  The properties of principal null directions of a perturbed black hole are
investigated. It shown that principal null directions are directly observable
quantities characterizing the space-time. A definition of a perturbed
space-time, generalizing that given by Stewart and Walker is proposed. This
more general framework allows one to include descriptions of a given space-time
other than by a pair $(M,g)$ where $M$ is a four-dimensional differential
manifold and $g$ a Lorentz metric. Examples of alternative characterizations
are the curvature representation of Karlhede and others, the Newman-Penrose
representation or observable quantities involving principal null directions.
The conditions are studied under which the various alternative choices of
observables provide equivalent descriptions of the space-time.
