  The Newman-Penrose formalism may be used in numerical relativity to extract
coordinate-invariant information about gravitational radiation emitted in
strong-field dynamical scenarios. The main challenge in doing so is to identify
a null tetrad appropriately adapted to the simulated geometry such that
Newman-Penrose quantities computed relative to it have an invariant physical
meaning. In black hole perturbation theory, the Teukolsky formalism uses such
adapted tetrads, those which differ only perturbatively from the background
Kinnersley tetrad. At late times, numerical simulations of astrophysical
processes producing isolated black holes ought to admit descriptions in the
Teukolsky formalism. However, adapted tetrads in this context must be
identified using only the numerically computed metric, since no background Kerr
geometry is known a priori. To do this, this paper introduces the notion of a
quasi-Kinnersley frame. This frame, when space-time is perturbatively close to
Kerr, approximates the background Kinnersley frame. However, it remains
calculable much more generally, in space-times non-perturbatively different
from Kerr. We give an explicit solution for the tetrad transformation which is
required in order to find this frame in a general space-time.
