  Recent discoveries in the field of observational cosmology have provided
increasing evidence that the Universe is undergoing a late time acceleration,
which has also stimulated speculations on the nature of the dark component
responsible for such a phenomenon. Among several candidates discussed in the
current literature, phantom fields, an exotic scalar field with a negative
kinetic term and that violates most of the classical energy conditions, appear
as a real possibility according to recent observational analysis. In this paper
we examine the invariant characterization for the energy-momentum tensor of
phantom fields through the Segre algebraic classification in the framework of
general relativity. We also discuss some constraints which are imposed on the
values of $V(\phi)$ from the classical energy conditions.
