This paper presents an analytic approach to spontaneous emission in re
sonators with distributed Bragg reflectors (DBR's). The foundation of
our analysis is the hard mirror (or penetration depth) approximation,
which we extend to radiation with both angular and frequency distribut
ions. This has allowed us to derive approximate analytic expressions f
or the divergence angle of the spontaneous emission, the spontaneous e
mission rate and the spontaneous emission coupling factor in planar DB
R resonators. These analytic tools provide insight into the considerab
le limitations to controlling spontaneous emission with DBR boundaries
. We also explore cavity controlled spontaneous emission with the clas
sical tools of intracavity field profiles, the induced EMF Method and
millimeter wave experiments-all of which are applied to distributed mi
rror boundaries.