  After decades of searching, the true nature of dark matter still eludes us.
One potential probe of the form of dark matter in galaxy clusters is to search
for microlensing variability in the giant arcs and arclets. In this paper, a
simple method is introduced to characterize pixel variability in the limit of
high optical depth to microlensing. Expanding on earlier work, the expected
microlensing signal for two massive clusters, A2218 & A370 is calculated. It is
found that the microlensing signal depends sensitively upon the mix of smooth
and compact dark matter in the cluster. Comparison of two deep exposures taken
with James Webb Space Telescope or two hour long exposures taken with a
30-metre class telescope in two epochs separated by a few years will possibly
detect about a few dozen pixels which show strong variability due to
microlensing at five sigma level, revealing wealth of information on the
microlensing population.
