  The solution of the FUV radiative transfer equation can be complicated if the
most relevant radiative processes such as dust scattering and gas line
absorption are included, and have realistic (non-uniform) properties. We have
extended the spherical harmonics method to solve for the FUV radiation field in
illuminated clouds taking into account gas absorption and coherent,
nonconservative and anisotropic scattering by dust grains. Our formalism allows
us to consistently include: (i) varying dust populations and (ii) gas lines in
the FUV radiative transfer. The FUV penetration depth rises for increasing dust
albedo and anisotropy of the scattered radiation (e.g. when grains grow towards
cloud interiors). Illustrative models of illuminated clouds where only the dust
populations are varied confirm earlier predictions for the FUV penetration in
diffuse clouds (A_V<1). For denser and more embedded sources (A_V>1) we show
that the FUV radiation field inside the cloud can differ by orders of magnitude
depending on the grain properties. We show that the photochemical and thermal
gradients can be very different depending on grain growth. Therefore, the
assumption of uniform dust properties and averaged extinction curves can be a
crude approximation to determine the resulting scattering properties,
prevailing chemistry and atomic/molecular abundances in ISM clouds or
protoplanetary disks.
