Photon statistics are the case of Bose-Einstein statistics
with an unspecified total number of particles. The average
number of particles in a state s
is given by the
<ns> = 1/(exp(Es/
kBT) - 1)
where Es is the energy of a particle in s,
T is the temperature,
and kB is Boltzmann's constant.
Black body radiation may be described through photon statistics:
the continuous absorption and re-emission of photons by the
walls of the body, which has nonzero volume and temperature.