  Multiply-imaged quasar lenses can be used to constrain the substructure mass
fraction in galaxy-sized dark matter halos via anomalous flux ratios in lensed
images. The flux ratios, however, can be affected by both the substructure in
the lens halo and by isolated small-mass halos along the entire line-of-sight
to the lensed source. While lensing by dark matter clumps near the lens galaxy
is more efficient than elsewhere, the cumulative effect of all objects along
the line-of-sight could be significant. Here we estimate the potential
contribution of isolated clumps to the substructure lensing signal using a
simple model motivated by cosmological simulations. We find that the
contribution of isolated clumps to the total lensing optical depth ranges from
a few to tens percent, depending on assumptions and the particular
configuration of the lens. Therefore, although the contribution of isolated
clumps to the lensing signal is not dominant, it should not be neglected in
detailed analyses of substructure lensing. For the currently favored Lambda-CDM
model, the total calculated optical depth for lensing is high, tau ~ 0.2-20 and
could, therefore, naturally explain the high frequency of anomalous flux ratios
in observed lenses. The prediction, however, is highly sensitive to the spatial
distribution of substructure halos in the innermost regions of the lens halo,
which is still very uncertain. Therefore, constraints on the properties of the
substructure population or accurate cosmological constraints, such as the mass
of the warm dark matter particle, are difficult if not impossible to derive at
this point.
