  Small distortions in the images of Einstein rings or giant arcs offer the
exciting prospect of detecting dark matter haloes or subhaloes of mass below
$10^9$M$_{\odot}$, most of which are too small to have made a visible galaxy. A
very large number of such haloes are predicted to exist in the cold dark matter
model of cosmogony; in contrast other models, such as warm dark matter, predict
no haloes below a mass of this order which depends on the properties of the
warm dark matter particle. Attempting to detect these small perturbers could
therefore discriminate between different kinds of dark matter particles, and
even rule out the cold dark matter model altogether. Globular clusters in the
lens galaxy also induce distortions in the image which could, in principle,
contaminate the test. Here, we investigate the population of globular clusters
in six early type galaxies in the Virgo cluster. We find that the number
density of globular clusters of mass $\sim10^6$M$_{\odot}$ is comparable to
that of the dark matter perturbers (including subhaloes in the lens and haloes
along the line-of-sight). We show that the very different degrees of mass
concentration in globular clusters and dark matter haloes result in different
lensing distortions. These are detectable with milli-arcsecond resolution
imaging which can distinguish between globular cluster and dark matter halo
signals.
