  The large-scale Galactic interstellar radiation field (ISRF) is the result of
stellar emission and dust re-processing of starlight. Where the energy density
of the ISRF is high (e.g., the Galactic Centre), the dominant gamma-ray
emission in individual supernova remnants (SNRs), such as G0.9+0.1, may come
from inverse Compton (IC) scattering of the ISRF. Several models of the ISRF
exist. The most recent one, which has been calculated by us, predicts a
significantly higher ISRF than the well-used model of Mathis, Mezger, and
Panagia. However,comparison with data is limited to local observations. Based
on our current estimate of the ISRF we predict the gamma-ray emission in the
SNRs G0.9+0.1 and RXJ1713, and pair-production absorption features above 20 TeV
in the spectra of G0.9+0.1, J1713-381, and J1634-472. We discuss how GLAST,
along with current and future very high energy instruments, may be able to
provide upper bounds on the large-scale ISRF.
