The birefringence of an anisotropic material can be estimated when observed and/or photographed in a polarised light microscope. A relationship between interference colour and retardation can be graphically illustrated in the classical Michel-Levy interference colour chart. The graph plots retardation on the abscissa and specimen thickness on the ordinate. Birefringence is determined by a family of lines that emanate radially from the origin, each with a different measured value of birefringence corresponding to thickness and interference colour.