  The local observables of the quantised electromagnetic field near a
mirror-coated interface depend strongly on the properties of the media on {\em
both} sides. In macroscopic quantum electrodynamics, this fact is taken into
account with the help of optical Green's functions which correlate the position
of an observer with all other spatial positions and photon frequencies. Here we
present an alternative, more intuitive approach and obtain the local field
observables with the help of a quantum mirror image detector method
[Furtak-Wells et al., Phys. Rev. A 97, 043827 (2018)]. In order to correctly
normalise electric field operators, we demand that spontaneous atomic decay
rates simplify to their respective free space values far away from the
reflecting surface. Our approach is interesting, since mirror-coated interfaces
constitute a common basic building block for quantum photonic devices.
