In several standards such as EN 1288 test scenarios for the determination of strength of glass is described. On the one hand there is the ring-on-ring test for the determination of the strength of glass without and on the other hand there is the four-point-bending test for the determination of the strength of glass with the influence of the edge strength. All these tests are described having perfect conditions. For example glass is homogeneous or test set-ups are without any imperfection. One interesting part is the distribution of the residual stress over the thickness and along the surface. Residual stress is caused by thermal or chemical pre-stress of glass. In case of heat strengthened or fully tempered glass photo-elastic studies of the stress distribution showed that the distribution over the surface is not constant. This not constant distribution is shown in a grey pattern, which can be visualized with the principle of polarized light. This paper is a discussion about this grey pattern, imperfections and measured stress with the help of SCALP stress measurement device. This investigation helps to understand much better the reason of the location of the origin of fracture.