The electromagnetic implementation of cloaking, the hiding of
objects
from sight by di-
verting and reassembling illuminating electromagnetic fields has now been with us ten years,
while the notion of hiding
events
is now five. Both schemes as initially presented neglected
the inevitable dispersion that arises when a designed medium replaces vacuum under trans-
formation. Here we define a transformation design protocol that incorporates both spacetime
transformations and dispersive material responses in a natural and rigorous way. We show
how this methodology is applied to an event cloak designed to appear as a homogeneous
and isotropic but
dispersive
medium. The consequences for spacetime transformation de-
sign in dispersive materials are discussed, and some parameter and bandwidth constraints
identified.