  Several authors, most notably Heckler, have claimed that the observable
Hawking emission from a microscopic black hole is significantly modified by the
formation of a photosphere around the black hole due to QED or QCD interactions
between the emitted particles. In this paper we analyze these claims and
identify a number of physical and geometrical effects which invalidate these
scenarios. We point out two key problems. First, the interacting particles must
be causally connected to interact, and this condition is satisfied by only a
small fraction of the emitted particles close to the black hole. Second, a
scattered particle requires a distance ~ E/m_e^2 for completing each
bremsstrahlung interaction, with the consequence that it is improbable for
there to be more than one complete bremsstrahlung interaction per particle near
the black hole. These two effects have not been included in previous analyses.
We conclude that the emitted particles do not interact sufficiently to form a
QED photosphere. Similar arguments apply in the QCD case and prevent a QCD
photosphere (chromosphere) from developing when the black hole temperature is
much greater than Lambda_QCD, the threshold for QCD particle emission.
Additional QCD phenomenological arguments rule out the development of a
chromosphere around black hole temperatures of order Lambda_QCD. In all cases,
the observational signatures of a cosmic or Galactic halo background of
primordial black holes or an individual black hole remain essentially those of
the standard Hawking model, with little change to the detection probability. We
also consider the possibility, as proposed by Belyanin et al. and D. Cline et
al., that plasma interactions between the emitted particles form a photosphere,
and we conclude that this scenario too is not supported.
