  We derive a non-linear sigma-model for the transport of light (classical
waves) through a disordered medium. We compare this extension of the model with
the well-established non-linear sigma-model for the transport of electrons
(Schroedinger waves) and display similarities of and differences between both
cases. Motivated by experimental work (M. van Albada et al., Phys. Rev. Lett.
66 (1991) 3132), we then generalize the non-linear sigma-model further to
include resonance scattering. We find that the form of the effective action is
unchanged but that a parameter of the effective action, the mean level density,
is modified in a manner which correctly accounts for the data.
