The present invention provides a tool for and method of using an infrared transmission technique to extract the full stress components of the in-plane residual stresses in thin, multi crystalline silicon wafers including in situ measurement of residual stress for large cast wafers. The shear difference method is used to obtain full stress components by integrating the shear stress map from the boundaries. System ambiguity at the boundaries is resolved completely by introducing a new analytical function. A new anisotropic stress optic law is provided, and stress optic coefficients are calibrated for different crystal grain orientations and stress orientations.