  Heckler has recently argued that the Hawking radiation emitted from
microscopic black holes has sufficiently strong interactions above a certain
critical temperature that it forms a photosphere, analogous to that of the sun.
In this case, the visible radiation is much cooler than the central temperature
at the Schwarzschild radius, in contrast to the naive expectation for the
observable spectrum. We investigate these ideas more quantitatively by solving
the Boltzmann equation using the test particle method. We confirm that at least
two kinds of photospheres may form: a quark-gluon plasma for black holes of
mass M_{BH} < 5 times 10^{14} g and an electron-positron-photon plasma for
M_{BH} < 2 times 10^{12} g. The QCD photosphere extends from the black hole
horizon to a distance of 0.2--4.0 fm for 10^9 g < M_{BH} < 5 10^{14} g, at
which point quarks and gluons with average energy of order \Lambda_{QCD}
hadronize. The QED photosphere starts at a distance of approximately 700 black
hole radii and dissipates at about 400 fm, where the average energy of the
emitted electrons, positrons and photons is inversely proportional to the black
hole temperature, and significantly higher than was found by Heckler. The
consequences of these photospheres for the cosmic diffuse gamma ray and
antiproton backgrounds are discussed: bounds on the black hole contribution to
the density of the universe are slightly weakened.
