During uniaxial deformation of amorphous PEN poly (ethylene 2,6 naphthalate) films, necking is developed even at rubbery temperatures above the glass transition temperature. To elucidate the structural changes occurring during neck formation and further strain hardening, a real time spectral birefringence technique together with a true stress / true strain measurement technique is applied. These techniques are able to track the changes in birefringence and stress levels as the film is being deformed. The results obtained by these two techniques permit determination of the stress optical constant and the limits at which it starts deviating from the linear behavior as well as large deformation behavior. Further investigation by X-ray and DSC measurements helps to understand and clarify the structure developed during the deformation process of the material.