  The diffuse reflectance (albedo) and transmittance of a Raman random gain
medium are calculated via semi-analytic two-stream equations with
power-dependent coefficients. The results show good agreement with the
experimental data for barium nitrate powder. Both the Raman albedo AR and Raman
transmittance TR diverge at a critical gain gc, interpreted as the threshold
for diffusive Raman laser generation. However, the ratio TR/AR approaches a
finite limiting value dependent on particle scattering albedo v and scattering
asymmetry g. The dependence of the generation threshold on the scattering
parameters is analysed and the feedback effect of Fresnel reflection at the
gain boundaries evaluated. The addition of external mirrors, particularly at
the pumped surface, significantly reduces the threshold gain.
