We present an experimental study of real time true stress-strain-birefringence measurements to elucidate the sequence of structural mechanisms that occur during simultaneous biaxial stretching of PET films from amorphous precursors in rubbery state. Stress birefringence relationship, wide angle Xray diffraction, Raman spectroscopy and DSC thermal analysis were used to identify the stages of the mechano-optical behavior of the films during stretching, and to identify their structural origins. The measurements revealed four regimes for the relationship between the stress and birefringence. In the first regime the stress has a linear relationship with birefringence where the linear stress optical rule holds and the stress optical constant for PET is 5.8 GPa(-1) (5800 Brewster). In the second regime, the relationship is also nearly linear with a steeper positive slope, and in the third regime the relationship is nonlinear. At very high stretching rates, a fourth stage could be seen, where the stress increases while the birefringence reaches a plateau. This stage is reached when the polymer chains attain their finite extensibilities. This stage was not observed if low rates of stretching employed, where high relaxation movements dominate the orientation effects. The deviation from the initial linear stress optical rule coincides with the onset of the stress-induced crystallization as revealed by the off line measurements. This transition was found to be rate dependent and increased rate delays this transition to higher stresses. (C) 2013 Elsevier Ltd. All rights reserved.