  In this paper, first we present a review of experimental data corresponding
to phase retardation per reflection of interferential mirrors. Then, we report
our new measurements on both commercial and tailor-made mirrors. To be able to
measure the phase retardation as a function of the number of layers, additional
pairs of layers are deposited on some of the mirrors. The results obtained with
this special set of mirrors allow us to fully characterise the waveplate
associated with the additional pairs of layers. We finally implemented a
computational study whose results are compared with the experimental ones.
Thanks to the additional layers, we have achieved reflectivity never measured
before at $\lambda=1064$~nm, with an associated finesse of
$\mathcal{F}=895~000$.
