(Science: procedure) Although all image formation depends on interference, the term is generally restricted to systems in which contrast comes from the recombination of a reference beam with light that has been retarded by passing through the object. Because the phase retardation is a consequence of the difference in refractive index between specimen and medium and because the the refractive increment is almost the same for all biological molecules, it is possible to measure the amount of dry Mass per unit area of the specimen by measuring the phase retardation. Quantification of the phase retardation is usually done by using a compensator to reduce the bright object to darkness (see Senarmont and Ehrlinghaus compensators). Two major optical systems have been used the Jamin Lebedeff system and the mach zehnder system. These instruments are often referred to as interferometers, since they are designed for measuring phase retardation. Although their use has passed out of fashion, it may be that they will be employed more frequently in future in conjunction with image analysing systems.