  Semiclassical black holes emit radiation called Hawking radiation. Such
radiation, as seen by an asymptotic observer far outside the black hole,
differs from original radiation near the horizon of the black hole by a
redshift factor and the so-called "greybody factor". In this paper, we
concentrate on the greybody factor-various bounds for the greybody factors of
non-rotating black holes are obtained, concentrating on charged
Reissner-Nordstrom and Reissner-Nordstrom-de Sitter black holes. These bounds
can be derived by using a 2x2 transfer matrix formalism. It is found that the
charges of black holes act as efficient barriers. Furthermore, adding extra
dimensions to spacetime can shield Hawking radiation. Finally, the cosmological
constant can increase the emission rate of Hawking radiation.
