Photoelastic stress analysis. A perforated plastic sheet undergoing stress analysis by photopolari- metry. The plastic sheet is stretched under weight, and the stress effects are detected as birefringence patterns in the plastic (seen as colours). A round filter (blue) polarises the light in order to visualise these stress patterns. The instrument used is a polarimeter. This phenomenon, called photoelasticity, is used in stress calculations in engineering design by building plastic scale models of a subject (a bridge, for example) and examining the stress pattern produced through polarised light.