in displacement measurement.
► The ratio of f to Tz can be used to determine if the camera calibration is precise, since it should be equal to the magnification factor of the microscopic system.