  We prove that the boundary of the future of a surface $K$ consists precisely
of the points $p$ that lie on a null geodesic orthogonal to $K$ such that
between $K$ and $p$ there are no points conjugate to $K$ nor intersections with
another such geodesic. Our theorem has applications to holographic screens and
their associated light sheets and in particular enters the proof that
holographic screens satisfy an area law.
