  The Polchinski exact renormalization group equation for a scalar field theory
in arbitrary dimensions is translated, by means of a covariant Hamiltonian
formalism, into a partial differential equation for an effective Hamiltonian
density that depends on an infinite tower of momenta fields with higher spin. A
natural approximation scheme is then expanding the Hamiltonian in momenta with
increasing rank. The first order of this expansion, one next to the local
potential approximation, is regulator-independent and already includes
infinitely many derivative interactions. Further truncating this down to a
quadratic dependence on the momenta leads to an alternative to the first order
of the derivative expansion, which is used to compute $\eta=0.03616(1)$ for the
critical exponent of the three dimensional Ising model.
