Coloured light interference patterns on a plastic ruler & shallow dish (manufactured by an injection moulding process), used to illustrate the principal of photo-elasticity as applied to stress distribution in objects under complex systems of loading. The object is photographed using cross polarised white light; the pattern of colours provides an isochromatic contour map of stress conditions throughout the object (isochromatic areas have the same colour & indicate similar conditions of stress). Transparent plastic models with stress loads proportional to real objects are made to determine stress distributions.