Transitivity in polarization optics and the diagonalization of quadratic   forms
  Any one measurement with polarized light makes it possible to fix the Mueller matrices of the Lorentz type with up to four arbitrary numeric parameters (x, u; z, w). These parameters are subject to the quadratic condition. It is demonstrated that the quadratic form can be diagonalized; in the case of partially polarized light four diagonal coefficients turn out to benon-zero and positive; in the case of completely polarized light two diagonal coefficients equal to zero. 
E. M. Ovsiyuk, N. V. Gutsko, V. M. Red'kov
physics.optics