We also recorded the Mueller matrix of the set up with the glass substrate only. Up to a normalization factor we deduced which satisfies M glass ≃ I with I the identity matrix. It implies that the optical set up do not induce depolarization and that consequently we can rely on our measurement procedure for obtaining M . The precise form of the theoretical Mueller matrice M $_{L}$th. deduced from equation (1) is with M th. 00 = (2 | A | 2 + | B | 2 + | C | $^{2}$) / 2, M th. 01 = Re [ BA ∗ + AC $^{∗}$], M th. 02 = Im [ AB ∗ + CA $^{∗}$], M th. 03 = ( | C | $^{2}$−  | B | $^{2}$) / 2, M th. 11 = | A | 2 + Re [ B $^{∗}$C ], M th. 12 = Im [ B $^{∗}$C ], M th. 13 = Re [ AC $^{∗}$− BA $^{∗}$], M th. 22 = | A | $^{2}$− Re [ B $^{∗}$C ], M th. 23 = Re [ CA $^{∗}$− AB $^{∗}$], M th. 33 = (2 | A | $^{2}$−  | B | $^{2}$−  | C | $^{2}$) / 2. Similar formula are obtained for M $_{R}$th. after permuting B and C . From the previous relations we deduce the useful equations (valid for M $_{L}$th. ) Together with equation (3) equation (6) allow us to fit B/A and C/A if we replace M th. by M $_{L}$exp. (a similar procedure is applicable to M $_{R}$exp. after permuting B and C ). M glass =     1 . 0000 0 . 0060 − 0 . 0040 − 0 . 0070 − 0 . 0030 0 . 9851 − 0 . 0010 0 . 0020 − 0 . 0020 0 . 0020 0 . 9965 0 . 0030 − 0 . 0050 − 0 . 0040 0 . 0030 0 . 9821     (4) M $_{L}$th. =     M th. 00 M th. 01 M th. 02 M th. 03 M th. 01 M th. 11 M th. 12 M th. 13 M th. 02 M th. 12 M th. 22 M th. 23 −M th. 03 −M th. 13 −M th. 23 M th. 33    $^{.}$ (5) B/A = M th. 01 − M th. 13 th. 00 + M th. 33 + i M th. 23 − M th. 02 th. 00 + M th. 33 C/A = M th. 01 + th. 13 M th. 00 + M th. 33 + i M th. 23 + th. 02 th. 00 + M th. 33 . (6) 11 VI. APPENDIX C: THEORETICAL MUELLER MATRICES.