Birefringence patterns in heated and stretched perspex shapes on top of stretched polythene bags. When polarised light is passed through a clear medium such as plastic, areas under stress rotate the plane of polarisation. Placing a second polarising filter in front of the camera lens creates coloured interference patterns by the process of birefringence. This allows stress patterns to be visualised, and is used in engineering and production research. Each colour and intensity relates to a different level and type of stress (ie compressive or tensile).