Optical anisotropy is studied in the polarised light microscope with accessory
plates that are divided into two primary categories: retardation plates that have a fixed optical path difference and compensators, which have variable optical path lengths. Addition of a retardation plate or compensator to the polarised light microscope produces a highly accurate analytical instrument that can be employed to determine the relative retardation (often symbolised by the Greek letter G) or optical path difference between the orthogonal wavefronts (termed ordinary and extraordinary) that are introduced into the optical system by specimen birefringence. The terms relative retardation, used extensively in polarised light microscopy, and optical path difference (D or OPD), are both formally defined as the relative phase shift between the orthogonal wavefronts, expressed in nanometers.