A Depolarization Criterion in Mueller Matrices

A necessary and sufficient scalar condition for a Mueller matrix M to describe a non-depolarizing optical system is obtained. By computing only one scalar parameter, it can be determined whether a given Mueller matrix M describes a non-depolarizing, or a depolarizing optical system. The following theorem is stated: a necessary and sufficient condition for a given Mueller matrix M to describe a non-depolarizing optical system is Tr(M T M) = 4m 2 00.