ESPSI or shearography has been successfully used in non-destructive testing (NDT) for measuring the derivative out-of-plane displacement, w/x and/or w/y, and recently a few investigations of derivative in-plain displacement, u/x, v/y, u/y and v/x have been reported. The demand of quantitative measurement has encouraged manufacturers to produce a quantitative shearography system. However, errors arising from numerous sources, including wavefront divergence and object geometry are not taken into account when calculating the interferometer fringe function and quantifying the data. In this paper the novel approach in quantified error that propagates from the divergence illumination wavefront is presented. The theoretical error is formulated by means of mathematical approach that comprises of three dependent variables, the inspected object distance, the object size (field area covered by the CCD camera) and the illumination angle. The error defined by the difference of phase data using divergence illumination object wavefront compared to phase data that is measured by using collimated illumination wavefront. Theoretical analysis and experimental validation indicates that the magnitude of the maximum phase change difference due to the divergence of illumination wavefront to exceed 10% for out-of-plane and 40% for in-plane analysis.