  We derive the phase space density of bosons from a general boson
interferometry formula.
  We find that the phase space density is connected with the two-particles and
the single particle density distribution functions. If the boson density is
large, the two particles density distribution function can not be expressed as
a product of two single particle density distributions. However, if the boson
density is so small that two particles density distribution function can be
expressed as a product of two single particle density distributions, then
Bertsch's formula is recovered. For a Gaussian model, the effects of
multi-particles Bose-Einstein correlations on the mean phase space density are
studied.
