  Atom-field interactions near optical interfaces have a wide range of
applications in quantum technology. Motivated by this, this paper revisits the
spontaneous emission of atomic dipoles in the presence of a two sided
semi-transparent mirror. First we review the main properties of the quantised
electromagnetic field near a semi-transparent mirror. To do so, we employ a
quantum mirror image detector method which maps the experimental setup which we
consider here onto analogous free space scenarios. We emphasise that the local
density of states of the electromagnetic field depends on the reflection rates
of both sides of the mirror surface. Hence it is not surprising that also the
spontaneous decay rate of an atomic dipole in front of a semi-transparent
mirror depends on both reflectance rates. Although the effect which we describe
here only holds for relatively short atom-mirror distances, it can aid the
design of novel photonics devices.
