  Negative indices are revisited through the thin-film admittance formalism.
Effective indices and phase delay associated with wave propagation through
negative index layers are carefully defined and computational rules easily
implementable in standard thin-film software are derived from this approach.
This admittance formalism is then used to recover the main features of the
perfect lens and to highlight the benefit of such negative index materials to
improve the performances of quarter-wavelength Bragg mirrors and Fabry-Perot
band-pass filters.
