Stress patterns in plastic geometry instruments revealed by photopolarimetry. When plastic objects are manufactured, internal stresses may build up. These stresses are induced both at the time that the object is pressed and by differential cooling. Areas under different stresses rotate polarised light by different amounts. By placing the object between two crossed polarising filters, the stress patterns show as various colours. An individual shade of each colour represents a discrete level of stress. The birefringence patterns, as they are called, may be used to predict stresses in large structures by applying suitable forces to a scale model, e.g. of a bridge component.