  Observations of quasars (QSOs) shining through or close to galaxies offer a
way to probe the properties of the foreground matter through dust extinction
and gravitational lensing. In this paper the feasibility of measuring the dust
extinction properties is investigated using the backlitting of QSOs. We test
our method to search for QSOs affected by intervening extinction, by matching
the coordinates in the SDSS QSO DR3 catalogue with the New York University
Value-Added Galaxy Catalog. In total, 164 QSO-galaxy pairs were found with a
distance of less than 30 kpc between the galaxy centre and the QSO
line-of-sight at the galaxy redshift. Investigating the QSO colours with
multiband SDSS photometry, two pairs with galaxy redshifts z < 0.08 were found
to be particularly interesting in that the QSOs show evidence of heavy Galactic
type extinction with R_v ~ 3.1 at very large optical radii in the foreground
spiral galaxies. With the available data, it remains inconclusive whether the
two pairs can be explained as statistical colour outliers, by host extinction
or if they provide evidence of dust in the outskirts of spiral galaxies. Deeper
galaxy catalogues and/or higher resolution follow-up QSO spectra would help in
resolving this problem. We also analyse five QSOs reported in the literature
with spectroscopic absorption features originating from an intervening system.
These systems are at higher redshifts than the other two and we find in most
cases significantly lower best fit values of R_v. The wide range of preferred
values of R_v found, although affected by substantial uncertainties, already
indicates that the dust properties in other galaxies may be different from the
Milky Way. Furthermore, the available data suggests a possible evolution in the
dust properties with redshift, with lower R_v at high z.
