Stress Birefringence in Safety Glass

This is the edge of a fragment of broken safety glass viewed with crossed polarized light. It shows the change in interference color as a function of the thickness of the glass. The stress is the constant but the path length through the glass changes as a result of wedge shape of this fragment. The red plate compensator was inserted so that the first order red becomes black and the retardation and associated interference color increases in both directions away from the black band.