Normal strain measurement by machine vision

•A detailed study on precious strain measurement by machine vision.•The non-coplanarity of the imaging and measured plane is fully considered.•The method of calibrating the parameters of the model is provided.•The test of the factors that will affect the precision of the measurement were carefully designed.

A machine vision method for measuring normal strain with high precision is presented. In the method, planar projection is modeled in order to enable the measurement without requiring coplanarity of the imaging and measured plane. Some of the model parameters are determined using translational displacements of known value. Along the single direction according to the need of measurement, the correction of distortion is involved in the method to improve the precision. Experiments were designed to validate the accuracy of the proposed method and confirm factors affecting the measurement results.