  Through a simple procedure based on the Lu-Chipman decomposition [S-Y. Lu and
R. C. Chipman, J. Opt. Soc. Am A 13, 1106 (1996)] any depolarizing Mueller
matrix can be transformed into a reduced form which accumulates the
depolarization and polarizance properties into a set of six parameters. The
simple structure of this reduced form provides straightforward ways for the
general characterization of Mueller matrices as well as for the analysis of
singular Mueller matrices.
